Flux compensation for interleaved parallel-connected multilevel converters using cross-coupled inductors

  • Author / Creator
    Zhang, Chenhui
  • To meet the continuously growing energy demand for both industrial and civilian use, parallel-connected voltage source converters (VSCs) using interleaved switching techniques have become increasingly popular over the single module VSC for a variety of applications: electric vehicles, aerospace, and renewable energy systems. With poorly designed switching patterns, the inductors required to connect paralleled converters together can have a size and weight higher than they need to be. The switching patterns used in parallel modules can also produce low-quality pulse-width-modulated (PWM) voltage outputs, which adversely affect the system output currents and result in higher than necessary power losses in the output filter inductors and machine loads.
    Pulse-width-modulated (PWM) switching patterns are presented for parallel-connected VSCs that are used to reduce the size and weight of the converter output inductors. These inductors are magnetically coupled to reduce the flux in their magnetic cores, resulting in a much smaller size and weight. The winding connections of these cores are magnetically cross-coupled. This results in a large inductance between the converter outputs, reducing circulating currents that increase the converter power losses. Conversely, the cross-coupled windings also result in a very low series out impedance, hence lowering fundamental voltage drops and increasing the voltage reaching the load. Switching patterns, using interleaved carriers and either: (a) carrier swapping techniques; or (b) reference signal modification, are used to lower the output voltage harmonics, hence improve the quality of the output multi-level line voltages in 3-phase systems.
    For the switching patterns described, the flux in the coupled inductor cores can experience rapid jumps that increase the peak flux experienced in the cores. The most significant contribution of this thesis is the modified switching patterns presented that suppress these flux jumps, referred to as flux jump compensation techniques. Controlled predictable flux patterns allow the core cross-sectional area to be significantly reduced with the core size and weight.
    In addition to modified PWM control using continuous switching, discontinuous switching patterns are presented (DPWM), which can be used to reduce the converter average switching frequency and significantly reducing the converter switching losses. The inverter switching patterns in DPWM are held high or low for two 60o periods in a fundamental cycle. The transitions from no switching to continuous switching cause flux jumps in the cores. The reference signals are modified to provide flux jump compensation for these transitions as well as for carrier swapping. The modified DPWM control lowers the peak flux in the CI core, hence reduces the core size and weight.
    The effectiveness of the various proposed PWM methods described are verified using both simulations and experimental results for a 3-phase parallel-connected coupled inductor converter prototype.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.