Grounding Transformer Transient Analysis Caused by Earth Fault Clearance in a Subtransmission Substation and Proposing Appropriate Solutions

Power transformers used in the subtransmission substations of the Iranian power grid have YNd connection. In addition, a grounding transformer with YNd connection is used to create an artificial neutral on the medium voltage side. The phase to ground capacitor may have a high value in the medium voltage network. In some conditions, such as interruption of the phase-to-ground fault current in one of the output feeders, a kind of resonance is created in the network that not only damages the grounding transformer but also causes protection problems related to the misalignment of ground fault relays. In this paper, the resonance between the grounding transformer and the phase-to-ground capacitor of the medium voltage output feeders of a sample subtransmission substation is investigated, considering that the phase-to-ground fault is interrupted. In this regard, based on available currents and voltages, several indicators are defined to adjust the surge arrester and other elements of the subtransmission network. In addition, appropriate solutions are presented in order to damp the aforementioned resonance and avoid unwanted challenges.