AC and DC Fault Ride-Through Strategies for Hybrid MMC Based HVDC Link and Interaction With Multimachine Power System Dynamics

This paper proposes a set of operational strategies implemented through suitable control characteristics for a point-to-point (PTP) HVDC link using hybrid modular multilevel converters (hybrid MMCs). The proposed strategies ensure stable operation of the PTP system under various system conditions, like active power reversals, and dc and ac side faults while maintaining average submodule (SM) capacitor voltages to their nominal values. The sudden discontinuation of active power exchange with the ac grid during dc fault may lead to voltage instability. However, with the proposed strategy, the hybrid MMC can inject reactive power to maintain the voltage of the ac bus even during dc side short circuit faults, thus helping to maintain the stability of the ac grid. During ac side fault, the control characteristics are adjusted to regulate the average SM capacitor voltages, which helps quick restoration of power flow after fault clearance. Moreover, because of the presence of SM capacitors in two MMCs, the power exchange by the HVDC can be modulated for a brief period after the fault clearance to dampen the oscillations in frequency. The performance of the proposed strategy is verified under different operating conditions through simulations in the PSCAD/EMTDC platform including a multimachine power system.