transformer protection overview

8/8/2019 Transformer Protection Overview

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Transformer Protection Overview

Ahmed Ibraheem


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Transformers Main Faults

Overloads

Short Circuit

Frame Fault


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Overload

Resulted from:

Increase in the number of loads being fedsimultaneously.

increase in the power absorbed by one or

more loads.


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Overload

Effect:

It results in an overcurrrent of long durationcausing a rise in temperature that isdetrimental to the preservation of insulationand to the service life of the transformer.


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Overload

Protection:

The secondary circuit breaker (B) LV Side should trip at time tb.

In case of malfunction of circuit

breaker (B), circuit breaker (A)should trip at time ta=tb+t.


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Short Circuit

Resulted from:

Internal Short Circuit

External Short Circuit


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Short Circuit

Internal Short Circuit:

Faults occurring between winding conductors

with different phases.

Faults in the same winding


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Short Circuit

Effect:

Winding damage and then fire.

The arc causes the emission of decompositiongas in oil transformers.

That can destroy the winding and also the tankframe by the spread of burning oil.


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Short Circuit

External Short Circuit:

Faults occurring in the downstream

connections:

Phase to phase fault

Phase to ground fault


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Short Circuit

Effect:

Produces electrodynamic forces In the

transformer that are liable to affect thewindings mechanically and then develop inthe form of internal faults.


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Short Circuit

Protection:

a Buchholz relay, DGPT or DMCR that are sensitive

to gas release or oil movement is used.


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Short Circuit

Protection:

Differential transformer

protection ensures rapidprotection against phase-to-phase faults. It is sensitiveto fault currents in therange of 0.5 In and is usedfor important transformers.


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Short Circuit

Protection:

An instantaneous overcurrent

protection device located at thetransformer primary is used.

Discrimination between upstream& downstream protection relaysshould be considered


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Short Circuit

Protection:

for low power transformers, a fuse is used for

overcurrent protection.


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Tank frame faults

Resulted from:

Between the winding and the tank frame.

Between the winding and the magnetic core.


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Tank frame faults

Effect:

Causes gas emission in oil transformers. Like

internal short circuits, it can causetransformer damage and fire.

The amplitude of the fault current dependson the grounding systems, and also on theposition of the fault within the winding.


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Tank frame faults

Delta Connection the frame current

varies between 50and 100% of themaximum valuedepending onwhether the faultis in the middle or

at the end of thewinding.

Star Connection The frame

current variesbetween 0 andthe maximumvalue dependingon whether thefault is at theneutral or phaseend of thewinding.

Effect:


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Tank frame faults

Protection:

Using an instantaneous

overcurrent protectiondevice installed on theframe earthingconnection.

The transformer tank hasto be isolated from theground.


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Tank frame faults

Protection:

Earth protection located on the upstreamsystem for frame faults affecting thetransformer primary.

Earth fault protection located on the incomingline, if the neutral of the downstream systemis earthed on the busbars.


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Tank frame faults

Protection:

They are only sensitive to phase-to-earthfaults located in the transformer or on theupstream and downstream connections.


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Tank frame faults

Protection:

Restricted earth protection if the neutral of

the downstream system is earthed at thetransformer.


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Tank frame faults

Protection:

This is a high impedance differential

protection system which detects thedifference in residual currents measured atthe grounding point and at the three phasetransformer outlet.


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Tank frame faults

Protection:

Neutral earth

protection if thedownstream systemis earthed at thetransformer


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Protection Summary

MV/LV Transformers


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Protection Summary

MV/MV Transformers

transformer protection overview

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