Time discreteness of commutation failure in HVDC transmission

Commutation failure caused by AC system failure is most common phenomenon of AC/DC dynamic interaction. The research about the DC commutation failure simply caused by AC system failure at a certain moment may not accurately reflect the complexity of HVDC commutation failure. This paper points out that the commutation voltage amplitude and phase angle and the closing time of fault are the main factors of commutation failure time discreteness in the HVDC transmission system, from the perspective of the physical nature of valve commutation process and the commutation voltage time area. Then, theoretically analyzed the features of the three-phase fault and single phase fault occur on AC bus, the paper concludes the change regularity of line voltage amplitude and line voltage phase which after Y-Y and Y-D1 transformer along with the change of transition resistance. Finally, Using PSCAD/EMTDC, the typical bipolar 12 Pulses DC system model with detailed control system is set up. The regularity of critical transition resistance in different fault moment of two fault types is analyzed to conclude the influence factors and regularity of commutation failure time discreteness in the HVDC transmission system. The results show that the most unfavorable condition to the commutation is when the AC system fault time corresponds to the pulse triggering time of commutation valve.