Design of Nonlinear Droop Control in DC Microgrid for Desired Voltage Regulation and Current Sharing Accuracy

It is well known that there is a design tradeoff between voltage regulation and load current sharing in conventional linear droop control for several parallel-connected distributed sources in a DC microgrid. To improve both performance areas, this article proposes a design method to identify the nonlinear droop curve coefficient with the desired bus voltage regulation and current sharing accuracy in a specified heavy load range, while a linear droop function with a negative droop resistance is used to tighten the bus voltage regulation in the light load range. The design procedure of load boundary and the effect of negative droop resistances on the system stability are analyzed in detail. Also, a boost-up and curve-fitting method can further improve the voltage regulation and achieve smooth transition between light load and heavy load. In this way, no additional voltage compensation loop is needed. Two parallel-connected DC converters are finally built to verify the theoretical analysis.