Small-Signal analysis of an isolated power system controlled by a virtual synchronous machine

Abstract

This paper investigates the small-signal dynamics and stability of an isolated power system where voltage, frequency and power balance are controlled by a Voltage Source Converter (VSC) operated as a Virtual Synchronous Machine (VSM). The analysis is based on a linearized state-space model of the isolated power system, including the VSM with its control loops, a grid equivalent model with local loads, and a power electronic converter relying on a Phase Locked Loop (PLL) for grid voltage synchronization. The validity of the small-signal state-space model is verified by comparison to a simulation model in Matlab/Simulink. The presented results demonstrate how a VSM can effectively ensure stable operation in an isolated power system. The small-signal analysis also reveals that the critical eigenvalues of the system are sensitive to the inertia time constant of the VSM, and that a low inertia can cause instability while high inertia can lead to a relatively slow response. However, the small-signal stability of the system is robust with respect to variations in load and operating conditions if the virtual inertia is set within the common range for the inertia of synchronous machines.