Research On The Intelligent Reclosing Of The True-bipolar VSC-HVDC Grid

The VSC-HVDC grid based on the DC circuit breakers(DCCB)and overhead transmission lines can realize the grid connection of distributed energy,and is one of the important technical means to solve problems such as energy shortage and environmental pollution.The external environment of the overhead line is bad,so the probability of failure is high.The nature of the fault is mostly transient.A reasonable reclosing scheme for faulty lines can quickly restore power supply,which is of great significance for ensuring the reliability of DC system power supply.This paper first analyzes the basic structure of the hybrid DC circuit breaker,the parameter characteristics of the components and the basic working principle.On this basis,the specific breaking process of the hybrid DC circuit breaker and the variation characteristics of each electric quantity in the process are analyzed by theoretical analysis and modeling and simulation.The difference of the fault line end voltage under different fault characteristics after the isolation switch of the hybrid DC circuit breaker on the faulty line after the single pole grounding is studied.The fault nature identification method of the overhead line single pole ground fault based on the fault line terminal voltage is proposed.And the calculation method of the corresponding criterion setting value;after the bipolar fault is solved,the isolation switches of the DC circuit breaker are overlapped.The difference of positive and negative voltages under different fault properties is proposed.The fault property identification method of overhead line bipolar fault based on positive and negative voltage difference is proposed.The oscillating overvoltage phenomenon on the transmission line on the DC circuit breakers on both sides of the faulty line is directly overlapped under transient faults.The causes and influencing factors of the line overlap overvoltage are analyzed,and the internal structure of the hybrid DC breaker is combined.The parameter characteristics,the coincidence coordination scheme of overhead line single-pole ground fault and the coincidence coordination scheme of overhead line bipolar fault are proposed.This scheme can effectively reduce the coincidence overvoltage on the transmission line,improve the safety of the DC system and the reliability of the power supply.