INTRODUCTION TO POWER SECTOR

The prosperity of any country is determined by its inherent power generation capacity. Power is now used in the form of electricity and its availability has been found to be the most powerful driving force in the implementation of economic development and social change. Modernization, the productivity of industry and agriculture and improvement in the per capita income of people is now measured in per capita consumption of power. Naturally the planning, setting up of power generation units, transmission and generation has been of utmost national importance.

The country’s generation and power distribution system has been vastly divided into 6 regions namely northern, eastern, western, southern, north-eastern and islands which has been found to be quite extensive. Every state implements their own state electricity board to promote integrated operation of generation, transmission and distribution of power in their respective states. The central government has control over generating stations, transmission lines and substations through undertakings such as National Thermal Power Corporation, Nuclear Power Corporation and Power Grid Corporation of India Limited.

ORGANISATIONAL PROFILE

NTPC is the largest power generating company of India. A public sector company, it was incorporated in the year 1975 to accelerate power development in the country as a solely owned company of the Government of India. At present, Government of India holds 89.5% of the total equity shares of the company and the balance 10.5% is held by FIIs, domestic banks, public and others. Within a span of 36 years, NTPC has emerged as a true national power company, with power generating facilities in all the major regions of the country.

A consistent high level performer in operations, the premier power utility provides the bench mark for Indian power sector. With excellent operation and maintenance, the power plants of the company are achieving high level of availability and plant load factor year after year. With less than one-fifth installed capacity, NTPC generates more than one-fourth of the country’s total power. Recognising its excellent performance and vast potential, Government of India has identified NTPC as one of the jewels of public sector-“NAVARATNA” a potential global giant. Inspired by the glorious past and vibrant present, NTPC is well on its way to realise its vision of being one of the world’s largest and best power utilities, powering India’s growth. NTPC is committed to comply with all relevant statutory, regulation and consumer requirements to develop and maintain high level of competency in all levels of business.

The current installed capacity of NTPC reaches 34,194MW (including joint ventures with a capacity of 3364MW). An additional 14,748MW is under construction at 14 locations. The company has prepared a Corporate Plan with the target of having a total installed capacity of 1, 28,000MW by 2032 with 28% capacity coming from non-fossil sources.

AN OVERVIEW OF RGCCPP KAYAMKULAM

Rajiv Gandhi Combined Cycle Power project is the first naphtha based power plant in India. It was commissioned in 2000 with a total capacity of 350 MW.

It has 3 units, with 2 gas turbines of 115 MW capacity and a steam turbine of 120 MW capacity. The fuels used here are naphtha, HSD and natural gas. The gas turbines works on the basis of Brayton cycle and steam turbine works on the basis of Rankine cycle.

The directly coupled compressor of gas turbine sucks air from atmosphere through specially designed filter and sends to combustion chamber. Hot product of combustion chamber at 1124° C is made to expand in turbine section where thermal energy is converted to mechanical energy which drives the turbine and hence the generator.

The temperature of exhaust gas from turbine at 550°C still has considerable heat energy and is capable of producing power. Waste Heat Recovery Steam Generators (WHRSG) are used to recover this valuable heat energy. In HRSG the Deminaralised Water (DM) is heated by hot turbine exhaust gases to produce steam before gases are let out to atmosphere. A bypass stack is also provided to let out hot gases directly to atmosphere in case Steam Generator is shut down. In HRSG, steam is produced in two pressure levels viz. low pressure of 6ksc. and high pressure of 80ksc. which are separately piped to LP/HP cylinders of steam turbine respectively. In the steam turbine the thermal energy is converted to mechanical energy which drives the turbine which is coupled to generator to produce electricity.

The steam after expansion in steam turbine is condensed in a condenser using circulating water as a cooling medium, and this circulating water is cooled at cooling towers where its temperature falls from 45°C to 35°C. The condensed steam called condensate is pumped back to high/low pressure circuits of HRSG by condensate extraction pumps and HP/LP boiler feed pumps.

In both gas turbines and steam turbine electricity is produced at 10.5kv which is stepped to 220kv by Generator Transformer (GT1 and GT2) .Through Gas Insulated Switchyard(GIS) the power is evacuated through 4 lines to Edappon, Kundra and Pallom substations of KSEB. The plant is also equipped with 2 Unit Auxiliary Tranformers (UAT) of 10.5/6.6kv for auxiliary uses.

GAS TURBINE

INTRODUCTION

The gas turbine is a common form of heat engine working with a series of processes consisting of compression of air from atmosphere, increase of working medium temperature by constant pressure ignition of fuel in combustion chamber, expansion of SI and IC engines in working medium and combustion.

Since gas turbines were introduced in the industry some 25 years back, the power generated is used only for peaking load service as the cost of fuel is very high. However, with increase in efficiency and reliability, gas turbines are being used more and more in base load generation. Availability of state of art gas turbine technology and concept of combined cycles it is projected that the efficiency will increase to 60% in the next couple of years. The work developed by the gas turbine may be used as mechanical energy or may be converted to electricity by coupling to a generator. In air craft jet engines the gas turbine’s useful work is produced as thrust from the exhaust of the turbine. Today the largest commercial gas turbine is a Siemens manufactured 340MW for Berlin with an efficiency of more than 60%.

BRAYTON CYCLE

Gas turbine plants operate on the cycle in which air is compressed (process 1-2 in P-V diagram). This compressed air is heated in the combustion chamber by burning fuel, a part of the compressed air is used for combustion (process 2-3) and the flue gases produced are allowed to expand in the turbine (process 3-4) which is coupled with the generator. The temperature of the exhaust is about 773-823K.

RANKINE CYCLE

The conversion of heat energy to mechanical energy with the aid of steam is based on this thermo- dynamic cycle. In its simplest way the cycle works as follows.

The initial state of the working fluid is water (point 3 of figure) which at a certain temperature is pressurized by a pump (process 3-4) and fed to the boiler.

In the boiler the pressurized water is heated at a constant pressure (process 4-5-6-1) Super heated steam (generated at point-1) is expanded in the turbine (process 1-2) which is

coupled with a generator.

