Short Answer:

A single-line-to-ground fault occurs when one phase conductor in a power system accidentally comes into contact with the ground or any grounded object. This is the most common type of fault in electrical systems and usually happens due to insulation failure, lightning strikes, or physical damage to the line.

During this fault, current flows from the affected phase directly to the ground, creating an unbalanced condition in the system. It leads to voltage disturbances, overheating, and potential equipment damage if not detected and cleared quickly by protective devices like relays and circuit breakers.

Detailed Explanation:

Single-Line-to-Ground Fault

In a three-phase electrical system, the system is considered balanced when all three phases carry equal currents and voltages are symmetrically spaced. A single-line-to-ground (SLG) fault disturbs this balance by connecting only one of the phases directly to the ground, either through a conductive path or accidentally.

This type of fault is common in transmission and distribution networks, especially in overhead lines where weather conditions or falling objects can cause one conductor to touch the ground.

How a Single-Line-to-Ground Fault Occurs

• A tree branch falls on a power line and creates a contact between the phase conductor and the ground.
• Lightning strikes a phase wire and causes it to flash over to the grounded tower.
• Insulation breakdown in cables or equipment allows the phase to contact the grounded body.

In all these situations, the current from the faulted phase bypasses its normal path and flows directly into the earth, creating a ground fault.

Characteristics of Single-Line-to-Ground Fault

1. Unbalanced System:
   • Only one phase is involved in the fault, so the system becomes asymmetrical.
   • The voltages and currents in the remaining two healthy phases are affected and become unbalanced.
2. Fault Current Path:
   • The current flows from the source through the faulted phase to the ground and returns via the grounding system or the neutral.
3. Magnitude of Fault Current:
   • Depends on the grounding method used (solid, resistance, reactance, or isolated).
   • Solidly grounded systems will have higher fault current, while resistance-grounded systems limit the current to safer levels.
4. Protection Operation:
   • Protective devices like ground fault relays or residual current devices detect the imbalance or overcurrent and isolate the faulted line.

Effects of a Single-Line-to-Ground Fault

• Voltage drop on the faulted phase.
• Overvoltage on the healthy phases due to imbalance.
• Damage to insulation and electrical equipment if the fault current is not quickly cleared.
• Fire risk in case of arcing or persistent fault.
• Disturbance in connected loads, especially motors and sensitive electronics.

Analysis and Detection

• SLG faults are analyzed using the symmetrical component method, where the unbalanced system is broken into positive, negative, and zero sequence components.
• Relays detect the zero-sequence current or voltage to identify the fault.
• Fault indicators and SCADA systems help in locating the fault quickly on the network.

Importance of Clearing SLG Faults

Even though the fault involves only one phase, it must be addressed quickly because:

• It causes imbalance in the system.
• It may lead to a double or three-phase fault if left unattended.
• It poses safety risks to maintenance workers and the public.
• It reduces the life of insulation and equipment due to overheating and stress.

Conclusion

A single-line-to-ground fault is a common and critical fault in power systems where one phase comes into contact with the ground. It leads to unbalanced voltages and currents, can damage equipment, and poses safety risks. Proper grounding, fast detection, and effective isolation through protection systems are essential to handle such faults. Understanding and managing SLG faults ensures reliable and safe operation of the electrical power system.