ARIJIT DASDepartment of Electrical

Engineering

Faculty of Engineering and Technology

JADAVPUR UNIVERSITY

ARIJIT DAS (JADAVPUR UNIVERSITY)

SIDDHARTA ROY (JADAVPUR UNIVERSITY)

AVISHEK SINGHA

(BENGAL ENGINEERING & SCIENCE UNIVERSITY)

SAIKAT KUMAR

(BENGAL ENGINEERING & SCIENCE UNIVERSITY)

AMI SAHA

(BENGAL ENGINEERING & SCIENCE UNIVERSITY)

POWER SYSTEM PROTECTION

APPROVAL CERTIFICATE

This is hereby certified that Arijit Das,

Department of Electrical Engineering, Faculty of

Engineering and Technology, Jadavpur University,

Siddharta Roy, Department of Electrical Engineering,

Faculty of Engineering and Technology, Jadavpur

University, Avishek Singha, Department of Electrical

Engineering , Bengal Engineering and Science University,

Saikat Kumar, Department of Electrical Engineering ,

Bengal Engineering and Science University and Ami

Saha, Department of Electrical Engineering , Bengal

Engineering and Science University, have successfully

undergone their Vocational Training at National Thermal

Power Corporation, Farakka and submitted the project

report on “PROTECTION OF ELECTRICAL

EQUIPMENTS IN A POWER PLANT”.

………………………………………..

Ms. Bhaswati Mukherjee

SR. ENGINEER (O & M / EM)

TRAINING GUIDE

ACKNOWLEDGEMENT

We have undergone vocational training at NATIONAL THERMAL POWER STATION Corporation (NTPC), Farakka, and we wish to acknowledge the support and helping hands extended by the entire TRAINING DEPARTMENT and Engineers who helped & guided us on our visit to the various sections of FSTPS.

Any successful work is accompanied by the Helping & Co-operation of well-wisher. Whatever we have tried to present in our project cum training report would remains incomplete unless & until we extend our heartiest thanks to all the people who have spent their valuable time to help & explain us all that we wanted to know. My words will fall short to describe their importance to us, our gratefulness to them & also to their kind & co-operative attitude throughout the course of our training in NTPC (Farakka).

No matter wherever we will stand in our life & career in the end, these glorious days of our short stay with all the people connected directly or indirectly to NTPC (Farakka) will never fade away from my memory.

CONTENTS

BRIEF OVERVIEW OF A

THERMAL POWER PLANT

POWER SYSTEM PROTECTION

GENERATOR STATOR PROTECTION

GENERATOR ROTOR PROTECTION

TRANSFORMER PROTECTION

BUSBAR PROTECTION

LIGHTNING PROTECTION

CONTENTS

BRIEF OVERVIEW OF A

THERMAL POWER PLANT

POWER SYSTEM PROTECTION

GENERATOR STATOR PROTECTION

GENERATOR ROTOR PROTECTION

TRANSFORMER PROTECTION

BUSBAR PROTECTION

LIGHTNING PROTECTION

BRIEF OVERVIEW OF ATHERMAL POWER PLANT

INTRODUCTION

NTPC, the mega power utility, generates around one fourth of the total power produced in India has been established in 1975 with primary objective of developing thermal power in the country. Since independence, the install power generating capacity was mere 1342 megawatt, which has grown to fifty folds in last 48 years to reach 118,000 megawatt. The principal purpose of the power station is to produce reliable electricity for public and private enterprises and also for public utilities.

Farakka Super Thermal Power Station is situated at a distance of

300 km away from Kolkata in North Bengal in the tusks of nature. It is

spread on both side of holy Ganga covering total area of 4330 acres. Its

coal source is eastern coalfields from the Rajmahal coalmines by the

assistance of merry go round. Farakka NTPC gets water from feeder canal

of the Farakka Barrage Project. The more Unit of 500MW project is yet to

come by next year.

Salient Features:Location: Farakka in Murshidabad district of West Bengal.

Installed Capacity: 1600MW(Stage I-3 X 200, Stage II –2 X 500)

Coal source: Hurra Block of Mines in the Rajmahal Coal Field of ECL.

Coal consumption: 10.8 million Tones per annum at ultimate capacity.

Heavy oil fuel: By Indian Railways.

Total area: 4330 acre.

Height of chimney: 194m for 200MW units and 275m for 500MW.

Water source: Feeder cannel of Farakka.

Beneficiary states: West Bengal, Bihar, Orissa, D.V.C, Sikim.

Unit Synchronization: Unit 1 - 01/01/1986

Unit 2 – 24/12/1986

Unit 3 – 06/08/1987

Unit 4 – 25/09/1992

SAFETY

ACCIDENT in industrial sector defines any incident which has potential to cause injury to human, loss of property and damage to environment.

Causes for occurrence of accident

Unsafe Act Unsafe Condition

Hazards

Conditions prevailing in work place finally leading to accidents.

Types

Electrical Mechanical Chemical Environmental

Precautions

Look overhead Watch steps Wear shoes and helmets Take care of the flow opening Avoid lose clothing

SIMPLIFIED DIAGRAM OF THERMAL POWER PLANT

Figure 1 SIMPLIFIED DIAGRAM OF A THERMAL POWER PLANT

EFFICIENCY CYCLE OF THERMAL POWER PLANT

Efficiency of Thermal power plant is based on the Rankine cycle. The Temperature-Entropy diagram shows how the efficiency of a thermal power plant can be increased by regularly cooling and then again re-heating the steam.

The overall efficiency of Thermal power plant is quite low(about 29%) due to mainly two reasons. Firstly, a huge amount of heat is lost in condenser and secondly heat losses occur at various stages of the plant. The heat loss in the condenser cannot be avoided. It is because heat energy cannot be converted into mechanical energy without temperature difference. The greater is the temperature difference the greater is the heat energy converted into mechanical energy. This necessitates to keep the steam in the condenser at the lowest temperature. But we know the greater is the amount of temperature difference the greater is the heat lost. This explains for low efficiency of such plants.

RANKINE CYCLE

TURBOGENERATORFSTPP has 5 units (stage-I : 3 x 200 and stage-II : 2 x 500)of 3-phase synchronous generators which convert mechanical energy to electrical energy.The generators are coupled to the steam turbine shaft.

Specification:

The following table shows the rating of the Generators of stage-I and stage-II.

Rated parameters Stage-I Stage-II

MAX. CONTINUOUS KVA RATING 247000 588000

MAX. CONTINUOUS KW RATING 210000 500000

RATED POWER FACTOR 0.83 lag 0.83 lag

STATOR Voltage 15750 21000

Current 9050 16200

ROTOR Voltage 310 240

Current 2600 4030

RATED SPEED 3000 3000

RATED FREQUENCY 50 50

PHASE CONNECTION YY YY

NO. OF TERMINALS BROUGHT OUT OF STATOR

9 9

COOLANT Water & Hydrogen

Water & Hydrogen

GAS PRESSURE 3.5 kg/cm2 3.5 kg/cm2

INSULATION CLASS B B

MAKER’S NAME BHARAT HEAVY ELECTRICALS LTD.

DIFFERENT PARTS

1. Stator - Stator Frame (Fabrication & Machining)2. Core Assembly - Stator Core, Core Suspension Arrangement3. End Shield4. Stator Winding Assembly - Stator Winding , Winding Assembly,Connecting Bus bar5. Rotor - Rotor Shaft, Rotor Wedges, Rotor Coils, Wound Rotor, Rotor Assembly6. Completing Assembly - Bearing Assembly, Shaft Seal Assembly, Oil Catchers,Insert Cover etc7. Exciter8. Auxiliary System

The machine usually consists of two main parts: 1. STATOR 1) FRAME 2) MAGNET CORE 3) WINDINGS

STATOR FRAME

It is a fabricated gas tight steel structure suitably ribbed internally. It can withstand explosion pressure of hydrogen air mixture without any residual deformation. H2 gas coolers are housed longitudinally inside stator body.

Figure 2 STATOR WINDINGS

STATOR CORE

Stator core is made up of insulated punchings of CRGO Si steel and is laminated to minimize eddy current loss. It provides path for machines’ magnetic flux and has slots in which windings are assembled. Core bars are designed to provide elastic suspension of core in stator.

STATOR WINDINGS

The windings are three phase fractional pitched distributed in two layers of individual bars. Generator voltage is induced in the stator windings and use of water cooling permits a high value of current density in the machine. The use of combination of solid and hollow conductors effectively reduces the depth of the slot which affect the losses in the winding, and better utilization of slots.

2. ROTOR

It is the rotating part and houses the field windings. It is a cylindrical type rotor. Rotor body is a high strength alloy steel single forging prepared by vacuum cast steel,containing slots for housing field windings and is supported on two bearings. The coils are held against centrifugal forces by means of wedges and by means of non-magnetic retaining ring on the overhang part of the winding.

FIELD WINDING

These are made from hard drawn silver bearing copper. Gas(H2) pickup system is employed for complete cooling of rotor. Two propeller type fans are shaft mounted on either side of rotor body for circulating cooling gas inside generator. Special ducts (fins) are provided in the rotor body, through which the cooling gas flows to the rotor end windings.

SLIP RINGS

Helically grooved alloy steel rings are shrunk on rotor shafts and insulated from it. Slip rings are connected to field windings through semi flexible copper lead.

COOLING SYSTEM OF THE GENERATOR

Generator auxiliary system are broadly classified into 3 parts:

1. STATOR WATER SYSTEM

Stator water cooling is a closed loop system. There are two full capacity single stage centrifugal pumps with change over facility. The pumps are driven by 3Ph. 415V A.C. motors. The stator water cooler is shell and tube

Figure 3 CYLINDRICAl ROTOR

type heat exchanger. DM water flows through the shell. There are two mechanical filters and one magnetic filters. Mechanical filters are of wire mesh type. Magnetic filter is having permanent magnet. The expansion tank is a hermetically sealed container made of S.S. Float valve is there in the expansion tank to maintain water level in the tank which act as suction storage tank for stator water pumps. Polishing unit (mixed bed ion exchanger) is there to maintain conductivity of stator water to desired level

2. SEAL OIL SYSTEM

Generator shaft seals are supplied with pressurized seal oil to prevent hydrogen escape at the shaft

POLISH UNIT

FILTERS

COOLERSS

PUMPS

GAS TRAP

FLOW METERS

CCCCCC CONDUCTIVITY

CONDUCTIVITY

CTIVITYO

EXPANSION TANK

TANK

MAKE UP

up

UP

FSII

DIFF PRESS.

MAGNET FILTER

Oil pressure is kept higher than the gas pressure. There are one AC seal oil pp. and one DC seal oil pp. which feed oil to the seal through cooler and filter.

