Grid-forming Hybrid Energy Storage with PLL for Improved Deadband Performance in Primary Frequency Regulation

With the wide adoption of environment-friendly renewable energy sources, the power converter needs to provide primary frequency regulation (PFR). Energy storage (ES) with grid-forming (GFM) control and fast frequency response are required for PFR. However, in GFM mode, active power is determined by the phase difference between the inverter terminal voltage and the point of common coupling voltage, causing spontaneous active power response in frequency deadband (FDB). In this paper, a strategy that adds a phase-locked loop (PLL) in the power synchronization control loop is proposed to restrict active power response in FDB, which can extend the lifespan of ES. Then, the proposed strategy is used in DC-coupled hybrid energy storage (HES), the slow-acting energy storage (SES) with larger capacity provides droop and damping for long-term frequency support with the control of GFM with PLL, and the fast-acting energy storage (FES) with smaller capacity provides inertia for short-term frequency support by following the SES's synchronization information. In addition, a step active power response with hysteresis is used in SES to maximize its advantages and decrease the times of the synchronous generator participation in PFR. Finally, the simulation results are provided to verify the proposed strategy's effectiveness.