Rockwell Automation 865 Differential Protection Relay - Manual (Page 100)

4-8 Supporting
Functions
865-UM001A-EN-P – July 2009
Any actual interrupted current will be logarithmically weighted for
the two given alarm current levels and the number of operations left
at the alarm points is decreased accordingly. When the "operations
left" i.e. the number of remaining operations, goes under the given
alarm limit, an alarm signal is issued to the output matrix. Also an
event is generated depending on the event enabling.
Clearing "Operations Left" Counters
After the breaker curve table is filled and the alarm currents are
defined, the wearing function can be initialized by clearing the
decreasing operation counters with parameter "Clear" (Clear oper.
left cntrs). After clearing the relay will show the maximum allowed
operations for the defined alarm current levels.
Operation Counters to Monitor the Wearing
The operations left can be read from the counters "Al1Ln" (Alarm 1)
and "Al2Ln" (Alarm2). There are three values for both alarms, one
for each phase. The smallest of three is supervised by the two alarm
functions.
Logarithmic Interpolation
The permitted number of operations for currents in between the
defined points is logarithmically interpolated using equation
Equation 4.1
n I
a
C =
, where
C = permitted operations
I = interrupted current
a = constant according Equation 4.2
n = constant according Equation 4.2
Equation 4.2
k
k
k
k
I
I
C
C
n
1
1
ln
ln
+
+ =
Equation 4.3
2
k k
I C a =
ln = natural logarithm function
C
k
= permitted operations. k = row 2...7 in Table 4.7.
I
k
= corresponding current. k = row 2...7 in Table 4.7.
C
k+1
= permitted operations. k = row 2...7 in Table 4.7.
I
k+1
= corresponding current. k = row 2...7 in Table 4.7.