Analysis on Arresters as the Protection of Secondary Winding Neutral-point of Voltage Transformer

In order to define the necessity and the technical qualification of the overvoltage protection of secondary winding neutral point of voltage transformer,both of the impact transfer voltage test in the lab and the numerical simulation analysis based on the ATP software were carried out to study the lightning transfer in the neutral point of the secondary winding and potential difference between two points of ground network under the situation of lightning invasion,direct lightning stroke and power frequency grounding fault.As a result,overvoltage level of lightning transmission at the neutral point of the secondary winding,and the potential difference between neutral points of secondary windings under both transient and steady-state situations are determined.The transfer overvoltage in secondary circuit of voltage transformer especially in capacitive one will largely exceed the lightning impulse level of the neutral point in secondary circuit when the amplitude of lightning overvoltage is high or the rise time is fast.It is necessary to install ZnO arrester on site to protect the transformer from the overvoltage of the neutral point of the secondary winding.Six kinds of developed ZnO arresters are installed in neutral point of secondary winding to test the protection effectiveness through the the impact transfer voltage test in the lab and the numerical simulation analysis.The results show it is an effective protection in the neutral point insulation of secondary even in a worst situation with large amplitude and steep waveform of overvoltage.