Multi-level voltage and current reinjection ac-dc conversion
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis describes a new concept of multi-level reinjection ac-dc conversion, its main purpose being a further reduction of the harmonic content, a solution of dynamic voltage balancing for direct series connected switching devices and an improvement of high power converter efficiency and reliability. It is a combination of the multi-level, soft switching and reinjection concepts. A variety of configurations are proposed, based on the new concept, to achieve efficient voltage and current conversion. For each configuration the firing sequences, waveform analysis, steady and dynamic performances and close-loop control strategies are presented, and particular applications suggested. The ideal reinjection waveforms are first derived for perfect harmonic cancellation and then fully symmetrical approximations are made for more practical implementations. This is followed by a description and comparison of the generation circuits required for the implementation of the multi-level symmetrical reinjection waveforms. A three-level voltage reinjection scheme, implemented by adding a reinjection bridge and a reinj ection transformer to the standard twelve-pulse converter, is discussed in great detail, both for the series and parallel connections. This is followed by an investigation into the possible application of these converters to Back to Back VSC HV de interconnection; the analysis is validated by EMTDC simulations. A multi-level voltage reinjection VSC is also proposed, which uses a controllable de voltage divider to distribute the de source voltage to the two main bridges and produces high quality output waveforms. The voltage and current waveforms, the firing sequences and the capacitor voltage balancing are analyzed and verified by EMTDC simulations. In particular, the proposed VSC is shown to be an ideal solution for the STATCOM application. The multi-level reinjection CSC alternative is also described and shown to exhibit an excellent performance in the STATCOM application.