Case study of ANSI standard delta-wye distribution transformer

An over current relay (51N) of a distribution feeder that feeds a branchy overhead line failed to eliminate a fault, which resulted from a fallen conductor at the primary side of the delta-wye distribution transformer. This was linked to the earth's high resistance. In the first part of this paper, the aforementioned reason will be proven to be not credible and that the load-side fallen conductor resulted in a very small return current that cannot be detected. In the second part of this paper, the fact that the voltage will build up at the distribution transformer primary side in case the transformer has a primary single phase blown fuse and unbalance loads will be presented.