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Technical description
4 Measurement functions
4.5 Primary, secondary and per unit
scaling
VM55.EN005
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107
Per unit [pu] scaling of zero sequence voltage
secondary
per unit
3
1
SEC
c
b
a
SEC
PU
U
U
U
VT
U
per unit
secondary
SEC
PU
SEC
c
b
a
VT
U
U
U
U
3
Example 1: Secondary to per unit.
U
0SEC
= 110 V (This is a configuration value corresponding to
U
0
at full earth fault.)
Voltage connected to the device's input U
C
is 22 V.
Per unit voltage is
U
PU
= 22/110 = 0.20 pu = 20 %
Example 2: Secondary to per unit.
VT = 12000/110
Voltage connected to the device's input U
A
is 38.1 V, while
U
A
= U
B
= 0.
Per unit voltage is
U
PU
= (38.1+0+0)/(
3x110) = 0.20 pu = 20 %
Example 3: Per unit to secondary.
U
0SEC
= 110 V (This is a configuration value corresponding to
U
0
at full earth fault.)
The device displays U
0
= 20 %.
Secondary voltage at input Uc is
U
SEC
= 0.20x110 = 22 V
Example 4: Per unit to secondary.
VT = 12000/110
The device displays U
0
= 20 %.
If U
B
= U
C
= 0, then secondary voltages at U
A
is
U
SEC
=
3x0.2x110 = 38.1 V