A Government of India enterprise)

PROJECT REPORT ONELECTRICAL ENGG(TRAIING) WITH RESPECT TO N.H.P.C

Submitted to CHANDIGARH University, Chandigarhin partial fulfillment for the Award of the degree of

B.TECH(EEE) (BECHLOR Of TECHNOLOGY)

Reporting to: Mr. Jamil Akhtar ,

Pervesh Thakur,Dm(e&c By:

Ajay Kumar Rana(358), Aja Btech EEE

CGC GHARUAN MOHALI,

NHPC - AN OVERVIEW

•Set up in 1975 with an authorized capital of Rs. 200 Crores.•Single largest organization for hydro power development in India in the central sector •Capabilities to undertake all the activities from conceptualization to commissioning in relation to setting up of hydro power projects •Schedule ‘A’ Enterprise w.e.f. 15.5.1990.•NHPC has been granted ISO 9001 Certificate for its quality system.•Installed capacity : 2475 MW•Projects constructed on Turnkey/deposit basis : 3•Projects under O&M : 7•Projects under construction : 11•Projects under investigation : 20•Projects under joint ventures : 4•Projects under construction•On turnkey/deposit basis : 2

SALAL AN OVERVIEWState : Jammu & KashmirDistrict : ReasiDam Site : Dhyangarh in Reasi River : Chenab Capacity : 6 X 115 MWAnnual Generation Target : 3100 MURated Head : 94.5 mTotal Reservoir Capacity : 285 million Cumecs Rockfill Dam : 1Concrete Dam : 1Penstocks : 6 Tail Race Tunnels : 2Tariff Rate : 41.56 paise/unit

TECHNICAL FEATURES

•118 m high, 630m long rockfill dam.•113 m high, 450 m long concrete dam•11 m dia, 2.46 km long tail race tunnel.•6 nos. Penstocks 5.23 m dia, 279 m long each.•Sub-surface power house containing 6 units of 115 MW each.

Salal Hydroelectric Power Station Stage-I of 345 MW (115X3) and Stage-II of 345 MW (115X3) is constructed on river Chenab in the state of Jammu & Kashmir. The project was conceived in 1920. The feasibility studies on the project commenced in 1961 by the state government of J&K and construction was started in 1970 by Central Hydroelectric Project Control Board under Ministry of Irrigation and Power. In 1978 construction of the projects was entrusted to NHPC on agency basis. After completion of project, it was handed over to NHPC on ownership basis for Operation and Maintenance. Stage-I of the project was commissioned in 1987. First unit of Stage-II of the project was commissioned in 1993, second in 1994 and third in 1995. At the peak of construction, about 12,000 personnel including contractors worked around the clock to complete the project from 1970s to present day, under proper guidance and maintenance of India's most successful company NHPC (NATIONAL HYDROELECTRIC POWER CORPORATION. The project was taken by HCC and various contractors.

GENERATOR There are 3 Synchronous generators in stage-1 and 3 in stage -2.The synchronous generators are the alternating generators whose rotor rotate at a a speed called synchronous speed. They run at this speed or not at all.They convert mechanical power into electrical power. Synchronous generators are usually of 3-phase because of several advantages of three phase generation, transmission and distribution. It is also called alternator. It mainly consists of two parts, a stator and a rotor. 1.GENERAL•Manufacturer : BHEL•Place of manufacture : Haridwar•Type of design : Vertical suspension•Applicable Standards : IS:4772-19682.Main data•No. of phases : 3•Rated P.F. : 0.9 lag•Generator continuous rating at : 127.8 MVARated frequency and voltage•Generated rated voltage : 11 KV•Rated Frequency : 50 Hz +5%•No. of Poles : 32•Rated Synchronous speed : 187.5 rpm•Design runaway speed : 375 rpm•Tolerance time of runaway speed : 2 min.•Class of insulation for stator winding : F class•Class of insulation for rotor winding : F class

3.Efficiency•At 100% continuous rated output (MVA) : 98%•At 75% continuous rated output (MVA) : 97.7%•At 50% continuous rated output (MVA) : 97%4.Temperature•Stator Winding : 60 C•Rotor Winding : 75 C•Max. temp at stator : 150 C•Max temp at rotor : 120 C 5.Excitation Conditions•Field Current : 1365 A•Field Voltage : 243 V•Max. permissible continuous field : 1600 A current•Main exciter rated capacity : 1500 A•Main exciter rated voltage : 243 V•Main exciter ceiling voltage ratio : 1.86.Stator•Material : high quality si steel•Insulation of laminations : Varnished•Stator frame construction : fabricated

