New line current differential relay using GPS synchronization

The "numerical line current differential relay" has proved to be one of the most successful types of transmission line protection system, and advances in communication technology have facilitated its ever wider application. However, more recently, the adoption of SDH (synchronous digital hierarchy) as the international standard for high speed data communication over optical fiber and microwave has given rise to communications links with characteristics which are unsuitable for traditional numerical line current differential relays. Therefore, we have developed a new "line current differential relay" using GPS (Global Positioning System) synchronization that is applicable for use over SDH communication networks. Following successful completion of type testing and twelve month field trials with NG, (National Grid) in the United Kingdom, type registration was achieved in April 2001. Subsequently, the first commercial units were delivered in September 2001 and commissioned in June 2002.