electrical system in power station

7/31/2019 Electrical System in Power Station

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ELECTRICAL SYSTEM IN POWER STATION


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CONSTITUENTS OF AN ELECTRICAL SYSTEM

BREAKER SFU BUS BAR C.T P.T AUX. RELAY

POWER CONTACTOR THERMAL OVER LOAD RELAY

SUPPORT INSULATOR PROTECTIVE RELAY CABLING & WIRING


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BREAKERS

INDOOR

415V, 3.3KV, 6.6KV,

11 KV, 33 KVOUTDOOR

33KV, 66 KV, 132KV,

220KV, 400KV, 765KV


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ELECTRICAL EQUIPMENTS

Equipment Purpose Remarks

Voltage Transformer Steps down voltage formeasurement, protectionand control.

Normally connected to thebus. At incoming /outgoingpoint for higher KV systemfor protection purpose.

Circuit Breaker For switching duringnormal and abnormal

condition. Interrupts shortcircuit currents.

SF6Vacuum

Air blastMinimum Oil

Transformers /AutoTransformers

To step up or downA.C.voltages.To transferpower from one voltagelevel to another.

Heart of the electricalsystem.Generally filled withtransformer oil.

Lightning arresters Discharges the over-voltage surges to the earthand protect theequipment insulation fromSwitching/Lightning

surges

Connected between liveconductor and earth.First equipment at theincoming/outgoing point.Always placed beside the

transformer


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Lightning Shield/Mast To protect the station fromthe direct lightning stroke.

Lightning mast around thesubstation

Isolated Phase BusSystem

Interconnection betweenGenerator and Generatortransformer for very highcurrent

Aluminium Enclosure foreach phase .

Neutral Grounding resistor To limit short circuitcurrent during ground fault

Connected betweenneutral point and ground

Station earthing To give low earthresistance

Earth mat. earth electrodeplaced under ground andall the equipments bodyconnected there.

Wave Trap Inductive coil mounted

outdoor

For communication

through PLCC

Power cable For power flow 1-core/3-core XLPE /PVCinsulated

Control Cable For control, protection andmeasurement purpose

Multicore,copper,armoured and

PVC insulated


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Equipment Purpose RemarksCarrier Equipment For

communication,Scadaetc

Wavetrap,CVT,Couplingcapacitor,PLCC Panels

Control and relay panel To housemeasuring/controlinstruments andprotective relays

Indoor equipment.

D.C. system/ For control andprotection supply andemergency lightingsystem.

Battery bank withcharger.

Medium voltage and lowvoltage a,c.switchgears

To provide a.c. power tothe auxiliaries/stationlighting etc

Indoor equipment


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CURRENT TRANSFORMER

Tests on CTs:1) Ratio test 2) Polarity test 3) CT sec.

Resistance

4) V-I characteristics 5) IR value

Three basic types of CTs used inSwitchgears:1) Measuring CT2) Protection CT3) Protection CT for special application


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CURRENT TRANSFORMER

1) Measuring CT: specified in terms ofaccuracy class, VA rating Relevant IS2705-1992Eg Cl 1, VA=15 has ratio error of +/- 1% at

100 to 120% of rated current and +/- 1.5%error at 20O% of rated current

2) Protection CT: specified by class,accuracy limit factor, VA rating Eg. 5P10,15VA CT means composite error of +/- 5%at10 times rated current, ratio error of +/- 1% atrated current with rated connected burden of15VA


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CURRENT TRANSFORMER

3) Protection CT for special application: PSclass used where balance is required betweenassociated CTs such as for differential relaysspecified by a) knee point voltage b)

magnetising currentc) CT secondary resistanceFor PS class turns ratio error is limited to 0.25%

which help maintain balance during maxm. faultcondition.

Typical specification of PS class CTratio 100/1 A, Vk >100V , Im


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VOLTAGE TRANSFORMER

Three Uses of VTs :1) metering VTs eg Cl 0.1,0.2,0.5,1,32) protection VTs eg 3P,6P ( ratio error +/-3%, +/-6% respectively)3)residual VTs - a) V-V b) Y-Y c) Y-open delta

Specifications in terms of Voltage ratio Accuracy class Rated VA burden Rated voltage factorEg. 11KV/110V, Cl: 1, VA 50, VF 1.2 continuous/ 1.5 for 30

sec


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D.C. SYSTEM

Backbone of the Substation.

Needs utmost care and preservation.

Comprises of a battery bank of lead acidstorage cells, Float/Boost charger andD.C.distribution board.

Feeds control power to the protectivecircuitry and operating circuits.


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BATTERY TROUBLE SHOOTING

Troubles Symptoms Remedies

Overcharging 1.Gassing2.Falling of active materialfrom + plates3.Plate buckling

4. Increased Temperature

1. Reduce Chargingcurrent. To sp.gr. 1200

2. Add distill water

Undercharging 1.Low sp. Gravity2 Lighter colour of plates3.Reversal of cell Voltage.4.Plate buckling5. Oxidisation of batterylinks

1. Increase chargingcurrent. To sp.gr.1200

2. Remove and clean inwarm water

Loss of capacity Abnormal drop of voltageduring normal dischargewithout correspondingtemperature rise.

Contact manufacturer,needs special treatment


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BATTERY TROUBLE SHOOTING

Troubles Symptoms Remedies

Excessive sulphation 1. Undercharging2. Too strong/weak acid3. Over discharge4. Local action

5. Short circuits6. Too rapid discharge7. Plates exposed to air

due to low level ofelectrolyte

1.Sulphation at initialstage can be cured by lowrate repeated chargingand discharging

2. Sulphation at advancedstage needsmanufacturers attention

Internal discharge (localaction)

1.Gassing in idle condition2.Abnormal gassingduring charging.3.Reduce capacity forelectrolytedecomposition/impurity coming in contact

with active materials

Use pure electrolyte androom should be clean andairy.


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EARTHING

Safety to personnel ,step and touch potentialshould be within safe tolerable limits.

Min. Damage to the equipt. During heavyfault current.

So resistance of the earthing system shouldbe as low as possible.


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GOOD EARTHING

To meet these requirements, an earthingsystem comprising an earthing mat buried atsuitable depth below ground, supplemented

with ground rods at suitable points isprovided in the switchyard. All the noncurrent carrying parts of the equipments in

the substation are connected to that earthingmat so as to ensure that under fault condition,non of theses parts is at a potential higher

than that of the E/mat.

electrical system in power station

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