VAMP 265 - Page 124

4.7 Primary, secondary and per unit
scaling
4 Measurement functions
Technical description
124
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Equation 4.7.1-1
N
N
N
U
S
I


3
Where
I
N
=
The rated current 1 pu.
S
N
=
Rated apparent power of the protected device
U
N
=
Rated line-to-line voltage of the protected device
.
For residual currents and ArcI> stage
1 pu = 1xCT
SEC
for secondary side and
1 pu = 1xCT
PRI
for primary side.
Phase current scaling
excluding ArcI> stage
Residual current (3I
0
)
scaling and phase
current scaling for ArcI>
stage
secondary  per unit
NSEC
PRISEC
PU
ICT
CTI
I



SEC
SEC
PU
CT
I
I 
per unit  secondary
PRI
N
SECPUSEC
CT
I
CTII 
SECPUSEC
CTII 
Example 1: Secondary to per unit and percent for phase
currents excluding ArcI>.
CT
PRI
= 150/1
CT
SEC
= 800/5
S
N
= 25 MVA
U
N
= 110 kV
U'
N
= 21 kV
Current injected to the relay's primary side input is 175 mA
and 859 mA for the secondary side input.
The rated current on HV and LV side will be
I
N
= 25 MVA/(3 x 110 kV) = 131.2 A
I'
N
= 25 MVA/(3 x 21 kV) = 687.3 A
 Per unit currents are
I
PU
= 0.175x150/(1x131.2) = 0.20 pu = 20 %xI
N
(HV side)
I'
PU
= 0.859x800/(5x687.3) = 0.20 pu = 20 % (LV side)
Example 2: Secondary to per unit for ArcI>.
CT = 750/5
Current injected to the relay's inputs is 7 A.
 Per unit current is
I
PU
= 7/5 = 1.4 pu = 140 %