Power management in islanded and grid-connected microgrids

Abstract

A microgrid (MG) is generally defined as a low or medium voltage distribution network comprising various distributed generation sources, energy storage elements, and controllable loads. A given microgrid can be connected or disconnected to/from the main grid. A hierarchical control architecture consisting of primary, secondary, and tertiary control is often adopted for these microgrids. The secondary control and tertiary control levels are essential for the microgrid key parameters (frequency and amplitude) restoration to their nominal values, for synchronization with the grid, and management of the power flow between the MG and the main grid. However, these control layers often use the phase-locked loop (PLL) or the frequency locked loop (FLL), which are highly sensitive to the load and the grid disturbances, especially to the presence of the DC component in their voltage input. Due to these disturbances, oscillations can occur in the fundamental frequency of the PLLs (FLLs) output. Of course, these oscillations affect the performance of the control layers and therefore the stability of the inverters-based MG and the grid-tied inverters. Despite, some solutions to deal with this issue have been proposed in the literature, especially for the case of single-phase systems, the integration of such techniques into the control layers has not been considered. In this regard, the design of the control schemes based on such techniques, which may lead to the enhancement of the system performance, is imperative. Ensuring the stability of the microgrid systems incorporating the considered control layers is an important task that requires appropriate mathematical models for effective stability analysis and proper control design. The present thesis focuses on designing secondary and tertiary control schemes using an Enhanced SOGI-FLL (ESOGI-FLL) that is suitable for the DC offset rejection, developing accurate modeling approaches, and providing systematic guidelines to tune the parameters of the proposed controllers. The case of single-phase droop-operated microgrid during islanded and grid-tied operation modes is considered. The main objective of the developed approaches is to ensure effective and optimal control of the power flow among the distributed generation units inside an islanded MG on one hand, and between the microgrid and the main grid during the grid-connected mode on the other hand. The stability analysis of the microgrid system incorporating the designed control schemes is provided. Furthermore, the robustness of the secondary and the tertiary controllers against the system parameter variations is investigated. Simulations and experimental tests are conducted to verify the performance of the proposed secondary and tertiary control strategies for single-phase autonomous and grid-tied MGs. The obtained results demonstrate the effectiveness of the developed control approaches in achieving effective frequency and amplitude restoration, seamless synchronization, and optimal power flow control under various operating conditions.