Restrictive earth faults 2

[bottler]

Hi

I am told my supply from a transformer is tripping on a restrictive earth fault detector ,
could someone please help me out and explain what a Restrictive earth fault is and how it could occur

any info is greatly appreciated

bottler

 

[ScottyUK]

It's actually Restricted Earth Fault, not restrictive.

The REF relay is basically a differential relay scheme which is employed on equipment with an earthed neutral point. It is common on both solid-earthed and impedance-earthed equipment, typically transformers and occasionally on small generators where the cost of a full differential scheme is not justified. The REF scheme provides protection within a clearly defined zone, where the zone boundaries are determined by the location of the CTs. Tyically these would be on the phase and neutral bushings.

The main benefit from using a REF relay is that it can be made very sensitive to faults occurring within the protected zone, and thus offers protection against developing faults before they become more severe. The REF scheme is insensitive to faults outside of the protected zone, allowing distance or other protection to clear an external fault without tripping the equipment protected by the REF scheme.

The REF scheme uses CTs on the phase windings and on the neutral point of the protected equipment. The CTs are connected in parallel and are phased so that in normal operation the currents have a vector sum of zero. A sensitive relay is connected across the CT group. In the event of a fault occurring within the protected zone, an imbalance current flows in the relay coil, operating the relay and tripping the circuit breaker.

A stabilising resistor is normally fitted in the relay coil circuit which reduces the likelihood of false operation of the relay if CT saturation occurs under a heavy through-fault. The resistor basically converts the relay to a voltage-operated device, and the operating voltage is calculated to ensure stability even assuming that CT saturation has occurred.





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