On the integration of low frequency transformer into modular multilevel converter

Abstract

Abstract-Various high power applications require power converters with large voltage step ratios, easily achieved by inclusion of a single or multiple transformers, which provide galvanic isolation at the same time. This paper presents, in a systematic manner, the necessary steps for the integration of a Low Frequency Transformer (LFT) into the Modular Multilevel Converter (MMC). Unlike the classical MMC that requires an external transformer for galvanic isolation, this work considers a transformer integration at the arm level resulting in a complete replacement of the arm inductors. Such galvanically isolated modular converters can be realized either in interleaved or stacked arrangements. The properties of each variant are discussed and compared with the classical MMC with external LFT, on the system design level but also from the control point of view.

Insulation coordination for a modular multilevel converter prototype

“…The MMC prototype has ratings of 0.5 MVA and is designed for connection to 10 kV on the MVDC side and 400 V on the LVAC side. The discrete branch inductors and the line frequency transformer are combined in a dual-primary transformer, resulting in the so-called galvanically isolated modular converter (GIMC) [3,4]. Nevertheless, the transformer primary windings are rated for 6.6 kV, similarly to the classical MMC.…”