REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-275
Time multiplier ‘k1-Setting’
Discriminative operation of the relays along a line is achieved by
time-grading. Assuming all the relays to be set to the same IB,
this involves setting the time multiplier in equal steps (grading
time), increasing from the load towards the source.
For example, in the case of the “very inverse” characteristic, the
constant c = 1 and the factor k1
£ 13.5. The operating time t is
then given by
t
k
I
I
B
=
-
1
3
1
0
Assuming the grading time of the protection functions to be 0.5 s
at 6 x I
B
, the settings of k1 according to the formula
k1 = 5 t
for operating times between 0.5 and 2.5 s become:
t [s] k1 [s]
0.5 2.5
15
1.5 7.5
210
2.5 12.5
The characteristics according to BS 142 are set as follows:
“normal inverse” : k1 = 0.14 s
“very inverse” : k1 = 13.5 s
“extremely inverse” : k1 = 80 s
“long time earth fault” : k1 = 120 s.
Definite minimum time ‘t-min.’
Where the IDMT function is being applied as back-up protection
for a directional E/F protection, the definite minimum time ‘t-min.’
must be set as follows
t-min. = t basic + t comp
t basic = basic time of the E/F function
t comp = comparison time of the E/F function (1 s).
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