⎯
11
⎯
6 F 2 S 0 7 5 8
T
C
T
C
A
B
C
Operate time
Figure 2.1.2 Time-distance Characteristics of Inverse Time Protection
The inverse time overcurrent protection elements have the IDMT characteristics defined by
equation (1):
( )
⎪⎭
⎪
⎬
⎫
⎪⎩
⎪
⎨
⎧
+
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
−
×
=
c
Is
I
k
TMS
t
a
1
where:
t = operating time for constant current I (seconds),
I = energising current (amps),
Is = overcurrent setting (amps),
TMS = time multiplier setting,
k, a, c = constants defining curve.
Nine curve types are available as defined in Table 2.1.1. They are illustrated in Figure 2.1.3.
Any one curve can be selected for each IDMT element by scheme switch [M
∗∗∗
C].
Table 2.1.1 Specification of IDMT Curves
Curve
Description
k a c kr b
IEC Normal Inverse (NI)
0.14
0.02
0
-
-
IEC Very Inverse (VI)
13.5
1
0
-
-
IEC Extremely Inverse (EI)
80
2
0
-
-
UK Long Time Inverse (LTI)
120
1
0
-
-
IEEE Moderately Inverse (MI)
0.0515
0.02
0.114
4.85
2
IEEE Very Inverse (VI)
19.61
2
0.491
21.6
2
IEEE Extremely Inverse (EI)
28.2
2
0.1217
29.1
2
US CO8 Inverse
5.95
2
0.18
5.95
2
US CO2 Short Time Inverse
0.02394
0.02
0.01694
2.261
2
Note: kr, b are used to define the reset characteristic. Refer to equation (2).
In addition to above nine curve types, GRD140 can provide a user configurable IDMT curve. If
required, set the scheme switch [M
∗∗∗
C] to “CON” and set the curve defining constants k,
a
, c.
(1)
