Modeling of Reverse Recovery Process of Series Thyristor Based on Single Thyristor Level Circuit Topology and Study of Commutation Over-voltage

In high-voltage direct-current (HVDC) converters, series-connected thyristors are usually adopted as switching devices, which produce commutation overshoots at the end of commutation processes. The commutation overvoltage is an important reference index for the insulation co-ordination of the converter station as well as the thyristor voltage capacity. Although the dispersity of reverse recovery charges of thyristors may affect the commutation overvoltage, especially the reverse recovery voltages of thyristors, the literatures detailing the issue are seldomly available. Firstly, we compared common reverse recovery models of an independent thyristor and analyzed their applicability to the modeling of series-connected thyristors. Then, we proposed an equivalent model of series-connected thyristors, based on which the turn-off model of a converter valve was also established. Subsequently, the validity of the proposed model was verified by simulations, and it was applied to the analysis of commutation overvoltage. The results show that although the uniform model can be used for estimating the valve voltage stresses, it may lead to great errors when used for that of thyristors. And setting the reverse recovery charge distribution mode as 1:n-1 can lead to excessive safety margin, which may reduce the economy of the converter valve. © 2020, High Voltage Engineering Editorial Department of CEPRI. All right reserved.