The Basics of Transmission Line Protection

Transmission line protection schemes have evolved over the years. Direct connection of instrument transformer secondary wires to protection relays was mostly applicable to short transmission lines. In medium to long haul transmission lines this scheme was not feasible due to line signal attenuation caused by increased line impedance among other factors. Technological advances have provided unique solutions that have been deployed in a variety of transmission lines from short to long haul lines. This paper discusses how the use of microprocessor based protection relays and modern digital communication technologies have simplified transmission line protection. The paper also discusses line current differential and distance relaying schemes as applied to transmission line protection. Communication assisted tripping schemes will also be discussed and how they apply to distance relaying schemes.