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Tuesday, 6 January 2015

Classification of Electrical Faults

As mentioned in my previous blog 
"a fault is the defect in the electrical circuit due to which the current in the circuit is diverted from the intended path."
Because of a fault the value of current and voltage at various points in the network changes giving rise to abnormal operating conditions.

Electrical faults may be broadly classified into two groups:
1.      Symmetrical faults, and
2.      Unsymmetrical faults.

Symmetrical faults

In symmetrical faults, also called three phase short circuits, all the three phases are short circuited to each other and often to earth also. Such faults are balanced and symmetrical as the system remains balanced even after the occurrence of the fault. During such a fault the fault current in the three lines of a 3-phase circuit are equal in magnitude and displaced by 120 electrical degrees from one another.

Though the symmetrical faults are rare, but when occurs they generally lead to most severe fault current flow. Balanced short circuit calculations are performed to find these large currents. A power network comprises of synchronous generators, transformers, transmission and distribution lines and loads. Loads are often neglected during faults, as the voltage drops down to such a low value that current drawn by loads can be neglected in comparison to fault currents.

Unsymmetrical faults

The majority of faults that occur in a power system are unsymmetrical faults involving only one or two phases. The most common type of unsymmetrical fault is a short circuit between a phase and the earth. In case of unsymmetrical faults, voltages and currents in the network become unbalanced and each phase is to be treated individually for computational purpose.

The magnitude of fault currents in the three lines is different having unequal phase displacements. The calculation procedure called as “method of symmetrical components” is used to find the currents and voltages during this type of fault.       
Electrical faults can also be classified as:
1.      Shunt faults or short faults, and
2.      Series faults or open faults.

Shunt or the short faults involve short circuit between power conductors or power conductors to earth. Shunt fault in a 3-phase line or system can be classified as:

1.      Single line to ground (LG) fault,
2.      Line to line (LL) fault,
3.      Double line to ground (LLG) fault,
4.      Three phase short circuits (LLL), and
5.      Three phase to ground (LLLG) fault.

Of the above mentioned faults, single line to ground, line to line and double line to ground faults are unsymmetrical faults, whereas three phase short circuits and three phase to ground faults are symmetrical faults. These faults may occur at the terminals of the generator and or transformer, on the conductors of a line or any other part of the power system. Often the path to earth contains resistance in the form of arc. Shunt faults are characterized by increase in current and fall in voltage and frequency.

A series fault is an unbalance in the line impedances mainly due to open conductors. It does not involve any connection between lines or between lines or between line and ground at the fault point and therefore called as series fault. These faults disturb the symmetry in one or two phases and are therefore unbalanced faults.