In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronousgenerator(SG)-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system. Index Terms--DC-AC power converters, distributed power generation, droop control, microgrids, power control, power system dynamics, power system modeling, renewable energy sources, state-space methods, virtual synchronous generator. 0885-8993 (c) .Currently, he is a professor of Division of Electrical, Electronic and Information Engineering at Graduate School of Engineering, Osaka University. His research interests are in the areas of power electronics and applied superconductivity for power systems including superconducting magnetic energy storage (SMES) and future power systems including many distributed generations.
