Rearch On Fault Clearing Technology For Hybrid MMC Based-dC System

Modular multilevel converter(MMC)not only can avoid the commutation failure,but also has features of decoupled control of active and reactive power,and excellent harmonic characteristics.Therefore,the MMC has become the preferred topology for the construction of flexible DC system.Building a DC system based on MMC is of great significance for solving the problem of distributed energy accommodation and improving power quality,and it is also one of the mainstream forms of future grid development.However,the development of short-circuit faults is extremely fast due to the low-damping characteristics of the DC system.Meanwhile,as overhead lines are mostly used in DC systems,the probability of DC line short-circuit fault increases.Therefore,the DC system urgently needs effective short-circuit fault clearing technology to ensure its safe and reliable operation.The main patrs of this dissertation are as follows:(1)From the perspective of short-circuit fault blocking technology,an equipment-level hybrid MMC with DC fault blocking capability is proposed,which possesses the blocking characteristics of the improved submodule-based MMC and the DC circuit breaker.On the basis of the traditional half-bridge MMC,this topological scheme only requires very minor additional IGBTs and a small number of thyristors and diodes.Hence,it can resolve the problems of high equipment cost and large operating loss in the existing fault blocking schemes.The control strategy of the proposed scheme is studied and designed,and the fault blocking process and mechanism are analyzed in detail.Then,the economical comparison of different blocking schemes is made,and the simulation model is built to verify that the proposed scheme can clear the DC short-circuit fault by blocking the converter.(2)Based on the research of the first part,and in view of the defect of the improved half-full-bridge MMC that the fault range expands in the blocking process,another equipment-level hybrid scheme with DC fault blocking capability—active current transferring MMC is proposed.Current breaking branch,bridge arm blocking branch and energy absorbing branch are added to the topology.Once the DC fault occurs,the current breaking branch is disconnected to transfer the fault current actively and reverse the arm current simultaneously,and then the fault energy in MMC can be absorbed with half-bridge submodules.Meanwhile,capacitors also operate to absorb the fault energy on the line side during the blocking process of full-bridge submodules in the energy absorbing branch,thus fast blocking of short circuit fault is realized.The fault blocking mechanism is analyzed in detail by establishing the active current transferring MMC model,and a single-end simulation model is built to verify the feasibility and effectiveness of the proposed topology.(3)From the perspective of fault fast recovery technology,a fast recovery control strategy of submodule capacitor voltage based on arm current is proposed for hybrid MMC,which ensures the rapid reconstruction of the submodule capacitor voltage in the arms.Therefore,the recovery speed of the converter greatly improved.In addition,a suppression control strategy for arm overcurrent is proposed to avoid the potential secondary overcurrent problem during the recovery process and make the unlocking process carry on smoothly.Finally,the simulation results validate the effectiveness and feasibility of the proposed method.