KSEB Training Reporty

Industrial Training Report

INDUSTRIAL TRAINING REPORT

Submitted in partial fulfilment of the requirements for the award of degree of Bachelor of Technology in

ELECTRICAL & ELECTRONICS ENGINEERING

Of Mahatma Gandhi University, Kottayam

ByPONNU CHANDRAN

DEPARTMENT OF ELECTRICAL ENGINEERING

RAJIV GANDHI INSTITUTE OF TECHNOLOGY

GOVERNMENT ENGINEERING COLLEGE

KOTTAYAM 686501

2013 – 2017

DEPARTMENT OF ELECTRICAL ENGINEERING

IDept. Of EEE RIT Kottayam

Industrial Training Report RAJIV GANDHI INSTITUTE OF TECHNOLOGY

GOVERNMENT ENGINEERING COLLEGE

KOTTAYAM 686501

2013 - 2017

This is to certify that the Industrial Training report is an authentic report presented by Ms. PONNU CHANDRAN, Reg. No. 13013878, during the year 2015 in partial fulfilment of the requirements for the award of Degree of Bachelor of Technology in Electrical Engineering of Mahatma Gandhi University, Kottayam.

Radhika R. Prof. Mary George

Staff Advisor Head of the Department

Internal Examiner External Examiner

IIDept. Of EEE RIT Kottayam

Industrial Training Report ACKNOWLEDGEMNENT

I express my sincere gratitude towards Prof. Mary George, Head of the Department of Electrical and Electronics Engineering, for giving us her invaluable knowledge and excellent technical guidance.

I would like to express my sincere thanks to Mrs. Radhika R. for her kind cooperation and guidance.

I also thank all other faculty members of the Electrical and Electronics Department and my friends for their help and support.

I wish to express my deep sense of gratitude to all the employees of Lower Periyar Power Station and 220kV Substation, Poovanthuruthu for their overall direction and guidance during the training program.

Last but not the least I thank the god almighty for having made my endeavour successful.

PONNU CHANDRAN

CONTENTSIII

Dept. Of EEE RIT Kottayam

Industrial Training Report 1. Section 1: Lower Periyar Power House.............................................................11.1 Introduction........................................................................................................3

1.2 Power station......................................................................................................4

1.3 Elements of power station............................................................................5

1.4 Lower Periyar Power Station......................................................................13

1.4.1 Introduction................................................................................ 13

1.4.2 Hydraulic system of lower periyar..............................................13

1.4.3 Hydro turbine..............................................................................15

1.4.4 Guide vane servo motors.............................................................17

1.4.5 Governer mechanism...................................................................18

1.4.6 Electo hydraulic transducer.........................................................18

1.4.7 Valve gallery................................................................................18

1.4.8 Generator.....................................................................................20

1.4.9 Bearings.......................................................................................24

1.4.10 Hydro static lubrication system..................................................25

1.4.11 Static excitation..........................................................................26

1.4.12 Cooling system...........................................................................29

1.4.13 Circuit breakers and generator transformers..............................31

1.4.14 Protection system.......................................................................33

1.4.15 Communication...................................................................................35

1.4.16 Operation of different equipment.....................................................35

1.5 Single Line Diagram..................................................................................40

2. Section 2:-220 kV Substation Poovanthuruthu........................................................41IV

Dept. Of EEE RIT Kottayam

Industrial Training Report 2.1 Introduction................................................................................................................43

2.2 Substation................................................. ................................................................44

2.3 Elements of Substation............................ .................................................................45

2.3.1. Power Transformer............................ .................................................................45

2.3.2. Circuit Breaker....................... ............................................................................48

2. 3.2.1. SF6 Circuit Breaker.....................................................................................50

2.3.2.2. Vacuum Circuit Breaker..............................................................................51

2.3.3. Instrumental Transformers.......................................................................................51

2.3.3.1. Potential Transformer.................................................................................52

2.3.3.2. Current Transformer...................................................................................52

2.3.4. Isolator.....................................................................................................................53

2.3.5. Insulator...................................................................................................................54

2.3.6. Wave Trap................................................................................................................55

2.3.7. Bus Bars...................................................................................................................56

2.3.8. Relays......................................................................................................................56

2.3.9. Lightning Arresters..................................................................................................58

2.3.10. DC Supply.............................................................................................................59

2.3.11. Battery Charger.....................................................................................................59

2.3.12. Switch Yard...........................................................................................................59

2.3.13. Steel Towers..........................................................................................................60

2.4. 220 KV Substation Poovanthuruthu...............................................................................................................61

2.5. Single Line Diagram.................................................................................................62

2.6. Details of 220 KV Substation Poovanthuruthu...............................................................................................................63

3. Conclusion ...................................................................................................................66

VDept. Of EEE RIT Kottayam


VIDept. Of EEE RIT Kottayam


SECTION:-1

LOWER PERIYAR POWER STATION

1Dept. Of EEE RIT Kottayam


AcknowledgementWe the students of Govt. Rajiv Gandhi Institute of Technology, Kottayam, have undertaken practical training at Lower Periyar Power Station, Karimanal, under the guidance and supervision of Mr. Jayaraj (Executive engineer). We are thankful to all the employees of this power-station who helped us to gain the practical knowledge and answered our queries to the best of our satisfaction. We feel obliged by gaining knowledge under the esteemed guidance of able personals at Lower Periyar Power Station.

During the training from 08-6-2015 to 12-6-2015 we have prepared this report of practical training, which gives insight information about instruments and apparatus used in this system and their working in brief. We can say that this report is a summary of what we have observed and learned there.



1.1. INTRODUCTION The Hydroelectric Power Plant, also called as dam or hydro power plant, is used

for generation of electricity from water on large scale basis. The dam is built across the large river that has sufficient quantity of water throughout the river. In certain cases where the river is very large, more than one dam can built across the river at different locations .among the various renewable natural energy resources; the hydropower generation has emerged as the most potential option in terms of environmental cleanliness and cost-effective high capacity generation. The hydel power s t a t i o n have the inherent ability for instantaneous starting, stopping and load variations, which ensures a high reliability of power system. Therefore, hydel power stations are the best option for meeting the peak demand. Further, the generation cost in hydroelectric projects is inflation free and reduces substantially over time after repayment of debt. With 41 rivers, flowing down (westward) from the Western Ghats joining the backwaters and the Arabian Sea, Kerala has tremendous potential for hydel-power generation.

Power generation started in Kerala in 194 7 with the commissioning of the Pallivasal hydro-electric project at the Ramaswami Ayer Headwork close to the tea county of Munnar in the erstwhile princely State of Travancore. The Kerala power system consists of 17 hydel stations including 2 captive power plants, 2 thermal stations,

3 independent power producers, 5 major inter-state transmission lines, one 400 KV sub• section, and two 220 KV substations with the interconnecting grid. Kerala has a storage capacity of 3843mu and the present storage is about 72% of the full capacity.

Mullaperiyar dam, Idukki Hydro-electric project, Idamalayar Hydro electric project and the Lower Periyar are constructed across the Periyar. Kundala Dam, Mattupetty Dam, Munnar head works, Ponmudi dam and the Kallarkutty Dam are constructed across the various tributaries of Periyar.

Lower Periyar hydroelectric project (180 MW) envisages utilization of the tail waters from the existing Neriamangalam power station and the spill from Kallarkutty head works.

The Sengulam hydroelectric project is situated downstream of Pallivasal Project in Mudirampuzha river, which is an important tributary of Periyar river. Panniyar hydroelectric project is developed on Panniyar, a tributary of Mudirampuzha river.



1.2. POWER STATION

A power station (also referred to as a generating station, power plant, powerhouse or generating plant) is an industrial facility for the generation of electric power. At the centre of nearly all power stations is a generator, a rotating machine that converts mechanical power into electrical power by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on which fuels are easily available, cheap enough and on the types of technology that the power company has access to. Most power stations in the world burn fossil fuels such as coal, oil, and natural gas to generate electricity, and some use nuclear power, but there is an increasing use of cleaner renewable sources such as solar, wind, wave and hydroelectric.

1.3.ELEMENTS OF POWER STATION



Hydroelectric power plant requires various components for generating electrical power. Some of the major components in hydroelectric power plants are: Reservoirs, Dam, Trash Rack, Forebay, Surge Tank, Penstock, Spillway, Prime Mover and Generator, Draft Tube. The functions of all major components are discussed.The basic requirement of a hydroelectric power station is a reservoir where large quantity of water is stored during rainy season and used during the dry season. The reservoir is built by constructing a dam across the river. The water from the reservoir is drawn by the forebay through an open canal or tunnel. The water from the forebay is supplied to the water prime mover through the penstock which is located at the much lower level than the height of the water in the reservoir. Thus potential energy of water stored in reservoir is converted into kinetic energy and made to rotate the turbine. Turbine shaft is connected to synchronous generator or alternator for generating electricity. This generated power is stepped up using step-up transformer and delivered to load centers or grid. The regulation of water flow to the turbine depending on the electrical load demand is carried out by the governor system.

fig 1.1

Reservoir:

The function or purpose of reservoir is to store the water during rainy season and supply the same during dry season. This is in simple, water storage area.

Dam:



Dams are structures built over rivers to stop the water flow and form a reservoir. The reservoir stores the water flowing down the river. This water is diverted to turbines in power stations. The dams collect water during the rainy season and stores it, thus allowing for a steady flow through the turbines throughout the year. Dams are also used for controlling floods and irrigation. The dams should be water-tight and should be able to withstand the pressure exerted by the water on it. There are different types of dams such as arch dams, gravity dams and buttress dams. The height of water in the dam is called head race.Trash Rack:The water intake from the dam or from the forebay are provided with trash rack. The main function of trash rack is to prevent the entry of any debris which may damage the wicket gates and turbine runners or choke-up the nozzles of impulse turbine. During winter season when water forms ice, to prevent the ice from clinging to the trash racks, they are often heated electrically. Sometimes air bubbling system is provided in the vicinity of the trash racks which brings warmer water to the surface of the trash racks.

Surge Tank:

The main function of surge tank is to reduce the water hammering effect. When there is a sudden increase of pressure in the penstock which can be due sudden decrease in the load demand on the generator. When there is sudden decrease in the load, the turbine gates admitting water to the turbine closes suddenly owing to the action of the governor. This sudden rise in the pressure in the penstock will cause the positive water hammering effect. This may lead to burst of the penstock because of high pressures.

fig 1.2

When there is sudden increase in the load, governor valves opens and accepts more water to the turbine. This results in creation of vacuum in the penstock resulting into the negative water hammering effect. Therefore the penstock should have to withstand both positive water hammering effect created due to close of governor valve and negative water hammering effect due to opening of governor valve. In order to protect the penstock from these water hammering effects, surge tank is used in hydroelectric power station.

Penstock:

Penstock is a pipe between the surge tank and the prime-mover. The structural design of the penstock is same as for any other pipe expect it has to bear high pressure on the inside surface during sudden decease in the load and increase in the load. Penstocks are made of steel through reinforced concrete. Penstocks are usually equipped with the head gates at the inlet which can be closed during the repair of the penstocks, A sufficient water head should be provided above the penstock entrance in the forebay or surge tank to avoid the formation of



vortices which may carry air in to the penstock and resulting in lower turbine blade efficiency.

fig 1.3

Spillway:

The function of spillway is to provide safety of the dam. Spillway should have the capacity to discharge major floods without damage to the dam and at the same time keeps the reservoir levels below some predetermined maximum level.

Power House:

A power house consists of two main parts, a sub-structure to support the hydraulic and electrical equipment and a superstructure to house and protect this equipment. The superstructure of most power plants is the buildings that house all the operating equipment. The generating unit and the exciter is located in the ground floor. The turbines which rotate on vertical axis are placed below the floor level while those rotating on a horizontal axis are placed on the ground floor alongside of the generator.Power station contains a turbine coupled to a generator. The water brought to the power station rotates the vanes of the turbine producing torque and rotation of turbine shaft. This rotational torque is transferred to the generator and is converted into electricity. The used water is released through the tail race.

Prime movers or Hydro Turbines:

The main function of prime movers or hydro turbines is to convert the kinetic energy of the water in to the mechanical energy to produce the electric power. The prime movers which are in common use are Pelton wheel, Francis turbine and Kaplan turbines.

