Frequency Support From a DC-Grid Offshore Wind Farm Connected Through an HVDC Link: A Communication-Free Approach

DC collection grid and high-voltage direct-current (HVDC) transmission lines provide promising solutions for an offshore wind farm (OWF). However, due to the decoupling effect of the dc collection grid and an HVDC link, an HVDC-connected dc-grid OWF cannot directly participate in the frequency regulation of the onshore ac grid. To address this issue, this paper presents a communication-free control scheme, which allows an HVDC-connected all-dc OWF to contribute to the onshore ac grid frequency regulation. This scheme consists of making the offshore dc collection grid voltage and onshore frequency deviations proportional. It relies on the HVDC voltage control at the onshore converter, together with the dc collection grid voltage control at the offshore converter. Furthermore, by continuously adjusting the droop of each wind generator in response to its local wind speed, this scheme takes into account the wind speed difference among wind generators, thereby improving the frequency nadir/peak and guaranteeing the wind generator stability. Simulation results verify the effectiveness of the proposed scheme.