transformer protections in thermal power plant

8/13/2019 Transformer Protections in Thermal Power Plant

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27 December 2013 PMI Revision 00 1

TransformerProtections


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Presentation outline

Need of transformerTypes of Transformers

Transformers AccessoriesMajor Transformers in Power Plants

Protection of transformers


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TRANSFORMERS


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WHY TRANSFORMER TO OPTIMISE COST OF BULK TRANSMISSION

OF POWER FROM GENERATORS TO

CONSUMERS

REDUCTION IN TRANSMISSION LOSS

TO REDUCE OR INCREASE VOLTAGE IN AC

SYSTEM

ENABLES SAFE SUPPLY VOLTAGE TOCONSUMERS

ISOLATION OF TWO SYSTEMS FOR VOLTAGE

REGULATION


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IEC 60076-1

A static piece of apparatus with two or more windingswhich, by electromagnetic induction, transforms asystem of alternating voltage and current into another

system of voltage and current usually of differentvalues and at the same frequency for the purpose oftransmitting electrical power.

IEEE C57.12.80-2002

A static device consisting of a winding, or two or morecoupled windings with or without a magnetic core forintroducing mutual coupling between electricalcircuits.

Definition of Transformer as per Stds


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What are the Types of

Transformers Power transformers : Used in transmissionnetwork of higher voltages, deployed for step-up

and step down transformer application (400 kV,200 kV, 110 kV, 66 kV, 33kV,22kV)

Distribution transformers : Used for lowervoltage distribution networks as a means to end

user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V,230V)


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Classifications

Transformers are adapted to numerous engineeringapplications and may be classified in many ways:

By power level:(from fraction of a volt-ampere(VA) to over a thousand

MVA), By application:(power supply, impedance matching, circuit isolation),

By frequency range:

(power, audio, radio frequency(RF)) By voltage class:(a few volts to about 765 kilovolts)


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Classifications

By cooling type:(air cooled, oil filled, fan cooled, water cooled (Natural/Forced) etc.)

By purpose:(distribution, rectifier, arc furnace, amplifier output, etc.).

By ratio of the number of turns in the coilsStep-up The secondary has more turns than the primary.Step-down The secondary has fewer turns than the

primary.

Connection :Single phase, Star / star , Star delta etc


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Cur ren t con duc to r s P r im ary& Seco nd ary Wind ing s Magn et ic f lux co nd uc to r -

Laminated Steel Core Insula t ionCool ingProtec t ionSup po r t ing acc ess or ies

Basic Components of aTransformer


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Tap Changer(s)-(On load/Off

load) Tank Radiators cooling fans, oil pumps, oil to

water heat exchangers (CoolingONAN / ONAF/ OFAF/ OFWFexternal coolers)

Bushings

Accessories & Auxiliaries


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Protection

Objective: To quickly isolate a faulty section fromboth ends so that the rest of the system can functionsatisfactorily.

THE FUNCTIONAL REQUIRMENTS OF THE RELAY:

1. Reliability :2. Selectivity:3. Sensitivity:

4. Speed :


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ELECRICAL FAULT OPEN CIRCUIT FAULT SHORT CIRCUIT FAULT THROUGH FAULT INTERNAL FAULT INCIPIENT FAULT



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Important Elements:

Switch gear:

Circuit breaker bulk oil, minimum oil,SF6 ,air blast , vacuum etc. depending

on medium used for quenching the arc.Different operating mechanism such assolenoid, spring, pneumatic,hydraulics etc. are employed.

.


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Protection Gear:


Relays(current , voltage,

impedance, power , frequencyetc. based on operatingparameter, definite time ,

stepped etc. as per operatingcharacteristic , logic wisesuch as differential , over

fluxing etc.


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NORMS OF PROTECTION FOR EHV CLASS POWERTRANSFORMER

Voltage ratio &capacity

HV side LV side Common relays

11/132 KV GT 3-non-dir O/L +1-non-dir E/LRelay and/or standbyE/F + REF

--- Differential or overalldifferential, overflux,buchholz, OLTCbuchholz, PRV ,OT , WT

13.8/220 KV15.75/220 KV18/400 KV21/400 KVGenerator T/Fs

3-non-dir O/L + 1-non-dirE/L relay and/or standbyE/F + REF

220/6.6 KV Station T/Fs 3-non-dir O/L + 1-non-dirE/L relay and/or standbyE/F + REF

3-non-dir. O/L relays Differential, overallbuchholz, OLTCBuchholz, PRV, OT, WT

Gen-volt/6.6KV UAT 3-non-dir O/L relays 3-non-dir O/L relays Differential, overallbuchholz, OLTCBuchholz, PRV, OT, WT

POWER STATION


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SUBSTATION

132/33/11 KV upto8 MVA

3 O/L relays + 1E/L relay 2 O/L relays + 1 E/Lrelay

Buchholz, OLTCBuchholz, OT, WT

132/33/11KV above8 MVA and below 31.5MVA

3 O/L relays + 1 dir. E/Lrelay

3 O/L relays + 1 E/Lrelay

Differential , Buchholz,OLTC Buchholz, OT, WT

132/33 KV, 31.5 MVA &above

3 O/L relays + 1 dir.E/Lrelay


Differential , Buchholz,OLTC Buchholz, PRV,OT, WT

220/33 KV, 31.5MVA &50MVA 220/132KV ,100MVA

3 O/L relay + 1 dir. E/Lrelay


Differential , Buchholz,OLTC Buchholz, PRV,OT, WT

400/220KV 315MVA 3 directional O/L relays(with dir.. Highest) + 1dir. E/L relays. RestrictedE/F relay + 3 dir. O/Lrelays for action.

