Thyristor voltage equalising network for crowbar application

Abstract

Many high-voltage applications are realised with series connected thyristors. Voltage imbalance among series connected thyristors during steady state as well as in transients is one of the major concerns. This voltage imbalance is mitigated by using static and dynamic balancing network. Dynamic balancing networks are typically designed based on reverse recovery charge of the thyristor during turn-off, which suits many applications. But this is not the case for a crowbar application, where turn-off of the thyristor is not a major circuit constraint. This study proposes the design method for dynamic balancing network considering gate turn-on delay time and the balancing network component tolerances. The study derives two models for the dynamic balancing network based on its charge-discharge cycle. The importance of charge-discharge cycle in the design of dynamic balancing network during high di/dt operation is emphasised. Influence of dynamic balancing resistance and crowbar current limiting inductance on voltage imbalance, charging current and discharging current is studied using the analytical model. The proposed design method also offers flexibility to incorporate differences in propagation delays among the thyristor drivers that are used to trigger individual thyristors. Such delays cannot be directly incorporated in the conventional balancing network design method based on reverse recovery. Further, it is also analytically shown that designing the dynamic balancing network based on reverse recovery charge makes the balancing network lossy and bulky for crowbar application. Simulation studies and experimental results on a 12 kV, 1 kA crowbar consisting of six series connected thyristors confirm the theoretical analysis and validate the proposed approach for crowbar applications.