A two-relay differential protective scheme for Le-Blanc transformers

A Le-Blanc transformer is a special winding connection transformer for the single-phase power supply of an electrified railway and it has been installed in the electrified railway of Taiwan (Tairail) for more than twenty years. It is well-known that the main electrical protection of large capacity transformers is differential protection. As the special winding connection and current relationship of Le-Blanc, differential relays are not easy to apply directly without using the special turn ratio current transformers. Therefore, the purpose of this paper is to design a differential protective scheme that is built by normal turn ratio current transformers connected for Le-Blanc transformers. According to the phasor combination method, a two-relay differential protective scheme that combines the primary line currents to match the secondary line current phases of Le-Blanc is proposed in this paper. For evaluating the performance of the proposed scheme, a practical design example using the parameters of a Tairail's Le-Blanc transformer and some scaled down data of the field test system in our laboratory are also presented in this paper.