Improved droop control strategy for grid-connected inverters

An improved control strategy for grid-connected inverters within microgrids is presented in this paper. The strategy is based on the classical P−ωP−ω and Q−VQ−V droop method. The improvement in the proposed control strategy is twofold: Firstly, the transient response of the droop controller is improved by replacing the traditional method of measuring average power, which is based on using a first order low pass filter, by a real time integration filter. This is shown to reduce the imported transient energy when connecting to the grid. Secondly, the steady state output current quality is improved by utilising a virtual inductance, which is shown to reject grid voltage harmonics disturbance and thus improve the output current THD. A small signal model of the inverter based on the transfer function approach is developed to analyse its stability and determine droop gains. Simulation and experimental results are presented to validate the model and demonstrate the controller capabilities.