Draft tube:



The draft tube is a part of the reaction turbine. The draft tube is a diverging discharge passage connecting the running with tailrace. It is shaped to decelerate the flow with a minimum loss so that the remaining kinetic energy of the water coming out of the runner is efficiently regained by converting into suction head., thereby increasing the total pressure difference on the runner. This regain of kinetic energy of the water coming out from the reaction turbine is the primary function of the draft tube. The regain of static suction head in case where the runner is located above the tail water level is the secondary purpose of the draft tube.

Generator:

The generator converts the rotational energy from the turbine shaft into electricity. Efficiency is important at this stage too, but most modern, well-built generators deliver good efficiency. Direct current (DC) generators, or alternators with rectifiers, are typically used with small household systems, and are usually augmented with batteries for reserve capacity, as well as in8verters for converting the electricity into the AC required by most appliances. DC generators are available in a variety of voltages and power outputs.

fig 1.4

AC generators are typically used with systems producing about 3 KW or more. AC voltage is also easily changed using transformers, which can improve efficiency with long transmission lines. Depending on your requirements, you can choose either single-phase or three-phase AC generators in a variety of voltages.

Frequency is determined by the rotational speed of the generator shaft; faster rotation generates a higher frequency

Turbine:

The turbine is the heart of the hydro system, where water power is converted into the rotational force that drives the generator. For maximum efficiency, the turbine should be designed to match your specific head and flow. There are many different types of turbines, and proper selection requires considerable expertise. A Pelton design, for example, works best with medium to high heads. A cross flow design works better with lower head but higher



flow. Other turbine types, such as Francis, turgo, and propeller, each have optimum applications.

fig 1.5

Turbines can be divided into two major types. Reaction turbines use runners (the rotating portion that receives the water) that operate fully immersed in water, and are typically used in low to moderate head systems with high flow. Examples include Francis, propeller, and Kaplan.

Wicket Gates are the key components in Hydroelectric Kaplan turbine that controls the flow of water from penstock to turbine (runner). There are 16 wicket gates used for each turbine. The closing and opening of these wicket gates are controlled by a governor mechanism which is activated by a servomotor

Impulse turbines use runners that operate without being immersed, driven by one or more high-velocity jets of water. Examples include Pelton and turgo. Impulse turbines are typically used with moderate-to-high head systems, and use nozzles to produce the high-velocity jets. Some impulse turbines can operate efficiently with as little as 5 feet (1.5 m) of head.

The cross flow turbine is a special case. Although technically classified as an impulse turbine because the runner is not entirely immersed in water, this “squirrel cage” type of runner is used in applications with low to moderate head and high flow. The water passes through a large, rectangular opening to drive the turbine blades, in contrast to the small, high-pressure

jets used for Pelton and Turgo turbines.

Regardless of the turbine type, efficiency is in the details. Each turbine type can be designed to meet vastly different requirements. The turbine system is designed around net head and



design flow. These criteria not only influence which type of turbine to use, but are critical to the design of the entire turbine system.

Minor differences in specifications can significantly impact energy transfer efficiency. The diameter of the runner, front and back curvatures of its buckets or blades, casting materials, nozzle (if used), turbine housing, and quality of components all affect efficiency and reliability.

Drive system:The drive system couples the turbine to the generator. At one end, it allows the turbine to spin at the rpm that delivers best efficiency. At the other, it drives the generator at the rpm that produces correct voltage and frequency—frequency applies to alternating current (AC) systems only. The most efficient and reliable drive system is a direct, 1:1 coupling between the turbine and generator.

This is possible for many sites, but not for all head and flow combinations. In many situations, especially with AC systems, it is necessary to adjust the transfer ratio so that both turbine and generator run at their optimum (but different) speeds. These types of drive systems can use either gears, chains, or belts, each of which introduces additional efficiency losses into the system. Belt systems tend to be more popular because of their lower cost.

Governing:

The purpose of governor pack is opening and closing of Main Inlet Valve and Wicket Gates (guide vanes). The PLC is connected to a governor. It regulates the speed of the system. The PLC command opens or energises the valve. Opening the valve causes the oil pressure to affect the servomotor, which in turn changes the wicket gates and the runner blade accordingly. This has a feedback mechanism. It require certain amount of power for control, normally it is 125 bar. This pressure is maintained using a servomotor. An accumulator is connected to governor. Accumulator is used to store nitrogen at a pressure range between 110-140 bar. The wicket gate is mounted on a ring. The pressurised nitrogen pushes the piston, this causes the wicket gate to open and close.

AC Controls:-Pure AC hydro systems have no batteries or inverter. AC is used by loads directly from the generator, and surplus electricity is burned off in dump loads—usually resistance heaters.

Governors and other controls help ensure that an AC generator constantly spins at its correct speed. The most common types of governors for small hydro systems accomplish this by managing the load on the generator. With no load, the generator would “freewheel,” and run at a very high rpm. By adding progressively higher loads, you can eventually slow the generator until it reaches the exact rpm for proper AC voltage and frequency. As long as you maintain this “perfect” load, known as the design load, electrical output will be correct. You might be able to maintain the correct load yourself by manually switching devices on and off, but a governor can do a better job—automatically.



By connecting your hydro system to the utility grid, you can draw energy from the grid during peak usage times when your hydro system can’t keep up, and feed excess electricity back into the grid when your usage is low. In effect, the grid acts as a large battery with infinite capacity.

If you choose to connect to the grid, however, keep in mind that significant synchronization and safeguards must be in place. Grid interconnection controls do both. They will monitor the grid and ensure that your system is generating compatible voltage, frequency, and phase. They will also instantly disconnect from the grid if major fluctuations occur on either end. Automatic disconnection is critical to the safety of all parties. At the same time, emergency shutdown systems interrupt the water flow to the turbine, causing the system to coast to a stop, and protecting the turbine from over speed.

DC Controls:-A DC hydro system works very differently from an AC system. The alternator or generator output charges batteries. A diversion controller shunts excess energy to a dump load. An inverter converts DC electricity to AC electricity for home use. DC systems make sense for smaller streams with potential of less than 3 KW.AC systems are limited to a peak load that is equivalent to the output of the generator. With a battery bank and large inverter, DC systems can supply a high peak load from the batteries even though the generating capacity is lower.

Series charge controllers, like those used with solar-electric systems, are not used with hydro systems since the generators cannot run without a load (open circuit). This can potentially damage the alternator windings and bearings from over speeding. Instead, a diversion (or shunt) controller must be used. These normally divert energy from the battery to a resistance heater (air or water), to keep the battery voltage at the desired level while maintaining a constant load on the generator.

The inverter and battery bank in a DC hydro system are exactly the same as those used in battery-based, solar-electric or wind-electric systems. No other special equipment is needed. Charge controller settings may be lower than used in typical PV and wind systems, since hydro systems are constant and tend to run with full batteries much of the time.

Generator Transformer:

Transformers connected to generator usually supply power to a transmission line which run from the generating plant to a bulk power load center located a considerable distance away. Some of the general requirements of a generator transformer are as follow :

1. No voltage regulating windings, because the voltage is regulated by the field of the generator.2. Fairly uniform load - the new units of high efficiency in particular are kept loaded to

maximum capacity.3. Least need for high efficiency or quiet operation - power for losses is cheapest at a generator

station, and other equipment makes more noise than the transformer.4. Construction can be such as to require the type of supervision and maintenance available in a

generating station.



fig 1.6

Switch Yard:

Switch yard is the most important part of a substation. In switch yard most of the part is laid with metals to reduce earthed voltage. In the switch yard the supply taken from incoming feeders are transferred to one or more bus bars from which they are switched on or off to various incomers and distribution auxiliary supply etc.

fig 1.7

1.4.LOWER PERIYAR POWER STATION

1.4.1 INTRODUCTION

Lower Periyar Power House which is situated at Karimanal is the third biggest generating station of K.S.E.B. The installed capacity of lower Periyar generating station is 3x60MW and there are 6 nos. 220kV out going feeders. This is the first generating station in KSEB using microprocessor



controlled logic circuit for the automatic operation of the generators from shutdown status to generator status and from generator status to shutdown status. It is the second generating station in Kerala where static excitation system is adopted. These machines are designed for synchronous condenser operation also. It forms one of the most important tie station in the power grid of Kerala .The 220 kV feeders from Lower Periyar powerhouse are 1) double circuit feeder to Idukki power house, 2) double circuit feeder to 400 kV substation Madakkathra, and 3) double circuit feeder to 220 kV substation Bhrahmapuram.During the tied operation of these lines, the 220kV bus will be the main inter linking bus for the 4 most important major grid stations of KSE Board viz. ldukki power house, 400kV substation Madakkathra, and 220 k V substations Bhrahmapuram which is directly tied with Kayamkulam Thermal station.

1.4.2 HYDRAULIC SYSTEM O F L O WER PERIYAR Average annual generation at the power station is approximately 69MW or 609Mu. Reservoir at Pambla along Mudirampuzha river basin with dam of 3 lm high above nominal riverbed and 244m long across river Periyar about 5km downstream of Panamkutty Power House form the water conductor system. Storage level of reservoir is approximately 4.55 MCM. The dam is of concrete gravity type with a FRL of 253m.there are 5 motorized upper vents and 2 hydraulic lower vents for the operation of dam. The intake arrangement consists of an intake well provided with a trash rack, an intake gate and also an emergency gate. There is a level difference between dam level and intake well level. The system also comprises 6.05 m d ia, D Shaped, 12.79 km long circular concrete lined Power Tunnel, a restricted orifice Surge Shaft of 18 meter diameter, a 5.25 meter finished diameter, pressure shaft of length 378 meter, branching in to three steel lined pressure shafts each of 2.96 meter diameter and of average length of 207 meter.A surface Power House with three machines located at Karimanal a b o u t 18km downstream of Mudirampuzha, Periyar confluence. The power house is of l 80MW capacity with 3 units of 60 MW each mechanically coupled to Francis turbines. The generator output is stepped up to 220KV by a 66.6 MVA power transformer and is distributed among 6 feeder lines, two each to Idukki, Bhrahmapuram and the 400KV Madakkathra.... .Specifications of the Hydraulic System:-

Reservoir-PamblaRiver basin MudirampuzhaStorage 455MCMWater usage 2.17MCM/MU

DamType Scheme concrete gravity runoff river



Maximum water 256mlevel Full reservoir 253mlevel Minimum 237.76mDraw Down level

Power TunnelSize and shape 6.05m,D shapelength 12.791 kmSill level at inlet 229.00 mSill level at surge shaft 186.55mMaximum velocity in tunnel for 434 m/seca discharge of 124.7m3 /sec

Surge ShaftType restricted orificeSize 18 m diaTop level of surge shaft 285.00 mMinimum down surge level 197.99mBottom level of surge shaft 194.10mControl gate vertical lift gate

Pressure shaftNo. of pressure shaft 1Size and shape 5.25 m,circularLength 378 mManifold (steel lined) size and shape 5.25 m, dia

Branch linesNo. of shafts 3 NosSize 2.96 m dia,circularAverage length 207 m

1.4.3HYDRO-TURBINE



Francis turbine

fig 1.8

The Lower Periyar Hydroelectric project employs the Francis Turbine. Francis Turbine has a circular plate fixed to the rotating shaft perpendicular to its surface and passing through its center. This circular plate has curved channels on it; the plate with channels is collectively called as runner. The runner is encircled by a ring of stationary channels called as guide vanes. Water is brought to the turbine and directed to guide vanes or wicket gates. Guide vanes are housed in a spiral casing called as volute. The exit of the Francis turbine is at the center of the runner plate. There is a draft tube attached to the central exit of the runner. The design parameters such as, radius of the runner, curvature of channel, angle of vanes and the size of the turbine as whole depend on the available head and type of application altogether.The modem Francis Turbine is an inward mixed flow reaction turbine i.e., the water under pressure enters the runner from the guide vanes towards the centre in radial direction and discharge out of the runner axially. The Francis turbine operates under medium heads and also requires medium quantity of water. The head acting on the turbine is transformed into kinetic energy and pressure head. Due to the difference of p7ressure between guide vanes and the runner (called reaction pressure), the motion of runner occurs. That is why a Francis turbine is also known as reaction turbine.

The pressure at inlet is more than that at outlet. In Francis turbine runner is always full of water. The moment of runner is affected by the change of both the potential and kinetic energies of water. After doing the work the water is discharged to the tail race through a closed tube called draft tube.