Modern steam power plants have steam temperature in the range of 773-823K at the inlet of turbine.

COMBINED CYCLE

A combined cycle block consists of two gas turbines, two waste heat recovery steam generation units and one steam turbine. The combined cycle integrates a gas cycle and a steam cycle, the Rankine cycle and the Brayton cycle with the principle objective of increasing overall plant power generation efficiency.

COMBINING TWO CYCLES TO IMPROVE EFFICIENCY

The gas turbine’s exhaust heat can be recovered using Waste Heat Recovery Boiler tpo run a steam turbine on Rankine cycle. If the efficiency of gas turbine cycle is 30% and the efficiency of Rankine cycle is 35%, then the overall efficiency becomes more than 45%. Conventional fossil fuel fired boiler of the steam power plant is replaced with a Heat Recovery Steam Generator (HRSG). The exhaust gas from the gas turbine is led to the HRSG where the heat of exhaust gases utilized to produce steam at desired parameters as required by the steam turbine.

ADVANTAGES OF GAS TURBINE PLANT

Low installed cost makes the installation of the station easier. Transport-easier shipping, handling because of its robustness. Low stand by cost. Maximum Application flexibility-plant can be operated either in parallel with existing plants

or as a completely isolated station. Controlled reliability-uses micro computer based control with an integrated temperature

system.

OPERATIONAL STAGES OF GAS TURBINE

The gas turbine has namely 4 stages

1. Air inlet and exhaust system2. Compressor section3. Combustion Section4. Turbine Section

1. AIR INLET AND EXHAUST SYSTEM

The internal arrangement of the inlet is to protect the compressor from debris, which consists of inlet air filters, silencing, ducting and trash screens. This arrangement generally is housed over the control and accessory compartments, comes out from the back of the inlet air house and down to the inlet plenum, which is mounted on the turbine base.

The exhaust system includes ducting, silencing and necessary expansion joints. This system leads the exhaust gases to the bypass stack or to the Waste Heat Recovery Steam Generator, which is mounted separately on its own base.

2. COMPRESSOR SECTION

The compressor section which has an axial flow design consists of the compressor rotor and the casing, within which there are inlet guide vanes, the 17 stages of rotor and stator blading, and the exit guide vanes. In the compressor air is confined to the space between the rotor and stator blading, where it is compressed in stages by a series of alternate rotating and stationary air foil shape blades.

The rotor blades supply the thrust needed to compress the air each stage and the stator blades guide the compressed air so that it enters the next rotor stage. At the proper angle the compressed air exits through the compressor discharge casing to the compressing chambers. Air is extracted from the compressor for turbine cooling, bearing sealing and during start up for pulsation control (to avoid surging).i. Compressor Rotor

The compressor rotor is an assembly of 15 individual wheels, tow stutshaft, each with an integral wheel, a speed ring, tie bolts and the compressor rotor blades. Each wheel and the wheel portion of each stutshaft have slots broached around its periphery. The rotor blades and spacers are inserted into the slots and are held in axial position by stacking each end of the slot. The wheel and stutshaft are assembled to each of the mating rabbets for concentricity control and held together with tie bolts. Selective positioning of the wheels is made during assembly to reduce balance correction. After assembling, the rotor is dynamically balanced to a fine limit.

ii. Compressor Stator

The stator (casing) area of the compressor section is composed of four major subassemblies.

a. Inlet casingb. Forward compressor casingc. Air compressor casingd. Compressor discharge casing

The inlet casing’s prime function is to direct the air uniformly from the inlet plenum to the compressor. It is located at the forward end of the gas turbine. The forward compressor casing houses the first 4 stages of the compressor. The air compressor contains the 5th to 10th compressor stages

iii. Blading

The compressor rotor blades are air foil shaped and are shaped and are designed to compressor air efficiency at high blade tip velocities.

3. COMBUSTON SECTION

The combustion system is of the reverse-flow type with 14 combustion chambers arranged around the periphery of the compressor discharge casing. This system also includes fuel nozzles, sparkplug, ignition system, flame detector, and cross fire tubes. Hot gases generated from burning in the combustion chamber are used to drive the turbine.

4. COMBUSTION WRAPPER

The combustion wrapper forms a plenum in which the compressor discharge air flow is directed to the combustion chambers. Discharge air from the axial flow compressor flows in to the each combustion flow sleeve from the combustion wrapper. All 14 combustion chambers are connected by means of cross fire tubes.

i. IgnitorCombustion is ignited by means of the discharge from high voltage, retractable-electro spark plugs installed in adjacent combustion chambers. The spring injected and pressure retractable plugs receive their energy from ignition transformers.

ii. Flame detectorsDuring the start up sequence a flame monitoring system consisting of 4 sensors for the detection and transmit to the control system.

5. TURBINE SECTION

The turbine section area in which energy contained in the hot pressurized gas produced by compressor and combustion section is converted into mechanical energy. The turbine section consists of rotor, turbine shell, exhaust frame, exhaust diffuser, nozzles, and diaphragms, stationary shrouds.

6. STARTING SYSTEM

Before the gas turbine can be fired and started, it must be rotated or cranked by the accessory equipment. This is accomplished by an induction motor, operating through a torque converter to provide the cranking torque and speed required by the turbine for start-up. The starting system consists of an induction motor and torque converter coupled to the accessory gear.A motor driven torque adjustor drive, which is an integral part of the torque converter system, provides the means for adjusting torque output within specified ranges.

7. LUBRICATION SYSTEM

The lubricating requirements for the gas turbine power plant are furnished by a common forced-feed lubrication system. This lubrication system, complete with tanks, pumps, cooler filters valves and various

control and protection devices, furnishes normal lubrication and absorption of heat load of the bearing of gas turbine. Lubricating fluid is circulated to the three main turbine bearings, generator bearings and to the turbine accessory gear and fuel pump.

