VAMP 259 - Primary, Secondary and Per Unit Scaling; Current Scaling

Technical description
4 Measurement functions
4.11 Primary, secondary and per unit
scaling
VM259.EN007
VAMP 24h support phone +358 (0)20 753 3264
197
4.11. Primary, secondary and per unit
scaling
Many measurement values are shown as primary values
although the device 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, VT ratios in the
―Scaling‖ of the relay.
Following scaling equations seen in the sub chapters are useful
when doing secondary testing.
4.11.1. Current scaling
NOTE! The rated value of the device's current input, 5 A, 1A or 0.2 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 currents 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 device's input is 4 A.
 Primary current is I
PRI
= 4x500/5 = 400 A
Example 2: Primary to secondary.
CT = 500/5
The device displays I
PRI
= 400 A
 Injected current is I
SEC
= 400x5/500 = 4 A