A Method to Detect the Deionization Margin Angle and to Prevent the Commutation Failure of an Inverter for DC Transmission

A method is suggested to detect directly the deionization margin angle and to prevent the commutation failure of an inverter. The deionization margin angle is detected by anode- cathode voltage of a valve and ac phase voltage. The method to prevent the commutation failure, namely the one to hold the critical deionization angle keeps a necessary deionization angle for the alternating voltage drop, thus enabling safer valve operation.