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Equation 3.4
k
A
=
t
B
⎛
⎞
I
⎜
⎟
−
1
⎜
⎟
I
⎝
⎠
pickup
t
=
Operation delay in seconds
k
=
User's multiplier
I
=
Measured value
Ipickup
=
User's pick up setting
A, B
=
Constants parameters according Table 3.51.
There are three different delay types according IEC 60255-3, Normal
inverse (NI), Extremely inverse (EI), Very inverse (VI) and a VI
extension. Additionally there is a de facto standard Long time
inverse (LTI).
Table 3.51 – Constants for IEC Inverse Delay Equation
Delay type
NI
Normal inverse
EI
Extremely inverse
VI
Very inverse
LTI
Long time inverse
Example for Delay type "Normal inverse (NI) ":
k
=
0.50
I
=
4 pu (constant current)
I
=
2 pu
pickup
A
=
0.14
B
=
0.02
⋅
. 0
50
. 0
14
=
=
t
5
0 .
. 0
02
⎛
⎞
4
−
⎜
⎟
1
⎝
⎠
2
The operation time in this example will be 5 seconds. The same
result can be read from Figure 3.40.
Protection Functions
Parameter
A
B
0.14
0.02
80
2
13.5
1
120
1
857-UM001A-EN-P – July 2009
3-81
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