What is Ferranti Effect?
- A lengthy transmission line consumes a considerable amount of charging current. Receiving end voltage being higher than sending end voltage in a transmission line is known as the Ferranti effect. This can happen if such a line is open-circuited or only weakly loaded at the receiving end.
- All electrical loads are inductive, therefore they use a lot of the transmission lines’ reactive power. There is a voltage drop in the lines as a result.
- At the receiving end, capacitors that produce reactive power are parallel-connected to the transmission lines in order to make up for the reactive power that the inductive loads consume.
- More capacitors are connected in parallel via electronic switching as the inductive load rises.
- As a result, the capacitors provide the reactive power needed by inductive loads, eliminating the need for transmission line reactive power.
- However, the capacitors can still be in an ON state when the inductive loads are turned off. Because there is no inductance, the capacitors’ reactive power added to the transmission lines.
- As a result, the voltage at the consumer end, also known as the receiving end, rises and surpasses the voltage at the supply end. This effect is called as Ferranti effect
Why does voltage rise on a long, unloaded transmission line?
- The Ferranti Effect happens when transmission line distributed capacitance draws more current than the load at the receiving end. Thus, the Ferranti effect is worse on weakly laden lines, especially underground cable connections with higher shunt capacitance than overhead lines.
- Due to charging current, the line inductance voltage drop is in phase with the transmitting end voltages. The outgoing end voltage drops, raising the receiving end voltage. Longer lines and greater voltages increase the Ferranti Effect.
- The Ferranti impact does not affect loaded lines because line capacitive impact is constant whereas inductance varies with load. Inductive load consumes line capacitance-generated VAR.
Why the Ferranti effect occurs ?
- There are three different types of loads: resistive, capacitive, and inductive. While resistive loads are more common than inductive ones, they are nonetheless connected to transmission lines.
- The power requirement thus shifts into two categories. Reactive (capacitive/inductive) power is also known as real or resistive power. Through transmission lines, power produced by the power plants provides the actual power. Therefore, some actions are taken and some alterations are made to the transmission lines in order to obtain the necessary reactive power.
- The power factor correction is the most typical of them all. By connecting capacitors in parallel with the transmission lines, the power factor can be fixed. These capacitors will give the network the necessary reactive power.
- When the need for reactive power is zero or very low, the capacitors continue to generate reactive power, which is added to the transmission lines and ultimately raises the voltage at the receiving end.
- When the voltage at the receiving end is higher than the voltage at the sending end, the phenomenon is known as the Ferranti effect. The receiving end voltage is higher when the transmission line is at no load or weakly loaded, which is the main source of this occurrence.
How to Reduce Ferranti Effect? Shunt Reactors and Series Capacitors:
- When lengthy power transmission lines for system voltages 220 kV and greater were constructed, the need for big shunt reactors became apparent. The series inductance (caused by the magnetic field around the conductors) and shunt capacitance (caused by the electrostatic field to earth) are the defining characteristics of a line.
- The capacitance and inductance are dispersed evenly along the length of the line. Both the admission to Earth and the series resistance are also. Due to the series inductance and resistance, there is a voltage drop along the line when the line is loaded. The line’s voltage increases (the Ferranti effect) when it is energized but not loaded or just loaded with a minor current.
- In this case, the capacitance to earth generates a potential capacitive current through the line. There will be a rise in voltage down the line when a capacitive current runs through the line inductance.
- Series capacitors and shunt reactors can be used to balance the line capacitance to earth and the line inductance, respectively, in order to stabilize the line voltage. Shunt reactors are frequently built in the stations at the line’s ends, whereas series capacitors are positioned at various points along the line.
- The voltage differential between the line’s endpoints is lessened in this way in terms of both amplitude and phase angle.
- Shunt reactors can also be linked to the electrical network at intersections of multiple lines or to the secondary windings of transformers.
- Compared to overhead lines, transmission cables have a substantially larger capacitance to earth. Shunt reactors are required for long undersea cables with system voltages of 100 KV or more. In order to avoid an excessive voltage surge when a high load unexpectedly drops out owing to a failure, the same applies to large urban networks.
- Shunt reactors can be produced in transformer factories since they have the same windings, core, tank, bushings, and insulating oil as power transformers.