Virtual Impedance Shaping for low voltage Microgrids

AC Microgrids, in presence of highly non-linear loads, require a tighter regulation of line voltage to maintain power quality. This article proposes an outer virtual impedance loop to shape the output voltage in such microgrids. In order to achieve this, a virtual impedance extracting and shaping filter is first proposed to extract and weigh selected frequency components of the line current. Secondly, design of impedances to weigh the extracted frequency components is obtained in order to achieve harmonic current sharing and reducing total harmonic distortion (THD) even in scenarios of asymmetric sharing of highly non-linear loads. An isochronous voltage-active power half droop architecture is also employed to achieve active, reactive power sharing and to keep circulating currents minimum in the low-voltage microgrid. Analytical results on designing virtual impedances and their dependence on network line parameters are obtained. Finally, MATLAB/SIMULINK-based simulations are performed on a 4.4 kVA microgrid and implemented on a low-cost controller in a controller-hardware-in-the-loop (CHIL) setup to validate the viability of implementation and efficacy of the proposed virtual impedance shaping method.