Fault tolerant cell design for MMC-based multiport power converters

Abstract

The Modular Multilevel Converter (MMC) is a promising technology for medium-high voltage DC/AC converters, being adequate for HVDC transmission systems. Among the appealing characteristics of the MMC are their modularity, and consequently their scalability, as well as the fact that there is no bulk storage element. One key aspect for the operation of the MMC is the response in the event of a short circuit in the DC link. Conventional MMC cells consist of a half-bridge and a capacitor, and have no capability to block the short circuit in the DC side, meaning that expensive and bulky circuit breakers might be needed in this case. Several fault tolerant cell designs have been proposed. However, these desings always bring an increase in the number of power devices and losses. Conventional MMC design can be enhanced to provide added functionalities, e.g. multiport power converters and solid state transformers (SST). A mean to achieve this is by providing the cells the capability to transfer power. This enhancements will imply an increase in the number of power devices and passives, as well as further complexity of the control. However, the resulting cells structures can offer new opportunities regarding fault tolerance. This paper revises the fault tolerance capability of MMCs, and analyzes the behavior of MMC-based multiport power converters in the event of faults. A new cell structure will be proposed capable of blocking the DC short circuit current, therefore protecting the power converter with reduced extra elements.1