Pilot protection communication channel requirements

Pilot relaying has been applied for transmission line protection since the 30's. The well known communication channels (pilot wire or power line carrier) are increasingly being replaced by digital channels. Dark fiber (dedicated fiber optic cable), multiplexed fiber optic systems (T1 and SONET) and 56 kbps phone lines (DDS - digital data service) are now made available for pilot protection purposes. The new channels provide much higher data transfer rate but reliability and security performance criteria developed for the telecommunications industry are not easily translated to teleprotection applications. A number of new terms are introduced to the protection engineer and it might not be all that obvious what precautions need to be taken from a protective relaying point of view. This paper discusses the requirements for the communication channel for common pilot schemes, direct transfer trip and current differential relaying. It addresses issues such as channel asymmetry and channel switching in T1 and SONET networks and the affect on pilot relaying performance. It also provides basic description of digital communication techniques and terminology that the relay engineer may encounter in his work.