Time/Current Characteristics - ABB REJ 527 Technical Reference Manual

ABB Substation Automation
Products and Systems
4.1.3.5.
1MRS750616-MUM
Technical Reference Manual

Time/current characteristics

The low-set earth-fault current stage I
definite minimum time operation characteristic. The settings of switches SGF4/1...6
determine the operation mode of stage I
from page 23. At the IDMT characteristic, the operate time of the stage is a function
of the current: the higher the current, the shorter the operate time. Six time/current
curve groups are available. Four of these comply with the standards IEC 60255 and
BS 142 and two curve groups, RI and RD, are special type curve groups according
to ABB praxis.
Characteristics according to IEC 60255 and BS 142
The relationship between time and current is in accordance with the standards IEC
60255-3 and BS 142, and can be expressed as follows:
k β ×
t s [ ]
= -------------------------- -
α
 
Io
1
-------- - –
 
Io>
where t = operate time
k = time multiplier
I = phase current value
0
I > = set current value.
0
Table 1: The values of constants α and β.
Time/current curve group
Normal inverse
Very inverse
Extremely inverse
Long-time inverse
The standard BS 142.1966 defines the normal current range to be 2...20 times the
setting value. In addition, the relay has to start at the latest when the current exceeds
the setting value 1.3 times if the time/current characteristic is normal inverse, very
inverse or extremely inverse. For the long-time inverse characteristic the normal
current range is specified to be 2...7 times the setting, and the relay is to start when
the current exceeds the setting value 1.1 times.
> can be given a definite-time or an inverse
0
>. Refer to section "Settings" beginning
0
α
0.02 0.14
1.0 13.5
2.0 80.0
1.0
REJ 527
β
120
19