Improved Design Methods for Robust Single- and Three-Phase ac-dc-ac Power Converters

After a century of fast developing, the society is facing energy issues again, e.g. the exhaustion of fossil fuel, emission caused air pollution, radiation leakage of nuclear generation, and so on. How to produce and use electricity in a more sustainable, efficient, and cost-effective way thus becomes a emerging challenge. Accordingly, installation of sustainable power generators like wind turbines and solar panels has experienced a large increase during the last decades. Meanwhile, power electronics converters, as interfaces in electrical system, are delivering approximately 80 % electricity to users. Their performances including cost, efficiency, reliability, and so on, therefore are more important concerns than they were. The objective of this thesis is to study and propose advanced design methods for robust ac-dc-ac converters, which are widely used interfaces in energy conversion system. The approaches for improving their performance, in terms of the voltage stress, efficiency, power density, cost, loss distribution, and temperature, will be studied. The structure of the thesis is as follows,

Chapter 1 presents the introduction and motivation of the whole project as well as the background, the emerging challenges, and the structure of the thesis. The main content of the thesis starts with single-phase converters: Chapter 2 and Chapter 3 propose new modulation methods for single-phase B6 and H6 converters, respectively, in order to retain the same dc link voltage with two full-bridges connected back-to-back, and meanwhile improve the harmonics, control flexibility, and thermal distribution between the switches. Afterwards, active power decoupling methods for single-phase inverters or rectifiers that are similar to the single-phase ac-dc-ac converter, are studied in Chapter 4. With the proposed new active power decoupling method, the ripple power in the converter can be compensated in a more efficient and more compact way. Then, Chapter 5 changes the scope of the thesis to three-phase converters, and the nine-switch converter, as a reduced switch version of two three-phase full-bridges connected back-to-back, is studied. Application criteria of the nine-switch converter are investigated for reducing the relatively high stress introduced by the less number of switches. In Chapter 6 a rotating speed controller design method is proposed for improving the thermal loading of the three-phase wind power converter in a system level. The thesis is finally concluded in Chapter 7, before the proposal for the future research plan.

The contributions of this project include several approaches proposed for improved performance of ac-dc-ac power converters, and they can be categorized into three aspects as: control methods, auxiliary circuits, and application criteria. The details are as follows: 1. new modulation schemes of single-phase B6 and H6 converters for improved performance, 2. an optimal active power decoupling approach for kW-scale single-phase converters to achieve high power density and high efficiency, 3. application criteria of nine-switch converters for improved performance in terms of loss and temperature, 4. a new rotating speed controller design method for power levelling of wind power converters.