Short Answer:

The Ferranti effect in transmission lines is a condition where the voltage at the receiving end becomes higher than the voltage at the sending end, especially during light load or no-load conditions in long transmission lines. This happens due to the charging current caused by the line’s capacitance, which leads to voltage rise along the length of the line.

This effect is more noticeable in high-voltage, long-distance AC transmission lines. It can lead to overvoltage problems and damage sensitive equipment if not managed properly. Engineers use devices like shunt reactors to control and reduce the Ferranti effect in practical systems.

Detailed Explanation:

Ferranti Effect in Transmission Lines

The Ferranti effect is an unusual but important phenomenon that occurs mainly in long AC transmission lines. When the transmission line is lightly loaded or open at the receiving end, the voltage measured at the receiving side can be higher than the voltage sent from the source. This condition is not a fault but a natural outcome of how capacitance and inductance behave in long lines, particularly under low current flow.

This effect is named after Sebastian Ziani de Ferranti, a British electrical engineer who first observed and explained it. While it is not dangerous under controlled conditions, the Ferranti effect must be accounted for in the design and protection of power systems.

Causes of Ferranti Effect

1. Line Capacitance
   Every transmission line has capacitance between the conductors and the ground. In long lines, this capacitance becomes significant. Under light load or no-load conditions, the line still draws charging current because of this capacitance.
2. Low Load Current
   When the line carries little or no load current, the charging current dominates the system. This current flows in a leading phase with respect to the voltage and results in voltage rise along the line.
3. Voltage Drop Due to Inductive Reactance
   The charging current flows through the line’s inductive reactance. Since the current is leading and the reactance is inductive, this creates a leading voltage drop, which adds to the sending end voltage, resulting in a higher receiving end voltage.
4. Line Length and Voltage Level
   The longer the transmission line and the higher the operating voltage, the more noticeable the Ferranti effect becomes. It is more common in EHV (Extra High Voltage) and UHV (Ultra High Voltage) transmission systems.

Effects of Ferranti Effect

1. Overvoltage at the Receiving End
   The most direct outcome is an increase in voltage at the receiving terminal of the line, which can be harmful to connected equipment and reduce the life of insulation.
2. Stress on Equipment
   Switchgear, transformers, and other components may not be rated to handle the unexpected rise in voltage, leading to premature failure or damage.
3. Reduced Power Quality
   The voltage instability due to the Ferranti effect can affect power quality, particularly in sensitive industrial or electronic loads.
4. Reactive Power Flow Issues
   The line behaves like a reactive power source under no load, sending leading VARs back to the generator, which can disturb system balance.

Methods to Control Ferranti Effect

1. Shunt Reactors
   These are inductive devices connected at the receiving end or along the line to absorb the excess reactive power produced by the line capacitance, thus preventing voltage rise.
2. Line Termination with Load
   Keeping the line loaded with even a small resistive load can help control the voltage rise.
3. Shorter Line Segments
   For extremely long distances, transmission lines are broken into sections using intermediate substations to minimize capacitive effects.
4. Use of Controlled Switching
   Switching equipment and compensation devices based on system load conditions can dynamically manage the effect.
5. FACTS Devices
   Flexible AC Transmission Systems such as STATCOMs and SVCs can absorb or inject reactive power and help regulate voltage levels effectively.

Practical Examples

• In long-distance rural power lines, especially during the night when demand is low, Ferranti effect can become quite visible.
• It is a critical design consideration in submarine and underground cables, which have high capacitance due to close conductor spacing and insulating materials.

Conclusion

The Ferranti effect in transmission lines is a voltage rise at the receiving end caused by the line’s capacitance under light or no-load conditions. It is more pronounced in long, high-voltage AC lines. This effect can lead to equipment stress, insulation failure, and reactive power issues. Proper compensation using shunt reactors and voltage control techniques is essential to keep the system stable and safe. Understanding and mitigating the Ferranti effect ensures better voltage regulation and reliable power delivery.