Ferranti Effect

As we know electricity is generated at power generation plants using huge electromechanical generators by conversion from different types of energy. After that, a long-distance transmission line carries this electrical energy to the end users. In order to maximize the effectiveness of the power transmission and distribution system and ensure the safety of the connected loads and personnel, the electrical power transmission lines require a variety of safety devices and components. The transmission line faces different types of losses and characteristics that influence its efficiency. The Ferranti Effect is one such phenomenon that has a significant impact on the transmission line.

For the most part, we assume that the voltage generally drops in the transmission lines because of the line losses. In a long-distance transmission line with a very low load or no load at all, the Ferranti Effect causes the receiving voltage to be higher than the sending voltage.

In this article, we will be going through the Ferranti effect, First, we will start with the basics of the Ferranti Effect, Then we will go through the Causes of the Ferranti Effect, After that we will go through the Ferranti Effect in Transmission Lines and ways to reduce Ferranti effect, At last, we will conclude our Article with Advantages, Disadvantages, and characteristics of Ferranti effect.

What is the Ferranti Effect?

The Ferranti effect is a phenomenon that describes the Increase in voltage that happens at the receiving end of a long transmission line compared with the voltage at the sending end. The Ferranti effect is more common when the load is low, or no load. The Ferranti effect can be expressed as a factor or as a percentage increase in voltage when the load is connected, such as in the case of an open circuit.

In general practice we know, that for all electrical systems current flows from the region of higher potential to the region of lower potential, to compensate for the electrical potential difference present in the system. In all operational cases, the sending end voltage is higher than at the receiving end because of line losses, so current flows from the source or the supply end to the load.

Sir S.Z. Ferranti, in the year 1890, came up with the theory about medium transmission line or long distance transmission lines proposing that in the case of low loading or no-load activity of the transmission system, the voltage at the receiving end often exceeds the voltage at the sending end. This Phenomenon, known as the Ferranti effect, became a significant aspect of power system analysis.

Terminologies of Ferranti Effect

• Ferranti Effect: The Ferranti effect is a phenomenon where the voltage at the receiving end of a long transmission line is higher than the voltage at the sending end. It happens because of the capacitance of the transmission line, causing reactive power flow.
• Capacitance: Capacitance is the capacity of a system to store an electric charge. With respect to the Ferranti effect, it plays a major role as the capacitance of transmission lines used for the gathering of reactive power, causing voltage rise.
• Surge Impedance Loading (SIL): The maximum power transfer capacity of a transmission line without causing overvoltage is known as surge impedance loading. It is a basic parameter in preventing unnecessary voltage rise related with the Ferranti effect.
• Reactive Power: Power oscillations between a system’s load and source are referred to as reactive power. In the Ferranti effect, the transmission line’s capacitance induces reactive power flow, adding to voltage rise.

Causes of Ferranti Effect

Ferranti effect basically happens due to the presence of an high charging current because of the capacitance of the transmission line. Although various elements affect the current in the transmission line. However, Ferranti effect happens due to the following three reasons;

• Transmission line capacitance
• Load at the receiving end
• Supply Frequency

Transmission Line Capacitance

The conductors in the transmission line are placed in closed proximity, particularly in underground cables that develop capacitance between them. As a matter of fact, the transmission cable comprises many shunt capacitors and series inductors similarly distributed along the length of the cable. The capacitance increases with an expansion in the length of the transmission line. The capacitors draw a lot of charging current that courses through the entire length of the line. Reactive power is produced by the capacitor and flows in the opposite direction of the source. The inductors in the line consume the receptive power causing a voltage drop across them. The sending voltage is synchronized with the voltage drop. Consequently the voltages add up and the receiving voltage is increased.

Load at the Receiving End

The connected load at the receiving end is another factor that affects the Ferranti effect. The load can be in one of three states:

• No load
• Light load
• Full load

In the no load state, the transmission line only receives a charging current and no load current. The charging current is drawn by the shunt capacitors the line. It produces reactive power that causes a voltage drop across the inductor that is in-phase with the sending voltage and expands the voltage at the less than receiving end.

