Feasibility study of the dual active bridge as a low-frequency sine wave inverter

Ortlepp, Kerren Mark
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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.
A dissertation submitted n fulfilment of the requirements for the degree of Master of Science in Engineering to the Faculty of Engineering and the Built Environment, University of Witwatersrand, Johannesburg, 2018
Ortlepp, Kerren Mark (2018) Feasibility study of the dual active bridge as a low-frequency sinewave inverter,University of the Witwatersrand, Johannesburg, https://hdl.handle.net/10539/25687