Control and Modulation Techniques for the Five-Phase Coupled Inductor Inverter

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Copyright: Tan, Cheng
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Abstract
Multilevel multiphase voltage source inverters have been increasingly attractive to the industry and academia with the development of multiphase machine drive systems. Although the additional phase legs and fast switching semiconductors complicate the control strategies, the advantages of multiphase multilevel inverters are considerable: the voltage and current rating on the semiconductors are reduced, fault tolerance is realized without extra hardware and harmonic spectrum is improved compared with two-level voltage source inverters. This thesis focuses on the development and practical assessment of a five-phase three-level pulse-width modulated (PWM) voltage source inverter (VSI) and associated modulation techniques. The PWM VSI employs a coupled inductor in each phase-leg to provide three-level output voltages. Compared with the popular three-level neutral-point-clamped inverter, the coupled inductor inverter (CII) uses fewer switching devices, has no dead-time and associated distortion, and does not require dc-link capacitor voltage balancing. The five-phase system increases the possible magnetic configurations. The thesis explores various combinations of magnetic cores—toroidal cores and three-limb cores. The design of the coupled inductors is illustrated, and the core loss is computed. Conventional modulation techniques, both carrier-based PWM techniques and space vector modulation techniques, cannot be directly applied to the CII as they do not achieve volt-second balance of coupled inductor windings. Thus, a new space vector modulation technique is proposed for the five-phase CII, aimed at eliminating the d2q2 sub-plane voltage. Two carrier-based PWM techniques aimed at reducing the common-mode voltage are proposed for multiphase inverters in general and then modified for use in the CII system in order to achieve volt-second balance in the coupled inductors. Mathematical models of the coupled inductors, total inverter current and current stress on the dc-link capacitors are established, based on sinusoidal PWM and the two proposed carrier-based PWM techniques for the CII. This provides an insight into the influence of the coupled inductors on the system, in terms of system efficiency and current stress in the dc-link capacitors. The five-phase CII is compared with the five-phase neutral-point-clamped inverter with the implementation of several carrier-based PWM techniques, namely sinusoidal PWM, the proposed common-mode voltage reduction techniques, and harmonic injection PWM, in terms of the efficiency, total harmonic distortion and the load current ripple. A five-phase CII test rig is developed, controlled by a combination of DSP and FPGA. The effectiveness of the proposed modulation strategies is verified by experiment on a passive RL load but also on a five phase motor load with closed-loop control (vector control and hysteresis control).
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Author(s)
Tan, Cheng
Supervisor(s)
Fletcher, John
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Publication Year
2016
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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