Primary control in DC microgrids: a review

Abstract

These days, microgrid (MG) is considered to be an alternative scheme to tackle the problems of power generation in distribution energy. Direct current (DC) MG is preferred over alternating current (AC) MG because of its simplicity in the control mechanism, low cost, high efficiency, and power quality. There is a challenge to provide good power-sharing performance and voltage support in DC MG having several different sources connected in parallel. Deviation in voltage and differences in load sharing cannot be reduced simultaneously by adopting a conventional droop control scheme due to cable resistances and sensor errors. In this paper, improved methods for primary control are compared and discussed in detail where variable droop resistance is used instead of fixed one. In the non- linear droop control methods, the droop resistance depends on the output current. The value of droop resistance changes with a change in the output current of the converter. Also, a piecewise linear droop control method is reviewed, in which, the presence of oscillations in the system and size of the converters settle conclusively the number of segments. They are entirely decentralized methods and make necessary only provincial facts of the individual converter. The proportional distribution of current can be enhanced by adjusting the droop coefficient as per the line impedance. By using Optimization programming, the optimum droop parameters can be derived which help in eliminating the effects of cable resistance and improve voltage quality and load sharing error. Inner loop and droop control approaches are introduced. Finally, a comparison among different droop control techniques is presented.