Design of virtual synchronous generators with enhanced frequency regulation and reduced voltage distortions

In modern power systems where the penetration of renewable energy resources is normally high, functions of voltage and frequency support are supposed to be taken over by virtual synchronous generators (VSGs) or virtual synchronous machines (VSMs), i.e., grid-connected inverters which are controlled to mimic the terminal characteristics of synchronous generators. In this paper, the comprehensive design of VSGs is provided in detail. The proposed control scheme consists of two control loops. The inner-loop voltage and current controller is designed in the discrete j-domain to guarantee system stability and voltage control with low distortions, while the design of the outer-loop frequency and power controller is performed in the continuous s-domain to be in consistence with the conventional power system frequency regulation models. With the proposed design method, various control objectives, e.g., resonance damping, voltage support, primary frequency regulation, secondary frequency regulation, and virtual inertia, can easily be achieved and evaluated. Finally, experimental results obtained from a 500-W three-phase virtual synchronous generator prototype are presented to verify the effectiveness of the proposed design method.