HEAT RECOVERY STEAM GENERATOR

STEAM TURBINE

GENERAL DESCRIPTION

Construction Steam Flow

The turbine is a tandem compound machine with HP and LPP sections. The HP section is a single flow turbine where as the LP section is double flow. The individual turbine rotors and the generator rotor are connected by rigid couplings.The steam from HP exhaust is led to the LP turbine through cross around pipes. Additional steam from LP stage is waste heat recovery generator is passed through the LP turbine via two combined LP stop and control valves. Bleeds are arranged at several points of the turbine for regenerative feed heating.

H P TURBINEThe HP turbine is of single flow, double shell construction horizontally split casting. The main stream enters inner casing from top and bottom. The provision of inner casing confines high steam inlet temperature and pressure conditions to the flange of the outer casing is subjected only to the lower pressure and temperature effective at the exhaust from the inner casing.

L P TURBINE

The casing of the double flow LP turbine is of three shell design. The shells are of horizontally split welded construction. The inner casing which carries the first rows of stationary blades is supported on the inner-outer casing rests at four points on longitudinal girders, independent of the outer casing.

BEARINGSThe HP rotor is supported on two bearings, a combined journal and thrust bearing at its front and a journal bearing close to the coupling with LP rotor. The bearing metal temperatures are measured by thermocouples fitted directly under the babbit lining.The HP cylinder rests with its lateral support horns on the bearing pedestrals at the turbine centre line level.

VALVESThe HP turbine is fitted with two main stop and control valves. The LP turbine has two induction steam stop and control valves. These valve combinations are located in an easily accessible position at both sides of the LP turbine and are supported on the foundations.

TURBINE GOVERNING SYSTEM

The turbine has an electro-hydraulic governing system backed up with an ordinary hydraulic governing system. An electric system measures and controls speed and output in conjunction with an electro-hydraulic convertor. It provides the run up control of the turbine up to rated speed swings following certain load shedding operation.

TURBINE MONITORING SYSTEMIt basically measures and indicates pressures, temperatures, valve, positions and speed. It also includes instruments and indicators for:

i. Differential expansion between the shafting and turbine casingii. Bearing pedestal vibrations at all turbine bearingsiii. Relative shaft vibration(of bearing pedestal relative to shaft)

OIL SUPPLY SYSTEM

A common oil system lubricates and cools the bearings and operates the hydraulic actuators and the safety and protective devices. The main oil pump draws oil from the main oil tank. Auxiliary oil pumps maintain the oil supply on start-up shut down, during tuning gear operation and when the main oil

supply is faulted. The lubricating and cooling oil is passed through oil coolers before being supplied to the bearings.

JOURNAL BEARINGS

The function of the journal bearing is to support the turbine rotor. Essentially, the journal bearing consists of the upper and lower shells, bearing cap, spherical block, spherical seat and the keys. The bearing shells are provided with the babbit face. The mating surfaces of this bearing are machined and additional scraping is neither necessary nor permissible. Both bearing shells are held in position by means of paper pins and bolted together. A spherical block prevents the vertical movement of the bearing shells and centering of the bearing shell in the vertical plane is achieved by means of keys fitted on both sides of the projection.

PROTECTIVE DEVICES

i. MAIN TRIP VALVES

Functions: The function of the main trip valves is to amplify and store the hydraulic or mechanical (manually initiated local) trip signal. It must respond in the course of every successful protective device test.

Operation: Each main trip valve is kept in its operating position by auxiliary trip oil pressure. If a protective device is actuated, the auxiliary trip oil circuit is de-pressurized and the main trip valve is activated. This connects the trip oil and auxiliary trip oil circuits to drain and shuts off the control oil supply to the turbine valves. At the same time, limit switch is actuated.

If manual trip-out lever is actuated, the same happens as for turbine trip-out, except that limit switch is also actuated. This provides a separate annunciation for manual trip-out.

ii. OVERSPEED TRIPS

Function: The over speed trips are provided to protect the turbine against over speeding in the event of load rejection, coincident with faults in the control system. As these devices are of extremely great importance for the protection function, provision is also made for their

testing at rated speed during turbine operation. For this, testing is activated locally via the over speed trip test device (hydraulic signal transmitter).

Operation: When the over speed setting is reached, the striker of each over speed trip activates the piston and the limit switch via the pawl. This connects the auxiliary trip oil circuit to drain, there by depressurizing it. The loss of auxiliary trip oil pressure causes the main trip valve to drop which in turn causes the trip oil pressure to collapse. To activate over speed trip at rated speed, as per test routine performed by the automatic turbine tester requires, a specific force, equivalent to the increase in centrifugal force, between rated speed and the preset trip over speed, is needed for testing this force is exerted by the test oil pressure, acting on the head of striker.

iii. ELECTRICAL LOW VACCUM TRIP

The function of electrical low vaccum trip is to protect condenser MAG10 AC001 against pressure increase, the exhaust end of the turbine against heating resulting due to increase in condenser pressure and the turbine blading against vibration excitation resulting from loss in vaccum.

DEMINARALISED WATER PLANT

The main water source of RGCCPP, Kayamkulam is the Achankovil river which is a tributary of Periyar. From there the water is pumped to Make Up Water Pump House and then to the reservoir. From the reservoir it is pumped to Raw water Pump House and then to stilling chamber.

At the stilling chamber Chlorine is dosed to water in vaccum through service water. This is done for disinfection of the raw water. Here lime is also added to boost the pH and to remove the temporary hardness. From stilling chamber water reaches the Clarifier Water Storage Tank after passing through clarifiers. Clarifier is a structure with 3 concentric cylinders. Flocculation and Coagulation (sledging down of the mud and dust particles) of the water takes place at the clarifiers. In the clarifiers poly Aluminium Chloride and Alum is added for coagulation.

80% of the stored water is used for the makeup of lost water in the cooling tower due to evaporation and blow down. And the remaining 20% is stored in filter water storage pump after passing through gravity sand filter and it meets the water requirements of the plant and the townships.

The DM plant which is PLC operated has 3 streams. And each stream passes through the following steps.

1. Activated Carbon Filter

Removes chlorine present in the water because Chlorine is harmful for the resins present in the oncoming steps.

