A Control Algorithm Based on the Conservative Power Theory for Cooperative Sharing of Imbalances in Four-Wire Systems

A cooperative control scheme based on the conservative power theory (CPT) is proposed, which can share imbalances in three-phase four-wire droop-controlled systems. By utilizing the CPT, the balanced, unbalanced, and distorted components of the currents and powers in a microgrid can be identified. Using control loops based on virtual impedances and implemented in the stationary a-b-c frame, the imbalances and harmonics are shared between the different four-leg inverters in the microgrid. A secondary control loop is implemented to regulate the maximum voltage imbalance/distortion at the point of common coupling or any other point in the microgrid. The theoretical background of the method is presented, and experimental validation is demonstrated using a laboratory-scale microgrid with two inverters operating at 5 kW.