What is Ferranti Effect in Power Transmission Lines?

The Ferranti effect causes an increment in voltage received by the other end of longer ac transmission lines as compared to the voltage at the sending point if there is no load connected or very little load on the receiving point. The first time this fact was observed was during the connection of underground cables by Sebastian Ziani de Ferranti’s for the installation of a 10,000-volt AC system in 1887. The voltage losses occur due to capacitive line charging over the line inductance that is in phase with the sending voltage. So both line inductance and capacitance are the main factors in this phenomenon.

It can be discussed if the line, like a transmission line, has a source impedance lower than the load impedance. The effect is like a short-circuit configuration of a quarter-wave impedance transformer with low voltage transformation.

The Ferranti effect is highly effective for longer-distance lines and high voltage at the sending point. The voltage increase is directly proportional to the square of the line length and the frequency square. So let’s get started. What is the Ferranti Effect in Power Transmission Lines?

What is the Ferranti Effect?

What is Ferranti Effect in Power Transmission Lines

In this effect, the voltage at the sending end is lower than the receiving end. it caused due to less load or no load at the receiving point.  The Ferrenti effect is caused by the charging current in the transmission line. The charging current increases in the transmission line if the receiving end voltage is higher than the sending point. This effect is the result of the reaction of capacitance and inductance in the transmission line, which are the main components of lines.

The main reason for the Ferri effect is voltage losses in line inductance due to line sharing current and charging current in phase with voltage connected at the sending point.

In small-distance transmission lines, the capacitance and charging current are very low and almost legible. As a result of the transition line, there is no effect on Ferranti.

For medium- and long-distance transmission, the Ferranti effect exists. In a transmission line having a length of 300km and functioning on a 50 hertz supply, there is a five percent increase in receiving point voltage at the sending end.

For a line of 240 km or more, capacitance and inductance are the main factors. If there is no load or very little load on the line, then the current used by the load is less than the line capacitor.

Main Causes of Ferranti Effects

The main cause of the Ferrint effect is the existence of a high charging current due to the capacitance of the line. However, the three main causes of Ferranti’s effects are explained here.

Transmission Line Capacitance

In transmission lines, conductors are configured in close proximity, especially for underground cables that cause capacitance between them. In the transmission lines, there are many shunt capacitors and series inductors over the cable length. There is an increase in capacitance if the length of the line increases. The capacitor uses a larger charging current that passes through the line length.

The reactive power produced by the generator moves in the reverse direction where the source is connected. The inductors in line use reactive power, resulting in voltage loss. There is the same phase for voltage loss with sending voltage. So this voltage is added, and at the receiving end, we get a higher voltage than the sending point.

Connected Load

Normally, there are three types of loads connected with lines: no load, light load, and full load. That also affects the Ferrant effect.

  1. If there is no load condition, then there is a charging current, and no load current passes through the line. The charging current is used by the shunt capacitor in line. It causes reactive power that results in voltage losses about the inductor, which is in phase with the sending voltage, and, as a result, high voltage at the receiving point.
  2. If there is a light load connected, then a low load current and a charging current pass through the line. Due to line capacitance, the charging current is leading. The capacitor produces reactive power that passes through the inductor, which is higher than the reactive power used by the inductor due to the lower load current. The voltage losses around the inductor are in phase with the supply voltage and proportional to the charging current. So a higher charging current than the load current causes the Ferranti effect.
  3. For full load conditions, the load current is higher than the charging current used by a capacitor. As high load current passes through series inductors, the reactive power used by the inductor is higher than the reactive power produced by a capacitor. So total reactive power is negative, and voltage reduces at the receiving point.

Supply Frequency

The cause of the Ferranti effect is reactive power produced in the shunt capacitance of power lines. Reactive power exists if voltage and current come at the same frequency. As DC has 0 frequency, there is no Ferranti effect. It can be said that transmission lines that function at high frequencies have a high chance of being affected by the Ferranti effect.

What is a short, medium, and long transmission line?

  • The transition line that has a length of 80km is called a short transmission line.
  • The length of a transmission line from 80km to 250km is called a medium transmission line.
  • If the length is more than 250 km, the line is called a long transmission line.
Resistance, capacitor, and inductance are the main factors that affect the current flow in the transmission line. There is a series combination of inductance resistance and capacitance in parallel with the phase conductor. These features cause power losses in transmission lines.

Why does the Ferranti Effect occur?

During the transfer of power from the generation point to the substation, imaginary capacitor forms are created between two conductors. The two conductors and the air gap between the two conductors work as a dielectric medium, creating an imaginary capacitor.
As we know, a capacitor is a charging device. If the current passes through an overhead transmission line, similar power losses occur in capacitors. This causes a reduction in current, and as a result, voltage increases at the receiving point.
Another cause is that insulators are connected with metallic rods at the top point of the tower. The air gap between the tower and the metallic rod works as a capacitor. There will be some charges and losses, and the current will increase and the voltage will reduce. So Ferranti effect power losses occur in transmission lines.

Advantages of Ferranti Effects

Advantages

  1. The Ferranti effect reduces copper losses. The copper loss is power loss due to current flow. These power losses are shown in heat losses. It is based on the current flow in line, and the formula is

Copper loss =I2R

As we know, the charging current is leading and the inductive current is lagging. Their vector responses are such that they cancel each other and reduce the load current.

For no or light load conditions, capacitive current is higher than inductive current. So the total load current is moving to the captive current. As load current is reduced, copper losses are reduced

Ferantti Effect Current Phasor

    • This effect also increases the power factor that occurs due to the phase angle of capacitive current and inductive current. Since capacitive current decreases the effect of inductive current, total current comes with a resistive current that increases the power factor.

    Ferranti effect disadvantages

    1. This effect increases the voltage at the receiving point of the line. The voltage increase can affect any load attached to the system. Each load is made to function at a nominal voltage. Overvoltage can affect insulation, the working life of instruments, and the load.
    2. It also increases the value of load regulation, which should be 0.
    3. The efficiency of the transmission line is reduced by affecting the load current that passes through it. Charging current increases the ampacity of line conductors, which decreases efficiency.

    How do you reduce the Ferranti effect?

    Shunt Reactor

    The Ferranti effect occurs due to the production of reactive power in the system, and no load absorbs this reactive power generated. So a shunt reactor is used to absorb this reactive power. Normally, these reactors are connected at the load side.

    A medium transmission line shunt reactor is connected at the receiving point. However, for a longer line, it connected at some distance or midpoint of the line. But underground cables come with high capacitance and need a reactor at a 15-kilometer distance.

    High Load

    The main cause of the Ferrant effect is that there is no load or light connected to the line. So to reduce this effect, the load current must be higher than the charging current.

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