Kim, Minjeong (2017-12). Commutation Technique for High Frequency Link Inverter without Operational Limitations and Dead Time. Master's Thesis.
An improved commutation technique for the ac-ac output converter circuit of a pulse width modulated high frequency link (HF-link) inverter has been investigated. The high frequency link inverter converts a DC input voltage into line frequency AC output voltage using a high-frequency transformer for voltage step-up and galvanic isolation, without an intermediate rectification and DC bus. In this topology, there is a direct ac-ac converter, which processes the HF-link square-wave voltage into the desired sinusoidal ac output voltage. To do this requires a commutation method to prevent shoot-through when output current changes direction or commutates from one switch to the next. Conventionally, dead time is used but this adds distortion to the output waveform. Previously a commutation technique without dead time was introduced, but it required a number of assumptions on the inverter load impedance and link voltage characteristics that made it useful for a stand-alone R-L load but not practical for grid connection. The commutation method in this paper does not require dead time and does not impose any limitation on the output inductance and link voltage magnitude and frequency. Simulations, results experimental results and detailed analysis of output current THD values are presented.