It is employed in the medium head power plants. This type of turbine covers a wide



range of heads (30m to 450m). Francis turbine doesn't allow the water to fall freely to the tailrace level as in the case of Pelton turbine. The free end of the draft tube is submerged deep in the tail water, thus making the entire water passage, right from the head race up to the tail race totally enclosed.The draft tube converts kinetic head to pressure head. About 70% conversion is possible. By recovering pressure head in the draft tube the pressure at the runner exit is reduced below atmosphere. This makes it possible to install the turbine above the tail race without any loss in available head. This is an important advantage in the reaction over Pelton turbine.The turbine has its own thmst bearing capable of carrying the additional load of turbine shaft, runner and hydraulic thrust making a total of three guide bearings for the complete unit.

Specification:Type Vertical FrancisRated/Max 61300/67400 kwOutput designNet head 184 mMax gross head 204.58 mMin net head 165 mRated/Max discharge 36.2/40.2 cub m3/secRated speed 333.33 rpmRun away speed 585 rpmDirection of rotation clockwiseMax pressure rise 50%Max speed rise 50%The vertical shaft Francis type turbine comprise of a draft tube, spiral casing and stay rings, guide apparatus, shaft, runner, guide bearing, shaft seal and auxiliary items. The guide apparatus regulates the flow of water with, change in load and also serves as a closing device. It includes top cover, pivot ring, guide vanes and turning machinery. The mechanism for turning the guide vanes (regulating ring) is designed to ensure simultaneous turning of guide vanes during opening or closing of guide apparatus. Two servomotors, housed inside the pit liner, actuate the regulating ring which in tum operates the guide vanes through regulating gear.To facilitate atmospheric arr supply below the runner during part load operation of turbine, the necessary connections from the aeration valve are made in the upper cone.The shaft sealing prevents leakage of water through clearance b e t w e e n top cover and shaft sleeve. It is located b e l o w turbine g u i d e bearing.To prevent the abrasive particles and dirty water corning in contact with the rubber-sealing ring, water at a pressure slightly higher than that above the runner is supplied at three points of the shaft seal through a micro filter from the main cooling water system.



Oil level relay is provided on the bearing housing to indicate high and low oil levels of the bearings at Unit control board [UCB]. Temperatures o f guide bearing pads are monitored by a set of resistance temperature detectors [RTD] and dial type thermometers [DTT]. Out of eight pads, temperatures of four pads are measured by RIDS and the remaining four by DTTS. Two RTDs measure temperature in the oil bath.

1.4.4 GUIDE VANE SERVO-MOTORS

Guide vanesfig 1.9

Guide vanes are fixed aerofoils that direct air, gas, or water into the moving blades of a turbine or into or around bends in ducts with minimum loss of energy. The runner of turbine is encircled by a ring of guide vanes. Guide Vanes are installed in the turbine to regulate the quantity of water to the runner with change in load. These are operated by two servomotors through guide vane operating mechanism via links & levers. The servomotors get signals from Governor. The guide vanes are of aero flow section, which allows the flow of water without formation of eddies in all positions. Depending upon silt flow, the guide vanes may be made of mild steel or stainless steel with integral machined stems, which are drilled for grease lubrication of bushes.

Two servomotors are provided for turning the regulating ring during regulation of load on turbine and closing /opening of the guide apparatus. When the turbine load changes during generating operation, the servo motor shall operate the guide vane smoothly coordinating with the speed governor.

1.4.5 GOVERNER MECHANISM



The primary purpose of a governor for a hydroelectric unit is to control the speed and loading of the unit. It accomplishes this by controlling the flow of water through the turbine by adjusting the opening of the Needles I Guide vanes and by sensing the Speed of the Machine.

The governing system consists of two parts (i) the sensing and signal processing part. (ii) The operational part. In the operational pan hydraulic oil pressure isused for operating vanes and valves.

1.4.6 ELECTRO HYDRAULIC TRANSDUCERThe electro- hydraulic transducer is the interface between the electronic signal processing part and the hydraulic operating part. This transducer receives the electric signal from electronic part and converts the signal into a hydraulic flow. This hydraulic signal is hydraulically amplified and used for operating the vanes or the jets and deflectors.When an opening signal is received from the electronic governor, the actuator will pull the floating valve piston to go down and pressure oil is admitted to opening side of servomotor and servomotor gradually opens. As the servomotor opens, the feedback lever pushes the floating lever upwards. When feedback push equals the feed forward pull, the distributing valve piston will return to the original position and steady state is achieved.

1.4.7 VALVE GALLERY

Valve Galleryfig 1.10

On the upstream side there is valve gallery throughout the length of the floor. The main equipments on this floor are Butterfly [BF] valve, water operated servomotors, oil leakage units and the pipelines for the same. The access to the draft tube cone and the removal of the runner for maintenance is also from this floor. The station drainage system is installed on the left hand side of the Power



Station when viewed from the downstream side.A 2.2 m dia. double door BF valve has been provided as main inlet valve on each penstock branch. Water operated double acting servomotor(20 kg/cm) has been provided on the left hand side of the BF valve and is mechanically connected with a lever and keyed to the door turn-on of the BF valve.

A 100 NB drain valve is provided on the bottom side of the BF valve to drain the water in between the two doors of the BF valves and is connected to the penstock drain pipe.The servomotor is water operated. An oil operated control valve (40 kg/cm") is provided to adjust opening and closing of the valve. For the opening of the main inlet valve [MTV], water under pressure is taken from the spiral side and for the closing the same is taken from the penstock side through isolating 40 ~B valves and duplex strainers. Time of closing is 50-55 sec. The operation of the control valve is carried out by oil pressure through a solenoid valve mounted on the MIV control panel. If the oil pressure is low due to control failure or any other fault, when the MIV is open, the control spring will force the operating piston down to its closed position. This will close the MIV automatically.

All these assembly has been provided on the left side of BF valve. From the upstream of inlet pipe of the BF valve tapping and connections are taken with isolating valves, for operating control valve, ejector, and pressure gauges.

Bypass valvefig 1.11

Oil operated by-pass valve and piping are provided over the top of the BF valve for balancing the pressure on either side of the BF valve. The opening and dosing of the valve is carried out with the help of pressurized oil taken from the oil pressure system through a solenoid valve which is mounted on the MIV control panel. Limit switches are provided to get the opening and closing indications for the by-pass valve and BF valve.1.4.8 GENERATOR



An alternator is an electromechanical d e v i c e that converts mechanical energy to electrical energy in the form of alternating current.Alternators generate electricity based on the principle that, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet, called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an induced emf (electromotive fo rce ), as the mechanical input causes the rotor to tum.

The rotating magnetic field induces an AC voltage in the stator windings. Often there are three sets of stator windings, physically offset so that the rotating magnetic field produces a three phase current, displaced by one-third of a period with respect to each other.

The rotors magnetic field may be produced by induction (as in a "brush-less" alternator), by permanent magnets (as in very small machines), or by a rotor winding energized with direct current through slip rings and brushes.In alternators, the armature may be the rotor or stator. The rotating-field alternator has a stationary armature winding and a rotating-field winding. The advantage of having a stationary armature winding is that the generated voltage can be connected directly to the load. The stationary armature, or stator, of this type of alternator holds the windings that are cut by the rotating magnetic field .. Rotating-field ac generator consists of an alternator and a static excitation system. In the case of a machine with field coils, a current must flow in the coils to generate the field; otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation. The output of the alternator section supplies alternating voltage to the load. The only purpose for the exciter is to supply the direct current required to maintain the alternator field. Thus, a fixed-polarity magnetic field is maintained at all times in the alternator field windings. When the alternator field is rotated, its magnetic flux is passed through and across the alternator armature windings. There are two types of rotors used in rotating-field alternators. They are called the turbine-driven and salient-pole rotors.

The windings can be lap or wave. Generators can be installed horizontally as well as vertically based on the weight.The Generators installed at Lower Periyar Power House are of vertical type, salient pole, and suspended type construction. The stator winding is of two-layer bar type wave winding. The Generator has a guide bearing positioned above the rotor, and one guide bearing below the rotor.

Hydro Static [HS] lubrication system for injection of oil to the thrust bearing pads



have been provided for use during starting and stopping. The generator slip rings and speed signal generators are located at the top. The generator excitation is provided by separate static excitation equipment.

SpecificationMaximum continuous rating 66.67 MVARated power 60 MWRated voltage 11000VRated power factor 0.9 lagging Rated frequency 50 Hz Rated speed 333.33 rpmNo. of poles 18 Direction of rotation clockwiseAir gap at po le centre 26mmStator Resistance 0.00505 ohmPhase Stator winding connection star(wave)Field winding Resistance 0.14255 ohm Excitation current at no load 607 ampsExcitation current at rated load 1250 A, 230 V, 287.5 KWStator current at rated load 3500 A

Stator:-The different parts are:-Frame-The stator frame is used to hold the armature windings in alternators, and in case of larger diameter alternators (which are slow speed) the stator frame is cast out of sections and there are holes for ventilation in the casting itself. The recent trends to such stator construction are more in favour of using mild steel plates which are welded together rather than castings. The stator frame is built of welded steel structure and to facilitate transport, it is dispatched from the factory in three parts. It has adequate depth to prevent distortion during transport and under any operating conditions.Core- Another integral part of the stator is the stator core. The core is constructed in the form of laminations and the material used for the same is either magnetic iron or steel alloy. The main purpose of lamination is to prevent loss of energy in the form of eddy currents. There are different types of armature slots provided in the core to insert the conductors and the three various types are as follows.

• Wide open type slots • Semi closed type slots • Close type slots

The core is securely clamped by a large number of studs. Ventilation ducts are provided at intervals along the stator core, being formed by means of non magnetic steel spacing is securely welded to adjacent steel stampings. Jacking



screws are provided at the outer edge of end plates to enable the pressure of the teeth to be adjusted.

Windings:-The stator winding is of two layer bar type wave winding. All the bars are formed, insulated and tested before being placed in the slots. Each bar consists of a number of individual copper strands of rectangular section to minimize eddy current losses. Each strand is insulated with polyesterimide varnished glass brainding. The bars are insulated along the slot portion by adequate presses and consolidated in a heated press. This ensures complete elimination of voids and high factor of safety against breakdown..

S ta to rf ig 1 .12

The end portion of the bar have flexible insulation consisting of polyester film and glass backed mica flake tape, reinforced at intervals with layers of varnish treated terylene tape and with glass tape for protection and finish. The joints between the bars are made by brazing and are insulated. All connections between bars and terminals are securely clamped. Both ends of each phase windings are brought out to suit the terminals near the top of the stator frame..

Rotor:-The rotor consists of a coil of wire wrapped around an iron core. Current through the wire coil - called "field" current - produces a magnetic field around the core. The strength of the field current determines the strength of the magnetic field. The field current is DIC, or direct current. In other words, the current flows in one direction only, and is supplied to the wire coil by a set of brushes and slip rings. The magnetic field produced has, as any magnet, a north and a south pole. The rotor is driven by the alternator pulley, rotating as the engine runs, hence the name "rotor." The rotor is constructed with a high strength alloy steel shaft forging that is precision machined, ground and finished to exact tolerances.



Rotorfig 1.13

Poles:-There are 18 magnet blocks on each rotor. Each magnet block has a north pole and a south pole. The poles are arranged alternately, so north faces the stator on one block and south on the next. The poles on the other magnet rotor are arranged in the opposite polarity so that the north poles face south poles across the stator. In this way, a strong magnetic flux is created through the stator between the magnet rotors. The coils embedded in the stator are dimensioned such as to encircle the flux from one magnet pole at a time. As the magnet blocks pass a coil, the flux through the coil alternates in direction. This induces an alternating voltage in each turn of the coil. The voltage is proportional to the rate of change of flux..

Damper Winding:-The rotor is equipped with damper windings. They stabilize the speed of AC generator to reduce hunting under changing loads. If speed tends to increase induction-generator action occurs in damper winding.

This action places a load on the rotor tending to slow down the machine. In case of speed decrease induction-motor action takes place.The damper winding is of major importance to the stable operation of the generator. While the generator is operating in exact synchronism with the power system, rotating field and rotor speed exactly matched, there is no current in the damper winding and it essentially has no effect on the generator operation. If there is a small disturbance in the power system, and the frequency tends to change slightly, the rotor speed and the rotating field speed will be slightly different. This may result in oscillation, which can result in generator pulling out of step with possible consequential damage.

