Assuming a grading time of 0.5 s at 6 times the base current I
B
is required, the factor k
1
for
each of the relays is given by,
k
1
= 5 t
For operating times between 0.5 and 2.5 s, this results in the following settings for k
1
:
t [s]
k
1
[s]
0.5
2.5
1
5
1.5
7.5
2
10
2,5
12.5
The characteristics according to BS 142 are set as follows:
normal inverse:
k
1
= 0.14 s
very inverse:
k
1
= 13.5 s
extremely inverse:
k
1
= 80 s
long time earth fault:
k
1
= 120 s
Characteristic angle
Determining the phase-angle of the current provides an additional criterion for preserving
discrimination compared with non-directional overcurrent protection. The directional
sensitivity is ±180° in relation to the reference voltage. This is illustrated in the following
diagram.
1MRK 505 406-UEN B
Section 5
Bay protection functions
Bay protection functions REB500
109
Technical manual
© Copyright 2019 ABB. All rights reserved