Author: Xu, Wenzheng
Title: Innovative control of bidirectional converters with wide load range in microgrid application
Advisors: Chan, Ka Wing Kevin (EE)
Degree: Ph.D.
Year: 2020
Subject: Hong Kong Polytechnic University -- Dissertations
Electric current converters -- Design and construction
Microgrids (Smart power grids)
Department: Department of Electrical Engineering
Pages: xiii, 175 pages : color illustrations
Language: English
Abstract: With increasing installation of clean and renewable energy sources, power electronics technology and converters play an important role in the energy transfer between the grid and each distributed sources. Stable, high-efficiency, fast-response and low-cost converters are desired and utilized everywhere. In a microgrid system, bidirectional DC-DC converters and DC-AC converters play a key role. Phase-shifted DC-DC converters are widely used and practically important to provide high conversion efficiencies through soft-switching techniques. However, the efficiency would undoubtedly drop with the output power level and is known to be poor at light load when fixed phase-shift control is adopted. Meanwhile, there is a minimum load constraint for those converters requiring the inductor current flowing through the active switch during the Zero Voltage Switching (ZVS) transition to be large enough in order to charge and discharge the energy stored in stray capacitances of the resonant rank. In this thesis, a new power electronics control technique to enable the dual features of bidirectional power flow is proposed. It provides an extended load range for soft switching in phase-shifted DC-DC converters. The proposed technique utilizes two identical full bridge converters and inverters in conjunction with a new control logic for gate-driving signals to facilitate both Zero Current Switching (ZCS) and ZVS in a single phase-shift-controlled DC-DC converter. The additional ZCS is designed for light load conditions at which the minimum load current cannot be attained. The proposed control method also has an advantage to give the converter a linear output voltage relationship with the phase shift. An experimental prototype of 1.5kW DC-DC converter with the proposed control method was built to demonstrate its capability of bidirectional power flow and show the higher conversion efficiency especially at light load compared to conventional phase-shifted converters. For bidirectional DC-AC converters, Z-Source Inverters (ZSIs) are investigated because of their capability of voltage buck/boosting by a single-stage topology and eliminating the need of dead time with the help of the added impedance network. Because of its unique feature and advantages, Z-source topology and corresponding converters have been widely investigated in areas of PV solar energy system, motor controller, EV charging and wireless transfer. However, the inserted shoot-through states unavoidably bring problems of output harmonics and the control methods are normally very complicated. In this thesis, a series of new control methods for single-phase Z-source inverters are proposed. A detailed description of the concept and principle of both methods is presented, and a comparison between each of them is conducted comprehensively. Based on this, an optimized closed loop control scheme with better harmonic elimination performance is derived. Simulations have demonstrated that the output voltage magnitude could catch the reference very well and accurately with a wide load range. The Total Harmonics Distortion (THD) of output voltage is suppressed to within 3% when the output voltage gain is lower than 2. A 1kW prototype was built and experiments were conducted to verify the theory. The proposed method keeps its simplicity to achieve a satisfying result without any additional circuitry or complicated algorithm. Compared to conventional simple/maximum boost control, it has better performance with lower harmonics and flexible voltage gain. To further extend its application, further investigation on bidirectional three-phase Z-source converters has been conducted. The proposed control scheme for Z-source inverter could be adopted in grid-connected mode and islanded mode. Bidirectional operation of both single-phase and three-phase Z-source converters was simulated and tested. Experiments conducted in OPAL-RT OP4570 simulator were used to validate the proposed control of bidirectional three-phase Z-source converters. Overall, the proposed control methods for bidirectional phase-shifted DC-DC converters and Z-source DC-AC converters have enriched the body of knowledge on converters' application in microgrids, extended load range and improved the performance on soft-switchingat light load and harmonics suppression.
Rights: All rights reserved
Access: open access

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