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IEEE/ANSI Inverse Time Operation
There are three different delay types according IEEE Std C37.112-
1996 (MI, VI, EI) and many de facto versions according Table 3.52.
The IEEE standard defines inverse delay for both trip and release
operations. However, in the Allen-Bradley relay only the trip time is
inverse according the standard but the release time is constant.
The operation delay depends on the measured value and other
parameters according to Equation 3.5. 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 relay for
real time usage.
Equation 3.5:
⎡
⎢
⎢
=
⎢
t
k
⎛
⎢
I
⎜
⎢
⎜
⎢
I
⎝
⎣
pickup
t
=
k
=
I
=
I
=
pickup
A,B,C
=
Table 3.52 – Constants for IEEE/ASI Inverse Delay Equation
Delay type
LTI
Long time inverse
LTVI
Long time very inverse
LTEI
Long time extremely inverse
MI
Moderately inverse
VI
Very inverse
EI
Extremely inverse
STI
Short time inverse
STEI
Short time extremely inverse
⎤
⎥
⎥
A
+
⎥
B
C
⎞
⎥
⎟
−
⎥
1
⎟
⎥
⎠
⎦
Operation delay in seconds
User's multiplier
Measured value
User's pick up setting
Constant parameter according to Table 3.52.
A
0.086
28.55
64.07
0.0515
19.61
28.2
0.16758
1.281
3-83
Protection Functions
Parameter
B
C
0.185
0.02
0.712
2
0.250
2
0.1140
0.02
0.491
2
0.1217
2
0.11858
0.02
0.005
2
857-UM001A-EN-P – July 2009
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