Testing sensitivity of restricted earth fault protection by simulation of faults inside transformers

摘要

Restricted earth fault protection covers faults inside of a transformer, which are located close to the star-point of wye-windings. Those faults cannot be detected by conventional transformer differential protection, because they do not cause high enough differential current on the transformer terminals, so that a trip with a percentage restraint characteristic is possible. Differential currents caused by faults inside of a transformer are highly dependent on the location of the fault, the star-point grounding of the wye-winding (solidly grounded, resistive or impedance grounding) and on fault resistance, both for conventional transformer differential and restricted earth fault elements. For protection engineers it is important to know about the sensitivity of both protection principles, so that the settings for the restraint characteristics can be optimized. Using a simulation model for turn-to-ground and turn-to-turn faults allows for testing and analysing the sensitivity of these transformer protection elements. Simulation of faults located at transformer turns does require the modelling of the leakage inductance caused by the splitting of the faulty winding. The leakage inductance is difficult to calculate exactly and depends on construction and detailed geometry data of the windings, which are usually not known to system operators or protection engineers. Therefore, different approaches to estimate the leakage inductance, based only on nameplate data from the transformer, are discussed.