Model predictive control of a doubly fed induction generator.

Masters Degree. University of KwaZulu- Natal, Durban.The world is currently is energy despair. For years, the world has relied on fossil fuels as the main energy source to produce electricity. At the start, this worked well as there was an abundance. However, due to the increase in population, urbanisation and the birth of many industries, this fuel source has been put under strain. Furthermore, the harmful emissions from the use of fossil fuels has been a great contributor to the destruction of our precious ozone layer. This in turn has gradually increased the harmful effects of global warming on Earth. The need for clean, reliable sources of energy has increased over time, and in a few years, it is expected to be the only source of energy utilized in the production of electrical energy. The research undertaken in this project involves the control of the doubly fed induction generator, which is used in wind energy conversion systems. Commonly termed DFIG, this generator has gained worldwide popularity and is used in majority of wind energy conversion systems. It provides direct grid connection and uses only a partially rated converter. However, the conventional control methods used in the control of the DFIG are either difficult to implement or inefficient. Some require complex tuning of proportional-integral controllers while some produce distorted results. The aim of this research was to investigate and evaluate the application of model predictive control to the control of the DFIG. There exist various different control strategies for the control of the DFIG. This research involved implementing all of the different control strategies via conventional methods and then via the use of model predictive control. Despite there being various methods to implement model predictive control, due to its simplicity and strong suitability, finite control set model predictive control was used in this research. Each of the control strategies implemented both conventionally and via model predictive control were thoroughly analysed in terms of the steady state response, dynamic response and quality of stator current. A comparison between the corresponding control methods is also presented

Control of Asymmetric Permanent Magnet Synchronous Generator Systems

The thesis focuses on the control of asymmetric permanent magnet synchronous generator (PMSG) system, with particular reference to the suppression of its second harmonic (2h) power, DC bus voltage and torque ripples. The asymmetries include the unbalanced resistances, unbalanced inductances, and unbalanced 3-phase back-electromotive forces (EMFs). The mathematical model of the general asymmetries in the PMSG system is firstly presented. The power ripple and torque ripple due to the asymmetries without/with negative-(N-) sequence currents are then analysed in detail. It shows that there are 2h impedances in the synchronous dq-axis frame. Consequently, the N-sequence currents emerge under the conventional current proportional and integral (PI) control, which will result in undesired 2h power, DC bus voltage and torque ripples. To suppress the 2h torque resulted from the N-sequence currents, three typical methods aiming for balanced currents without N-sequence currents are reviewed, evaluated and their relationship is revealed. It shows that all these three methods are capable of suppressing the N-sequence currents as verified by experiments. However, the 2h power and DC bus voltage cannot be suppressed. To suppress the undesired 2h power and DC bus voltage, an improved power control without any sequential component decomposers under general unbalanced conditions is proposed. Its effectiveness is validated by elaborated experiments on a prototype PMSG with inherent asymmetry and deliberately introduced asymmetries. However, the 2h torque is compromised. To solve the 2h torque, power and DC bus voltage simultaneously, the compensation in parallel with the DC bus is investigated in the PMSG system with asymmetric impedances. The undesired 2h power from the PMSG is compensated by the 2h power from the compensation unit. Two topologies of the compensation unit and corresponding control methods are investigated, while the compensation effectiveness is validated by experiments. Furthermore, the compensation unit with external circuits in series with the asymmetric PMSG is investigated. By the compensation in series, the original unbalanced system is modified to a balanced system in theory. Therefore, the N-sequence currents, 2h power, DC bus voltage, and torque ripple can be naturally suppressed. The feasibility of this compensation method is verified by experiments at different speeds and load conditions, although the effectiveness may be slightly affected by the non-linearity of the compensation inductors in practice. Finally, the research of suppressing the 2h DC bus voltage and torque ripple is extended to the dual 3-phase PMSG system with one channel failed. By utilizing the windings, rectifier or inverter in the faulty channel which are still functional, three methods designated as two sets in parallel, two DC buses in parallel and N-sequence currents compensation are investigated, which require minimum extra hardware investment compared with the compensation in parallel and in series