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 voltages considering the line resistance as negligible. 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.[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]

In general practice we know, that for all electrical systems current flows from the region of higher potential to the region of lower potential, to compensate for the electrical potential difference that exists in the system. In all practical cases the sending end voltage is higher than the receiving end, so current flows from the source or the supply end to the load. But Sir S.Z. Ferranti, in the year 1890, came up with an astonishing theory about medium distance transmission line or long distance transmission lines suggesting that in case of light loading or no load operation of transmission system, the receiving end voltage often increases beyond the sending end voltage, leading to a phenomena known as Ferranti effect in power system.