Neutral-Point-Shift-Based Active Thermal Control for a Modular Multilevel Converter Under a Single-Phase-to-Ground Fault

The thermal design of a highly reliable modular multilevel converter (MMC) is significant for the voltage-source-converter-based high-voltage direct current system, especially under the unbalanced ac grid fault. In this paper, the analytical thermal model of the MMC considering the fault ride-throug...

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Bibliographic Details
Published in:IEEE transactions on industrial electronics (1982) 2019-03, Vol.66 (3), p.2474-2484
Main Authors: Dong, Yufei, Yang, Heya, Li, Wuhua, He, Xiangning
Format: Article
Language:English
Subjects:
Active control
Active thermal control
Circuit faults
Direct current
Electric converters
Electric potential
High voltages
Mathematical models
modular multilevel converter (MMC)
Modular multilevel converters
Phases
Power conversion
Rectifiers
Reliability
Thermal analysis
Thermal design
Thermodynamic properties
unbalanced ac grid fault
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Summary:The thermal design of a highly reliable modular multilevel converter (MMC) is significant for the voltage-source-converter-based high-voltage direct current system, especially under the unbalanced ac grid fault. In this paper, the analytical thermal model of the MMC considering the fault ride-through strategy is established to explore the MMC dynamic thermal behavior under the single-phase-to-ground fault. With the established thermal model, the proportional relationship between the MMC dc current and submodule thermal imbalance factor is derived. By exploring the features of dc current, it is discovered that the asymmetrical ac voltage makes the dc current different among three phases, leading to the thermal imbalance among three phases. To solve this issue, the neutral-point-shift-based active thermal control method, which can reshape the MMC ac voltage and balance the three-phase dc current, is proposed. With the proposed control method, the thermal distribution among three phases becomes balanced and the junction temperature of the most stressed power device is significantly reduced. Furthermore, the performance analysis shows that the proposed method brings little effects on the circulating current suppression. Finally, the major theoretical conclusions are verified by a laboratory MMC test bench.
ISSN:0278-0046
1557-9948
DOI:10.1109/TIE.2018.2833019