A vacuum pump is provided to maintain vacuum in seal oil tank The seal oil pressure to the seal is controlled by DPR which maintain specified DP between oil and hydrogen. There is provision for thrust oil to hold the seal ring in position against H2 pressure (0.5kg/cm2 more than seal oil pressure). There are 2 oil coolers to cool the hot oil.

BEARGDRAINS

TG

1 2

EE

BEARING

TE

BEARING

GAS

EXHAUST

FROM GAS

SYSTEM

IOT

VACUUM

TANK

SOST

SOPSOP-3 DC

DPSW

TS ES FR

FLOW M. PS PG

3. GAS SYSTEM Generator gas system constitutes of hydrogen gas used to cool the rotor and certain parts of stator. H2-air mixture is explosive. So filling the generator

with H2 by replacing air is dangerous. So initially air is replaced by CO2 and

since CO2 is heavier than air CO2 is being filled from the bottom. Purging of

air with CO2 is being done till the purity of CO2 inside casing reaches above

95%.

Now H2 is dried and then passed from the top to replace the CO2. Purging

of H2 is continued till purity of H2 reaches 98%.

Advantage of using H2 as a coolant

H2 is lightest gas with 0.09 gm / litre while air’s 1.3 i.e. 14.4 times & high thermal capacity

Thermal conductivity of H2 is 5 times that of air. Its specific heat is 3.42 at 0°C, as compared to 0.237 of air.

2 H2 + O2 mixture ignites on adiabatic compression at 526°C, 3H2 + O2 at 544°C. H2 and O2 combine slowly at 180°C or in bright sunlight. Explosion occurs with moist gases at 550 - 700°C. Ignition temp. of H2 in air is 538°C & Calorific value of H2 is 136 k Cal / gm.

For filling in TGs, 99.9% v/v purity gas is used. Traces of SO2 & NH3 shall not be detectable

Two propeller type fans are shaft mounted on either side of rotor body for circulating H2 in the generator.The H2 is itself cooled by DM water

circulation.TRANSFORMERS A static electromagnetic device with two or more windings ,which transforms a system of alternating voltage and current into another system of voltage and current usually of different values and at the same frequency for the purpose of transmitting electrical power.TYPES OF TRANSFORMERS: Power transformersUsed in transmission network of higher voltages, deployed for step-up and step down transformer application (400 kV, 200 kV, 110 kV, 66 kV, 33kV,22kV)Distribution transformersUsed for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V,

230V).Transformer insulations Minor insulation Like inter turn insulation, is achieved using cellulogic paper.

Major insulation Between primary and secondary, phase to phase and inner coil to

core. This is achieved by Bakelite, wooden blocks, cellulogic paper cylinders.

Transformer Oil Derivative of petroleum crude. This has good dielectric strength. Also

a good cooling medium and absorbs heat from the windings in transformer. The mineral oil has a flash point of 140°C and 160°C fire point. This also 'can Sustain the combustion with its own energy, once it catches fire. Thus this is unsuitable for the transformer located indoors. •The indoor transformers are filled with a synthetic liquid known as silicate liquid. This is fire assistant and has flash point well above 300°C.

Types of transformers in FSTPP:

Generator Transformer: Figure 4 TRANSFORMER

The generator is connected to this transformer by means of isolated bus ducts. This transformer is used to step up the generating voltage of around 15KV to grid voltage. This transformer is generally provided with OFAF cooling. It is also provided with off circuit/on load taps on the high voltage side. This transformer has elaborate cooling system consisting of number of oil pumps and cooling fans apart from various accessories. For 500 MW units there are 3 individual single phase transformers for each phase. The rated input voltage is 21KV.

RATED O/P 250MVA

RATED VOLT. (HV) 420KV

RATED VOLT. (LV) 15.75KV

RATED CURRENT(HV) 344A

RATED CURRENT(LV) 9175A

VECTOR GROUP Y n d11

Unit auxiliary transformer

The UAT draws its input from the main bus-duct connecting generator to the generator Transformer. The total KVA capacity of unit auxiliary transformer required can be determined by assuming 0.85 power factor and 0.9 efficiency for total auxiliary motor load. It is safe and desirable to provide about 20% excess capacity than circulate so as to provide for miscellaneous auxiliaries and possible increase in auxiliary load. With higher unit ratings and higher steam conditions, the auxiliary power required also increases and limitations imposed by the switchgear voltages available, indicate the maximum size of unit auxiliary transformer which can be used.

Station transformer

The station transformer is required to feed power to the auxiliaries during start ups. This transformer is normally rated for the initial auxiliary load requirements of unit. In typical cases, this load is of the order of 60% of the load at full generating capacity. But in large stations where more than one units are operating, the station transformers should have sufficient capacity to start two units at a time in addition to feeding the common auxiliaries. It is also provided with on load tap changer to cater to the fluctuating voltage of the grid.

RATED OUTPUT(LV):6.6KV RATED INPUT( HV) : 33KV

Excitation transformer

The excitation transformer is used in the static excitation system of the generator .The output of the generator is fed to the primary of the excitation transformer. The secondary is the input to 4 thyristor banks .In addition there are protective relays for the excitation transformer.

Auxiliary transformers

They are used to supply power to the LT auxiliary units like ESPs, lubricating oil pumps, seal oil pumps etc.

Tie / Auto transformer Tie transformers are connected to the 400 KV bus. They are used to

step down the voltage to 33 KV. Then the station transformers step down the voltage to 6.6 KV (station buses). The unit buses are connected to the station buses through circuit breakers and isolaters. When the generator trips, there is no supply to the unit buses and the tie-transformers supply the station buses. Then the circuit breakers and the isolaters are closed. Thus the supply to the auxiliaries is maintained.

Instrument transformers

Potential Transformers step down values to safe levels for measurement. They are also called voltage transformers. Their standard output is 120V.

Current Transformers have standard output of 1 or 5 amps. They can produce high voltages if open circuited.

SWITCHYARD

MAIN FUNCTIONS

1) Providing a link between Generating plant and transmission system.2) Stepping up or stepping down voltage as required.3) Controlling reactive power which has effect on quality of power.

MAIN COMPONENTS

Lightning arrestor Current transformers (CT)

Voltage transformer (VT) Power transformer/ICT

Busbar and clamp fittings Support structures

Isolators Circuit breakers (CB)

Wave traps Earthing switches

Control rooms Control relay panels

LIGHTNING ARRESTOR

Discharging highvoltage surges in the power stations due to lightning to the ground

Protecting overhead lines,substations, hv equipments like transformers.

CURRENT TRANSFORMERS (CTs)

Stepping down of high magnitude currents to a safer value for measurement purposes.

Used for metering and instrumentation purposes in power system. The meters are calibrated in terms of the original high values of current while

they take only a minute fraction of the original current hence the meters get protected but serve their purpose at the same time.

VOLTAGE TRANSFORMERS (VTs)

Stepping down of high magnitude voltage to a safer value for measurement purposes.

Used for metering and instrumentation purposes in power system. These are also used in relay protective system. In conjunction with current transformers (CTs) they can be used in

measuring power. They feed synchronizing equipments. They can be used as coupling capacitors in PLCC (Power Line Carrier

Communication).POWER TRANSFORMERS/ ICT

Stepping up of voltages for transmitting power along the line.

ISOLATORS

They can break electric circuit when the circuit is to be switched ON no-load. Isolators perform the following functions.

Interrupting transformer magnetized currents. Interrupting line charging currents Load transfer switching & helps in isolating a part of the line during

maintenance

CIRCUIT BREAKERS

They can break or make a circuit on-load even during on-fault condition. Circuit breakers are always used in conjunction with the isolators. Circuit breakers perform the following functions.

Protection of equipment during fault condition.

ONE AND A HALF CIRCUIT BREAKER SCHEME

Main 1Main 2

Feeder 2

WAVETRAP

It is used for the protection of the transmission line and communication between the substation.

VHF signal is transmitted from one end to other through the same power line.

EARTH SWITCHES

Earth switches are devices which are normally used to earth a particular system to avoid accident, which may happen due to induction on account of live adjoining circuit. These do not handle any appreciable current at all.

Used for protection purposes.

Tie CB Feeder 1

SWITCHGEAR

“The apparatus used for Switching, Controlling and Protecting the Electrical Circuits and equipment” is known as Switch gear

Need of Switchgear : Switching during normal operating conditions for the purpose of Operation

and Maintenance. Switching during Faults and Abnormal conditions and interrupting the fault

currents.

Parts of switchgear

Switching device:

Power circuit Control circuitMeasurement and display ProtectionPower

Circuit:

Circuit breakers / contactors Isolators

Earthing switch

Control Circuit :

Service / test /isolated position selectors Tripping and closing circuit

Spring charging, anti pumping arrangement Supply monitoring , space heaters , indications

Measurement and Protection:

Ammeter, voltmeter, energy meter Relays, CT, PT,

Classification of switchgears:

Method of arc quenching :

Bulk oil, Min. oil, Air Break, Air Blast, SF6 , Vacuum

Working voltage :

440V, 6.6 kV, 11 kV, 400 kV etc.

Indoor / out door

SOME INTERLOCKS :

Check synchronization for closing

Master relay contacts for trip and close

1.) HV & LV Breaker interlocks 2.) Main / Reserve supply change over

Switchgear: Basic Design Aspects• The Auxiliary power system in a power plant must form a RELIABLE source of power to all unit and Station auxiliaries. The basic function of Switchgear is to control supply of electric power and to protect the equipment in the event of abnormal conditions. Hence the switchgears have to be RELIABLE, SAFE, and ADEQUATE.

•

Defining the reliability, safety aspects and adequacy aspects in terms of Quantitative parameters forms the essential part in “SPECIFICATIONS”

• 33KV, 11KV, 6.6KV and 3.3KV Switchgears

• Indoor, metal clad single front and fully Compartmentalized, with degree of protection IP42 and IP52 for metering compartments. For 33 KV the switchgears can be metal enclosed either.

•

Circuit Breakers are of either SF6 or Vacuum type. They shall comprise of three separate identical single pole interrupting units operated through a common shaft by a sturdy mechanism.

• Breakers are suitable for Switching transformers and motors at any load and also for starting 3.3 KV - Above 200 KW to1500 KW, 11 KV- above 1500 KW for 500MW units and 6.6 KV- above 200KW for 210MW units.

• Surge arresters are provided for all motor feeders to limit the over voltages. For Motors where frequent start/stop of motors is called for HRC fuse backed contactors are provided.

• Suitable Interlocks are provided to ensure that Breaker is off before opening the rear doors/covers.

Basic design features: Control and Safety

• Circuit Breakers/contactors are being normally operated from remote through Distributed Digital Control & Management of Information System (DDCMIS)/ Programmable Logic Controller (PLC). The control Switch located on the Switchgear is normally used only for testing.

