Distributed Hierarchical Control for Fast Frequency Restoration in VSG-Controlled Islanded Microgrids

Abstract

As microgrids develop rapidly, more inverters are adopted to achieve DC/AC or AC/DC/AC conversion of distributed generators (DGs). The virtual synchronous generator (VSG) control has started to replace the traditional droop control for inverters. In order to restore the frequency to its nominal value, most existing secondary frequency control (SFC) methods are based on frequency measurements. However, while reducing the rate of change of frequency (ROCOF), virtual inertia also slows down the convergence of frequency-based SFC. Therefore, this paper proposes a new distributed hierarchical control for fast frequency restoration. Based on the real-time VSG control at the bottom level, a novel frequency restoration control is designed. The power reference values generated by the proposed control can accelerate the frequency restoration with accurate power sharing. Meanwhile, by designing event-triggering conditions, parallel inverter controllers only need to communicate with neighbors at the event-triggered moments. Simulations have been performed in MATLAB/Simulink environment. Furthermore, the proposed control has also been tested on the experiment platform, which contains practical physical circuits and real-time controllers. Both simulation and experiment results verify the effectiveness of the proposed control strategy.