Network errors and their influence on current differential relaying

While a point-to-point fiber is the preferred communication link for a protective relaying engineer it is not always available. An OPGW (Optical Ground Wire) on the power line itself could provide a dedicated fiber pair for protection, but often the utility communications network does not follow the power system topology. In addition, multiplexing data over that fiber pair optimizes the use of available communications media. The same physical fiber pair that carries a single channel 64 kbps relaying data could be used for up to OC-192 (10 Gbps) or GigaBit Ethernet. To use an available communications network for relaying makes sense from an economical standpoint. However, even with digital communications, the network is not error free. While many links are optical fiber which are immune to electrical interference, most networks incorporate wired links or microwave paths in their design. Attenuation on long fibers, synchronous clock inaccuracies causing frame slips, etc., contributes to the errors. For evaluation, protective relays are subjected to extensive bench or simulator testing with injected currents and voltages it is less common to include the communications link in the test. Typically, the relays have a direct connection of the communications ports back to back. This excludes the vital communications link that is integral to the operation of a current differential relay. The results are thus valid for a direct fiber application but not necessarily so over networked communications. While a relay engineer typically is not directly involved in the design of the utility communications network, the ability to explain how and why critical pilot relaying channel requirements may differ from other data communication needs could go a long way towards obtaining reliable communications services for protective relaying. This paper examines the type of errors that can be expected over a modern digital communications network and what the consequences for current differential relaying may be. It looks at factors influencing protection system availability and how problems could be mitigated by relay design, proper network configuration and circuit conditioning. Actual testing with current differential relays and a BER (bit error rate) generator illustrate the points.