Reliability of Metal Oxide Varistors Under Lightning Overvoltages
Xiaoqing Zhang · Gao dianya jishu · 2013
To predict the reliability of metal oxide varistors(MOVs) under lightning overvoltage,a new assessment model for MOV was established based on Markov process.The Markov process was used for analyzing the transfer behavior of operating MOV in different states.The computational formula of the mean time to failure(MTTF) for MOV under lightning impulses was derived.Based on this formula,a realistic model was set up for predicting the reliability of MOVs.By simulating lightning transient of a distribution system,the lightning current probability was calculated in three operation states,and the reliability and lifetime of MOVs in different conditions was also estimated.For a 34S621 MOV which has a conversion rate of 1/10,a rated current capability of 40 kA,and a rated breakdown voltage of 620 V,when the conversion rate changes to 1/50,the breakdown voltage and the current capability are selected to be 910 V and 40 kA,respectively,the lifetime of the MOV will be more than 4,5,and 3 years,respectively.It is concluded that the lifetime of MOV is inversely proportional to the conversion rate,and it is directly proportional to the breakdown voltage and current capability.