Earth Fault Protection with Restrictions
Many medium and small-sized transformer windings are protected by restricted earth fault (REF) systems. The illustration depicts the REF protection principle.
The Fundamental Procedure
The sum of currents in both current transformers (CTs) equals zero under normal conditions and when Kirchhoff’s laws are applied. If an earth fault exists between the CTs, some current will bypass the CTs, and the total current will not be zero. Faults between CTs can be easily identified and quickly cleared by measuring the current imbalance.
The detection of faults is limited to the zone between the two CTs, hence the name ‘Restricted Earth Fault.’
REF protection is quick and can isolate winding faults quickly, limiting damage and subsequent repair costs. Only the winding is protected if CTs are installed on the transformer terminals. However, the secondary CT is frequently installed in the distribution switchboard, extending the protection zone to include the main cable.
Faults in the transformer star secondary winding must be detected on the transformer’s primary by the reflected current in the absence of REF. The magnitude of the current seen on the primary rapidly decreases as the winding fault position moves towards neutral, and it may not be detected (limiting the amount of winding which can be protected). Because the magnitude of the currents on the secondary remains relatively large (especially if solidly earthed), nearly the entire winding can be protected using REF.
It should be noted that the illustrated protection only protects the transformer’s secondary. REF protection is sometimes added to the primary as well (although if primary protection is required it may be preferable to consider full differential protection).
Because it is critical that the current in the CTs be balanced under normal conditions (and through faults), REF has traditionally been implemented using High Impedance Relays. CTs have also been specified as matched pairs, and the impedance of leads/wires and interconnecting cables has had a significant impact on the relay’s operation.
Mismatched CTs, saturation, and varying lead resistance have all been blamed for nuisance tripping, and the system can be difficult to commission. This could be why some people avoid using REF. Recent advancements in numerical relay technology have virtually eliminated these issues, making REF implementation relatively simple, ensuring no nuisance tripping, and simplifying commissioning.
The ANSI Device Number for a restricted earth fault is shown in the diagram as 87N.