Development of control algorithms for renewable energy sources inverters in a microgrid environment

Abstract

In the islanded microgrids (MGs), it is important to maintain system stability and achieve proper active and reactive power-sharing between the parallel-connected distributed generation (DG) units. This objective is the responsibility of the primary control (PC) which consists of multiple control loops: an estimation-based power calculation unit, a droop control loop, a virtual impedance loop, and an inner voltage control loop. However, the problem of poor active and reactive power sharing, due to the impact of the impedance mismatch of the DG units¡¦ feeders, is inevitable when adopting the conventional control loops-based PC scheme. Furthermore, the presence of nonlinear and unbalanced loads may severely affect the accuracy of the reactive power sharing when controlling the active power and it is difficult to share the load current harmonics only by using these conventional control schemes. Therefore, the design of advanced control schemes, included in the primary control, is imperative in order to improve the MG performance in terms of power-sharing accuracy, stability, and robustness against load disturbances. In this regard, the present thesis focuses on developing advanced control strategies that are involved in the PC as well as providing detailed mathematical analysis. The main contributions made in this thesis to the PC intended for single-phase islanded AC MG, are: „h An enhanced SOGI-FLL scheme; called ESOGI-FLL; suitable for DC-offset rejection is proposed for the aim of ensuring proper estimation of the voltage key parameters in single-phase systems. „h A scheme of the double-loop inner control is designed. A Proportional-Integral (PI), with feedforward control strategy, is used in the voltage control loop and a Proportional (P) control is used in the current control loop. „h An improved power sharing control scheme, that includes an enhanced power calculation unit and a frequency-adaptive virtual impedance loop, is proposed. The obtained results through simulations and experimental tests demonstrate the effectiveness and the robustness of the proposed control schemes in achieving stable MG operation and accurate power-sharing among single-phase paralleled DG units even under nonlinear loads operating conditions.