Influence Of High-voltage Dc Commutation Failure On Sending-side Grid Voltage

With the development of ultra-high voltage direct current(UHVDC)transmission technology and a large number of DC transmissions connected to the grid,the power grid is gradually developing towards a large-scale,large-capacity AC/DC hybrid connection.Commutation failure is the most common fault in HVDC transmission.The main reason of commutation failure is external AC grid failure.Faults in the rectifier-side grid of the HVDC transmission cause commutation failures,which in turn cause DC grid fluctuations due to DC transmission to the sending-side grid.The access of a large number of HVDC transmission projects has made the regional grid connection at both ends of the DC closer,and the fault coupling has become more intense.It is of great significance to study the influence of the failure of HVDC transmission phase commutation on the voltage of the transmission grid to the planning and operation of the HVDC system and to ensure the security and stability of the grid.First,based on the operating characteristics of HVDC transmission and the principle of commutation failure,the change of DC electrical quantity during commutation failure was analyzed,and the reactive power variation characteristics of rectifier-side converters during commutation failure were studied.The reactive power consumed by the rectifier undergoes a process of decreasing and then increasing,which causes the grid voltage at the sendingside to decrease and then increase.The principle of voltage drop at the sending-side caused by commutation failure was analyzed.Based on the equivalent model of the DC sending-side,the quantitative analysis formula of the transient voltage drop at the sending-side was deduced,and the characteristics and changes of the transient voltage drop were studied in detail.In addition,the influencing factors of the voltage drop at the sending-side caused by commutation failure were obtained,that is the strength of the ending-side AC system and the transmission power of the high voltage DC system.The impact of AC system strength on transient low voltage is obvious,decrease in system strength causes increase in transient voltage drop,the transmission power of DC system has a smaller effect on the decrease of AC system voltage at the sending-side.Then by studying the characteristics of reactive power consumption of the rectifier-side converter during the phase of DC commutation failure reduction,the principle of transient overvoltage at the rectifier-side caused by commutation failure was analyzed.The calculation formula of the transient overvoltage at the rectifier-side grid when the commutation fails is derived,and the quantitative analysis of the transient overvoltage is carried out.The transient overvoltage is mainly related to the DC transmission power and the short-circuit ratio of the sending-side system.The transient overvoltage is positively related to the DC transmission power.The greater the DC transmission power,the larger the transient overvoltage caused.It is negatively related to the short-circuit ratio of the sending-side system.The larger the shortcircuit ratio,the smaller its equivalent reactance,the smaller the transient overvoltage.Finally,based on Hyperim’s electromagnetic transient digital-analog hybrid simulation model,the transient process of HVDC commutation failure was analyzed.In the case of different system short-circuit ratios and DC transmission power,the transient low voltage and overvoltage of the rectifier-side grid caused by commutation failure were simulated and verified,and the correctness and effectiveness of the proposed transient voltage quantitative analysis method are verified.