An Enabling Resynchronization Technique for Grid-Connected Voltage-Source Converters

Abstract

A novel control methodology for voltage-source converters (VSCs) is proposed. The control scheme mimics the functions of the synchronous generator (SG); or so-called virtual-synchronous-generator controlled converter (VSGCC), this function essentially allows VSCs to operate in both islanding and grid-connected modes. When a problem occurs at the main grid, the VSGCC can automatically transfer from the grid-connected mode to islanding mode. To stabilize the converter’s voltage amplitude and frequency until the utility system is restored. Nevertheless, the amplitude and phase of the converter remain deviated from the main grid. This causes enormous inrush currents that bring harm to the equipment. As a solution, this paper presents a simple yet flexible resynchronization mechanism for a VSGCC operating in islanding mode. The scheme is based on the use of vector Phase-Lock-Loop (PLL) and the terminal voltage control to compensate the internal control loop of VSGCC by altering frequency and voltage reference setpoint until the frequency, phase-angle, and voltage amplitude difference between a converter and the main grid are synchronized. The results are then simulated to prove the vector PLL’s abilities and the terminal voltage control to robustly regulate the voltage amplitude, the frequency, and the phase angle of the converter so that the difference with the main grid complies with IEEE Std. 1547-2018 for microgrid synchronization.