A control strategy for islanded operation of a Voltage Source Converter (VSC) based distributed resource unit under unbalanced conditions

This paper proposes a new control strategy for the Voltage Source Converter based Distributed Generation units operating in a Microgrid under unbalanced load conditions. The objective of the proposed control strategy is to control the positive sequence components and the negative sequence components of the Voltage at the Point of Common Coupling (PCC) in dq - frame, so that the Voltage will be controlled to the reference command rapidly. The load in a distribution system is usually unbalanced and highly unpredictable. The main objective behind controlling the negative sequence components of the PCC Voltage is to make sure that the Voltage at the PCC is balanced always irrespective of the unbalance in this load. Therefore the negative sequence components at the load will be minimized to a great extent, such that the Voltage supplied to the loads is nearly balanced. The proposed controller employs feed forward compensation in order to compensate for the effects of the cross coupling terms and the dynamics of the load, thereby the control structure becomes completely decoupled. The effectiveness of the proposed control structure has been implemented on the detailed switched model representing the Distributed Generation unit modelled in PSCAD/EMTDC.