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Ee 308

The document describes a generic three-phase power system and its representation using single line diagrams, impedance diagrams, and reactance diagrams. It discusses how each component such as generators, transformers, transmission lines, motors, and loads can be represented by their equivalent circuits. The document also introduces the per unit system for representing power system quantities like voltage, current, impedance, etc. within a narrow range. It describes how to develop a per unit impedance diagram and convert impedance values between different bases corresponding to equipment ratings. An example is provided to illustrate creating a per-phase per unit equivalent circuit of a power system.

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76 views19 pages

Ee 308

The document describes a generic three-phase power system and its representation using single line diagrams, impedance diagrams, and reactance diagrams. It discusses how each component such as generators, transformers, transmission lines, motors, and loads can be represented by their equivalent circuits. The document also introduces the per unit system for representing power system quantities like voltage, current, impedance, etc. within a narrow range. It describes how to develop a per unit impedance diagram and convert impedance values between different bases corresponding to equipment ratings. An example is provided to illustrate creating a per-phase per unit equivalent circuit of a power system.

Uploaded by

catdogsunmoon12
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We take content rights seriously. If you suspect this is your content, claim it here.
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You are on page 1/ 19

11-Jan-2024

Power Systems
Course Code: EE 308
(2-1-0-3)

Lecture - 3

Electrical Engineering, IIT Indore


A Generic Three-Phase Power System

Image Source: Internet


Single Line Diagram

• a pictorial representation of connection of various components of power system.


• All the components are depicted using symbols.
• Machine ratings and impedances are also included.
• Also, called one-line diagram.

Image Source: Internet


Representation of Different Power System Components

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Impedance Diagram
When we need to perform analysis, it is desirable to obtain an equivalent circuit of the power
system. This requires each component to be represented by its own equivalent circuit and then
connect them together as per the single line diagram.

Reactance Diagram
• Impedances are replaced by reactances
Equivalent Circuits

Generator Transformer Transmission Line

Motor Load
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Impedance Diagram from Single Line Diagram

Single Line Diagram

Impedance Diagram
Image Source: Internet
Single Line Diagram from Impedance Diagram

Image Source: Internet


Single Line Diagram from Impedance Diagram

Image Source: Internet


Modifications in Impedance and Reactance Diagrams
Per Unit System
Power system quantities such as current, voltage, impedance and power are often expressed in
per unit values.

actual value of any quantity in a unit


per unit value =
base value of the quantity in the same unit

Why p.u.?
• To bring down the range of these parameters power, voltage, current, impedance within a narrow
range.
• Advantageous with respect to specifying impedance values referred to particular side of
transformer.
Per Unit Values
• The main parameters are voltage, current, power and impedance. Base values of any two
parameters should be chosen and the base values of the other two parameters should be
obtained using the relationship with other parameters.
• Usually, we choose the base values of voltage and power and obtain the base values of current
and impedance.

Single phase system:


Choose Base kV and Base MVA
Per Unit Values for three-phase System
Conversion of pu values from one base to another base
• This is required because each equipment’s pu values are specified on the basis of its own rating.
When we are connecting these equipments together, their pu values should be specified on some
common base. Therefore, we need to change the base values and hence pu values need to be
converted from one base to other base.
Per Unit Impedance Diagram
• Choose a common system base MVA

• Divide the network into different sections on the basis of the location of transformer.

• Choose a suitable voltage in any one of the section.

• Obtain the base values of voltage in other sections using the voltage ratios of the transformers.

• Convert all impedances in p.u. corresponding to the base values in that section.
Example
A power system consists of one synchronous generator and one synchronous motor connected by two
transformers and a transmission line. Create a per-phase per unit equivalent circuit.

G1 : 100 MVA, 13.8 kV, R = 0.1 pu, Xs = 0.9 pu


T1 : 100 MVA, 13.8/110 kV, R = 0.01 pu, Xs = 0.05 pu
T2 : 50 MVA, 120/14.4 kV, R = 0.01 pu, Xs = 0.05 pu
M: 50 MVA, 13.8 kV, R = 0.1 pu, Xs = 1.1 pu
TL: R = 15 ohm, X = 75 ohm
Solution
Impedance Diagram

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