Symmetric double cascade DC conversion unit with circulating current reverse self-elimination capability

Abstract

The present invention discloses a symmetric double cascade DC conversion unit with circulating current reverse self-elimination capability. The unit can be used for DC voltage interconnection of different voltage levels under the condition of high voltage and high power. The DC conversion unit comprises two cascade modules, wherein input ends of the two cascade modules are in series connection, and output ends of the two cascade modules are also in series connection. Each cascade module consists of two modular multilevel converter (MMC) structures and an intermediate frequency transformer. The DC conversion unit is subjected to series-parallel modular combination to obtain a high voltage and high power DC converter, and finally the object of DC voltage conversion is realized. According to the unit, energy is transmitted dispersedly and the application of the single intermediate frequency transformer that may become the bottleneck of system application in a high voltage and large capacity situation is avoided, so that the unit has the advantages that the intermediate frequency transformer is easier to wind as compared with a conventional MMC type DC converter. According to the unit, a symmetric double cascade structure is used, so that the circulating current generated by the two MMCs in each cascade module is reversely self-eliminated via control; and DC conversion of more wide transformation ratio is realized by using a structure of the MMCs combined with series-parallel modular combination.

Description

technical field [0001] The invention relates to the field of modularized multilevel converter technology and DC conversion technology, in particular to a symmetrical dual-cascade DC conversion unit with the capability of reverse self-elimination of circulating current. Background technique [0002] With the increasing shortage of traditional fossil energy and the intensification of environmental pollution, the requirements for new technologies such as clean power energy transmission and wide-area balance of energy supply and demand have gradually emerged and increased, which has brought major challenges to traditional grid-related technologies. In order to further improve the stability and reliability of the power grid, improve the power quality of the power grid, increase the power supply capacity, isolate fault areas, and facilitate the flexible access of renewable energy and energy storage devices, domestic and foreign scholars have carried out relevant research on DC powe...

AI Technical Summary

Problems solved by technology

In recent years, with the maturity of high-voltage flexible DC power transmission technology represented by Modular Multilevel Converter (MMC), it has been applied to the development of DC transformers to play a role in the development of modular multilevel converters. The advantages of good redundancy, low switching frequency, low loss, and less output harmonics have become a new technology development trend. However, from the existing results, the relevant research on this type of structure is still in the early stage, and there is no practical There are still reports on industrial prototypes, and there are still certain bottlenecks in system capacity and reliability, making it difficult to adapt to the application requirements of wide voltage ratios in DC power grids

[0004] The document "A Versatile DC-DC Converter for Energy Collection and Distribution using the Modular Multilevel Converter" first proposed a topology concept of a DC converter based on traditional MMC, but in applications where the DC voltage ratio is relatively high, it will lead to two The side transformation ratio is too large, and the excessive low-voltage side current requires thicker windings, which is not conducive to the winding of intermediate frequency transformers. Moreover, since the current maximum capacity of a single intermediate frequency transformer is about 5MW, it is also necessary to use a single intermediate frequency transformer. limits its capacity to further increase

[0005] The invention patent "Power Electronic Transformer Based on Cascade Modular Multilevel CN104638940A" discloses a series-input-parallel-output (Input-SeriesOutput- Parallel, ISOP) DC converter, although it disperses the energy of the AC side and reduces the difficulty of winding the intermediate frequency transformer, but due to the low voltage level of the DC parallel output terminal, it can only be applied to the case of large transformer ratio

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