• All the logic for incomers, bus couplers, ties, transformer feeders and motor feeders is being generated in DDCMIS only. The reverse blocking schemes are still incorporated in Switchgear (hardwired).

Power System Protection

NEED FOR POWER SYSTEM PROTECTION

Modern power systems are growing fast with more generators, transformers and large network in the system. For system operation, a high degree of reliability is required. In order to protect the system from damage due to undue currents and abnormal voltages due to faults, the need for reliable protective devices such as relays and circuit breakers, arises.

As a rule, on the occurrence of short circuits which may lead to heavy disturbances in normal operation, the protective scheme is designed to disconnect or isolate the faulty section from the system without any delay.

The main functions of protective relaying are to detect the presence of faults, their locations and to initiate the action for quick removal from service of any element of power system when it suffers a short circuit or when it starts to operate in any abnormal manner that might cause damage or interfere with the effective operation of the rest of the system.

The relaying equipment is added in this task by the circuit breakers that are capable of disconnecting the faulty equipment, when they are called upon to do so by the relaying equipment. Circuit breakers are generally located so that each generator, transformer, bus, transmission line can be completely disconnected from the rest of the system.

Fusing is employed where protective relaying and circuit breakers are not economically justifiable.

RELAYSRelays are a device that detects the fault and initiates the operation of the Circuit breaker to

isolate the defective element from the rest of the system”. The relay detects the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and faulty conditions.

Classification of relays

Protection relays can be classified in accordance with the function which they carry out, their construction, the incoming signal and the type of functioning.

1. General function· Auxiliary.· Protection.· Monitoring.· Control.

2. Construction· Electromagnetic.· Solid state.· Microprocessor.· Computerized.· Non-electric (thermal, pressure ......etc.).

 3. Incoming signal

· Current.· Voltage.· Frequency.· Temperature.· Pressure.· Velocity.· Others.

4. Type of protection· Over current.· Directional over current.· Distance.· Over voltage.· Differential.· Reverse power.

· Other.

Figure 5 ARMATURE TYPE RELAY

ELEC TR OMAGNETIC RELAYS

Electromagnetic relays are constructed with electrical, magnetic and mechanical components, have an operating coil and various contacts and are very robust and reliable. The construction characteristics can be classified in three groups, as detailed below.

ATTRACTION RELAYS

Attraction relays can be supplied by AC or DC, and operate by the movement of a piece of metal when it is attracted by the magnetic field produced by a coil. There are two main types of relay in this class.

The attracted armature relay, which is shown in figure 1, consists of a bar or plate of metal which pivots when it is attracted towards the coil.

The armature carries the moving part of the contact, which is closed or opened according to the design when the armature is attracted to the coil. The other type is the piston or solenoid relay, illustrated in Figure 2, in which α bar or piston is attracted axially within the field of the solenoid. In this case, the piston also carries the operating contacts.It can be shown that the force of attraction is equal to K1I2 - K2, where Κ1

depends upon the number of turns on the operating solenoid, the air gap, the effective area and the reluctance of the magnetic circuit, among other factors. K2 is the restraining force, usually produced by a spring. When the relay is balanced, the resultant force is zero and therefore Κ112 = K2,

So that    I = √(K2/K1) = constant

In order to control the value at which the relay starts to operate, the restraining tension of the spring or the resistance of the solenoid circuit can be varied, thus modifying the restricting force. Attraction relays effectively have no time delay and, for that reason, are widely used when instantaneous operations are required.

RELAYS WITH MOVEABLE COILS

This type of relay consists of a rotating movement with a small coil suspended or pivoted with the freedom to rotate between the poles of a permanent magnet. The coil is restrained by two springs which also serve as connections to carry the current to the coil.The torque produced in the coil is given by:

T = B.l.a.N.i

Where:

 T= torqueB = flux densityL =length of the coila = diameter of the coilN= number of turns on the coil i = current flowing through the coil

Figure 6 SOLENOID TYPE RELAY

Figure 7 INVERSE TIME CHARACTERISTIC

From the above equation it will be noted that the torque developed is proportional to the current. The speed of movement is controlled by the damping action, which is proportional to the torque. It thus follows that the relay has an inverse time characteristic similar to that illustrated in Figure 3. The relay can be designed so that the coil makes a large angular movement, for example 80º.

INDUCTION RELAYS

An induction relay works only with alternating current. It consists of an electromagnetic system which operates on a moving conductor, generally in the form of a disc or cup, and functions through the interaction of electromagnetic fluxes with the parasitic Fault currents which are induced in the rotor by these fluxes. These two fluxes, which are mutually displaced both in angle and in position, produce a torque that can be expressed by

T= Κ1.Φ1.Φ2 .sin θ,

Where Φ1 and Φ2 are the interacting fluxes and θ is the phase angle between Φ1 and Φ2. It should be noted that the torque is a maximum when the fluxes are out of phase by 90º, and zero when they are in phase.

Figure 8 ELECTROMAGNETIC FORCES IN INDUCTION RELAYS

It can be shown that Φ1= Φ1sin ωt, and Φ2= Φ2 sin (ωt+ θ) , where θ is the angle by which Φ2 leads Φ1. Then:

And

Figure 4 shows the interrelationship between the currents and the opposing forces. Thus:

F= (F 1 - F 2 ) α (Φ2 iΦ1+ Φ1

iΦ2 )

F α Φ2 Φ1

sin θ α T

Induction relays can be grouped into three classes as set out below.

SHADED POLE RELAY

In this case a portion of the electromagnetic section is short-circuited by means of a copper ring or coil. This creates a flux in the area influenced by the short circuited section (the so-called shaded section) which lags the flux in the non-shaded section.

Figure 7 SHADED-POLE RELAY

WATTMET RI C TYPE RELAY

In its more common form, this type of relay uses an arrangement of coils above and below the disc with the upper and lower coils fed by different values or, in some cases, with just one supply for the top coil, which induces an out-of-phase flux in the lower coil because of the air gap. Figure 6 illust rates a typical arrangement.

Figure 8 WATTMETRIC-TYPE RELAY

CUP TYPE RELAY

This type of relay has a cylinder similar to a cu which can rotate in the annular air gap between the poles of the coils, and has a fixed central core. The operation of this relay is very similar to that of an induction motor with salient poles for the windings of the stator. Configurations with four or eight poles spaced symmetrically around the circumference of the cup are often used. The movement of the cylinder is limited to a small amount by the contact and the stops. Α special spring provides the restraining torque.

The torque is a function of the product of the two currents through the coils and the cosine of the angle between them. The torque equation is

T= ( KI1I2 cos (θ12 – Φ) – Ks ),

Where K, .Κs and Φ are design constants, Ι1 and I2 are the currents through the two coils and θ12 is the angle between I1 and I2.

Figure 9 CUP-TYPE RELAY

In the first two types of relay mentioned above, which are provided with a disc, the inertia of the disc provides the time-delay characteristic. The time delay can be increased by the addition of a permanent magnet. The cup-type relay has a small inertia and is therefore principally used when high speed operation is required, for example in instantaneous units.

CIRCUIT BREAKERS A Circuit Breaker is a switching device which can open or close a

circuit in a small fraction of second. This is when the movable contacts start to separate. As a opening of the circuit allows to establish or to interrupt the circulation of current through the circuit under usual or unusual working conditions, such as short circuits.

Figure 10

The interruption process of the current in a CB, begins achieved due to its separable contacts. The closing and consequence, the contact area is reduced and the current density gets larger, until the energy causes the metal to begin vaporizing and an arc appears. In spite of the existence of a physical separation of the switching contacts, the established arc makes possible that the current continues flowing . The interruption of the circulating current will be achieved when the interrupting medium gets to turn the carrying arc plasma into an isolating medium.

In the beginning of the arc phenomena the main source of charged particles is the electrode vapour. However, as the contact separation increases the ionization degree of the arc column is also influenced by the characteristics of the surrounding medium, except for Vacuum CBs.

CLASSIFICATION OF CIRCUIT BREAKERS

According to the medium and the method used for the interruption of the current, CBs can be grouped in the following types:

Air Magnetic Circuit Breakers

Air Blast Circuit Breakers

Oil Circuit Breakers

Sulphur Hexafluoride (SF6) Circuit Breakers

Vacuum Circuit Breakers

Oil circuit breakers were the first CB type in the grid, due to its ability to interrupt large currents. The interruption process is based on the creation of hydrogen and acetylene gas bubble, as the oil decomposes as a result of the arc established between the switching contacts. The disadvantages of using oil as quenching media in circuit breakers, as flammability and a high maintenance cost, forced to search for different mediums of quenching. Air Blast and Magnetic Air circuit breakers were developed but did not sustain in the market due to some disadvantages, as the fact that they are bulky and cumbersome. In the middle of the century, and being considered as the new generation of CBs, SF6 and Vacuum CBs appeared.

Figure 11 OIL CIRCUIT BREAKER

SF6 CBs started quickly to replace oil and air CBs for HV applications, as most SF6 properties are superior to other interrupting mediums, such as its high dielectric strength or higher thermal conductivity. In contrast Vacuum CBs started to spread in the Medium Voltage level up to rated voltages of around 36kV.

Even though many CBs from the first generation, with oil or air as quenching medium, are still working, SF6 CB is undoubtedly the most common CB used nowadays for used for medium voltage levels (5-38 kV), has emerged limitations and disadvantages. Vacuum switching, widely HV applications worldwide. But, it also has its as an alternative for high voltage applications due to its environmental friendliness. Latest research composition magnetic field application material insulation, multi-gap or long gap technology has lead to the development of various prototypes for higher voltage levels.

SF6 C IRCUIT BREAKER

             In an SF6 circuit-breaker, the current continues to flow after

contact separation through the arc whose plasma consists of ionized SF6

gas. For, as long as it is burning, the arc is subjected to a constant flow of

gas which extracts heat from it. The arc is extinguished at a current zero,

when the heat is extracted by the falling current. The continuing flow of gas

finally de-ionises the contact gap and establishes the dielectric strength

required to prevent a re-strike.              

The direction of the gas flow, i.e., whether it is parallel to or across

the axis of the arc, has a decisive influence on the efficiency of the arc

interruption process. Research has shown that an axial flow of gas creates a

turbulence which causes an intensive and continuous interaction between

the gas and the plasma as the current approaches zero. Cross-gas-flow

cooling of the arc is generally achieved in practice by making the arc move

in the stationary gas. This interruption process can however, lead to arc

instability and resulting great fluctuations in the interrupting capability of

the circuit breaker.   

In order to achieve a flow of gas axially to the arc a pressure

differential must be created along the arc. The first generation of the SF6

circuit breakers used the two-pressure principle of the air-blast circuit-

breaker. Here a certain quantity of gas was kept stored at a high pressure

and released into the arcing chamber. At the moment high pressure gas and

the associated compressor was eliminated by the second generation design.

