| Abstract |
In the currently predominant AC-Grid, energy reserves are provided by the inertia of rotating synchronousgenerators. In a potential DC-Grid, there is no such inherent energy reserve and thus additionalenergy storage (ES) is required to ensure grid stability. The Modular Multilevel Converter (MMC) is anemerging technology for HVDC-Grid applications and offers a convenient way of connecting ES to thegrid. Short strings of batteries or supercapacitors can be integrated directly into the MMC submodules,creating an ES enabled MMC, which reduces the susceptibility of the system to an individual ES cellfailure.Since connecting ES to the submodule cells requires additional electrical devices (switches and/or inductors),reducing the number of submodules with integrated ES can significantly reduce the additionalcost for ES integration. In most research conducted on this topic so far, the existence of ES in all MMCmodules is assumed. However, the amount of ES in an ES enabled MMC can be reduced by utilizingcircular currents to achieve energy balance between submodules with and without integrated ES. By inducingcircular currents, energy can be transferred between any phase arms, and phase arms with ES cansupply those without. The focus of this examination is the additional semiconductor load caused by thecircular currents. It is shown that there is only a very small increase in the semiconductor load, whenonly a few arms supply the whole output power. |
