Year

2021

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

This thesis investigates grid integration techniques of the distributed energy resources (DERs) with a magnetic linked converter (MLC). Rapid penetration of the DERs, e.g., solar, wind, and wave in the existing power grid introduces several power quality problems. Moreover, the future power grid should be robust enough to handle diverse non-linear loads and random charging/discharging of electric vehicles. This thesis aims to develop an MLC-based grid integration technique with improved controller architecture, where the distributed energy resources and various loads can be integrated into the grid directly without requiring any intermediate energy conversion unit. In this way, the entire energy conversion structure becomes compact and energy efficient. Moreover, the developed grid integration technique solves the power quality problems and achieves high stability and resiliency by incorporating a coordinated control algorithm.

The MLC has found diverse applications in solid-state transformer (SST), electric vehicles, electric aircraft, shipboard power system, pulsed-power system, and renewable integration. The MLC-based SST can perform as an embedded energy router in a power grid. Such a framework removes the complexities associated with the interconnection of the traditional distribution grid and the DERs from the viewpoints of reliability, controller performance, and communication functionalities. The MLC-based SST has the potential to achieve several unique functionalities, such as improvement of the grid power quality, regulation of the voltage and power factor, support of the grid reactive power, provision of real-time communication, and intelligent management of the energy flow. The design of the MLC-based SST requires multidisciplinary expertise from the field of communication, power electronics, magnetics, and control to harness the benefits mentioned above.

At first, this thesis investigates the grid integration issues of the DERs with the MLC, different MLC-based SST topologies and their prospective application areas, multilevel converters for the MLC-based SST, development of an optimized magnetic link for the high-frequency operation of the MLC, solid-state switching devices for the MLC, and control techniques for the MLC-based SST. Later, the selection methods and development procedure of an optimized high-frequency magnetic link with advanced magnetic materials in the laboratory environment are covered. For the energy efficient design and high energy density operation of the MLC, the high-frequency magnetic link should exhibit low specific core loss at high frequency and high saturation flux density. A shape and a size optimization procedure are carried out to select a suitable high-frequency magnetic link. Two prototype cores are developed in the laboratory based on amorphous and nanocrystalline magnetic materials due to their excellent magnetic properties. The cores are characterized under high-frequency non-sinusoidal excitations from the high-frequency inverter, suitable for the MLC applications. A detailed core-loss model for the non-sinusoidal excitations is developed including the dead-band of the high-frequency inverter and validated using experimental core loss measurements.

After that, an improved control-oriented modeling method is developed for a multiport MLC-based SST application. The improved large- and small-signal-based average models facilitate to understand the underlying control dynamics of the multiport MLC and design a robust control architecture. The power transfer characteristics of the MLC can also be defined with the developed average models. The core loss characteristics of the magnetic link under MLC-based SST application are also investigated by experimental measurement of core loss and examining the hysteresis curves.

Afterward, a data-driven coordinated controller architecture for an MLC-based SST is developed, which can ensure improved power quality of the distribution grid. A step-by-step controller design methodology is developed, which can coordinate several control data according to the various operation functionalities. The simulation and experimental results validate the effectiveness of the proposed coordinated control approach.

Next, a novel MLC-based wind energy technology and a novel wind-wave hybrid ocean energy technology (HOET) are proposed. With the proposed novel MLC-based wind energy technology, the wind energy system can be directly integrated to the grid with the multiport MLC. An improved modulation scheme is proposed for the converter over-modulation operation to reduce the switching loss and improve the voltage harmonic profile. Necessary loss analysis is carried out in the grid connected and the islanded mode with the core loss characteristics of the magnetic link. Moreover, it is shown that the ocean energy system can be integrated effectively with the wind energy system by the proposed wind-wave HOET. The proposed HOET works on multiport magnetic bus concept, which can be operated as the multiport MLC. The detailed modelling method of the Archimedes wave swing-based wave energy converter is developed to design a damping controller for the maximum power extraction from the wave. Furthermore, a voltage and current control architecture is proposed for the power balance in the multiport magnetic bus.

Subsequently, an improved voltage balance controller and a novel multi-loop load-disturbance rejection controller are proposed for the smooth integration of the DERs and local dc/ac loads to the distribution grid. The improved voltage balance controller shows high robustness to dc-link capacitance mismatch and high ripple suppression capability in the cascaded structure of the MLC-based energy conversion unit. The proposed multi-loop load-disturbance rejection controller can achieve high load-disturbance rejection. The controller showing low sensitivity to the load-disturbance ensures robust voltage regulation performance. This facilitates plug-n-play connectivity and the dc-port of the MLC can be configured to form a dc fast-charging platform for electric vehicles or a strict dc-bus for a dc microgrid. Moreover, an improved decoupled power balancing controller architecture is proposed to balance the power in the multiple ports of the MLC under leakage parameter mismatch of the magnetic link indicating a controller safe operation area.

Finally, the thesis is concluded by summarizing the contributions with relevant outcomes and indicating the future research prospects in the MLC-based grid integration technique for the DERs with associated modelling and controller design methodologies.

FoR codes (2008)

0906 ELECTRICAL AND ELECTRONIC ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.