This thesis aims to extend the body of knowledge about the modulation
and control of converters to cover multilevel converters.
A general definition of a (power) converter, and subsequently a multilevel
converter, is given based on a unified converter theory. Different converter
structures are shown to have common features under this unified set of
rules. The most common multilevel topologies found in the published literature
are examined and compared. Some other example converters are also
developed theoretically from the basic definition and by using the concept
of duality.
Starting from the essential requirements, the different approaches to switching power converter modulation are explained and compared. In particular, aspects of modulation which are required or desirable for multilevel converters are discussed. Sine-triangle carrier modulation is identified as the most promising technique to pursue for both technical and pedagogical reasons.
Natural and uniform sampled sine-triangle modulation are examined and contrasted in detail. The modulators' transfer function and spectral properties are examined mathematically, and then confirmed both with simulations and software and hardware implementations. The natural sampling process is shown to have a flat, distortionless transfer function, which uniform sampling cannot achieve. Uniform sampling is invariably implemented digitally, either in software or hardware, with many associated advantages, while natural sampling does not lend itself to a digital implementation. The advantages and short-comings of each technique are generally exacerbated by multilevel converters. A digital implementation of naturally sampled PWM would seem the perfect solution.
Re-sampled Uniform, a technique for achieving natural like sampling
digitally is suggested. The success of this approach is demonstrated as
improvements are shown for three implementations, one in software and two
in hardware. Some unexpected
effects are commented on in the subsequent analysis of the results.
Further improvements to carrier based PWM are possible by randomising the carrier period, and by modulating the zero sequence component in a three phase converter. The carrier period can be varied without causing distortion of the desired modulating waveform. It blurs the sharp spectral harmonic terms associated with the carrier into a continuous spectrum with lower peak amplitudes. The successful technique for generating multilevel random modulation also demonstrates that the separate sub-cycles can be treated as PWM switch periods in their own right. This has significance for space vector modulation, re-sampled uniform, and alternative modulation techniques.
In a three phase converter with no neutral connection, any voltage which is common to all three phases does not cause any current to flow. Such a zero sequence component may be deliberately introduced to increase the maximum possible modulation depth and hence converter utilisation, lower the switching rate, and possibly lower distortion. A number of possible zero sequence components for multilevel modulation are tested using simulations. Only the increase in maximum modulation depth is a significant advantage. The distortion (spectral) benefits are minimal, and discontinuous switching cannot be implemented in some topologies.
These different approaches to improving the modulation of multilevel converters have been applied using sine-triangle PWM. The outcomes of this research can be generalised to other methods of PWM, and also to conventional two-level converters.
Geoff Walker / School of Computer Science and Electrical Engineering / walkerg@csee.uq.edu.au