| Thyristor,as the core equipment of DC transmission converter valve,suffers from performance degradation during long-term operation,which may lead to system failure in serious cases.The reverse recovery charge is a key parameter of thyristor performance degradation,which not only affects the series dynamic characteristics of the converter valve itself,but also has a high correlation with other degradation parameters.However,in practice,it is challenging to keep track of the current of a large number of thyristors in a converter valve,so it is difficult to measure the reverse recovery charge.In this paper,we propose an online monitoring method for thyristor reverse recovery characteristics of converter valves,and conduct research around three aspects:online measurement of reverse recovery charge,evaluation of reverse recovery characteristics based on charge measurement,and experimental validation of the measurement method.First,the converter valve phase-change process and turn-off equivalent circuit are analyzed,and the turn-off process of the converter valve thyristor is analyzed in detail by combining the reverse recovery current-voltage waveform and current flow path.The dynamic current-voltage correlation of the converter valve series thyristor is revealed,the correlation between the series dynamic characteristics and the reverse recovery charge is analyzed,the online measurement method of the converter valve thyristor reverse recovery charge based on the turn-off voltage is proposed,and the implementation steps of the measurement method are introduced.The proposed measurement method requires only one additional current sensor,which is more economical and feasible than the traditional method.Secondly,the external characteristics of the converter valve operation are analyzed,and the thyristor reverse recovery charge model based on multiple influencing factors is established,and the validity of the model is verified by using the charge curves provided in the datasheet.Based on the analysis of the external characteristics of the converter valve and the charge model,the reverse recovery charge normalization coefficient is derived,and the charge measurement values under different operation modes are normalized.A method for evaluating the reverse recovery characteristics of the thyristor is proposed,and the implementation steps of the evaluation method are introduced.Finally,to verify the feasibility of the proposed measurement method,a thyristor turn-off experimental platform is built and an experimental scheme is designed.Two thyristor shutdown experiments and series thyristor shutdown experiments are conducted to measure the reverse recovery current and voltage waveforms,and the proposed measurement method is used to calculate the reverse recovery charge,and the results show that the error of reverse recovery charge of the proposed method is only 0.1%,and the error of reverse recovery charge difference is only 0.3%.On this basis,the effects of capacitor aging,current sensor accuracy and voltage sensor accuracy on the measurement method are further analyzed,and the error results are within 10.7%. |
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