Investigation of a New Alternate Arm Modular Multilevel Converter Topology for HVDC Applications

This paper introduces an improved hybrid voltage source converter topology that belongs to the alternate arm converter topologies family proposed for HVDC applications. It constitutes cascaded switches in the converter upper arms and string of submodules made up of an active switch and a diode in the lower converter arms. The proposed converter topology features lower number of active switches compared to the conventional modular multilevel converter and other alternate arm converter topologies proposed for HVDC applications. Therefore, it provides significant reduction in the converter power loss, footprint, and cost. The basic operation principle of the proposed converter and its associated control strategy that regulate the power exchange between the converter, the ac network and the dc network is explained. The dimensioning of the converter (the number and voltage rating of the devices required for the converter) is discussed in detail. In addition, comparison of the proposed converter topology with other similar converter topologies in terms of device count, output voltage level, dc fault blocking capability and converter power loss is presented. Preliminary experimental results validating the basic operation of the converter are also presented.