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Technical description
4 Measurement functions
4.5 Primary, secondary and per unit
scaling
VM55.EN005
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105
4.5.
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 VT.
The following scaling equations are useful when doing
secondary testing.
4.5.1.
Voltage scaling
Primary/secondary scaling of line-to-line voltages
secondary
primary
SEC
PRI
SEC
PRI
VT
VT
U
U
3
primary
secondary
PRI
SEC
PRI
SEC
VT
VT
U
U
3
Example 1: Secondary to primary.
VT = 12000/110
Voltage connected to the device's input U
A
or U
B
is 100 V.
Primary voltage is U
PRI
= 100x12000/110 = 10909 V
Example 2: Secondary to primary.
VT = 12000/110
Three phase symmetric voltages connected to the device's
inputs U
A
, U
B
and U
C
are 57.7 V.
Primary voltage is U
PRI
=
3x58x12000/110 = 10902 V
Example 3: Primary to secondary.
VT = 12000/110
The device displays U
PRI
= 10910 V.
Secondary voltage is U
SEC
= 10910x110/12000 = 100 V
Example 4: Primary to secondary.
VT = 12000/110
The device displays U
12
= U
23
= U
31
= 10910 V.
Symmetric secondary voltages at U
A
, U
B
and U
C
are
U
SEC
= 10910/
3x110/12000 = 57.7 V4