Research On Reclosing Of Overhead Line DC Grid Based On Hybrid MMC

Using new energy to generate electricity is an effective way to solve the problems about energy crisis and climate warming.There are abundant renewable energy resources in China,but the load center and the primary energy center are distributed in reverse.Therefore,it is necessary to carry out long-distance transmission of clean power.Voltage source converter based high voltage direct current(VSC-HVDC)grid using overhead lines is an effective solution to the large-scale transmission of the clean energy.However,the probability of the DC short-circuit fault occurrence on the overhead lines is high.The practical recovery strategy should be presented to enhance the security of the VSC-HVDC grid.The traditional reclosing method is harmful when reclosing with permanent fault,and the key equipment in the DC power grid will bear the overcurrent shock again.The control mode of hybrid MMC is highly expandable to make it has a good application prospect in dealing with DC faults.Therefore,it is necessary to study the reclosing method of VSC-HVDC grid based on hybrid MMC.In this context,the reclosing method after DC short-circuit fault of VSC-HVDC grid based on hybrid MMC is studied in this paper.The main research contents and results are as follows:(1)Aiming at the DC grid based on hybrid MMC and fast vacuum switch,the method of recovery after DC short-circuit fault is proposed.The control capability of the hybrid MMC is extended to ensure that the converter is not locked during DC failure.It makes the VSC-HVDC grid can quickly and flexibly return to normal working state after the DC fault is cleared.The electromagnetic transient model was built on PSCAD/EMTDC platform to verify the correctness of the proposed method.(2)Aiming at the two-terminal VSC-HVDC system based on hybrid MMC and DC circuit breaker(DCCB),the DC fault adaptive reclosing method based on active signal injection of hybrid MMC was proposed.After fault isolation,the system was proactively injected with characteristic signals to identify fault properties,so as to avoid reclosing to permanent fault and improve system security.The electromagnetic transient model of DC transmission system with mechanical DCCB was built on PSCAD/EMTDC platform to verify the correctness and effect of the proposed method.(3)Aiming at the MMC-HVDC grid with mechanical DCCB,an adaptive reclosing strategy with signal injection is presented.The detection signal is injected into the fault line of DC grid by means of superposition signal.The fault property is identified before reclosing.Therefore,second strike of reclosing to permanent fault is avoided.Furthermore,this strategy is robust to variable fault resistance.The electromagnetic transient model of MMC-HVDC grid with mechanical DCCB is built on PSCAD/EMTDC to validate the availability of the strategy.The feasibility of injecting signals at different converter stations is analyzed.