2.24 Inverse time operation
2 Protection functions
Technical description
134
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VM259.EN007
Table 2.24.1-1 Available standard delay families and the
available delay types within each family.
Delay type
Curve family
DT
IEC
IEEE
IEEE2
RI
DT
Definite time
X
NI1
Normal inverse
X
X
VI
Very inverse
X
X
X
EI
Extremely inverse
X
X
X
LTI
Long time inverse
X
X
LTEI
Long time extremely inverse
X
LTVI
Long time very inverse
X
MI
Moderately inverse
X
X
STI
Short time inverse
X
STEI
Short time extremely inverse
X
RI
Old ASEA type
X
RXIDG Old ASEA type
X
IEC inverse time operation
The operation time depends on the measured value and other
parameters according Equation 2.24.1-1. Actually this equation
can only be used to draw graphs or when the measured value I
is constant during the fault. A modified version is implemented
in the device for real time usage.
Equation 2.24.1-1
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 2.24.1-1.
There are three different delay types according IEC 60255-3,
Normal inverse (NI), Extremely inverse (EI), Very inverse (VI)
and a VI extension. Additional there is a de facto standard
Long time inverse (LTI).
