3. Power Transformer Protection by HK Rajput..ppt

TRANSFORMER PROTECTIONPRESENTATIONBY

HITESH KUMAR MANAGER (T) PROTECTION DELHI TRANSCO LIMITED

What is a Transformer? Transformer is a static piece of apparatus by means of which electric power is transferred from one circuit to another without change in frequency. It can raise or lower the voltages with a corresponding decrease or increase in current. It is accomplished by Faradays law of electromagnetic induction. In simplest form, a transformer consists of two conducting coils having mutual inductance. The primary winding is one, which receives electric power from the supply, and the secondary winding is the one, which delivers to the load. The coils are wound on a laminated core of magnetic material. It operates on the principle of mutual inductance between the two circuits linked by a common magnetic flux through a path of low reluctance.

TRANSFORMER PROTECTION

Failure of Transformers:Insulation failure due to:-Aging ( function of time and temperature).Heating (Over voltages/over loading/loose connections/poor cooling). High voltage surges (lightning/Switching surges).Mechanical forces due to external faults.


NRPC recommendations:400/220 kV and 220/66/33kV Power Transformers.All windings of power transformers of EHT class shall be protected by differential relays, REF relay for HV & LV windings. Two separate relays to be provided for Main Protection e.g. differential and REF and Back up relay should be separate. Over-fluxing relays shall be provided for EHT transformers. In addition there shall be back up time lag over current and earth fault protection. For parallel operation such as back up protection shall have a directional feature. For protection against heavy short circuits, the over current relays should incorporate with a high set instantaneous element. In addition to electrical protection, gas operated relays, sudden pressure/PRV winding temperature protection and oil temperature protection shall be provided.


TRANSFORMER PROTECTION400/220 kV Transformers (HV side):Directional 2O/C and 1 directional E/F relay, REF relay.400/220 kV Transformers (LV side):Directional 2O/C and 1 directional E/F relay, REF relay.Common relays:Numerical differential relay and separate REFs, over load, Over flux, Buchholz, OLTC Buchholz (OSR), PRV, OTI, WTI.SPR.

220 / 66-33 / 11kV Transformers (HV side):2O/C and 1 E/F relay, REF.220 / 66-33 / 11kV Transformers (LV side):2O/C and 1 E/F relay, REFCommon relays:Numerical differential relay and separate REFs, Over flux, Buchholz, OLTC Buchholz, PRV, OTI, WTI.SPR.

TRANSFORMER PROTECTIONMagnetic oil gauge with low level alarm contact.

Indicates oil level in transformer conservator tank. Shows relation between temperature & volume. Low oil level alarm & trip facility. Installed generally in transformers of rating 3.15 MVA.

TRANSFORMER PROTECTIONMagnetic oil gauge with low level alarm/trip contact.

TRANSFORMER PROTECTIONBuchholz relay with alarm & trip contacts.

A protective device for oil immersed transformers. Provides alarm & trip facility. Generated gases is collected in this relay and gives alarm. Installed generally in transformers of 1600 KVA rating.

TRANSFORMER PROTECTIONOil surge relay for OLTC with trip contact.

Alarm element Operates When a specified volume of gas gets collected in Chamber during Broken down core bolt insulation Shorted Laminations Bad Contacts Overheating of winding partsTrip element Operates by Oil surge in the event of serious faultShort Circuit between Winding Phases or within WindingsPuncture of Bushing

TRANSFORMER PROTECTIONPressure relief valve.

A safety device.Provides alarm & trip facility.Releases pressure excess to pre-set value ( 0. 50 0.60 kg/cm ). Installed in transformers of rating 500KVA.

TRANSFORMER PROTECTIONSudden pressure relay

Very sensitive safety device.Provides alarm & trip facility.Works on the principle of differential pressure.Senses rate of change of pressure( App. 0.25 kg/cm 20% , Fukunda Instrument co. ).Transformer trips on abnormality.Used in large EHV class transformers.

TRANSFORMER PROTECTIONOTI with alarm & trip contacts.WTI with alarm & trip contacts.