2. Strong Acid Cation Exchanger Bed Here the +ve ions of Magnesium, Sodium etc. are removed.

3. Degasser Tower Here the bicarbonates are removed as carbon dioxide mechanically.

4. Strong Acid Anion Exchanger Bed Here the anions like sulphates, chlorides, Bromides etc. are removed.

5. Mixed Bed Unit This is a polishing unit which removes the anions and cations which are still present in the water.

The water which is passed through all the above units will have .1µs/cm conductivity. But when it is stored in the DM water tank its conductivity rises to 1µs/cm due to the mixing of CO2. To a certain percentage this is avoided by keeping alkali at the breather of the storage tank.

The 20% of the water stored in filter water storage tank is pumped to the following units1. Portable colony pump2. Portable plant pump3. Heating ventilation and air conditioning of the colony and plant4. Service water5. Feed to DM plant

And the Demineralised Water is stored in 2 tanks of 2000cm3 capacity each. The main purpose of this DM water is for the denoxification process at the gas turbine (DM water is injected to the gas turbine to prevent the formation of NOX, the poisonous oxide of nitrogen).

GAS INSULATED SUBSTATIONS (GIS)

INTRODUCTION

SF6 gas insulated substations (GIS) are preferred for voltage ratings of 72.5 KV, 145 KV, 245 KV, 420 KV and above. In such a situation, the various equipments like circuit breakers, bus bars, isolators and load break switches, current transformers, voltage transformers, earthing switches etc. are housed in metal enclosed modules filled with SF6 gas. The SF6 gas provides the phase to ground insulation. As the dielectric strength of SF6 is higher than the air, the clearances required are smaller. Hence, the overall size of the equipment and the complete substation is reduced to about less than half of conventional air–insulated substations. As a rule GIS are installed indoors. However outdoor GIS can also be installed.

The various modules of GIS are factory assembled and re filled with SF6 gas at a pressure of about 3k¿cm2. Thereafter they are taken for final assembly. Such substations are compact and can be installed conveniently on any floor of a storied building or in an underground substation.

As the units are factory assembled, the installation time is substantially reduced. Such installations are preferred in cosmopolitan cities, industrial townships etc, where of land is very high and higher cost of SF6 insulated switchgear (GIS) is justified by saving due to reduction in floor area

requirement. They are also preferred in heavily polluted areas where dust, chemical fumes and salt layers can cause frequent flashovers in conventional outdoor air-insulated substations. The GIS has a monitoring system in which the gas density in each compartment is monitored. If pressure decreases, initially a warning is sounded after which the circuit breaker trips with further leakage.

DESIGN AND DEVELOPMENT OF GIS

Design, planning and layout of gas insulated switchgear substation are carried out by experts using modern computer aided tools like SCADA (Supervisory Control And Data Acquisition). Model based computer graphics are used for creating substation models.

In particular, of all the advanced calculation and simulation techniques used during the development stage of the various components the ones given below are to be noted:

2 dimensional and 3 dimensional finite element software’s for evaluation of the dielectric strength.

Finite element software for stress analysis of GIS enclosures. 2 dimensional and 3 dimensional software for gas flow analysis for evaluation of the

performances of the interrupting chambers during various network operating and fault conditions.

Modelling of hydraulic drives:All the calculations and simulations are fully integrated by experimental data obtained during type and research tests.

WHY SF6 IS USED?

Physical properties

Molecular weight 146.05

Melting point -50.8°C

Sublimation temp. -63.9°C

Density(liquid at 50°C) 1.98g/ml

Critical temp. 45.6°C

Critical pressure 36.557atm

Thermal conductivity 3.36E4cal/s/sq.cm/K/cm

Boiling point -63.0°C

Specific Heat (at 30°C) .143cal/g

Relative density 5.10

Vapour pressure(at 20°C) 10.62

Viscosity(gas at 25°C) 1.61E4poise

Surface Tension 11.63dyne/cm

Critical density 0755g/ml

Density(liquid at 25°C) 1.329g/ml

Density(gas at 1 bar and 20°C) 6.164g/L

Color, odour Colorless, odorless

Pressure-Temperature relationThe variation of pressure with temperature is linear and relatively small in the range of service temperature (-25 to +50°C).

Specific Heat The volumetric specific heat of SF6 is 3.7 times that of air. This has important consequences for reducing the effects of heating within electrical equipment.

Thermal ConductivityThe thermal conductivity of air is below that of air but its overall heat transfer capability in particular, when convection is taken into account, is excellent, being similar to that of the gases such as hydrogen and helium and that of air. At higher temperature the thermal conductivity curve of SF6 reveals one of the exceptional qualities of gas, which allows it to be used for extinguishing arcs by thermal transport.

Electrical Properties The dielectric strength of SF6 is about 2.5 times higher than that of air under same conditions. Because of its low dissociation temperature and high dissociation energy, SF6 is an excellent arc quenching gas. When an electric arc cools in SF6, it remains conductive to a relatively low temperature, thus minimising current chopping before current zero, and thereby avoiding high over voltages.

Sonic CharacteristicsThe speed of sound in SF6 is one third of that in air, making SF6 a good phonic insulator.

Chemical Properties SF6 can be heated without decomposition to 500°C in the absence of catalytic

metals. SF6 is non inflammable Hydrogen, Chlorine and oxygen have no action on it. SF6 is insoluble in water It is not attacked by acids. In its pure state SF6 has no toxicity.

