Cause Analysis of Sent-in-Error Thyristor Protective Trigger on HVDC

On HVDC open-line, valve control monitoring forward voltage of thyristors is prone to send protective trigger in error and cause DC block. For this case, this paper analyses operating condition of open HVDC line and principle of protective trigger. Taking an actual 800kV HVDC as example, cause analysis is carried out. The causes and conditions of sent-in-error valve control protective trigger are proposed. Affected by waveform of actual voltage, near the point where thyristor voltage phase angle is 180°, valve electronics are prone to send built voltage pulse signal when receiving trigger pulse signal. And valve interface electronics would take this signal as protective trigger pulse signalin mistake. Because of the limit of trigger delay angle by converter unit control, the valve electronics would not receive the trigger pulse signal near the point where the thyristor voltage phase angle is 180° on non-open HVDC line. The case of sent-in-error valve control protective trigger would only happen on open HVDC line. Experimental test of the valve control was carried out. Fault phenomenon and experimental test result are consistent with the proposed causes and conditions.