Here the pressure differential was created by a piston attached to the

moving contacts which compresses the gas in a small cylinder as the

contact opens. A disadvantage is that this puffer system requires a

relatively powerful operating mechanism.

Figure 12 SF6 CIRCUIT BREAKER

Neither of the two types of circuit breakers described was able to

compete with the oil circuit breakers price wise. A major cost component of

the puffer circuit-breaker is the operating mechanism; consequently

developments followed which were aimed at reducing or eliminating this

additional cost factor. These developments concentrated on employing the

arc energy itself to create directly the pressure-differential needed. This

research led to the development of the self-pressuring circuit-breaker in

which the over – pressure is created by using the arc energy to heat the gas

under controlled conditions. During the initial stages of development, an

auxiliary piston was included in the interrupting mechanism, in order to

ensure the satisfactory breaking of small currents. Subsequent

improvements in this technology have eliminated this requirement and in

the latest designs the operating mechanism must only provide the energy

needed to move the contacts.          

Parallel to the development of the self-pressuring design, other work

resulted in the rotating – arc SF6 gas circuit breaker. In this design the arc

is caused to move through, in effect the stationery gas. The relative

movement between the arc and the gas is no longer axial but radial, i.e., it

is a cross-flow mechanism. The operating energy required by circuit

breakers of this design is also minimal.

VACUUM CIRCUIT BREAKERS

 In a Vacuum circuit breaker, vacuum interrupters are used for

breaking and making load and fault currents. When the contacts in vacuum

interrupter separate, the current to be interrupted  initiates a metal vapour

arc discharge and flows through the plasma until the next current zero. The

arc is then extinguished and the conductive metal vapour condenses on the

metal surfaces within a matter of micro seconds. As a result the dielectric

strength in the breaker builds up very rapidly.

The properties of a vacuum interrupter depend largely on the material

and form of the contacts. Over the period of their development, various

types of contact material have been used. At the moment it is accepted that

an oxygen free copper chromium alloy is the best material for High voltage

circuit breaker. In this alloy, chromium is distributed through copper in the

form of fine grains. This material combines good arc extinguishing

characteristic with a reduced tendency to contact welding and low chopping

current when switching inductive current. The use of this special material is

that the current chopping is limited to 4 to 5 Amps.

At current under 10KA, the Vacuum arc burns as a diffuse discharge.

At high values of current the arc changes to a constricted form with an

anode spot. A  constricted arc that remain on one spot for too long can

thermically over stress the contacts to such a degree that the deionization

of the contact zone at current zero can no longer be guaranteed . To

overcome this problem the arc root must be made to move over the contact

surface. In order to achieve this, contacts are so shaped that the current

flow through them results in a magnetic field being established which is at

right angles to the arc axis. This radial field causes the arc root to rotate

rapidly around the contact resulting in a uniform distribution of the heat

over its surface. Contacts of this type are called radial magnetic field

electrodes and they are used in the majority of circuit breakers for medium

voltage application.

A new design has come in Vacuum interrupter, in which switching

over the arc from diffusion to constricted state by subjecting the arc to an

axial magnetic field. Such a field can be provided by leading the arc current

through a coil suitably arranged outside the vacuum chamber. Alternatively

the field can be provided by designing the contact to give the required

contact path. Such contacts are called axial magnetic field electrodes. This

principle has advantages when the short circuit current is in excess of 31.5

KA.

Table 1. Characteristics of the SF6 and vacuum current interrupting technologies.

SF6 Circuit Breakers Vacuum Circuit Breakers

Criteria Puffer Circuit Breaker Self-pressuring circuit-breaker

Contact material-Chrome-Copper

Operating energy requirements

Operating Energy requirements are high,

Operating Energy requirements are low,

Operating energy requirements are low,

because the mechanism must supply the energy needed to compress the gas.

because the mechanism must move only relatively small masses at moderate speed, over short distances. The mechanism does not have to provide the energy to create the gas flow

because the mechanism must move only relatively small masses at moderate speed, over very short distances.

Arc Energy Because of the high conductivity of the arc in the SF6 gas, the arc energy is low. (arc voltage is between 150 and 200V.)

Because of the very low voltage across the metal vapour arc, energy is very low. (Arc voltage is between 50 and 100V.)

Contact Erosion Due to the low energy the contact erosion is small. Due to the very low arc energy, the rapid movement of the arc root over the contact and to the fact that most of the metal vapour re-condenses on the contact, contact erosion is extremely small.

Arc extinguishing media

The gaseous medium SF6 possesses excellent dielectric and arc quenching properties. After arc extinction, the dissociated gas molecules recombine almost completely to reform SF6. This means that practically no loss/consumption of the quenching medium occurs. The gas pressure can be very simply and permanently supervised. This function is not needed where the interrupters are sealed for life.

No additional extinguishing medium is required. A vacuum at a pressure of 10-7 bar or less is an almost ideal extinguishing medium. The interrupters are ‘sealed for life’ so that supervision of the vacuum is not required.

Switching behavior in relation to current chopping

The pressure build-up and therefore the flow of gas is independent of the value of the current. Large or small currents are

The pressure build-up and therefore the flow of gas is dependent upon the value of the current

No flow of an ‘extinguishing’ medium needed to extinguish the vacuum arc. An extremely rapid de-ionization of the

cooled with the same intensity. Only small values of high frequency, transient currents, if any, will be interrupted. The de-ionization of the contact gap proceeds very rapidly, due to the electro-negative characteristic of the SF6 gas and the arc products.

to be interrupted. Large currents are cooled intensely, small currents gently. High frequency transient currents will not, in general, be interrupted. The de-ionization of the contact gap proceeds very rapidly due to the electro-negative characteristic of the SF6 gas and the products.

contact gap, ensures the interruption of all currents whether large or small. High frequency transient currents can be interrupted. The value of the chopped current is determined by the type of contact material used. The presence of chrome in the contact alloy with vacuum also.

No. of short-circuit operation

10---50 10---50 30---100

No. full load operation

5000---10000 5000---10000 10000---20000

No. of mechanical operation

5000---20000 5000---20000 10000---30000

TABLE 2. COMPARISON OF THE SF6 AND VACUUM TECHNOLOGIES IN RELATION TO OPERATIONAL ASPECTS.

Criteria SF6 Breaker VCB

Summated current cumulative

10-50 times rated short circuit current

30-100 times rated short circuit current

Breaking current capacity of interrupter

5000-10000 times 10000-20000 times

Mechanical operating life 5000-20000 C-O operations 10000-30000 C-O operations

No operation before maintenance

5000-20000 C-O operations 10000-30000 C-O operations

Time interval between servicing Mechanism

5-10 years 5-10 years

Outlay for maintenance Labour cost High, Material cost Low

Labour cost Low, Material cost High

Reliability High High

Dielectric withstand strength of the contact gap

High Very high

TABLE 3. COMPARISON OF THE SF6 AND VACUUM SWITCHING TECHNOLOGIES IN RELATION TO SWITCHING APPLICATIONS

Criteria

SF6 Breaker VCB

Switching of Short circuit current with High DC component

Well suited Well suited

Switching of Short circuit current with High RRV

Well suited under certain conditions (RRV>1-2 kV per Milli seconds

Very well suited

Switching of transformers Well suited. Well suited

Switching of reactors Well suited Well suited. Steps to be taken when current <600A. to avoid over voltage due to current chopping

Switching of capacitors Well suited. Re-strike free Well suited. Re-strike free

Switching of capacitors back to back

Suited. In some cases current limiting reactors required to limit inrush current

Suited. In some cases current limiting reactors required to limit inrush current

Switching of arc furnace Suitable for limited operation Well suited. Steps to be taken to limit over voltage.

GENERATOR STATOR PROTECTION

STATOR WINDINGS FAULTS

Stator winding faults: These types of faults occur due to the insulation breakdown of the stator coils. Different types of stator windings faults are:

a) phase to earth faultb) phase to phase faultc) inter turn fault

Phase to earth fault are limited by resistance of the neutral grounding resistor. There are fewer chances for the occurrence of the phase to phase and inter-turn faults. The insulation between the two phases is at least twice as thick as the insulation between one coil and the iron core, so phase to phase fault is less likely to occur. Inter turn fault occurs due the incoming current surges with steep wave front.

STATOR PROTECTION

Differential protection for generators: Differential protection is used for protection of the generator against phase to earth and phase to phase fault. Differential protection is based on the circulating current principle.

Figure 13 Differential Protection for Generators

In this type of protection scheme currents at two ends of the protection system are compared. Under normal conditions, currents at two ends will be same. But when the fault occurs, current at one end will be different from the current at the end and this difference of current is made to flow through relay operating coils. The relays then loses its contacts and makes the circuit breaker to trip, thus isolate the faulty section. This type of protection is called the merz price circulating current system.

Limitations of this method:

The earth fault is limited by the resistance of the neural earthing. When the fault occurs near the neutral point, this causes a small current to flow through the operating coil and it is further reduced by the neutral

resistance. Thus this current is not sufficient to trip the circuit breaker. By this protection scheme, one can protect only 80 to 85 percent of the stator winding. If the relays with low settings are used then it will not provide desire stability. This difficulty is overcome by using the modified differential protection.

Modified differential protection:

In modified differential protection setting of the earth faults can be reduced without any effect on the stability.

Figure 14 Modified differential protection for the generators

In this method two relays are used for the phase to phase fault and one relay is used for the protection of earth fault. In this method the two relays and the balancing resistance are connected in star and the phase fault relay is connected between the star point and the neutral pilot wire. The star connected circuit is symmetrical in terms of impedance. So when the fault current occurs due to the phase to phase fault, it cancels at the star point due to the equal impedance. Thus it is possible with this scheme to operate with the sensitive earth fault relays. Thus this scheme provides protection to the greater percentage of the stator winding.

Biased circulating current protection (percentage differential relay protection):

With the differential protection relaying, the CTs at both end of the stator windings must be same. If there is any difference in the accuracy of the CTs the mal-operation of the relay will occurs. To overcome this difficulty, biased circulating current protection is used. In this protection system we can automatically increase the relay setting in proportion to the fault current. By suitable proportioning of the ratio of the relay restraining coil to the relay operating coil any biased can be achieved.

Figure 15 Biased protection of the stator winding

Under normal operating condition current in secondary of the line CTs will be same as the current in the secondary of the CTs at the neutral end. Hence there are balanced current flows in the restraining coils and no current flows in the operating coil. If there is any phase to phase or phase to earth fault occurs then it causes the differences in the secondary current of the two CTs. Thus the current flows through the operating coil and make the circuit breaker to trip.

Advantages of this method:

a) It does not require the CTs with balancing features.b) It also permit the low fault setting of the relay, thus protects the greater percentage of the stator winding.

