Short Answer:

In a power system, faults are abnormal conditions that interrupt normal current flow. Symmetrical faults affect all three phases equally, like a three-phase short circuit, and are balanced in nature. They are rare but very severe. Unsymmetrical faults, on the other hand, affect one or two phases only and cause imbalance in the system.

Unsymmetrical faults are more common and include single line-to-ground, line-to-line, and double line-to-ground faults. Identifying the type of fault helps in selecting the right protection method and restoring the system quickly and safely.

Detailed Explanation:

Symmetrical and Unsymmetrical Faults in Power Systems

Power systems are designed to operate under balanced, three-phase conditions. However, due to insulation failure, weather effects, equipment damage, or human error, faults can occur. Faults disturb the normal operation of the system by causing overcurrent, voltage dips, equipment damage, and sometimes blackouts.

To maintain reliability and safety, it’s important to understand the types of faults. They are mainly divided into two categories: symmetrical faults and unsymmetrical faults, based on how they affect the phases of the system.

Symmetrical Faults

A symmetrical fault affects all three phases equally and maintains the balance of the system, although it introduces a large disturbance. The most common type is the three-phase-to-ground fault or three-phase short circuit.

Characteristics:

All three phases are involved.
Fault current is very high.
System remains balanced but disturbed.
Easier to analyze using symmetrical components.

Example:

A metallic short circuit that connects all three phase conductors directly or through the ground.

Impact:

Although rare, symmetrical faults are the most severe in terms of fault current.
They can cause major equipment damage if not cleared quickly.

Protection:

Circuit breakers and relays are set to trip instantly in case of a symmetrical fault.
Requires strong fault current withstand capacity.

Unsymmetrical Faults

Unsymmetrical faults affect one or two phases, creating an unbalanced condition in the system. These are more common and are further classified into three main types:

Single Line-to-Ground Fault (LG):

One phase touches the ground.
Most common type of fault (approx. 70–80%).
Causes unbalanced voltage and current.

Line-to-Line Fault (LL):

Two phases come in contact with each other.
Found often in overhead lines due to swaying conductors.

Double Line-to-Ground Fault (LLG):

Two phases touch the ground simultaneously.
Less common but still a serious fault condition.

Characteristics:

Causes imbalance in voltages and currents.
Fault currents are lower than symmetrical faults.
Analyzed using symmetrical components (positive, negative, and zero sequence).

Impact:

Can lead to overheating, unbalanced loads, motor vibration, and damage to sensitive equipment.
May result in poor power quality if not cleared.

Protection:

Relays are set to detect current or voltage imbalance.
Differential and distance protection schemes are used.

Importance of Distinguishing the Two

Symmetrical faults are simpler to analyze but require stronger equipment to handle high fault currents.
Unsymmetrical faults are more complex due to imbalance but are more common and must be analyzed carefully.
Fault analysis helps in designing the protection system, setting relay values, and ensuring quick fault isolation to keep the system running safely.

Conclusion

In power systems, symmetrical and unsymmetrical faults represent different types of disturbances. Symmetrical faults affect all phases equally and are rare but highly destructive. Unsymmetrical faults affect only one or two phases, are more frequent, and cause imbalance in the system. Understanding both types is essential for designing effective protection systems, maintaining system stability, and ensuring safe and continuous electricity supply during fault conditions.