ABB RELION REX640, Техническое руководство, Страница: 1329 / 1960

6.4.5 Application
Communication principle
Analog samples, trip-, start- 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.
Figure 756: Measuring sampling latency
P
T T T T
d
=
− + −
( ) ( )
2 1 5 4
2
(Equation 317)
S P T T T T
d d
= + − + −
( ) ( )
4 3 6 5
(Equation 318)
1MRS759142 F Supervision functions
REX640
Technical Manual
1329