Neutral Point Overvoltages in Wye-Wye Connected Distribution Transformer Caused by Lightning Current in Low-Voltage Winding

Lightning strokes are a main source of transformer failures in distribution systems. The discharge of a lightning current through a distribution transformer's high-voltage (HV) side arresters leads to a ground potential rise of the transformer tank. In the case of a wye-wye connected transformer with ungrounded HV neutral point, the ground potential rise can drive a current through the low-voltage (LV) side windings that leads to an excessive voltage in the HV side neutral point by inductive transformation. Simulations by a measurement-based model demonstrate that a voltage several times the lightning impulse test voltage can arise in the neutral point, in the case of a close lightning strike to the HV side overhead line. The use of transformer LV side surge arresters in combination with proper grounding on the HV-side are effective ways of reducing the neutral point overvoltage.