In general, sending end voltage is higher than the receiving end voltage due to line losses. However, Sir Ferranti in year 1890 came up with theory that the receiving end voltage will be higher than the sending end voltage in medium and long transmission lines during light and no loaded conditions. This phenomenon is given the name Ferranti effect. In this article, we will discuss this effect in detail.
How Ferranti Effect occurs ?
The conductors of transmission line and their spacing forms capacitance and inductance among them. In a long transmission line, the value of capacitance and inductance is very high and they are distributed across the entire length of transmission line. Inductance is in series with the line whereas capacitance forms between line to line or line to ground.
During light load or no load condition in a long (more than 240km) transmission line, the effect of capacitance becomes dominant as compared to the inductive effect. Actually during low load, the current is very low and so does the variation of flux due to which inductance of line reduces. In that case, capacitance effect will dominate and generates maximum reactive power. Similarly, reactive power consumed by the inductors is very low. In other words, generated reactive power is more than the absorbed reactive power during light or no loaded conditions. Since voltage and reactive power have a direct relation. Hence increase in a reactive power will leads to an increase in the terminal voltage of system. This makes a receiving end voltage of the transmission line greater than the sending end voltage which is known as Ferranti effect. In simple words, we can define as :
When receiving end voltage (Vr) is higher than the sending end voltage (Vs), it is known as Ferranti effect.
How to reduce Ferranti Effect ?
Ferranti effect can be reduced by installing the shunt reactors which are basically parallel inductors at the receiving end of transmission line. Shunt reactor absorb the excess reactive power generated by the capacitance of line during light or no loaded conditions. In this way, it reduces the voltage level and keeps the voltages same at both (receiving and sending) ends of the transmission line. It is always connected in parallel with the line. In case of very long transmission line, compensation is required after every 250 to 300 km. It is found that a 300Km line operating at 50Hz receives 5% higher voltage than the sending end voltage.
Disadvantages of Ferranti Effect :
Over voltage at the receiving end may damage the connected equipment. Ferranti effect will leads to insulation failure. Ferranti effect also increases the voltage regulation which must be closer to zero.
Voltage at the receiving end may become too high during no load time period and needs to be lowered. Increase in voltage is directly proportional to the square of the length of transmission line. To overcome Ferranti effect, shunt reactor is connected at the receiving end of transmission line. In short transmission line, line capacitance is negligible and they do not produce considerable amount of reactive power. Hence Ferranti effect only occurs in medium and long transmission lines. Moreover, shunt reactor is disconnected at full load.
That’s all. Hope this will helps you.