Introduction
We all are familiar and encountered at least once the phenomenon of fault occurrence in electrical system.
A power system failure across the town that happened due to a storm
breakout or an internal equipment fault that disrupted your local power
supply – these are all essentially the cases of faults in electrical
systems.
Through this article let us try to examine this phenomenon in little more detail.
What is an Electrical Fault?
An
Electrical System fault can be defined as a condition in the electrical
system that causes failure of the electrical equipment in the circuit
such as: Generators, Transformers, Busbars, Cables and all other
equipments in the system that operate at given voltage level.
Principal Fault Types
By nature of electrical systems, at the basic level, electrical faults can be categorized as:
- Short-Circuit
Faults: This is caused when there is a failure of insulation causing a
short-circuit condition. This is by far the most common cause of
failure.
- Open Circuit Faults: This fault occurs when a failure happens in the conduction path of electricity
Besides
this, there could be combination (Simultaneous) fault situations as
well and equipment level winding faults. We will not look in to them in
detail over here.
Since Short-Circuit Faults are the most common causes of faults in Electrical distribution systems, let us study them in detail:
Short-circuit Faults
A
short circuit fault occurs when there is an insulation failure between
phase conductors or between phase conductor(s) and earth or both. An
insulation failure results into formation of a short-circuit path that
triggers a short-circuit conditions in the circuit (i.e. abnormally high
current situations followed by visible effects like arcing, flashing).
Figure 1.0 below depicts a three phase-to-earth balanced fault condition:
Fig 1.0: Three Phase-to-Earth Balanced Fault Condition
Two other most common unbalanced fault conditions seen in a balanced three phase electrical system are:
- Phase-to-Phase fault:
In this, only two of the three phases get short-circuited, causing an
unbalanced fault condition in the system. Figure 2.0 below depicts this
fault condition.
Fig 2.0: Phase-to-Phase unbalanced Fault Condition
- Single phase-to-earth fault:
In this, one of the three phases get short-circuited with ground,
causing an unbalanced fault condition in the system. Figure 3.0 below
depicts this fault condition.
Fig 3.0: Single Phase-to-Earth unbalanced Fault Condition
Normally
during operations, the fault situations may be dynamic and change the
fault types rapidly based on local conditions. (e.g. a single
phase-to-earth fault may in turn change to a two phase-to-earth fault)
Conclusion
A
fault’s severity and magnitude depends on variety of factors like the
location of fault in the electrical system, damage caused due to fault.
While analyzing a given fault’s severity, it is a usual practice to
refer to a standard fault condition (three phase fault) for given
voltage level.
A three phase fault is considered as the most
severe fault that can occur in the system and hence its short-circuit
ratings are used while determining the required switchgear system.
Besides this, a single phase-to-earth fault current ratings are also
considered in short-circuit calculations.
