The Delta Configured Modular Multilevel Converter
The modular multilevel converter (MMC) has become an attractive option for dc/ac conversion in many power system applications, in particular for HVDC transmission. Due to its parallel arrangement of phase legs the MMC is known to be wye configured architecture. In this project, the equivalent delta formulation of the conventional wye architecture is introduced. The delta configured MMC, enabled by series-stacking phase legs, is an alternative realization to the wye configured MMC and has significantly different operating characteristics. Introduction of the delta MMC enables greater flexibility in converter voltage/current scaling for a given set of sub modules, and may be an appealing option for dc/ac applications requiring a power tap-off from a high-voltage dc rail. The single-ended and differential delta topologies are derived, with the latter seen as the preferred topology due to its reduced energy storage requirements.
Modular multilevel cascaded converters employing delta configured SMs have been proposed they are limited to using isolated dc sources. That is, they lack a common dc link as depicted in the family of MMC structures in use a common dc link only for wye configured SMs). These topologies are thus constrained to applications such as reactive compensation (i.e. statcom), battery energy storage systems and ac motor drives. In differential dc/ac converter structures employing cascaded SMs have been proposed, however a delta equivalent realization has not been established.
The theory of operation of the differential delta MMC is discussed in detail. Unique operating characteristics of the delta MMC include inherent modulation of line-to-line ac voltages, and the presence of significant circulating ac currents at the third harmonic. A mathematical model is derived to provide additional insight into converter steady-state and dynamic performance. Based on the converter model, a dynamic controller is developed for the differential delta MMC that regulates grid power injections while suppressing unwanted circulating ac currents. A comprehensive switched model of the differential delta MMC is implemented in PLECS. Presented simulation results validate the operating principle of the differential delta MMC as well as the developed controls.
TOOLS AND SOFTWARE USED:
- MP LAB
- J. Kish, Student Member, IEEE, R. Shi, Student Member, IEEE, and P. W. Lehn, Senior Member, IEEE, “The Delta Configured Modular Multilevel Converter”, IEEE Transactions on Power Delivery 2015.