Research On Transient Characteristics Of Commutation Failures Of HVDC Systems Induced By AC Faults

With the development of HVDC technology,China has formed a hybrid structure of AC and DC grids.Connecting AC and DC grids regionally is an inevitable choice for the optimal allocation of national resources and an important strategic deployment to achieve green development.Commutation failure is an important interaction form of AC-DC grids.Therefore,it is of great significance to research into the influence of different fault types as well as different fault moments on the commutation process of inverter valves and critical voltage of DC system.In addition,it is beneficial to conduct analysis on the characteristics of current fed into the AC system by the DC system,in order to acquire a better understanding of the interaction between the AC and DC grids,put forward the criterion to judge the occurrence of commutation failures and to analyze the adaptability of traditional protection and control schemes in hybrid grids.This paper makes a detailed analysis about the influence of AC system fault moments on commutation process of inverter valves and commutation failures.Taking into account the time-varying characteristic of the commutation voltage effective value after a three-phase fault of the AC system,a new method to calculate critical voltage drop is proposed.Also,the relationship between different fault moments and the critical voltage drop is deduced theoretically.In addition,considering the regulatory effect of the control strategy,detailed analysis is elaborated mathematically about the influence of different fault moments on the commutation process of valves connected in a 12-pulse pair at the inverter station.According to this,the variation curve of the critical voltage along with different fault moments is developed.As to the single-phase fault,this paper takes a close research on the impact of voltage reduction in a single phase as well as its occurring moments on commutation failures.When there is a sudden voltage reduction in a single phase,the change rule of post-fault magnitudes and phases of commutation voltages is researched theoretically.On the basis of the change characteristic,valves that are most probable to suffer from commutation failures are carefully elaborated.Moreover,a method for calculating critical voltage of a single phase considering different fault moments is developed.In addition,the change regularity of critical voltage along with different fault moments and phases is given.Besides,analysis on the commutation process of inverter valves after a metallic fault or a grounding fault via different resitances is made,based on the conduction time series of valves and the corresponding equivalent circuit.Also,the method to analyze the current fed into the AC system by the DC system is proposed,which may act as a theoretical basis for making improvements of traditional protection and control scheme in hybrid grids.