Influence of the output impedance of an inverter on its droop control strategies in a microgrid

The objective of this paper is to present a comprehensive review of the various droop control strategies employed to control the operation of the parallel inverters present in a microgrid, based onits output impedance. Predominantly, the output impedances of the inverters are inductive due to the line impedance and filter but in low power systems, the inverter has a resistive output impedance. The various categories under which the droop control strategies of inverters with inductive output impedance fall have been reviewed, along with its pros and cons. Although most of the inverters have inductive output impedance, inverters with resistive output impedance are superior due to the easier compensation of harmonics. Along with their advantages the various disadvantages present in the droop control strategies utilized for inverters with resistive output impedance and the solutions to overcome these problems are presented. Recent studies have shown that inverters with capacitive output impedance provide the lowest Total Harmonic Distortion along with reliable regulation of voltage and accurate power sharing. The technique of obtaining an inverter with capacitive output impedance along with its various advantages has also been presented. The universal droop strategy utilized for all types of inverters without prior knowledge of their output impedances is reviewed along with an example, to overcome the problem of having to change the droop equations according to the output impedance of the inverter. Lastly a few case studies related to microgrid implementation have been analyzed along with its challenges as well as standards and policies.