VAMP 59 - Page 138

4.9 Primary, secondary and per
unit scaling
4 Measurement functions
Technical description
138
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VM59.EN001
Phase current scaling
Residual current (3I
0
)
scaling
secondary  per unit
MODESEC
PRISEC
PU
ICT
CTI
I



SEC
SEC
PU
CT
I
I 
per unit  secondary
PRI
MODE
SECPUSEC
CT
I
CTII 
SECPUSEC
CTII 
Example 1: Secondary to per unit for phase currents.
CT = 750/5
I
MODE
= 525 A
Current injected to the relay's inputs is 7 A.
 Per unit current is
I
PU
= 7x750/(5x525) = 2.00 pu = 2.00 xI
MODE
= 200 %
Example 2: Per unit to secondary for phase currents.
CT = 750/5
I
MODE
= 525 A
The relay setting is 2xI
MODE
= 2 pu = 200 %.
 Secondary current is
I
SEC
= 2x5x525/750 = 7 A
Example 3: Secondary to per unit for residual current.
Input is I
01
.
CT
0
= 50/1
Current injected to the relay'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 relay setting is 0.03 pu = 3 %.
 Secondary current is
I
SEC
= 0.03x1 = 30 mA