VAMP 59 - Primary, Secondary and Per Unit Scaling; Current Scaling

Technical description
4 Measurement functions
4.9 Primary, secondary and per
unit scaling
VM59.EN001
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4.9. Primary, secondary and per unit
scaling
Many measurement values are shown as primary values
although the relay is connected to secondary signals. Some
measurement values are shown as relative values - per unit or
per cent. Almost all pick-up setting values are using relative
scaling. The scaling is done using the given CT in feeder mode.
The following scaling equations are useful when doing
secondary testing.
4.9.1. Current scaling
NOTE! The rated value of the relay's current input 5 A, does not have any effect
in the scaling equations, but it defines the measurement range and the
maximum allowed continuous current. See chapter 9.1.1 for details.
Primary and secondary scaling
Current scaling
secondary  primary
SEC
PRI
SECPRI
CT
CT
II 
primary  secondary
PRI
SEC
PRISEC
CT
CT
II 
For residual current to input I
01
use the corresponding CT
PRI
and CT
SEC
values. For earth fault stages using I
0Calc
signals use
the phase current CT values for CT
PRI
and CT
SEC
.
Example 1: Secondary to primary.
CT = 500/5
Current to the relay's input is 4 A.
 Primary current is I
PRI
= 4x500/5 = 400 A
Example 2: Primary to secondary.
CT = 500/5
The relay displays I
PRI
= 400 A
 Injected current is I
SEC
= 400x5/500 = 4 A
Per unit [pu] scaling
For phase currents
1 pu = 1xI
MODE
= 100 %, where
I
MODE
is the nominal value of the feeder.
For residual currents
1 pu = 1xCT
SEC
for secondary side and
1 pu = 1xCT
PRI
for primary side.