The two types of Faults are;
- Symmetrical Faults
- Unsymmetrical Faults
Symmetrical Types Of Faults
A three phase fault is commonly called as symmetrical fault.
- All the three lines are short circuited without an earth connection at the fault (L-L-L)
- All the three lines short circuited with an earth connection at the fault (L-L-L-G)
While such a faults occur, it give rises to symmetrical fault currents i.e., fault currents in all the three lines are identical in magnitude and displaced 120o electrical from one another. Although symmetrical faults are the most severe and imposed heavy duty on the CB (circuit-breakers), however the analysis on such type of faults can be made with a fair degree of ease. It is because the balanced nature of fault premits to consider only one phases in calculations, the conditions in the other two phases being similar.
The great majority of the faults in the power system is of unsymmetrical nature; the most common type being a short-circuit from line to ground fault. Whenever such faults occur, it gives increase to unsymmetrical currents i.e., the magnitude of fault currents in the three lines are different having unequal phase displacement. The calculation procedure known as method of symmetrical components is used to determine the currents and voltage on the occurrence of an unsymmetrical fault.
Unsymmetrical Types Of Faults
Those faults on the power system which give rise ti unsymmetrical fault current (i.e, unequal fault currents in the lines with unequal phase displacement) are known as unsymmetrical faults.
On the occurrence of an unsymmetrical fault, the current s in the three line becomes unequal and so is the displacement among them. It might be noted that the run term ‘unsymmetry’ applies only to the fault itself and the resulting line currents. However, the system impedances and the source voltages are always symmetrical through its main element viz. generators, transmission lines, synchronous reactors etc.
There are three ways in which unsymmetrical faults may occur in a power system;
- Single line to ground fault (L-G)
- Line to line fault (L-L)
- Double line to ground fault (L-L-G)
The solution for symmetrical fault problems can be obtained by either (a) kirchoff’s laws or (b) Symmetrical components method. The latter method is preferred because of the following reasons;
- It is a simple method and gives generality to be given to fault performance studies
- It provides a useful tool for the protection engineers, particularly in connection with finding out fault currents.
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