AC Fault Ride-through Control Strategy and Fault Characteristics Analysis of AC / DC System Based on MMC-MTDC

In the Multi-Terminal Direct Current based on Modular Multilevel Converter ( MMC-MTDC ) system, the fault characteristics become more complex when an AC fault occurs in the receiving end system connected to the multi-terminal DC transmission system. When a fault occurs on the AC side of converter station 4, the grid-connected point voltage drops, and the input and output power of the converter station are out of balance. This unbalanced power causes fluctuations in the DC voltage of the MMC-MTDC system, resulting in overvoltage in the DC system. So, to ensure the safe and stable operation of the system, a coordinated control strategy under various combinations of reactive power priority, voltage droop, suppression of negative sequence combined with energy dissipation resistance is proposed to maintain DC voltage stability, Furthermore, the fault characteristics and influencing factors under different types of faults are further discussed. To prove the availability of the coordinated control strategy, symmetric and asymmetric faults are considered. When asymmetric faults occur, the negative sequence component fed into the AC side is reduced by suppressing the negative sequence current; when a three-phase short-circuit fault occurs, the DC side voltage exceeds 1.05 p.u., at this time, the energy dissipation resistor is activated to consume the surplus power of the system, so that the DC voltage is controlled within the range of 1.0 ~ 1.05 p.u.. In addition, based on the voltage droop control strategy, by adjusting the active power transmitted by the MMC, the surplus power of the DC line can be absorbed, and the MMC can be controlled to restore power balance as soon as possible during the fault. Finally, the effectiveness of the AC fault ride-through scheme is verified by RTDS simulation experiments, and the consistency between the fault characteristics and the theoretical analysis conclusions under this control scheme is confirmed.