Control And Protection Strategy For The HVDC Fault Of Wind Power Transmission

Wind power is hard to consume locally.So it is necessary to employ the technology of large-scale and long-distance HVDC to solve this problem.LCC-HVDC and VSC-HVDC are two different systems suitable for wind power connection at the present stage.MMC-HVDC has gradually drawn much attention for it keeps the VSC-HVDC's advantages and overcomes the shortcomings like higher switching losses.The faults of LCC-HVDC or MMC-HVDC could threaten wind turbines operating environment and the whole system safety.DC block and line-to-line fault need further study because they are the most severe problems of LCC-HVDC and MMC-HVDC respectively.For the LCC-HVDC,Firstly the main reasons of transient overvoltage caused by DC block and its impact on wind turbines are analyzed in this paper,and the factors that influence the overvoltage level are summarized.Then dynamic overvoltage and operating process of control and protection are expounded,and the trigger mode of DC block is explained.Besides,the transient overvoltage of the grid-connected bus is suppressed by improving the emergency switch off sequence strategy that trigger DC block and changing the action sequence of stability control and pole control system.Then the steady operation of wind turbines is guaranteed.Finally,the wind-thermal-bundled HVDC model is built based on PSCAD/EMTDC and the effectiveness of the method mentioned is proved by example simulation.For the MMC-HVDC,this paper firstly analyzes the fault current caused by line-to-line fault with the method of equivalent simplification.Then the method is chosen to block the fault current by installing hybrid HVDC breaker.The fault type is detected by constructing a new fault criterion.And the breaking moment orders of switch units in breaker is setting by combining the fault current with the breaker switch characteristics.The orders are input into the time sequence control system in breaker as controlled variables.Finally the wind power MMC-HVDC with hybrid HVDC breaker model is built based on PSCAD/EMTDC.Simulation results verify that the strategies proposed can effectively isolate the fault and prevent the damage of switch devices in the converter,and that ensure the safety of HVDC system and normal transmission of wind power.