Power Conversion Techniques Using Dual Inverter Structures

  • Author / Creator
    Perera, Lokugonaduwage S. C.
  • Dual inverter topologies can produce multilevel output voltage waveforms and have multiple degrees of freedom in control. Due to their versatility and modular nature, they find many different applications in power electronics. Although these topologies have been in existence for almost three decades, there is still room for improvement in terms of their utilization. The aim of this work is to further utilize the capabilities of these topologies in existing dual inverter applications and to explore new opportunities in applications that could benefit from their use.
    Two dual inverter structures are explored in this thesis. The first is an existing topology: the dual inverter drive using a floating capacitor bridge. This topology can provide induction machines with a reactive voltage boost extending their speed range without increasing supply voltage requirements. However, this presents some unique challenges with respect to drive modulation and capacitor voltage control. These challenges are addressed through the development of a new carrier based PWM scheme and a control strategy. The PWM scheme is able to produce high quality 5-level PWM line voltage waveforms regardless of the difference in phase or modulation depth between inverters. This improves the motor current THD and harmonic volt-seconds, reducing high frequency motor losses. The proposed control strategy uses a stator current reference frame that is able to decouple the motor and capacitor dynamics such that the capacitor voltage can be maintained constant during motor transients. This allows the floating bridge capacitor size to be reduced by a factor of more than ten compared to existing works. Additionally, the use of this reference frame provides an extra voltage boost that reduces supply voltage requirements by 26%.
    The second dual inverter structure studied utilizes a center-tapped open-ended winding grid transformer to produce new topologies with enhanced capabilities. Two new multiport topologies are derived for application in the emerging landscape of hybrid AC/DC grids where high penetration of renewable DC energy sources and storage is needed. Dual inverter topologies are ideal for such applications due to their multiple AC/DC ports and degrees of freedom in control. The first topology derived is for bipolar DC distribution that is capable of performing the two tasks of exchanging power between the AC grid and bipolar DC grid, while keeping the DC pole voltages balanced without the use of additional components or penalties on component ratings. The second topology is a multiport DC/DC/AC converter which is suitable for applications such as renewable energy integration with battery energy storage, or DC fast charging. The topology significantly increases the power transfer limits of the ports compared to using multiple separate converters. Both topologies allow bidirectional single stage power transfer between their ports, reducing the number of conversion stages and losses. This is made possible by a control strategy developed to decouple AC and DC power transfer mechanisms. Through the use of these mechanisms and the multi-tasking of inverters, the capabilities of the two dual inverter topologies are significantly enhanced compared to two inverters operated independently.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Doctor of Philosophy
  • 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.