A Study on the Effect of Grid Impedance on the Performance of a Virtual Synchronous Generator

Abstract

A Voltage Source Inverter (VSI) is widely used to interface distributed generating sources to AC grid. In a grid connected VSI, the active and reactive powers injected to the grid are controlled. The control dynamics of a synchronous machine may be mimicked to control the active power injection by a VSI, resulting in a Virtual Synchronous Machine(VSM) operation. Here the reactive power injection is controlled using a simple PI controller. This paper evaluates the performance of a virtual synchronous machine in a weak grid. Simulation results show that the virtual synchronous machine is able to track the active and reactive power references when the grid impedance is less. It is observed that when the distributed generator is far away from the grid, the effective grid impedance seen by the VSM will be larger and it adversely affects the performance of conventional VSM.