ADVANTAGES OF SF6

i. Compactness The space required by the SF6 installation is about 8 to 10% of the conventional outdoor substation. High cost of installation is partially compensated by saving in cost of space. (A typical 420/525kv SF6 GIS requires only 920m2 against 30,000m2 for an air insulated substation).

ii. Reduced installation time The modules are factory assembled, tested and dispatched with nominal SF6 gas filling. Therefore installation takes only a few weeks.

iii. Protection from pollution In conventional outdoor air insulated substations atmospheric pollution (dust, chemical fumes, shoreline layers, carbon layers, moisture etc.) produce corrosion of metallic parts

and conducting layers of low resistivity on insulator surfaces. These problems are avoided in a SF6 insulated substation.

iv. Outdoor SF6 GIS versus indoor SF6 GIS SF6 substation can be installed indoor or outdoor. For outdoor GIS the following special measures are essential,

Sealed opening mechanism housings. Protection of the flanges outside the seals to prevent the entry of moisture and

corrosion. Rust protection of bolts and supporting structures. The use of a special paint composition, light colours for reflection of sun radiation. Roof for sun shading if high normal currents are involved. Depending on the ambient conditions, paint of an outdoor installation may have to

inspected periodically, touched up or renewed. During erection the section where assembly takes place is to be protected against

pollution by covering During rainy season, erection may be avoided or should be prohibited.

v. Increased safety As the enclosures are at earth potential there is no possibility of accidental contact by service personal to live parts. While in conventional substations either shutdown or isolation is necessary for maintenance.

vi. Immunity from flashovers due to birds, bees, dust, lightning Flashovers due to above causes are eliminated in SF6 GIS.

APPLICATION OF GIS

1. Switchgear installation for higher security requirements.2. Indoor switchgear installations with minimum space requirements therefore especially suitable

for congested area.3. Protected installation in areas of contamination and corrosion by sea or desert climate and in

industrial plants.4. Cavern switchgear installations for hydro and pump storage water plants.5. In power plants, with the possibility of accommodation in the immediate vicinity of the

transformers to achieve an optimum overall concept.6. Extension of existing conventional outdoor installations on limited ground area.7. Replacement of existing conventional switchgear installations enabling increased voltage level,

without additional space requirements.8. Hybrid solution as a combination of metal-enclosed switchgear components together with

equipment of conventional design, for reducing dimensions.

9. Mobile stations10. On hilly terrains.

SF6 AND GLOBAL ENVIRONMENT

Green House Effect

The contribution of SF6 is less than one part in ten thousand of the total contribution of the other agents and is thus negligible.

Ozone Depletion

Any fluorine atom that is released has tendency to combine with ozone thus causing depletion in ozone layer. However SF6 is not photo-decomposed at ozone layer altitudes (32-44km), so very little atomic fluorine is released.

GIS Vs CONVENTIONAL SUBSTATION

FACTORS GIS CONVENTIONAL

Area requirement (Including power transformer)

3648 m2

14300 m2

Clearance

Phase – Phase

Phase – Earth

0.23 m

0.15 m

4.30 m

2.08 m

Environmental pollution Insensitive Corrosion of exposed conductors, clamps and connectors

Installation time Less due to assembling of main components at factory

High due to erection of more number of components at site.

Maintenance Less routine maintenance, almost maintenance free

Routine maintenance will be more

Requirement of special foundation

Special foundation may be required for GIS building

For all structures special foundation required.

Seismic resistance High Nil

A STUDY OF 220 KV KAYAMKULAM GAS INSULATED SWITCHGEAR

The GIS in NTPC Kayamkulam was supplied by Hyundai Heavy Electricals Industries, Korea. Following are the features of the 220KV Kayamkulam GIS.

1. The GIS is of double bus bar configuration2. There are 3 incoming cable feeders from the 3 units of the plant3. There are 4 outgoing line feeders

2 lines to Pallom 1 line to Edamon 1 line to Kundra

4. There is one outgoing line feeder maintained for future use.

MAIN FEATURES

Type:HS300R Rated Voltage: 245kv Nominal Voltage: 220kv Rated SF6 pressure:5ksc Rated continuous current of the busbar:2500A Rated short time current: 40kA rms, 1sec Peak withstand current:100kA peak

Rated Insulation level Power frequency voltage: 4600KV rms

Lighting Impulse w/s voltage: 1050KV peak

BUS SECTIONS

Main bus bar: 3 phase enclosure

Bus coupler and feeder lines: I phase enclosure

CIRCUIT BREAKER

The interrupters operate on puffer principle Type: 300SR-K Rated Current:1600A for lines and 2500A for bus coupler Hydraulic operated Close time: 100ms max Trip time: <30ms Normal SF6 pressure: 6ksc Low alarm: 5.5 ksc Lock out: 5ksc

DISCONNECTOR SWITCH

Type:300 KLP for bus DS and 300DSM for line DS 300KLP-motor spring operated 300DSM-motor operated Rated SF6 pr. : 5ksc

EARTH SWITCH

Type : 300EAP for bus ES and maintenance ES 300EYP for line ES All motor spring operated Rated SF6 pr. : 5ksc

VOLTAGE TRANSFORMER

Type: 1 phase gas insulated and inductive type 220kv/110V,100A and 5cl. SF6 pr.: 5ksc

CURENT TRANSFORMER

Bushing Type SF6 pr.: 5 ksc 800/1A Metering CT: .5 cl and 4 VA

CABLE HEAD

SF6-AIR BUSHINGS

Bushing are used for interconnections ivolving SF^ to air terminations Rated voltage: 245KV Rated currnt: 1600A SF6 pr.: 5 ksc

LOCAL CONTROL CUBICLE

One LCC for each bay.

The LCC comprises of

Control switches for operating circuit brakers,disconnectors and arthing switches Gas alarm Indication Lamps

SF6 GAS MONITIORING

Each gas compartment is provided with its own gas pressure gauge Stop valves, pipings to fill and recover gas are also provided

PRESURE RELEASE DEVICE AND ABSOBENT

A pressure release device is provide in each gas compartment to protect during occurrence of abnormal pressure increase or shock waves generated by internal electrical fault arcs.

Operating pressure: >10ksc Absorbent zeolites filter to absorb moisture and gas decomposed products.

SF6 FILLING UNIT

AUXILIARY POWER FOR GIS

Auxiliary power for GIS met through Transformers-7A and 7-B fed from NTPC end. Tfr.-7A and 7-B ratings: 630 KVA, 6.6KV/433V DG set: Cummins-Kirloskar make, 250KVA,415V,347A

MODULES OF A TYPICAL GIS

The major parts that form a typical GIS are the circuit breaker, disconnecting switch, earthing switch, current transformer, voltage transformer, cable head, lightning arrestor, bus sections, support insulators and bushings. All these modules are SF6 gas filled at sufficient safe pressure maintaining the desired insulation between the live parts and the grounded outer enclosure.

