Types of faults that occur in electrical system
Electrical energy or power is generated at various power stations and is conveyed by transmission and distribution system before being delivered to the consumers. There are various faults which may occur in the system during transmission and distribution of electric power.these fault reduce the reliability of the electric supply. To ensure better services every effort is made to design the power system in which the frequency of faults is minimum however if faults occurs efforts are made to remove it as early as possible. A fault in a circuit is a failure that interfares with the normal flow of current. A short circuit fault occurs when the insulation of the system fails resulting in low impedence path either between phase or phase to ground.This cause excessively high currents to flow in the circuit requiring the operation of protective equipments to prevent damage of the equipments.
Types of faults in overhead system
Short circuit faults is the most common fault in an overhead system. This may occur in the system due to the insulation failure caused by over voltage or due to the broken conductors. The various faults may occur in a 3-phase overhead system.
Single phase to earth-the single phase to earth is when insulators of two line conductor fail simultaneously or when one of the line conductor breaks and falls on the ground or ground wire
Phase to phase-this may occur when one of the line conductor breaks and falls on the other line conductor.
Two phase to earth-this may occur when insulators of two line conductor fail simultaneously or when two line conductors break and fall on the ground or guard wire simultaneously.
Phase to phase and third phase to earth-this may occur in when one of the line conductor breaks and fall on the other conductor simultaneously the insulator of the third conductor fails.
All the three phase short circuited-this may occur when two line conductors break and fall on the third conductor.
All three phase to earth- this may occur when insulators of all three line conductor fall simultaneously.
The first four types of faults is called unsymmetrical fault because these faults give rise to unsymmetrical currents. The last two faults are called symmetrical fault as they give rise to the symmetrical currents.
Whenever any one of the above fault occurs the line disconnected from the power system by protection devices placed at the substation. Sometimes there may be a temporary fault which may be developed by birds therefore circuit breaker is operated to energize the lines. If the circuit breakers could not hold it means the fault is permanent.
Types of faults in underground cables
The main faults which are likely to occur in underground cables are:-
Ground or earth faults-when the insulation of the cable between core and armouring fails or get damaged current starts flowing from core to the cable sheath or earth. such faults are known as ground or earth faults.
Short circuited faults-when the insulation between two cores of the multicore cable fails or get damaged current start flowing from one core to other instead of flowing through the load.such faults are known as short circuit faults. However in single core cables such faults only occur when the insulation between the two cables fails and get damaged.
Open circuit faults-when the joint of the cable conductor is pulled out or when the cable conductor breaks, no current flows in the cable. Such faults are known as open circuit fault. Before locating the point of fault in the cables it is necessary to determine the nature of fault. To determine the fault the insulation resistance of each core to ground and between the core and the earth that indicates ground fault. Whereas if megger shows how insulation resistance between any two power system faults.
Cores keeping for ends of the cable isolated from load, that indicates short circuit fault.
To determine open circuit fault the far end of a single core cable is earthed whereas the far ends of the two core of a multicore cable are interconnected and an ammeter is connected across the core and earth or between two cores. If the ammeter gives zero deflection that indicates open circuit fault in the cable
The short circuit faults can be classified as
- Symmetrical faults
- Unsymmetrical faults
A three phase symmetrical fault is caused by application of three equal fault impedances Zf to the three phase as shown in fig. If Zf =0 then fault is called a solid or bolted fault. These faults can be of two types i.e Line to Line to Line to Ground(LLLG fault) and Line to Line to Line (LLL faults).since three phase equally affected, the system remains balanced this fault is called symmetrical fault or balanced fault and the fault analysis is done on per phase basis.The behavior of LLLG fault and LLL fault is identical due to the balanced nature of the fault is usually the most serve fault that can occur in the system.
Faults in which the balanced state of the network is disturbed are called unsymmetrical or unbalanced faults. The most common type of fault in unsymmetrical or unbalanced fault in a system is single line to ground fault (LG fault). Almost 60 to 75% of faults in a system are LG faults. The other types of unbalanced faults are Line to Line faults (LL faults) and Line to Line to Ground (LLG faults). About 15 to 25% faults are LLG faults and 5 to 15% are LL faults. These faults are shown in figure below.