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Harmonic distortion in power stations due to ferroresonance

In a three-phase system with isolated neutral ferroresonance is a periodic displacement of the neutral potential. These so called relaxation oscillations are caused by discharging and recharging of the capacitances to the ground via magnetising inductances of e.g. single-pole insulated voltage transformers and the periodic repetition of this process. As a consequence, high saturation currents flow through the primary windings of the voltage transformers heating up these windings and often leading to the destruction of the voltage transformer. The non-linear phase-to-ground inductance of voltage transformers and the phase-to-ground capacitance form an oscillatory circuit which may be kicked on by a switching event, e.g. an earth-fault followed by a self-extinction of the fault current. The present paper specifically investigates the possible occurrence of ferroresonance in power stations where a generator circuit-breaker is employed. The effects of harmonic distortion due to ferroresonance are thoroughly investigated and different mitigation measures are analysed.