At the point when light load is connected, there is an very low load current when contrasted with the charging current flowing through the lines. The charging current is naturally leading because of the line capacitance. The capacitor generates reactive power that moves through the inductors which is more prominent than the reactive power consumed by the inductors because of low load current. The voltage drop across the inductors is practically in phase with the source voltage and it is relative to the charging current. As the charging current is higher than the load current, the Ferranti effect happens.

In full load conditions, the load current is higher than the charging current drawn by the capacitor (it is practically steady). Since a large load current is moving through the series inductors, the reactive power consumed by the inductor is larger than the reactive power produced by the capacitor. In this way the net reactive power is negative and the voltage decreases at the less than receiving end.

Supply Frequency

As we know that the Ferranti effect happens because of the reactive power produced in the shunt capacitance of the electrical cables. Anyway reactive power possibly happens if supply voltage and current has some frequency. Since DC has zero frequency, there is no Ferranti effect. The Ferranti effect is more likely to occur in transmission lines that operate at a high frequency.

Ferranti Effect in Transmission Line

Let’s consider the equivalent circuit diagram of a long transmission line. Since a long transmission line is made out of high capacitance and inductance distributed through the entire length of the line, this diagram addresses the parameters per kilometer of length. In this manner the capacitance and inductance are proportional to the length of the line. The inductors are connected to the power lines in series, while the shunt capacitors are connected in parallel.

Circuit-Diagram-of-Ferranti-Effect

The parameters in the given circuit diagram are :

• Vs = Sending or source Voltage
• Vr = Receiving Voltage
• Is = Sending or Source Current
• Ir = Receiving Current
• Ic = Capacitive or Charging Current
• R = Resistance of line
• Xc = Capacitive reactance of the line
• XL = inductive reactance of the line
• C = Capacitance of line
• L = inductance of the line

Phasor diagram of the given circuit is shown in below. During the Ferranti effect, there is no load subsequently

Phasor-Diagram-of-Ferranti-Effect

Receiving current Ir = 0

The receiving voltage Vr is taken as reference OA where the capacitive current Ic is addressed by opposite line OD driving the Vr by 90°. The capacitive current Ic has a voltage drop across the line resistance R and the line inductance L, as shown by the equation:

Voltage drop across the resistor = IcR = AB

Voltage drop across the inductor = IcXL = BC

The inductive voltage drop IcXL (BC) is 90 degrees higher than the resistive voltage drop IcR (AB). Where the sending voltage Vs is the amount of all voltage drops + receiving voltage represented by OC.

Vs = Vr + Resistive voltage drop + Inductive voltage drop

Vs = OA + AB + BC = OC

As shown in the phasor diagram, the receiving voltage Vr at the load side is grater than the sending voltage Vs at the source side.

Now let’s derive the equation for the Ferranti effect using the same circuit diagram of the transmission line.

Vs = Vr + resistive drop + inductive drop

Vs = Vr + IcR + IcX_L

Vs = Vr + Ic (R + X_L)

Since capacitive current, Ic = jwCVr

Vs = Vr + jwCVr (R + X_L)

Since X_L = jwL

Vs = Vr + jwCVr (R + jwL)

Vs = Vr + jwCVrR + j ²w ²CLVr

Vs = Vr + jwCVrR – w ²CLVr

Vs – Vr = jwCVrR – w ²CLVr

In long transmission lines, the line resistance is a more smaller than the line reactance. Therefore the resistance R, as well as resistive voltage drop is neglected

Vs – Vr = – w ²CLVr

Now assume the capacitance and inductance per km of the length are Co and Lo respectively and the length of the transmission line is l. The equation becomes

Vs – Vr = -w2(C_O I)(L_O l)Vr

Vs – Vr = -w² l ² CoLoVr

Since the line capacitance is distributed throughout the entire length (l) of the transmission line, the charging current as well as the voltage drop associated with it is taken as an average.

Now assume Therefore;

Vs – Vr = – (1/2) w² l ² CoLoVr

The voltage distinction between the sending and receiving voltage is negative which implies the voltage rises. Additionally, it is directly proportional to the squares of line length (l) and frequency (w). This condition demonstrates that the Ferranti effect increases with an expansion in the length of the transmission line and supply frequency. Consequently little transmission lines and HVDC transmission are not impacted by the Ferranti effect.

