Technical description
2 Protection functions
2.13 Inverse time operation
VM265.EN011
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Equation 2.13.1-1
1
B
pickup
I
I
A
k
t
t
= Operation delay in seconds
k
= User‟s multiplier
I
= Measured value
Ipickup = User‟s pick up setting
A, B
= Constants parameters according Table.
There are three different delay types according IEC 60255-3,
Normal inverse (NI), Extremely inverse (EI), Very inverse (VI)
and a VI extension, Long time inverse (LTI).
Table 2.13.1-2 Constants for IEC inverse delay equation
Delay type
Parameter
A
B
NI
Normal inverse
0.14
0.02
EI
Extremely inverse
80
2
VI
Very inverse
13.5
1
LTI
Long time inverse
120
1
Example for Delay type "Normal inverse (NI) ":
k
= 0.50
I
= 4 pu (constant current)
I
pickup
= 2 pu
A
= 0.14
B
= 0.02
0
.
5
1
2
4
14
.
0
50
.
0
02
.
0
t
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