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F.2 TIME OVERCURRENT ELEMENTS
F.2TIME OVERCURRENT ELEMENTS
Follow on an example will be defined to explain the timing accuracy for 51 element.
According to the manual, the general formula for the IEC Very Inverse Curve is as follows:
Since IEC Standard does not fix time error limits for current ratios below 2 (see F.4 of this appendix for more information),
GE MII Family Relays fixes a maximum error of 5% maximum permitted in all range for I / Ipu > 2. Care must be taken to
use (in the formula) the current measured by the relay.
F
The lower limit means 1.5 times the pickup multiplied by a coefficient for the error:
For the upper limit:
Using a real case, where the time dial is 0.45 for an injected current of 4 A and a pickup of 3.5 A, the limits are as follows:
F-2
Applied Current
MIFII
Measured current
Resultant
Operation Time
Figure F–1: MAXIMUM ERROR
T
=
T
13
5 .
*
. 0
45
=
=
T
30
1
⎛
⎞
4
−
⎜
⎟
*
. 1
05
1
⎝
⎠
3
5 .
MIFII Digital Feeder Relay
13
5 .
*
D
=
T
1
⎛
⎞
I
⎜
⎟
−
1
⎜
⎟
I
⎝
⎠
pu
5%max. Error for I/Ipu<2
3%max. Error for I/Ipu>2
13
5 .
1 *
=
=
23
4 .
(
)
−
1
1
5 .
*
. 1
05
1
13
5 .
1 *
=
31
8 .
s
(
)
−
1
1
5 .
*
. 0
95
1
13
5 .
=
.
375
s
T
⎛
4
⎜
*
⎝
3
5 .
s
*
. 0
45
=
70
.
875
s
1
⎞
−
⎟
. 0
95
1
⎠
APPENDIX F
GEK-106237P
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