•No. of sectors : 4•No. of slot : 456•No. of pole : 32•No. of slots per phase : 4.75•Air gap : 25 mm•No. of line terminals : 6•No. of neutral terminals : 67.Generator Rotor•Constructor of field poles : laminated•Method of pole fixing : T-Heads•Rotor rim Construction : punched steel segments•Rotor spider construction : Fabricated •Damper winding construction : cu, short circuited cage type•OD of rotor : 7650 mm•Type of collector ring brushes : carbon•No. of slip rings : 28.Generator Shaft•Material of the shaft : Steel forging•OD : 1000mm•Length : 7.9 m•Dia of coupling flange : 1570 mm•No. of coupling holes : 20

Transformer

1.General Stage I and Stage II•Manufacturer CG and BHEL•Design OFWF•Applicable Standards IS2026

4.Dimensions

Overall length 5.5m

Overall width 5.3m

Overall height 7.5m

2.Main data•No. of phases : 1•Rated power : 43.33 MVA•Rated Frequency : 50 Hz•Rated Voltage(No load) HV side : 220/sqrt 3 KV LV side : 11 KV•Rated Line Current HV side : 340.7 A LV side : 3939.1 A•Temperature Rise (oil) : 55 C•Temperature rise (winding) : 65 C •HV winding : star•LV winding : delta3.Losses•Load losses at rated power, : 45 KW frequency and at principal tapping, referred to winding •Core loss : 180 KW•Total Loss : 185 KW

6.HV neutral bushing

Type Oil communicating

Current rating

630 A

 

Voltage

36 Kv

7. bushings/bus duct termination

Current rating3150 A

Voltage rating17.5 KV

5.Transformer bushingsManufacturer S&S Bushing ltd.

TypeOil filled condenser

Current rating800 A

Voltage rating245 Kv

2.Main dataGovernor operating oil pressure minimum 31Kg/cm2 maximum 40Kg/cm2

Rated 40Kg/cm2

Minimum wicket gate setting

Closing 10 sec

Opening 11 sec

Adjustmnt range in Governer opening and closing times3 - 30 sec.

Sensitivity of governor to respond to minimumspeed change of rated

speed 0.02%

Governing system1.GeneralManufacturer •Control Unit(Electro) : BHEL•Hydraulic Unit(Hydraulic) : BHEL

Turbines

The turbine used at salal power plant was the Francis turbine.Francis turbines are most widely used among water turbines and the development of the Francis turbines in the last decade has opened up a large range of new application possibilities for this type. These advances, motivated by a search for maximum profitability, have become possible as the result of improved knowledge of the water flows in turbines and other hydraulic phenomena.A complete investigation and intensive research are carried out and efforts are put forth in the improvement of turbine performance, the selection of suitable materials, and the construction design in consideration of difficulties imposed by mechanical, manufacturing, and maintenance factors at the design stage.TOSHIBA is always making best efforts to design and manufacture highly efficient runners to meet all requirements or Specifications. The runner is designed in consideration of various parameters for computation by both theoretical analysis of internal flow and experimental investigation by model tests. The runner is usually made of carbon steel castings and overlay coating of stainless steel welding will be made on critical areas of cavitations if necessary. For higher head machines, the runner is made of stainless steel castings. Especially 13%a Chrome steel with enriched Nickel content becomes widely used for its excellent anti-cavitations corrosion characteristics and mechanical strength.

The hydraulic research of the water passage through the spiral

case to stay vanes becomes very important in diminishing the losses of the flow and the angle and the shape of stay vane cascades are carefully designed. A new type of construction (parallel type) advantageous in structural design is applied to all the stay rings. The spiral case is made of steel plates for welded structures or high tensile strength steel plates.TOSHIBA has used 60 or 80 kg/mm' high tensile strength steel plates for spiral case shell plates of a big turbine or a high-head strength turbine. From the view point of field welding, the spiral case shells are accurately cut out by automatic gas-cutting machines, contributing to perfect field assembly even without a temporary shop assembly check.

