Per Unit [Pu] Scaling Of Line-To-Line Voltages - Allen-Bradley Rockwell Automation 857 User Manual

5-16
Measurement Functions
857-UM001A-EN-P – July 2009

Per Unit [pu] Scaling of Line-to-Line Voltages

One per unit = 1 pu = 1xU
the VT.
Table 5.18 – Scaling of Line-to-Line Voltages
Voltage measurement mode =
"2LL+Uo", "1LL+Uo/LLy", "2LL/LLy",
"LL/LLy/LLz"
secondary
U
=
SEC
⇒ per U
PU
VT
unit
SEC
per unit
=
⇒ U U
SEC PU
secondary
Example 1 – Secondary to per unit. Voltage measurement mode
is "2LL+Uo"
VT = 12000/110
Voltage connected to the device's input U
⇒ Per unit voltage is
U = 110/110 = 1.00 pu = 1.00xU
PU
Example 2 – Secondary to per unit. Voltage measurement mode
is "3LN"
VT = 12000/110
Three symmetric phase-to-neutral voltages connected to the device's
inputs U ,U and U are 63.5 V
a b c
⇒ Per unit voltage is
= √3x63.5/110x12000/11000 = 1.00 pu = 1.00xU
U
PU
Example 3 – Per unit to secondary. Voltage measurement mode
is "2LL+Uo"
VT = 12000/110
The device displays 1.00 pu = 100 %.
⇒ Secondary voltage is
U = 1.00x110x11000/12000 = 100.8 V
SEC
Example 4 – Per unit to secondary. Voltage measurement mode
is "3LN"
VT = 12000/110
U = 11000 V
N
The device displays 1.00 pu = 100 %.
⇒ Three symmetric phase-to-neutral voltages connected to the
device's inputs U ,U
a b
U = 1.00x110/√3x11000/12000 = 58.2 V
SEC
= 100 %, where U
N
Line-to-line voltage scaling
Voltage measurement mode = "3LN"
VT
⋅
PRI
U
PU
U
N
U
⋅ ⋅
N
VT U
SEC SEC
VT
PRI
or U
a
= 100 %
N
and U are.
c
= rated voltage of
N
U VT
= 3 ⋅ ⋅
SEC PRI
VT U
SEC N
VT U
= ⋅ ⋅
SEC N
U
PU
VT
3
PRI
is 110 V.
b
= 100 %
N