Research On DC Power Flow Controller With Fault Blocking Capability And Its Optimal Operation

DC grid can integrate the complex power characteristics of renewable energy power generation and new load,and realize multi energy complementary,especially suitable for large-scale renewable energy power transmission and consumption.However,DC grid is still faced with the problems of insufficient freedom of power flow control and serious harm of DC fault.DC power flow controller(DCPFC)can assist the converter station to control the power flow of DC lines and supplement the degree of freedom of power flow control for DC grid;fault blocking device can effectively suppress the fault current and quickly isolate the fault,reducing the harm of DC short circuit fault to DC grid.It can be seen that DC power flow controller and fault blocking device are the key equipment of DC grid,but independent design and installation of the two devices will greatly increase the cost of the system.In order to improve the power flow control and fault blocking capability of DC grid,and take into account the system cost,it is urgent to study an integrated device integrating power flow control and fault blocking function.In this paper,an interline DC power flow controller with fault blocking function is proposed,and its topology and control strategy,component parameter configuration,optimal operation and other aspects are studied.(1)A dual-port interline DC power flow controller with fault current limitation capability(FCL-PFC)is proposed.In view of the lack of freedom of DC power flow control,the topology of dual-port interline DC power flow controller based on full bridge submodule is constructed,and then the steady-state working principle and control strategy are proposed.Through the bridge arm voltage to control the power flow of DC power grid and balance its own power,the power flow distribution of DC grid is improved.In view of the serious damage of DC short circuit fault,the topology of FCL-PFC is modified and the action strategy during the fault is given.By locking the full bridge submodule and changing the fault current path,the fault circuit voltage is increased and the rise rate and peak value of fault current are reduced.The simulation results show that FCL-PFC can control the current of a single line to the target value in steady state,and has the advantages of small harmonic,low insulation cost and no need to take energy from the outside;it can reduce the fault current by 40% in case of fault,greatly reducing the harm of DC short circuit fault to the system.(2)Based on FCL-PFC,an integrated multi-port DC power flow controller with DC circuit breaker(M-PFCCB)is proposed.In view of the need to supplement multiple degrees of freedom for power flow control in complex DC grid,the topology of multiport interline DC power flow controller is constructed,the working principle and control strategy are proposed,and the control parameters are optimized.Through the mutual coupling of multiple bridge arm voltages,the converter station is assisted to control the power flow of all DC lines and balance its own power,and the power flow control of DC power grid is realized.For the DC short circuit fault is difficult to remove,the M-PFCCB topology is modified and the action strategy during the fault is given.The capacitor is used as the circuit breaker to suppress and remove the fault current,which reduces the harm of DC fault to the system.The simulation results show that M-PFCCB can supplement multiple control degrees of freedom for DC grid,suppress and cut off short-circuit current in case of fault,avoid installing DC power flow controller and DC circuit breaker on each line,and save a lot of power electronic devices.(3)This paper presents an optimal configuration method of current limiting equipment parameters in DC grid with DC power flow controller.Based on the fault equivalent circuit of modular multilevel converter,the suppression characteristics of DC reactor and power flow controller to fault current are calculated and analyzed;the converter station equivalent circuit and fault current solution method are extended to DC grid,and the optimal configuration model with the minimum of fault current and DC reactor is constructed,and the components parameters of DC reactor and power flow controller are obtained.The simulation results show that,compared with only using DC reactor,the combined use of DC reactor and power flow controller can greatly reduce the fault current without prolonging the fault clearing time.(4)After configuring the parameters of key equipment in DC grid,a probabilistic optimal power flow calculation method for AC/DC hybrid system with multiport DC power flow controller is proposed.The mathematical model of multiport DC power flow controller is established,and the power flow calculation method of AC/ DC system with DC power flow controller is proposed.The probability distribution of PV and load is accurately described by using the nonparametric kernel density estimation with adaptive bandwidth,and the joint probability distribution of them is established by copula function.The probabilistic optimal power flow is calculated with the objective of minimizing system loss and DC load flow rate.The simulation results show that the DC power flow controller can greatly reduce the DC line ampacity and improve the static security of the system;the nonparametric kernel density model with adaptive bandwidth can better adapt to the uncertainty of random components.