A Coordinated Frequency Support Scheme for Offshore Wind Farms Connected with Low Frequency Transmission System

In the grid connection of an offshore wind farm, flexible low-frequency AC transmission technology has received special attention. With the increasing penetration of grid-connected offshore wind farms, it is of great importance that the offshore wind farm can provide frequency support to the bulk power grid to help solve its frequency stability problem. However, the offshore wind turbine is decoupled from the onshore bulk power system by the back-to-back converter, which plays the central role in the low-frequency power transmission technology. Therefore, the offshore wind farm cannot react to the change in frequency of the shore power grid. To this end, this paper proposes a frequency support strategy for a low-frequency transmission system integrated offshore wind farm, which has seldom been studied in existing literature. First, a mapping strategy is developed to establish the connection between the frequencies of the onshore bulk power system and the low-frequency transmission system. A dual loop based strategy is developed for the back-to-back converter. Finally, a droop control based strategy is developed to control the active power of the wind turbine, which will provide frequency support for the bulk power system. The simulation results show that with the method proposed in this paper, the offshore wind farm can provide effective frequency support in response to the frequency change of the onshore bulk power system.