Damping bars are of circular sections of copper which are semi closed in the pole faces. The ends of the bars are short circuited together by copper stampings. The damper winding is inter-connected between poles.



Field Winding:-The magnetic field in the synchronous generator is created by field winding. The field coils are square ended being fabricated from a straight length of copper strips dove tailed and braced at the ends. At intervals down each coil the copper is increased in width to give fins for cooling purposes..All connections between adjacent field coils and also between field coils and slip rings are firmly secured to the rotor.

Temperature Detectors:-Resistance temperature detectors are built into the generator stator core and windings. The detectors are of three wire resistance type having 100 ohms resistance at o0c and 138.5 ohms at 100°c. The loads from the detectors are brought out to a metal clad terminal box located in a conveniently accessible position from which cables could be run to the indicating instrument via generator marshalling box.

1.4.9 B EARINGS Conventional alternators comprise of top-mounted thrust and guide bearing supported on heavy brackets, capable of supporting total weight of generator. A guide bearing is a plain bearing used to guide a machine element in its lengthwise motion, usually without rotation of the element. A bottom guide bearing combined with turbine shaft is usually provided. This conventional design is used for high speeds (up to 1000 rpm) generators..

Thrust bearing:-Thrust bearing in any turbo machine is used to prevent axial tolerance on the shaft. The thrust bearing is a spring supported type in which the stationary part consist segmental pads supported on mattress of helical springs. The rotating bearing surface is machined accurately perpendicular to the axis of the shaft. The bearing surface is polished to fine surface finish. The thrust pads are of stress relived mild steel and are faced with a high quality white metal. Each pad rests on a number of springs which are pre-compressed by a permanently locked centre screw and finished to a standard overall length.

The springs are assembled on a heavy fabricated spring plate which is an integral part of the thrust bearing housing. The thrust pads are prevented from moving circumferentially by pad stops secured to the spring plate. Radial movement is prevented by-dog damps which would also prevent the pad from rising with the thrust block during rotor jacking operation. The thrust bearing pads are completely immersed in oil bath. The oil is cooled by plug in oil coolers.



Top Guide Bearing:-The top and bottom guide bearings are of the pivoted pad type consisting of a row of white metalloid pads arranged in a support ring. Top guide bearing is located above the thrust bearing, on a journal surface machined on the periphery of the thrust collar. Sufficient insulation and protection is provided in top guide bearing to prevent flow of shaft current through the bearing pads. The same oil bath for the thrust pads is used for the guide bearing.

Bottom Guide Bearing:-Bottom guide bearing is located on a journal integrally forged with the shaft. A pivot bar is bolted to the back or each guide bearing pad to enable the pad to rock slightly to take up a suitable position and facilitate formation of the oil film, when running. The clearance between individual pads and the journal is set by adjusting the shims between the back of the pad and the pivot bar. The pads are cooled by an oil bath with plug in type coolers.

1.4.10 H Y DRO STATIC [ HS] LUBRICATION SYS T E M

Lubricants (solid or fluid film) are deliberately applied to produce low friction and low wear. In hydrostatic lubrication, a thick fluid film is maintained between two surfaces, with little or no relative motion, by an external pumping agency: a pump, which feeds pressurized fluid to the film.

Hydrostatic lubrication requires an external pumping agency. HS bearings provide high load-carrying capacity. Since HS bearings do not require relative motion of the bearing surfaces to build up the load-supporting pressures as necessary in hydrodynamic(HD) bearings by viscous shear/drag, HS bearings can be used in applications with little or no relative motion between the surfaces.The hydro static lubrication system has been designed to provide an oil film between the thrust pads and the runner disc during starting and stopping when there is little likelihood of formation of hydrodynamic oil film.

Therefore, it should always be put on service before starting the unit. However, if for any reason the HS lubrication system is out of order, the rotor shall be jacked up and released just before starting the unit, to ensure formation of oil film. This operation is not necessary If the machine has been at stand still for less than 12 hours.

Brakes and Jacks:-

The generator brakes consists of a number of 'Ferodo' lined shoes which operates against a polished circular steel brake track to the underside of the



rotor spider hub. Each brake shoe is mounted on a vertical piston moving in a small cylinder. To apply/release the brakes, air would be forced into the brake cylinder in appropriate direction from the station compressed air supply. The brake cylinders are mounted on the bottom bracket.

The brakes are to be applied continuously starting from 30 rpm, with HS. Lube ON and with air pressure of 4 to 5 bars for minimizing brake -dust problems. When the machine has come to a full stop, the brake should be left on for about 5 minutes more, to flow static friction to be established between the rotating parts and the bearing pads. If sufficient time is not allowed for the oil to squeeze out from between the bearing surface to establish static the friction, turbine gate leakage torque may cause the rotor to creep, which could cause damage to the thrust bearing pads.

1.4.11 STATIC EXCITATIONLower Periyar is the second power house using static excitation in Kerala State Electricity Board. The static excitation system consists of excitation transformer, thyristor converter and voltage regulator. A complete system also includes control and de-excitation circuits. It is called static excitation when you make use of solid state components like diode and thyristors to convert to pure de and to use this de for field excitation of synchronous generators.The Thyristor-type static excitation system, due to its many advantages, excellent response characteristics, easy maintenance and simplified main machine construction, is now extensively used for medium-and large-capacity hydro-or steam-turbine generators.In the case of synchronous machine protection circuit's activation the automatic quick field suppression via a de-excitation D.C. circuit breakerand a discharge resistor is accomplished. The excitation system is equipped with a microprocessor control system that enables voltage control, supervision, protection, communication and signalization. The system is completely automated and adapted for no-crew plants and for remote control from the superimposed control centre.

The main types of Exciters are:

1) Conventional D.C. Exciter.2) Static Exciter.3) Brushless Exciter

In modern generators, magnetic field is produced by an electromagnet. Equipments required to produce a controlled amount of field current is known as Excitation System.

Static Excitation Equipment:-

It consists of Regulation Cubicle, field flashing & field breaker cubicle, thyristor cubicles, and transformer cubicle. All excitation power is normally derived from the



synchronous machine terminals through the step down excitation transformer of 850 kVA rating, generally termed as the rectifier Transformer or the Excitation Transformer, housed inside a cubicle and the thyristor converter. The voltage regulator via pulse• triggering unit controls the thyristor converter.As synchronous machine has low remnant voltage, the voltage built-up in the self excitation mode is accomplished by flashing the field from an external D.C. supply (station battery) or with AC. supply (station auxiliary) through a diode rectifier. The control circuit is suitable to accept supervisory command signal contacts from remote Supervisory control equipment.The AC input supply of all electronic power supplies are given from the secondary of the Excitation Transformer through suitable intermediate transformers. The secondary of the Excitation Transformer feeds the thyristor bridge which consists of parallel connected bridges to meet the field current requirement of the Machine.The DC output of the Thyristor Bridge is fed to the generator field through field breakers. The discharge resistance in the field circuit enables faster suppression of stored energy in the field.

Static Excitationfig 1.14

Power Rectifier:-Three phase 6-pulse fully controlled thyristor bridges with fuse RC circuit, gate circuit and de coupling reactors are provided with conduction monitoring unit to indicate with the help of LEDs the non-conduction of any thyristor in the bridge. De• coupling reactors provided in each arm of the bridge for di/dt protection also improves the paralleled sharing between thyristor bridges. One redundant bridge is built in the system such that in the event of -failure of one bridge rest of the bridges can carry full rated excitation requirement of the machine. With 2 bridges out of service, machine can be operated at reduced load with the remaining bridge.

Voltage Build up/field flashing:-Electrical generators that are self excited depend on residual magnetism in the field to start generating. If the residual magnetism has been lost, it may be restored by



briefly applying power from an external source. The brief application of power for that purpose is called "flashing the field." Flashing is sometimes done manually to start a small generator.

Voltage build up (field flashing) can be done either with the help of station battery supply through a dropping resistor and blocking diodes stack or Station Auxiliary supply through a step down transformer and diode bridge. At 30% of the rated generator voltage pulses to the thyristors in the main circuits are released and they take over the build process at about 40% of the rated generator voltage. For checking the healthiness of the main circuit, the field flashing is kept in circuit up to 70% of the rated generator voltage after which the field flashing circuit is automatically disconnected.

If a successful start up is not achieved during this period of time, a timer provided in the excitation circuit, switches off the field flashing process. It is to be noted that a minimum period of 10 minutes must elapse before field flashing is resorted once again. For AC field flashing a diode bridge stack consists of six screws in type diode mounted on suitable heat sink assembled side by side and can be easily replaced from the front. The six diodes are connected to from a three-phase bridge.

Modes of Operation:-

Two independent modes of operation are envisaged namely1. Automatic mode2. Manual mode.

Automatic mode:-In the Automatic Mode excitation is regulated by the AVR. The AVR compares the actual value of generator voltage which is sensed through PT after suitably stepping down and converting into DC with the reference value set on the Auto Reference Potentiometer. The amplified error (output of AVR) is used as control signal to control the Grid control unit (Firing Circuit) for the Auto Channel. The output pulse of the Grid control unit is amplified to boost the voltage level in the pulse Intermediate amplifying stage and power supply unit. The power supply unit of me pulse Intermediate Amplifying Stage feeds the A'VR and the Auto; I and is termed as supply A.Manual mode:-In the manual mode the Grid control unit of the Manual channel is directly controlled by the Manual reference potentiometer. The pulse generated by the Manual Grid Control unit is amplified in the pulse Intermediate stage and power supply. unit of the manual channel. The power supply unit of the pulse Intermediate Amplifying stage feeds reference voltage to the manual channel etc., and is termed as supply 'M'.



1.4.12 COOLING SYSTEMNormally cooling water is tapped from the penstocks and connected to a common inlet header through a duplex strainer with isolating valves on either side. The inlet header is connected to an outlet header through many numbers of cooling water pumps and a non-return valve. Isolating valves are provided on either side of the pumps.

Cooling Water systemfig 1.15

Normally cooling water for generator and transformer is taken from the outlet header through valves. The cooling water pressure at outlet header is sufficient as the same is tapped from the penstock; hence the cooling water pumps are normally not started.The cooling water system will be used for the following service. 1..Cooling water for turbine bearing and shaft seal.2.Cooling water for Generator coolers and bearing.3. Station services.4. Transformers

Cooling water for the above requirements is taken from cooling water pit which is connected to the tail races. The cooling water from the pit pushes through a duplex strainer pump motor sets with non return valve [NRV]. Pressure switch has been provided in each line which helps in the automatic start/stop of main and stand by pump. Discharge of each pump is connected to common header. Cooling water is supplied to Generator and turbine components through motor (4 Nos. llOHP) operated valve. Cooling water connection for transformer and station services are provided on the common header. Out of 4 pumps 3 pumps works as main pump for each unit and one pump is common as stand by. An emergency cooling water system is also provided to feed the cooling during total shutdown of the power supply..

High and Low Pressure Air System:-H.P Air system consists of two H.P. compressor sets with air-cooled systems. Air from compressors pass through non-return valve, isolating valve, air cooled after coolers and finally to the H.P air receiver. Isolating valve in the H.P. airline shall be kept open. The H.P receiver has pressure gauge and safety valve mounted on it. Low pressure (LP) air receivers are provided to supply low-pressure air to shaft seal,



Brake & Jack panel and station service. The feeding of air to the, LP receiver is earned out from the H.P. receiver through a pressure reducer. Pressure switches have been provided on the H.P. receiver to work the compressors automatically. The main compressor starts when the pressure drops below 37kg/sq cm and stop at 40kg/sq cm. The stand by compressor will start at 34kg/sq cm and stop at 40kg/sq cm. One pressure switch is set to give alarm at 32kg/sq cm.

Dewatering System:-

The de-watering system has been provided to remove water passage via. a de• watering pump to tailrace. The de-watering sump has two oil lubricated vertical turbine pump set (11 OHP) placed at turbine floor on the left hand side (near Unit-3) of the Power House. The discharge from the two pumps is connected to a common header via non-return valve and is lead to the tailrace. Level control relays nave been provided for the automatic operation or the pump sets. Pumps can also be operated manually by push buttons provided in the starter panels. A high level alarm is also provided in the sump to avoid flooding of the sump.

