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Publication Date: 2016-May-17
Document File: 6 page(s) / 145K

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The invention proposes a system and method to obtain redundancy in modular multi-level converter (MMC) based power systems. Upon detection of failure of one cell, the arm current in the affected arm is ramped down within a few hundred microseconds and shortly shifted to the parallel connected MMCs. Concurrently, the slow bypass switch is triggered and is configured to close and bypass the current. The invention is built on a combination of features of components being used in the MMC such as the thermal time constants and overcurrent capabilities of IGBT modules, the switching times of conventional slow electro-mechanical switches, and the controllability of the MMC topology (full bridge and half-bridge based) to bring the arm current to zero in a short time frame. The risk of damage of adjacent cells by plasma blow-out is highly mitigated using a design that avoids ultrafast switches and expensive enclosure designs.

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Page 01 of 6



The invention relates to modular multi-level converters (MMCs) and methods to achieve redundancy in MMC converter systems. MMCs have gained an increasing market share in high power applications, especially grid-connected applications like HVDC and DSVC, and are now extending into applications like variable speed-pumped storage plants or railway interties, as well as into MV drives market. Generally, cell redundancy in MMC converters is typically based on ultrafast mechanical bypass switches in the MMC cells to avoid catastrophic destruction in case of cell failures. A failed cell that is a part of a chain of series-connected cells should be able to continuously carry current after its failure. Typically, used IGBT modules would usually fail into an open circuit in all likelihood, resulting in electric arcing and potentially even explosion of the cell. Thus, a bypass circuit needs to be created to carry the arm current after IGBT failure. This bypass is created by a fast mechanical switch connected between the AC terminals of the cell. However, these ultrafast mechanical bypass switches are still single source components that are not freely available in the market. A disadvantage of some of the explosives used in these switches is the limited lifetime of the explosives that would need to be replaced during the mid-life of the application or system. One way to avoid bypass switches is by using press-pack IGBTs (PPIs) that are inherently conduct-on-fail devices. However, PPIs are more expensive than IGBT modules and cannot compete with the cost of an IGBT module-based cell. An alternative device, IGCTs, require a snubber circuit to operate, which adds complexity and volume to the cell construction. To overcome the above disadvantages, there is a need to provide a simple means to achieve redundancy without the need of ultrafast switches.


The invention discloses a power conversion system in MMC converters and method to achieve redundancy in an MMC converter, wherein:

FIG. 1 illustrates a double-star MMC topology and full-bridge cells with bypass switch.

FIG. 2 illustrates a flow chart for handling of the cell bypass.

FIG. 3 illustrates a parallel connection of two double-star MMC converters.

FIG. 4 illustrates a parallel connection of three delta MMC converters.


The invention proposes a system and method to obtain MMC redundancy without the need of superfast mechanical bypass switches. The invention provides a redundancy solution for systems of parallel-connected MMC converters based on cells with IGBT modules using simple slow mechanical bypass switches.

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The system and method are illustrated with reference to FIG. 1, showing MMC converter arms that are built with n+x cells for redundancy, where each cell (MMC) is equipped with a conventional slow electro-mechanical switch for bypassing the cell. Upon detection of failure...