MAIN SHAFT

The main shaft for the turbine is made of high grade forged carbon steel. The main shaft is connected to the generator shaft or the intermediate shaft by a flange coupling. The shaft surface passing through the shaft seal is protected with a stainless steel shaft sleeve to prevent the main shaft from wearing. DRAFT TUBE LINER

The most commonly employed draft tube is of elbow type, in a concrete structure, fixed with anchoring materials. Overall configuration of the draft tube is thoroughly checked at Toshiba Research Laboratory to ensure effective use of the head energy. The draft tube liner, of welded construction, is made of steel plate for general structure. If necessary due to theoretical and practical reasons, a special air admission system is provided with a draft tube liner to reduce water-pressure pulsation in the draft tube.

SPECIFICATION OF TURBINE AT SALAL POWER STATION•Type Vertical Shaft Francis •Rated Speed : 187.5 RPM•Rated Net Head : 94.5 m•Minimum Head : 94.5 m•Maximum Net Head : 102 m•Rated Discharge : 137 Cumecs•Turbine Rated Output At rated head and rated discharge : 117.5 MWMaximum Output of turbine at

Head > Maximum Head 123.5 MWOptimum Efficiency of Turbine at90% Part Load 92.5%Maximum Runaway Speed 375 RPMSpecific Speed 229.2 m/KWDirection of Rotation from Top Clock wiseMaximum Axial Hydraulic Thrust 382 TonnesFlywheel Effect of turbine Rotating Parts 13000 Tonnes-m2

Total Thrust from turbine Carried by Generator 382 TonnesMass of Turbine rotating parts 52.4 TonnesMaximum permissible pressure rise 139 m (Max. water hammer)Speed rise at 100% load throw off 55%Pressure rise at 100% load throw off 35%

220KV OUT DOOR SWITCHYARD:Equipment in the switchyard •Circuit breakers •Isolators•Current transformer•Potential transformer•Capacitive voltage transformer • Lightening arrestor•Bus Bar & conductor•Earth switch•Wave trap•Insulators (post & Suspension)•Control & power cables•Clamps •Fire fighting equipments

SINGLE LINE DIAGRAM OF 220 KV SWITCHYARD OF SALAL POWER STATION

Circuit Breaker

Isolator

C.T.

P.T.

L.A.

Capacitive Voltage Transformer

Earthing

UNIT#3UNIT#1 UNIT#2

BUS-A

BUS-B

UNIT#5UNIT#4 UNIT#6

BUS-A

BUS-A P.T.

BUS-B P.T.

K-2 J-2K-1 K-3 K-4 J-1

Wave Trap

1.Circuit breakersIn this switchyard mainly two types of circuit breakers were used, one is SF6 and another one is MOCB. SF6 breakers in Unit# 4,5,6 and outgoing feeders Kishanpur # 2, Jammu #1,2. The MOCB is used in Unit # 1,2,3, Bus coupler and Kishanpur # 1,3,4 feedersSF6 CIRCUIT BREAKERSF6 GAS: gas in pure state is inert, exhibits exceptional, thermal stability and has excellent arc quenching property as well as exceptional high insulating properties. It is one of most stable compounds, inert, non-flammable, and non-toxic and odorless. The density of gas is about 5 times that of air and heat dissipation in it is also much more than in air. At atmospheric pressure the dielectric strength is about 2.4 times that of air and at 3Kg/cm2 it is same as that of oil.PHYSICAL PROPERTIES OF SF6 GAS Molecular weight : 146.07Melting point : - 50.7Dielectric constant : 1.002 (at 760mm of Hg pressure)CONSTRUCTION:This is single-phase auto reclosing type circuit breaker. The three phases have their own mechanism and air reservoir inter connected electrically and pneumatically. Single pole of the breaker consisting of interrupting unit, supporting unit and mechanism housing. INTERRUPTING UNIT: The interrupter is a single-phase puffer type, which consists of puffer cylinder and piston, stationary contact and its assembly. Over conductors are connected to the upper and lower terminal pads.

