Short Answer:
A symmetrical fault is a type of fault where all three phases of a power system are equally affected, such as a three-phase short circuit. It causes very high fault current but is easier to analyze because the system remains balanced during the fault.
An unsymmetrical fault, on the other hand, affects only one or two phases, like a line-to-ground or line-to-line fault. It creates an unbalanced system condition, leading to unequal voltages and currents in different phases and requires more complex analysis using symmetrical components.
Detailed Explanation:
Symmetrical fault and unsymmetrical fault
In power systems, faults refer to abnormal electrical conditions that disturb the normal flow of current. These faults can be classified into symmetrical and unsymmetrical based on how many phases are affected and whether the system stays balanced.
Understanding the difference between these two faults is important for designing reliable protection systems and ensuring the power system stays stable and secure during faults.
Symmetrical fault
A symmetrical fault is also known as a balanced fault. In this condition, all three phases of the power system are equally short-circuited either to each other or to ground.
- Most common example: Three-phase-to-ground fault (L-L-L-G)
- Less frequent in practice, but it causes the most severe fault current
- System remains balanced, meaning the magnitude and phase angle of each phase remain equal
- Can be analyzed using simple single-line diagrams
Key features:
- Maximum current flows through the system
- Causes high mechanical and thermal stress on equipment
- Easiest to analyze mathematically
- Often used for designing circuit breaker ratings
When it occurs:
- Due to equipment breakdown, failure in busbars, or severe insulation damage
Unsymmetrical fault
An unsymmetrical fault is also called an unbalanced fault. It affects only one or two phases of the power system, creating imbalance in current and voltage.
- Types of unsymmetrical faults:
- Single line-to-ground fault (L-G) – one phase touches the ground
- Line-to-line fault (L-L) – two phases shorted together
- Double line-to-ground fault (L-L-G) – two phases touch ground
Key features:
- Occurs more frequently than symmetrical faults (especially L-G faults)
- System becomes unbalanced — voltages and currents are not equal in all phases
- Analysis is more complex and requires the use of symmetrical components method
When it occurs:
- Due to lightning strikes, animal contact, tree branches, or faulty insulation
Impact:
- Can lead to voltage dips in specific parts of the system
- May cause motors to vibrate or overheat due to unbalanced current
- Can trigger relay misoperations if not properly detected
Protection and analysis
- Symmetrical faults are easier to detect and isolate using standard protection schemes.
- Unsymmetrical faults need careful detection and separation using advanced relays that can analyze each phase separately.
- Fault current values are lower in unsymmetrical faults, but the complexity of system behavior increases.
Conclusion:
A symmetrical fault affects all three phases equally and keeps the system balanced, while an unsymmetrical fault affects only one or two phases and causes system imbalance. Symmetrical faults are severe but simpler to analyze, whereas unsymmetrical faults are more common and require detailed study for protection. Understanding both is essential for maintaining the safety and stability of the power system.