Single Stage Transformer Isolated High Frequency AC Link Inverters without the Problem of Leakage Energy Commutation

Abstract

High frequency AC link based converters offer several desirable features such as improved power density, galvanic isolation and improved reliability due to the absence of an electrolytic capacitor. The power conversion happens through a high frequency link which operates in the tens of kilohertz range and consequently, the size of magnetic components is reduced. An AC or DC voltage source is ’chopped’ by a converter to generate the high frequency voltage which is applied to a transformer that can be used to provide the necessary voltage transformation. On the secondary winding of the transformer, the high frequency voltage is directly converted to synthesize the desired output voltages which are at a much lower frequency than the AC link. Unlike DC bus based topologies, there is no need for a power stage to first rectify the AC link before generating the output voltage. Despite several advantages, transformer isolated single stage power conversion re- quires the use of a voltage clamping circuit across the AC link for its operation due to the presence of transformer leakage inductance. When high frequency AC link volt- age is directly used to synthesize the output voltages, the link currents exhibit sudden polarity reversals. Due to the presence of leakage inductance, which is unavoidable in transformers, large voltage spikes result during the current reversal instants and neces- sitate the use of additional circuitry to limit such overvoltages. This limitation, which is termed the problem of leakage energy commutation, needs to be overcome to pre- vent the link voltage from exceeding the breakdown voltages of the switches, damaging winding insulation or causing unwanted distortion in the output voltage. This dissertation proposes two implementations of transformer isolated high frequency link inverters that overcome the problem of leakage energy commutation. The inverters consist of a H-Bridge to generate the high frequency voltage from a DC voltage source and a cycloconverter to generate the output voltages. In the first implementation, a parallel LC tank is proposed to be connected across the secondary winding of the transformer to ensure the current through the leakage inductance does not have sudden reversals. The high frequency transformer along with the LC tank results in a sinusoidal voltage which is applied across the cycloconverter. A discrete space vector modulation technique is used to synthesize three phase output voltages and any change of switch states in the cycloconverter is restricted to the zero crossing regions to avoid switching losses. The second approach proposes using an additional leg across the cycloconverter to induce a resonant current transition when the current flowing through the transformer has to reverse its polarity. The additional leg, comprising of a bidirectional switch and a small capacitor, is only used for a short interval during which it resonates with the leakage inductance. The operation and analysis of the proposed ideas are discussed in this thesis and have been validated through computer simulations. A hardware prototype was constructed to demonstrate the operation of a high frequency link inverter using the LC tank and the results are found to be in agreement.