Fallback algorithms for line current differential protection applied with asymmetrical channels upon the loss of time reference

When applied over asymmetrical or potentially asymmetrical channels (unequal channel propagation times in the transmitting and receiving directions), such as commercial-class SDH/SONET networks, line current differential schemes need an external time reference for current data alignment if the sensitivity of the differential scheme is to be preserved. Modern line differential schemes can handle a limited amount of asymmetry (2 to 3 milliseconds) without the need for an external time source, but they sacrifice sensitivity in the process. Such reliance on external time sources increases both the cost and complexity of line current differential scheme applications. The usage of external time, however, may be unavailable in some situations. Line current differential schemes that use external time sources must provide a well-defined response that suits user preferences in situations when the time source is lost or degraded beyond the point of safe usage in protection applications. This is often referred to as time fallback logic. This paper presents several time fallback modes, varying with respect to balancing protection security and dependability.