Comparison of Dynamic Characteristics Between Virtual Synchronous Generator and Droop Control in Inverter-Based Distributed Generators

TLDR

Inverter‑based distributed generators lack grid‑forming ability and inertia, leading to droop control and virtual synchronous generator (VSG) methods, the latter adding virtual inertia via a swing‑equation emulation. This study compares the dynamic behavior of droop and VSG controls in stand‑alone and grid‑connected modes to elucidate the impact of the swing equation. Small‑signal and state‑space models evaluate frequency transients and active‑power oscillations, while delay effects are examined and an inertial droop control scheme is proposed. Simulations and experiments confirm that VSG and the proposed inertial droop control retain droop advantages while supplying inertia support to the system.

Abstract

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 synchronous-generator-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.