Study On Reignition Monitor Of Vacuum Circuit Breaker As Switching Shunt Capacitor Banks

The application of vacuum circuit breaker in medium voltage systems and their powerdistribution net works is getting more and more popular. And its performance has beenimproved significantly in past decades. But ignition over voltage produced when itswitches on or off the capacitor group can be harmful. Such over-voltages could severelythreaten the safety of shunt devices and the whole system, the life of capacitors and theinsulating component of the system. To avoid this problem, thyristor is mainly adopted byother developed countries to switch capacitor banks. Never the less, the control system ofthyristoris complicated and the price is high, which could be approximately20to30timesthe price of a vacuum switch. Chinese scholars developed two schemes when usingvacuum switches, namely, protection program and prevention program. Even though withthe most comprehensive protective and preventive measures, it is still unavoidable thatreignition occurs. Thus, real-time monitoring and analysis of the switches of vacuumbreakers is necessary.Based on the principle that reignition certain to be accompanied by over voltages, inthis thesis it is proposed of an on-line monitoring scheme including the monitoring of arcvoltage and current during the process of the vacuum circuit breaker switching capacitorbanks. However, in practice, it’s a challenge to detect the transient capacitance voltage atthe fracture of vacuum breakers. And at the side of the breaker connecting with the circuitsystem, the contact voltage changes along with the power grid. On the other side, thevoltage changes of the capacitor group-to-ground can be used to represent the capacitancevoltage changes at the fracture of breaker. According to such an idea, the voltage borne byits insulating part was obtained directly when the vacuum circuit breaker switchedcapacitor banks, which is diversified the content of the on-line monitoring scheme in thisthesis.Firstly, by the simplified model of the35kV system and circuit equations, the modelof circuit breaker resecting capacitor was established with the help of EMTDC/PSCAD.Secondly, different scenarios were simulated and the failure conditions were analyzed inthe vacuum circuit breaker, including the single-phase reignition with synchronous openingof the breaker, the two-phase reignition with synchronous opening of the breaker, thesingle-phase reignition with asynchronous opening of the breaker, and the two-phasereignition with asynchronous opening of the breaker. Then, all the four scenarios werecompared with the no-reignition scenario. Thirdly, the features of the voltage waveformswere concluded during the reignition caused by circuit breaker switching capacitor banks.Some of the important parameters were analyzed theoretically as well, i.e., the size andfrequency of inrush current. Based on the above analysis, the evidences for on-linediagnosis were determined where failure occurred during vacuum circuit breaker switched capacitor banks. Furthermore, in this thesis, the wave record system was completedcombined with the actual situation of the high pressure vacuum switchgear. And by meansof the FFT analysis, the frequency and the peak of inrush current were calculated.Meanwhile, the parameters concerning the overvoltage and the time when the breakerswitched on/off were also determined. Then the final reliable identification of the reignitionwas obtained. Finally, field measurements were carried out at different medium to lowvoltage level systems to verify its usefulness of the on-line monitoring instrument. And theportability in field was also checked. In conclusion, the proposed on-line monitoringscheme in this thesis was proved to be feasible.