Global | July 18, 2025

The Ferranti Effect on Long-Distance High‑Voltage AC Transmission Lines

The Ferranti effect was first observed in 1887 by Sebastian Ziani de Ferranti in London.

 

Image for illustration purposes.

In long high‑voltage AC transmission lines, a counterintuitive phenomenon occurs under low or no‑load conditions: the voltage at the receiving end can exceed that at the sending end.

This voltage rise stems from the interplay between a line’s inductance and capacitance. When lightly loaded, the line’s shunt capacitance disturbs the voltage profile. Charging currents flow through the line’s inductance, producing a voltage in phase with the source, effectively boosting the receiving‑end voltage. The increase varies directly with the square of both the line length and the system frequency.

The effect is more evident on longer lines—typically over 200 km—and at higher voltages. Underground or submarine cables, due to their higher capacitance, exhibit the Ferranti effect even over much shorter distances.

In typical practice, overhead AC lines are limited to 600–700 km at 50 Hz (or 500–600 km at 60 Hz) to prevent excessive overvoltage. If lines must exceed such lengths, shunt reactors—additional inductance elements—are installed at the receiving end. These absorb reactive power and counteract the capacitive charging current, effectively stabilising voltage.

Underground cables, with significantly higher capacitance (20–60 times that of overhead lines), are particularly vulnerable. Their combined inductance–capacitance values can be 10–30 times larger, intensifying the Ferranti effect.

In summary, the Ferranti effect highlights a vital consideration in power‑system design and regulation. Failure to account for it risks overvoltage stress on cables, switchgear, insulation, and other equipment, potentially leading to premature failure or unsafe conditions.

Source: EEPower