OPENING OPERATION: opening is affected by pulling down the insulating rod, piston rod, puffer cylinder, moving contact, moving arc contact and the nozzle. After some contact wiping the stationary and moving arc contacts separate there by generating arc. During down ward movement the gas pressure in puffer cylinder builds up and high-pressure gas flows through nozzle and quenches the arcCLOSING OPERATION: in closing operation insulating rod is pushed up and all the parts move in reverse order of opening operation. Also sf6 is gas taken in to the puffer cylinder.SUPPORT UNIT ASSEMBLY: It consists of support porcelain and insulating pull rod, this provides insulation between live parts and ground SF6 gas filled in containers in interrupting and supporting unit at rated pressure.AIR SYSTEM: Compressed air is required for opening operation of the breaker. It is stored in inter connected chamber 70 Lt each which act as local source. This system is provided with motor compressor unit. The capacity of compressor is 90 L/M. This governor switch compressor system maintains the pressure between 14.5 to 15.5 Kg/cm2

PNEUMATIC OPERATING MECHANISM: The pneumatic operating mechanism consists of opening piston, air cylinder, control valve, tripping coil& closing coil, latch, trigger, closing spring, shock absorber and others.OPENING OPERATION: The opening operation is affected by compressor air, when opening signal energies trip coil, latch and cam are released by trip coil plunger and control valve is open by spring which allows the compressed air, to push down the piston of the pneumatic operating mechanism. The movement of opening piston is transmitted to the puffer cylinder through livers and rods and moving contacts of all all three poles are pulled down together. The movement of opening piston

compresses the closing spring. At the final position of the opening operation the control valve is closed by the linkage and the air in the cylinder is exhausted through the control valve to the out side atmosphere. The piston is held in opening position against forcing of the closing spring by holding latch.CLOSING OPERATION: Closing operation made by the closing spring. When closing signal energies the closing coil trigger and holding latch released by the closing coil plunger, and closing spring closes the breaker.SPECIFICATIONSRated voltage : 245 KVRated insulation level : 1050 KV (peak)Rated frequency : 50 HZRated normal current : 3150 ARated short circuit breaking current : 40 KARated short circuit making current : 100 KARated break time : 60 ms (3 cycles)Rated short time current : 40 KA for 3 secOperating mechanism : spring closing, pneumatic trippingOperating pressure(air) :15 kg/cm2 SF6 gas pressure :6kg/cm 2

Opening time : 30ms Closing time : 100msPole discrepancy : 3.3msMake : CGL

SF6 Circuit Breaker

MOCB (MINIMUM OIL CIRCUIT BREAKER)220KV MOCB having two breaking units per pole, normally this can vary between one and four, depending on the, rated voltage, insulation level and short circuit current breaking capacity. The double breaking unit per pole is connected together to form V shaped breaking unit. The breaking units are mounted on post insulator, which constitute the insulation of the pole to earth. Each [post insulator supports one double breaking unit. The post insulator erected on a low concrete pedestal. The breaking unit consists of an oil container with an arc control device of cross blast type. The oil container consists of a porcelain tube reinforced with a glass fiber tube on the inside, fixed contact mechanism housing cast in light metal and gas tight top cap. The breaking unit is hermetically sealed and pressurized with N2 gas. The top cap is, therefore fitted with a pressure control valve. The circuit breaker is operated with a motor operated spring – closing device of the type BLG. The operating device is connected with mechanism of the breaking units via a pull rod system, link gears and operating insulators. The circuit is closed by operating device, which contains the closing spring. When closing, the opening springs, which are tightly joined, to the outer end of the rod system are charged. The opening catch in the operating device keeps the CB in the closed position for opening all that is required is to release the opening latch. Each breaking unit contains 53 lt.

MOCB SPECICATIONS:Type : HLR 245/2502 E Rated voltage :245 KvImpulse with stand voltage : 1050 KvRated normal current : 2500ARated short time current : 40 KABreaking capacity : 40 KARated making current :100KA Gas pressure : 0.3 to 0.7 Mpa Make : BHEL

MOCB Circuit Breaker

2.Isolators•Make : AMEI/WSI/BIMCO•Type : Triple pole single circuit Breaker•Standard : IS9921•Rated Continuous Current : 1600 A•Rated voltage : 245 KV•No of operations possible : 2500•Rated short time current(1 sec) : 40 kA•Clearance (Live and Ground) : 2300 mm•Clearance(phase to phase) : 4500 mm•Clearance(fixed contact and blades) : 2700 mm3.Current transformerCurrent transformers are instrument transformers to measure current in high circuit and for protection purpose. The current transformer connected always in series with line. The CT s installed in switchyard is oil filled hairpin type of 220kv. The hair pin arises out of fact that the HV primary winding is of that shape, comprising a tube having graded insulation of oil impregnated in which metal foils are inter posed at sutiblr intervals. The outer most of these coils is connected to earth via an insulated terminal and link enabling dielectric loss angle measurements be taken to check up the quality of insulation. This foil further acts as earth shield between primary and secondary winding. The secondary winding with own cores are mounted over the legs of hairpin. In this type of design up to 4 secondary cores can be accommodated, an alternative design allowing accomadation forf ive secondaries.

