The document discusses the per unit system which is used to analyze power systems. It defines the per unit system and provides the following key points: 1) The per unit system uses a common base value for voltage, current, impedance, and other values to simplify calculations since actual power system values are very large. 2) Impedance and other values in per unit are represented as percentages of the defined base value. 3) The per unit system provides a standard reference for comparing different power systems or components within a power system.

1. THE PER UNIT SYSTEM
Introduction:
The per unit system is fundamental in the analysis of power systems
since:
a. Parameters such as voltage and power are usually in the kilo or
mega range in a power system.
b. This is due to the large amount of power transmitted .
c. Other values such as current and impedance are usually
represented as a percent or per unit.
d. Determined by using a base or reference value.

2. The per unit system is an International Standard that has several
advantages. These are as follows:
1) System calculations are simplified.
2) Determining equipment rating.
3) Provides a reference to voltages in the power system.
Usually the base is specified in MVA and kV such that:
impedancebasetheisZwherephaseper
I3
kV
Z(1) b
L)-b(L
L)-b(L
b 
)(
MVA
)(kV
b
2
L)-b(L
bZ 
L)-b(L
b
b
L)-b(LL)-b(Lb)(3b
kV3
MVA
I
IkV3MVASBut



3. (2) Per Unit Conversion
MVAorIV,beeithercanAwhere
A''ofvalueBase
A''ofvalueActual
Apu 
Note: Apu is dimensionlessNote: Apu is dimensionless
  





 2
b
b
given
b
given
pu
kV
MVA
Z
Z
Z
Z
(3) P.U. Impedance
MVA
)(kV
b
2
L)-b(L
bZWhere 
%Z100xZ:Note pu 

4.  
MVA
1
αkVαZ
2
b
(4) To compare two power systems with different MVA
.
 
MVAα
kV
1
αZ 2pu
MVA
MVA
.
kV
kV
ZαZ
old
new
new
old
oldnew
b
b
2
b
b
pupu
















1
kV
kV
if:Note
new
old
b
b









MVA
MVA
.ZZhence
old
new
oldnew
b
b
pupu










5. Example:
MVA
MVA
.ZZusing
old
new
oldnew
b
b
pupu









Given a transformer with the following specifications: 100 MVA, 66/12 kV,
0.1 p.u. (10%).
If the base value chosen is 200 MVA, then
0.2
100
200
0.1Z newpu 






Note:
Zp.u. for transformer is the same whether referred to primary or secondary side since the
transformer has the same MVA rating on both sides and turns ratio cancels