| Modular Multilevel Converter based High Voltage Direct Current(MMC-HVDC)is playing an more and more important role in the power system of China,and it has become the development trend of the Voltage Source Converter based HVDC(VSC-HVDC),due to its special technical advantages.MMC-HVDC is developing in the direction of higher voltage,greater capacity and longer transmission distance.As a result,the overhead line transmission(O HL)will be the inevitable choice in the future MMC-HVDC project.However,high DC fault probability is unavoidable for the OHL.Thus there are urgent needs of fast and reliable removal of DC faults and minimizing the harm causing by the transient over current/voltage for the OHL MMC-HVDC.To combine the DC fault current clear ance capability and good economy of the converter,cell-hybrid MMC consisting of the half-bridge sub-module(FBSM,with the smallest number of devices used but no DC fault current clamping capability)and new sub-module with DC fault clamping capability ha s attracted wide attention in academia and engineering.This thesis will propose solutions against the fault over current/voltage with designing topologies or control strategies for the OHL MMC-HVDC system based on the cell-hybrid MMC.(1)Solution against the DC fault current for the MMC-HVDC ——Topology research of cell-hybridMMCBasic operating principle and control strategy of MMC-HVDC is first presented.Then three typical sub-module topologies are focused on: Half Bridge Sub-Modules(HBSM),Full Bridge Sub-Modules(FBSM)and C lamp Double Sub-Modules(C DSM).Comparing their charictoristics,it is HBSM and FBSM that are selected to constituting the cell-hybrid MMC.To clamp the DC fault current,blocking fault current paths are analyzed during dc single-pole-to-ground short-circuit fault and pole to pole short-circuit fault.According to the mechanism of riding-through DC fault,SM configuration principle for cell-hybrid MMC is designed.The lowest ratio for FBSM is 0.5 to clamping the DC fault current.At last,based on the conventional HBSM and FBSM,a new cell-hybrid MMC is proposed consisting of two new sub-module with parallel IGBTsplit running: Double Half-bridge Sub-module(D-HBSM)and Paralleled Full-bridge Sub-module(P-FBSM).In accordance with the law of SM voltage balancing of D-HBSM and P-FBSM,the Sub-module triggering strategy is designed for the new cell-hybrid MMC.With 50% P-FBSM and 50% D-HBSM in each arm,the time complexity of sorting algorithm is deduced to 1/16 than before in cell-hybrid MMC with 50% FBSM and 50% HBSM.(2)Solution against the fault over current for the MMC-HVDC ——Over current suppression control for MMC faults based on virtual impedanceTo deal with the over current suppression for MMC faults,both the AC and DC faults mechanism are analyzed.Relations between the circuit parameters and the over current level are derived.According to the result of derivation,characteristics of AC and DC outlet impedance are mathematically mapped to the control scheme respectively,added to the vector current controller.Evaluation indexes both in average and transient are proposed to measure the effect of the over current suppression control:the average indexes AAM-Current,the heat coefficient Gh,and the arm instantaneous current.A detailed 2-terminal 11 level MMC model with AC and DC faults is simulated on PSCAD/EMTDC.Taking the evaluation indexes as references,the time-domain simulation shows the control sc heme woks well.(3)Solution against the DC fault over voltage for the MMC-HVDC ——Separated DC Polar Control for MMC decoupling voltage off-set on AC and DC sideThe concept of Voltage O ffset Rate is defined.By analysising upper and lower arms symmetrically,with the conventional MMC control strategy,Voltage Offset Rate of AC side and has a p roportional relationship with the DC side,the ratio is modulation ratio M.With remodelling MMC by analysising upper and lower arms respectively,the active component of AC current is direct ratio with the DC bus voltage and the power flowing to upper and lower arms.It is the theory foundation to design the control strategy.A new control strategy is designed to decouple the AC/DC voltage-offset based on this theory foundation.Simulation validate the basic availability,working as the same as the converntional control strategy.It can also superess the circulating current in arms..(4)Solution against the DC fault over voltage for the MMC-HVDC ——Asymmetrical configuration and operatio n for cell-hybrid MMC-HVDCAsymmetrical configuration and operation for cell-hybrid MMC-HVDC are specific application and development of the separated DC polar control for MMC decoupling voltage off-set on AC and DC side.To control and operate the upper a nd lower arms respectively and independently,configuration of asymmetrical MMC is ntroduced.Two principles for arm SM configuration are proposed to satisfy the requirements to decouple the AC /DC voltage-offse,with both FBSM and HBSM.With proposed configuration principle and control strategy,conventional symmetrical MMC can operates on both symmetrical and asymmetrical states.MMC with asymmetrical configuration can also operate steady.Simulations validate the availability. |
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