Power System Representation
using One Line Diagrams
• One Line Diagram or Single Line Diagram
One Line Diagram Symbols
OLD Symbols
Impedance Diagram from One line
diagram
Four Bus power system
One Line Diagram
Reactance Diagram
Admittance Diagram
Reactance Diagram
Admittance Diagram
• In power system engineering we are
concerned with the analysis of the power
system down to the “major loads” and this
will mean that the analysis will stop at the
distribution system voltages.
• Many distributions “feeders” attached to a
single bus in a distribution substation are
lumped into a single “load”.
Per unit system
• Sbase: “System MVA base” is the same
throughout the entire power system (say
100 MVA)
• The MVA base is a number used to
convert loads, generated power and loss
quantities to per unit power quantities.
Per Unit quantity
•
Per Unit Quanitities
Analysis of a power system in
per unit: Steps
• 1) Convert all three phase bus load MW
and MVAR, generator MW and MVAR, to
three phase per unit quantities using the
3phase system base.
• 2) Convert all actual resistance, inductive
and capacitive reactance values to per unit
using the Zbase for each particular part of
the power system where each r, x etc. is
located.
• 3) Carry out all calculations in per unit,
solving for the per unit bus voltage
magnitudes and bus phase angles, line
currents, and transmission losses.
• 4) Convert from per unit back to actual
MW, MVAR, MVA, and KV.
T1
T2
G1
Line 1
G2
Section D
Section A
Line 2
Section B
Line 3
T3
Section C
M
Seven Bus Case: One-line
System has
three areas
44 MW
-42 MW
-31 MW
0.99 PU
3
1.05 PU
1
106 MW -37 MW
AGC ON
62 MW
1.00 PU
-32 MW
Case Hourly Cost
16933 $/MWH
94 MW
AGC ON
-14 MW
32 MW
79 MW
Top Area Cost
2
40 MW
20 MVR
80 MW
30 MVR
4
110 MW
40 MVR
38 MW
-61 MW
1.04 PU
31 MW
-77 MW
5
8029 $/MWH
-39 MW
40 MW
1.01 PU
130 MW
40 MVR
168 MW AGC ON
-40 MW
1.04 PU
6
Area left
has one
bus
20 MW
-20 MW
40 MW
1.04 PU
20 MW
200 MW
0 MVR Left Area Cost
4189 $/MWH
200 MW AGC ON
7
-20 MW
Right Area Cost
4715 $/MWH
200 MW
0 MVR
201 MW AGC ON
Area right has one
bus
Area top
has five
buses
Summary
• One Line Diagram or Single Line Diagram
is a graphical representation of the power
system
• OLD is the starting point for Power System
Analysis
• Equivalent circuits are different from OLDs
• Power system analysis programs give
different techniques to create OLDs