Load Sharing in Medium Voltage Islanded Microgrids With Advanced Angle Droop Control

Abstract

Demand for converter-fed distributed energy resources are rapidly growing in microgrids and these energy sources are mainly responsible for sharing loads in islanded mode of operation. In high voltage lines, sharing real and reactive load power requirements can be achieved by conventional droop control methods, nevertheless strong coupling between active and reactive power in medium voltage lines with low X/R ratio degrades rating-based load sharing among converters. A transformation matrix which incorporates the line impedance effect in droop parameter calculation by modifying droop coefficients can be used to overcome this drawback. However, inclusion of power decoupling term in reference voltage calculation, has found to be more powerful to overcome coupling effects in load power sharing. By utilizing lesser steady state frequency deviation of angle droop in comparison with frequency droop, this paper adopts angle droop in real power loop. Angle droop concept along with power decoupling factor has an added advantage of improved reactive power sharing with surplus accuracy in real power distribution, which in turn, allows harmonic power distribution based on converter rating under different load conditions. Moreover, discrete state feedback control together with third harmonic injection-based pulse width modulation scheme provides additional robustness to individual converters.