A Postfault Strategy to Control the Modular Multilevel Converter Under Submodule Failure

Abstract

The Modular Multilevel Converter (MMC) has become one of the most promising topologies for high-voltage applications. Although the MMC has a modular and redundant structure, which can significantly enhance its reliability and faulttolerance, its fault handling capability can be further improved by adopting a post-fault strategy. In this paper, a post-fault strategy based on the neutral-shift method is proposed, which, subsequent to bypassing the faulty SM(s), aims at improving the maximum attainable line-to-line voltages of the MMC. Based on a newly developed internal variable analysis method, the impact of the neutral-shift strategy on the operation of the MMC is analyzed. In conjunction with the proposed post-fault strategy, a modified circulating current suppression (CCS) method is used to mitigate the double-frequency component of the circulating currents. Performance and effectiveness of the proposed neutralshift strategy with and without the CCS method is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. Index Terms-Modular Multilevel Converter (MMC), Post-Fault Strategy, Neutral-Shift. 0885-8977 (c)

Recent contributions and future prospects of the modular multilevel converters: A comprehensive review

“…Circulating current in MMC could cause power losses and phase current increment as long as it is not thoroughly diminished . Circulating current is suppressed as introduced briefly in Section .…”

Characteristic Analysis and Fault-Tolerant Control of Circulating Current for Modular Multilevel Converters under Sub-Module Faults

“…Each phase unit is comprised of the upper arm unit and the lower arm unit, where N identical SMs and an arm inductor Lm in the same arm comprise an arm unit. The resistance Rm in each arm represents inner inductor resistance and the arm power losses; Ls and Rs are the AC-side inductor and resistance, respectively [13]. Udc is the total DC-link voltage.…”

“…Compared to the fault-tolerant control method based on a PI controller [13], the suppressing strategy based on a PR controller can achieve the independent control of each phase. In addition, it can avoid the acb-dq decoupling control.…”

“…The three-phase reference voltage vj * (j = a, b, c) is produced by adopting the traditional vector control method [13]. For realizing fault-tolerant control of the MMC, the correction amount Δujz generated by the proposed CCS fault-tolerant controller, can be added to the modulation waves for the upper and lower arms of phase j in the opposite direction.…”

“…The mathematical model and fault characteristics analysis with faulty SMs are not yet involved. In [13], the zero-sequence voltage-injection technique is improved based on neutral-point shift control, and a circulating current-suppressing (CCS) control method based on acb-dq and abc-dq dual frames is also designed in this paper. It can achieve the suppression of unbalanced circulating current, but it needs positive-, negative-, and zero-sequence controllers simultaneously, and can only suppress the second-order frequency AC components of the circulating current.…”