Current differential relaying - coping with communications channel asymmetry

Line current differential relays calculate the difference between the currents entering and leaving the protected zone. The protection operates when the current difference exceeds a threshold. Line differential relaying requires that the information of the current flowing through each line terminal is made known to all other terminals. To evaluate the differential function, current samples need therefore to be time aligned. This requires that either the relay clocks are synchronized, or current phasors are aligned using propagation delay measurements. Any time difference between the relays´ clocks or errors in propagation delay measurement will translate into a differential current that may cause a relay to misoperate. Line differential relays (87L) operating on asymmetrical communications channels present a more difficult task. If the difference in TX and RX times are not taken into account, the relays´ clocks will present synchronization errors, therefore differential current errors will arise, and as a result the differential function may misoperate even for steady state conditions. This paper describes a GPS based mechanism that guarantees reliable operation of current differential relays under channel asymmetry conditions.