Investigation on no-load mechanical endurance and electrical degradation of a circuit breaker model under short circuit current interruption

摘要

The demand on reducing circuit breaker failure, extending service life, increasing equipment reliability and lowering the related operating and maintenance costs are nowadays of prime importance for the electricity supply network. According to statistics, the major failure mode is originated from operating mechanism, whereas interrupter has the highest percentage of failure in high voltage component. Thus, the method to facilitate the assessment of internal mechanical conditions by using commercially available devices without opening major parts and the experimental investigation on electrical erosion inside the interrupter were performed in this work.The experimental investigation on the mechanical endurance of operating mechanism up to 6,000 no-load switching was performed with 110kV puffer type circuit breakers due to its requirement of large driving energy for hydraulic operating mechanism. The conventional measurement, e.g. coil current, contact timing, dynamic contact resistance and pumping motor current as well as additional vibration technique were carried out in both laboratory and field test. The investigation on electrical erosion focusing on aging of nozzle and electrode under current interruption was performed with a reduced-size SF6 self-blast circuit breaker model and synthetic test circuit, providing the current density comparable to that in the real circuit breaker, because of the extensive use of nozzle ablation, the possibility for parameters variation and the reduction of cost and time.For mechanical investigation, the parameters were measured at the beginning as a reference and also thereafter every 500 switching under normal and variable operating conditions, e.g. variation of auxiliary supply voltage, hydraulic oil pressure and SF6 gas pressure. The test result was verified by that from two of the same type circuit breakers and the theoretical calculation for vibration result in the frequency-domain analysis. Subsequently, the on-site measurement before and after maintenance and some simulated defects were executed. The electrical aging investigation was performed by successive tests with constant energy stresses until interrupting failure occurred. The effect of current amplitude, nozzle diameter, expansion volume, arcing time and recovery voltage on mass losses, geometry changes of nozzle and its influence on pressure build-up were investigated.The analysis of vibration signals in a combination with the other supervised parameters provides the useful information regarding the significant events occurring inside during circuit breaker operation, e.g. motion start of mechanism and within pole, contact touching and motion stop. The deviations of these measured values from the reference according to number of no-load switching operations, variable operating conditions and some incorporated defects were comprehensively determined. The relationship between the gradual changes in technical status of the supervised components and the number of no-load operations is helpful for selecting the proper maintenance method.Regarding the electrical erosion of the nozzle and the electrodes, the mass losses per switching and per energy; and the geometry change of nozzle especially between the arcing column and the expansion chamber were investigated. Moreover, volume loss and mass loss along the nozzle column; its effect on the pressure build-up reduction; and the dependences of mass losses on interrupting current amplitude, charge and generated heat have been determined.Furthermore by applying the wear equivalent law, the allowable number of interruption in function of the breaking current to rated short-circuit current ratio and the remaining lifetime can be determined. This result will be used to verify the accuracy of simulation tools in order to visualize the aging process, to evaluate the interrupter condition and to minimize the expensive experiment tests.