Novel Hybrid Unidirectional Three-Phase
AC-DC Converter Systems
by
KAZUAKI MINO
A dissertation submitted to
Swiss Federal Institute of Technology (ETH) Zurich
EIDGENÖSSISCHE TECHNISCHE HOCHSCHULE
ZÜRICH
for the degree of
Doctor of Sciences
Presented by
Kazuaki Mino
E.E., M.Sc., Tokyo Denki University
born December 23, 1968
citizen of Japan
Accepted
Accepted on the recommendation of
Prof. Dr. J. W. Kolar, Examiner
Prof. H. Akagi, Co-Examiner
Abstract
In order to reduce harmonics, many rectifier topologies have been developing. Passive rectifiers, which employ only passive components, e.g. phase shifting transformers, diode bridges, and inductors etc., show some advantages concerning high efficiency, low EMC, low complexity, and high reliability. The passive components could be compact if mains frequency is high as in aircraft and micro gas turbine applications. However, the output voltage is unregulated. Furthermore, the input current of the passive rectifiers results in a staircase waveform which is not high quality if compared to active rectifiers. In this thesis, the drawbacks of the passive rectifiers are reduced.
A diode bridge rectifier whose harmonics are reduced by adding an inductor to a diode bridge is widely used in motor drive applications. Mains current quality of the diode bridge rectifier is improved if a large inductance of the passive inductor is employed. However, the inductor is bulky, heavy and occupies a large space. In this thesis, the power density of the diode bridge rectifier is improved. The Electronic Smoothing Inductor, which is able to control a current to a constant value, is applied to the diode bridge output to act as a passive inductor. A control scheme for the DC-link voltage and active damping control for filter resonances are proposed. Moreover, the filtering concept is established to effectively attenuate EMI emission. The system dimensioning of the rectifier system is also introduced. A 5kW prototype shows a significant improvement in power density. The behaviours of the proposed rectifier system are tested by assuming practical conditions, e.g. not only ideal but also unbalanced and distorted input conditions. The dynamic behaviours are also evaluated. From the results, it is verified that the Electronic Smoothing Inductor has a similar characteristic to a passive inductor having a large inductance. Therefore, the proposed rectifier system brings a significant improvement in power density without impairing any features of a diode bridge rectifier.
On the other hand, the passive 12-pulse rectifier can be extended to a hybrid rectifier having two active switches operated in an interleaved manner. The proposed topology
ensures a controlled output voltage. Furthermore, modulation schemes to realize a purely sinusoidal input current are proposed. A 10kW prototype has been build with respect to future more-electric aircraft applications. The design procedure including the magnetic components and the active parts is introduced in this thesis. The proposed hybrid rectifier and the control schemes are verified by numerical simulations and experimental results. The output voltage is regulated. Furthermore, the input current is improved from a 12-pulse staircase shape to sinusoidal by the proposed triangular modulation. Moreover, the proposed closed loop control of input currents is performed to track a reference independently of mains voltages e.g. unbalanced and distorted input voltages. Therefore, both output voltage regulation and improvement of input current quality for the 12-pulse rectifier have been achieved by the proposed schemes.
This thesis presents two rectifier systems which perform successfully. Both systems are hybrid and allow output power to flow without switching behaviours. Therefore, the proposed rectifier systems have not only high quality characteristics but also a high reliability. The system configurations, control schemes and their features are introduced.
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