Self balance protection system:

This type of protection is employed for earth fault and also for the phase to phase fault.

Figure 16 Self biasing protection of the stator windings

In this type of protection two cables are required which is connected to the two ends of the each phase. These two cables are passed through the

circular aperture of the ring type CTs. Under normal conditions the current flowing in the two leads of the cable will be in the same direction and no magnetisation occurs in the ring type CTs. When the earth fault occurs in any phase the fault current occurs only once through the CTs and thus magnetic flux induced, this induces the emf in the relay circuit causes the circuit breaker to trip.

This is very sensitive type earth fault protection but it also has some limitations:a) A different design of the cable lead is required in this scheme.b) Large electromagnetic forces are developed in the CT ring under the condition of heavy short circuit.

Stator ground fault protection:

The method of grounding effect the degree of protection which is employed by the differential protection. High impedance reduces the fault current and thus it is very difficult to detect the high impedance faults. So the differential protection does not work for the high impedance grounding. The separate relay to the ground neutral provides the sensitive protection. But ground relay can also detect the fault beyond the generator, it the time co-ordination is necessary to overcome this difficulty.

If we use the star- delta transformer bank, then it will block the flow of ground currents, thus preventing the occurrence of the fault on other side of the bank from operating ground relays. In unit protection scheme the transformer bank limits the operation of the fault relay to the generator.

Unit connected schemes:

In this scheme high resistance grounding is used and system is grounded through the transformer bank and through the resistors.

95% scheme: Relay which uses in the unit connected schemes must be insensitive to the normal third harmonics voltage that may be present between the neutral and the ground, and it must be sensitive to the fundamental harmonics voltage that is the cause of the fault. The magnitude of the neutral shift depends upon its location in the winding of the ground fault. And the general choice of the relay sensitive and distribution transformer voltage provide 95% protection of the winding so this scheme is called 95% scheme.

Neutral third harmonic under voltage: There is the third harmonic present between the neutral and the ground , and other schemes takes advantages of this and respond to the under voltage between the neutral and the ground.

100% scheme: This scheme provides complete protection of the stator winding by injecting the signal between the stator winding and monitors it for change. 95% scheme and third harmonics protection scheme provide protection only at rated speed and rated voltage but it 100% scheme also provide protection at standstill.

Stators inter-turn fault protection: Differential protection for stator does not provide protection against the inter-turn faults on the same phase winding of the stator. The reason is that the current produced by the turn to turn fault flows in the local circuit between the turns involved and thus it does not create any difference between the current entering and leaving the windings at its two ends where the CTs are mounted.

The coils of the modern turbo generator are single- turn, so there is no need to provide inter–turn fault protection for the turbo generator. But the inter turn protection is necessary for the multi turn generator like hydro electric generator. Sometimes stator windings are duplicated to carry heavy current. In this case stator winding have two different paths.

In this type of protection primaries of the CTs are inserted in the parallel paths and secondaries are inter-connected. Under the normal condition current flowing through the two parallel path of the stator winding will be same and no current flowing through the relay operating coil. Under the inter turn fault, current flowing through the two parallel path will be different and this difference in current flowing through the operating coil and thus causes the circuit breaker to trip and disconnect the faulty section. This type of protection is very sensitive.

Figure 17 Inter turn protection of the stator winding

Stator over heating protection: Stator over heating is caused due to the overloads and failure in cooling system. It is very difficult to detect the overheating due to the short circuiting of the lamination before any serious damage is caused. Temperature rise depend upon I2Rt and also on the cooling. Over current relays cannot detect the winding temperature because electrical protection cannot detect the failure of the cooling system.

So to protect the stator against overheating, embed resistance temperature detector or thermocouples are used in the slots below the stator coils. These detectors are located on the different places in the windings so that to detect the temperature throughout the stator. Detectors which provide the indication of temperature change are arranged to operate the temperature relay to sound an alarm.

UNDER/OVER FREQUENCY PROTECTION:

Over frequency operation: Over frequency results from the excess generation and it can easily be corrected by reduction in the power outputs with the help of the governor or manual control.

Under frequency operation: Under frequency occurs due to the excess. During an overload, generation capability of the generator increases and reduction in frequency occurs. The power system survives only if we drop the load so that the generator output becomes equal or greater than the connected load. If the load increases the generation, then frequency will drop and load need to shed down to create the balance between the generator and the connected load. The rate at which frequency drops depend on the time, amount of overload and also on the load and generator variations as the frequency changes. Frequency decay occurs within the seconds so we cannot correct it manually. Therefore automatic load shedding facility needs to be applied.

These schemes drops load in steps as the frequency decays. Generally load shedding drops 20 to 50% of load in four to six frequency steps. Load shedding scheme works by tripping the substation feeders to decrease the system load. Generally automatic load shedding schemes are designed to maintain the balance between the load connected and the generator. The present practice is to use the under frequency relays at various load points so as to drop the load in steps until the declined frequency return to normal. Non essential load is removed first when decline in frequency occurs. The setting of the under frequency relays based on the most probable condition occurs and also depend upon the worst case possibilities.

During the overload conditions, load shedding must occur before the operation of the under frequency relays. In other words load must be shed before the generators are tripped.

UNDER/OVER VOLTAGE PROTECTION

Over voltage protection:

Over voltage occurs because of the increase in the speed of the prime mover due to sudden loss in the load on the generator. Generator over voltage does not occur in the turbo generator because the control governors of the turbo generators are very sensitive to the speed variation. But the over voltage protection is required for the hydro generator or gas turbine generators. The over voltage protection is provided by two over voltage relays have two units – one is the instantaneous relays which is set to pick up at 130 to 150% of the rated voltage and another unit is IDMT which is set to pick up at 110% of rated voltage. Over voltage may occur due to the defective voltage regulator and also due to manual control errors.

Under voltage protection:

If more than one generators supply the load and due to some reason one generator is suddenly trip , then another generators try to supply the load. Each of these generators will experience a sudden increase in current and thus decreases the terminal voltage. Automatic voltage regulator connected to the system try to restore the voltage. And under voltage relay type-27 is also used for the under voltage protection.

PROTECTION OF THE GENERATOR DUE TO UNBALANCE LOADING:

Due to fault there is an imbalance in the three phase stator currents and due to these imbalance currents, double frequency currents are induced in the rotor core. This causes the over heating of the rotor and thus the rotor damage. Unbalanced stator currents also damage the stator.

Negative sequence filter provided with the over current relay is used for the protection against unbalance loading. From the theory of the symmetrical components, we know that an unbalanced three phase currents contain the negative sequence component. This negative phase sequence current causes heating of the stator. The negative heating follows the resistance law so it is proportional to the square of the current. The heating time constant usually depend upon the cooling system used and is equal to I²t=k where I is the negative sequence current and t is the current duration in seconds and k is the constant usually lies between 3 and 20.

Its general practice to use negative current relays which matches with the above heating characteristics of the generator. In this type of protection three CTs are connected to three phases and the output from the secondaries of the CTs is fed to the coil of over current relay through negative sequence filter. Negative sequence circuit consists of the resistors and capacitors and these are connected in such way that negative sequence currents flows through the relay coil. The relay can be set to operate at any particular value of the unbalance currents or the negative sequence component current.

Figure 18 Protection against unbalance loading

GENERATOR ROTOR PROTECTION

ROTOR WINDINGS FAULTS

In the rotor mainly faults are earth faults or inter turn faults these are caused by thermal and mechanical stresses.

The field system is normally ungrounded so when a single fault between field winding and rotor body exist does not give rise to any fault current. But when second earth fault exist it short circuit the rotor winding and therefore produce an unsymmetrical field system and unbalance force on the rotor. This causes vibration of the rotor and damage to the bearings. So rotor earth fault protection is provided to restrict the fault spreading.

Because of this fault, unbalance in three phase stator currents exists. As unbalance three phase currents have a negative sequence component, it rotates in a opposite direction at synchronous speed giving rise to double frequency currents. This results in overheating of the rotor and other damage to the rotor. So rotor temperature indicators are used for detecting the rotor overheating.

Rotor open circuit faults are very rare to exist causes arcing problems and reduced excitation. Loss of excitation or field failure occurs due to short circuit or open circuit in field winding. In this case generator start running as induction generator, supplying power at leading power factor. Due to loss of excitation loss of synchronism and system stability occur. In this case rotor protection can be done by using tripping scheme which opens field circuit breaker which will trip the generator unit breaker.

ROTOR PROTECTION

Rotor earth fault protection:

As the field circuit are operating unearthed a single earth fault does not affect the operation of the generator. But this fault increases the stress to the ground because stator transients induce an extra voltage in the field winding. So if there is only single earth fault but relay should be provided to give the knowledge that fault has occurred so the generator may take out of service until second fault occurs and cause serious damage. Two methods are used for rotor earth fault protection.

Method I: In this method a high resistance is connected across the rotor circuit and its mid point is grounded through a sensitive relay. This relay detects the earth fault for whole circuit except the rotor centre point.

Figure 19 Rotor Earth Fault Protection Method I

Method II: In this method dc injection or ac injection method is used. In it either dc or ac voltage is connected between the field circuit and ground through a sensitive over voltage relay and current limiting resistor or capacitor. A single earth fault in the rotor circuit will complete the circuit including voltage source, sensitive over voltage relay and earth fault. DC injection method is simple and has no problems of leakage currents. If we use dc the over voltage relay will be more sensitive than if we use ac because in case when we use ac the relay not picking up the current that flows normally through capacitance to ground and also care should be taken to avoid resonance between capacitance and inductance.

Figure 20 Rotor Earth Fault Protection Method II

Rotor overheating Protection:

Negative sequence component of the unbalanced currents of the stator winding causes double frequency current to be induced in the rotor winding due to this component overheating of the rotor take place. In case of over current due to over excitation in the rotor circuit, a dc relay is used. This relay senses and initiates alarm. Application of such relay is limited because relaying of dc quantities is relatively uncommon.

Rotor Temperature Alarm :

This kind of protection is only provided in case of large generators. It gives the level of temperature. In it resistance is measured by comparing voltage and current by a double actuating quantity moving coil relay. The operating coil being used as voltage coil and restraining coil used as current

coil. The relay measures the ratio of voltage and current because resistance gives the measure of rotor temperature.

Figure 21 Rotor Overheating Protection

Automatic Field Suppression and Use of Neutral circuit Breaker:

When a fault on the generator winding exist even through the generator circuit breaker is tripped, the fault continues to be fed as long as the excitation will exist. For the quick removal of the fault, it is necessary to disconnect the field simultaneously with disconnection of the generator. So it is very necessary to discharge its magnetic field as soon as possible in short duration. Hence it should be ensured that all protection system not only the trip the generator circuit breaker but also trip the automatic field discharge switch.

