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The Ferranti effect causes voltage to rise at the end of an AC transmission line, and is more common in buried lines. Shunt reactors and capacitors can stabilize power flow and compensate for the effect. Power factor must be considered when compensating for the effect.
The Ferranti effect is a term that describes an electrical phenomenon that causes the voltage to rise at the end of an alternating current (AC) transmission line. This is likely to occur when a transmission line is very lightly loaded or is disrupted due to a power outage. The Ferranti effect takes its name from the electrical engineer Sabastian Zianni de Ferranti, who designed the power station and distribution network for an AC plant in England in 1887, shortly after the War of the Currents. The war of the currents was the debate between Nikola Tesla, who advocated the use of AC power distribution, and Thomas Edison, who advocated the use of direct current (DC) power distribution. The newly built power station at Deptford, England first exhibited the Ferranti effect in its AC distribution lines when it opened in 1891.
Observations of the Ferranti effect occur more frequently along buried transmission lines in contact with the earth, such as at Deptford Power Station, rather than overhead transmission lines. Burying transmission lines contributes to the load increasing as it approaches the end of its line; and for this reason shunt reactors must be applied to underground lines to stabilize the power flow. Since the desired distribution is for the reduced supply voltage at the receiving end of the transmission lines relative to the raw supply voltages at the sending source, the Ferranti effect must be compensated for along the line in order not to avoid supplying an excessively high voltage at households or industrial appliances.
The length of the transmission line also contributes to the Ferranti effect. Transmission lines produce inductive charges when their load is light, and as these charges build up, capacitors must be connected along the length of the lines at intervals to reduce power. Using shunt reactors and capacitors in combination, and activating more capacitors along the line as needed, keeps the voltages more in phase or synchronization between the source and the ends of the line.
In AC power distribution networks, power factor is a term that indicates real power versus apparent power. True power is the power in the transmission line that produces the working power to the appliances at the receiving end. When compensating for the Ferranti effect, real power must be differentiated from apparent power and the required amount of power reduction must be applied across the shunt reactors to provide the reverse effect of voltage buildup. In addition, line engineers can use the electronic switching on and off of the capacitors to make necessary adjustments when monitoring informs of Ferranti effect overloads.
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