Control of power electronic converters for microgrids

Date

2019-08

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Abstract

Power systems are going through a paradigm change from centralized generation to distributed generation and further to smart grids with growing deployments of distributed energy resources (DER) such as renewables. Microgrids are identified as the key components of the Smart Grids for improving the power reliability and quality, and increasing energy efficiency, as microgrids are locally controlled systems, and can connect and disconnect from the utility-grid with the operation of grid-connected or islanded mode. Though microgrids have broad applications nowadays, there are still many technical challenges faced by the operation of the microgrids, particularly for grid-independence, with high penetration of renewables. For example, the intermittency of renewables and fluctuating loads can easily cause the unstable voltage or frequency regulation of the microgrids with low system inertia. Power sharing and power regulation need to be well conducted among DER to avoid circulation currents of power electronic converters and inverse power flow of renewable resources. Power regulation also affects the energy efficiency in a microgrid. The fault ride-through capabilities are expected to enhance the system resilience and have fast system recovery during the faults. In grid-connected mode, the power converters are usually operated as current sources; while in islanded mode, they should be operated as voltage sources to stabilize the microgrid. It is difficult to achieve mode change with these two incompatible sources for the operation of microgrids.

This dissertation aims to address the above challenges by developing advanced control technologies of power electronic converters to achieve fully autonomous microgrids. The following five technical aspects are investigated: voltage and frequency regulation, accurate power sharing and power regulation, energy efficiency enhancement, fault ride-through enhancement, and seamless mode change. An improved droop control strategy named the uncertainty and disturbance estimator (UDE)-based robust droop control is proposed to achieve voltage and frequency regulations, power sharing, and power balance without the requirement of communication networks. For the integration of renewables, the robust power flow control is developed to achieve accurate power regulation, and some fault ride-through capabilities are also demonstrated. The energy efficiency is enhanced through robust control designs, which are investigated in variable-speed wind turbine systems to achieve both maximum power point tracking (MPPT) and robust integration of wind resources. The fault ride-through enhancement is designed and demonstrated in PV systems, where both MPPT and robust integration of solar resources are also achieved. A unified control framework for seamless mode change between islanded mode and grid-connected mode is proposed and investigated in PV systems as well. The fast responses of the power electronic converters are further demonstrated to handle the intermittency of renewables and load change. Both a lab-scale microgrid and a residential-scale microgrid are built for the validation. The results have shown that with the proposed technologies, the fully distributed operation of the microgrids is achieved with a stable, efficient, reliable, secure, and flexible manner. The results provide a promising way for the development and the implementation of microgrids, which can further improve the reliability and resiliency of the grid, enhance the grid-independence of individual end-users, promote electrification in rural and remote areas, help communities better prepare for future extreme weather events, and keep the nation moving toward a clean energy future.


This dissertation won 2nd Place in the Texas Tech University Outstanding Thesis and Dissertation Award, Mathematics, Physical Sciences & Engineering, 2020.

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Keywords

Power electronic converters, Advanced control technologies, Microgrids

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