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 electrified railway and it has been installed in the electrified railway of Taiwan(Tairail) for more than twenty years. I t is well-known that the main electrical protection of large capacity trans formers is differential protection. As the special winding connection and c urrent relationship of Le-Blanc, differential relays are not easy to apply directly without using the special turn ratio current transformers. Therefo re, the purpose of this paper is to design a differential protective scheme that is built by normal turn ratio current transformers connected for Le-B lanc transformers. According to the phasor combination method, a two-relay differential protective scheme that combines the primary line currents to m atch the secondary line current phases of Le-Blanc is proposed in this pape r. For evaluating the performance of the proposed scheme, a practical desig n 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 prese nted in this paper.