1. CIRCUIT BREAKER

A circuit breaker is a device that automatically breaks an electrical circuit whenever the circuit becomes overloaded or an unintentional short circuit occurs. Circuit have a fixed electric

current load capacity which when breached results in automatic shutdown. Overloading occurs when there is a shorting between two cables or there is an earth fault.When this happens the cables heat up which might result in an insulation breakdown (in which a short circuit may occur) or an electrical fire.

A sulphur hexafluoride circuit breaker uses contacts surrounded by sulphur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to liquefaction of SF6 gas. Use of SF6 gas for insulation in GIS as interrupting medium in circuit breaking match well, and thus are the obvious choice for these substations. The circuit breakers operate at a pressure of about 6 to 6.5 bar. The overall size of the CB in a GIS is considerably reduced due to the absence of the porcelain insulators and the direct short terminal connections with the breaker chamber at earth potential. The requirements of energy from the operating mechanisms are reduced in the case of a GIS circuit breaker.

Breaker shall be trip free, designed for 3 phase auto reclosing day duty (0-0.3sec) equipped with anti-pumping protection. Two independent trip coils to be provided.

It should be possible to reduce the gas pressure within the breaker to a value not exceeding 8 millibar within 4 hours. Circuit breakers including its enclosure shall be capable of withstanding this degree of vacuum without distortion or failure of any part.

2. CIRCUIT-BREAKING MECHANISMBasically, a circuit breaker has a switch and a moving, conductive contact plate that moves with the switch. When the switch is placed on an ‘on’ position, the circuit breaker’s moving contact plate touches a stationary plate that is connected to the rest of the circuit so electric current can flow. A circuit breaker has a mechanism for automatically breaking an electrical circuit. There are three main types of circuit breaker according to the mechanism used:

i. The first type uses electromagnetism. It uses the concept that when current increases in an electromagnet which causes the magnet to trip once the prescribed load is exceeded (once the electromagnet become powerful enough to force the circuit breaker’s lever down).

ii. The second type uses heat to break a circuit. Basically it is a thermostat where the circuit is closed when current is in the prescribed limit and trips when current exceeds prescribed value leading to heating, the strip bends by an angle severing the contact.

iii. The third type uses both heat and electromagnetism to protect electrical systems. An electromagnet pulls the lever in case of sudden jumps in electric load, and a bimetallic strip protects the system from prolonged cases of over current that results in overheating.

3. CABLE HEAD

Cable head is used for interconnection of the high voltage cable and the GIS.

4. SF6 AIR BUSHINGS

A gas insulated bushing used for the interconnection involving SF6 to air terminations. Suitable reducers are incorporated for mounting the standard bushing to the GIS equipment.

5. BUS SECTIONS

The buses interconnect the bays and the phase of equipment. Buses are made either with 1Ø or 3Ø enclosure. The current conducting buses are made of tubular sections. The surface finishing of the live parts and shield built and at the joints ensures partial discharge free

operation with the proper choice of the bus section and the T enclosure any configuration can be realized by assembling the modules accordingly.

6. SUPPORT INSULATORS

These are of epoxy tube with alumina as filler material. Alumina filled spacer insulator are more resistant to attack by decomposition products of SF6 compared to silica filled ones that have poor resistance. The non commutating type insulator seals the enclosure with adjacent sections and are used in CB, VT sections. The commutating type insulators are used in isolator, earth switch, CT etc.

7. EARTHING SWITCH

Earthing Switch is necessary to earth the conducting parts before maintenance and also to provide deliberate short-current while testing. There can be 3 types of earthing switches in metal-clad switches manually operated automatic high speed Earthing Switch, protective Earthing Switch for earthing the installation.

There are several versions of Earthing Switch for following applications

Maintenance Earthing Switches. These are single pole or 3 pole units: manually operating mechanism with a provision of filling motor mechanism

High Speed Earthing Switch. These are operated by spring energy. Spring is charged by motor mechanism.

The earth switch is mounted direct on the enclosure. Earthing switch has to satisfy various requirements. For earthing isolated sections of switchgear for protection of personnel during maintenance and over-hauls or erection, the maintenance earthing switches are employed. For earthing higher capacitances (cables, overhead lines etc.) high speed earthing switches are employed. Depending on the substation scheme, the bus bars may be earthed either by maintenance or high-speed switches.

8. CABLES

This is used to transfer power from generation site to the switchgear. Usually the power carried by the cable is at very high potential, which may be either from the power transformers or from other substations.The cables are either of aluminium alloys or of high conducting copper and are generally of tubular cross-section. In NTPC at Kayamkulam rating of generator transformer is 10.5 KV/220 KV. The cable thus used here is XLPE designed to handle this voltage rating.

9. INSTRUMENT TRANSFORMERS

Transformers are used in ac system for the measurement of current, voltage, power and energy. They are also used in connection with measurement of power factor, frequency and for indication of synchronism. Instrument transformers find a wide application in protection circuits of power systems of over current, under voltage, earth fault and various other types of relays.

In all the above applications, the transformer is used for measurement purposes, the actual measurement being done by measuring instruments. Transformers used in conjunction with measuring instruments for measuring purposes are called ‘Instrument Transformers’. The transformer used for the measurement of current is called ‘Current transformer’ and that for voltage is called ‘Voltage transformer’.

Use of instrument transformers: The extension of instrument range, so that current, voltage, power and energy can be measured with instruments or meters of moderate size is of very great importance in commercial metering. In power systems, currents and voltages are handled and therefore, direct measurements are not possible as these currents and voltages are too large for any meter of reasonable size and cost. The solution lies in stepping down these currents and voltages with the help of instrument transformers so that they could be metered with instrument of moderate size.

