Parallel operation of inverters with different types of output impedance

It is well known that inverters are of different types of output impedance and that the widely-used droop control strategy takes a different form when the impedance involved changes its type. As a result, it is difficult to operate inverters with different types of output impedance in parallel. In this paper, a universal droop control strategy that can be applied to inverters with any type of output impedance with a phase angle between -π/2 rad and π/2 rad is presented. It takes the form of E ~ P and f ~ Q droop relationships, which is the same as the conventional droop control principle for inverters with resistive output impedance (R-inverters). Furthermore, a corresponding robust droop control structure is adopted to develop the universal droop controller. Simulation results are provided to demonstrate the feasibility of the universal droop controller.