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5.11 Primary, secondary and per
unit scaling
5 Measurement functions
VAMP 321
Example 5: Secondary to per unit for residual current.
Input is I01.
CT0 = 50/1
Current injected to the relay's input is 30 mA.
 Per unit current is
IPU = 0.03/1 = 0.03 pu = 3 %
Example 6: Per unit to secondary for residual current.
Input is I01.
CT0 = 50/1
The relay setting is 0.03 pu = 3 %.
 Secondary current is
ISEC = 0.03x1 = 30 mA
Example 7: Secondary to per unit for residual current.
Input is I0Calc.
CT = 750/5
Currents injected to the relay's IL1 input is 0.5 A.
IL2 = IL3 = 0.
 Per unit current is
IPU = 0.5/5 = 0.1 pu = 10 %
Example 8: Per unit to secondary for residual current.
Input is I0Calc.
CT = 750/5
The relay setting is 0.1 pu = 10 %.
 If IL2 = IL3 = 0, then secondary current to IL1 is
ISEC = 0.1x5 = 0.5 A
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VM321.EN001_B