南方能源建设 (Sep 2023)
Capacitor Voltage Ripple and Harmonics Analysis in MMC
Abstract
[Introduction] The rapid development of VSC-HVDC provides practical support for MMC (Modular Multi-Level Converter) topology. Takes a half bridge MMC converter as an example, the relationship between steady-state capacitor voltage fluctuation, harmonic interaction, and bridge arm switching function of MMC is studied in order to provide theoretical support for MMC system design and control strategy. [Method] By establishing analytical expressions for the sub module capacitor voltage and bridge arm switch function, the mathematical approximate expressions for the energy fluctuation and capacitor voltage fluctuation of the upper and lower bridge arms of the MMC converter were obtained. Furthermore, the distribution law of harmonics in the upper and lower bridge arms current, AC (Alternating Current ) valve side, and DC (Direct Current) side, as well as the positive and negative sequence relationship of inter phase circulation were studied. Based on the harmonic formula, an estimation method for the secondary circulation was obtained; Furthermore, the variable comparison method was used to study the relationship between the switching function of the bridge arm and the voltage of the submodule capacitance with the valve side power factor and the size of the submodule capacitance. Finally, a detailed PSCAD/EMTDC model was built based on actual engineering parameters, and a consistency comparison between theoretical analysis and offline simulation was conducted. [Result] The research has shown that the inter phase circulating current of MMC only contains even harmonics, with 6k+2 circulating currents exhibiting negative sequence, 6k+4 circulating currents exhibiting positive sequence, and 6k circulating currents exhibiting zero sequence characteristics. At the same time, under steady-state conditions, the AC voltage and current flow at the outlet of MMC valve side only contains odd harmonics, while the DC voltage and current flow at MMC DC side only contains even harmonics, and at the power factor angle φ<0, the peak value of the bridge arm switch function will be greater than 1, and the peak voltage fluctuation of the module capacitor at low power factor is greater than that at high power factor. [Conclusion] Based on this, we can select and design capacitor for MMC inverters and carry out the transient stability strategies, also, master the operational characteristics of MMC topology through theoretical analysis.
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