Calculation and Analysis of Transformer Transient Over-Voltage in no-Load Switching

Transformer no-load closing may result in various types of over-voltage, which may affect the stability of power system. To analyze this problem, a 500kV transformer no-load closing model in Bailianhe Pumped Storage Power Station is built by EMTP/ATP, the closing over-voltage on primary equipment is calculated, and the effect of closing angle on over-voltage value is analyzed. Meanwhile, the cable sheath induced voltage in transient process is analyzed. The results show that when three-phase are simultaneously closed and no resonance occurs, no-load closing over-voltage is small, and closing angle has larger impact on the overvoltage, the overvoltage multiple of transformer on low-voltage side is generally greater than that on high-voltage side. In the transformer no-load closing process, transient current of cable core is coupled to the outer sheath by inducing, which results in inducted voltage, and may cause sheath voltage in ungrounded part exceeding the insulation withstand level.