DC Fault Traversal Strategy For Flexible And Direct Grid Connected Systems Of Large-scale Photovoltaic Power Stations

China’s photovoltaic power generation is mainly concentrated in areas with abundant solar energy resources and weak power absorption capacity.It is necessary to transmit photovoltaic power generation to areas with large power loads and few power resources over a long distance.Using DC overhead lines for large-scale photovoltaic grid connection through flexible DC grid connection systems is one of the trends of new energy grid connection in the future.However,the probability of DC short circuit faults on overhead lines is high,which can easily lead to disconnection of photovoltaic power stations or damage to power electronic components in converter stations.Studying the DC fault traversal problem of flexible and direct photovoltaic grid connection has become one of the key issues to improve the stability of photovoltaic energy integration into the grid.Firstly,according to the characteristics of flexible DC transmission system and photovoltaic power station,the corresponding mathematical model and steady-state model of PSCAD/EMTDC simulation platform are established,and their working principles and control methods are studied;Secondly,the fault characteristics of high voltage DC power transmission systems with large capacity photovoltaic(PV)via bipolar modular multilevel converters(MMC)were analyzed when a single pole short circuit to ground fault occurred on DC overhead lines;Finally,a coordinated control strategy for DC fault traversal is proposed,which comprehensively considers DC current circuit breaker(DCCB),converter station control mode,and photovoltaic power output control.That is,when a fault occurs,use a non fault pole converter station to continue power transmission,calculate the unbalanced power based on the rated power of the converter station and the output power of the photovoltaic power station,fully utilize the power output characteristics of the photovoltaic array itself,optimize the DC line voltage within the photovoltaic power station,and achieve control over the output power load shedding of the photovoltaic power station;For the fault traversal problem of DC overhead lines under transient fault conditions,load shedding of photovoltaic power stations and power feed forward increment control of converter stations are proposed to maintain system power balance and improve grid connection stability of the system.Secondly,aiming at the fluctuation of photovoltaic output,a combined optical storage system is established to suppress the variation of power output.Then,based on the system power balance relationship,the analysis of system power and capacity configuration is proposed;Aiming at the power changes of photovoltaic systems affected by the environment,a control mode switching method for optical storage joint systems is proposed.In addition,considering the occurrence of DC short circuit faults in the system,a coordinated control strategy for fault traversal under transient and permanent fault conditions with DC single pole grounding is proposed to ensure the stability of the system during faults.Finally,based on the PSCAD/EMTDC simulation platform,the above fault traversal strategy is simulated and verified.By comparing the system parameters before and after applying fault traversal measures,it is shown that the proposed method can effectively maintain the operating characteristics of photovoltaic power plants and flexible DC systems,and smoothly achieve fault traversal.