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In this paper, the performance of wireless operation of four-leg parallel inverters, at the presence of unbalanced nonlinear loads, is investigated. In order to control the parallel inverters, an inner current and external voltage control loops should be designed. In this paper, a proportional controller for the current internal loop as well as a proportional-resonant one for the voltage external loop are investigated and designed to ensure the proper performance of the system at the presence of unbalanced and nonlinear loads. In this paper, droop control and virtual output impedance loops have formed the power sharing control system for the parallel inverters. The proposed system is able to feed balanced, unbalanced, and nonlinear loads and provides an appropriate sinusoidal voltage waveform for loads by accurately sharing power between the parallel inverters. Simulation results verify the accurate and proper performance of the proposed system.

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Polymer electrolyte membrane fuel cells (PEMFC) have been considered by researchers due to their high efficiency, low pollution, and high-power density in distributed generation systems. In this paper, the connected PEMFC fuel cell power recovery system with an LCL filter is evaluated in the harmonic Grid. LCL filters, despite their greater ability to attenuate harmonics, can lead to system resonance and instability. In this research, a transformer has been used to connect the fuel cell inverter to the Grid and its leakage inductance has been used as the inductor on the network side. Besides, for optimal resonance damping, and attenuation of current ripple caused by grids voltage harmonics, capacitor voltage through feedback control has been used. Complete control of capacitor voltage feedback includes proportional, derivative, and second-order components. In the proposed control scheme, the capacitor voltage derivative component opposes the capacitor current feedback due to identical and symmetrical loop gain. Therefore, both of them can be deleted. Thus, the capacitor current sensor is saved. Instead, the LCL filter resonance is damped by a proportional component and a second-order derivative of the capacitor voltage. A low-pass filter is also added to the second-order derivative in the controllable frequency range to ensure system stability. The simulation results of the PEMFC power recovery system in different conditions confirm the proper attenuation of the grid-connected inverter, the injection of current of suitable quality into the contaminated and harmonic grid, the stability, and the appropriate dynamic response of the proposed system.