Overvoltage Analysis And Protection Of High Frequency Discharge Thyristor For DC Circuit Breakers

As an important power device,thyristor is widely used in high voltage DC circuit breaker.Thyristor generally works in LC discharge circuit of high frequency.Under the working condition of high voltage DC circuit breaker,the thyristor is subject to high voltage and large current on and off.Once the voltage exceeds the breakdown voltage,the thyristor will be damaged,resulting in the equipment can not operate normally.This requires more stringent voltage equalized in the thyristor,and the research of voltage limiting means is particularly important.In the actual circuit,in order to withstand higher voltage,more than one thyristor series form,so that the thyristor in the opening,closing and reverse recovery process will appear very high overvoltage phenomenon.The existing overvoltage suppression measures are to parallel RC buffer circuit or parallel gap arrester at both ends of the thyristor.However,the buffer circuit has some problems such as insufficient operation time,and the parallel arrester will reduce the circuit resistance,resulting in too large protection margin and lower voltage utilization rate.Based on the high speed discharge circuit of high voltage DC circuit breaker,the electric stress of thyristor at the moment of opening and closing is simulated.The causes of uneven voltage and overvoltage are analyzed theoretically,and the limiting measures of parallel gap arrester at both ends of thyristor are proposed.Finally,an equivalent experimental circuit is built,and the experiments verify that the shunt gap arrester can effectively limit the phenomenon of uneven voltage and overvoltage,and improve the voltage utilization rate.Firstly,the high speed discharge circuit containing thyristor in hybrid high voltage DC circuit breaker,coupled negative voltage DC circuit breaker and fast mechanical switch is analyzed.According to the simulation waveform,the electrical stress received by the thyristor is analyzed,and then summed up into two kinds of simplified equivalent circuits.Secondly,the mechanism model of thyristor with rated voltage of 5200V and the equivalent circuit model of two basic high speed discharge circuits are established.Firstly,the mechanism model of thyristor is simulated and verified,and the change rule of carrier in thyristor under various working conditions is analyzed.After that,the phenomenon of thyristor overvoltage in high speed discharge circuit and its causes are analyzed.Finally,in order to solve the phenomenon of overvoltage at both ends of thyristor,the overvoltage limitation measure of shunt gap arrester is put forward.The gap arrester was adjusted and configured in the laboratory,and the equivalent experiment was designed.In the experiment,the overvoltage phenomenon of two basic circuits was reproduced without parallel arrester,which was basically consistent with the simulation results.Then the gap arrester was connected in parallel at both ends of the thyristor,and the experiment was conducted again.Through analysis,it can be concluded that the gap arrester played a good role in limiting overvoltage,and compared with the direct shunt MOV voltage limiting measure,the parallel gap arrester increased the voltage utilization rate from less than 70%to 84%.