What is ferranti effect ?

In general when the receiving end voltage is greater than the sending end voltage then that is called Ferranti effect.

It is mainly a part of transmission line. When a transmission line is lightly loaded or not loaded, then the receiving end voltage exceeds the sending end voltage. The loads are normally inductive in nature, which draws a huge amount of reactive power. Typically capacitors are connected in parallel to the transmission lines to supply this reactive power but when the transmission lines are not loaded or lightly loaded, the reactive power supplied by the capacitors adds on to the transmission line and as a result the receiving end voltage is found to be greater than the sending end voltage which is termed as Ferranti effect.

Ferranti Effect on Transmission Lines

Power triangle describes the relationship between real power and reactive power. Real power is the power, which flows through the circuit without being stored or interrupted. Reactive power is that power which is stored in the circuit for some time and the power returns to the source in each cycle. Now returning to the transmission lines, the loads connected with the transmission lines are inductive in nature and they require reactive power. To introduce reactive power in to the circuit, the angle between real power and apparent power is increased or power factor is reduced. To do this, capacitors are connected in parallel with the transmission lines which store energy in one cycle and release in the other cycle, hence fulfilling the requirement of the reactive power. But when the transmission line is at no load or lightly loaded condition there is excess reactive power in the network which is added with the real power and at the receiving end we get more voltage than the supply end voltage, this phenomenon is termed as Ferranti effect. So in a single sentence Ferranti effect can be understood as the incidence when the receiving end voltage is greater than the sending end voltage.

Causes of the Ferranti Effect

Loads can be divided in three types: resistive load, capacitive load and capacitive load. Normally we are familiar with resistive loads but most of the loads connected with the transmission lines are inductive in nature. So the power requirement changes in to two type’s-

• Resistive or real power.
• Reactive (capacitive/inductive) power.

Power generated by the power plants supplies the actual power through the transmission lines. So, to get the required reactive power some steps are taken and some changes are made in the transmission lines. The most common of them is the power factor correction. The power factor can be corrected by introducing capacitors in parallel with the transmission lines. These capacitors will supply the required reactive power to the network. Now suppose the transmission lines are at no load or in very lightly loaded condition. Then reactive power requirement is zero or very low but the capacitors keep on supplying reactive power which will be added on to the transmission lines and ultimately increasing the receiving end voltage. The term Ferranti effect describes the phenomenon when the receiving end voltage is greater than the sending end voltage.

Hence the main cause of this phenomenon is when the transmission line is at no load or lightly loaded condition and then the receiving end voltage is higher.