Ferranti Effect in PI Model

The Ferranti effect in extensive transmission line where OE-signifies the receiving end voltage, OH-signifies the flow of current in the capacitor at the receiving end. A decrease in voltage across the resistance R is indicated by the FE-phasor, and a decrease in voltage across the inductance (X) is indicated by the FG. The OG-phasor means the sending end voltage in a condition of no-load. The nominal Pi model of the transmission line at no load condition circuit is shown in below.

PI-model-with-Circuit-Diagram

In the following phasor representation, OE is greater than OG(OE>OG). In different terms, the voltage at the less than receiving end is better than the voltage at the transmitting end when the transmission line is at no load condition. Here the Ferranti effect phasor diagram is shown below.

Phasor-Diagram-of-PI-model

Vs = (1+ZY/2)Vr + ZIr

Where, Ir =0 at no load condition

Vs = (1+ZY/2)Vr + Z (0)

= (1+ZY/2) Vr

Vs – Vr = (1+ZY/2)Vr-Vr

Vs – Vr = Vr [1+ZY/2-1]

Vs – Vr = (ZY/2) Vr

Z= (r + jwl)S, and Y = (jwc)S

Assuming that the transmission line’s resistance is unnoticed

Vs – Vr = (ZY/2) Vr

Substitute Z= (r + jwl)S, and Y = (jwc)S in the above Vs

Vs – Vr= ½ ( jwls) (jwcs) Vr

Vs – Vr= – ½ (W2S2) lcVr

For the lines of above, 1/√LC = 3×108m/s (velocity of electromagnetic wave transmission on the transmission lines).

1/√LC = 3×108m/s

√LC = 1/3×108

LC = 1/(3×108 )2

Vs – VR = – ½ W2S2 . ( 1/(3×108 )2) Vr

W = 2πf

Vs – VR = – ((4π2/18)* 10-16) f2S2Vr

That’s what the above equation represents (Vs – Vr) is negative, that means Vr is greater than Vs This is also illustrated that this effect will also determine by the electrical period of the transmission lines and frequency.

Generally, for each line

Vs = AVr + BLr

On no load condition

Ir =0, Vr = Vrnl

Versus = AVrnl

|Vrnl| = |Vs|/|A|

For a extensive transmission line, A is<1 and it reduces with the increase in the extent of the transmission line. Hence, the voltage at no load is more greater than the voltage at the load (Vrnl > Vs) As the length of line in the voltage on the collecting end, then at no load acts as the main.

How to Reduce Ferranti Effect?

Ferranti effect makes voltage instability in the electrical system that cause a dangerous hazardous for equipment and personnel at the load side. There are specific measures taken to limit the Ferranti effects.

Shunt Reactor

The Ferranti effect arises when reactive power accumulates in the power system without loads to absorb it. We must install something that can absorb this excessive reactive power in the transmission line. This reactive power can be absorbed by installing a shunt reactor in the transmission line. Usually, it is installed in at the load end.

A medium transmission line requires the shunt reactor to be placed at the receiving end. However long transmission lines require it to be installed at a periodic distance or in the line. While underground cables have very high capacitance and require short distance of around 10 miles or 15 km intervals.

Load Management

Ferranti effect happens when there is no or light load associated with the lines. To reduce the Ferranti effect, it should satisfy the given condition.

Load current > Charging current.

A transmission line’s loads should be continuously monitored, and the load must be maintained above the limit. It tends to be finished by setting different light load lines onto a single line.

Characteristics of Ferranti Effect

The following characteristics are shown below related to Ferranti effect :

• Voltage Increase
• Capacitive Nature of Transmission
• Surge Impedance Loading
• Voltage Profile and Compensation
• Dependency
• Importance in Power System Analysis

Voltage Increase

• Under Load Conditions: The Ferranti Effect is such a big thingy when the transmission is like kind a lightly loaded or has a low power. Like seriously, such cases, the voltage at receiving end of the transmission tends to be way than the voltage at the end.

Capacitive Nature of Transmission

• Capacitance Effect: The lines inherently possess like we know, capacitance to the insulation and spacing between the conduct. Like, when line is lightly loaded, the capacitive nature dominates and stuff, you know leading to an overvoltage condition like big time at receiving end.
• Capacitive Charging Current During Low Power Demand: With like power demand and not much power, the charging current through the capacitance the transmission line dominates the system, kind you know. This charging current leads to voltage rise at the receiving end and stuff.

