| Circuit breaker is the most important control and protection equipment in the power system,which plays a vital role in the safe operation of the power system.The SF6 circuit breakers have been widely used in the power system due to the excellent insulation and arc cooling performance.Electrical lifetime of SF6 circuit breaker is one of the main factors affecting the service life.While SF6 circuit breaker breaking current,the arc on the contact can produce the ablation which make the electrical lifetime of contact degraded gradually.Once circuit breaker is used beyond the life range,circuit opening failure will be easy to occur,and the operation stability and power system security will be threatened.Therefore,in-depth study on breaking current process of SF6 circuit breakers and accurate assessment on its electrical life can provide a reference basis for condition-based maintenance of SF6 circuit breakers timely,so that SF6 circuit breakers can maintain a good electrical lifetime for operation.The conclusions of this paper have a great significance to guarantee the power system reliability.In this paper,the changes of the contact state parameters and the current commutation time are analyzed during the process of electrical lifetime degrading with the degree of arc ablation on contacts.The relationship between the current commutation process of contact and the electrical lifetime degradation is obtained by analyzing the degree of contact degradation and influence of the contact degradation of on the current commutation process.Then the threshold values of electrical lifetime assessment parameters of the circuit breaker are proposed.Firstly,the ablation experiment of 126kV and 40.5kV SF6 circuit breakers were carried out with 50Hz large current,which simulate the process of actual operation that SF6 circuit breakers lifetime degrade from the excellent state to complete failure.Four sets of complete SF6 circuit breaker state parameters from the dynamic resistance curves were obtained,such as the main contact resistance,arc contact resistance,contact over-travel and overlapping time which change with the contact degradation.It is found that the resistances of main contact and arc contact slightly increase with the increase of the number of breaking current when the contacts are in good condition.After the contacts are seriously ablated,the resistances of main and arc contact increase rapidly.At the same time,SF6 circuit breaker contact travel and overlapping time reduce with the number of breaking current increases,and the greater the breaking current is,the faster the contact travel and overlapping time reduce.Based on the lifetime degradation test of circuit breaker,the simulation model of current commutation process is established by using MATLAB software.Then the influence of contact state degradation on current commutation process is simulated.Results show that the current commutation time of SF6 circuit breaker increases with the increase of main and arc contact resistances.The main contact resistance has little influence on the current commutation time,but the arc contact resistance has a significant influence on the value.When the state of the circuit breaker contacts keeps good,the current commutation time increases slowly with the increase of the degree of ablation.When the main and arc contacts resistances increase steeply,the current commutation increases steeply almost at the same time,and the commutation time is longer than overlapping time in the end.This study shows that the current commutation time of contact depends on the degree of electrical lifetime degradation,and has a direct contact with the circuit breaker which breaking the short-circuit current successfully next time.According to the SF6 electrical lifetime degradation experiment and the current commutation process simulation calculation,this paper finally puts forward four basis for evaluating the good lifetime state of SF6 circuit breaker:the main contact resistance is less than 100??;arc contact resistance is less than 400??;contact over-travel is greater than the half of initial value;overlapping time is greater than the two thirds of the initial value.If there are two or more of the above conditions are not satisfied at the same time,SF6 circuit breaker contacts have been ablated severely.The electrical lifetime of SF6 circuit breaker is in complete failure when the contact current commutation time is greater than the overlapping time. |
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