Drainage System:-

Water from the seepage, turbine leakage delivery water during the operation of BF valve and ejectors are taken to the drainage sump. This sump has got two vertical turbine pumps, (2x20HP) set with motors. The discharge from the two pumps is connected to a common header and leads to the tailrace. Level control relays have been provided for automatic starting and stopping of pump sets and can also operate manually by push buttons. A high level alarm is also provided in the sump..

Centralized Grease Lubrication System:-

To facilitate grease lubrication of every moving part of Inlet valve and turbine, a centralized grease lubrication system has been provided. The system is completely automatic with a synchronous time adjustable between 6 to 120 hours for repeating greasing cycles. The system consist of a heavy duty reciprocating pump drives plungers type pump with built in reduction gear, a four way solenoid valve, a set of dose feeders with high pressure pipes and fittings. The grease lubrication have provided on the valve door turnings on both sides with non return valves, servomotor lines of BF valve, guide vane lower bushing through non-return valves, upper bushings, guide vane servo motor pins and the regulating ring supporting bushes.Lubrication systems increase the life span of machine components and they protect from wear and corrosion. As a result, they are an inevitable part of modern service and maintenance concepts. Lubrication systems have the task of bringing the lubricant to the appropriate point in an exact measured quantity, at the



right time. In the field, single-line and progressive lubrication systems are largely used. The choice of a suitable lubricant is largely dependent on the operational method of the lubrication system and the application. This is why both these factors need to be carefully scrutinized.

Synchronous Condenser Operation:-A synchronous condenser (sometimes synchronous capacitor or synchronous compensator) is a device identical to a synchronous motor, whose shaft is not connected to anything but spin freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage, or to improve power factor.Increasing the device's field excitation, results in furnishing magnetizing power (kVARs) to the system. Its principal advantage is the ease with which the amount of correction can be adjusted.

The energy stored in the rotor of the machine can also help stabilize a power system during short circuits or rapidly fluctuating loads such as electric arc furnaces. Large installations of synchronous condensers are sometimes used in association with high-voltage direct current converter stations to supply reactive power.

1.4.13 CIRCUIT BREAKERS AND GENERATOR TRANSFORMERS

11 KV/220 KV Transformerfig 1.16

The combined electrical, physical, chemical and thermal properties offer many advantages when used in power switchgears. Some of the outstanding properties of SF6 making it desirable to use in power applications are:



• High dielectric strength • Unique arc-quenching ability • Excellent thermal stability • Good thermal conductivity

SF6 circuit breakers of capacity 1250 A, 40 kA, 245 KV are used in this power house. These are of air operated single break, with individual operating mechanism with one common air compressor unit coupled to the three limbs with pipe. AH control equipment and compressor are housed in the centre limb. The opening of the breaker is done by 15 Kg/sq. cm air pressure. While opening, the closing spring is automatically charged this is used for subsequent closing. The breaker can be operated locally or remotely according to the switch position.

GENERATOR TRANSFORMER

Specification:Technical dataMake Crompton greaves makeNo load voltage ratio 11KV/220KVTap changing circuit OFF load provided on HV sideVector simple YndlType of cooling ODWF (oil driven water forced)Constructional detailsHV line end 3 Nos,245 KV oil forcedBushing condenser typeLV line end HV 3Nos,24KV,4000ANeutral end Out door type bushingSupervisory Apparatus 1 out door type bushing

• A double float type Buchholz relay with a set of alarm and trip contacts

• A dial type oil temperature indicator with two sets of contacts for alarm and trip and maximum reading pointer• A winding temperature indicator with maximum reading pointer, heater bulb/ and four sets of contacts for alarm, trip, fan control, and oil pump.• A magnetic oil gauge, 2 oil flow indicators, 2 water flow indicators.• A No pressure release valve, pressure gauges in oil and water circuits.• Differential pressure gauge with a set of alarm contacts

The oil is pumped through heat exchangers using motor. Water to the heat exchangers is taken from the cooling water system controlled by motor operated valve followed by gate valve. While putting the transformer in service first oil pump must be started and then



only the cooling water valve is opened.1.4.14 PROTECTION SYSTEM

Protection of Generator and LineIn this powerhouse modern solid-state type protection relays are installed for generators and feeders. All the protective relays are of ABB make.

Generator - Transformer differential relay:-It is s three phase differential relay intended for all types of autotransformers, multi winding transformers, generator with step up transformer over all protection, often including the auxiliary transformer in the protected zone. In our power house overall protection of generator and transformer is adopted. The CT wiring is taken from the generator neutral side and from 220 kV side of the corresponding unit. A differential relay is connected so that it is supplied with current proportional to the current to the power transformer, and current out from the transformer. The relay is connected to the current transformers and possible auxiliary current transformers. For transformers with tap- changers for voltage control, the average ratio of the taps should be used for calculation. During normal operating conditions, small current flows through the differential circuit of the relay. This current corresponds to the excitation current of the transformer and to a current depending on the ratio error to the current transformers. Normally these two currents only comprise a small percentage of the rated current. The duty of the relay is to detect the internal faults (that is the faults within the generator, power transformer, or on the connecting lines and bus duct etc) and then rapidly initiate disconnection of the power supply. The internal faults that can occur are

1. Short circuit.

2. Ground faults

3. Turn-to- turn faults.

When faults arise outside the current transformer, the differential circuit of the relay maybe supplied with a relatively large current, which can be caused by ratio errors in the current transformers or by the tap changer not being in the centre tap position. If the tap changer is in a position 20% from the centre tap position, and the short circuit current is 10 times the rated current/a differential current of twice the rated current is obtained. The differential shall not operate for this differential current. In order to make an operate value setting for such high over current unnecessary, the differential relay is provided with a through fault restraint with restraining circuits. The relay then will not react for the absolute value of the differential current, but for a certain percentage differential current related to the current through the power transformer. When energizing a power transformer, it is possible to obtain a large inrush current in the exciting winding and then proportionally large-current in the



differential circuits of the relay. The magnitude and direction of the inrush current depends on the instant of switching in the power transformer, power transformer remanance, the design of the transformer, the type of the transformer connection, the method of neutral grounding, the fault MVA rating of the power system and power transformers connected in parallel.In modem system the current can be 5 to 10 times the rated current when switching in into the high voltage side, and 10- 20 times the rated current when switching to the low voltage side. To prevent the relay from operating when energizing power transformer, it is not possible, as a rule, to delay the operation during such a long time as required. Thus an instantaneous relay must have a magnetizing Inrush restraint and there by utilize a certain characteristic difference between the inrush current and the fault current.Auxiliary CTs are used to balance the current to the relay. In addition auxiliary CT may be used to reduce the effective leakage burden of the long secondary leads. The differential zone of the relay can include up to one kilometre of high voltage cable since adequate filtering provides security against high current oscillations.

Bus Bar Protection Differential Relay:-

Internal bus faults occur less frequently than line- faults. On the other hand, a bus fault tends to be appreciably more severe, both with respect to the safety of personnel, system stability and the damage at the point of fault. The fact that bus faults occur relatively seldom is therefore of little comfort to the engineer in-charge subsequent to a major system shutdown caused by the Sack of adequate bus relay.When an internal bus fault occurs the magnitude of the fault current and its D.C. component may be so large that the line CT's (current transformers) saturate within 2-3ms. In such cases it is essential that the bus differential relay operates and seals in within 2ms, i.e. Prior to the saturation of the line CT's. This high speed is necessary because when a line CT saturates its output e.m.f. tend to drop to zero.ln the event of an external fault, just outside the line CT's of a relatively small feeder, the fault current may in an extreme case be as large as 500 times the rating of the feeder. The line CT's of the faulty feeder are then likely to saturate at an even higher speed, particularly so if the remanence left in the core from a previous fault has an unfavourable polarity. The response of the restraint circuit of the differential relay must therefore be at least the same high speed as that of the operating circuit, if mal-operation is to be avoided.

Distance relay for feeder protection:-Distance relaying is used to a large extent to provide protection against ground and phase faults on HV and EHV networks, The operation of all distance relays is based on information available through main current and voltage transformers. Sometimes additional information may be required from other apparatus such as receiver equipment in a communication link between two distance relays.

But, the action of a protective relay cannot only be based on the sole estimation of currents and voltages in the primary system/ but must also take



into consideration the steady-state and transient characteristics of the relay input sources, namely the instrument current and voltage transformers. The demands made on protective relays are steadily increasing owing to such factors as the growing short-circuiting power and the demand of consumers for greater reliability in their power supply..

1.4.15 COMM U N ICATION

Power Line Carrier Communication (PLCC):-

PLCC System is established at LP Power Station through the 220 KV feeders LP• Brahmapuram, LP-Madakkathara & LP-Moo]amattom. Inter Circuit phase to phase coupling is used for the system. The feeders are provided with carrier inter trip protection coupler system. BPL make 9505 and 6515 model panels are used for the communication system. An exchange MDX 50 BPL make is used for linking PLCC phones to the panels. The station code of LP is '055'. The communication system works on power supply of 48V DC.The data and status of Generators and feeders are transmitted to the Load Despatch (LD) station Kalamassery through the PCNCOM make PLCC panel established the scheme up to Madakkathara and from there to LD station through optical fibre cable.AC Supply fail alarm for the PLCC Battery charger is wired to Unit No 1 annunciation panel. On initiation of this alarm in the CIR, the operator must inspect the carrier room for the reason of power failure.

1.4.16 OPERATION OF DIFFERENT E Q U IP M E N TS Procedure for Stand By Cooling Water Pump Operation:-

In case the main pump of any unit is not functional, Stand by Cooling Water Pump (CW4) can be used for starting and running of any machine. Stand by Cooling Water Pump runs on station auxiliary supply.

Sequence of Operation During Starting:-• Switch OFF the MCCB of the faulty C. W.P and put the selector switch in NORMAL position. Start the machine as usual. When the C.W. Valve of the machine is opened, Switch OK the Stand By Pump from the UCB of the respective machine. (The Stand By Pump can be Switched ON locally by putting switch to TEST position and pressing the START button locally from the C.W. Pump control panel at Turbine floor).

• At the same time short the terminals 64 & 67 in terminal block TB3 of the respective machine in Auto Sequencer Panel in Control Room, for getting the command from the sequencer for executing next step. When next step is executed



the shorting can be removed. The remaining procedures are same as usual for starting the machine.• While changing the machine supply Stand By Pump will not be affected.• After synchronization of the machine, if the Stand by pump is switched on in TEST position, the selector switch can be put back to normal.

Note: - If a machine is to be run using Stand By Pump it is better to put the machine in service as last one.

Sequence Of Operation During Stopping:-

While Stopping- Switch OFF the machine having Stand By Pump first.Do the Stopping procedure as Usual. After breaking, when the machine comes to stand still, Switch OFF the stand by pump either from the UCB (Stop command is to be given from the UCBs of all machines ) or by putting the Selector Switch in TEST position and press the OFF push button locally. When Machine comes to standstill change the selector switch, of Stand by Pump, back to NORMAL.

Starting Of Machine During 'Black Out' Using Emergency Cooling Water system:-

• Avail the Station Supply from DG Set.•To start a machine from Black Out, Switch OFF the C\VP MCCB of the concerned machine and Switch OFF the MCCB of Stand By Pump. The selector switch of both the Pumps should be in NORMAL position.• Initiate machine Start Up sequence from Control Desk.•When the CW Valve of the machine opens, OPEN the Emergency Cooling Water Valve fully. (If required the Emergency Cooling Water Pumps can be put in to service from DG supply)• Short the concerned terminals of the Main and Stand pumps at Sequencer panel. (Main pump-TB3 - 64,67, Stand by TB3 - 68,71- if required).• When the machine Voltage and Frequency reaches the required level (before synchronization) change the supply from GA to GB.• Put the Main Cooling Water Pump selector in TEST, Switch ON the Main Cooling Water Pump MCCB and put the selector to NORMAL (the CWP will start automatically).• Close the Emergency cooling water valve.• Synchronize the Machine and Normalize the Auxiliary supply.

