Rockwell Automation 865 Differential Protection Relay - Manual (Page 103)

Supporting Functions 4-11
865-UM001A-EN-P – July 2009
The internal clock of the relay is used to time stamp events and
disturbance recordings.
The system clock should be externally synchronized to get
comparable event time stamps for all the relays in the system.
The synchronizing is based on the difference of the internal time and
the synchronizing message or pulse. This deviation is filtered and the
internal time is corrected softly towards a zero deviation.
Adapting Auto Adjust
During tens of hours of synchronizing the device will learn its
average error and starts to make small corrections by itself. The
target is that when the next synchronizing message is received, the
deviation is already near zero. Parameters "AAIntv" and "AvDrft"
will show the adapted correction time interval of this
±1 ms auto-
adjust function.
Time Drift Correction Without External Sync
If any external synchronizing source is not available and the system
clock has a known steady drift, it is possible to roughly correct the
clock error by editing the parameters "AAIntv" and "AvDrft". The
following equation can be used if the previous "AAIntv" value has
been zero.
Week DriftInOne
AAIntv
8 . 604
=
If the auto-adjust interval "AAIntv" has not been zero, but further
trimming is still needed, the following equation can be used to
calculate a new auto-adjust interval.
8 . 604
1
1
Week DriftInOne
AAIntv
AAIntv
PREVIOUS
NEW
+
=
The
term
DriftInOneWeek/604.8 may be replaced with the relative drift
multiplied by 1000, if some other period than one week has been
used. For example if the drift has been 37 seconds in 14 days, the
relative drift is 37*1000/ (14*24*3600) = 0.0306 ms/s.
System Clock and
Synchronization