Protection and monitors shall be divided in two groups viz. Gr A and Gr B at 400kV.Given below is the way of grouping these protections:

Group A Group B

Transformer biased R.E.F Protection differential relay Buchholz Protection

Back up Protection(HV)Back up Protection(MV)

Over fluxing protection(HV) Over fluxing protection(MV)

Oil temperature high tripping Overload protection (Alarm only) winding, temperature high tripping


Group A Group B

Oil temperature high tripping Overload protection (Alarm only) winding, temperature high tripping

Pressure relief tripping OLTC Buchholz tripping

Delta winding protection Oil level high/low tripping

Group A and B protections shall be connected to separate DC source/ separately fused supplies.

DC sources shall be supervised.

Both Gr A and Gr B protections shall give out tripping impulses to HV, MV and LV (if applicable), circuit breakers.


A true Main-2 protection scheme is not provided in case of transformers. As operating requirements change the level of Main-2 protection may need to be re-evaluated Numerical relays offer the best characteristic since digital filters remove harmonics and DC offset currents from the inrush. Numerical back-up over current relays can therefore be set much more sensitive than conventional relays.Harmonic restraint will be required to obtain sensitive settings in case of zero sequence over current relays Sometimes security is a problem In case of Buchholz relays. The addition of an over current blocking relay, which blocks or slows down the tripping, may be used to improve security of the devices For mechanical type of fault detectors like Buchholz and PRDs, duplicating of the initiating contacts may be considered.


For grid transformers, application of over excitation protection is not considered as necessary.

Power transformers can withstand overloads for some time. Therefore selective protection and monitoring and load management should be done. Tripping of the transformer should be the last action. Fast clearance of faults in the transformer is important for stability of the system. Requirement of through fault withstand is also important. Protections provided should meet these requirements.


Where Fire protection is provided the contacts should be wired to trip both Group-A and Group-B trip relays.

For 132 kV level transformer where two separate DC sources are not provided, the protections may be grouped in two groups connected to separately fused DC sources. One gets to use two trip relays in this case, which is advantageous.


R1Y1B1R2Y2B2 Im/1 E2E1Normal Condition (No Ter Winding):V1


R1Y1B1R2Y2B2 Im/1 E2 I2F/F E1SLG Fault (No Ter Winding):V1 I1FF I2F/F


Normal Condition (No Ter Winding): IA=

IB=

IC=IA=

IB=

IC=


IA=

IB=

IC=IA=

IB=

IC=SLG Fault (No Ter Winding):


Normal Condition with Ter Winding: IA=

IB=

IC=IA=

IB=

IC=IA=

IB=

IC=


Line Fault with Ter Winding: IA=

IB=

IC=IA=

IB=

IC=IA=

IB=

IC=


Comparison(No Ter/ with Ter Winding): IA=

IB=

IC=IA=

IB=

IC=IA=

IB=

IC=IA=

IB=

IC=


R1Y1B1R2Y2B2R3Y3B3B3R3Y3 Im/1 E3E1E2Normal Condition with Ter Winding: V1Resultant Voltage in Delta Connected Ter Winding =zero


R1Y1B1R2Y2B2R3Y3B3B3R3Y3SLG Fault with Ter Winding: 0 I02Resultant Voltage V0 in Delta Connected Ter Winding will generate I0, 0 & V0 in other windings.V03 E32.=>=>V02I2F3I02I01I033I013I02

TransformerNormal conditionsV1Resultant Voltage in Delta Connected Ter Winding =zero I1/1 1 E2 I2/2 I2 2 I3/3 I3 3 E1E3

TransformerDuring SLG Fault in LV sideResultant Voltage V0 in Delta Connected Ter Winding will generate I0, 0 & V0 in other windings.V02 I03/0 V03 E30 V01E2/I2FV0/I0I0 0 V0/I0 V0/I0 V0/I0 0 I01I02

TRANSFORMER PROTECTIONFault Levels/Voltages LevelsTrans.: 220/66kVYNyn0Trans.: 66/11 kVDyn11

TRANSFORMER PROTECTIONDifferential Protection:

Idiff= (I1-I2) AmpI bias=(I1+I2)/2 Amp

-

-

-

-

1 -I I I I I I I I I I I 0.5 1 2 3 4 5 M1=20 %M2=80 %


It should not operate: with through faults.

