Electrical Fault Current
Electrical fault is an abnormal condition in our power system. It can be due to any type of equipment failures such as transformer or electric machines, human error or environmental conditions. All these type of faults can cause heavy electrical fault current, interruption of electricity, equipment damage and even can result in death of living organisms.
Electrical fault can be illustrate as:
“The failure in electrical system, that causes flow of electric current in abnormal manners “
Electrical system is developing in size and complexity in all segments. i.e. generation,transmission and distribution. Any type of fault in electrical system can cause extreme economic loss and reduces the reliability of the electrical system. In electrical power systems, protective devices can recognize fault conditions in system and operate circuit breakers and different devices to restrain the loss of service due to a failure.
Electrical Fault Current Types
In electrical power system, current and voltage remains in its nominal values. Electrical fault causes deviation of those values from its nominal values or state to abnormal behavior. The most usual and dangerous fault that occurs in power system is short circuit. In short circuit, heavy current flows through the circuit sidestepping the normal load.
There are mainly two types of faults in electrical power system.
- Symmetrical fault
- Unsymmetrical fault
“A fault that effects all the phases equally is called symmetrical fault.”
In symmetrical faults, all phases are short-circuited together or often to earth. These faults are also known as Balanced fault. These types of faults are of two types.
Line to line to line ( L-L-L ) fault
This type of fault occurs when all three phases short together due to environmental effect or insulation breakdown. Their appearance in power system is rarely 2% – 3%.
Line to line to line to ground( L-L-L-G ) fault
This type of fault occurs when all three phases short together with ground wire. This type of fault is very severe type of fault. Their appearance in power system is rarely 2% – 3%.
“The fault that effects all the phases unequally is known as unsymmetrical fault.”
Unsymmetrical faults only involves one or two phases. The phases in unsymmetrical faults gets unbalanced. Thus, also known as unbalanced fault. These type of faults are further classified into four types.
Single line to ground ( L-G ) fault
It mainly occurs when any single phase line get in contact with ground or neutral conductor due to breakdown or environmental effect. The changes of these type of faults in electrical power system are reasonably high.
Double line to ground ( L-L-G ) fault
This type of fault occurs when two phases together get in contact with ground or neutral conductor. 15% – 20% of faults in electrical systems are usually L-L-G faults.
Line to line ( L-L ) fault
Line to line faults are when two phases contact together. These faults usually occurs by conductor swinging due to wind. The changes of these type of faults in electrical power system are approximately 10% – 12%.
Line to line and Line to ground fault
This type of fault occurs when two phases contact together and third phase contact with ground. Such type of fault occurs due to breakdown of conductor and insulation. Changes for such type of faults are hardly 1% – 2%.
Destructive Effects Of Fault Current
On the occurrence of these faults, heavy short circuit current flows in the circuit. This fault current have the following destructive effects on electrical power systems.
a) Excessive flow of current: When fault occurs, it provides a low impedance path for the current to flow. This results in very high current drawing from the source.
b) Electrical fire: Sometimes short circuit causes sparks due to air ionization round the conductor. This arc may further leads to fire that may cause considerable damage to electrical power system.
c) Instability: The instability of power system may occurs due to short circuit that may even lead to complete shutdown of electric power system.
d) Equipment loss: Sometimes heavy current flow due to short circuit may damage or completely burnt the electrical equipment. This lead to improper working of electrical power system.
Detecting And Locating Faults
Electrical Faults in overhead transmission lines are easy to investigate. Since, the problems in it will be pretty much obvious. Fault location in a cable system can either be done by de-energizing circuit or by the circuit with power.
There are two techniques use broadly for detecting electrical fault location.
- Terminal Method
- Tracer Method
Terminal method which measures current and voltage at the corner end of cables. This method only locates the general location of electrical fault. This method can be useful for reducing fault cost expenditure of long buried underground cables.
Tracer method which uses time-domain reflector to locate the electric fault location. In time-domain reflector a pulse is send down the wire to analyze the returning reflected pulse. The mathematical calculations are done to trace the exact location of fault. This method is usually adopted in complex wiring system.
The Murray Loop and Varley loops are the old techniques used for locating faults in cables.
Electrical Fault Current Limiting Devices
We can reduce human error faults but we have no control over environmental changes. Tracing fault location is very tough in complex power system. So, if we can manage to break the circuit if fault arises, this would reduce the percentage of maximum equipment damage loss.
Most commonly used electrical fault current limiting devices are circuit breakers, fuses and relays.
Circuit breakers is an automatically operating electrical switch device to protect power system from over current due to short circuit. It interrupt current flow by tripping down when electrical fault current arises.
Fuses are the most commonly used protecting device. It melts down due to heavy short circuit current when arises in system thus breaks the power supply between the supply end and load.
Relays are the protecting devices which operates electromagnetically. When electrical fault current arises, it energize the coil within the relay thus results in tripping down of the circuit by opening the circuit.
Transmission Lines are consist of Lightening arrestors and grounding wires to protect the system against lightening and surge voltages.