Short Answer:

The Ferranti effect is a phenomenon where the voltage at the receiving end of a long, lightly loaded or open-circuited transmission line becomes higher than the sending end voltage. It usually occurs in high-voltage AC transmission lines due to the line’s capacitance.

This happens because the charging current caused by the line’s capacitance leads to a voltage rise along the line. The Ferranti effect becomes more significant as the line length and voltage level increase, especially under light load or no-load conditions.

Detailed Explanation:

Ferranti effect in transmission lines

The Ferranti effect is an important concept in power system engineering, especially when dealing with long-distance high-voltage transmission lines. It refers to the situation where the voltage at the receiving end of the transmission line is greater than at the sending end, which may seem unusual. This effect occurs mainly under light load or no-load conditions and is more prominent in long AC transmission lines due to their distributed capacitance and inductance.

It is named after Sebastian Ziani de Ferranti, who first observed this behavior in the early 20th century while studying transmission systems.

Cause of the Ferranti Effect

In long transmission lines, the capacitance between the conductors and between conductors and ground causes a charging current to flow even when the line is lightly loaded or open. This charging current leads to a buildup of reactive power along the line, which results in a voltage rise toward the receiving end.

Since the load is very small or zero, there is not enough current drawn to balance or consume the reactive power generated by the line’s capacitance. This unabsorbed reactive power causes a voltage rise, creating the Ferranti effect.

Factors Affecting the Ferranti Effect

1. Line Length:
   The longer the transmission line, the greater the total capacitance, leading to a higher Ferranti effect.
2. Line Voltage:
   Higher operating voltages result in more charging current, increasing the effect.
3. Load Condition:
   The effect is most prominent under light load or open-circuit conditions because reactive power is not being used.
4. Frequency:
   Higher frequency increases the capacitive reactance, contributing to greater voltage rise.

Mathematical Explanation

Using simplified transmission line equations, the receiving end voltage VrV_rVr​ can exceed the sending end voltage VsV_sVs​ when the line’s shunt capacitance creates leading charging current, which flows through the series inductance. This causes a voltage drop that adds to the receiving end rather than subtracting, increasing the voltage.

This effect is more noticeable in medium (100–250 km) and long transmission lines (above 250 km).

Effects on the System

• Overvoltage Damage: Sensitive equipment at the receiving end may get damaged due to overvoltage.
• Voltage Control Issues: Makes voltage regulation difficult under light load.
• System Instability: Uncompensated Ferranti effect may lead to improper relay operation or overexcitation in generators and transformers.

How to Reduce the Ferranti Effect

• Shunt Reactors: Installing shunt reactors at the receiving end helps absorb the excess reactive power and limit the voltage rise.
• Loading the Line: Ensuring the line is properly loaded reduces the effect naturally.
• Voltage Regulators: Used to stabilize the voltage at the receiving end.

Conclusion

The Ferranti effect is a voltage rise that occurs at the receiving end of a long, lightly loaded or open transmission line due to line capacitance and low current draw. It is a result of reactive power buildup and is more severe at high voltages and longer distances. Understanding and managing the Ferranti effect is important for maintaining voltage stability and protecting equipment in high-voltage power systems.