Fault ride-through control for the sending-end AC system based on reactive power auxiliary in LCC-MMC hybrid HVDC system

The hybrid HVDC system based on line commutated converter and modular multilevel converter is a feasible method to address the common commutation failure at the receiving end of conventional HVDC systems. At present, project Gezhouba–Shanghai in China is under the demonstration scheme of the conventional receiving end upgrading, which introduces the difficulty in sending-end AC system failure riding through. For this purpose, an electromagnetic simulation model is established in the PSCAD/EMTDC simulation platform firstly, and then a transient analytical expression of DC current is deduced through the system equivalent circuit and verified through the simulation model. The expression deeply reveals the variation pattern of DC current and lays the theoretical foundation for fault ride-through control. Base on this expression, the decreasing level of MMC DC voltage after fault is deduced, however, it is difficult to realize the decreasing of MMC voltage due to the modulation index limitation of DC voltage. In order to solve this problem, this paper uses MMC to absorb reactive power from AC power grid to broaden the range of DC voltage drop. To solve this problem, the reactive power MMC absorbing from the AC power grid is used in this paper to broaden the decreasing range of DC voltage. On this basis, due to the conflict between the rapidity and stability of fault ride-through control, a current ride-through control based on reactive power auxiliary is proposed. This control strategy can dynamically regulate the reactive power transmission between MMC and AC grid according to fault states, assist to reduce the MMC DC voltage, and enable the constant current control so as to realize the sending-end AC system fault ride-through. Finally, the effectiveness of this control is verified with various simulation results. The sending-end AC system fault ride-through control method proposed in this paper has profound project application significance on upgrade transformation for the conventional receiving end