Design and Analysis of a Current-Controlled Virtual Synchronous Machine for Weak Grids

Emulation of inertia has drawn much attention in recent years due to the growing demand for network services from systems operators. In this scenario, the converter control method Virtual Synchronous Machine (VSM) has gained importance because of its inertial properties. However, in its original form VSMs do not have a current-controller and, therefore, the current-limiting functionality severely restricting their use in industrial applications. The current-controlled version of VSMs (CC-VSMs) removes this restriction. In this paper, a CC-VSM is proposed to interface Voltage Source Converters (VSCs) to weak grids. Dynamic aspects of CC-VSMs have been already studied in the literature. However, in this paper, a simplified small-signal model is derived. It is shown that this model is able to capt the essential dynamics of the CC-VSM. Also, a novel method to calculate the steady-state operating condition is developed. Differences between VSMs and CC-VSMs are highlighted, while robustness of CC-VSMs against parameter variations of the electricity network are studied. The control system improvements were demonstrated firstly in simulation and then on a 15 kVA prototype of a VSM.