Research On DC Short Circuit Active Current Limiting Control Method Of Half Bridge MMC

HVDC transmission based on modular multilevel converter（MMC）is the only way to vigorously develop green industry and clean energy,solve the disadvantages of traditional energy and build a supply system dominated by new energy.The flexible DC power grid based on half bridge MMC basically adopts the overhead line scheme with low cost,and has the characteristics of low inertia and low damping,which not only has the problems of high line fault rate and rapid evolution of fault current,but also brings challenges to the overload capacity of electrical equipment and fault current suppression.Therefore,how to effectively suppress the fault current of flexible DC power grid based on half bridge MMC not only has important theoretical value,More important engineering practical significance.This paper mainly studies and analyzes the DC fault characteristics of bipolar short circuit on the DC side of MMC,gives the influence degree of MMC with different distance from the fault location on the short-circuit current,deduces the theoretical basis of setting the parameters of current limiting controller according to different functional types of MMC,and obtains the leading factors affecting the rise rate of fault current according to the fault current expression after short circuit on the DC side of MMC and before converter locking,and carries out simulation analysis and verification.Secondly,the control system composition of half bridge MMC is studied in detail,and the active current limiting method is studied in detail.The advantages and disadvantages of two active current limiting methods are given,and the mechanism of fault current considering active current limiting is analyzed and studied.Then,based on the research of active current limiting.Considering the cost of current limiting,rapid action,reliable current limiting and high-performance current limiting,an active current limiting method based on two-stage active control of MMC sub module and the coordination strategy of DC power grid are proposed.The starting criteria of segmented current limiting are set as the real-time monitoring results of DC current change rate and the voltage change rate of current limiting inductance respectively.After the fault is detected,the fault current is restrained by 1 and2 segments of current limiting,At the same time,the preset maximum short-circuit current can protect the safety of MMC bridge arm,cooperate to realize the rapid reduction of the number of sub modules during fault,and the two-stage fault detection criterion effectively ensures the reliability and input accuracy of two-stage active control（stac）based on MMC sub module.Finally,the application of the proposed DC short-circuit current limiting method in DC power grid is comprehensively analyzed and verified.The simulation results show that the segmented current limiting can adapt to the converter station under different operating conditions and has good reliability to ensure that it can only be put into operation after fault.The current limiting effect is verified in the four terminal DC power grid.Compared with the infinite current control measures,the breaking current of DC circuit breaker（DCCB）is effectively reduced by 49.7%,the peak current of bridge arm is reduced by 23.15% after fault,and the DC voltage is restored 100 ms after fault.Compared with the additional current limiting device,it reduces the cost,steadystate loss and maintenance requirements.Only by reducing the input proportion of sub modules during the fault period,it will not affect the normal operation of DC power grid,and ensure the power transmission in the fault clearing stage.