Coordinated Control Strategy Of Fault Ride-through For DC Grid Based On Hybrid MMC

The DC grid with modular multilevel converter(MMC)has the advantages of flexible operation mode,multi-source power supply and multi-point power supply.It is an effective means to realize the collection,transmission and grid connection of large-scale renewable energy generation.The DC grid stations are closely linked and have strong interaction forces.Short-circuit faults occur on the DC side,and there are multiple branches providing short-circuit current.The fault current has the characteristics of large rising rate,high peak value,and fast fault propagation speed.It is easy to cause damage to power electronic equipment with poor tolerance and difficulty in fault isolation.Therefore,it is urgent to study the coordinated control strategy of fault ride through for DC grid to ensure the safe and stable operation of the system.This paper studies the fault characteristics analysis of DC grid,the design of DC fault current cooperative suppression strategy,and the DC fault current blocking scheme based on high speed switch(HSS).The main research work is as follows:Firstly,the overall framework of DC grid is constructed;the structure and working principle of hybrid MMC are analyzed.The influence of key technical parameters such as withstand current and terminal voltage of HSS on its working performance is analyzed.The evolution law of DC side short circuit fault current of single-ended MMC is studied.The equivalent circuit of shortcircuit current calculation is constructed.The expression of DC fault current is derived.The key factors affecting the peak value and rising rate of fault current are obtained,which lays a foundation for the design of DC fault ride-through scheme.Then,based on the analysis of the voltage regulation margin of the hybrid MMC arm,the fault line MMC(FLMMC)negative voltage control method is proposed,which can actively absorb the discharge current of adjacent lines and quickly suppress the fault line current while suppressing the discharge of the FLMMC capacitor.Then,in order to effectively suppress the discharge of nonfault line MMC(NFLMMC)to FLMMC,an active current limiting cooperative suppression method of NFLMMC is proposed,and the parameter selection principle adaptive to the change of DC bus voltage is designed to effectively reduce the discharge of NFLMMC to FLMMC while taking into account the recovery of DC voltage.The DC fault current blocking scheme for DC grid based on HSS interruption is designed,and the specific blocking process of short-circuit current is introduced.The simulation results show that the proposed DC fault current blocking control strategy can effectively suppress the rise rate and amplitude of fault current,reduce the overcurrent and over-voltage stress of the arm converter valve,and cooperate with HSS to isolate the fault quickly and reliably.At the same time,it can effectively reduce the spread range of the fault and enhance the safety and stability of the system.Finally,the principle of fault type determination by detecting fault current as fault clearance criterion is proposed,and the restart control strategy of DC grid system based on HSS action is designed.After the transient fault is identified,the system power can be quickly restored and the stability of DC grid operation can be improved.The power transfer control method suitable for permanent faults is studied.By reasonably switching the MMC control mode under fault conditions,the autonomous consumption of surplus power is realized.Simulation results show that the proposed method can effectively reduce the power fluctuation during system fault recovery and improve the fault ride-through capability of the system.