Procedure To Be Followed During Tripping Of All machines If auxiliary Supply is Available:-

Switch off the MCCBs of the Standby and any of the two (say #2 and #3) Main



Cooling Water Pumps immediately and then change the station supply (otherwise the LT Breaker may trip). Change the Auxiliary supply of all Machines. Open the Emergency Cooling Water valve (Emergency Cooling Water Pump can be put into service if required). Confirm that all the Governor Pumps are running. If not try to switch ON locally. Otherwise close the Isolation valve at Pressure Receiver Tank. Now stop the Machines one by one. After the machines comes to standstill, close the Emergency Cooling Water valve. Give stop command to the Cooling Water Pump of U#2 and U#3 from UCB. Close the cooling water valve from UCB and confirm. Check the Break Dust collector in OFF condition.

If auxiliary supply is not available:-

Switch off the MCCBs of the Standby and all the Main Cooling Water Pumps immediately. Then only avail DG Set Supply and change the station supply (otherwise the LT Breaker/DG Set may trip). Change the Auxiliary supply of all Machines. Open the Emergency Cooling Water valve (Emergency Cooling Water Pump can be put into service if required). Confirm that all the Governor Pumps are running. If not try to switch ON locally. Otherwise close the Isolation valve at Pressure Receiver Tank. Now stop the Machines one by one.After the machines comes to standstill, close the Emergency Cooling Water valve.Give stop command to the Cooling Water Pumps from UCB. Close the cooling water valve from UCB and confirm. Check the Break Dust collector in OFF condition.

Procedure for pneumatic breaking:-

Breaking of machine during stopping:-

When the Machine speed reaches 10 Hz (20% of rated speed) and getting confirmation from Chief Operator, (AE should confirm that the HS Pump is ON, if not, start locally). Fully open the Air valve Near the LP air receiver Tank in Turbine Floor. Open the Air admission Valve near the Brake & Jack panel in the Shaft Room. Press the RESET Button until the pressure inside the Brake cylinder fully released (the hissing sound stops). Apply brakes by pressing APPLY push button. Confirm that ANY ON and ALL ON indications are obtained. When the machine speed reaches Zero and Mechanical Brakes Off status is displayed on the CD, the Chief Operator should inform the concerned to release the Brakes.

For releasing the brakes, apply RESET button as above. Apply RELEASE push button until the ALL OFF indication is obtained. If ALL OFF indication is not getting, close the air valve at shaft room and apply RELEASE until the pressure gauge reads Zero, then conduct a visual check up inside the Barrel to confirm that, all brakes are released.



Procedures to be followed at Brake Jack panel before Starting of Machine:-

Fully open the Air valve Near the LP receiver Tank in Turbine Floor. Open the Air admission Valve near the Brake & Jack panel in the Shaft Room. Press the RESET Button and confirm that ANY ON and ALL ON indications are OFF and ALL OFF indication is ON. The indications GEN Start Not Ready and Syn. Start Not Ready in Control desk should get OFF. In the Display panel at CD "Air Pressure Normal" and "Mechanical Brakes reset" status will vanish. Now the Machine is ready for Starting.

Machine start procedure:-

Start procedure::-

• Close the 220 KV Isolator of machine from Control Desk.• Select Release - Close (A or B Bus)• Physical verification of lsolator contacts for proper closing must be done by AE.• Give direction to the Generator Floor AE to make ready the machine for Starting• OPEN Air Valve and RESET brakes.•The glowing "Gen. Start Not Ready" indication lamp in CD will fail.• The Machine is now READY for Starting.• Set the Speed Setting Indicator in its marked position using Raise-Lower Speed Setting push button• Switch ON the MCBs in the Transformer Annunciation Panel, Machine Annunciation panel and Vibration & Rotor Temperature Indicator Panel•Switch ON Transformer Oil Pump and Cooling water Valve from the Transformer Control Desk.

Synchronizing:-•Put the key, open the lock and put the Synchronising Selector S/W in CD to CHECK position. Select Release+ 'Synch'.•Adjust the voltage and frequency of the incoming M/c to that of Bus using excitation auto sel. and speed setting Raise-Lower Push Button• When the Incoming Machine frequency approaches the Bus frequency, Switch ON the Synchronoscope Selection Switch in Vertical panel.

• When the machine frequency matches system frequency and the green lamp of synchroscope glows steadily, Synchronize the machine .Select Release+ CB ON• Increase the Load Suddenly to l 5MW using R-L of Guide vane position (MW)•Increase the Load gradually to 45MW using G.V limit PB.



• Change the Auxiliary supply from Bus to Machine.

Shutdown Procedure:-• Load reduced to 45MW for All Machines• (Change the station auxiliary to other Machine or Karimanal Feeder)• Change Machine Auxiliary to Station Auxiliary• Select Display ON/OFF• Using Speed Setting Push Button Reduce the Load to 40 MW•By using Guide Vane Raise - Lower (MW) reduce the load to 20 M\V• (During this time the Output Setting must be >50)



1.5 SINGLE LINE DIAGRAM



SECTION: 2

220KV SUBSTATION POOVANTHURUTHU



AcknowledgementWe the students of Govt. Rajiv Gandhi Institute of Technology, Kottayam, have undertaken practical training at 110 kV Substation Poovanthuruthu, under the guidance and supervision of Mr. Jacob (Assistant Executive Engineer) of 220KV Substation Poovanthuruthu. We are thankful to all the employees of this substation who helped us to gain the practical knowledge and answered our queries to the best of our satisfaction. We feel obliged by gaining knowledge under the esteemed guidance of able personals at Poovanthuruthu substation.

During the training from 15-6-2015 to 19-6-2015 we have prepare d this report of practical training, which gives insight information about instruments and apparatus used in this system and their working in brief. We can say that this report is a summary of what we have observed and learned there.



2.1.INTRODUCTIONWe all know that electrical power systems are playing an important role in our daily routine. Electrical power is generated in power stations by different processes and from there it is transmitted to substations, which are located in different places through transmission lines. Then it is delivered to the consumer through a large network of transmission and distribution cables.

In electrical systems the most important components are the Power Transformers, circuit breakers, lightning arresters, instrument transformers, relays, wave traps, isolators, bus bars etc.



2.2. SUB–STATIONSubstation is a part of an electrical generation transmission and distribution system. Substation transforms voltage from high to low, or the reverse or performs any of the several other important functions. Between generating station and consumer electric power may flow through several substations at different voltage levels.

Substations may be owned and controlled by an electrical utility, or may be owned by a large industrial or commercial customer. Generally substations are unattended, relying on SCADA for remote supervision and control.

Substations may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. The word substation comes from the days before the distribution system became a grid. As central generating stations became larger, smaller generating plants were converted to distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station.

The electrical substation design is influenced by following aspects:

1. Rated voltage of incoming and outgoing lines

2. Total MVA to be transferred

3. Geographical area available

4. Step up and step down

5. Switching substation

6. Receiving substation

7. Distributing substation

8. Industrial substation



2.3.ELEMENTS OF SUBSTATION

2.3.1.POWER TRANSFORMER

fig 2.1

The Power Transformers are those transformers installed at the ending or receiving end of long high voltage transmission lines. The distribution transformers (generally pole mounted) are those installed in the location of the city to provide utilization voltage at the consumer terminals. Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.They have usually has one primary and one secondary, and one input and output.

Power transformers generally operate at nearly full – load. However, a distribution transformer operates at light loads during major parts of the day.The performance of the power transformers is generally judged from commercial efficiency. The rating of a high transformer is many times greater than that of distribution transformer and the flux density is also higher

Power transformer’s primary winding always connected in star and secondary winding in delta. In the Substation end of the transmission line, The power transformer connection is star-delta.( for the purpose of step down the voltage level)In the star up of the transmission line (H-T), the connection of the power transformer is delta – star (for the purpose of step up the voltage level)



Transformer Core:-

A physical core is not an absolute requisite and a functioning transformer can be produced by placing the windings near each other, an arrangement termed as ‘air-core’ transformer. The air which comprises the magnetic circuit is essentially lossless, and so and air core transformer eliminate loss due to hysteresis in the core material. The leakage inductance is inevitably high resulting in very poor regulation, and so such designs are unsuitable for usein power distribution. They have however very high bandwidth, and are frequently used in radio-frequency applications for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings. They are also used for resonant transformers such as tesla coils where they can achieve reasonably low loss in spite of the high leakage inductance.

Windings:-

The conducting materials used for the winding depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire, such are Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.

Bushings:-

Large transformers are provided with high voltage insulate bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control if the electric field gradient without letting transformer leak.

Tap Changer:-

A tap changer is a connection point selection mechanism along a power transformer winding that allows a variable number of turns to be selected in discrete steps. A transformer with a variable turns ratio is produced, enablingstepped voltage regulation of output. The tap selection may be made via an automatic or manual tap changer mechanism.

Cooling Equipment:-

ONAN Cooling of Transformer:-

This is the simplest form of cooling system. The full form of ONAN is “Oil Natural Air Natural”. Here natural convectional flow of hot oil is utilized for cooling. In convectional circulation of oil, the hot oil flows to the upper portion of the transformer tank and the vacant



place is occupied by cold oil. This hot oil which comes to the upper side will dissipate heat in the atmosphere by natural conduction, convection and radiation in air and will become cold.

In this way the oil in the transformer continually circulate when the transformer is put into load. As the rate of dissipation of heat in air depends on dissipating surface of the oil tank, it is essential to increase the effective surface area of the tank,so additional dissipating surface in the form of tubes or radiators are connected to the transformer tank. This is known as radiator bank of transformer.

ONAF Cooling of Transformer:-

Heat dissipation can obviously be increased by increased by increase in surface area, but it can be made further faster by applying forced air on that dissipating surface. Fans blowing air on cooling surfaces is employed. Forced air takes away the heat from the surface of the radiator and provides better cooling than natural air. The full form of ONAF is “Oil Natural Air Forced”. As the heat dissipation rate is faster and more in ONAF transformer cooling method than in ONAN cooling system, electrical power can be put into more load without crossing the permissible temperature limits.

OFAF Cooling of Transformer:-

The heat dissipation rate can be still improved if the oil circulation is accelerated by applying some force. In OFAF cooling system the oil is forced to circulate within the closed loop of the transformer tank by means of oil pumps. OFAF means “Oil Forced Air Force” cooling methods of transformer. The main advantage of this system is that it is a compact system and for same cooling capacity OFAF system occupies much lesser space than former two systems of transformer cooling. Actually in Oil Natural cooling systems , the heat comes out of the conducting part of the transformer is displaced form its position, is a slower rate due to convectional flow of oil but in oil forced cooling systems the heat is displaced from its origin as soon as it comes out in the oil, hence rate of cooling becomes faster.

For all T-6 models – 0.35to .84kg/cm2(any one value as per demand) For all T-3 models – 0.35to .84kg/cm2(any one value as per demand)

Port openings

For all T-6 models- about 150mm Dia. For all T-3 models- about 70mm Dia.

Oil/Winding Temperature indicator:-

Scientific Controls:-

Mechanical Instruments are incorporates proven design features acquired from many years of experience in providing Temperature Indicators/Controllers for Power& Distribution Transformers.



Oil Temperature Indicator:-

The Oil Temperature Indicator (OTI) measures the Top oil Temperature. It is used for control and protection for all transformers.

Winding Temperature Indicator:-

The winding is the one component with highest temperature within the transformer and, above all, the one subject to the fastest temperature increase as the load increases. Thus to have a total control of temperature parameter within the transformer, the temperature of winding as well as top oil must be measured. An indirect system is used to measure winding temperature as it is dangerous to place a sensor close to the winding due to heavy voltage. The indirect measurement is done by means of a built-in Thermal Image.

Winding Temperature Indicator is equipped with a specifically designed Heater which is placed around the operating bellows through which passes a current proportional to the current passing through the transformer winding subject to the given load. Winding temperature is measured by connecting the CT Secondary of the transformer through a shunt resistor inside the Winding Temperature Indicator to the Hater Coil around the operating Bellows. It is possible to adjust gradient by means of Shunt Resistor.

In this way the value of the winding temperature indicated by the instrument will be equal to the one planned by the transformer manufacturer for a given transformer load.

2.3.2.CIRCUIT BREAKER

fig 2.2

A circuit breaker is a manually or automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse, which



operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Operation

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents and high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high voltage circuit breakers are self-contained with current transformers, protective devices and internal control power source.

Once a fault is detected, contacts within the circuit breaker must open interrupt the circuit; some mechanically stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers maybe manually operated, larger units have solenoids to trip the mechanism, and electric motors to store the energy to the springs.