CT Secondary connections.CT core selection


Triple pole feature with individual phase indicationUnrestrained instantaneous high set O/C units which should not operate during in rush.Adjustable for multi bias settingSecond harmonic or other inrush proof feature.One bias winding per phase and per CT inputAn adjustable operating currentAn operating time not greater than 30mSec at 5 times of settingFacility for ratio and phase angle correction

On-Line Monitoring 1. DGA to be monitored on all transformers periodically. Periodicity depends on the nature of evaluation of gases.

2. HYDRANE directly mounted on the transformer and in contact with transformer oil.The use of this device is to detect the change in fault gases, to monitor their evolution. As an alarm is triggered oil is sampled from the transformer to evaluate the nature and severity of the fault.

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

4Infrared thermographyWith the help of infrared thermo-vision camera, the thermo-image of transformer is preparedThis scanning helps in detecting overheated zones, loose & corroded connection.

1. Frequency Response AnalysisShort circuit forces could move the winding.Can also change winding inductance and capacitance.Conventional tests are not sensitive to winding movement.Such changes can be effectively detected by FRAMethodSweep frequency input is given in the winding.Input & response recorded.FFT analysis of response to obtain the finger print of the winding.These finger prints are compared periodically to assess the condition of winding.

2.Recovery voltages measurementIt indicates the content of water in the insulation.PRINCIPLESample of discharged insulationCharge with DC voltage and note timeSample to be short circuited for predetermined period of time ( approximately half of the charged time)Open the terminal and measure voltage.3.Tan- & Capacitance by standard bridge.

4.Resistance & magnetizing current.5.Maintenance schedule call for daily , quarterly, yearly, two yearly maintenance requirement. Over and above after 7 to 10 years, thorough overhauling and inspection of core-coil assembly by lifting tank top cover / core-coil assembly neededIn overhauling sludge deposited on coil is removed and coil assembly which may become loose is tightened. All other clampings & cleats etc. are also tightened

POWER SYSTEM Generation to Distribution

POWER SYSTEM 20 MVA 66/11 kV Dyn11 Power Transformer 400 KVA 11/.433 kV Dyn11 Distribution Transformer

POWER SYSTEM Power Transformer YNyn0d11

POWER SYSTEM TWO BUS ARRANGEMENTBUS-1BUS-2TRANSFORMERFEEDER BUS COUPLER

POWER TRANSFORMER BACKUP PRTECTIONTransformer220/66-33/11 kV262/875-1750/1750 A

CTs:400/1A262/0.655ACTs:2000/1A1750/0.875Ao/cE/Fo/cX

X

XX

X

XPickup setting to be decided for over loading:120-150% of Ib

POWER TRANSFORMER Normal loading (I < I base):Transformer220/66-33/11 kV262/875-1750/1750 A

CTs:400/1A262/0.655ACTs:2000/1A1750/0.875Ao/co/cE/Fo/cE/Fo/cPickup setting to be decided for over loading:120-150% of Ib

POWER TRANSFORMERThree phase short circuit (O/C) protection (I >> I base):CTs:400/1A1750/4.3754ACTs:2000/1A11664/5.832ATMS to be decided as per the short time rating/loading curve of the transformer.

POWER TRANSFORMERSingle phase short circuit (O/C) protection (I >> I base):CTs:400/1A1750/4.3754ACTs:2000/1A11664/5.832ATMS to be decided as per the short time rating/loading curve of the transformer.o/co/c

POWER SYSTEM REF Protection

POWER SYSTEM REF Protection Operation: NoX

X

XX

X

X

POWER SYSTEM REF ProtectionX

X

XX

X

X

POWER SYSTEM REF Protection Operation: Yes (LV)X

X

XX

X

XREF

POWER SYSTEM Differential Protection: Normal Loading

POWER SYSTEM Differential Protection Operation: NoX

X

XX

X

XYd11/Dy1Yd11/Dy16A6A

POWER SYSTEM Differential Protection Operation: Yes

X

X

XX

X

XYd11/Dy1Yd11/Dy1CTs:400/1A2620/6.55ACTs:2000/1A1750/0.875A6.55A/5.77A10A

POWER SYSTEM Differential Protection (Dyn11)Yd1/Dy11CTs:400/1A175/0.437ACTs:1200/.578A1050/0.506A0.437A/1A0.506A/0.578A1A1A

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3. Power Transformer Protection by HK Rajput..ppt

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