This formula is applicable only when measured current (for example, positive
sequence current) exceeds a pre-set value (typically in the range from 105 to 125%
of the generator rated current).
By defining parameter x equal to the per unit value for the desired pickup for the
overload IED in accordance with the following formula:
EQUATION1377 V2 EN-US (Equation 520)
formula 3.5can be re-written in the following way without changing the value for
the operate time of the generator stator overload IED:
2
2
2
1
1
op
m
r
k
x
t
I
x I x
×
=
æ ö
-
ç ÷
×
è ø
EQUATION1378 V1 EN-US (Equation 521)
In order to achieve such protection functionality with one CVGAPC functions the
following must be done:
1. Connect three-phase generator currents to one CVGAPC instance (for
example, GF01)
2. Set parameter CurrentInput to value PosSeq
3. Set base current value to the rated generator current in primary amperes
4. Enable one overcurrent step (for example OC1)
5. Select parameter CurveType_OC1 to value Programmable
op
P
A
t k B
M C
æ ö
= × +
ç ÷
-
è ø
EQUATION1375 V1 EN-US (Equation 522)
where:
t
op
is the operating time in seconds of the Inverse Time Overcurrent TOC/IDMT algorithm
k is time multiplier (parameter setting)
M is ratio between measured current magnitude and set pickup current level
A, B, C and P are user settable coefficients which determine the curve used for Inverse Time
Overcurrent TOC/IDMT calculation
When the equation 521 is compared with the equation 522 for the inverse time
characteristic of the OC1 step in it is obvious that if the following rules are
followed:
Section 12 1MRK 506 369-UEN B
Multipurpose protection
576 Line distance protection REL670 2.2 IEC
Application manual
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