Abstract
This paper investigates the impact of reference's bias on the high frequency voltage spectrum for the modular multilevel converters (MMC). Analytical solution of the voltage spectrum under carrier Phase Shifted Pulse Width Modulation (PSPWM) is derived using double Fourier integral analysis. The distribution of the switching frequency voltage harmonics is found to be directly related to the ratio between the dc-link voltage and the cell voltage. A control method utilizing this phenomenon is thereby proposed to suppress switching frequency harmonics in the Differential Mode Voltage (DMV) and Common Mode Voltage (CMV). Cell voltage control trade-offs considering different cell numbers, different ac voltages, 3rd harmonics injection, and cell voltage stress are also investigated. The analysis and control method are validated through experiments on a 1.25 megawatt MMC testbed.
Highlights
Due to the high modularity, high scalability, low semiconductor device stress, low switching losses, and low output harmonics, the Modular Multilevel Converter (MMC) is an increasingly attractive topology for high- and medium-voltage systems [1], [2]
Double Fourier integral analysis is utilized in this paper to determine the MMC voltage harmonics [18], the process for CMC and MMC half-bridge are shown in Fig. 3 and Fig. 4
MMC is operated at minimum Common Mode Voltage (CMV) mode in Fig. 19, the CMV harmonics are suppressed at 12 kHz, 36 kHz when Vcell = 1 kV, the Differential Mode Voltage (DMV) is at it’s peak value
Summary
Due to the high modularity, high scalability, low semiconductor device stress, low switching losses, and low output harmonics, the Modular Multilevel Converter (MMC) is an increasingly attractive topology for high- and medium-voltage systems [1], [2]. Many carrier based MMC modulation methods have been proposed over the last few decades [10]–[15], most of them are phase shifting and level shifting variations. Phase Shifted Pulse Width Modulation (PSPWM) distributes the switching events between different cells, so the power is evenly distributed in cells. Level-shifted Phase Disposition Pulse Width Modulation (PDPWM) is confirmed to have less output voltage distortion than PSPWM [12], [14]. WANG ET AL.: MMC SWITCHING FREQUENCY HARMONICS ANALYSIS AND SUPPRESSION THROUGH CELL VOLTAGE CONTROL converter performance, based on the insights gained from analytical solutions. Compared with the cascaded H-bridge multilevel converters [18], there is significant bias in the arm voltage reference of an MMC.
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