期刊
IEEE TRANSACTIONS ON POWER ELECTRONICS
卷 35, 期 3, 页码 2467-2479出版社
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
DOI: 10.1109/TPEL.2019.2932003
关键词
Circuit faults; Current control; Equivalent circuits; Modular multilevel converters; Stress; Temperature distribution; Voltage control; High-voltage direct current (HVdc) transmission; modular multilevel converter (MMC); nonuniform temperature distribution; unbalanced grid fault; zero-sequence voltage injection
资金
- Singapore Ministry of Education Academic Research Fund Tier 1 [2018-T1-001-150 (RG 90/18)]
Symmetrical ac-side current control is widely used for grid-connected modular multilevel converters (MMCs) under grid fault conditions. However, it is revealed that this control method can lead to unbalanced arm currents and consequently asymmetrical electrical stresses and nonuniform temperature distributions. This paper aims to analyze the mechanism of the unbalanced arm currents and propose an effective method to solve this problem. First, the electrical quantities inside MMCs are analyzed under unbalanced grid fault conditions. The analytical expressions of the unbalanced arm currents are derived considering different fault severities and operating conditions to reveal the side-effects of the symmetrical ac-side current control. After that, a zero-sequence ac voltage injection method is proposed. The injected voltage is directly derived from negative-sequence ac current controller and ac-side current with pure mathematical manipulations, which can be easily obtained without extra controllers. The effect of the injected voltage on the original modulation index is very limited according to an accurate quantitative analyzation. Moreover, the proposed method does not affect the already achieved symmetrical ac-side current control and the circulating current suppression. The effectiveness and performance of the proposed analysis and control method are verified by both simulation and experimental results.
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