A hierarchical control scheme for reactive power and harmonic current sharing in islanded microgrids

In this paper, a hierarchical control scheme consisting of primary and secondary levels is proposed for achieving accurate reactive power and harmonic currents sharing among interface inverters of distributed generators (DGs) in islanded microgrids. Firstly, fundamental and main harmonic components of each inverter output current are extracted at primary level and transmitted to the secondary controller. Then, instantaneous circulating currents at different frequencies are calculated and applied by the secondary level to generate proper control signals for accurate reactive power and harmonic current sharing among the inverters. Consequently, these signals are sent to the primary level and inserted as voltage references after passing the control blocks. In contrast to the conventional virtual impedance schemes, where reactive power and harmonic current sharing are realized at the expense of introducing additional voltage drop and distortion, the proposed control strategy effect on the amplitude and waveform quality of DGs' voltage is negligible. Meanwhile, it is able to provide accurate harmonic current sharing even if nonlinear loads are directly connected at the terminal of DG units. Control system design is described in detail and simulation results are provided to demonstrate the effectiveness of the proposed control method.