Novel Frequency Regulation Scheme of Grid-Forming MMC-HVDC with DC-ESS for Wind Farm Integration

The energy storage system (ESS) is recognized as a promising approach to enable renewables to participate in frequency regulation. This paper proposed a novel DC-ESS topology in the modular multilevel converter-high voltage direct current (MMC-HVDC) system for the wind energy conversion. The DC-ESS consists of series-connected sub-modules, which are composed battery model and DC-DC module. The basic control strategy of DC-ESS is investigated. Based on the DC-ESS topology, the grid-forming control structure and the frequency regulation scheme are well designed by coordinating the coupling interactions among the wind farm, MMC-HVDC and DC-ESS. The proposed topology and frequency regulation scheme has merits of: 1) The DC-ESS is easy to implement and has compact structure, independent ESS configuration, which is suitable for high voltage level and large ESS capacity application. 2) The proposed scheme can provide strong frequency support for the weak AC system without losing maximum power point tracking (MPPT) and need of phase-locked loop (PLL). Simulations are carried out under various network disturbances to validate the proposed topology and the frequency regulation scheme.