- Ferranti Effect Definition: The Ferranti effect is defined as the increase in voltage at the receiving end of a long transmission line compared to the sending end.
- Conditions for Occurrence: The Ferranti effect happens when the load is very small or there is no load (open circuit).
- Capacitance and Inductance: The effect is due to the significant capacitance and inductance distributed throughout the transmission line.
- Voltage Rise Mechanism: The current drawn by the line’s capacitance causes a voltage drop across the inductor, leading to a higher voltage at the receiving end.
- Proportionality to Line Length: The voltage increase is proportional to the square of the line length, making it significant in long transmission lines.
What is Ferranti Effect?
The Ferranti effect is the rise in voltage from the sending end to the receiving end of an energised long transmission line. It occurs under no-load or light-load conditions, when the receiving-end voltage can exceed the sending-end voltage. Engineers express the rise as a voltage ratio or percentage.
Under normal load, the line carries current to the receiving-end load and its series impedance produces a drop in electrical potential difference. Under light load, the line still draws capacitive charging current. The interaction between this current and the line’s series inductive reactance can reverse the usual voltage profile.
The effect is named after electrical engineer Sebastian Ziani de Ferranti, whose high-voltage AC distribution work exposed this behaviour in the late nineteenth century. In modern power-system terms, the voltage rise called the Ferranti effect in a power system is a steady-state operating effect of an unloaded or lightly loaded AC line.
Ferranti Effect in Transmission Line
A long line has distributed shunt capacitance and series inductance along its full length. The shunt capacitance draws charging current whenever the AC line is energised. At light load, this capacitive current can dominate the receiving-end current.
The charging current flows through the line’s series inductor. In the no-load phasor relationship, the resulting reactive drop makes the magnitude of the sending-end voltage lower than the receiving-end voltage. The distributed-parameter line model gives the same result.
Shunt capacitance and series inductive reactance produce this effect in a transmission line. A short transmission line has low total charging current and series reactance, so its voltage rise is usually small. No single percentage applies to every line: the rise depends on line length, frequency, construction and electrical parameters.
The phasor diagram above shows a no-load approximation of the effect.
Take the receiving-end voltage Vr as the reference phasor OA.
The line charging current Ic leads Vr by 90 degrees and is represented by phasor OC.
For this approximate derivation, assume the line electrical resistance is much smaller than its inductive reactance. The resistive component IcR is then neglected. The remaining inductive-reactance component accounts for the calculated voltage rise.
Let c0 be the capacitance per unit length, L0 the inductance per unit length and l the line length. The total capacitance and inductance then increase with l, as shown in the equation below.
Because the shunt capacitance is distributed along the line, the charging current varies with position. The simplified derivation uses the average current shown below.
Substituting that average current into the inductive-reactance drop gives the approximate receiving-end voltage rise:
Within this low-loss approximation, the voltage rise is proportional to the square of line length. The result explains why the effect becomes more pronounced as an unloaded line becomes longer. To practise the underlying power-system concepts, use the power system MCQ (Multiple Choice Questions).
Utilities limit this light-load overvoltage with measures such as shunt reactors, which absorb part of the line’s capacitive reactive power. The required compensation depends on the network and operating condition. After reviewing the mitigation principle, check your understanding with the power system MCQ (Multiple Choice Questions).
