Dynamic Avalanche and Current Filaments in Voltage Commutated Phase Control Thyristors

The operation of silicon 8.5 kV phase control thyristor under fast voltage commutation is shown to lead to dynamic avalanche and current filamentation. The measured waveforms are analyzed using device simulation to demonstrate the origin and dynamics of current filaments. The operation of thyristors under study is investigated for half sine current pulses between 40 and 1100 μs with amplitudes from 200 A to the order of tens of kiloamperes. The subsequent voltage commutation with reverse voltage of several kilovolts is studied for the commutation di/dt between 10 and 1000A/μs. Such conditions may occur, for example, during fault cases in the HVDC systems.