Lecture 11
Per unit in 1
φ
systems
Each device is assigned 2 base (or rated) values, V
B
and S
B
, the base voltage in Volts and
the base power in VA.
The voltage in pu is
pu
B
V
V
V
=
and the power is
pu
pu
B
P
jQ
Pj
Q
S
+
+=
The bases for the impedance and current are
,
BB
SV
IZ
VI
==
.
Form the 2 base quantities we define other base quantities so as to maintain KVL, KCL
and Ohm’s laws invariant.
Ohm’s Law.
VZ
I
=
or
I
YV
=
B
p
up
u
pu
B
B
B
B
Z
Z
Z
I
I
Z
I
VV
Z
V
Z
I
=
=
=
For KVL,
_
00
k
kk
p
u
loop
loop
loop
B
V
V
∆
∆=
→
=∆
=
∑∑
∑
Similarly for KCL,
_
p
u
node
node
II
=→
=
For the power formulae the same thing applies.
cos
cos
cos
cos
B
B
B
B
pu
pu
pu
pu
S
V
P
VI
V
PV
I
SS
V
I
V
I
φ
φφ
=
=
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View Full DocumentSimilarly,
2
_
sin
pu
transmitted
pu
pu
V
P
X
δ
=
Where
δ
is in radians.
Conversion from one pu base to another.
Suppose you are given two devices with impedance Y
1
and Y
2
.
Figure 1  Change of base
Y
1
is known in pu on a base V
B1
and S
B1
, Y
2
is known in pu on a base V
B2
and S
B2
. Since
Y
1
and Y
2
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 Spring '09
 FRANCISCODGALIANA
 Alternating Current, Volt, Conversion of units

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