Rockwell Automation 865 Differential Protection Relay - Manual (Page 101)

Supporting Functions 4-9
865-UM001A-EN-P – July 2009
Example of the Logarithmic Interpolation
Alarm 2 current is set to 6 kA. What is the maximum number of
operations according Table 4.7.
The current 6 kA lies between points 2 and 3 in the table. That gives
value for the index k. Using:
k = 2
C
k
= 10000
C
k+1
= 80
I k+1 = 31 kA
I
k
= 1.25 kA
and the Equation 4.2 and Equation 4.3, the relay calculates
5038 . 1
1250
31000
ln
80
10000
ln
= = n
6 5038 . 1 10 454 1250 10000 ⋅ = ⋅ = a
Using Equation 4.1 the relay gets the number of permitted
operations for current 6 kA.
945
6000
10 454
5038 . 1
6
=
⋅
= C
Thus the maximum number of current breaking at 6 kA is 945. This
can be verified with the original breaker curve in Figure 4.1. Indeed,
the figure shows that at 6 kA the operation count is between 900 and
1000. A useful alarm level for operation-left, could be in this case for
example 50 being about five per cent of the maximum.
Example of Operation Counter Decrementing when the CB is
Breaking a Current
Alarm2 is set to 6 kA. CBFP is supervising trip relay T1 and trip
signal of an overcurrent stage detecting a two phase fault is connected
to this trip relay T1. The interrupted phase currents are 12.5 kA, 12.5
kA and 1.5 kA. How much are Alarm2 counters decremented?
Using Equation 4.4 and values n and a from the previous example,
the relay gets the number of permitted operation at 10 kA.
313
12500
10 454
5038 . 1
6
10
=
⋅
=
kA
C
At alarm level 2, 6 kA, the corresponding number of operations
is calculated according