Types of Faults in the Power System

In power systems, faults occur due to various conditions such as insulation failure, overvoltage, short circuits, broken conductors caused by excessive mechanical damage, adverse climatic conditions, unbalanced currents, and more.

This article explains the different types of faults that occur in power systems and the reasons behind their occurrence.

In transmission systems, alternating current (AC) is primarily used for power transmission and distribution. Faults in power systems can be broadly classified into three major categories:

1. Symmetrical Faults
2. Unsymmetrical Faults
3. Simultaneous Faults

1. Symmetrical Faults

Symmetrical faults are characterized by the fault current being balanced across all three phases, maintaining a 120° phase displacement. These are rare but severe and easier to analyze due to their symmetry.

Types of Symmetrical Faults:

• L-L-L Fault (Three-Phase Fault)
• L-L-L-G Fault (Three-Phase to Ground Fault)

In these faults, all three phases are either short-circuited together or short-circuited with the ground. The fault current rises symmetrically, distributing equally among the three phases or flowing to the ground in case of a ground connection.

2. Unsymmetrical Faults

Unsymmetrical faults cause unequal current magnitudes and phase displacement among the phases. These faults result in unbalanced loads and can significantly affect the performance and efficiency of electrical equipment.

Types of Unsymmetrical Faults:

1. Line-to-Line (L-L) Fault
2. Line-to-Ground (L-G) Fault
3. Double Line-to-Ground (L-L-G) Fault
4. Open Circuit Fault

Occurs when two phase conductors are short-circuited together and both are connected to the ground. The fault current flows through the two phase lines into the ground, adversely affecting the connected equipment. The third phase may continue operating under certain conditions.

Occurs when any two phase conductors are short-circuited. The fault current flows through a low-resistance path between the two phases, with no current flowing to the ground. The third phase may remain unaffected.

Occurs when one phase conductor comes into contact with the ground due to insulation failure or physical damage. The fault current flows through the grounding conductor as it offers a low-resistance path.

This fault occurs when one or more phase conductors break due to mechanical stress or poor connection. It often happens at joints or terminals where connections are not secure. Circuit breakers with carbon deposits or weak springs may also cause this fault. Open circuit faults can lead to single-phasing, where a phase fails to deliver current properly.

Simultaneous faults occur when two or more faults happen at the same time on the same line. These are also referred to as multiple faults.

For example, a short circuit may cause excessive current, which increases conductor temperature, potentially leading to burnt conductors and open circuits. Simultaneously, these conductors may also come into contact with the ground, causing a line-to-ground fault. Such faults are often due to insulation failure.

Cross-country faults are common in systems grounded through high impedance (such as Petersen coils) but are rare in solidly grounded systems.

Apart from symmetrical and unsymmetrical faults, the following types of faults can also occur:

1. Unbalanced Loading Fault
2. Overvoltage Fault
3. Undervoltage Fault
4. Reverse Power Fault
5. Power Swing Fault
6. Under frequency Fault
7. Over frequency Fault