Protection of powerformers in neutral ineffectively earthed power systems

Neutral Ineffectively Earthed Power Systems (NIEPS) is common in China to limit earth fault current and eliminate the need for an immediate shutdown. A new type of high-voltage generator which offers a direct connection to the power network without the need for a step-up transformer is called powerformer. The neutral of the most powerformers is unearthed which belongs to NIEPS. When a stator single phase-to-ground fault occurs in a powerformer, every generator that is connected in parallel with the faulty powerformer has the same voltage. Traditional protection schemes using zero sequence voltage and third-harmonic voltage signals cannot identify the faulty powerformer. In order to solve this problem, a novel stator single phase-to-ground fault protection based on the direction of zero-sequence currents is proposed in this paper. Simulation results have shown that, under different fault conditions, the new scheme can distinguish reliably internal faults from external faults, and 100% of the winding can be fully protected.