Research On Control Strategy Of Voltage Balance And DC Fault Clearance Of Novel Hybrid MMC

In order to solve the imbalance between energy and load center and construct the global energy Internet,building long distance,high voltage and large capacity DC transmission lines to achieve coordinated energy dispatch,wide area optimization and configuration,clean and efficient utilization is to become the inevitable choice of power development.HVDC converter has undergone the development from line commutated converter(LCC)to voltage source based converter(VSC).Compared with LCC,VSC has no problem of commutation failure,and can independently control active and reactive power.As a VSC topology,the modular multilevel converter(MMC)proposed in recent years does not have the problem of voltage equalization in series switching devices.MMC has better waveform quality and less switching loss,which shows that it becomes the mainstream application of HVDC projects.However,the main drawback of MMC is that the common half-bridge sub-module topology can not clear the DC fault,while topology with DC fault clearance capability has more switch devices and thus poorer economy.Hybrid MMC converters take into account the DC fault clearance capability and better economy,showing greater research and application value.With the rapid increase of the number of sub-modules,in order to maintain the balance among sub-module capacitor voltages within a limited control cycle,the traditional balance method of sorting is faced with issues about more computational boards and voltage sensors for sub-module voltage.The double half-bridge sub-module(D-HBSM)has the function of partial capacitor voltage self-balancing and the parallel full bridge sub-module(P-FBSM)is able to realize overall capacitor voltage self-balancing.In this paper,a novel hybrid MMC consisting of D-HBSM and P-FBSM is proposed and its parallel voltage balancing characteristics are analyzed.On the basis of this,it is proposed that the sub-module capacitor voltage balancing control method between and within the converter valve sections.Furthermore,based on the analysis of sub-module blocking characteristics,DC fault control strategy is designed.Finally,electromagnetic transient simulation verification of that parallel voltage balancing characteristics,voltage self-banlance and DC fault ride-through control strategy is carried out in PSCAD/EMTDC.The results show that,(1)parallel voltage balancing has little power loss and helps keeping the good economy;(2)under the premise of effectively reducing the controller performance and sensor accuracy,the overall voltage balance of the novel hybrid MMC can be achieved by the proposed voltage balancing control;(3)novel hybrid MMC-HVDC is capable of DC fault ride-through and recovering.The simulation results verify the effectiveness of proposed voltage balancing and DC fault ride-through control strategy.