Loss of Excitation Protection:

Loss of field failure of or loss of excitation is same phenomena and same kind of protection is used. It is discussed here in the field failure topic.

Loss of Field Protection

Loss of field occurs due to tripping of the supply of the field current which occurs because of the reasons.

(i) Loss of field to the main exciter.(ii) Accidental tripping of the field breaker.(iii) Short circuit in field circuits.(iv) Poor brush contact in the exciter.(v) Loss of A.C supply to the excitation system.(vi) Operating errors.

Field Protection Phenomena:

When the field supply is tripped, it speed increased and it start behaving as induction generator so heavy currents are produced in the teeth and wedges of the rotor. Because of the drop in excitation voltage the generator output voltage drops slowly to compensate this voltage current start increasing then generator become under excited and start drawing reactive power 2 to 4 times the generator load. Before losing excitation, the generator is delivering power to the system. But when loss of field occur this large reactive load thrown on the system abruptly with loss of generator’s reactive power and it further causes voltage reduction and extensive instability.

Protection against Loss of field:

If the system has capability to tolerate the difference of reactive power then automatic protection is not required but if the system will be instable in this condition and has not capability to tolerate then automatic protection is required.

Under current Moving coil relay is connected across a shunt in series with field winding. But in case of large generators which operate over a wide range of field excitation then this relay will not work properly because field failure due to the failure of the excitation is not detected by it because it is held in by the ac induced from the stator.

Figure 22 Loss of Field Protection

The most valid type of protection in this case is by using directional-distance type relay operating by alternating current and voltage at the generator terminals. In offset-mho relay is used and its setting is like that when the excitation goes certain value then this relay start operating because machine start running asynchronously. Its characteristics are shown on R-X diagram. When excitation is lost the generator impedance start a curve from the first quadrant to the fourth quadrant. This region is enclosed in the operating area of the relay so the relay will operate when the generator starts to slip poles and will trip the field breaker and disconnect the generator from the system. The generator may then return to service when the cause of failure is cleared.

Figure 23 Loss of Field Relay And system characteristics

Effects produced by loss of field:

(i) It can endanger the generator , connected system or both.(ii) Loss of synchronism.(iii) Overheating of stator winding.(iv) Increased Rotor Losses.

Pole Slipping

When angular displacement of the rotor exceeds the stability limit then rotor slips a pole pitch or we can say rotor flux slips with respect to stator flux. This condition is called pole slipping.

Causes:

(i) Power system fault that persists for long duration.(ii) Connecting line between two systems is opened.(ii) Because of the insufficient electromagnetic torque that keep rotor in synchronism(iv) Faulty excitation system.(v) Operating errors.

Pole Slipping Phenomena:

Pole slipping does not occur very often when faults are cleared very fast. When pole slipping occurs due to this synchronising power will start flowing in reverse direction twice for every slip cycle. On drawing this synchronising power on the impedance plane the flow of it characterised by cyclic change in the load impedance and load impedance locus passes between +R and –R quadrants because real power flows in reverse direction. When the load impedance is very reactive in nature then two systems are 180 degree out of phase, this instant is when drawn on the jx axis the point corresponding to this instant is called transition point. At this stage only reactive power flows and system voltage reached to zero at the electrical midpoint of the two systems. Mid point is that point where pole slipping take place and its location can be determined from the apparent load impedance to the point where the locus crosses the jx axis. Three parameters magnitude, direction and rate of change of load impedance with respect to the generator terminals tell us about the pole slipping, that is it taking place.

Figure 24 Offset Mho type Pole slipping Protection

Need of Pole Slipping Protection:

High currents and torques can Loosen or causes wear off stator windings. Damage shaft and coupling. Stator and rotor overheating. Excitation system damage.

Figure 25 Bock Diagram of the method used

This is protection against loss of synchronism. Impedance type protective relays are used in this case. There are two types relays connected to both current transformers and voltage transformers on l.v. and h.v side of generator transformer respectively pole slipping detection relays and directional control relays. Protection can be done by two stages.

Stage I- Reaching through generator to neutral point into the transformer to cover the whole winding but not crossing the h.v. terminals. Stage II-Operation after adjustable no of pole slips whether the system centre is in the generator transformer unit or out of it.

TRANSFORMER PROTECTION

INTRODUCTION

The development of modern power systems has been reflected in the advances in transformer design. This has result in a wide range of transformers with sizes ranging from a few kVA to several hundred MVA being available for use in a wide variety of applications.

The considerations for a transformer protection package vary with the application and importance of the transformers. Small distribution transformers can be protected satisfactorily, from both technical and economic considerations, by the use of fuse or over-current relays. This results in time-delayed protection. However, time-delayed fault clearance is unacceptable on larger power transformers, due to system operation/stability and cost.

Transformer faults are generally classified in to five categories:

Winding Faults Core faults Tank and transformer accessory faults On-load tap changer faults Abnormal operation conditions Sustained or uncleared external faults

WINDING FAULTS

A fault on transformer winding is controlled in magnitude by the following factor:

Source impedance Neutral earthing impedance Transformer leakage reactance Fault voltage Winding connection

Star-connected winding with neutral point earthed through an impedance

The fault current for the fault is dependent on the value of earthing impedance and is also proportional to the distance of the fault from neutral point as the voltage at the fault point will be directly proportional to this distance. The fault current in the primary winding depends on the transformation ratio between primary winding and short circuited turns, which varies with the position of fault in the winding.

Star connected winding with neutral point solidly earthed

In this case, fault current is controlled by the leakage reactance of the winding, which varies with the position of fault on the winding. As in the earlier case, the voltage available for fault current varies with the position of fault on the winding. It is seen that the reactance decreases very rapidly for the fault point approaching the neutral and hence the fault current is highest for a fault near the neutral end of the winding.

Delta connected winding

No part of a delta-connected winding operates with a voltage to earth of less than 50% of the phase voltage, and the impedance of a delta winding is particularly high to fault currents flowing to a centrally placed fault on one leg.

The earth fault current may be no more than the rated current, or even less than this value if the source or system earthing impedance is appreciable.

The current will flow to the fault from each side to the two half windings, and will be divided between two phases of the system. The individual phase currents may therefore be relatively low, resulting in difficulties in providing protection.

Phase-to-phase Fault

Phase-to-phase faults in the transformer are rare. If such a fault does occur, it will give rise to substantial current to operate instantaneous over current relay on the primary side as well as the differential relay.

Inter-turn Fault

Inter-turn fault occurs because of mechanical forces on the winding due to external short circuit. Transformer differential protection protects against short-circuits between turns of a winding and between windings that correspond to phase-to-phase or three phase type short-circuits.

If there is no earthing connection at the transformer location point, this protection can also be used to protect against earth faults. If the earth fault current is limited by an impedance, it is generally not possible to set the current threshold to a value less than the limiting current. The protection must be then carried out by a high impedance differential protection. Transformer differential protection operates very quickly, roughly 30 ms, which allows any transformer deterioration in the event of a short-circuit between windings to be avoided.

CORE FAULT

If any portion of the core insulation becomes defective or the laminated structure of the core is bridged by any conducting material, which can permit sufficient eddy current to flow, it will cause serious over-heating. The insulated core bolts are used for tightening the core. If the insulation of these bolts fails and provides easy path for eddy current, this will lead to over-heating. This additional core-loss, although it causes severe local heating, will not produce a noticeable change in the input current and cannot be detected by normal electrical protection. However, it is desirable to detect over-heating condition before a major fault has been created.

Excessive heating of core will break down transformer oil with evolution of gas. This gas rises to the conservator.

All faults in transformer core and winding result in the localised heating and breakdown of oil. When the fault is of very minor type such as hot joint, gas is released slowly and rises towards conservator. A major fault where severe arcing takes place causes rapid release of large volume of gas and oil vapour. This violent evolution of gas and oil vapour does not have time to escape and instead builds up pressure and bodily displaces the oil, causing surge of oil to pass up the relief pipe to the conservator.

TANK FAULTS

Loss of oil through tank leaks will ultimately produce a dangerous condition, either because of a reduction in winding insulation or because of overheating on load due to the loss of cooling..

Externally Applied ConditionsSources of abnormal stress in a transformer are

Overload System faults Over voltage Reduced system frequency

OVERLOAD

Overload causes increased ‘copper loss’ and a consequent temperature rise.

SYSTEM FAULTS

System short circuits produce a relatively intense rate of heating of the feeding transformers, the copper loss increasing in proportion to square of the per unit fault current.

OVER VOLTAGE

Transient surge voltages

Transient over-voltages arise from faults, switching and lightning disturbances and are liable to cause inter-turn faults.

Power frequency overvoltage

Power frequency overvoltage causes both an increase in stress on the insulation and a proportionate increase in the working flux, this lead to a rapid temperature rise in the bolts, destroying their insulation if the condition continues.

REDUCED SYSTEM FREQUENCY

Reduction of system frequency has an effect with regard to flux density, similar to that of overvoltage. If follows that a transformer can operate with some degree of overvoltage with a corresponding increase in frequency, but operation must not be continued with a high voltage input and low frequency. Operation cannot be sustained when the ratio of voltage to frequency with these quantities given values in per unit of their rated valued, exceeds unity by more than a small amount, for instance if V/f = 1.1.

MAGNETISING INRUSH CURRENT

The phenomenon of magnetising inrush is a transient condition that occurs primarily when a transformer is energized. It is not a fault condition, and therefore transformer protection must remain stable during the inrush transient.

Under normal steady-state conditions the magnetising current associated with the operation flux level is relative small. However, if a transformer winding is energized at a voltage zero, with no remanent flux, the flux level during the first voltage cycle (2* normal flux) will result in core saturation and a high non-sinusoidal magnetising current waveform.

The energizing conditions that result in an offset current produce a waveform that is asymmetrical. Such a wave typically contains both even and odd harmonics. Typical inrush currents contain substantial amounts of second and third harmonics and diminishing amounts of higher order. This current is referred to as magnetising inrush and may persist for several cycles.

STANDARD PROTECTIONS USED FOR TRANSFORMERS

BUCHHOLZ RELAY

Buchholz relay is a gas- actuated relay installed in oil-immersed

transformers for protection against all kind of faults. It is used to gives an

alarm in case of slow developing faults or incipient faults in the transformer

and to disconnect the transformer from the supply in the event of severe

internal faults. It is installed in the pipe between the conservator and main

tank as shown in fig11 below. This relay is used in oil-immersed

transformers of rating above 750 kVA.

CONSTRUCTION

It consists of a domed vessel placed in the pipe between the

conservator and main tank of the transformer. The device has two elements.

The upper element consists of a mercury type switch attached to a float.

