ABB REJ 527 Technical Reference Manual page 21

ABB Substation Automation
Products and Systems
1MRS750616-MUM
Technical Reference Manual
RD-type characteristic
The RD-type characteristic is a special characteristic that is principally used in earth-
fault protection requiring a high degree of selectivity even at high resistance faults.
In this case, the protection can operate in a selective way even if it is not directional.
Mathematically, the time/current characteristic can be expressed as follows:
t s [ ] ×
= 5.8 1.35 – log
where t = operate time in seconds
k = time multiplier
I = phase current
0
I > = set start current.
0
The RD-type characteristic is illustrated in Fig. 4.1.3.5.-3.
t/s
70
60
50
40
30
20
10
9
8
7
6
5
4
3
2
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
1 2 3 4 5 6 7 8 9 10 20 Io/Io>
Fig. 4.1.3.5.-1 Normal inverse and very inverse time characteristics
 
Io
----------------- -
 
×
e
k Io>
t/s
70
60
50
40
30
20
10
9
8
7
6
5
4
3
k
1.0
2
0.9
0.8
0.7
0.6
0.5
1
0.9
0.4
0.8
0.7
0.3
0.6
0.5
0.2
0.4
0.3
0.1
0.2
0.05
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
1 2
REJ 527
k
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.05
20 Io/Io>
3 4 5 6 7 8 9 10
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