Power Flow and Power Reduction Control Using Variable Frequency of Offshore AC Grids

In the "integrated strategy" for connecting offshore wind farms to onshore grids, an offshore AC grid is formed by interconnecting offshore wind farms and point-to-point VSC-HVDC links using an AC network. The advantage of this strategy is that in the case of a DC fault, AC circuit breakers are used to isolate the faulted HVDC link without the need for DC circuit breakers. This paper presents a control system that regulates power sharing between the HVDC links by varying the operating frequency of the offshore AC grid. Primary and secondary power regulation controllers are designed to achieve automatic power flow coordination between HVDC links. Increase in the offshore AC grid frequency during onshore AC grid fault or DC fault signals power flow imbalance. This signal is used to suppress the transient DC over-voltage through the coordination control between the wind farm side converters of HVDC links and the power reduction control of wind turbines. Importantly, because the inputs of the controllers are from local frequency signals only, fast communication is not required under the proposed control strategy. Simulation results are presented to verify the functioning of the proposed control strategy and illustrate variable frequency operation of an offshore four-node AC grid.