Elucidation of DC Magnetic Deviation in Converter Transformers Used for a Self-Commutated BTB System during Single-Line-to-Ground Faults

This paper deals with a 50-MW self-commutated BTB (Back-To-Back) system intended for power-flow control between transmission networks. It focuses on the dynamic behavior of the BTB system during single-line-to-ground (SLG) faults. During an SLG fault, a dc magnetic deviation appears in the converter-transformers used for the BTB system just after the occurrence and restoration of the fault. It is indispensable to understand an amount of deviation because it may bring magnetic saturation as well as a large amount of magnetizing current to the transformers. This paper derives theoretical equations related to the deviation during the SLG fault. The theoretical analysis developed in this paper would make significant contributions to designing the transformers.