Protecting transmission lines terminated into transformers

Transmission lines that are terminated into transformers are applications where the line and transformer cannot be separated by a circuit breaker and are therefore in the same zone of protection. These applications may be addressed with either current differential or line distance protection. In either case the implementation of separate transformer differential protection isolating the protection zone is recommended. The line protection application needs to consider the instrument transformer locations and transformer winding connections at the transformer end. This paper will discuss the application considerations for both line differential and distance schemes for lines terminated into transformers and provide application guidelines.