Each unit is completely oil filled and sealed to ensure that the oil cannot come in to contact the atmosphere and hermetically sealed with N2 gas at 0.35 kg/cm2 The flexible oil reservoir in the top housing above the high voltage terminal chamber ensure this by preventing the creation of undue pressure or depressions.Unit side CTs having 4 core and outgoing feeder CTs having 5 core. Unit CTs core 1 is for link line protection, core 2 is for LBB protection, core 3 is for Bus differential main zone protection and core 4 is for metering, Bus differential check zone protection. Ratio is 800/600/400/: 1. The feeder side CTs core 1 is for main 1 distance protection and fault locator, core 2 is for main 2 distance protection and disturbance recorder, core 3 is for Bus bar main zone protection, core 4 is for Bus bar check zone protection and core 5 is for LBB protection and metering. CTs ratio is 800: 1, make BHEL and CGL.

Current transformer4.VOLTAGE TRANSFORMERVoltage transformer is an instrument transformer for high voltage measurement and protection purpose. In this switch yard, two types of voltage transformers are used. For Bus Bar potential transformer and for out going feeders capacitive voltage transformer. The Bus bar voltage transformer having one primary and two secondary winding. The primary winding is connected across phase and earth through neutral link. The secondary winding is for measurement and another one is for disturbance recorder. The secondary winding + ve terminal connected through 15a fuse and – ve earthed through neutral link. The PT is oil filled with 300 lt.

SPECIFICATION: Insulation level : 460/1050V Primary volts : 127KV Sec-wdg 1 : 63.5V Burden : 500VA Sec-wdg 2 : 63.5V Burden :50 VA Class : 5.0 Make : BHEL5.CAPACITIVE VOLTAGE TRANSFORMER:When the higher voltage of 110KV and above the electro magnetic design is a costly item, and an economical is found in the capacitive voltage transformer. This comprises a capacitor divider unit in which capacitors are connected in series between line earths. A taping is made at convenient voltage (inter mediate voltage), which is usually at 20/√3 or 12 KV. The secondary burden is connected to this taping via a reactor and inter mediate transformer, the later being is electro magnetic type. The reactor and transformer combined in one unit and oil filled, hermetically sealed. The CVT is using as a filter circuit for PLCC (Power line carrier communication). The inductive reactance of combination is such as to balance the capacitive reactance of the coupler at rated frequency i.e. there is a resonating circuit. The primary and inter mediate voltage capacitors being housed in the ceramic insulator and sealed to prevent air and moisture coming in to contact with the insulation oil.

The HF terminal of CVT is for to send or receive high frequency communication signal. the CVT and Wave trap and line impedeance combined form as a ∏ filter for PLCC. SPECIFICATIONS: Primary capacitance :4840 PF Secondary capacitance : 48400 PF HF capacitance :4400PF

Rated burden :100 VA LIGHTENIG ARRESTER OR SURGE ARRESTEROver voltages, which appear on electrical systems, are internal and external over voltages. The internal over voltages are due to operation of circuit breaker when switching inductive or capacitive loads, sudden earthing of one phase of system, disconnection of generator which over speeds, sudden load removing etc. these over voltages rarely exceeds 3to 5 times of normal voltage. External over voltages are caused by atmospheric discharges. These voltages are cause considerable stress on insulation. The need to avoid breakdown of primary insulation due to over voltages on the system, requires the CB, Transformer,post and tension insulators and other apparatus have a with stand voltage level greater than the protective levels of devices provided to give over voltage protection, e.g. surge diverters. Diverters which provide these feature comprises one or more air gaps mounted in series with non liner resistors, the function of air gap being to keep the circuit through the arrestor to earth open under all normal power frequency to flash over and close the circuit when abnormal voltages appear at the diverter terminal. In this

switchyard we used Zinc oxide non-linear gap less arrestor. Elpro make arresters having 3 blocks each 66Kv, W.S. make having 4 arrester with 49.5 Kv voltage. SPECIFICATIONS:Make : ELPRO INSULATORSRated discharge current : 10 KARating voltage : 66Kv Make :W.S. INSULATORSRated discharge current :10 KARated voltage :49.5 Kv

