S-shaped droop control method with secondary frequency characteristics for inverters in microgrid

Microgrid technologies have been studied for various goals including enabling renewable energy penetration. Traditional droop control is important for microgrids to achieve plug and play characteristics, but it may not be suitable for storage units with time-varying capacity levels due to the constant slope in droop control. In order to address the potential challenges of dynamic adjustment in linear droop control applied to microgrid inverters powered by limited energy sources, this study presents an S-shaped droop control. This control method tends to make the inverter to output more power when the frequency is close to the nominal operating point, while outputting less power when the frequency is away from the nominal operating point such that the other generators will pick up more load. Also, power quality problems can be addressed with the improvement provided by the characteristic similar to the secondary frequency control which brings the frequency closer to the rated frequency. The proposed mathematical model is built by curve fitting and theoretic analysis, and is simulated in Matlab/Simulink. The results verified the ability to improve power quality and adjust the load share among inverters as opposed to conventional droop control.