tripping schemes, since receipt of an incorrect signal must coincide with a ‘start’ of the receiving end protection for
a trip operation to take place. The intention of these schemes is to speed up tripping for faults occurring within the
protected zone.
Blocking Scheme
Blocking commands are initiated by a protection element that detects faults external to the protected zone.
Detection of an external fault at the local end of a protected circuit results in a blocking signal being transmitted to
the remote end. At the remote end, receipt of the blocking signal prevents the remote end protection operating if it
had detected the external fault. Loss of the communications channel is less serious for this scheme than in others
as loss of the channel does not result in a failure to trip when required. However, the risk of a spurious trip is higher.
2.3
TRANSMISSION MEDIA AND INTERFERENCE
The transmission media that provide the communication links involved in protection signalling can be:
● Private pilots
● Rented pilots or channels
● Power line carrier
● Radio
● Optical fibres
Historically, pilot wires and channels (discontinuous pilot wires with isolation transformers or repeaters along the
route between signalling points) have been the most widely used due to their availability, followed by Power Line
Carrier Communications (PLCC) techniques and radio. In recent years, fibre-optic systems have become the usual
choice for new installations, primarily due to their complete immunity from electrical interference. The use of fibre-
optic cables also greatly increases the number of communication channels available for each physical fibre
connection and thus enables more comprehensive monitoring of the power system to be achieved by the
provision of a large number of communication channels.
P543i/P545i Chapter 20 - Fibre Teleprotection
P54x1i-TM-EN-1 501
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