Abstract
This paper concentrates on the simplest quasi-two-level PWM operation mode for modular multilevel converters, where the internal currents within the converter are not controlled. The model of the converter is derived and the properties of the inherent resonance circuit within the converter are discussed. The paper proposes an optimized design approach for the converter parameters and shows several challenges in the practical design. The main problem of the studied converter operation mode is the dependency of the converter behavior on many parasitic parameters that can significantly vary in the converter production. Moreover, the paper shows that when the converter losses are low, the optimized converter inductances are below the values expected for realistic converter construction. Consequently, the module capacitances have to be significantly increased or the converter internal currents reach exceedingly high values. Furthermore, a comparison is drawn to the quasi-two-level PWM operation mode in which the leg currents and branch energies are controlled, showing several crucial disadvantages of the studied passively damped operation mode. The utilized models and the basic concepts are validated experimentally on a downscaled converter prototype.
Highlights
Quasi-two-level pulse-width modulation (PWM) operation of modular multilevel converters (MMCs) is beneficial for an application in medium-voltage low-speed drives
As a trade-off, the quasi-two-level waveforms lead to higher harmonic distortion of the voltages and currents at the converter input and output
This requires the use of an additional capacitive filter at the converter input, the output currents are filtered by the inductances of the low-speed machine
Summary
Quasi-two-level PWM operation of modular multilevel converters (MMCs) is beneficial for an application in medium-voltage low-speed drives. In their research [10], [11], they propose to change the shift of the modulation carriers non-optimally during the low machine speeds This way, quasi-two-level voltage waveforms are generated at low speeds, thereby stabilizing the converter behavior, and the conventional multilevel operation is applied at high speeds. Since the main motivation of the approach was the application at low speeds only, the module capacitance is not significantly reduced compared to that of conventional MMCs. The quasi-two-level PWM operation for MMCs that applies the quasi-two-level voltage waveforms over the whole operating range to minimize the installed module capacitance was first proposed by the authors of this paper in [1], [12]. The paper provides a deeper analysis of the converter behavior and proposes a novel optimized design process for this quasi-two-level-operated MMC in order to minimize the required module capacitance. Comparisons to the approaches of the second group are drawn and discussed
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