In order to limit the overvoltage of VSC caused by AC power grid faults at receiving end of hybrid cascaded HVDC, coordinated suppression strategies of sending end and receiving end for the overvoltage of VSC were proposed. Firstly, the mechanism of overvoltage of VSC caused by AC power grid fault at the receiving end was analyzed. On this basis, an improved current control method based on dynamic parameters was proposed. According to the change of DC current, the current controller parameters are dynamically adjusted to limit the overcurrent and its duration during commutation failure. Therefore, the active power flowing into the VSC is reduced, and the overvoltage of the VSC is reduced. The reactive power supporting, and dynamic current limiting methods based on the voltage change rate of VSC were proposed. The active power output of the VSC group is increased by dynamically adjusting the ratio of active current and reactive current of VSC. Thus, the overvoltage of VSC is reduced. Finally, the simulation model of hybrid cascaded UHVDC was built based on PSCAD/EMTDC. The effectiveness of the proposed method was verified by comparative experiments.
Compared with alternating current(AC) power grid, flexible direct current(DC) power grid has obvious advantages such as good controllability, flexible operation mode, and multiple applications, which also makes it the preferred scheme for new energy grid connection. In order to ensure the reliability of flexible DC power grid, it is necessary to use high-voltage direct current(HVDC) circuit breaker for fault isolation, and it is urgent to develop and study the reliability evaluation method of HVDC circuit breaker. This paper gives the circuit topology of HVDC circuit breakers firstly, then analyzes the relationship between single component failure and overall failure in combination with component redundancy.On this basis, this paper proposes the stability evaluation index system of DC circuit breaker and establishes the stability evaluation model.
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