陕西省国华锦界电厂低压配电柜中400V断路器自投运以来发生两起短路烧毁事故。根据现场情况详细分析，事故原因是低压系统出现了过电压。断路器是系统中重要设备，若能从原理上掌握400V母线过电压产生的原因及防治方法，可以对此类事故进行有效防范，提高系统运行的安全性。 本文通过现场测试和仿真实验相结合的方法，对过电压产生原因进行了详细分析，并提出了相应的限制措施。 系统中有大量风机负荷且均采用变频器控制，变频器的使用会在系统引入谐波。对400V母线电压、电流进行测量，发现波形畸变严重，谐波电压电流、最大畸变率分别为10.46%、25.82%。。主要谐波次数为5次、7次、11次、13次等，谐波含量严重超标。若系统的电感、电容参数刚好和某次谐波频率一致，就会出现谐振过电压，这是母线出现过电压一方面的原因。在每台变压器下安装2×BLUEWAVE 100-400-3有源滤波装置1套，波形畸变率变小符合国标要求。 本系统400V母线上装有电磁型电压互感器（PT），且故障发生在断路器合闸时，这种情况下会激发铁磁谐振过电压，这是母线出现过电压另一方面原因。本文利用电磁暂态程序（EMTP）对一个典型配电系统进行仿真计算，发现单相接地故障消失易激发分频谐振，间隙性弧光接地可激发较严重的谐振；PT高压侧中性点经非线性电阻或消谐器接地以及4PT法是抑制谐振的有效措施，但应注意使非线性电阻或消谐器的热容量满足要求。

The circuit breakers of 400V voltage class in Guohua Jinjie power plant have been distroied by short circuit since its running. Based on the dissectional analysis of the accident, it can be known that the short circuit is caused by some kind of over-voltages. Circuit breakers are of great important in power system, to find the reasons of over-voltages can keep away such accident and improve the security of the whole power system. The reasons caused the over-voltages are studied detailedly in this paper by field testing and computer simulating experiment. The protective measures are also studied. The transducers which control the exhaust fans in the plant caused resonance. The testing results show that the voltage and current Total Harmonics Distortions(THD) are 10.46% and 25.82% respectively. The fifth, seventh, eleventh and thirteenth harmonics exceed the correlative standard. If the parameters of capacitance and inductance match the frequence of some harmonics, resonance over-voltages can be preduce. To install active power filters in each transformers can limite the THD within permited. Most of the ferroresonance caused by saturation of PT in distribution network could hapen affter the disappearance of single-phase ground fault and interval arc grounding. Connecting a nonlinear resistance or resonance eliminator between the neutral point on high voltage side of PT and 4PT connection are effective measures to eliminate or suppress ferroresonance, the heat capacity of nonlinear resistance or resonance eliminator should meet the requirement.