Resistive sub-module hybrid MMC and direct current fault processing strategy thereof

A direct current fault processing strategy applied to a resistive sub-module hybrid modular multilevel converter (MMC), the resistive sub-module hybrid MMC comprising:

• a three-phase six-bridge arm structure, wherein each phase comprises an upper bridge arm and a lower bridge arm, wherein the upper bridge arm comprising N1 half-bridge arm sub-modules, N2 resistive sub-modules and a bridge arm reactor in series from a high voltage terminal to a low voltage terminal;
• the lower bridge arm comprising a bridge arm reactor, N2 resistive sub-modules and N1 half-bridge sub-modules in series from the high voltage terminal to the low voltage terminal, where N1 and N2 are natural numbers greater than 1;
• a middle node of the upper bridge arm and the lower bridge arm of phase A is connected with a first end of an alternating current breaker BR1, and a second end of the alternating current breaker BR1 is grounded;
• a middle node of the upper bridge arm and the lower bridge arm of phase B is connected with a first end of an alternating current breaker BR2, and a second end of the alternating current breaker BR2 is grounded;
• a middle node of the upper bridge arm and the lower bridge arm of phase C is connected with a first end of an alternating current breaker BR3, and a second end of the alternating current breaker BR3 is grounded;
• wherein the high voltage terminal of a direct current side of the MMC is connected with a direct current line through a smoothing reactor and a mechanical switch in sequence;
• wherein the direct current fault processing strategy comprises a method comprising the following steps:
• (1) during normal operation, keeping the alternating current circuit breakers BR1, BR2 and BR3 of the MMC in open state, keeping the mechanical switches in closed state, keeping all resistive sub-modules in bypass state, and monitoring the bridge arm current of each bridge arm of the MMC at the same time;
• (2) after the direct current fault occurs, if a converter station where the MMC is located first detects that the bridge arm current exceeds a threshold, immediately locking all half-bridge sub-modules in the MMC and conducting all resistive sub-modules, closing alternating current circuit breakers BR1, BR2 and BR3 at the same time, and then sending a direct current fault processing instruction to the converter station at the other end connected with the MMC;
• (3) when the converter station at the other end receives the direct current fault processing instruction or detects that the current of its own MMC bridge arm exceeds the threshold, similarly, immediately locking all half-bridge sub-modules in the MMC and conducting all resistive sub-modules, and closing the alternating current circuit breakers BR1, BR2 and BR3;
• (4) after a certain period of time t3, when the current flowing through the mechanical switches at both ends of a fault line is reduced to a certain size, the converter stations at both ends issuing an opening signal to their respective mechanical switches, the mechanical switches complete turning on to realize a physical isolation of the fault line, and at the same time, the converter stations at both ends issuing an opening signal to the alternating current circuit breakers BR1, BR2 and BR3 of their respective MMC;
• (5) after a certain time t4, the alternating current circuit breakers BR1, BR2 and BR3 of the MMC at both ends returning to the open state, and completing the direct current fault processing.

A resistive sub-module hybrid modular multilevel converter (MMC) comprising:

• a three-phase six-bridge arm structure, wherein each phase comprises an upper bridge arm and a lower bridge arm, wherein the upper bridge arm comprising N1 half-bridge arm sub-modules, N2 resistive sub-modules and a bridge arm reactor in series from a high voltage terminal to a low voltage terminal;
• the lower bridge arm comprising a bridge arm reactor, N2 resistive sub-modules and N1 half-bridge sub-modules in series from the high voltage terminal to the low voltage terminal, where N1 and N2 are natural numbers greater than 1, wherein the high voltage terminal of a direct current side of the MMC is connected with a direct current line through a smoothing reactor and a mechanical switch in sequence;
• a middle node of the upper bridge arm and the lower bridge arm of phase A is connected with a first end of alternating current breaker BR1, and a second end of the alternating current breaker BR1 is grounded;
• a middle node of the upper bridge arm and the lower bridge arm of phase B is connected with a first end of an alternating current breaker BR2, and a second end of the alternating current breaker BR2 is grounded;
• a middle node of the upper bridge arm and the lower bridge arm of phase C is connected with a first end of an alternating current breaker BR3, and a second end of the alternating current breaker BR3 is grounded.