PWM and Control Strategies for AC-DC Matrix Converters

• Author / Creator

  Fang, Fanxiu

• Alternating Current (AC)-Direct Current (DC) matrix converters can realize buck type rectifying and boost type inverting without bulky and vulnerable intermediate DC link capacitors. Thus, the topology shows great advances in applications like vehicle to grid, DC microgrid, data center, and telecom equipment, etc. According to the applications’ requirements, AC-DC matrix converters can be built with or without a high frequency transformer (HFT) for isolation. The non-isolated AC-DC matrix converter has a simple structure and can effectively reduce the system size and cost, while the isolated AC-DC matrix converter can provide galvanic isolation for safety. However, both types of AC-DC matrix converter face some challenges, such as reliability of commutation, current distortions, and control complexity.
  The purpose of this thesis is to develop modulation and control strategies for AC-DC matrix converters; therefore, current qualities can be improved while reliable commutation is ensured.
  Firstly, the challenges in non-isolated AC-DC matrix converters are addressed. The multiple objectives control of the non-isolated AC-DC matrix converter is a major challenge since the single-stage converter shall regulate both AC current and DC current. To improve the quality of both currents and avoid narrow pulses, the finite control set model predictive control (FCS-MPC) with virtual space vectors, featuring multiple objectives capabilities and direct generation of gating signals without the pulse width modulation (PWM) scheme, is proposed.
  Secondly, the challenges of isolated AC-DC matrix converters are addressed. To enhance the commutation safety while achieving sinusoidal AC current, a 9-segment space vector modulation (SVM) strategy is proposed to avoid the commutation between two phases with close voltage magnitudes. The strategy can achieve high current quality even under a relatively large commutation duty ratio (commutation time divided by switching period). Generally, the commutation between bidirectional switches will require a long commutation time; however, with the development of fast-switching devices, the commutation time is reduced significantly. When the switching frequency is not very high, the commutation impact becomes smaller. In this case, another SVM method is developed to reduce low order harmonics with a reduced number of switching actions for higher efficiency and high quality AC current. To further improve the performance under light load condition based on the proposed SVM method, a new control method to coordinate modulation index and phase-shift angle is proposed to reduce current stress and improve system efficiency.
  Finally, with the galvanic isolation provided by the HFT, isolated AC-DC matrix converters can be operated in parallel with increased power rating and mitigated circulating current. To achieve high performance for paralleled configuration, an interleaved sine pulse width modulation (SPWM) strategy is developed for unidirectional isolated AC-DC matrix converters. The interleaved SPWM can significantly suppress DC current ripples and it has good scalability and ease of implementation. Also, the duty cycle loss is compensated to reduce low order harmonics. As a result, the converters can achieve better current quality at both the AC and DC sides.

• Subjects / Keywords

  • AC-DC matrix converter
  • modulation strategy
  • control strategy

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-462p-ts98

• 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.