Channel Asymmetry Compensation For Line Current Differential Relays Without External Time Sources

Line current differential relays exchange line current data samples to perform differential protection. The data received from the remote end are required to be precisely time-aligned with the locally sampled data to accurately calculate the operate and restraint currents. For data alignment, there are two widely used methods, namely, time-based synchronization and channel-based synchronization. The time-based synchronization method relies on external time sources and their time quality. It offers the advantage of estimating the channel asymmetry using the measured round trip delay. The channel-based synchronization does not rely on the need for external time sources. However, it is not dependable for applications using data multiplexers for long distance line differential protection in which the channel asymmetry resulting from communication 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. This paper describes the existing data alignment methods widely used by line current differential relays, proposes a novel method for data alignment, evaluates the performance of the novel method using real-time power system simulator test results obtained using an industry standard data multiplexer, and compares the performance of the proposed novel method against the data alignment schemes used by other commercially available relays.