Different Types of Faults in Overhead Transmission Lines

Electrical energy is generated at power stations and then transmitted and distributed to consumers. However, various faults can occur in the system during the transmission and distribution of electricity.

These faults can disrupt the transmission and distribution of power, leading to potential issues for consumers and the overall system. Common faults include short circuits, overloads, ground faults and line-to-line faults.

Types of Faults in Overhead System

The most common fault in an overhead system is a short circuit. This may occur in the system due to insulation failure caused by over-voltage or due to broken conductors.

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The various faults occur in a three phase overhead system are :

1. Single Phase to Earth

This type of fault occurs when one of the conductors of the transmission line breaks and falls on the ground or guard wire. It is a common fault caused by environmental conditions such as storms, heavy rain or damage due to external objects like falling trees.

2. Phase to Phase

This fault occurs when one of the line conductors breaks and falls on the other line conductor. The major cause of this type of fault is heavy wind, which may cause the line conductors to swing and touch each other, resulting in a short circuit. This can also happen due to mechanical failure or improper spacing between conductors.

3. Two Phase to Earth

This may occur when two conductors of three phase transmission line break and fall on the ground or guard wire. It causes a large flow of current and can damage the system.

4. Phase to Phase and Third Phase to Earth

This fault takes place when one line conductor breaks and falls on another line conductor, while the third conductor falls on the ground or guard wire. This type of fault combines the effects of a phase to phase short circuit and a single-phase to earth fault, making it complex to handle.

5. All the Three Phase Short Circuited

This fault occurs when two conductors of the transmission line fall on the third line conductor, causing a short-circuit between all three phases. It is usually caused by extreme weather conditions or damage to the transmission line.

6. All the Three Phase to Earth

This fault occurs when all three conductors break and fall on the earth or guard wire or on any other grounded object. This type of fault results in a complete system shutdown and requires immediate attention to restore normal operations.

When a fault occurs in a transmission line, the protective relays at the substation disconnect the line from the power system to prevent further damage. The six types of faults are shown in the figure. Out of all the six faults, the first four are called unsymmetrical faults because they result in unequal (unsymmetrical) currents in the system. Examples of such faults include single-line-to-ground, line-to-line, and double-line-to-ground faults.

The remaining two faults (types 5 and 6) are known as symmetrical faults as they produce equal (symmetrical) currents in all phases. Symmetrical faults are rare but are more severe compared to unsymmetrical faults

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