Power sharing control strategies for a three-phase microgrid in different operating condition with droop control and damping factor investigation

In this study, a microgrid with two voltage source inverters (VSIs), operating in islanded or grid-connected mode is analysed. The active power control by droop coefficients and reference frequencies is demonstrated. In addition, the inductive and resistive droop strategies are compared by considering the complex line impedance. By measuring the line impedance in the prototype, it was found that, even for the low-voltage microgrid, the line impedance was not predominantly resistive, but complex. For this line impedance condition, an investigation to determine the best droop control law considering the partial derivatives. Observing the partial derivatives and the experimental results obtained it is demonstrated that inductive droop control is better to control the active power for complex line impedance. For a scenario with resistive unbalanced loads and different power sharing between the inverters, the proposed control, implemented in the a-b-c reference frame, was validated. A novel absolute damping factor is developed to improve the transient response and reduce the reactive power flow. The VSIs are connected to the grid without transformers to reduce the connection costs. Simulation and experimental results are presented to confirm the improvements achieved using the implemented control method.