This article proposes a novel bipolar-type dc system suitable for both distribution and transmission systems based on modular multilevel series/parallel converters (MMSPCs). The system features decoupled operations of each pole of the bipolar system, being able to operate in both asymmetrical and regenerative modes. This enables two independent dc systems by using a single grid-tied converter. The MMSPC is based on a three-switch cell configuration and enables a simple balancing mechanism in combination with a wide range of output voltage frequencies. The simple balancing mechanism is the key to enable the dc operation and lead to simpler scalability for different voltage levels. Theoretical studies and experimental results are provided to verify and characterize the proposed system.

The Modular Multilevel Series Parallel Converter (MMSPC) not only allows dynamic interconnection among the modules in series connection, like the traditional Modular Multilevel Converter (MMC), but also in parallel connection. This extra switching state among the modules allows new features and operation modes. The parallel connection between modules enabled by the MMSPC, is not only limited to those modules belonging to the same arm, but can also be extended to the parallel connection of modules from different arms or even from different phases. All these different interconnections brings operational benefits as well as some constraints. In this paper, the power topology, operation and comparison of different MMSPC configurations are presented. Specifically, this paper describes in detail the traditional MMC (without parallel interconnection), the MMSPC with parallel connection of modules of the same arm only, the MMSPC with the option of paralleling connection among the phases of the system, the MMSPC with parallel connection between arms of the same phase and finally, the MMSPC with full-parallel capability.