Sensorless Control and Large Signal Stability Analysis of Microgrids

Abstract

Microgrids in modern power networks have been gaining attention as a result of the global shift to sustainable energy systems. Microgrids, which are localised energy systems that can operate in either grid-connected or islanded modes, are essential for the development of renewable-powered solutions that are both flexible and reliable. But as these systems develop, they encounter major control, stability, and reliability challenges, especially when operating in dynamic environments and integrating more renewable energy sources. For microgrid technology to advance and remain viable over the long term, these challenges must be resolved. A comprehensive literature review of microgrids, sensorless control and large signal stability analysis is presented in chapter 2. The microgrid configurations, types of sources used, challenges of integrating renewable energy sources are discussed. In addition, the importance and types of voltage and current estimation techniques and the large signal stability analysis are studied to help understand the concept and current advancement in this research field. Additionally, the primary research gaps are highlighted after each subsection to ensure a clearer understanding of the specific challenges within each topic and helps accurately attribute the contributions made in addressing these gaps. Sensorless control of microgrids is presented in Chapter 3. The use of voltage and current estimators, typically used in conventional systems, to a master-slave distributed generator configuration is shown. The proposed method eliminates the need for expensive dc-side voltage and current sensors necessary for maximum power point tracking (MPPT) of the connected photovoltaic (PV) modules. Moreover, in medium and high-power applications, this method can provide a valuable substitute to one or more faulty current or voltage sensors which ensure system continuous operation while locating the faulty sensor(s). The introduced method doesn’t affect the PV normal operation even in the presence of partial shading, battery charging / discharging and fluctuating loads. The introduced method utilizes voltage and current estimators that rely on the ac-side measurements and a proposed switching function to isolate and calculate the voltage and current of each connected DG. The structure and the design details of these estimators are discussed in detail. The simulation and practical results conducted using series-connected converter configured by master (battery) – slave (PV) DG proved the validity of the proposed method in maintaining normal operation below 2% estimation error. In chapter 4 of the thesis, a new method to define the boundaries of large signal stability of a virtual-synchronous-generator-controlled converter is introduced. The new method estimates the region of attraction (ROA) around the converter’s stable equilibrium point (SEP) using vector perturbation and convex hull algorithm. It provides a mathematical representation of the converter’s stability boundaries, which can be used to evaluate its frequency and power angle stability under large disturbances. A case study, simulation results, and experimental results are provided to demonstrate the validity and the accuracy of the proposed method. Finally, conclusions and future work are presented. In final chapter of the thesis, the conclusion and the future work are discussed where it summarizes the main research gaps and contributions and draws on the potential future research directions driven from this thesis.