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.
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