Mechanism and Suppression of Frequency-Decrease Effect Related to the Low Modulation Frequency Ratio in a High-Ratio MMC-Based DC Transformer

The high-ratio MMC-based DC transformer is the crucial device for the interconnection between different DC distribution networks. Compared to the typical MMC, the transformer has a similar structure and low modulation frequency ratios (FR). Thus, it also has the severe frequency-decrease effect and similar operating performance deterioration. However, because of the extremely lower FRs that are close to 1, the feature of the frequency-decrease effect in the transformer has different manifestations. And because of the two-stage structure which combines the MMC and rectifiers, more factors emerged in the transformer to determine the frequency-decrease effect. Therefore, this study further develops the theory and elaborates the frequency effect's mechanism that is originally from the MMC to adapt to the more complex structure of the transformer. A quantitative analysis of the frequency-decrease effect is then provided, demonstrating that because of the transformer's two-stage structures, two types of FRs and concomitant factors exist in the transformer, thus having a combined effect on the harmonic distributions of capacitor voltages and causing more proportional voltages' harmonics distorting into the slope harmonics. Based on the analysis, the study entirely redesigns the FRs and regulates the modulation strategy of the transformer to fundamentally eliminate the frequency-decrease effect. The study also confirms that the FRs in the two sides cannot be set to the same, and their decimal part cannot cause extra low-frequency harmonics. The frequency-decrease effect, the deteriorations, and the efficiency and comparisons of the proposed methods are finally demonstrated via simulations and experiments.