Stability Analysis of Power-Converter-Dominated Microgrid

Power systems are going through a paradigm shift from electric machine-based to power electronics-based, with a huge number of different players on the supply side. Nowadays, thousands of distributed energy resources (DERs) are being integrated into power systems through power electronics components such as solar panels, wind turbines, and energy storage systems; however, the integration of numerous power electronic components and constant power loads (CPLs) destabilizes power systems and leads to critical oscillations. Consequently, one of the crucial challenges of this new paradigm is to keep the whole power system stable. The stability issues faced by DC microgrids are especially severe and urgent due to their unique properties. First, the low inertia of DC microgrids sharply weakens their stability; and second, owing to their advantage of smooth control, DC microgrids are unprecedentedly more promising than AC power systems given the increasing penetration of DERs. Therefore, the main purpose of this research is to solve the stability issues in power-converter-dominated DC microgrids. Considering the limited applicable ranges of traditional small-signal stability analysis, this research develops stability analysis of power-converter-dominated DC microgrids from the perspective of large-signal stability analysis. The main contributions of this research can be summarized as follows: 1) We model the DC microgrids with high penetration of power electronic devices and CPLs based on Brayton-Moser’s potential theory, including a proposed novel current-mode controller of power converters and a more realistic model of CPL. 2) We present a rigorous derivation of sufficient criteria for large-signal stability in DC microgrids with multiple power converters and CPLs. It is worth mentioning that this derivation works for many different types of power converters. In this research, the stability of each equilibrium point and the convergence of state trajectories with different starting points are discussed in detail. We integrate the discussion of the local stability of each equilibrium point into the large-signal stability analysis of the system. 3) We develop a novel approach to ROA estimation with less conservativeness using a potential-based approach. The approach tackles the common conflict between model accuracy of ROA estimation and computational overhead. 4) We resolve misunderstandings and emphasize the key points of potential theory, which plays a fundamental role in large-signal stability analysis in nonlinear power grids. Additionally, this dissertation develops a preliminary study about the microgrid control and voltage stability in AC/DC hybrid microgrids in Chapter 5, which investigates the control techniques, simulation modeling, and voltage stability analysis when a DC microgrid is connected to an AC microgrid. We will extend the preliminary study from the perspectives of stability-aware power flow management, fault detection, and grid protection in our future work.Ph.D.College of Engineering & Computer ScienceUniversity of Michigan-Dearbornhttp://deepblue.lib.umich.edu/bitstream/2027.42/174676/1/Fangyuan Chang Final Dissertation.pdfDescription of Fangyuan Chang Final Dissertation.pdf : Dissertatio