BUS BAR AND CONDUCTORSAlternators are used in a power system where they are in parallel with many other alternators. Bus Bar main function is to connect different alternators with common voltage and frequency to deliver power. In this sub station double Bus system. , i.e. Bus A& Bus B . Bus A and Bus B coupled through Bus coupler CB. Here three machines are connected in one bus and another 3 machines are connected in another Bus. The out going feeders are connected on Bus A and Bus B. Due to different loads on different feeders the buses are not operating on split mode. The Bus conductor is twin tarantula (All Aluminium) conductor. Each phase of bus having two conductors (bundle conductor).Specifications: No. Of strands: :37 Outer dia :36.67 mm

Conductor :AAC (tarantula)The conductor from machine to Bus and Bus to out going feeder are zebra conductor.SPECIFICATIONS: •Conductor : ACSR (Aluminium conductor steel reinforced)•Nominal size : Zebra•Strands : 54 Al+ 7 steel (3.18 mm) •Dia : 28.62 mm•Area : 484. 480mm2

MAINTENANCE OF VARIOUS EQUIPMENTS IN SWITCHYARDMaintenance of switchyard equipment is of two types:1.Preventive maintenance.2.Breakdown maintenance.Preventive maintenance schedule is followed daily/monthly or yearly to avoid any outage due to breakdown.CB Monitoring & Maintenance:3.Checking of SF6 gas pressure4.Checking of air pressure5.Drain water from air reservoir6.Checking of looseness of compressor and motor mounting bolts and belt.7.Oil level in crank caseShutdown maintenance:8.Cracks and contamination of porcelain clean with emery paper9.Main terminal cleaning with emery paper10.Tightness all external bolts11.Rusting area painting12.Stroke and contact wipe measurement13.Setting check of gas density and air pressure switch, mesurement of dew point14.Checking of opening and closing time etcMOCB: •Checking of leakage oil level•Checking of oil leakage from grading capacitor

Shut down maintenance:•B.D.V of oil•Cleaning of breaker poles•Checking of pole discrepancy relay•Capacitance and tan delta measurement of grading capacitor•Dynamic contact resistance and contact travel measurementCURRENT TRANSFORMER:1.Checking of oil leakage2.Checking of any sparking at CT bushes3.The rmovison scanning of CT4.Cleaning and tightness CT primary bushing clamps and healthiness of bush5.Checking of oil level and gas pressure6.Capacitance and tan delta testingCVT:7.Checking of oil leakage8.Thermo vision scanning of capacitance of stacks9.Checking of any cracks10.Capacitance and tan delta measurement11.Testing of oil BDVISOLATORS AND EARTH SWITCH: 12.Maintenance of link gears 13.Cleaning of auxiliary contacts and greasing with silicon grease14.Lubrication of operating mechanism 15.Alignment 16.Cleaning of support insulators and check for cracks17.Contact resistance measure ment of isolator and earth switch

LIGHTENING ARRESTER:1.Daily checking of surge counter and leakage counter2.Cleaning of insulator3.If required capacitance and tan delta testsBUS BAR AND BUS INSULATOR:4.Cleaning of insulators5.Checking of insulators for cracks6.Thermovison scanning of all clamps and joints7.De weeding of switchyard8.Checking of switchyard lightsWAVE TRAP:9.Tightness and cleaning10.General inspection and cleaning of tuning units THERMO VISION SCANNING OF SWITCHYARD:Thermo vision scanning of switchyard carrying out at the time of peak season for every June and August. At every clamps and joints should be scanned through camera on peak seasonBUS BAR AND FEEDER PROTECTIONBus bar protection works as back up protection to the connected generators & feeders .In case of any fault on generator or feeder which was not cleared by its primary protection scheme and the fault was being feed by other generators and feeders connected to same bus to witch the faulty generator or feeder is connected, in this case bus bar protection work as back up protection and thus trip all the generator and feeder connected to faulty portion through this bus bar.