POWER SYSTEM

1. SYMMETRICAL COMPONENTS IN FAULT
STUDIES
Paper Name: POWERSYSTEM II
Paper Code: PC-EE 601
CO Number : (CO1 & CO II)
Module Number: 1
Bloom’s Level of Taxonomy : (Understanding)

2. CONTENT
 Introduction
 Symmetrical component theory
 Calculation
 Conclusion

3. Introduction
• A fault in a power system or circuit is a failure which interferes with the normal flow of current. In
general faults occur in power system networks due to insulation failure of equipment, flashover of
lines initiated by a lightning stroke, or due to accidental faulty operation
• Symmetrical Fault = This is also called balance fault. After failure the fault currents in various
lines are still balance (i.e. equal fault currents in the lines with 1200 displacement). 3 phase fault
(LLL) is the example of this category.
• Unsymmetrical Fault = This is also called unbalanced fault. After failure the fault current in
various lines are not balance (i.e. not same in magnitude and not displaced by 120°).
Unsymmetrical faults are three types;
• (1) Single line to Ground fault (LG)
• (2) Double line fault (LL)
• (3) Double line to ground fault (LLG)
• An unbalanced system consisting of ‘n’ number of phasors can be divided into ‘n’ systems of
balanced phasor. These ‘n’ systems of balanced systems are known as symmetrical
components.

4. SYMMETRICAL COMPONENT THEORY
In 1918, Dr. C.L. Fortescue, an American scientist, shown that any unbalanced system of 3-
phase currents (or voltages) may be regarded as being composed of three separate sets of
balanced vectors. This has come to be known as symmetrical component theory. This is a
general theory and is applicable to any n vector system whose resultant is zero.
Positive sequence components = Three phasors equal in magnitude and displaced each
other by 1200 and has the same phase sequence as the original phasors.
Negative sequence components = Three phasors equal in magnitude and displaced each
other by 1200 and has the opposite phase sequence as the original phasors.
Zero sequence components = consisting of thee phasors equal in magnitude and has zero
phase displacement from each other.

5. CALCULATION
Since each of the original unbalanced phasors are the sum of its positive, negative and zero
sequence components defined above.

6. CALCULATION
Matrix form of the equations

7. CONCLUSIO
N
 We can see those symmetrical components are a way of analyzing
unbalanced phasors which arise due to faults. Converting a set of
unbalanced phasors into sets of balanced components greatly simplifies our
analysis and is helpful in performing fault calculations, power flow studies,
and stability studies.
 Moreover, we can calculate sequence impedances using symmetrical
components, from which we can derive sequence circuits of transmission
lines and transformers and ultimately sequence networks of a complete
power system. This helps us analyze the behavior of our power system
under different types of faults.