Advanced Selective Harmonic Elimination Pulse Width Modulation for High-Power Medium-Voltage Converters
Creators: Wu, Mingzhe
Publisher: name: University of Alberta Library
Year: 2023

<p>High-power medium-voltage converter play an indispensable role in various industrial applications. In high-power scenarios, there is a strict limitation on the switching frequency of the pulse width modulation (PWM) scheme to avoid excessive switching losses. The minimization of switching frequency and the design of cost-effective solutions that can well-balance the switching losses and output performance are important tasks for high-power medium-voltage converters.</p>

<p>Among the existing PWM methods, selective harmonic elimination PWM (SHE-PWM) is considered as one of the most attractive solutions for high-power medium-voltage converters, which demonstrates several advantages such as good output waveform quality with low ratio of switching frequency to fundamental frequency, reduced filtering requirement, and low switching losses with tight control of harmonics, etc. Despite the numerous merits of SHE-PWM, its off-line operation nature with low switching frequency can induce a number of challenges on its implementations.</p>

<p>In this thesis, advanced SHE-PWM formulations, capacitor voltage balancing methods, and closed-loop control strategy are investigated for further improving the performance of SHE-PWM operated high-power medium-voltage converters. Firstly, three advanced SHE-PWM formulations are presented, including the unified SHE-PWM formulation with optimal output waveform quality, generalized SHE-PWM formulation with common-mode voltage (CMV) reduction ability, and generalized SHE-PWM formulation with natural capacitor voltage balancing ability. As for the unified SHE-PWM formulation for even-level multilevel converters, it explores all existing switching patterns for each modulation index and selects the optimal one based on a specifically designed index. Therefore, the optimal output waveform quality can be guaranteed. Then, the advanced SHE-PWM formulation with CMV reduction ability for all voltage source converters is proposed. The mathematical model of CMV under SHE-PWM is established firstly, based on which the SHE-PWM formulation with CMV reduction ability is proposed by including the third-order harmonics into the SHE-PWM model. As for the generalized SHE-PWM formulation with natural capacitor voltage balancing ability, it is applicable for all four-level neutral-point clamped and flying capacitor converters, especially those without redundant switching states that cannot be controlled by conventional PWM methods. The capacitor charge equation over one fundamental period under SHE-PWM is first derived, which is incorporated into the proposed formulation to achieve natural capacitor voltage balancing as well as fundamental control and harmonic eliminations.</p>

<p>Secondly, to address the capacitor voltage balancing issue for complex multilevel converters with a large number of redundant switching states under SHE-PWM, a composite SHE-PWM and model predictive control (MPC) strategy is proposed in this thesis. After receiving the output voltage level signal from the SHE-PWM modulator, the optimal switching state that can minimize the cost function is searched out by the MPC module, where the cost function is designed to simultaneously balance capacitor voltages and regulate switching frequency. Besides, a dynamic weighting factor design is proposed to improve the overall control performance.</p>

<p>Thirdly, to improve the closed-loop control dynamics of current source rectifiers (CSRs) under SHE and selective harmonic compensation (SHC) PWM, a model-based closed-loop control scheme is proposed in this thesis. When the DC current reference of CSR changes in transient, the new input references of the SHE/SHC-PWM module, i.e., modulation index and/or delay angle, can be calculated directly based on the derived mathematical model of CSRs and the input reference signal, then updated in real-time without being slowly adjusted by the linear controller to achieve a faster dynamic response.</p>