ABB REL 551-C1*2.5, Technical Reference Manual

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Line differential protection, phase segregated
(DIFL) Chapter 4
Line differential protection
1 Line differential protection, phase segregated
(DIFL)
1.1 Application
Current line-differential protection compares the currents entering and leaving the protected
overhead line or cable. The differential function offers phase-segregated true current differential
protection for all networks. Current comparison on a per phase basis obviates the problem of the
current summation approach and provides phase selection information for single-pole tripping.
A dependable communication link is needed to allow exchange of information between the ter-
minals at the line ends. Direct optical fiber or galvanic communication link are supported, as
well as digital communication systems like multiplexed and route switched networks. The trans-
mission time is measured in short intervals to provide correct synchronization of local clocks.
The transmission time compensation is based on the assumption that the transmission time is the
same in both directions.
The line differential function in the protection of version 2.5 is compatible with earlier versions
1.1, 1.2, 2.0 and 2.3.
Two independent binary signals can be transmitted from one line side to the other through the
differential communication link for tripping, control or information purposes.
1.2 Functionality
The current differential function is of master/master design. Each terminal evaluates the three
phase currents related to its line end, in terms of amplitude and phase angle, and sends them to
the other terminal through the communication channel. At the same time it receives the three
current information from the other terminal and performs locally the phase segregated current
comparison.
All currents are Fourier filtered in order to extract the sine and cosine components. The six com-
ponents, two per phase, are included in a message that is transmitted every 5 ms to the remote
terminal over a synchronous 56/64 kbit/s data channel. Also included in the message is informa-
tion for differential function supervision, CT saturation detection, synchronisation of terminals,
transfer trip signals etc.
The differential measurement is stabilised phase by phase with the current scalar sum, see
figure 15 . The degree of stabilisation is settable.
All currents are individually supervised by the patented CT saturation detection algorithm, to
minimise the requirements on the CTs. In case of CT saturation, the degree of stabilisation is
increased in the affected phase in the differential protections at both ends, see
figure 15 .
Note!
Wrong setting might cause the protection to misoperate.