Analysis and improvement of the biased stator earth fault 100% protection

A kind of high resistance grounding methods of generators is few applied by a 3-phase displacement transformer, which is called Pohl (or Bütow) transformer. The secondary asymmetry of the Pohl transformer yields a fundamental zero-sequence displacement voltage so that the biased stator earth fault (SEF) 100% protection can be applied. The law of the fundamental zero-sequence voltage variation under the normal and SEF conditions is derived from an equivalent circuit. For protecting 100% stator windings, the grounding resistance should be not great than √3 times of the capacitive impedance. The characteristic surface of the biased SEF relay is analyzed, and a novel calculating method of the operational voltage via the generator voltage and frequency compensation is proposed. The miss operation zone in the low voltage and low frequency could be reduced by the improved scheme.