Adaptive integrated control strategy for MMC-MTDC transmission system considering dynamic frequency response and power sharing

Due to the lack of grid inertia caused by the increasing penetration of renewable energy sources, the ability of system to withstand disturbance is reduced. In attempt to address this challenge, an adaptive virtual inertial frequency control (AVIFC) strategy for multi-terminal dc (MMC-MTDC) transmission system considering dy-namic frequency response is proposed. Based on the frequency variation characteristics in inertia response and primary response stages, the virtual inertia coefficient can be automatically adjusted. In the inertia response stage, considering the limitation of DC voltage, the virtual inertia coefficient adaptively decreases according to the frequency deviation to reduce the maximum frequency deviation. In the primary response stage, the virtual inertia coefficient decreases to 0 rapidly following the frequency deviation to speed up the frequency recovery. Furthermore, aiming at the problem of coordinated power sharing between converter stations of weak AC sys-tems after disturbance, an adaptive droop control (ADC) strategy based on DC voltage deviation and converter station power margin is studied to realize the optimal distribution of unbalanced power so that DC voltage deviation is reduced, DC voltage regulation time is shortened and the stability of DC voltage is maintained at the same time. Finally, based on PSCAD / EMTDC simulation platform, the control model of a five-terminal MMC-MTDC system is established. Three working conditions, namely the load reduction, the load increment and the converter station out of operation, are selected to verify the effectiveness and feasibility of the proposed adaptive integrated control (AIC) strategy including AVIFC and ADC.