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Novel methods of synchronous modulation for the medium-power and high-power converters

Final Activity Report Summary - MODULATED CONVERTERS (Novel Methods of Synchronous Modulation for the Medium-Power and High-Power Converters)

Novel methods of synchronous space-vector pulsewidth modulation were developed with potential application to perspective topologies of power electronic converters and adjustable speed electric drives with increased power rating. In particular, basic schemes and algorithms of synchronous modulation were disseminated for the control of:
1. three-level neutral point clamped inverters with cascaded configuration;
2. cascaded two-level converters with standard and non-standard control modes;
3. multilevel dual inverter-fed systems with two separate direct current (dc) sources and power balancing capability;
4. asymmetrical split-phase, i.e. dual three-phase, converters and drives with single dc source;
5. asymmetrical six-phase motor drives with two dc sources with different voltages;
6. five-phase voltage source inverters with a wide control range;
7. symmetrical dual three-phase converters and drives with single dc source;
8. symmetrical split-phase systems with two dc sources with different voltages.

The results of theoretical and practical investigations proved the fact that the spectra of the phase and line voltages of power conversion systems, controlled in accordance with algorithms of synchronous modulation, did not contain even harmonics and sub-harmonics for any ratios between the switching frequency and fundamental frequency, contributing to increase the efficiency of operation of the medium-power and high-power converters and electric drives. Specialised schemes of synchronous modulation were elaborated and investigated, allowing for minimisation of undesirable common-mode voltages and currents in some topologies of converters and drives, therefore contributing to prolongation of the life span of electric drives fed by modulated power converters.

It was also shown that the proposed methods of synchronous pulsewidth modulation were well-suited for the effective control of power conversion systems in the zone of overmodulation, providing smooth symmetrical voltage adjustment up to the highest fundamental frequencies. All algorithms of synchronous modulation, based on space-vector approach for determination of the pulse patterns, provided minimum number of switchings and minimal switching losses in power conversion systems with increased power rating.

Furthermore, on the basis of the comparative analysis of operation of the power conversion systems with basic versions of synchronous modulation, some recommendations regarding selection of the rational scheme of modulation, in dependence of the system topology and its control mode, were formulated.

Along with the remarked practical relevance, the proposed and developed methodology of synchronous space-vector modulation could provide, because of its physical clarity, an improvement of the understanding of complicated processes of pulsewidth modulation in modern power conversion systems. Therefore, it could be used in teaching and learning.