Abstract
In high-voltage applications, the magnitude of total semiconductor losses (on-state and switching) determines the viability of modular-type multilevel converters. Therefore, this paper presents a new cell arrangement that aims to lower total semiconductor loss of the modular multilevel converter (MMC) to less than that of the half-bridge modular multilevel converter (HB-MMC). Additional attributes of the proposed cell are: it eliminates the protective thyristors used in conventional half-bridge cells that deviate part of the dc-fault current away from the antiparallel diode of the main switch when the converter is blocked during a dc short-circuit fault, and it can facilitate continued operation of the MMC during cell failures without the need for a mechanical bypass switch. Thus, the MMC that uses the proposed cell retains all advantages of the HB-MMC such as full modularity of the power circuit and internal fault management. The claimed attributes of the proposed cell are verified using illustrative simulations and reduced scale experimentations. Additionally, this paper provides brief and critical discussions that highlight the attributes and limitations of popular MMC control methods and different MMC cells structures proposed in the literature, considering the power electronic system perspective.
Highlights
R APID developments of voltage-source converter highvoltage direct-current (VSC-high-voltage direct-current (HVDC)) transmission systems in recent years have attracted significant research interest in high-voltage high-power converters, dc switchgear, and dc protection systems [1], [2]
This paper presents a new cell arrangement that can reduce modular multilevel converter (MMC) semiconductor losses beyond that of the half-bridge modular multilevel converter (HB-MMC), eliminate the need for the protective thyristor used in HB-MMC to deviate part of the fault current from the freewheeling diodes of the main switches that bypass the cell capacitors when the converter is blocked during dc fault, and facilitate continued operation of the MMC during internal cell failure, without the need for mechanical bypass switches
It has been shown that the proposed MMC is promising as it has lower semiconductor loss compared to HB-MMC, and its unique cell structure enables dc short-circuit survival over an extended period, without the need for protective thyristor as in the HB-MMC
Summary
R APID developments of voltage-source converter highvoltage direct-current (VSC-HVDC) transmission systems in recent years have attracted significant research interest in high-voltage high-power converters, dc switchgear, and dc protection systems [1], [2]. An improved version of the method is discussed in [7], [13], and [18]–[20], which includes two additional cascaded control loops that regulate the average cell capacitor voltage per phase leg and common-mode current [21]–[23] This control method could be used with MMCs that employ half- or full-bridge cells and other symmetrical and asymmetrical bipolar cells in Fig. 1 in order to decouple the control of cell capacitor voltages from the dc-link voltage. While the positive sequence of the common-mode current at fundamental frequency is used to ensure energy balance between the upper and lower arms of each phase leg This approach is interesting, the choice of capacitor voltage ripple instead of the suppression of the second-order harmonic currents in MMC arms is not appropriate for HVDC applications, where the semiconductor losses supersede the capacitor voltage ripple, especially as all the above control methods are able to keep the capacitor voltage ripples well within the tolerable limits. It has been shown that the proposed MMC is promising as it has lower semiconductor loss compared to HB-MMC, and its unique cell structure enables dc short-circuit survival over an extended period, without the need for protective thyristor as in the HB-MMC
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