Ferranti effect

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In electrical engineering, the Ferranti effect is an increase in voltage occurring at the receiving end of a long transmission line, above the voltage at the sending end. This occurs when the line is energized, but there is a very light load or the load is disconnected. The capacitive line charging current produces a voltage drop across the line inductance that is in-phase with the sending-end voltage, assuming negligible line resistance. Therefore, both line inductance and capacitance are responsible for this phenomenon.[1]

Illustration of the Ferranti Effect; addition of voltages across the line inductance

The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied.[2] The relative voltage rise is proportional to the square of the line length and the square of frequency.[3]

The Ferranti effect is much more pronounced in underground cables, even in short lengths, because of their high capacitance.

It was first observed during the installation of underground cables in Sebastian Ziani de Ferranti's 10,000 volt distribution system in 1887.[4]