Electrical Studys

 The Ferranti Effect is a phenomenon in long AC transmission lines where the receiving-end voltage becomes higher than the sending-end voltage, especially when the line is lightly loaded or operating under no-load conditions.

This effect is mainly caused by the capacitance of the transmission line.

Why Does It Occur?

Every transmission line has:

Resistance (R)

Inductance (L)

Capacitance (C)

In long transmission lines, the line capacitance draws a charging current even when there is little or no load connected.

Long transmission lines exhibit significant capacitance, 

leading to the Ferranti Effect under light-load conditions.

The charging current leads the voltage by 90°, causing a voltage rise across the line inductance. As a result:

Receiving End Voltage (Vr) > Sending End Voltage (Vs)

This increase in voltage is called the Ferranti Effect.

The Ferranti Effect is primarily caused by

 the distributed capacitance of the transmission line.

Under no-load conditions, the receiving-end voltage
may exceed the sending-end voltage due to the Ferranti Effect.

Important Formula

For a lightly loaded long transmission line:

Percentage of Ferranti Effect: 

Example Calculation:

Suppose : -

Effects of Ferranti Effect

⚠ Overvoltage at receiving end

⚠ Stress on insulation

⚠ Reduced equipment life

⚠ Risk of transformer overheating

⚠ Damage to connected loads

⚠ Voltage regulation problems

Precautions to Reduce Ferranti Effect

Shunt reactors absorb excess reactive power and
help reduce voltage rise caused by the Ferranti Effect.

1. Use Shunt Reactors

Shunt reactors absorb the excess reactive power produced by line capacitance.

2. Maintain Proper Loading

Operate transmission lines with adequate load whenever possible.

3. Use FACTS Devices

Devices such as SVC and STATCOM help control voltage levels.

4. Voltage Monitoring

Continuously monitor receiving-end voltage in EHV systems.

5. Reactive Power Compensation

Install suitable compensation equipment at substations.

Voltage regulation equipment in substations
helps maintain system stability.

Key Points for Exams

✅ Occurs mainly in long transmission lines

✅ More severe under light-load or no-load conditions

✅ Caused by line capacitance

✅ Receiving-end voltage becomes greater than sending-end voltage

✅ Reduced using shunt reactors

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