Active Damping Control of Multiport DC Power Flow Controller for MMC-MTDC With Unbalanced AC Grid

Multiport dc power flow controller (M-DCPFC) has become one of the most attractive solutions to regulate the power flow in the modular multilevel converter-based multiterminal dc (MMC-MTDC) transmission system. However, the current studies on the M-DCPFC mainly focus on the power flow control under the normal operation in the transmission system. To eliminate the impact of the power oscillation which frequently happens because of the unbalanced ac grid, this article proposes an integration to calibrate the active damping control into the M-DCPFC. It introduces the complementary virtual damping to the transmission lines in order to reduce the dc current fluctuations, which suppresses the dc-side power oscillations. In this article, the influence of the unbalanced ac grid on the MMC-MTDC transmission system is investigated and the active damping control of M-DCPFC is presented in detail. A four-terminal MMC-MTDC transmission system is developed as the test platform in the real-time model-in-the-loop (MIL) simulation. The proposed active damping control is validated and shows its effectiveness under different unbalanced conditions.