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 104)
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 105)
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 106)
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 105 is compared with the equation 106 for the inverse time characteristic
of the OC1 step in it is obvious that if the following rules are followed:
1. set k equal to the IEC or ANSI standard generator capability value
2. set parameter
A_OC1
equal to the value 1/x2
3. set parameter
C_OC1
equal to the value 1/x2
4. set parameters
B_OC1
= 0.0 and
P_OC1
=2.0
5. set
StartCurr_OC1
equal to the value x
then the OC1 step of the CVGAPC function can be used for generator negative sequence
inverse overcurrent protection.
Section 10 1MRK 511 358-UEN A
Multipurpose protection
208
Application manual
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