Technical description
4 Measurement functions
4.5 Primary, secondary and per unit
scaling
VM55.EN005
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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
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