3 directional O/L relays(with dir.. Highest) + 1dir. E/L relays. RestrictedE/F relay

Differential , overflux,Buchholz, OLTCBuchholz, OT, WT andoverload (alarm) relay


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The following relays areemployed to protect the

transformer against internalfaults.

i. Buchholz Relays

ii.Differential Relays

iii.REF relays.

iv.Over fluxing relays


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Buchholz Relay


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Buchholtz relay


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Alarm element Operates When a specified volume ofgas gets collected in Chamber during

Broken down core bolt insulation

Shorted Laminations

Bad Contacts

Overheating of winding parts

Trip element Operates by Oil surge in the event ofserious fault

Short Circuit between Winding Phases orwithin Windings

Puncture of Bushing

BUCHHOLZ PROTECTION


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Transformer connections :

- Delta/Starused in Generating stations forStep-up.

- Star/Deltaused in Receiving stations forStep-down

* All GTs are Delta/Star connected.

All Tie T/Fs are Star/Star connected.


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STAR / DELTA connection

STAR / STAR connection


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The relay is operative only for the internal faults of the transformerand thus fast operating time can be achieved.


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OTHER PROTECTIVE DEVICE PRESSURE RELIEF VALVE

WINDING TEMPERATURE

OIL TEMPRETURE

OLTC BUCHHOLZ LIGHTNINIG ARRESTER FIRE PROTECTION



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Accessories & AuxiliariesAccessories &

Auxiliaries

Oil Level Indicators Marshalling Box/Control

cubicle Oil Preservation Systems:

Conservators

gas sealed, BellowsBreathers


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SILICA GEL BREATHER

conservator


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Winding / Oil Temperature Indicator


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Pressure Relief Device PRD)


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Different transformers in a power

Plant Generator Transformer (GT)

Station Transformer (ST)

Unit Auxiliary Transformer (UAT)Excitation TransformerNeutral Grounding Transformer

Auxiliary transformersTie / Auto transformer


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SPECIFICATION

STAGE -1 GT-VindhyachalRATED O/P 250MVARATED VOLT. (HV) 420KVRATED VOLT. (LV) 15.75KVRATED CURRENT(HV) 344ARATED CURRENT(LV) 9175AVECTOR GROUP Ynd11


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GENERATOR

7UA 7UB

SA SB

GT (3X200 MVA)

Unit Tx 21/11 K.V. (50 MVA)

IN THE EVENT OF UNIT TRIPPING AUTO CHANGEOVERFROM UT TO STATION WILL TAKE PLACE


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TESTING OF TRANSFORMER

THE FOLLOWING TESTS ARE GENERALLY PERFORMED ON THETRANSFORMER REFERENCE STANDARD : IEC 60076

ROUTINE TESTS

MEASUREMENT OF WINDING RESISTANCE

MEASUREMENT OF VOLTAGE RATIO, POLARITY AND CHECK OFVOLTAGE VECTOR RELATIONSHIP

MEASUREMENT OF NO-LOAD LOSS AND EXCITATION CURRENT

MEASUREMENT OF INSULATION RESISTANCE

SWITCHING IMPULSE VOLTAGE WITHSTAND TEST

LIGHTNING IMPULSE VOLTAGE WITHSTAND TEST


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ON LINE MONITORING EQUIPMENT ON TRANSFORMER

1. DGA to be monitored on all transformers periodically.2. Probable Reason of gas3. Arcing

CoronaOver HeatingPartial Discharge

4. Advance warning of developing faults5. Determining the improper use of units6. Status checks on new and repaired units7. Convenient scheduling of repairs8. Monitoring of units under overload
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GASES IN DGA 1. Corona a. In Oil H 2 b. In Cellulose H 2 , CO , CO 2

Oil Low temperature CH 4 , C 2H6 Oil High temperature C 2H4 , H 2 ( CH 4 , C 2H6 Cellulose Low temperature CO 2 ( CO ) temperature CO ( CO 2 ) In Arcing H 2, C 2H2 (CH 4, C 2H6, C 2H4)



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7. ON LINE MONITORING EQUIPMENT ON TRANSFORMERContd.

3. Acoustic PD measurement.

In case DGA results indicate the presence of high dischargesAcoustic PD measurement is done. Acoustic PD detector consists of sensor which can sense soundvibrations produced due to occurrence of discharges in oil. The sensor has pre-amplifier and filter circuit to eliminateenvironmental noise

4 Infrared thermographyWith the help of infrared thermo-vision camera, the thermo-image

of transformer is preparedThis scanning helps in detecting overheated zones, loose &corroded connection.


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OFF LINE MONITORING1. Frequency Response Analysis

Short circuit forces could move the winding.

Can also change winding inductance andcapacitance.

Conventional tests are not sensitive to windingmovement

Such changes can be effectively detected byFRA

.


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THANK YOU

transformer protections in thermal power plant

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