A Circulating Current Calculation Method for Modular Multilevel Converter with Arbitrary Level

Abstract

The invention belongs to the technical field of DC power transmission and specifically relates to an any-level modular multilevel converter circulation current calculating method. The method comprises establishment of a submodule switch model and a circulation current calculating model, and detailed process of circulation current calculation; and writing of a voltage and current equation of each electric quantity of a modular multilevel converter for a circulation current model by utilizing a circuit theory. By analyzing a first-order differential equation met by bridge arm circulation current of the modular multilevel converter, a specific mathematical expression of the bridge arm circulation is solved; calculation of frequency-doubled circulation current is derived in detail, and finally, a specific computational formula is provided; and meanwhile, a calculation result obtained with only the DC component being considered as the circulation current is prevented, thereby improving circulation current calculating accuracy.

Description

technical field [0001] The invention belongs to the technical field of direct current transmission, in particular, it relates to a calculation method for circulating current of an arbitrary-level modular multi-level converter. Background technique [0002] Modular multilevel converter (MMC) has attracted extensive attention from scholars all over the world since it was first proposed in 2002. Compared with traditional multilevel converters, modular multilevel converters rely on highly modular The design makes it have good voltage expansion capability and power expansion capability and the output voltage waveform quality is good, but the biggest advantage of the modular multilevel converter is that it has a common DC bus, so that the energy can be transferred between the three-phase bridge arms. exchange, which is beneficial to three-phase balance control. [0003] The energy of the modular multilevel converter is distributed and stored in the capacitors of each sub-module (...

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Problems solved by technology

[0003] The energy of the modular multilevel converter is distributed and stored in the capacitors of each sub-module (SM). Theoretically, the parameters of each sub-module are exactly the same, but in practice, due to the small difference in the capacitance parameters, the different investment time of the sub-modules and the loss of the device and other problems, resulting in unbalanced energy distribution of the modular multilevel converter, and due to the structural characteristics of the three-phase bridge arms connected in parallel on the DC side, it will cause circulating currents between the bridge arms

[0004] At present, the analysis and calculation of the circulating current between the bridge arms considers that the circulating current is indirectly given from the current of the upper and lower bridge arms of the modular multilevel converter, and the specific expression of the circulating current is not deduced in detail; or only the public DC side is considered in each The DC component generated on the phase bridge arm is roughly considered to be one-third of the DC side current. However, due to the complex coupling relationship between the DC side capacitance of the sub-module and the AC output side, the DC component is simply used as the circulating current. The calculation results will bring a large error

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