Research On Technical And Economical Improvement Of Hybrid DCCB Adopted In Mmc Based Multi-terminal HVDC Grid

Multi-terminal DC grid(MTDC)based on modular multi-level converter(MMC)has great advantages in terms of stability and flexibility in the transmission of renewable energy.But due to its remarkable low short-circuit capacity and weak damping characteristics,it is trapped by how to clear the DC faults.DC circuit breakers(DCCBs)are one of the most promising schemes for fault isolation.Among them,the hybrid DCCBs(HCBs)with fast speed and low loss have gained much attention.However,the high cost restricts the wide adoption of HCBs.Therefore,this thesis focuses on the power electronic device branches(PEs)with the highest investment in HCBs,and proposes several feasible strategies to reduce the power electronic devices consumption from two aspects.In terms of the performance requirements,the fault clearance is divided into two parts.One is the fault isolation including the fault detection,the breaking of the ultrafast disconnector(UFD)and the insertion of the energy-absorbing branch.The other is the fault recovery(reclosing).During these processes,reducing the transient overvoltage and overcurrent can reduce the number of the power electronic devices in single PE branch.In terms of the topology,multiple single-port HCBs can share only one PE branch to reduce the total number of PE branches required for the MTDC grid.Specifically,this paper conducts in-depth research from the following aspects:(1)In terms of the performance requirements,aiming at how to quickly calculate the overvoltage and overcurrent during the fault isolation,a numerical calculation method suitable for MTDC grids is proposed.Based on the phase-mode transformation,MTDC grid is decomposed into the common-mode and differential-mode networks.The fault voltage and current can be computed by listing the state matrix equations.Thus the problems of the asymmetry between the poles caused by DC faults as well as the coupling effects of the DC lines are solved.The relative error between the proposed method and simulation is within 3 %,which means high accuracy.(2)In terms of the performance requirements,aiming at how to suppress the fault current during the fault detection and UFD breaking process,an MMC active fault current suppression method is proposed.The principle of suppressing the DC fault current by lowering arm DC reference voltage is analyzed.Then the way to reconstruct arm total reference voltage is prposed.By the proposed method,the peak DC fault current can be reduced by more than 30 %.Compared with the existing MMC active current limiting methods,the proposed one can ensure that the valve-side output voltage of MMC will not be affected.Therefore,the risk of MMC blocking is greatly reduced.(3)In terms of the performance requirements,aiming at how to reduce the overvoltage during the insertion of the energy-absorbing branch,an energy-absorbing branch with parallel arrester structure is proposed.Combined with the sequential insertion strategy,a multi-objective optimization mathematical model is established.A parameter selection scheme is given to simultaneously reduce the switching overvoltage,fault current peak value,fault clearance time and absorbed energy.By the proposed method,the peak switching overvoltage can be limited to 1.33 times the rated DC voltage,and the peak fault current can be reduced by another 30 %.(4)In terms of the performance requirements,aiming at how to eliminate the overvoltage and overcurrent when the reclosing fails,an adaptive sequential reclosing strategy with the fault identification method based on the local voltage is proposed.The model of sequential insertion is established,and the reason why the existing methods cannot judge the nature of the fault in time is analyzed.According to the differences between the voltages at the local end of the fault line under different types of the faults,a method for judging the nature of the fault is proposed.Thus when reclosing fails,the system will hardly suffer from overvoltage and overcurrent.(5)In terms of the topology,aiming at how to reduce the number of PE branches in the MTDC grid,a multi-port HCB integrated with multi-port DC power flow control function is proposed.The working principle,control strategy and parameter configuration method are illustrated in detail.Compared with the existing multi-port HCBs and multi-port DC power flow controllers,the proposed topology can realize all the functions of fault isolation,UFD failure protection,reclosing and power flow control with higher technical economy.The economic analysis of the whole process shows that combining the above research results,the investment of the PEs can be saved by 45.1 % from the aspect of reducing the performance requirements,and saved by 77.5 % from the aspect of optimizing the topology.The overall economy can thus be increased by 89.3 %.