An Improved Grid-Forming Control Strategy of Wind Turbine Generators with the Supercapacitor for Optimizing Primary Frequency Regulation Ability

To optimize the primary frequency regulation (PFR) ability of the Type-IV wind turbine generator (WTG) system equipped with a supercapacitor energy storage system (SC-ESS), this paper proposes an expanded DC-link energy-based grid-forming (GFM) control strategy of WTG system. This strategy utilizes the energy stored in the WT rotor kinetic and the DC-link capacitance flexibly to maintain voltage stability and provide frequency support to the power grid in the grid-side converter (GSC) and SC-ESS, while the machine-side converter (MSC) can serve as the energy source. To flexibly utilize the power output ability of SC, an increased-power outer loop-based cascade control scheme of SC-ESS is designed to coordinate with GSC for unified voltage and power control. Meanwhile, a decoupling mechanism between DC-link voltage and power is designed to reduce their mutual interference. Besides, a coordinated PFR control method of WTG and SC is presented to comprehensively consider the interdependency among equipment safety, PFR ability indexes, and PFR costs. This method optimizes the depth of WTG and SC-ESS participating in PFR under different frequency stages, which can ensure the above issues. Case studies on a standard IEEE four-machine two-area system verify the superiority of the proposed control strategy.