A Push-Pull Series Connected Modular Multilevel Converter for HVdc Applications

This article introduces a push-pull series connected (PPSC) ac-dc converter intended for high-voltage direct current applications. The proposed topology requires fewer submodules compared to a standard modular multilevel converter (MMC) to withstand the same dc voltage. Each converter phase unit is connected to its corresponding phase on the ac side via a center-tapped single-phase transformer which also provides the galvanic isolation needed in most applications. The required arm inductance can be merged into the transformer leakage inductance avoiding the need for additional inductors. In this article, both the design and operation of the converter are discussed in detail. Furthermore, a comparison of the PPSC and MMC in terms of energy storage requirements and efficiency is presented. The converter concept and the control strategy proposed is validated by computer simulations using the piecewise linear electrical circuit simulation (PLECS) simulation package as well as through experiments on a small-scale, 300-V, 2-kW laboratory prototype.