The circuit breaker contacts must carry the load current without heating, and must also with stand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature circuit breakers (MCB) and moulded-case circuit breakers (MCCB) are usually discarded when the contacts have worn, but power circuit breakers and high voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in. Different techniques are used to extinguish the arc including.

Lengthening /deflection of the arc Intensive cooling ( in jet chamber) Division into partial arcs Zero point quenching (Contacts open at the zero current time crossing of the AC

waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency, i.e. 100 times per second for 50 Hz and 120 times per second for 60 Hz)

Connecting capacitors in parallel with contacts in DC circuits



Finally, once the fault condition has been cleared, the contact must again be closed to restore power to the interrupted circuit.

2.3.2.1.SF6 Circuit Breaker

fig 2.3

A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as an SF6 Circuit Breaker.

SF6 has an excellent insulating property. SF6 has high electro-negativity. That means it has high affinity of absorbing free electron. Whenever a free electron collides with SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion.

The attachment of electron with SF6 gas molecule may occur in two different ways

1)SF6+e=SF6-

2)SF6+ e = SF5-+F

These negative ions obviously much heavier than a free electron and therefore over all mobility of the charged particle in the SF6 gas is much less as compared to other common gases. We know that mobility of charged particle is responsible for conducting current through a gas.

Hence for heavier and less mobile charged particle in SF6 gas, it acquires gas very high dielectric strength. Not only the gas has a good dielectric strength but also it has the unique property of fast recombination after the source energizing the spark is removed. The gas has also very good heat transfer property. Due to its low gaseous viscosity (because of less molecular mobility) SF6 gas can efficiently transfer heat by convection. So due to its high dielectric strength and high cooling effect SF6 gas is approximately 100 times more effective arc quenching media than air. Due to these unique properties of this gas SF6 Circuit Breaker is used in complete range of medium voltage and high voltage electrical power system. These circuit breakers are available for the voltage ranges from 33KV to 800KV and even more.



2.3.2.2.Vacuum Circuit Breaker

fig 2.4

A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption takes place in a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10-6 bar.

2.3.3.INSTRUMENT TRANSFORMERS

Instrument Transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformer is to operate instruments or metering from high voltage or high current circuits, safely isolating the secondary control circuitry from the high voltages or currents. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit.

Instrument transformers may also be used as an isolation transformer so that secondary quantities may be used in phase shifting without affecting other primary connected devices.

In Poovanthuruthu Substation there are a total of current transformers and potential transformers. They are mainly used for metering purpose and protection by operation of various relays.

TYPES:



2.3.3.1. Potential Transformer:-

fig 2.5

Potential transformers are also called voltage transformers (VT) are a parallel connected type of instrument transformer. They are designed to present negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering

.2.3.3.2.Current Transformer

fig 2.6

Current transformers (CT) are a series connected type of instrument transformed. They are designed to present negligible load to the supply being measured and have an accurate current ratio and phase relationship to enable accurate secondary connected metering.



Current transformers are often constructed by passing a single primary turn either an insulated cable or an uninsulated bus bar through a well-insulated toroidal core wrapped with many turns of wire. This affords easy implementation on high voltage bushing insulators and using the pass-through conductor as a single turn primary.

A current clamp uses a current transformer with a split core that can be easily wrapped around a conductor in circuit. This is a common method used in portable current measuring instruments but permanent installations use more economical types of current transformer. Specially constructed wireband CTs are also used, usually with an oscilloscope, to measure high frequency waveforms or pulsed currents within pulsed power systems. One type provides an IR voltage output that is proportional to the measured current; another called a Rogowski coil, requires an external integrator in order to provide a proportional output.

2.3.4. ISOLATOR

fig 2.7

Circuit breakers always trip the circuit but open contacts of breaker cannot be physically visible from outside of the breaker and that is why it is recommended not to touch any electrical circuit by just switching off the circuit breaker.

So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works. So definitions of isolator can be rewritten as Isolator is a manually operated mechanical switch which separates a part of the electrical power normally at off load condition.



Isolators are classified into two:

1.Line Isolator

2.Bus Isolator

Line Isolator:-

Line Isolator is the isolator which is situated in between potential transformer and current transformer. This is the isolator which is coming directly from line. This isolator has an inbuilt provision for grounding the main transmission lines so that work can be done on them.

Bus Isolator:-

This is the isolator which is coming directly from the bus. They are used to isolate the various buses present in the switch yard.

2.3.5.INSULATORS

fig 2.8

An insulator is a material whose internal electric charges do not flow freely, and therefore very hard to conduct an electric current under the influence of an electric field.

A perfect insulator does not exist, but some materials such as glass, paper and Teflon which have high resistivity, are very good electrical insulators. A much larger class of materials, even though they may have lower bulk resistivity, are still good enough to insulate wiring and cables. Examples include rubber like polymers and most plastics. Such materials can serve as practical and safe insulators for low to moderate voltages.



Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called insulation. The term insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers hold the conductors and prevent current through conductors to pass through the poles to the ground.

2.3.6. WAVE TRAP

fig 2.9

Wave traps in electrical systems are used for power plant communication through transmission wires. It is a combination of low inductances in series and high capacitances in parallel with transmission wires in power frequencies. But communication signals are very low amplitude and very high frequency signals. So, these signals are nearly open circuited by the inductance and short circuited by capacitance. That is how wave trap power and communication waves are used in PPC

2.3.7.BUS BARS

In electrical power distribution, a bus bar is also spelled as busbar or sometimes incorrectly as buss bar, with the term bus being a contraction of the Latin word omnibus. It is



a strip or bar of copper, brass or aluminium that conducts electricity within a switchboard, distribution, substation, battery bank or other electrical apparatus. Its main purpose is to conduct electricity, not to function as a structural member.

The cross-section size of the bus bar determines the maximum amount of current that can be safely carried. Bus bars can have cross sectional area as little as 10 mm2 but electrical substations may use metal tubes of 50 mm in diameter (20 mm2) or more as bus bars. An aluminium smelter will have very large bus bars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminium from molten salts.

fig 2.10

2.3.8.RELAYS

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used, where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming from one circuit and re-transmitting it to another. Relays were extensively used in telephone exchanges and early computer to perform logical operations.

A type of relay that can handle the high power required to directly control an electric motor or other loads is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload faults; in modern electrical power systems these functions are performed by digital instruments called “protective relays”.



Electromagnetic Relay:-

Electromagnetic relays are those relays which are operated by electromagnetic action. Modern electrical protection relays are mainly microprocessor based, but still electromagnetic relay holds its place. It will take much longer time to be replaced the all electromagnetic relays by microprocessor based static relays. So before going through details of protection relay system we should review various types of electromagnetic relays.

Electromagnetic Relay Working:-

Practically all the relaying device are based on rather one or more of the following types of electromagnetic relays.

a) Magnitude measurement

b) Comparison

c) Ratio measurement

Principle of electromagnetic relay working is on some basic principles. Depending upon working principle these can be divided into following types of electromagnetic relays.

i) Attracted Armature type relayii) Induction Disc type relayiii) Induction Cup type relayiv) Balanced Beam type relayv) Moving Coil type relayvi) Polarised Moving Iron type relay

Attraction Armature Type Relay:-

Attraction Armature type relay is the most simple in construction as well as its working principle. These types of electromagnetic relays can be utilized as either magnitude relay or ratio relay. These relays are employed as auxiliary relay, control relay, overcurrent, undercurrent, overvoltage, under-voltage and impedance measuring relays.

Hinged armature and plunger type constructions are most commonly used for these types of electromagnetic relays. Among these two constructional designs, hinged armature type is more commonly used.

2.3.9. LIGHTENING ARRESTERS

A lightning arrester, also known as lightning conductor, is a device used to on electrical power systems and telecommunication systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high



voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrester, in most cases to the earth

In telegraphy and telephony, a lighting arrester is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Smaller versions of lighting arresters also called surge protectors are devices that are connected between each electrical conductor in power and communication systems and the earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communication or power line is struck by lightning or is near to a lightning strike.

If the protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning produce extreme voltage spikes in incoming power lines can damage electrical home appliances.

fig 2.11

2.3.10. DC SUPPLY



fig 2.12

DC Supply is one of the most essential parts of a substation because all the relays are working on DC Supply. So even if AC Supply is not available the circuit breaker should work. For this DC Supply is very essential.

2.3.11.BATTERY CHARGER

The battery charger is used to provide dc supply to the substation. It is used to convert the AC signal to DC signal, for that purpose it employs a rectifier circuit.

2.3.12.SWITCH YARD

fig 2.13

Switch yard is the most important part of a substation. In switch yard most of the part is laid with metals to reduce earthed voltage

. In the switch yard the supply taken from incoming feeders are transferred to one or more bus bars from which they are switched on or off to various incomers and distribution auxiliary supply etc.

2.3.13.STEEL TOWERS (Transmission Towers):-



fig 2.14

A transmission tower (colloquially termed an electricity pylon in the United Kingdom and part of Europe, and a hydro tower in certain provinces of Canada where power generation is mainly hydroelectric) is a tall structure, usually a steel lattice tower, used to support an overhead power line. They are used in high voltage AC and DC systems, and come in a wide variety of shapes and sizes. Typical height ranges from 15 to 55 metres (49 to 180 ft) though the tallest are the 370 metre (1214 ft) towers of a 2700 metre long span of Zhoushan Island Overhead Power line Tie. In addition to steel, other materials may be used including concrete and wood.

There are four major categories of transmission towers: Suspension, Terminal, Tension, and Transposition. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual effect include undergrounding.

2.4.110 KV SUBSTATION POOVANTHURUTHU

Here the characteristic property ie voltage is changed and transmitted. That is 220 KV is changed and transmitted to 110 KV , 66 KV and 11 KV . This is a Transformer Substation.

fig 2.15



CHARACTERISTICS OF POOVANTHURUTHU SUBSTATION:-

The incoming 220 KV feeders in the substation are :

1. Brahmapuram(Diesel Plant)

2. Sabarigiri(Moozhiyar)

3. Kayamkulam(Feeder from new pallom)

4. Idukki(Feeder from new pallom)

Similarly, There are three voltage outgoing feeders

1. 110kV Outgoing:

1. Pallom-Chengannur 1(PLCE No.1)

2. Pallom-Chengannur 2(PLCE No.2)

3. MRF- Pallom(PL-MR)

4. Pallom –Chengalom(PL-CN)

5. Pallom-Ayarkunnam(PL-AY)

6. Singulam -Pallom(SCM-PL)

2. 66kV Outgoing:

1. Pallom- Mavelikkara No.1

2. Pallom- Mavelikkara No.2

3. Pallom- Ettumannoor No.1



4. Pallom- Ettumannoor No.2

5. Pallom-Gandhinagar

3.11kV Outgoing :

1. Puncha North

2. Puncha South 3. Puncha Central 4. Travancore Cements Ltd 5. Aymanam 6. Kottayam

MAIN COMPONENTS

Main components we observed in the visit are listed below:

1. Circuit Breakers

2. Lightning Arresters

3. wave-traps

4. Instrument Transformer

5. Transmission Lines

6. Isolators

7. Protection Relays

8. Transformers

CIRCUIT BREAKER



These are used as switching devices in high power lines. They are capable to make , brake and carry current under normal circuit conditions and abnormal conditions such as short circuit.

Properties of circuit breaker devices are :

Ability to minimize arching and its effects.

Minimum distortion to supply waveform during switching.

The different types of circuit breakers are :

SF6 circuit breaker

Air blast circuit breaker

Vaccum circuit breaker

Oil circuit breaker

And we observed SF6 Circuit Breaker and Air Blast Circuit Breakers

1. SF6circuit breaker

In such circuit breakers, sulphur hexaflouride (SF6) gas is used as the arc quenching medium. The SF6 is an electro-negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6 gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. The SF6 circuit breakers have been found to be very effective for high power and high voltage services.

2. Air blast circuit breaker

It is a comparatively high speed device and prevents re striking .Here , the arc quenching is by purely compressed air. The contacts are normally kept closed by spring pressure. when opens air is admitted in arc extinguish chamber at high pressure and it pushes the moving contact away and against spring pressure. The arc produced is extinguished by this air blast.

