Calculation of dc magnetic flux deviation in the converter-transformer of a self-commutated BTB system during single-line-to-ground faults

This paper deals with a 50-MW self-commutated Back-To-Back (BTB) 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 that often result in voltage sags. A non-negligible amount of dc magnetic flux deviation appears in the converter-transformers of the BTB system just after the occurrence and restoration of an SLG fault. The dc deviation may bring magnetic flux saturation and an excessive magnetizing current to the transformers. This paper derives theoretical equations related to the dc deviation with reasonable approximation. The theoretical results are confirmed by computer simulation. Index Terms--Back-To-Back (BTB) system, converter-transformer, dc magnetic flux deviation, multilevel converter, power flow control, single-line-to-ground (SLG) fault.