Control scheme design for power inverters working with linear and nonlinear loads in isolated microgrids

The concepts of smart grid and microgrid have recently gained an increasing interest because of their potential advantages, like their higher reliability when compared to conventional grids and the possibility of integrating distributed generators based on renewable sources. A microgrid can operate in grid connected or isolated mode. In the case of isolated microgrids, it is mandatory that power inverters connected to the microgrid generate a voltage waveform with a very low harmonic distortion, in order to fulfill international power quality standards. In this context, this paper presents the analysis and design of an innovative control scheme that significantly reduces the Voltage Total Harmonic Distortion (THDv) of power inverters connected to microgrids working in island mode. The proposed Repetitive Two-Degrees of Freedom (R2DOF) control scheme combines the advantages of both repetitive and two-degrees of freedom control techniques, both controllers reduce the THDv in the presence of nonlinear loads. As it is shown in the paper, the inverter performance is improved by the proposed concept even when strongly non linear loads are connected to the microgrid. The analysis and design of the whole control system is presented in detail.