Magnetic saturation in transformers used for a self-commutated BTB system with ride-through capability during voltage sags

This paper deals with a large-capacity self-commutated BTB(Back-To-Back) system rated at 50 MW for the purpose of power flow control between transmission networks. It focuses on the dynamic behavior of the BTB system during voltage sags such as a single-line-to-ground(1LG) fault. During the 1LG fault, a dc magnetic deviation occurs in the transformers used for the BTB system just after 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 converter-cells. This paper derives theoretical equations related to the deviation during the ILG fault. The theoretical analysis developed in this paper would make significant contributions to designing the transformers.