1

OUTLINE OF THE PRESENTATION
 INTRODUCTION
 NEED FOR THE SYMMETRICAL
COMPONENET METHOD
 TECHINQUES USED TO ANALYSIS
SYMMETRICAL
 SYMMETRICAL METHODS USED TO STUDY
POWER SYSTEM FAULTS
 CONCLUSION
 REFERENCES
2

INTRODUCTION
 The normal operating conditions of an electric power system are occasionally disrupted because of faults.


Analysis of power systems usually implies the computation of network voltages and currents under a given set of conditions.
Under many circumstances we tend to ignore the unbalanced operation in the system and unbalanced operation is always present.

3

An organization of power system analysis problem


Source (Reference #1)
4

Effects of faults on power system


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
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Flow of excessive current
Abnormal voltages
Voltage elevation of system neutral
Induce over voltages on neighbouring equipments .
Hazards to human, equipment and animals.
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Need for fault analysis




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Design of protection system requires the knowledge of fault current.
The information obtained from the fault studies are used: to select the sizes of circuit breaker, fuse and characteristic, setting of relay.
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Normal types of fault are:




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Fault due to lightning
Tree limbs falling on the line
Wind damage
Insulation deterioration
Vandalism
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Types of fault


Symmetrical fault :
Usually three phase to ground fault


Unsymmetrical fault
The fault is unbalanced in nature
8

Sources of Asymmetrical fault are:
(Reference#2)
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Sources of Asymmetrical fault are (cont.)

One phase open circuit

Unbalanced in load mainly the arc loads
10


Sources of Asymmetrical fault are (cont.)
One phase open circuit
11

Sources of Asymmetrical fault are (cont.)

SLG fault
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Need for the symmetrical component analysis

Negative sequence relay in generator has helped in protecting the generator from over heating in the event of unbalanced loads.


The positive sequence segregating network is used to supply the sensing voltage to generator voltage regulators
Certain connections of CT and PT develop zero sequence components that are used in protective ground relaying scheme.
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Method of adopted for symmetrical component analysis (cont)

Mathematically:
V a
= V a1+
V a2+
V a3+----------
V an





V b
= V b1+
V b2+
V b3+----------
V bn
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
V n
= V n1+
V n2+
V n3+----------
V nn
Where:
V a,
V b…….
V n are unbalanced set of phosors


V a1,
V b1……
V n1 first set of n balanced phasors with an angle 2pi/n between components a,b,…..n
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Method of adopted for symmetrical component analysis (cont)
Mathematically:
 V a2,
V b2……
V n2 second set of n balanced phasors with an angle
4pi/n between components a, b………,…..n

……………………………………………………….
 V a(n-1),
V b(n-1)……
V n(n-1)are (n-1)th set of n balanced phasors with an angle 2pi(n-1)/n between components a, b………,…..n
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Phasor a or a-operator

The phasor notation of a-operator
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Phasor a or a-operator
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Phasor a or a-operator
18

Equation in matrix form
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Symmetrical components for three Phase system

The n-phase system presented above is of academic interest only and only the practical three phase system will be emphasised.

Power is generated, transmitted and consumed mostly in three phase only.
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Symmetrical components for three Phase system
(cont)

The phasor representation of three phase system (source reference#2)
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21


Symmetrical components for three Phase system
(cont)

The positive sequence set consisting of three components of equal magnitude, displaced by 120 & 240 o respectively and having the phase sequence of abca.

The negative sequence set consisting of three components of equal magnitude displaced by 240 & 120 o respectively, having phase sequence of acba.
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The zero sequence set of the component of which being equal both in magnitude and and phase.
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22

Symmetrical components for three Phase system
(cont)
Relations of voltage components in matrix form

23

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Symmetrical components of generator
24


Symmetrical components of generator
25


Symmetrical components of transformer (zero)
26

Fault analysis using symmetrical components
The most common type of fault is the single line to ground fault:
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Fault analysis using symmetrical components
(cont)
The sequence component connection for the single line to ground fault:
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
Fault analysis using symmetrical components
(cont)
The sequence component connection for the single line to ground fault:

The sequence components are connected in series.

The three currents in the case of SLG fault are equal.
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Fault analysis using symmetrical components
(cont)

LL fault:
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Fault analysis using symmetrical components
(cont)


LL fault:
There is no zero sequence component due to absence of ground return path.

The positive and negative sequence components are connected in parallel.
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Fault analysis using symmetrical components
(cont)

LLG fault:
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Fault analysis using symmetrical components
(cont)


LLG fault:
For the zero sequence component it requires to add an external impedance of Z f
+3Z g

The networks are connected in parallel.
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Software available for carrying fault analysis
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The most commonly used soft ware are:

MATLAB

EDSA

ETAP

CYME
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Conclusion

Power system analysis using symmetrical component is very helpful in improving the reliability of the power system.

The principle adopted for the analysis of unbalanced fault system is symmetrical component method.

By knowing the principle, the results obtained from the computer can be analysed.
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References:
1. Paul M. Anderson, Analysis of Faulted Power
Systems.
2. W.D. Stevenson, Elements of Power System
Analysis.
3. A.P.S Meliopoulos, Power System Grounding and
Transients.
4. Olle. I. Elgerd, Electric Energy Systems Theory.
5. IEEE Transactions
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