Analysis And Protection Of Neutral Point Overvoltage In 220kV And 110kV Power Systems

Most 220 kV substations use part transformer neutral grounding operation mode in China.While 110 kV substations use both part transformer neutral grounding operation mode and all transformer neutral ungrounded operation mode.For a transformer with neutral point ungrounded,when the line meets errors,such as asymmetric short circuit of transmission line,non-full-operating and lightning strike,the original voltage balance of the three-phase is broken,and the point will generate severe overvoltage,which will damage the normal and stable working of the power system.The current measure for the protection of ungrounded neutral points is paralleling protection gaps and lightning arresters.When grounding fault happens and the system is ungrounded,the clearance works.When grounding fault happens and the system is still grounded,the clearance does not work.When the lightning wave intrudes into substation,the lightning arrester works.In actual project,the selection of the length of protection clearance and the type of the arrester is often carried out according to experience.However,because of the complexity of every system and the randomness of the action of protection gap,it is difficult to cooperate effectively between them,which leads to protective clearance's malfunction and lightning arrester's explosion.To solve the above problems,this article build typical 220 kV and 110 kV systems by using power system simulation software such as ATP-EMTP and PSCAD/EMTDC.Based on the simulation model,the way to select clearance and arrester in actual projects is given.This article also takes a real accident of protective clearance malfunctions as an example.It is pointed out that by properly setting operation time of zero sequence current protection and line protection action,the difficulty of neutral point protection gap's coordination with lightning arrester can be well solved.