Calculation And Simulation Of Overvoltage On Neutral Point Of 110kV And 220kV Power Transformers And Its Protection Exploration

Currently, the transformers in our 110kV and 220kV system run part of the neutral point grounding way.The overvoltage protection of Neutral point in transformers adopts measures such as arresters and parallel discharge gap,but is found insulation coordination disorders in field operation. When there is a single-phase earth fault in effective grounding system,the gap false breakdown is the most common,resulting in expanded scale power outages.Arrestor explosions and equipment insulation damage caused by rod gap tripping are also happened.Therefore,further research on neutral grounding of transformer is needed to ensure the safe and reliable operation of the power system.In this paper,two methods are discussed in depth,the first one is neutral point grounding of the transformer via small reactance.The other is parameters tuning based on current neutral grounding mode and some suggestions on gap improving is also given.Grounding via a small reactance can avoid’no grounding’in effective grounding system,limit short-circuit current of the neutral point of the transformer,and lower the damage caused by overvoltage to the system insulation.In this mode,neutral overvoltage protection can be done individually by zinc oxide arresters, without the need of gap,therefore the risks from gap can effectively avoided.Currently 500kV autotransformer’s neutral grounding via small reactance run well in field operation,while 110kV and 220kV ones are still on theoretical complement and field trials stage,the view differs in different literature.therefore,further research is necessary.In this paper, the zero sequence impedance changes of transformer and system after the access of small reactor is studied,and the impact on the short-circuit current oft he neutral point, system zero-sequence protection and neutral insulation level of transformer are analyzed. Small reactance grounding devices were designed and it is recommended small ranges reactance transformer zero sequence impedance of 1/3.This paper focuses on improving and optimizing the part grounding mode of current transformers.After analysis of the cause of many accidents,we find the parameters of the neutral protection should be based on the actual over-voltage of transformer neutral, but current reference of arrester and gap tuning is standard lightning wave and power frequency voltage discharge voltage, resulting in poor protection of neutral insulation from gaps and arrester.Therefore, under the considerations of operation conditions, the lightning overvoltage and single phase power frequency overvoltage waveform of the neutral point is simulated in EMTP simulation software, the conclusion is used for arrester and gap parameters tuning; however there is still poor distance adjustment and concentricity in rod gap,once there is a electric arc in gap, electrode ablation occurs, the distance changes,the frequency of the use of gap lowers, the paper also presents own proposals to the improvement of gap.