The lower element contains a mercury switch mounted on a hinged type flap

located on the direct path of flow of oil from the transformer to the

conservator. The upper element closes an alarm circuit during slow

developing faults whereas the lower element is arranged to trip the circuit

breaker in case of severe internal faults.

OPERATION

The operation of buchholz relay is as follows:

1. In case of slow developing faults within the transformer, the heat due

to the fault causes decomposition of some transformer

oil in the main tank. The products of decomposition mainly

contain 70 % of hydrogen gas. The hydrogen gas being light tries to

go into the conservator and in the process gets trapped in the upper

part of the relay chamber. When a predetermined amount of gas gets

accumulated, it exerts sufficient pressure on the float to cause it to tilt

and close the contacts of mercury switch attached to it. This

completes the alarm circuit to sound an alarm.

Alarms are given for incipient faults such as Hot spots on the core due to short circuiting of lamination

insulation. Core bolt insulation failure. Faulty joints. Interturn faults on other winding faults. Loss of oil due to leakage

2. If serious fault occur in the transformer, an enormous amount of gas

is generated in the main tank. The oil in the main tank rushes towards

the conservator via the Buchholz relay and in doing so it tilts the flap

to close the contacts of mercury switch. This completes the trip circuit

to open the circuit breaker controlling the transformer.

Major faults which will cause isolation of the transformer include All severe winding faults, either to earth or inter phase. Loss of oil if allowed to continue to a dangerous degree.

ADVANTAGES:

1. It is the simplest form of transformer protection.

2. It detects the slow developing faults at a stage much earlier than

other forms of protection.

DISADVANTAGES:

1. It can only be used with oil immersed transformers equipped with

conservators.

2. The device can detect only faults below oil leveling the transformer.

Therefore separate protection is needed for connecting cables.

DIFFERENTIAL PROTECTION

Differential protection is a unit scheme that compares the current on the primary side of a transformer with that on the secondary side. Where a difference exists (other than that due to the voltage ratio) it is assumed that the transformer has developed a fault and the plant is automatically disconnected by tripping the relevant circuit breakers. The principle of operation is made possible by virtue of the fact that large transformers are very efficient and hence under normal operation power-in equals power-out. Differential protection detects faults on all of the plant and equipment within the protected zone, including inter-turn short circuits.

PRINCIPLE OF OPERATION

The operating principle employed by transformer differential protection is the Merz-Price circulating current system as shown below. Under normal conditions I1and I2 are equal and opposite such that the resultant current through the relay is zero. An internal fault produces an unbalance or 'spill' current that is detected by the relay, leading to operation.

Figure 26 DIFFERENTIAL PROTECTION

BASIC CONSIDERATIONS FOR TRANSFORMER DIFFERENTIAL PROTECTION

1. Line Current Transformer Primary Ratings

The rated currents on the primary and secondary sides of the transformer depend on the Line Current. This current will be an inverse ratio of the primary or secondary voltages. Consequently, the line current transformers should have primary ratings equal to or greater than the rated line currents of the power transformer to which they are applied. Standard primary ratings will usually limit the choice and availability of the Current Transformers that are used.

2. Current Transformer Connections

In certain configurations of Power Transformers, the CT's must be connected such that they compensate for phase differences on each side of the Power Transformer. For a WYE-DELTA connected power transformer, the phase shift is 30 degrees. So a CT must be able to compensate for the phase shift on both sides, such that the resultant current that is seen by the relay in of Equal Phase and Magnitude.

So, for a DELTA-WYE connected power transformer, in order for the phase to be to be the same at both the primary and secondary CT, the CT's must be connected as WYE-DELTA. By doing this, the resultant phase that is seen by the relay is effectively Zero. (Similarly, for a WYE-DELTA connected power transformer, the CT's must be connected as DELTA-WYE).

The zero sequence current flowing on the star side of the Transformer will not produce current outside the delta on the other side. Thus, the zero sequence is eliminated on the star side by connecting the CT's in a Delta connection, and that on the delta side is connected as star. Also, by connecting the CT's as shown, the phase shift of 30 degrees is removed.

Finally, for the CT's connected in delta, the secondary ratings must reduce to 1/(√3) times the secondary rating of the Star connected CT's. This is to balance the currents on the delta side with that of the star side of the power transformer.

3. Magnetising Inrush Conditions

Magnetising Inrush produces current input to the energised winding which has no equivalent on the other sides of the transformer. The Inrush appears as an imbalance, not distinguishable from a fault. To prevent this, several methods have been employed:

(i) TIME DELAY

This is a transient phenomena, and as a result, the stability of the transformer system may be maintained by providing a small time delay after switching on the transformer. The use of an instantaneous kick fuse divertsmost of the current. Under transient conditions, the fuse does not blow. Under faulted conditions, the fuse blows and thereby allows the relays to operate.

(ii) SECOND HARMONIC FILTER

Tests have shown that the magnetising inrush current has a high second harmonic content. However, this component does not exist in fault currents. As a result, the CT's must be sufficiently large such that the harmonics produced does not delay relay operation.

The filter uses a circuit which extracts the second harmonic current. This differential current is then applied to another circuit which applies a restraining quantity, sufficient to overcome the operating tendency due to the whole of the inrush current which flows in the operating circuit. In effect the second harmonic component is used to prevent the relay from operating under transformer energising conditions.

(iii) BUCHOLZ RELAY

This relay is used to protect Oil Immersed Transformers. The relay comprises two floats contained in an enclosed housing located in the pipe from the transformer tank to the conservators.Any fault in the transformer causes the oil to decompose, generating a gas which passes up the pipe towards the conservator tank, and thus trapped in the relay. In the case of a heavy fault, bulk displacement of the oil takes place. In a two float relay, the upper float responds to the slow accumulation of gas due to mild incipient faults. The lower relay is deflected by oil surge caused by major faults. These floats control contacts, which in the first case generates an alarm, and in the second case, causes isolation of the transformer.

Alarms are given for incipient faults such as Hot spots on the core due to short circuiting of lamination

insulation. Core bolt insulation failure. Faulty joints. Inter-turn faults on other winding faults. Loss of oil due to leakage

Major faults which will cause isolation of the transformer include All severe winding faults, either to earth or inter phase. Loss of oil if allowed to continue to a dangerous degree.

Restricted Earth Fault Relay

Figure 27 RESTRICTED EARTH FAULT PROTECTION

Windings of many smaller transformers are protected by restricted earth fault (REF) systems. Even so, I have come across a lot of installations which would have benefited from this type of protection, but for whatever reason it has not bee installed.

The illustration shows the principal of REF protection.  Under normal conditions and by  application of Kirchhoff’s laws the sum of currents in both current transformers (CTs) equals  zero.  If there is an earth fault between the CTs then some current will bypass the CT's and the  sum of currents will not be zero.  By measuring this current imbalance faults between the CTs can be easily identified and quickly cleared.  Fault detection is confined to the zone between the two CTs hence the name 'Restricted Earth Fault'.

REF protection is fast and can isolate winding faults extremely quickly, thereby limiting damage and consequent repair costs.  If CTs are located on the transformer terminals only the winding is protected.  However, quite often the line CT is placed in the distribution switchboard, thereby extending the protection zone to include the main cable.

Without REF, faults in the transformer star secondary winding need to be detected on the primary of the transformer by reflected current.  As the winding fault position moves towards the neutral, the magnitude of the current seen on the primary rapidly decreases and could potentially not be detected (limiting the amount of winding which can be protected).  As the magnitude of the currents remain relatively large on the secondary (particularly if solidly earthed), nearly the entire winding can be protected using REF.

It should be remembered that the protection as illustrated covers only the secondary of the  transformer.  Sometimes REF protection is added to the primary as well (although if primary  protection is required I would prefer to consider full differential protection).

As it is essential that the current in the CTs be balanced during normal conditions (and through faults), historically REF has been implemented using High Impedance Relays.  CT's have also been specified as matched pairs and the impedance of leads/wires and interconnecting cables has had a large influence on the functioning of the relay.  Measurement errors associated with these issues have been responsible for nuisance tripping and the system could be difficult to commission.  This may be the reason some people avoid the use of REF.   Recent advances in numerical relay technology have all but eliminated these issues, making the implementation of REF relatively easy, ensuring no nuisance tripping and simplifying commissioning.

HARMONIC-CURRENT RESTRAINT

Figure 28 HARMONIC CURRENT RESTRAINT

The principle of harmonic-current restraint makes a differential relay self-desensitizing during the magnetizing-current-inrush period, but the relay is not desensitized if a short circuit should occur in the transformer during the magnetizing-inrush period. This relay is able to distinguish the difference between magnetizing-inrush current and short-circuit current by the difference in wave shape. Magnetizing-inrush current is characterized by large harmonic components that are not noticeably present in short-circuit current.

The above shows how the relay is arranged to take advantage of the harmonic content ofthe current wave to be selective between faults and magnetizing inrush. The restraining coil will receive from the through-current transformer the rectified sum of the fundamental and harmonic components. The operating coil will receive from the differential-current transformer only the fundamental component of the differential current, the harmonics being separated, rectified, and fed back into the restraining coil.

The direct-current component, present in both magnetizing-inrush and offset fault current, is largely blocked by the differential-current and the through-current transformers, and produces only a slight momentary restraining effect.

TRANSFORMER OVER CURRENT PROTECTION

Over-current relays are also used on larger transformers provided withstand circuit breaker control. The time delay characteristic should be chosen to discriminate with circuit protection on the secondary side.

WINDING HOT-SPOT TEMPERATURE PROTECTION

The transformer winding hot-spot temperature is another quantity that should be used for protection of transformers. Protection based on winding hot-spot temperature can potentially prevent short circuits and catastrophic transformer failure, as excessive winding hot-spot temperatures cause degradation and eventual failure of the winding insulation. The ambient temperature, transformer loading, and transformer design determine the winding temperature. Temperature based protection functions alarm or trip when certain temperature conditions are met.

OVERFLUX PROTECTION

Transformer overfluxing can be a result of

Overvoltage

Low system frequency

Geomagnetic disturbances

A transformer is designed to operate at or below a maximum magnetic flux density in the transformer core. Above this design limit the eddy currents in the core and nearby conductive components cause overheating which within a very short time may cause severe damage. The magnetic flux in the core is proportional to the voltage applied to the winding divided by the impedance of the winding. The flux in the core increases with either increasing voltage or decreasing frequency. During startup or shutdown of generator-connected transformers, or following a load rejection, the transformer may experience an excessive ratio of volts to hertz, that is,

become overexcited. When a transformer core is over-excited, the core is operating in a non-linear magnetic region, and creates harmonic components in the exciting current. A significant amount of current at the 5th harmonic is characteristic of over-excitation.

Since momentary system disturbances can cause transient over-fluxing that is not dangerous time delay tripping is required. The protection is initiated if a defined V/f threshold is exceeded.

