Study on Commutation Overshoots Based on a Novel Model for Series Thyristors During the Turn-off Process

The commutation overshoot is an important reference index for the insulation coordination of the converter station as well as the thyristor voltage capacity. Although the dispersity of reverse recovery charges of series thyristors may affect commutation overshoots, especially those at the thyristor level, little literature has tried to analyse it in detail. Firstly, this study compares the turn-off models of independent thyristor and elaborates their applicability to series thyristors. Then, a novel model for series thyristors during the turn-off process is proposed, based on which the turn-off model of converter valve is also established. Subsequently, the validity and efficiency of the proposed model are discussed by simulations and it is applied to the analysis of commutation overshoots. The analysis shows that though the average model can be used to estimate the valve voltage stresses, it may lead to large errors in the thyristor voltage stress. And selecting the 1:n − 1 distribution as the distribution mode of reverse recovery charge in series thyristors will result in very conservative results.