Enhanced control of grid forming VSCs in a micro-grid system under unbalanced conditions

Due to the technical advances in power electronic industry, employing inverter-based distributed energy resources (IBDERs) are increased in electric power systems. Distributed generators (DGs) with the rating capacity of few megawatts are commonly connected to the distribution level of power systems which are inherently unbalanced. A cluster of IBDERs and loads form a small-scale grid called microgrid (MG) which can operate independent of the main grid. In a MG, grid forming voltage source converters (VSCs) are used to control the voltages and frequency of the MG. However, grid forming VSCs with conventional control schemes cannot do this task properly when the MG is unbalanced. In this paper, a new control strategy is proposed to enable grid forming VSCs to work properly in unbalanced conditions. The proposed method does not need to increase the voltage control loop bandwidth, and therefore, is suitable for medium power VSCs applications operating at low switching frequencies. To examine the functionality of the proposed method, different scenarios are simulated on a test system in PSCAD/EMTDC environment. Also, a comparison with conventional control method is presented.