Modeling and coordination control of AC/DC hybrid microgrids

• Main Author: Zhang, Zhe
• Other Authors: Tang Yi
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Electrical and electronic engineering::Power electronics
• Online Access: https://hdl.handle.net/10356/161339
• Institution: Nanyang Technological University

Three-phase alternating current (AC) has dominated the global power systems for over a century due to advantages such as readily transforming voltages with different levels, high-efficiency generation, transmitting power over long distances, etc. However, direct current (DC) power systems have recently gained popularity because they enable the more efficient and dependable integration of renewable energy sources such as solar, wind, fuel cells, and battery energy storage systems (ESSs). Thus, interlinking AC and DC subgrid power systems to be hybrid AC/DC microgrids presents a promising solution to energy and environmental issues. Due to the intermittence characteristic of renewable energy resources, energy storage (ES) is important to help mitigate the power imbalance between generation and consumption in AC/DC microgrids and to maintain the AC grid frequency and DC bus voltage. The improvement in power conversion efficiency and the reduction of carbon emissions into the environment are the major benefits of AC/DC hybrid microgrids. To achieve comprehensive coordination for hybrid microgrids, the most common implementation for the entire system is a centralized energy management system (EMS) with a communication link between the central controller and other system components. The primary drawbacks of centralized control are relatively low system reliability due to the need for a communications network and relatively slow response time due to the communication delay. Therefore, proper coordination controls of AC/DC microgrids need to be implemented to ensure stable, reliable, and efficient system operation and supply the load with high power quality. The modeling for different typologies, such as bipolar DC networks and nine-switch-based configurations of hybrid microgrids, investigates the effectiveness of the associated decentralized and coordination control power management schemes. The objective is to minimize the power system frequency deviation of the AC subgrid and the voltage deviation of the DC subgrid caused by load changes and renewable power fluctuations. Advanced coordination control, extending the developed modeling and control techniques to a cluster of AC/DC hybrid microgrids with different network topologies, are presented in this thesis. Furthermore, in more-electronic power networks, the growing penetration of renewable sources reduces overall system inertia. Besides, the lack of inertia issue becomes increasingly obvious as synchronous generators are gradually replaced by renewable generators in more-electronic power networks. One efficient technique to enhance the overall inertia of a typical microgrid is to use grid-connected power converters with distributed virtual inertia. By proportionally coupling the grid frequency and DC-link bus voltages, power converter DC-link capacitors serve as energy buffers and inertia supplies. A system with high inertia, on the other hand, is expected because it can slow the dynamic response to frequency changes and reduce frequency deviations, avoiding large-scale blackouts or undesirable load-shedding due to frequency events or system contingencies. To fill the aforementioned research gap, a hybrid microgrid with bidirectional virtual inertia support has been proposed in this thesis to slow down DC voltage and AC frequency changes, thus enhancing system stability. With a standard hardware configuration, inertia is delivered to both AC and DC subgrids via a bidirectional interlinking converter (BIC). Specifically, the difference in per-unit DC bus voltage and AC frequency is controlled through a proportional-integral (PI) controller, forming the active power reference of the interlinking converter. In addition, other supportive functions, such as frequency and voltage droop and secondary control, can be further implemented. The superiority of the proposed bidirectional virtual inertia support in hybrid microgrids is validated by experiments. With the proposed bidirectional inertia support in hybrid AC/DC microgrids, improvements in frequency nadir and rate of change of frequency (RoCoF) are expected, resulting in better overall system-level stability. In summary, this thesis examines autonomous coordinate control techniques without information exchange via communication links for various microgrid topologies, the issues and challenges of power electronics in more-electronic power systems in terms of stability and power quality. In addition, to verify the feasibility and effectiveness of the proposed control strategies, a hybrid AC/DC microgrid with the integration of renewables, various types of energy sources, ESs, and loads has been designed and developed.