Steady-State Analysis and Optimal Power Routing of Standalone Unbalanced Hybrid AC/DC Microgrids
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Date
2018-08-03
Authors
Alsanbawy, Mahmoud Ahmed Allam Sayed
Journal Title
Journal ISSN
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Publisher
University of Waterloo
Abstract
The concept of ac microgrids was introduced to integrate distributed generators (DGs) and loads within one entity that can operate autonomously or connected to a utility grid. Furthermore, dc microgrids have received increasing attention as a potential solution to deliver power from DGs to modern dc loads with reduced conversion stages. Moreover, hybrid ac/dc microgrids have been introduced as a paradigm combining the benefits of the two types of microgrids by interconnecting them through interlinking converters (ICs).
Steady-state analysis is essential for planning and operation studies of electrical power systems. However, conventional analysis approaches cannot be applied to hybrid ac/dc microgrids due to their distinctive features, such as droop characteristics, lack of a slack bus, and coupling between the ac and dc variables. Additionally, the unbalanced nature of ac microgrids adds to the complexity of modeling and analysis in such networks. Therefore, this thesis is focused on developing steady-state modeling and analysis framework for standalone unbalanced hybrid ac/dc microgrids.
First, a steady-state analysis tool for unbalanced hybrid ac/dc microgrids is developed. The ac subgrid's components are modeled in phase coordinates. Furthermore, the dc subgrid's components are modeled and the coupling between the ac and dc variables is formulated. The models of the various system elements are incorporated into a unified power flow formulation, which is solved using a Newton-Trust Region (NTR) method. The developed power flow algorithm is verified through comparisons with time-domain simulations of test microgrids. The analysis tool is used to analyze a larger hybrid ac/dc microgrid through case studies. The case studies shed light on some challenges of these microgrids, namely, imposed limitations on microgrid loadability due to unbalanced ac subgrid's loading, effect of IC settings on microgrid operation, and trade-off between proportional loading of the ac and dc subgrids and proportional power-transfer sharing among ICs.
Second, based on the identified microgrid loadability limitation of unbalanced microgrids, a novel adaptive power routing (APR) scheme is proposed to maximize the microgrid loadability. The proposed scheme allows independent control of active and reactive powers flowing through IC phases, so that power can be routed among the ac subgrid's phases. The DPR scheme is integrated into an optimal power flow (OPF) formulation with the objective of minimizing load shedding. A supervisory controller is proposed to solve the OPF problem by adjusting the DG and IC settings. Several case studies are conducted to show the ineffectiveness of conventional supervisory controllers in resolving the loadability issue, and to verify the success of the proposed controller in solving the problem.
Third, a power flow approach based on sequence component analysis of the ac microgrid's elements is adopted for faster convergence and improved modeling accuracy as compared to conventional approaches in phase coordinates. This approach breaks down the system model into positive-, negative-, and zero-sequence subsystems that can be solved in parallel for enhanced performance. The positive-sequence power flow is solved using a Newton-Raphson (NR) method, while the negative- and zero-sequence voltages are obtained by solving linear complex equations. The approach is verified through comparisons with time-domain simulations. In addition, the algorithm is utilized to investigate the operation of droop-controlled DGs in larger-scale isochronous unbalanced ac microgrids, and to examine its limit-enforcement abilities at the same time. The algorithm demonstrates significant improvements in terms of accuracy and convergence time when compared against the conventional NTR-based approach in phase coordinates.
Finally, the power flow approach developed in the third part is extended to include the IC's and dc subgrid's models so that it can be applied to hybrid ac/dc microgrids. A power flow algorithm is proposed to solve the ac and dc power flows independently in a sequential manner, while maintaining the correlation between the two. The algorithm is verified through comparisons with time-domain models of test hybrid microgrids. Case studies are introduced to test the algorithm's effectiveness in enforcing the DG and IC limits in the power flow solution under various conditions. The algorithm also shows enhanced accuracy and solution speed with respect to the tool developed in the first stage.