LIGHTNING ARRESTER

A lightning arrester is a device used on electrical power systems and telecommunications systems to protect the insulation and conductorsof the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a



ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

In telegraphy and telephony, a lightning arrestor is placed where wires enter a structure, preventing damage to electronic instruments within and ensuring the safety of individuals near them. Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the Earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communications or power line is struck by lightning or is near to a lightning strike.

WAVE TRAP

A line trap (high-frequency stopper) is a maintenance-free parallel resonant circuit, mounted inline on high voltage AC transmission power lines to prevent the transmission of high frequency (40 kHz to 1000 kHz) carrier signals of power line communication to unwanted destinations. Line traps are cylinder like structures connected in series with HV transmission lines. A line trap is also called a wave trap.

The line trap acts as a barrier or filter to prevent signal losses. The inductive reactance of the line trap presents a high reactance to high-frequency signals but a low to mains frequency. What this does is prevent carrier signals from being dissipated in the substation or in a tap line/branch of the main transmission path and grounds in the case of anything happening outside of the carrier transmission path. The line trap is also used to attenuate the shunting effects of high voltage lines.

Line traps are connected in series with power line and thus their coils are rated to carry the full line current. The impedance of a line trap is very low at the power frequency and will not cause any significant voltage drop.

INSTRUMENT TRANSFORMERS A transformer intended to supply measuring instruments, meters , relays and other similar apparatus are called Instrument Transformer



Current Transformers

A current transformer is defined as an instrument transformer in which the secondary current, in the normal conditions of use is proportional to the primary current and differs in phase from it by an angle, which is approximately zero for an appropriate direction of the connections.

Current transformers are therefore used with the measuring instruments to:

(a) Isolate the instruments from the power circuits.

(b) Standardise the instruments, usually at 5 amps or 1 amp.

Primary current

The primary current of the CT depends on the load on the circuit at which CT is connected. The standard values are 10,15,20,30,50,75,100,150,200,300,400,600,800,1000 and 2000.The selection should be based on the load in the circuit.

Secondary current

The standard values of rated secondary current shall be 1A and 5A. For distribution applications 5A is the standard secondary current. For substations application 11kV metering CT is having 5A secondary current . For 66Kv to 400kV metering 1A secondary current is used.

Potential Transformer

Potential transformers (PT) (also called voltage transformers (VT)) are a parallel connected type of instrument transformer. They are designed to present negligible load to the supply

being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering. They are used for measurement of high voltages by means of

low range voltmeters or for energizing the potential coils of wattmeters and energymeters.

potential transformer may have several secondary windings on the same core as a primary winding, for use in different metering or protection circuits. The primary may be connected phase to ground or phase to phase. The secondary is usually grounded on one terminal. Potential transformers are of shell type because such a construction develops high degree of accuracy.

TRANSMISSION LINE

We were able to observe different types of transmission cables at poovanthuruthu. A transmission line is a specialized cable or other structure designed to carry alternating current



of radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account.

DIFFERENT TYPES OF TRANSMISSION LINES:-

1) Balance Two Wire Line

In this type of construction for two wire transmission lines the insulated spacers are used in order to maintain the distance between the transmission lines or between the two conducting wire equally throughout.

2) Coaxial cable

Coaxial lines confine virtually all of the electromagnetic wave to the area inside the cable. Coaxial lines can therefore be bent and twisted (subject to limits) without negative effects, and they can be strapped to conductive supports without inducing unwanted currents in them.

4.6. ISOLATOR

Isolator is used to ensure that an electrical circuit is completely de-energised for service or maintenance. Such switches were used substations, where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers, transformers, and transmission lines, for maintenance. The dis-connector is usually not intended for normal control of the circuit, but only for safety isolation. Dis-connector can be operated either manually or automatically (motorized dis-connector). We familiarized with different type of isolators.

4.7. PROTECTION

Feeder protection

The feeder may be an EHT transmission line (short, medium or long) or sub transmission line (in the distribution system).

The protection shall be

1. Overcurrent and earth fault protection

2. Distance protection

3. Pilot wire protection

Overcurrent protection

It is the simplest and cheapest means of protection. No voltage connection is required for this. Two phases of one earth fault protection provide complete protection of three phase circuit.

Distance protection



Whenever the overcurrent relaying is found slow or selective, distance protection is used. Distance provides primary or back up protection in single schemes. It may be single or three stepped.

Auto re-close schemes

The majority of fault occurring in HV transmission line are of transient in nature (ie. lightning) and can be cleared without affecting the system stability, if the circuit is momentarily tripped at both ends, and reclosed. In the feeders only short reclosing is recommended. But in radial feeders, multiple short reclosing can be attempted to eliminate semi-permanent fault (also). Single phase and three phase reclosing facility in conjunction with high speed distance schemes and carrier inter-trip can be provided for 220kV line. For 110kV, 66kV line only three phase reclosing has been provided (mainly due to breaker limitations).

Anti-pumping feature is incorporated in the circuit breaker or in the auto reclosing scheme to avoid repeated reclosing operation during permanent fault(s).

Conditions for auto re-close:-

1. Time to be allowed for the arc to de-ionize so that it will not restrike during reclosing.

2. The opening and closing times of circuit breaker at two ends.

3. The probability of transient fault that will allow high speed reclosure of faulty lines.

Transformer protection

Fault in a transformer may be in the external connection or in the internal circuit.

4.8.TRANSFORMER

A transformer is a static machine used for transforming power from one circuit to another without changing frequency. Since the invention of the first constant potential transformer in 1885, transformers have become essential for the AC transmission, distribution, and utilization of electrical energy.

4.8.1. USE OF POWER TRANSFORMER

Generation of electrical power in low voltage level is very much cost effective. Hence electrical power is generated in low voltage level. Theoretically, this low voltage level power can be transmitted to the receiving end. But if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low level power must be stepped up for efficient electrical power transmission. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at



the receiving end with the help of step down transformer. These are the uses of electrical power transformer in the electrical power system.

Two winding transformers are generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use auto transformer where the ratio between high voltage and low voltage is less than 2. Again three phase single unit transformer is more cost effective than a bank of three single phase transformer unit in a three phase system. But still it is preferable to use than the later where power dealing is very large since such large size of three phase single unit power transformer may not be easily transported from manufacturer's place to work site.

4.8.2 AT POOVANTHURUTHU SUBSTATION

Transformers at Poovanthuruthu substation were introduced to us by Mr. Biju , an employee of Kerala State Electricity Board. His expertise in the respective field helped us in understanding the transformer operation well. At Poovanthuruthu substation, two set of single phase autotransformers of are used in converting 220 kV to 110 kV. The station capacity is 400 MVA. It is cost effective to use autotransformer where the ratio between high voltage and low voltage is less than 2. At Poovanthuruthu, three phase transformers are used in stepping down the voltage level to 66 kV and 11 kV. But we concentrated our studies on the 220/110 kV transformer. At Poovanthuruthu, power transformers are star- star connected, distribution transformers are delta- star connected and tertiary winding is delta connected. Tertiary windings are used to avoid harmonics and to avoid unbalancing current created during faults. The main components of transformer which we studied are described below.

4.8.3 TRANSFORMER CONSTRUCTION

The core, which provides the magnetic path to channel the flux, consists of thin strips of high-grade steel, called laminations, which are electrically separated by a thin coating of insulating material. he core is held together by, but insulated from, mechanical structures and is grounded to a single point in order to dissipate electrostatic buildup. Core steel is cold-rolled, grain-oriented steel. In autotransformer, the single continuous winding is divided into a number of "tappings" to produce different voltages. An appropriate number of turns are provided between each tapping to produce the required voltage, based on the turns ratio between the complete winding and the tapping. A useful method of calculating unknown voltages on an autotransformer, if the numbers of turns on the various tappings are known, is to use the volts per turn method described on the Basic Transformer Operation page. Unlike a conventional transformer with primary and secondary windings, the autotransformer does not provide any isolation between input and output.

4.8.4 CONSERVATOR TANK OF A TRANSFORMER

Conservator tank at Poovanthuruthu is half oil filled. This is a cylindrical tank mounted on supporting structure on the roof the transformer main tank. The main function of conservator tank of transformer is to provide adequate space for expansion of oil inside the transformer. When the transformer is loaded and ambient temperature rises, the volume of oil



inside transformer increases. A conservator tank of transformer provides adequate space to this expanded transformer oil. It also acts as a reservoir for transformer insulating oil. When volume of transformer insulating oil increases due to load and ambient temperature, the vacant space above the oil level inside the conservator is partially occupied by the expanded oil. Consequently, corresponding quantity of air of that space is pushed away through breather. On other hand, when load of transformer decreases, the transformer is switched off and when the ambient temperature decreases, the oil inside the transformer contracts. This causes outside air to enter in the conservator tank of transformer through silica gel breather.

4.8.5 CONSTRUCTION OF CONSERVATOR TANK

This is a cylindrical shaped oil container closed from both ends. One large inspection cover is provided on either side of the container to facilitate maintenance and cleaning inside of the conservator. Conservator pipe, i.e. pipe comes from main transformer tank, is projected inside the conservator from bottom portion. Head of the conservator pipe inside the conservator is provided with a cap. This pipe is projected as well as provided with a cap because it prevents oil sludge and sediment to enter into main tank from conservator. Generally silica gel breather fixing pipe enters into the conservator from top. If it enters from bottom, it should be projected well above the level of oil inside the conservator. This arrangement ensures that oil does not enter the silica gel breather even at highest operating level.

4.8.6 BUCHHOLZ RELAY

The Buchholz relay working principle of is very simple. Buchholz relay function is based on very simple mechanical phenomenon. It is mechanically actuated. Whenever there will be a minor internal fault in the transformer such as an insulation faults between turns, break down of core of transformer, core heating, the transformer insulating oil will be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to decomposition of transformer insulating oil will accumulate in the upper part the Buchholz container which causes fall of oil level in it.

Fall of oil level means lowering the position of float and thereby tilting the mercury switch. The contacts of this mercury switch are closed and an alarm circuit energized. Sometime due to oil leakage on the main tank air bubbles may be accumulated in the upper part the Buchholz container which may also cause fall of oil level in it and alarm circuit will be energized. By collecting the accumulated gases from the gas release pockets on the top of the relay and by analyzing them one can predict the type of fault in the transformer.

More severe types of faults, such as short circuit between phases or to earth and faults in the tap changing equipment, are accompanied by a surge of oil which strikes the baffle plate and causes the mercury switch of the lower element to close. This switch energized the trip circuit of the circuit breakers associated with the transformer and immediately isolate the faulty transformer from the rest of the electrical power system by inter tripping the circuit breakers associated with both LV and HV sides of the transformer. This is how Buchholz relay functions.



4.8.7 SILICA GEL BREATHER

Whenever electrical power transformer is loaded, the temperature of the transformer insulating oil increases, consequently the volume of the oil is increased. As the volume of the oil is increased, the air above the oil level in conservator will come out. Again at low oil temperature; the volume of the oil is decreased, which causes the volume of the oil to be decreased which again causes air to enter into conservator tank. The natural air always consists of more or less moisture in it and this moisture can be mixed up with oil if it is allowed to enter into the transformer. The air moisture should be resisted during entering of the air into the transformer, because moisture is very harmful for transformer insulation. A silica gel breather is the most commonly used way of filtering air from moisture. Silica gel breather for transformer is connected with conservator tank by means of breathing pipe.

Silica gel crystal has tremendous capacity of absorbing moisture. When air passes through these crystals in the breather; the moisture of the air is absorbed by them. Therefore, the air reaches to the conservator is quite dry, the dust particles in the air get trapped by the oil in the oil seal cup. The oil in the oil sealing cup acts as barrier between silica gel crystal and air when there is no flow of air through silica gel breather. The color of silica gel crystal is dark blue but, when it absorbs moisture; it becomes pink. When there is sufficient difference between the air inside the conservator and the outside air, the oil level in two components of the oil seal changes until the lower oil level just reaches the rim of the inverted cup, the air then moves from high pressure compartment to the low pressure compartment of the oil seal. Both of these happen when the oil acts as core filter and removes the dust from the outside air.

CONCLUSION

We were able to get to know more about substations and the different components used in substation. We were able to know the difference between switching station and distribution substation. We got an overview of working of different sections in substations. We also familiarized with the working of different types of transformers, circuit breakers, lightning arrestors and other major power handling equipments of the substation. We learned about the steps of power transformation and various specifications to be taken care when doing so. We also learned about the protection of important machines in the substation


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