Adaptive clock offset estimation for protection devices without time-based synchronization

Line current differential relays exchange line current data samples to perform differential protection. The data received from the remote end are time-aligned with the locally sampled data to accurately calculate the differential current. For data alignment, line differential relays typically use two methods: namely, external time-based synchronization and channel-based synchronization. The time-based synchronization method relies on external time sources and reported time quality within the time message. It offers the advantage of estimating the channel asymmetry using the external time signal. The channel-based synchronization method does not rely on the need for external time sources. However, it is not reliable for applications using data multiplexers for long-distance line differential protection in which the channel asymmetry resulting from communications path switching is of concern. Therefore, a novel channel asymmetry detection and compensation method has been developed for precise data alignment independent of any external time sources.