Energy-Storage-Device-Enabled Adaptable Fast/Slow Synchronization Control Structure for Dual-Port Grid-Forming Voltage-Source Converters

Grid-forming (GFM) voltage-source converters (VSCs) are required to provide two functionalities: ensuring synchronization stability, which is vital for their operation, and supplying GFM capability, which provides inertia power to support AC grid frequency. However, these functionalities posed a dilemma on the rate of convergence of synchronization (RoCoS) of the VSC. The synchronization stability can be enhanced with a higher RoCoS, while enhancing GFM capability requires a lower RoCoS to provide more inertia active power. To reconcile this dilemma, a control structure capable of seamless transitions between the high RoCoS and the low RoCoS (i.e., the fast and slow synchronization transients) is desirable. Previous adaptive fast/slow synchronization control methods depended on stiff DC-side sources and may not work in scenarios with limited DC-side power capacity. To address this limitation, the paper introduces an adaptable fast/slow synchronization control structure for a dual-port grid-forming (DGFM) VSC with an energy storage device (ESD). The proposed control structure allows adjustment of RoCoS through the Pdc/vdc droop coefficient of the ESD. The droop coefficient is found to exhibit an inverse relationship with RoCoS, based on the small-signal model derived for the synchronization control loop. Simulation results validate the analytic findings and demonstrate the effectiveness of the proposed control structure.

I. Introduction

Voltage source converters (VSCs) have assumed a pivotal role in integrating renewable energy sources into the AC power grid. Traditional synchronous generators (SGs) can synchronize the frequency of their output voltage with the AC grid using their inherent swing equation, while grid-tie VSCs rely on digital control algorithms for synchronization [1]-[3]. Current grid-tie VSCs commonly employ the phase-locked loop (PLL) technique for synchronization, relying on the AC voltage at the point of common coupling (PCC) [2]. However, as power systems transition to more and more converter-based renewable generation, some of the grid-tie VSCs must employ the grid-forming (GFM) control to independently establish stable AC voltage [1], [3]. The GFM control techniques can be generally classified into the following two categories [3]: