VAMP 259 - Page 198

4.11 Primary, secondary and per unit
scaling
4 Measurement functions
Technical description
198
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VM259.EN007
Per unit [pu] scaling
For phase currents excluding ArcI> stage
1 pu = 1xI
n
= 100 %, where
I
n
is the rated current according to the mode (see chapter 10).
For residual currents and ArcI> stage
1 pu = 1xCT
SEC
for secondary side and
1 pu = 1xCT
PRI
for primary side.
Phase current scaling for
phase currents, ArcI>
stage and residual
current (3I
0
)
secondary  per unit
SEC
SEC
PU
CT
I
I 
per unit  secondary
SECPUSEC
CTII 
Example 1: Secondary to per unit for feeder mode and ArcI>.
CT = 750/5
Current injected to the device's inputs is 7 A.
 Per unit current is
I
PU
= 7/5 = 1.4 pu = 140 %
Example 2: Per unit to secondary for feeder mode and ArcI>.
CT = 750/5
The device setting is 2 pu = 200 %.
 Secondary current is
I
SEC
= 2x5 = 10 A
Example 3: Secondary to per unit for residual current.
Input is I
01
.
CT
0
= 50/1
Current injected to the device's input is 30 mA.
 Per unit current is
I
PU
= 0.03/1 = 0.03 pu = 3 %
Example 4: Per unit to secondary for residual current.
Input is I
01
CT
0
= 50/1
The device setting is 0.03 pu = 3 %.
 Secondary current is
I
SEC
= 0.03x1 = 30 mA