An adaptive fault clearing scheme for a MMC VSC-HVDC grid based on source-grid coordination, wherein the MMC VSC-HVDC grid comprises two source-side converter stations MMC1 and MMC3, two grid-side converter stations MMC2 and MMC4, wherein converters of the four converter stations are connected in a square shape by double-circuit DC overhead lines, and each converter station is equipped with two converters;
- a grid-side circuit breaker is disposed at each end of each DC line connected with the converters to coordinate with a source-side control strategy when a fault occurs to the DC line so as to isolate the fault together;
- the grid-side circuit breaker topology comprises:
- a steady-state low-loss branch, comprising a disconnector UFD1, an IGBT device and a disconnector UFD2 sequentially connected in series;
- a transfer branch, comprising a thyristor T0 disposed in parallel, wherein one end of the thyristor T0 is connected with a front end of the disconnector UFD1, and other end is connected to a line between the IGBT device and the disconnector UFD2;
- a charging circuit, disposed at a rear ends of the steady-state low-loss branch and the transfer branch and formed by sequentially connecting a pre-charging capacitor C, a switch RCB, a charging resistor RC and a charging inductor LC in series;
- wherein one end of the pre-charging capacitor C is connected with a rear end of the disconnector UFD2, and one end of the charging inductor LC is grounded;
- a discharging branch, comprising a thyristor T1 connected in parallel with the charging circuit, wherein one end of the thyristor T1 is connected to a ground terminal of the charging inductor LC, and other end is connected to a line between the pre-charging capacitor C and the switch RCB;
- and
- a flyback energy dissipation branch, connected in parallel with the charging circuit and disposed at a rear ends of the charging circuit and the discharging circuit, and comprising a flyback diode D and an energy dissipation resistor Re connected in series;
- wherein other end of the energy dissipation resistor Re is grounded, and other end of the flyback diode D is connected with the rear end of the disconnector UFD2 and a front end of a current limiting inductor Ldc disposed at a terminal;
- wherein the source-side control strategy is realized by a voltage regulation controller;
- when a system runs stably, the voltage regulation controller does not actuate, and an output voltage regulation coefficient K is kept at 1;
- after a fault occurs, the voltage regulation controller is started, and the output voltage regulation coefficient K decreases within the range of [0,1], so that voltage at outlets of the converters drops accordingly, and coordinates with voltage of the pre-charging capacitor of the grid-side circuit breaker topology to realize fault isolation based on the source-grid coordination;
- wherein the voltage regulation controller gives corresponding voltage regulation coefficients Kat different stages after the fault occurs, and the voltage regulation controller comprises a preliminary current limiting link and an adaptive voltage regulation link;
- the preliminary current limiting link is put into operation until the system detects the fault, while the adaptive voltage regulation link is put into operation only after the system detects the fault;
- after the fault occurs, the preliminary current limiting link is started immediately to preliminarily limit the development of fault current;
- after monitoring the fault, a DC grid protection system switches to the adaptive voltage regulation link, and adaptively coordinates with voltage of the grid-side pre-charging capacitor to isolate the fault together.