There are still many protection relays in service within Utility Companies that use 4-wire analogue interfaces with the cost of upgrading relays and associated communications infrastructure not justifying the cost of replacement.
In some cases utilities continue to use standard telephony circuits for communications between relays whilst others are deploying some form of pseudo-wire communications over Ethernet/MPLS networks to transport the 4-wire V.F signals.
It is important to ensure that the communications circuits used for protection relays are not susceptible to interference as this can cause spurious trips and isolation of primary systems. Traditional methods of testing V.F 4-Wire communications circuits send V.F tones at different frequencies and signal levels and then measure the V.F tones at the far end to measure signal attenuation.
Although this method does test the operation and frequency response of the V.F circuit under test, it does not test the stability of the V.F circuit over time, the signalling delay or asymmetry of the circuit. Signalling delay is an important factor in the overall relay fault clearance time, and this needs to be less than the maximum time for which faults can remain on the system to ensure minimum plant damage. Communications asymmetry is also an important consideration as it can lead to differential protection relay misoperation with relays at either end of a power line sampling the load current waveform at different moments in time.
The Ether.Genius, xGenius and Zeus communications test sets from Albedo Telecom can test V.F Signal level and frequency, BER, and delay (round-trip and One-Way Delay/Asymmetry), making them ideal tools for testing V.F protection circuits. Using the internal logging capability of the testers it is possible to log the received V.F signal at the far end of the circuit to determine the stability of the circuit under test and to identify any interference. This is an important capability as it allows users to identify sporadic issues on a circuit and determine when these events occur, whereas traditional frequency/level measurements often fail to identify problems.
Historically it has been difficult to measure V.F circuit delay, certainly one way delay and asymmetry. Using a ToD input signal such as GPS, the Albedo testers are able to measure the communications delay of a V.F circuit measuring both the round trip delay as well as the forward and return path delay, providing an asymmetry measurement.
As utility networks and sub-stations migrate to IP/IEC61850, equipment with legacy communications interfaces will continue to be used for many years to come. It it important therefore to be able to test both existing V.F circuits and those V.F circuits transported over packet networks for both performance and conformance.