Surge Impedance Loading

• Relation to Surge Impedance: The Ferranti Effect is like super closely to the surge impedance loading of the transmission line, you know. So, like when the power transmitted is less than the surge impedance loading, the line experiences a capacitive voltage rise.

Voltage Profile and Compensation

• Voltage Gradients: Voltage profiles along the transmission line may exhibit gradients, you know, and the Ferranti Effect contributes to this totally non-uniform voltage distribution kind thing.
• Compensation Devices: Engineers use various compensation devices, like, you know, such as shunt reactors or synchronous condensers, to mitigate the Ferranti Effect and control voltage levels within acceptable, like, limits and stuff.

Dependency

• Effect on System Frequency: So, you see, the Ferranti Effect is like so much more pronounced at lower frequencies. And like, power systems and stuff are designed to operate within specific frequency ranges, you know, and variations in frequency can impact the magnitude of the Ferranti Effect and stuff.

Importance in Power System Analysis

• Consideration in Planning: Power system planners and engineers must totally account for the Ferranti Effect when designing and analyzing long-distance transmission lines, kind, you know, to ensure stable and reliable operation

Advantages of Ferranti Effect

• Reduced Copper Losses: The copper loss is the power loss in the lines due to the current flowing through them. It appears in the form of heat. It relies upon how much current flowing through the line and is given by

Copper loss = I² R

• Voltage Regulation: When we manage a appropriately Ferranti Effect, then the voltage regulation is contribute by compensating for voltage drop along the transmission line.
• Power Factor Improvement: Power factor occurs due to the phase angle of the capacitive current and inductive current. As the capacitive current losses the impact of the inductive current. Hence, the net current is for the most part made out of resistive current which further improves the power factor.

Disadvantages of Ferranti Effect

• Over Voltage: The Ferranti impact builds the sending voltage when it is gotten at the heap end of the transmission line. The expansion in voltage can harm any heap associated with the framework. Since each heap is intended to work at an ostensible voltage. Overvoltage can harm protection and definitely diminish the lifetime of gear and it can forever harm the heap.
• Voltage Regulation: It expands the voltage regulation which should be almost zero.
• Efficiency: It reduces the efficiency of the transmission line by reducing the load current moving through it. The charging current takes a large portion of the ampacity of the line conductor thus reducing its efficiency.

Conclusion

In conclusion, the Ferranti effect plays a vital role in the domain of electrical power systems, introducing the two difficulties and opportunities. Understanding this peculiarity is fundamental for power engineers and system operators to ensure the dependable and effective activity of transmission networks.

The Ferranti effect’s favorable viewpoints lie in helping voltage regulation and reactive power management potential. When tackled appropriately, it can add to keeping up with reliable voltage levels, especially in regions served by lengthy transmission lines. This is significant for giving dependable capacity to distant districts and upgrading the general exhibition of the electrical systems.

Be that as it may, the Ferranti Impact additionally presents difficulties, especially as far as hardware stress, network shakiness, and dynamic circumstances. Uncontrolled voltage rise related with the Ferranti Impact can prompt mileage on electrical parts, network motions, and hardships in dealing with the impact under shifting load conditions. To losses the negative effects of the Ferranti Effect in practice, engineers use sophisticated methods like series compensation, shunt reactors, and voltage control devices. Finding some kind of harmony between utilizing its advantages and dealing with its difficulties is fundamental for ensuring the flexibility and soundness of modern day power systems.

FAQs on Ferranti Effect

What causes the Ferranti effect?

The Ferranti effect is fundamentally brought about by the capacitance of long transmission lines, prompting receptive power oscillations.

Ferranti effect can be beneficial ?

In controlled situations, the Ferranti effect can be used to improve voltage stability. However, there must be mitigation for uncontrollable situations.

How does the Ferranti effect influence transmission line efficiency?

The Ferranti effect, if uncontrolled, can prompt to increase power losses, reducing in general transmission line efficiency.

Are there standards methods to deal with the Ferranti effect in power systems?

Yes, engineers manage and reduce the Ferranti Effect in power systems using a variety of techniques like series compensation, shunt reactors, and voltage control devices.