DC current capability of high voltage apparatus based on the open-core concept

Modular Multilevel Converters (MMC) HVDC converter station topologies require the use of high-impedance grounding reactors on the converter side of power transformers. The main requirement for such units is the ability to withstand simultaneous AC and DC loading. The aim of this paper is to give insight on how to determine the DC capabilities of such units both through calculation and measurements. Additionally, the purpose of this paper is to show what the possibilities of different core designs in terms of DC capability are and what values can be expected and obtained.