Improved Active and Reactive Power Control for Symmetrical Bipolar MMC-HVDC Systems During DC Side PTG Fault

Symmetrical bipolar modular multilevel converter (MMC) combined with two monopolar MMCs has many benefits, such as high voltage and power level and flexible operation modes. However, the active power and reactive power of the MMC stations will reduce to half when direct current (DC) side pole-to-ground (PTG) happens, which will cause large disturbance to the alternating current (AC) grid. This study focuses on an improved active and reactive power control method for a symmetrical bipolar MMC-high-voltage direct current system during PTG fault. The healthy pole is adjusted to overload mode while the faulty pole switches to STATCOM mode. To maximise the utilisation of the operation region for healthy pole, it is analysed based on multi-restrictions of transformer capacity, arm current, sub-module (SM) voltage balance, SM voltage fluctuation and DC line overload capacity. By means of the improved active and reactive power control method, the disturbance to the AC grid is minimised. The feasibility of the proposed control method is verified by PSCAD/EMTDC simulation.