Research On The Influence Of DC Circuit Breaker On The Overvoltage Of Flexible DC Power Grid Under Single-pole-to-ground

The flexible DC grid can facilitate the transition from traditional energy to clean and renewable energy,realize wind,light,store much can complement,and it has good flexibility and controllability,it will become the development trend of future flexible DC transmission technology.DC circuit breaker is the essential core equipment in the construction of flexible DC power grid,its introduction will make the overvoltage characteristics of the flexible DC power grid more complex.In this paper,the overvoltage simulation model of a four-terminal ±500kV flexible DC power grid is built on the PSCAD platform,takes the single-pole-to-grounding fault on the DC side of the flexible DC power grid as an example,The transient characteristics of single-pole-to-grounding fault with DC circuit breaker are studied.The equivalent circuit of single-pole-to-grounding fault in flexible DC power grid before and after operation of DC circuit breaker is established,The analytical expressions of fault current and fault voltage are obtained.The transient process of DC circuit breaker before operation is mainly the discharge of MMC submodule capacitor through fault circuit.The transient process after operation is divided into the first converter and the second converter.Theoretical analysis and simulation show that the DC circuit breaker action before the fault current and fault busbar voltage rapidly rising and falling,The first converter after DC circuit breaker operation has little effect on fault current and fault voltage,But the second converter causes fault current drop and overvoltage,It is proposed that the second converter of DC circuit breaker is the key factor that affects the characteristics of single-pole-to-grounding fault in the flexible DC power grid.In this paper,through the simulation analysis of the DC circuit breaker breaking single-pole-to-grounding fault line,the following results can be obtained:The influence of DC circuit breaker on the overvoltage of the system is mainly reflected in the transfer branch module block,With the increase of the blocking time of the module in the transfer branch,the over-voltage level decreases at the key nodes in the converter station of the flexible DC power grid;With the increase of the capacitance of the transfer branch module,the single-pole-to-grounding overvoltage of the flexible DC power grid decreases,The rise rate of the fault voltage also becomes smaller,The overvoltage of DC bus,current-limiting reactor and metallic return is more serious when there is no module capacitance;With the increase of arrester voltage limiting factor,The overvoltage of DC bus,AC side of the converter,terminal of current-limiting reactor and metallic return of each converter station will increase,but the absorbed energy of arrester decreases with the increase of the value of y;The overvoltage between the terminal of current-limiting reactor produced by the input of surge arrester in sequence(with appropriate delay)is much smaller than that produced by the input of surge arrester at the same time,At the same time,the rising rate of circuit breaker voltage is reduced,but have little influence on overvoltage of other equipment.