Primary, Secondary And Per Unit Scaling; Current Scaling - Allen-Bradley Rockwell Automation 857 User Manual

5-12
Measurement Functions
Primary, Secondary and
per Unit Scaling
857-UM001A-EN-P – July 2009
Figure 5.5 shows a graphical solution. The input values have been
scaled with √3/100 to make the calculation easier.
Injected line-to-neutral voltages
U =0
L3
1
120°
1
U
L2
Figure 5.5 – Example of symmetric component calculation using line-to-neutral voltages
Unscaling the geometric results gives
U = 100/√3 x 2/3 = 38.5 %
1
U = 100√3 x 1/3 = 19.2 %
2
U /U = 1/3:2/3 = 50 %
2 1
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 in feeder mode and furthermore motor name plate
values in motor mode.
The following scaling equations are useful when doing secondary
testing.

Current Scaling

NOTE: The rated value of the device's current input, 5 A or 1A,
does not have any effect in the scaling equations, but it defines the
measurement range and the maximum allowed continuous current.
See "Measuring Circuitry" in Chapter 10 for details.
Table 5.15 – Primary and Secondary Scaling
secondary ⇒ primary
primary ⇒ secondary
2
a U
L2
120°
U U
L1 L1
120° aU
L2
U
L2
= ⋅
I I
PRI SEC
= ⋅
I I
SEC PRI
FortescueEx3
Positive sequence
U = 2/3
1
2
U +aU
L1
Negative sequence
2
2 U +a U
a U
L1 L2
L2
1
1
1
U
L1
U = 1/3
2
Current scaling
CT
PRI
CT
SEC
CT
SEC
CT
PRI
L2