Accurate design of controllers in a parallel single-phase inverter system of distributed generators sharing linear and non-linear loads

Renewable energy sources provide power in a form not directly deliverable to the grid, thereby requiring power electronic converters. Parallel inverters are generally used to achieve high reliability and reduce inverter rating. In this paper the inherent limitations of conventional droop control method for low voltage parallel inverters with mainly resistive tie line are shown, and multi-loop control strategy is proposed. Such strategy uses proportional plus multi-resonant controllers to achieve high quality of the voltage through harmonic distortion rejection. The design procedure of this controller is investigated thoroughly and it is shown that both controller resonant gain and bandwidth affect the stability of the system. Also a new configuration for virtual resistive impedance loop is proposed to improve the accuracy of both linear and non-linear load sharing without degrading voltage quality. Simulation results with two 4-kVA inverters are provided, showing proper stability margin, having sinusoidal voltage in the presence of nonlinear load, power sharing with a good accuracy and proper dynamic. Furthermore, some highlights on system reliability, and its possible improvements by proper redundancy, are briefly illustrated at the end of the paper.