Definition: Fault in electrical equipment or apparatus is defined as an imperfection in the electrical circuit due to which current is deflected from the intended path. In other words, the fault is the abnormal condition of the electrical system which damages the electrical equipment and disturbs the normal flow of the electric current.
The fault reduces the insulation strength between phase conductors and earth or any earthed screen surrounding the conductors. It also reduces the impedance (property of circuit to oppose the flow of current) between the conductors and earth due to which the heavy short circuit current flows through the system and damage the power system equipment.
The probability of the failure or occurrence of abnormal faults is more on the transmission lines. About one-half of the fault occurs on the power system is the transmission line fault. Because transmission lines are broadly branched, have a greater length, operate under variable weather conditions and are subjected to the action of atmospheric disturbances of an electrical nature.
According to the causes of incidence, the causes of failures may be classified, as mentioned below;
- Breakdown due to insulation may occur at normal voltage due to the decline or ageing of the insulation, and the damages are caused by the unpredictable happening such as blowing of heavy winds, tree falling across the line, vehicles colliding with the towers or poles, bird shorting out lines, aircraft colliding with the line, line breaks etc.,
- The breakdown may occur due to abnormal voltages caused by switching surges or lightning strokes which may be either direct or induced.
The line and insulation apparatus may also be damaged due to the transient overvoltage which is generated in the switching operation. During the switching operation, the voltage rises at a rapid rate and may achieve a peak value approaching three-time phase to neutral voltage. Hence the higher insulation level is provided for protecting the equipment from damage.
The fault can be minimised by improving the system design, better quality of the equipment and maintenance. But the fault cannot be eliminated completely.
Types of Electrical Fault
The most common and dangerous fault that occurs in a power system is the short circuit or shunt fault. On the occurrence of the short circuit fault, heavy or short-circuit current flow through the circuit which damages the insulation of current carrying phase conductors corresponding to earth or in the insulation between phases. The different types of the electrical fault are explained below:
- Single Phase to Ground Fault – It is also called a line-to-ground fault. It mainly occurs due to insulation breakdown between one of the phase and earth. Single-line-to-fault is most frequently occurs in the power system. Their chances of appearance in the power system are 70%.
- Phase-to-Phase Fault – Such type of fault rarely occurred on the power system. It is also called Line-to-line fault. It occurs when two conductors are short circuited. Their chance of appearance is hardly 15 % in the power system.
- Two Phases to Ground Fault – In this type of fault breakdowns of insulation between two phases and earth occur. It is the most severe type of fault but rarely occurs in the power system. It is also called Line-to-line-to-ground fault (L-L-G). Their chance of occurrence is hardly 10 %.
- Phase to phase and Third Phase to Ground Fault – It is the combination of phase to phase and phase to phase to ground fault. Such types of fault occur due to the breakdown of insulation between two phases and simultaneous breakdown of insulation between the third phase and earth. The chance of such type of fault is hardly 2 % to 3 %.
- All the Three Phases to Ground Fault – It is the most severe type of the fault and very rarely occurs in the power system. It occurs due to a breakdown of insulation between all the phases as well as to the earth. It is 2% to 3% in the power system.
- All the three Phases Short Circuited – This type of fault mainly occurs due to a breakdown of insulation between all the three phases. Their appearance is rarely 2 % to 3% in the power system.
The first four faults are of an unsymmetrical nature and give rise to unsymmetrical current, i.e., different currents in the three phases. The latter two faults are of symmetrical nature and give rise to symmetrical current, i.e., equal fault current in all the three phases with 120º displacement.
Harmful Effect of Faults on Power System
On the occurrence of the fault, heavy short circuit current is flowing in the circuit. This current have the following disadvantages. Their consequences are explained below in details.
- The heavy current due to fault causes excessive heating which may result in fire or explosion.
- Sometimes the short circuit current takes the form of an arc that may cause considerable damage to the element of the power system.
- The stability of the power system may be adversely affected, and even the complete shutdown of the power system may occur.
- Damage to other apparatus in the system may be caused due to overheating and due to abnormal mechanical forces set up.
A large number of unsymmetrical faults is temporary in nature and may vanish within a few cycles as would be the cause when a twig (stem) falls across a line and burn itself out or just fall. The symmetrical three-phase faults, generally occur due to the carelessness of operating personnel.