Protection and telecommunications interface problems for digital transfer relays

The authors discuss a protection application on SaskPower´s S3B line in which communications propagation time asymmetry could have resulted in false tripping of the line current differential protection scheme for out of zone faults and load currents. A line differential scheme was chosen as the secondary protection. Modern line differential relays which require the transfer of digitized quantities between both line terminals for decision making purposes are interfacing with telecommunications equipment which is not specifically designed for protection applications. Certain relays require precise time alignment for measured quantities from both ends to arrive at a correct decision. These relays are designed assuming bidirectional propagation times are essentially equal. Any significant asymmetry between the two times can result in misalignment of the data and cause incorrect operation of the protection scheme. The line differential relay and the impedance relay signals are multiplexed by a digital channel bank and transmitted via fiber optics. Bi-directional time symmetry is not a critical requirement for normal data transfer and there are no standards requiring time symmetry on communications equipment. The lack of definite communications standard incorporating allowable time asymmetry or relays designed without the capability to adapt to time asymmetry, will result in incorrect relay operations. A solution chosen by SaskPower to eliminate this bi-directional propagation time asymmetry is outlined.