Research On Erosion Of Arcing Contacts In Making Process In SF₆ Circuit Breakers Used For Capacitor Banks

Because inrush current in making process causes arc ablation and mechanical wear which cause serious arcing contacts erosion of SF6 circuit breakers used for switching capacitor banks,thus shortening the electrical life of circuit breakers,especially when the inrush current of circuit breakers of switching capacitor banks used for UHV AC system reaches 10kA and frequent operations cause serious erosion of arcing contacts in the making process.So far,circuit breakers produced by domestic and foreign manufacturers can only reach about 1000 times of opening and closing,which is far from the electrical life requirement of 3000 times.Therefore there is an urgent requirement to conduct research on arcing contacts erosion of circuit breakers used for switching capacitor banks.Firstly this paper researches the mass loss characteristics of the arcing contacts based on experiments and then focuses on the process of wear behavior.The mathimatical models of closing erosion are set up.The shape of the arcing contacts is optimized and the detection method of arcing contacts after erosion is studied.The main contents and conclusions are as follows:A test platform for mass loss of arcing caontact erosion in making process was established which included a test circuit of erosion experiment in the making process and a test circuit of arc ablation.By controlling the arcing time and arc current of the two tests,pre-strick arcing time of the erosion experiment in making process was equivalent to the arcing time of the arc ablation test.The mass loss of mechanical wear which was the difference between the mass loss of the two experiments mentioned above were obtained in the erosion experiment in the making process.Therefore,studies were conducted on the arc erosion and mechanical wear of arcing contacts respectively.More than 1000 tests were carried out on the arcing contacts with different interference radius and different test current to obtain how the arc erosion and mechanical wear mass loss of the arcing contacts vary with the interference radius and current.It was found that the mass loss caused by mechanical wear accounted for more than 72.6%of the total mass loss.The mechanical wear was the main cause of mass loss of the arcing contacts.Since mechanical wear is the main reason for mass loss of arcing contacts,an analysis was made of the mechanical wear bahavior process of arcing contacts based on Rabinowics wear theory.Firstly contact stress of the moving and static arcing contacts in making process was calculated.It was found that the maximum equivalent stress of arcing contacts was less than the yield strength of W at normal temperature,which can not meet the necessary requirement for adhesive wear.The wear behavior of arcing contacts were mainly abrasive wear at normal temperature.By simulating the change of physical parameters of arcing contact material at high temperature,it was found that theyield strength of Cu and W decreases significantly at high temperature and is less than the maximum equivalent stress of arcing contacts.The grinding radius for meeting adhesive wear increases with temperature,but is still less than partical radius tested by EDS energy spectrum.Both the yield strength and particle radius meet conditions of adhesive wear.It is indicated that arcing contacts will have adhesive wear behavior in the center of arc ablation,which will aggravate the mechanical wear of arcing contacts.The wear behavior gradually changes from adhesive wear to abrasive wear along with the increase of distance from the area of arc ablation.The analysis of wear behavior provides a basis for quantitative study of arcing contacts erosion in making process.Based on characteristics of wear behavior and Archard wear theory,a mathematical model of mass loss was established to characterize the arc erosion area,the transition area and area away from the center of arc ablation.The quantitative calculation of the erosion mass loss in the making process was realized in combination with mass loss model of J.Tepper arc ablation.A mathematical model for erosion shape of arcing contacts was established,which reflected the relationship between mass loss and axial contact travel loss and radial interference radius loss.The model calculation shows that the larger the interference radius is,the smaller the axial contact travel loss and radial interference radius loss of arcing contacts are,but there is a trend of saturation.The interference radius of the arcing contacts should be greater than 1 mm in order to achieve the goal of 3000 times closing operation of circuit breaker with closing current of 10kA.The detection method of arcing contact erosion condition based on dynamic contact resistance was proposed.The dynamic change curve of contact resistance was used to retrieve the axial erosion of the arcing contacts,and the average value of dynamic contact resistance were used to reflect the radial erosion.The arcing contact erosion condition was detected without disassembling circuit breakers.A dynamic contact resistance measurement instrument was developed.The measurement current should be greater than 1000A.It provided a way to detect the condition of arcing contacts of circuit breakers,and was applied to field test successfully.In conclusion,the main innnovations of this thesis are as follows:1)The establishement of a test platform for mass loss of circuit breaker erosion test in making process enables the mass loss of mechanical wear which is the difference between the mass loss of the two experiments mentioned above to seperate from arc ablation in the erosion experiment in the making process and helps to obtain the mass loss characteristic of arcing contacts of SF6 circuit breaker used for capacitor banks in the making process.It is revealed that main cause of closing erosion is mechanical wear,and the mass loss caused by it accounts for more than 72.6%of the total mass loss.2)The wear behavior process of arcing contacts was revealed.Wear behvior at room temperature is abrasive wear.Wear behvior at high temperature is adhesive wear in the area of arc ablation center,which will aggravate the mechanical wear erosion of arcing contacts.The wear behavior gradually changes from adhesive wear to abrasive wear along with from the increase of distance from the area of arc ablation.The analysis of wear behavior provides a basis for quantitative study of arcing contacts erosion in making process.3)A mathematical model of mass loss and a mathematical model of erosion shape of arcing contacts were established.The quantitative calculation of erosion process of arcing contacts is realized.An analysis was made of the quantitative relationship of the contact travel and the interference radius of arcing contacts with change of closing times.The interference radius of the arcing contacts should be greater than 1mm in order to achieve the goal of 3000 times closing operation of circuit breaker with closing current of 10kA.4)Proposals were made to detect arcing contact condition without disassembling circuit breakers based on dynamic contact resistance.It provides a way to detect the condition of arcing contacts of circuit breakers,and has been applied to field test successfully.