10. CURRENT TRANSFORMER

The current transformer is used with its primary winding connected in series with line carrying the current to be measured and, therefore, the primary current is dependent upon the load connected to the system and is not determined by the load connected on the secondary winding on the current transformer. The primary winding consists of very few turns and, therefore, there is no appreciable voltage drop across it. The secondary winding consists of the current transformer has larger number of turns, exact number being determined by the turns ratio. The ammeter or wattmeter current coil, are connected directly across the secondary winding terminals. Thus a current transformer operates its secondary winding nearly under short circuit conditions. One of the terminals of the secondary windings is earthed so as to protect equipment and personnel in the vicinity in the event of an insulation breakdown in the transformer.

Transformer ratio, R = primary winding / secondary winding current

Nominal ratio, Kn = rated primary winding current/ rated secondary winding currentSince primary current is larger than the secondary current there will be high transformation ratio.Primary current is in the order of 10-300A and secondary current is in the order of 5A, 1A, 0.1A etc.

Current transformers are classified mainly into two; protective current transformer and measuring transformers. Current transformers, which are used in sub-stations, are divided into many cores. There are mainly 5 core current transformers, two are used for bus protection, one for measuring purpose and remaining two for line protection. In 3 core current transformers 2 are for relay protection and 1 for measuring. The output of each core winding is 800/1A.

11. VOLTAGE TRANSFORMER

Voltage transformer or ‘Potential Transformers’ are used to operate voltmeters.The potential coils of watt meters and relays from high voltage lines. The primary winding of the transformer is connected across the line carrying the voltage to be measured and the voltage circuit connected across the secondary winding.

Transformation ratio, R = primary winding voltage/secondary winding voltageNominal ratio, Kn = rated primary winding voltage/rated secondary winding voltage Since secondary voltage is less than the primary voltage the transformation ratio is low.The potential transformer makes the ordinary low voltage instruments suitable for high voltage measurements and isolates them from high voltages.

12. BUS BAR

Bus bar is a long copper or aluminium conductor. A bus bar is assumed to have a constant voltage and infinite frequency. In bus bar the total incoming current is equal to the total outgoing current. The buses can be either 1Ø or 3Ø. Conductors of bus bars must be of aluminium or copper with sufficient cross sectional area for meeting the current rating requirement. Conductors shall be supported from the enclosure by means of suitable insulators shaped to ensure uniform electrical field distribution and zero corona at rated voltage.Bus bar and connections can be made with multi contact connectors to allow for axial thermal expansion of the bus. Each end of the main bus bar shall be designed for convenient future extension of the switchgear.

13. LOCAL CONTROL CUBICLEThe local control cubicle of self standing type is supplied with necessary control switch, signal lamps, mimic and relays for operating CBs Disconnector and earthing switches from the switchgear rather than controlling it from the remote control room.

OUTDOOR SWITCHYARD

LIGHTNING ARRESTORS

A lightning arrestor or a surge arrestor is a protective device which conducts the high voltage surge to the ground. The active parts of the arrestor are enclosed in a synthetic epoxy resin tube, which is surrounded by a gas tight aluminium enclosure. The active part is connected to the bus through a conducting bus connector. The gas tight sealing ensures prevention of damage to active parts by the SF6 gas and its decomposition products. The filling up and draining of nitrogen and SF6 gas can be controlled through the valves provided for the same.

The action of lightning arrestor is under Under normal operation, the lightning arrestor is offline, means it conducts nio current to

earth or the gap is non-conducting. On the occurrence of over voltage, the air insulation across the gap break down and an arc is

formed, providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to surge is harmlessly conducted through the arrestor to the ground.

It is worthwhile to mention the function of non-linear arrestor in the operation of arrestor. As the spark goes over due to over voltage, the arc would be a short circuit on the power system and may cause power follow current in the arrestor.

CAPACITIVE VOLTAGE TRANSFORMER

Capacitive Voltage Transformer converts transmission class voltages to standardized low and easily measurable values, which will be used for metering, protection and control of the high voltage system. Additionally CVT serve as a coupling capacitor for high frequency power line carrier signals to the transmission line. The capacitive unit consists of HV capacitor (C1) and intermediate voltages capacitor (C2).

AIR BUSHING

SF6 to Air Bushing is the structure which is the interface medium for the conductor in te GIS and the overhead lines. SF6 gas is partially filled in the SF6 to Air Bushing. And the remaining part of the bushing is filled with air. Insulation spacers are provide inside the bushing to prevent the flow of SF6 gas to air bushing.

GROUNDING

Enclosure of GIS shall be grounded at several points. Grounding system shall ensure that circulating current is minimised and potential rising is kept to an acceptable level where operating mechanism cabinets are mounted on the switchgear, grounding shall be made by separate conductor.

POWER LINE CARRIER COMMUNICATION

Power line carrier communication (PLCC) is a technique that involves high frequency signal transmission along the overhead power line. It is robust and therefore reliable, constituting a low loss transmission path that is fully controlled by the utility.

High voltage capacitors are used for the purpose of injecting the signal to and extracting it from the line. Injection can be carried out by impressing the carrier signal voltage between one conductor and earth or between any two phase conductors. The basic units can be build into a high pass or band pass filter.

PLCCs are intended for the transmission of speech, tele-metering, tele-printing, tele-control, tele- indication and tele-protection signals in the carrier frequency range between 30 Hz and 500 Hz over the following communication media, with suitable line coupling equipments. These equipments are:

High voltage and medium voltage power lines Open wire lines exposed to power lines

Message transmission is based on the single side band principles. Where the carrier power and one of the two side bands, generated as a result of modulation, are suppressed this mode operation offers the following advantages:

Optimum utilization of the available send power for signal transmission Minimum channel width to conserve spectrum space Large transmission range

In NTPC Kayamkulam, the model used is 9505PLC. I t provides single or twin channel voice grade channels for the transmission of speech or audio tunes over high voltage transmission lines. The transmitted audio tunes can be used for telemetering, supervisory control, protective relay, data or other purposes. Low pass filter can be supplied for ‘speech plus’ operation (simultaneous transmission of speech and audio tunes). When used for data only, each channel up to typically twenty-four 50 band telegraph channels or a smaller number of channels at higher band rates.

