Research On Fault Current Limiting Method Of DC Grid Based On Coupled Inductor Bidirectional Fault Current Limiter

At present,the problems of energy shortage and environmental deterioration are becoming more and more serious.The development and utilization of energy resources is gradually changing from traditional energy to renewable energy.Flexible DC power grid is an effective means to connect and consume renewable energy.In the future,the main use of arial conductor is DC power grid networking.However,a large number of overhead transmission lines also bring more risk of fault to the DC grid.Due to the low inertia and impedance of DC power grid composed of power electronic devices,its fault develops rapidly.The fault current of DC power network will reach the upper limit of IGBT’s working tolerance in milliseconds,which poses a huge hidden danger for the safe operation of converter station and DC system.Therefore,effective fault current limiting technology is the key to the construction of DC power network.This work mainly studies the topology of DC fault current limiter.A coupled inductance bidirectional fault current limiter（CIB-FCL）is presented in this work.Combined with hybrid dc circuit breaker（HDCCB）,which has been applied in practical projects,the combined operation strategy of HDCCB and CIB-FCL is studied to achieve better fault current suppression effect.Firstly,the basic working principle of modular multilevel converter（MMC）is analyzed.The fault characteristics of DC side short circuit in MMC based High Voltage Direct Current（MMC-HVDC）are deeply studied and analyzed.It provides important theoretical support for the efficient limitation of DC side fault current.Secondly,a coupled inductance bidirectional DC current limiter（CIB-FCL）topology is presented.The CIB-FCL’s structure includes main part（Branch）,auxiliary branch,current limiting branch and bypass branch.The four working modes are analyzed in detail,and the internal current paths in each working state are given.Mathematical model under CIB-FCL current limiting state is established.An experimental platform for the current limiting part of CIB-FCL is set up to verify its feasibility.The design method and main parameters of CIB-FCL’s topology are given.Taking ABB-HDCCB as the object,based on the switchable working state of CIB-FCL,the jointed operation strategy of CIB-FCL and HDCCB including current limiting interruption and fault coincidence is studied.A current limiting method based on CIB-FCL for DC power network faults is refined.Finally,the proposed joint operation strategy is applied to the simulation model of four-terminal DC power grid of 400 k V symmetrical unipolar structure,and the peak of fault current when a bipolar short-circuit fault occurs on the DC side of MMC-HVDC,the clearance time of DC side fault,the amplitude and fluctuation time of the system DC bus voltage fluctuation caused by fault,and the interference of DC system after DC fault coincident.Through the study of the fault current limiting method of the DC power grid,the high-efficiency suppression of the fault current rise rate on the DC side of the MMC-HVDC can be realized,the fault breaking speed can be accelerated,the harm caused by the DC side fault of the flexible DC system can be reduced,and the system reliability can be improved.