Control strategies for improving the performance of Micro-grid systems

Abstract

A Microgrid (MG) is commonly described as a distribution network operating at low or medium voltage levels, comprising various Distributed Generation (DG) sources, energy storage elements, and controllable loads. The majority of DG sources rely on Renewable Energy Sources (RESs) like wind and photovoltaics (PVs). MGs are designed to connect/disconnect to/from the main grid as needed. Typically, these MGs are managed using a hierarchical control structure with primary, secondary, and tertiary control levels. Two levels can be further distinguished within the primary control. The first level, also referred to as the inner control or zero level, includes an external voltage control loop and an internal current control loop, tasked with maintaining the power stage's voltage and frequency within desired references. The second level, known as power-sharing control, is responsible for sharing power among multiple parallel-connected converters feeding a common load. During this stage, the MG’s key parameters, such as: frequency and amplitude, may deviate; hence, the role of the secondary level is to restore them to their nominal values, facilitating synchronization with the grid. The tertiary, being the last level, plays a crucial role in regulating power flow between the MG and the main grid. The present thesis focuses on the design, modeling, analysis, and control of parallel-connected three-phase VSIs within an AC MG system, specifically on the design of the Photovoltaic MG (PVMG) system based on the hierarchical control. It involves the design of advanced control schemes, developing accurate modeling approaches, and providing systematic guidelines for tuning the parameters of the proposed controllers. Additionally, it covers the adaptation stage, involving the DC-DC converter with a Maximum Power Point Tracker (MPPT) controller, between the PV Generator (PVG) and the inverter. The main objective of these approaches is to ensure effective and optimal control of the PVMG. Simulations and tests are conducted to validate the performance of the proposed control strategies for three-phase PVMG, demonstrating their effectiveness in achieving frequency and amplitude references, restoration, seamless synchronization, and optimal power flow control under various operating conditions.