Rapid Power Compensation Based Flexible DC Transmission System Control Strategy for Supporting Frequency Stability of Islanded Grids

The large-scale islanded power grids decouples the frequency of public power system and causes function loss to clamp its frequency, threatening the stability of power systems. The flexible DC transmission system can transmit power to the islanded power grids and improves the frequency stability of islanded grids by adding frequency control, which can actively participate in the dynamic process of frequency quality improvement. However, the frequency instability of islanded grids is easy to occur, and the typical frequency controls (such as droop, inertia and PD control) cannot compensate the power deficiency quickly and sufficiently. Considering the limitation of conventional strategies, the rapid power compensation (RPC) based frequency control strategy was proposed in this paper, which quantifies the transmitted power between the inverter and the islanded grid to determine the maximum power that can be generated by the inverter. When the disturbance of the islanded grid exceeds the specified threshold, the maximum power is directly used to fully use the capacity of the inverter and fill the power gap of the islanded grid to the maximum extent, and hence reduce frequency deviation and frequency change rate quickly. Besides, the simulation results also show that the RPC control is more suitable for low inertia systems, because of its remarkable effect under large disturbances.