Power Line Carrier Communication Equipment

PLCC terminals are used as a pair, one at each end of the power line (between substations) Each terminal is designated for a set of transmit and receive frequencies (channel frequencies) The corresponding PLCC at the other end will be designated for the reverse value of the

transmission and reception frequencies. The channel frequency will be either in 4 KHz bandwidth or 8KHz bandwidth depending upon

single channel or twin channel equipments. PLCC converts an input signal of 300-4 KHz bandwidth to the RF range between 30-500 KHz and

amplifies this RF signal to the desired output power level (up to 40 Watts).

The various components can be explained as follows:

Wave trap(WT):Any high frequency signal entering into the substation should be blocked; wave traps are used for this purpose. That is they are used for blocking high frequency signals of PLC communication entering into the substation. They are designed in such a way that they will allow only high voltage power signal of 50Hz frequency to the substation.

Capacitance Voltage Transformer/ Coupling Capacitor (CVT/CC):CVT/CC is connected between the line matching unit (LMU) and power lines. The purpose of CVT/CC is for blocking the high voltage from entering into LMU or PLCC.

Line Matching Unit (LMU):The output of PLCC is connected to LMU and then to the power lines to achieve proper impedance matching between PLCC equipments and the power lines. The main purpose behind using LMU is to send signal at minimum impedance.

Drainage Coil And Lightning Arrester:\Any high voltage entering into LMU or PLCC can damage the device. So there is a need for preventing this ac high voltage spikes entering into LMU or PLCC. Drainage coil and lightning arresters are used for this purpose.

Earth Switch:At the time of maintenance of LMU, earth switches are used one of whose terminals is grounded.

Co-axial Cable:For carrying high frequency signals, there is a need for inter-connection between PLCC and LMU which are brought about by co-axial cables. The different types of coupling employed in PLCC are:

1. Phase to phase coupling.2. Phase to ground coupling.3. Interface coupling.

PROTECTION

CABLE FEEDER PROTECTION

This protection is given for different components in the feeder bay which includes incoming feeder and outgoing feeder. For feeder protection differential protection principle is used. Differential protection includes over current and over voltage protection.

Different components in the feeder including the incoming feeder and outgoing feeder are protected by cable feeder protection.

LINE PROTECTION

In traditional transformer differential schemes, the requirements for phase and ratio connection were met by the application of external interposing current transformer as a secondary replica of the main winding connections, or by a delta connection of the main CTs to provide phase connection only. Digital or numerical relays implement ratio or phase correction in the relay software instead. This avoids the additional space and cost requirements of hardware interposing CTs.

Basic considerations for transformer relay protection

1. Distance relay protection2. Restricted Earth Fault Protection 3. Differential Protection

DISTANCE RELAY PROTECTION

Distance relay protection is a non-unit system of protection, whose action depends upon the point of fault. Time of operation of this system is the function of the ratio of voltage and current and therefore the fault coverage of the protection circuit is virtually independent of source impedance variations. This is also known as ‘Impedance Relays’. Here, the torque produced by a current element is opposed by the torque produced by a voltage element. The relay operates when the ratio of V/I is less than the predetermined value.

Impedance relays are of 2 types:

Definite-distance relay It operates instantaneously for fault unto a pre-determined distance from the relay

Time-distance relay Here the time of operation is proportional to the distance of fault from the relay point. A fault nearer to the relay will operate it earlier than a fault farther away from the relay.

RESTRICTED EARTH FAULT PROTECTION

Conventional earth fault protection using over current elements fails to provide adequate protection for transformer winding. This particularly is the case for star connected windings with impedance earthed neutral.

The system of protection is very much improved by the application of restricted earth fault protection (REF Protection). This is a unit protection scheme for one winding of the transformer. It can be of high impedance as well as biased low impedance type. For the high impedance type the residual current of three line current transformer is balanced against the output of a current transformer in the neutral conductor. In the biased low impedance version the three phase currents and the neutral current become the bias inputs to a differential element.

The system is operative for fault within the region between current transformers. The system will remain stable for all faults outside the zone.

DIFFERETIAL PROTECTION

The restricted earth fault protection depend entirely upon the Kirchoff’s Principle that the sum of the current flowing into a conducting network is zero. A differential system can be arranged into the complete transformer, this is possible because of the high efficiency of transformer operation, and the close equivalence of ampere turns developed on the primary and secondary windings. Current transformers on the primary and secondary sides are connected to form circulating current system.

BUSBAR PROTECTION

Although not basically different from other circuit protection, the key position of the bus bar intensifies the emphasis put on the essential requirements of speed and stability.

Speed : Bus bar protection is concerned with limitation of consequential damage and removal of bus bar faults in lesser time.

Stability: The stability of bus bar protection is of great importance. Bearing in mind the low rate of fault incidence, amounting to no more than an average of one fault per bus bar in 20 years, it is clear that unless the stability of the protection is absolute, the degree of disturbance to which the power system is likely to be subjected may be increased by the installation of bus protection.

CONCLUSION

The generation of electrical power is being emphasized to be made more efficient and more environment friendly in today’s world. The use of gas powered stations has harbored this cause to a very large extent, removing the hassles of smoke treatment and ash disposal without discharging hot effluents into water bodies making gas powered power projects desirable in terms of power generation efficiency and adheres to environment protection norms.

The present power generation at NTPC Kayamkulam is approximately 150 MW which is much lower than its maximum power generation capacity owing to the fact that GT 1 is currently under maintenance and the power demand is low. As gas stations are only used for power generation at peak loads because power generated can be varied much faster than in thermal plants and adding to its high cost of production the power generated presently is low. The plant being of smaller size facilitates easier

understanding of power production unlike the usual size of NTPC projects making it easier for students to get a comprehensive view of power generation and distribution.

There is a move to expand the plant with a LNG (liquefied natural gas) fuelled generator which will add to the project’s capacity and will be crucial to an ever-developing state like Kerala industrially and financially making it independent of power requirements from neighboring states. This will make RGCCPP one of the most energy efficient and pollution free plants in the country.