| Summary: | Master of Science in Engineering by research only
A dissertation submitted to the Faculty of Engineering and the Built Environment,
University of Witwatersrand, Johannesburg, in fulfilment of the requirements
for the degree of Master of Science in Engineering.
Johannesburg 2018 === The conventional Sinusoidal PulseWidth Modulation (SPWM) inverter is limited
by the fact that it does not allow for Zero Voltage Switching. This means
that the switching frequency is kept low to reduce the switching losses. As
a consequence of holding these switching frequencies low, the distribution
of power over the frequency spectrum is kept closer to the fundamental frequency
(compared to higher switching frequencies) leading to larger reactive
components to filter out these harmonics. The use of high-frequency switching,
Zero Voltage Switching, and different modulation schemes can lead to
higher power densities. This research investigates under what conditions the
use of these techniques in a Dual Active Bridge (DAB) inverter might lead to
a higher power density than the SPWM.
Volumetric approximations for the different circuit components in the investigated
inverter topologies are demonstrated. These approximations are used
to design circuits using physical volume as the cost function where possible.
Additionally, a loss model is derived to determine the expected efficiency of
each topology being investigated. This model is related to the power density
since it is directly proportional to the size of heat sink required to cool the
inverter.
The techniques for improving power density mentioned above are presented,
and the impact that they have on power density is shown using the volumetric
approximation function. From this approximation, the volumes between the
DAB and the SPWM are compared and investigations into where the DAB
may have a higher power density have been performed. It was found that
the DAB was not smaller than the SPWM for frequencies less than 72kHz.
When simulating the converters operating at different frequencies, the general
trend is that the SPWM increases in volume as the frequency increases,
whereas, the DAB decreases in volume as the frequency increases. An exact
frequency at which the DAB would be smaller than the SPWM was not found
in this research. However, many conclusions have been drawn around the
use of a DAB as an inverter and the strengths and shortcomings it provides.
The modulation scheme would need to be modified to reduce the losses and
provide a more competitive volume. Additionally, multi-level and multi-stage
techniques could be used to reduce the volume further. === MT 2018
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