6.4.5
Application
Communication principle
Analog samples, trip, pickup and user programmable signals are transferred in each
protection telegram and the exchange of these protection telegrams is done eight times per
power system cycle (every 2.5 ms when F
n
= 50 Hz).
Master-Master communication arrangement is used in the two-terminal line differential
solution. Current samples are sent from both line ends and the protection algorithms are
also executed on both line ends. The direct-intertrip, however, ensures that both ends are
always operated simultaneously.
Time synchronization
In numerical line differential protection, the current samples from the protections which
are located geographically apart from each other must be time coordinated so that the
current samples from both ends of the protected line can be compared without introducing
irrelevant errors. The time coordination requires an extremely high accuracy.
As an example, an inaccuracy of 0.1 ms in a 50 Hz system gives a maximum amplitude
error of approximately around 3 percent. An inaccuracy of 1 ms gives a maximum
amplitude error of approximately 31 percent. The corresponding figures for a 60 Hz
system are 4 and 38 percent respectively.
In the protection relay, the time coordination is done with an echo method. The protection
relays create their own time reference between each other so that the system clocks do not
need to synchronize.
The figure shows that in the time synchronization the transmission time to send a message
from station B to station A, T1→T2, and the time to receive a message from A to B,
T4→T5, are measured. The station A protection relay delay from the sampling to the start
of send, T3→T4, and the local delay from receive to the station B protection relay
sampling T5→T6 time, are also measured for the station B protection relay, and vice
versa. This way the time alignment factor for the local and remote samples is achieved.
GUID-2DDF64E2-D635-4783-854A-A62E5EFB7186 V1 EN
Figure 446:
Measuring sampling latency
Section 6
1MAC059074-MB A
Supervision functions
864
615 series ANSI
Technical Manual
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