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UBC Theses and Dissertations

Low harmonic content three-phase-to-DC-conversion using AC-side switches and discontinuous conduction mode Carlton, Dan

Abstract

The quality of AC power is affected by the large number of nonlinear loads, particularly power converter systems. One way of improving the power factor of AC-DC converters in the 3-100 kW power range is by using Pulse Width Modulation rectifiers with low effects on the mains. The three-phase Power Factor Correction circuits process the whole amount of power transferred, using inductors whose current is controlled at switching frequencies above 20 kHz. Based on the inductor energy being fully or partially transferred to the output within a switching cycle, the operation is called Discontinuous or Continuous Conduction Mode. The definition comes from the shape of the inductor current, whether or not it reaches zero every switching cycle. The present work brings a contribution to the knowledge about three-phase Power Factor Correction in the Discontinuous Conduction Mode. These circuits are characterized by the small size of the inductors and simple voltage follower control with the downside of higher component stress and large input filters. The thesis investigates the performance features of circuits with AC-side switches. Two new circuits, the boost-delta and boost-star, with very competitive features emerge. The possibility of using bi-directional and quasi tri-directional switches is explored. New analytical tools are developed for the study of circuits operating in Discontinuous Conduction Mode. The average current space vector method brings a new insight into the operation of the circuits. Thus, the development of modulation techniques which improve the Total Harmonic Distortion down to zero becomes possible. Moreover, a sinusoidal current waveform in lightly unbalanced voltage systems is achievable. New circuits using two boost stages, series connected, are proposed. Advantageous features are derived without any compromise. The SEPIC converter with AC-side switches is also analyzed. A comparison is drawn among the investigated circuits. The possibility of staggered operation of several stages, which reduces the amount of ripple on the input is analyzed in this context. The contribution of the thesis consists in finding theoretically viable options for achieving high power factors with very low harmonic content in Discontinuous Conduction Mode.

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