The term “skin effect” has a wide role in AC power system, and people have many doubts regarding this topic, but this post will make this topic very easy for you, we will learn that What is Skin Effect?, Why skin effect occurs?, Why skin effect does not occur in DC?, and How to reduce Skin Effect?
What is Skin Effect?
When AC current pass through a conductor then it is observed that, the majority of the current will flow from the outer part/skin/surface of the conductor.
Definition: The tendency of alternating current to flow near the surface or skin of a conductor is known as Skin Effect.
You can see the phenomenon of skin effect in below figure, where charge concentration is present more near the surface or skin of the conductor, or we can say that, the whole cross-section of the conductor is not being used to flow the current through it.
We have seen that What is skin effect? But now question arises that Why or How does skin effect occur? Let’s discuss it.
Why does skin effect occur?
Let us take the cross-sectional view of a conductor and we assume this conductor to be made up of several concentric cylinders as shown in the fig. below, and each of its layer will carry some amount of current.
We know that, when an alternating current pass through a conductor/wire, then it produces a magnetic field around it and direction of this magnetic field is given by “Right-Hand Thumb Rule” as shown in fig. below.
As our conductor has several current carrying layers and each of its layer will create its own magnetic field around that layer.
Now, these magnetic field lines will create magnetic flux (ɸ). Also, we know that when magnetic flux is multiplied by the no. of turns/coils/layers (N ɸ), then it becomes Magnetic Flux Linkage (ψ).
It is observed that the magnetic flux at the core or central part of the conductor is stronger than the outer part of the conductor.
To understand the phenomenon of skin effect, we are going to study about two extreme layers of the conductor (central or inner layer and outer/surface layer).
As we have seen that, magnetic flux is stronger at the central part of the conductor then it is quite obvious that magnetic flux linkage (ψ) will also be more for the inner layer (layer 1) than outer layer (layer 2).
Now, we know that, Inductance is directly proportional to the magnetic flux linkage (ψ) by the formula given below.
By the above relation between magnetic flux linkage and Inductance (L), It is clear that, Inductance at the first layer is more than the layer 2 (outer layer).
Reactance (XL) is directly related to the inductance (L). As the formula of reactance is XL = 2π f L.
It means that, for layer 1 reactance will be high as compared to the layer 2.
Now Impedance is given by,
Z = R + j XL
here, Resistance is a uniformly distributed parameter and it does not depend on the value of flux, so Impedance is directly depending on the value of XL.Where Reactance is low, the Impedance will also be low.
We know that, I = V/Z
With the help of above formula, we can say that if the value of Z increases, then the value of Current will decrease.
For layer1, the Impedance is very High, so the current in the layer1 will be very low. And, for Layer 2, the Impedance is very low, so current in the outer layer will be high.
With the help of above table, we can see that Layer1 (Inner layer) has very less current and Layer2 (outer layer) has more current. And we called this phenomenon as Skin Effect.
Cause of skin effect: The non-uniform magnetic flux (linkage) is the root cause of the skin effect.
Why skin effect does not occur in DC?
In case of DC, the frequency is zero. (f=0). Hence, XL will also be zero, because the formula of XL is,
XL = 2π f L
XL = 2π (0) L = 0
Therefore, Impedance (Z) will only depend on the resistance,
Z = R + j XL
Z = R + 0
Z = R
And we know that, resistance is a uniformly distributed parameter, therefore the current flowing through the wire will also uniformly distributed and that’s why skin effect does not occur in case of DC.
Disadvantages of skin effect
Skin effect is a undesired phenomenon which leads to many disadvantages in the power system as:
1. Effective resistance increases (Reff ↑)
Due to skin effect, majority current flows near the surface of the conductor. Due to this phenomenon the whole cross-section of the conductor is not being used and this reduces the effective cross-sectional area (the area of the conductor through which the current is actually flowing).
As we know that resistance is inversely proportional to the area, If the effective cross-sectional area is reduced then the effective resistance will increase.
2. Copper losses Increases:
Copper losses or I2R losses will increase because the effective resistance has increased due to ski effect. Increment in copper losses also leads to Heat loss.
3. Voltage drop increases:
When resistance increases, it also increases the Voltage drop.
4. Voltage regulation decreases
5. Efficiency decreases
What are the factors which affect the skin effect?
1. Supply frequency:
Supply frequency is directly proportional to the skin effect. Because, Reactance is directly proportional the supply frequency (XL = 2π f L)
2. Diameter of the conductor:
Skin effect is directly proportional to the Diameter of the conductor. Skin effect is very less for thin conductors and very high for thick conductors.
3. Nature of material:
Relative permeability (µr) is a property of material which supports the formation of magnetic field and skin effect is directly proportional to the Relative permeability (µr).
If µr of a material has high value, then it will support the formation of magnetic flux which will increase the Inductance, Reactance and Impedance, and decrease the current in the inner section of conductor. In other words, we can say that Skin effect will be more if Relative permeability of a material is high.
4. Type of Conductor:
Skin effect is more in case of Solid conductors and Skin Effect is less in case of stranded conductors and Hollow Conductors.
Read here: How Stranded conductor or ACSR conductor reduces the Skin effect.
Skin effect increases by increasing the temperature. It means that Skin effect is directly proportional to the Temperature.
Methods to reduce skin effect
We can reduce the skin effect to some extent but we can not Finish the skin effect, There are few methods to reduce the Skin effect a follows:
1. By using Stranded conductors:
Stranded conductor consists of several thin wires of small cross-sectional area called as Strands. These conductors are more flexible than the solid conductors and they help to reduce the skin effect.
This is the most widely used method to reduce the skin effect. There are many types of Stranded conductors but generally we use ACSR (Aluminium Conductor Steel- Reinforced) conductor.
This conductor is made up of two layers, the inner layer has Steel strands which provide strength to the cable, and outer layer is made up of Aluminium strands which are used to take current as shown in the figure below.
2. Reducing the Diameter:
By reducing the thickness of conductor we can reduce the Skin effect.
3. Using hollow conductors:
As we know that, Due to skin effect the majority of the current flows through the outer surface of the conductor, so if we use the Hollow conductor then We can eliminate the waste material of the conductor. This method is only used in some applications like Bus-Bars in substation. As, Hollow conductors are very costly to design and they have very low mechanical strength, so they can not be used in the long-distance Transmission.
Few more methods like, Reducing the frequency, reducing the Temperature can also reduce skin effect.
I hope that this post cleared all your confusion and doubts regarding Skin Effect. If you still have any doubt related to this topic then You can watch the Video below.
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