Geomagnetic disturbance may result in over-fluxing without the V/f threshold being exceeded. Some relays provide a 5th harmonic detection feature, which can be used to detect such a condition, as levels of this harmonic rise under over-fluxing conditions.

BUSBAR PROTECTION

INTRODUCTION

In electrical power distribution, a busbar is a thick strip of copper or aluminium that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus. Busbars are used to carry very large currents, or to distribute current to multiple devices within switchgear or equipment.

Busbars are typically either flat strips or hollow tubes as these shapes allow heat to dissipate more efficiently due to their high surface area to cross-sectional area ratio. The skin effect makes 50–60 Hz AC busbars more than about 8 mm (1/3 in) thick inefficient, so hollow or flat shapes are prevalent in higher current applications. A hollow section has higher stiffness than a solid rod of equivalent current-carrying capacity, which allows a greater span between busbar supports in outdoor switchyards.

A busbar may either be supported on insulators, or else insulation may completely surround it. Busbars are protected from accidental contact either by a metal enclosure or by elevation out of normal reach. Neutral busbars may also be insulated. Earth busbars are typically bolted directly onto any metal chassis of their enclosure. Busbars may be enclosed in a metal housing, in the form of bus duct or busway, segregated-phase bus, or isolated-phase bus.

Busbars may be connected to each other and to electrical apparatus by bolted or clamp connections. Often joints between high-current bus sections have matching surfaces that are silver-plated to reduce the contact resistance. At extra-high voltages (more than 300 kV) in outdoor buses, corona around the connections becomes a source of radio-frequency interference and power loss, so connection fittings designed for these voltages are used.

The busbar needs special attention because of the following reasons.

Fault level at busbars is very high. The fault on the busbar would result in widespread supply

interruption. The system stability is adversely affected by fault in bus zone.

BUSBAR ARRANGEMENTS

There are several types of busbar arrangements. The choice of a particular arrangement depends upon various factors such as system voltage, position of substation in the system, reliability of supply, flexibility and cost.

The different types of busbar arrangements are illustrated as follows.

1. Single Busbar2. Double Busbar3. Double Busbar with U-connection4. Composite double bus/bypass bus5. Double busbars with draw-out circuit breaker6. Two-breaker method with draw-out circuit-breakers7. Double busbars with bypass busbar8. Triple (multiple) busbars9. Double busbars with shunt disconnector10. Two-breaker method with fixed switchgear11. One and a Half circuit breaker12. Cross-tie method13. Ring busbars

SINGLE BUSBAR

Suitable for smaller installations. A sectionalizer allows the station to be split into two separate parts and the parts to be disconnected for maintenance purposes.

DOUBLE BUSBAR

Preferred for larger installations. Advantages: cleaning and maintenance without interrupting supply. Separate operation of station sections possible from bus I and bus II. Busbar sectionalizing increases operational flexibility.

DOUBLE BUSBAR WITH U-CONNECTION

Low-cost, space-saving arrangement for installations with double busbars and branches to both sides.

COMPOSITE DOUBLE BUS/BYPASS BUS

This arrangement can be adapted to operational requirements. The station can be operated with a double bus, or with a single bus plus bypass bus.

DOUBLE BUSBARS WITH DRAW-OUT CIRCUIT BREAKER

In medium-voltage stations, draw-out breakers reduce downtime when servicing the switchgear; also, a feeder isolator is eliminated

TWO-BREAKER METHOD WITH DRAW-OUT CIRCUIT-BREAKERS

Draw-out circuit-breakers result in economical medium-voltage stations. There are no busbar isolators or feeder isolators. For station

operation, the draw-out breaker can be inserted in a cubicle for either bus I or bus II.

DOUBLE BUSBARS WITH BYPASS BUSBAR

The bypass bus is an additional busbar connected via the bypass branch. Advantage: each branch of the installation can be isolated for maintenance without interrupting supply.

TRIPLE (MULTIPLE) BUSBARS

For vital installations feeding electrically separate networks or if rapid sectionalizing is required in the event of a fault to limit the short-circuit power. This layout is frequently provided with a bypass bus.

DOUBLE BUSBARS WITH SHUNT DISCONNECTOR

Shunt disconnector “U” can disconnect each branch without supply interruption. In shunt operation, the tie breaker acts as the branch circuit-breaker.

TWO-BREAKER METHOD WITH FIXED SWITCHGEAR

Circuit-breaker, branch disconnector and instrument transformers are duplicated in each branch. Busbar interchange and isolation of one bus is possible, one branch breaker can be taken out for maintenance at any time without interrupting operation.

ONE AND A HALF CIRCUIT BREAKER

Fewer circuit-breakers are needed for the same flexibility as above. Isolation without interruption. All breakers are normally closed. Uninterrupted supply is thus maintained even if one busbar fails. The branches can be through-connected by means of linking breaker V.

CROSS-TIE METHOD

With cross-tie disconnector “DT”, the power of line A can be switched to branch A1, bypassing the busbar. The busbars are then accessible for maintenance.

RING BUSBARS

Each branch requires only one circuit breaker, and yet each breaker can be isolated without interrupting the power supply in the outgoing feeders. The ring busbar layout is often used as the first stage of One and a Half -breaker configurations.

BUSBAR PROTECTION

Busbars and lines are important elements of electric power system and require the immediate attention of protection engineers for safeguards against the possible faults occurring on them. The methods used for the

protection of generators and transformers can also be employed, with slight modifications, for the busbars and lines. The modifications are necessary to cope with the protection problems arising out of greater length of lines and a large number of circuits connected to a busbar. Although differential protection can be used, it becomes too expensive for longer lines due to the greater length of pilot wires required. Fortunately, less expensive methods are available which are reasonably effective in providing protection for the busbars and lines. In this chapter, we shall focus our attention on the various methods of protection of busbars and lines.

Busbars in the generating stations and sub-stations form important link between the incoming and outgoing circuits. If a fault occurs on a busbar, considerable damage and disruption of supply will occur unless some form of quick-acting automatic protection is provided to isolate the faulty busbar. The busbar zone, for the purpose of protection, includes not only the busbars themselves but also the isolating switches, circuit breakers and the associated connections. In the event of fault on any section of the busbar, all the circuit equipment's connected to that section must be tripped out to give complete isolation.

The standard of construction for busbars has been very high, with the result that bus faults are extremely rare. However, the possibility of damage and service interruption from even a rare bus fault is so great that more attention is now given to this form of protection. Improved relaying methods have been developed, reducing the possibility of incorrect operation. The two most commonly used schemes for busbar protection are:

(1) Differential protection(2) Fault bus protection.

DIFFERENTIAL PROTECTION

The basic method for busbar protection is the differential scheme in which currents entering and leaving the bus are totalised. During normal load condition, the sum of these currents is equal to zero. When a fault occurs, the fault current upsets the balance and produces a differential current to operate a relay.

Figure 29 Current differential scheme for a station busbar

The above figure shows the single line diagram of current differential scheme for a station busbar. A generator and supplies load to two lines feeds the busbar. The secondaries of current transformers in the generator lead, in line 1 and in line 2 are all connected in parallel. The protective relay is connected across this parallel connection. All CTs must be of the same ratio in the scheme regardless of the capacities of the various circuits. Under normal load conditions or external fault conditions, the sum of the currents entering the bus is equal to those leaving it and no current flows through the relay. If a fault occurs within the protected zone, the currents entering the bus will no longer be equal to those leaving it. The difference of these currents will flow through the relay and cause the opening of the generator, circuit breaker and each of the line circuit breakers.

FAULT BUS PROTECTION

It is possible to design a station so that the faults that develop are mostly earth-faults. This can be achieved by providing earthed metal barrier (known as fault bus, surrounding each conductor throughout its entire length in the bus structure. With this arrangement, every fault that might occur must involve a connection between a conductor and an earthed metal part. By directing the flow of earth-fault current, it is possible to detect the faults and determine their location. This type of protection is known as fault bus protection.

Figure 30 Schematic arrangement for fault bus protection

The above figure shows the schematic arrangement of fault bus protection. The metal supporting structure or fault bus is earthed through a

current transformer. A relay is connected across the secondary of this CT. Under normal operating conditions, there is no current flow from fault bus to ground and the relay remains inoperative. A fault involving a connection between a conductor and earthed supporting structure will result in current flow to ground through the fault bus, causing the relay to operate. The operation of relay will trip all breakers connecting equipment to the bus.

LIGHTNING PROTECTION

INTRODUCTION

Damage caused by lightning strikes cannot be completely prevented either technically or economically. For this reason, lightning protection facilities cannot be specified as obligatory. The probability of direct lightning strikes can be greatly reduced on the basis of model experiments, measurements and years of observation with the methods described below.

A distinction is made between external and internal lightning protection.

External lightning protection is all devices provided and installed outside and in the protected installation provided to intercept and divert the lightning strike to the earthing system.

Internal lightning protection is total of the measures taken to counteract the effects of lightning strike and its electrical and magnetic fields on metal installations and electrical systems in the area of the structure.

OVERHEAD EARTH WIRES

The protected zone should enclose all equipment and also the transformers.The sectional plane of the protected zone is bounded by an arc along an overhead earth wire, whose midpoint M is equal to twice the height H of the earth wire both from ground level and from the overhead earth wire B. The arc touches the ground at a distance √3 H from the footing point of the overhead earth wire.

Figure 31 sectional plane of the protected zone with one overhead earth wire

The sectional plane of the protected zone for two overhead earth wires, whose distance from each other is C≤2H. The outer boundary lines are the same as with an overhead earth wire. The sectional plane of the protected zone between the two overhead earth wires B is bounded by an arc whose midpoint M1 is equal to twice the height 2H of the earth wire from ground level and is in the middle of the two overhead earth wires. The radius R is the distance between the overhead earth wire B and the midpoint M1.

Figure 32 sectional plane of the protected zone with two overhead earth wires

The angle between the tangents to the two bounding lines is 2 X 30° at their point of

intersection. If an angle of around 2 X 20° is required in extreme cases, the distance1.5H must be selected instead of the distance 2H.

Figure 33 arrangement of the overhead earth wires and protected zone of an outdoor

LIGHTNING RODS

Experience and observation have shown that the protected zone formed by rods is larger than that formed by wires at the same height.

A lightning rod forms a roughly conical protected zone, which is bounded by the arc whose midpoint M is three times the height H of the rod both from ground level and the tip of the lightning rod. This arc touches the ground at distance √5 · H from the footing point of the lightning rod.

Figure 34 sectional plane of the protected zone with one lightning rod

The area between two lightning rods whose distance from each other is ≤ 3H forms another protected zone, which is bounded by an arc with

radius R and midpoint M1 at 3 · H, beginning at the tips of the lightning rods.

Figure 35 sectional plane of the protected zone with two lightning rods