basic transformer protection

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Transformer basics and more…

Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…

Power Transformer as an expensive device

Power Transformer Name Plate

Generator Step- Up

transformers

Step- Down transmission

substationtransformers

Distribution substation

transformer

Distribution transforme

Location

Transformer types• Generator Step-Up (GSU) transformer

• Transmission step down transformer

• Distribution substation transformer

• Distribution small transformers

• Autotransformers

• Voltage regulating transformers

• Scott transformers

• Reactors

• Split-Phase autotransformers

GENERATOR STEP- UP (GSU) TRANSFORMERS

GSU xfmr

• 1300 MVA

• 343 ±2x1.9% / 25 kV

• 60 Hz

• 3-phase

Aux. xfmr

STEP- DOWN DISTRIBUTION SUBSTATION TRANSFORMER

• 24 MVA

• 138 / 13.8 kV

• 60 Hz

• 3-phase

• FOA

AUTOTRANSFORMER

1000 MVA

346 ±17x3.66 kV / 225 kV

3-phase

OA/FA/FA (self-cooled/ forced air cooled)

100kVA, 14.4kV/240V/120V

100kVA, 14.4kV/347VPole-Mount

Single-phase

transformers

500kVA, 14.4kV/600VPlatform-Mount

SMALL DISTRIBUTION TRANSFORMERS

45kVA, 24.94kV/480V3-phase Pole Mount

14.4kV/240V/120VMini-Pad

Pole-Mount

Three-phase

transformers

Up to 300kVAPrim. voltage - up to 36kV3-phase substation installation

Platform-Mount

2 MVA, 13.8/600kVPad - Mount

Transformer componentsTransformer Basic & More…Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…

Transformer components – cooling system

OIL PUMPCOOLING FANS

Transformer components – tap changer

TAP-CHANGER

BUCHHOLZ RELAY

Transformer components –Buchholtz relayTransformer Basic & More…Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…

SUDDEN PRESSURE

Transformer components – gas pressure relayTransformer Basic & More…Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…

CHANGE PRESSURE RELEIF

DEVICE

Winding connections and phase shiftTransformer Basic & More…Transformer Basic & More…Transformer Basic & More…Transformer Basic & More…

Transformer electric parameters - relations

SSSPPP PIVIVP === **

Transformer losses

copper losses -I 2 R

Stray loss -stray capacitance and leakage inductance lossesApparent loss –loss due to magnetizing current in the primary winding

Iron loss –loss due to varying flux in the core( hysteresis and eddy losses)

- hysteresis loss

Transformer inrush

The steady state flux

lags the voltage by

90° degrees

The flux builds up from zero, when the voltage is applied at zero crossing, and can reach 2 times the maximum flux. The magnetizing current becomes even higher, if the transformer is energized at zero point of the voltage wave, and there is residual flux

As the flux builds,

the exciting

current grows with

the flux.

Transformer faults and detection

• EXTERNAL FAULTS

– Overloads

– Overvoltage

– Underfrequency

– External system short circuits

• INTERNAL FAULTS

– Incipient faults

• Overheating

• Over-fluxing

• Overpressure

– Active faults

• Short circuit in wye-connected

windings

• Short circuits in delta windings

• Phase-to-phase faults

• Turn-to-turn faults

• Core faults

• Tank faults

External faults:

OVERLOADSOVERLOADSOVERLOADSOVERLOADSIn most cases, no protection is provided, but an alarm is used to

warn the operating personnel of the conditions. A TOC

protection with definite time delay can be set.

OVERVOLTAGEOVERVOLTAGEOVERVOLTAGEOVERVOLTAGEIt can be either due to short-term transient conditions, or long

term power frequency conditions. Transient over-voltages,

cause end-turn stresses and possible breakdown. The conditions

are detected by Volts/HertzVolts/HertzVolts/HertzVolts/Hertz protection.

UNDERFREQUENCYUNDERFREQUENCYUNDERFREQUENCYUNDERFREQUENCY

Under-frequency is caused by some system disturbances

resulting in unbalance between generation and load. This low

frequency creates overfluxing in the transformer core, leading to

overheat. Volts/HertzVolts/HertzVolts/HertzVolts/Hertz protection is used with typically 1.1 pu

pickup ratio setting.

EXTERNAL SYSTEM SHORT EXTERNAL SYSTEM SHORT EXTERNAL SYSTEM SHORT EXTERNAL SYSTEM SHORT CIRCUITSCIRCUITSCIRCUITSCIRCUITS

Large external fault currents can cause high mechanical stress

in the transformer windings, with the maximum stress occurring

during the first cycle. The transformers are not protected during

such external conditions. It is a matter of transformer design,

and application, to deal with these conditions.

Incipient transformer internal faults

OVERHEATINGOVERHEATINGOVERHEATINGOVERHEATING

Caused by:

• poor internal connections, in either electric or magnetic circuit

• loss of coolant due to leakage

• blockage of coolant flow

• loss of fans or pumps

Buchholtz relay and thermal elements protections such as Hottest

Spot temperature, Aging Factor and Loss of Life are normally used

Continuous overfluxing can gradually lead to isolation breakdown. The detection is provided by Volts/Hertz Volts/Hertz Volts/Hertz Volts/Hertz protection

OVERFLUXINGOVERFLUXINGOVERFLUXINGOVERFLUXING

OVERPRESSUREOVERPRESSUREOVERPRESSUREOVERPRESSURE

Overpressure in the transformer tank occurs due to released gases that accompany localized heating. An example is the turn-to-turn fault, that can burn slowly, releasing bubbles of gases, which increase the pressure. Sudden Pressure relay, or Buchholtz relay

Protection elements

Active internal faults detected by 87T protection

* ** *

WYE connectionWYE connection

I1 prim I2 prim

Phase-to-phase faults

Three-phase faults

Ground faults

Core faults

Tank faults

i1 sec

i2 sec

DIFFERENTIAL SIGNAL:

I DIFF. = I1COMP + I2COMP

RESTRAINING SIGNAL:

I RESTR. = max ( |I1COMP| , | I2COMP|)

Id, pu

Ir, pu

Min. PKP

B 1 B 2

T60 BIASED DIFFERENTIAL(TRIP/NO TRIP DECISION)

Id > PKP

Ir < B1 Id/Ir%>S1YES

NO TRIP

B1 < Ir < B2 YES

Ir > B2

Id/Ir, % >S2 YES TRIP

Id/Ir,% >S1&S2

Restricted Ground Fault protection

� Fast detection of winding ground faults� Very secure performance on external ground faults� Configurable pickup, slope, and time delay

Igd, pu

I = max( IR1, IR2, IR0 ), pu

Min. PKP

S lope

gIIIgdρρ

+= 03)2,1,0max( IRIRIRIgr =

Questions

basic transformer protection

phase transformers500kva

primary windingiron

94kv480v3phase pole

plategenerator step

magnetizing current

varying flux

core hysteresis

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