Ref Protection of Transformers

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Restricted Earth Fault Protection of Transformers

Introduction:
Transformers and generators are voltage sources. They are traditionally protected by an Over current + Earth fault relay , normally mounted in the breaker panel. This is shown in figure below. It should be noted that this protection alone is not adequate.

When an earth fault occurs within the zone defined as A in the figure, or within the machine, the fault current will circulate within the zone or within the machine. The fault current will not flow through the CTs connected to the O/C +E/F relays near the breaker. This will cause a no trip situation when there is a fault in the zone A (Internal fault). Consequently, a separate scheme is required to detect internal earth faults in zone A. This scheme is called Restricted Earth Fault (REF) scheme. It should be noted that the fault currents in Zone A is limited by the impedance of the equipments in the zone – for transformers and generators it is very low – the fault currents can rise very fast and damage the equipment. Consequently REF protection is of utmost importance for generators and transformers. Now the question arises that

What is restricted earth fault and how we can get protection over it?

A Restricted Earth Fault (REF) means an earth fault from a restricted/localized zone of a circuit .Restricted earth fault (REF) protection is a sensitive way to protect a zone between two measuring points against earth faults. The principal of the Restricted Earth Fault is based upon the distinction between a fault current inside the protected zone and a fault current outside the protected zone.

Restricted earth fault protection is a protection scheme given in assistance to the differential protection. It is given to protect an earth fault which occurs very close to the neutral point in star connected windings of the transformer. This kind of fault is a very low fault current only and this current may not be sufficient to...
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