Analysis Of The Time Discreteness On HVDC Commutation Failure

In the AC/DC interconnected power grid, commutation failure is an importantform of AC/DC dynamic interaction, and the root problem of the stable operation ofAC/DC interconnected system. Among them, the DC commutation failure caused byAC system failure is most common. Generally, commutation failure can restore on itsown, but the secondary commutation failure and the continuous commutation failurecould easily lead to the DC system latch, which gives a tremendous impact on the grid,and even a direct threat to the safe and stable operation of power grid.Field operation experience shows that the AC system failure time is random, andthe influence on DC system is quite different, when the same type fault occurs in thesame location, characterized as discreteness. The research about the DC commutationfailure simply caused by AC system failure at a certain moment can not accuratelyreflect the complexity of DC commutation failure in the AC/DC interconnectedsystem.To address this problem, this paper points out that the commutation voltageamplitude and phase angle, the closing time of fault and the function of control systemare the main factors of commutation failure time discreteness in the HVDCtransmission system, from the perspective of the physical nature of valvecommutation process and the commutation voltage time area. Then, theoreticallyanalyzed the features of the three-phase fault, single phase fault, phase to phase faultand phase to ground fault occur on AC bus, the paper concludes the change regularityof line voltage amplitude and line voltage phase which after Y-Y and Y-D1transformer along with the change of transition resistance. Finally, UsingPSCAD/EMTDC, the typical bipolar12Pulse DC system model with detailed controlsystem is set up. The regularity of critical transition resistance in different faultmoment of different fault types and system capacity is analyzed to conclude theinfluence factors and regularity of commutation failure time discreteness in theHVDC transmission system.The results show that the commutation is the most unfavorable, when the ACsystem fault time corresponds to the pulse triggering time of commutation valve. Inaddition, it will be very different when fault types are different. In the practical engineering simulation, appropriate fault closing moments are selected according tothe actual situation to draw more accurate conclusions.This work is supported by the the National High Technology Research andDevelopment Program (“863” Program) of China (No.2012AA050209)， theNational Science Foundation of China (51377059) and the Science Foundation ofGuangdong Province (S2013010012885). Some of the research results have beenapplied to the operation of actual HVDC systems, obtaining great economic and socialbenefit.