Skin Effect in Transmission Lines

The distribution of current throughout the cross section of conductor is uniform only when the steady current(D.C.) is passing through it. However, an alternating current flowing through the conductor does not distribute normally, rather it has the tendency to concentrate near the surface of the conductor as shown in figure below.

What is skin effect

The tendency of alternating current to concentrate near the surface of a conductor is known as skin effect. 

This results in higher resistance to alternating current that to direct current and is more pronounced as frequency is increased. This is known as skin effect. It causes a larger power loss for a given rms ac than the loss when the same value of dc is flowing through the conductor. Consequently, a qualitative explanation of the phenomenon is given below.

" A conductor could be considered as composed of very thin filaments. The inner filaments carrying current gives rise to flux which links the inner filaments only when as the flux due to current carrying outer filaments enclose both the inner as well as the outer filaments. The flux linkages per ampere to inner strands is more as compared to outer strands. The inductance* of each strand will vary according to its position. Thus the strands near the center are surrounded by greater magnetic flux and hence have larger inductance than that near the surface. The high reactance of inner strands causes the alternating current to flow near the surface of conductor. This crowding of current near the conductor surface is the skin effect. "
With the increase of the frequency the non-uniformity of inductive reactance of the filaments becomes more pronounced, so also the non-uniformity of current distribution. For large solid conductors the skin effect is quite significant even at 50Hz. The analytical study of skin effect requires the use of Bessel's functions.
The skin effect depends upon the following factors:-

1. Nature of material
2. Diameter of wire- It increases with increase in diameter of wire
3. Frequency- It increase with increase in frequency.
4. Shape of wire- It is less for stranded conductor than solid conductor.

It may be noted that skin effect is negligible when the supply frequency is low (≤ 50Hz) and conductor diameter is small (< 1 cm).

Corona phenomenon in Transmission Lines

When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameter, there is no apparent change in the condition of atmospheric air surrounding the conductors, if the applied voltage is low. However when the voltage on line conductor is raised beyond a certain limit, called critical disruptive voltage, the conductors are surrounded by pale violet glow together with a slight hissing noise and a smell of ozone. This phenomena is called as corona.

In short corona phenomena is the ionization of air surrounding the power conductors. Free electrons are normally present in free space because of radioactivity and cosmic rays. As the potential between the conductors is increased, the gradient around the surface of the conductor increases. Assuming that the spacing between the conductors is large as compared with the diameter of the conductors. The free electrons will move with certain velocity depending upon the field strength. These electrons will collide with the molecules of air and in case the speed is large they will dislodge electron from the air thereby the number of electrons will increase. The process of ionization is thus cumulative and ultimately forms an electron avalanche. This results in localization of air surrounding the conductor and hence corona effect is occurred.

Corona occurrence is therefore defined as a self sustained electric discharge in which the filed intensified ionization is localized only over a portion of the distance between the conductors.

The phenomena of corona is accompanied by hissing sound, production of ozone, power loss and radio interference. The higher the voltage is raised, the larger and higher the luminous envelope becomes and greater the sound, the power loss and radio noise. If the applied voltage is increased to breakdown value, a flash over will occur between the conductors due to the breakdown of air insulation.

If the conductors are polished and smoothed, the corona glow will be uniform throughout the length of the conductors, otherwise the rough points will appear brighter. With d.c. voltage, there is difference in the appearance of the two wires. The positive wire has uniform glow about it, while the negative conductor has spotty glow. For a visual corona the line voltage has to be somewhat higher than critical disruptive voltage and is called visual critical voltage.

Turn ON methods of SCR/Thyristor Triggering

A thyristor can be switched from a non conducting state to a conducting state in several ways-

Forward Voltage Triggering (High Voltage)

When anode to cathode forward voltage is increased with gate circuit open, the leakage current of the thyristor increases. Due to internal current multiplication taking place inside, this current increases. As soon as the forward voltage reaches the breakover voltage (V_BO), the reverse biased junction J2 will have an avalanche breakdown. 

At this voltage, thyristor changes from OFF state to ON state characterized by a low forward voltage across it with large forward current. This type of turn ON may be destructive and should be avoided.

Thermal Triggering ( Temperature Triggering)

Like any other semiconductor, the width of depletion layer of a thyristor decreases on increasing junction temperature. When the temperature of thyristor is high, there is an increase in the number of electron-hole pairs which increases the leakage current. This increase in leakage current causes increase in current amplification factor ∝₁ and ∝₂. Due to the regenerative action, ∝₁+∝₂ may tend to be unity and the thyristor may be turned ON. This is called thermal triggering of thyristor. This type of turn ON may cause thermal runaway and is normally avoided.

Light Triggering (Radiation Triggering)

If light of adequate frequency and intensity is allowed to strike the thyristor junction, then the photons will strike the electrons and increase the number of electron-hole pairs. This leads to instantaneous flow of current within the device and the triggering of the device. For light triggering to occur, the device must have high value of rate of change of voltage (dV/dt).

dV/dt Triggering

With forward voltage across the anode and cathode of a device, junction J1 and J3 are forward biased whereas the junction J2 becomes reverse biased. This reverse biased junction J2 has the characteristic of a capacitor due to charge existing across the junction. If a forward voltage is suddenly applied, a charging current will flow tending to turn ON the device. If the voltage impressed across the device is denoted by V, the charge by Q and capacitance by C_j then,

The rate of change of junction capacitance may be negligible as the junction capacitance is almost constant. If the rate of change of voltage across the device is large, the device may turn ON even though the small voltage appearing across the device is small.

Gate Triggering

This is the most commonly used method for SCR triggering. The injection of gate current by applying positive gate voltage between the gate and cathode terminals turn ON the SCR much before the specified breakover voltage. The conduction period of the SCR can be controlled by varying the gate signal within the specified value of maximum and minimum gate current. Three types of signals can be used for triggering the SCR using gate. They are either a.c. signal, d.c. signal or pulse signal.

Silicon Controlled Rectifier (SCR)

Silicon Controlled Rectifier or SCR is one of the oldest type of solid state power device. It was invented in 1975 by the General Electric Research Laboratory. It has the highest power handling capacity of all the power semiconductor device. It has four layer construction with three user accessible terminals. SCR is a latching type device that can be turned ON by the gate terminal but once turned ON, the Gate loses control on it.

Important Features

1. It is latching type device.
2. It can handle a very large power.
3. It is a current controlled device because the gate current controls the SCR.
4. It acts as an open or closed switch.
5. The ON state voltage drop is very low.
6. It can handle thousands of ampere of current.

Construction

It is a four layer PNPN device with three terminals brought out for the user, namely Anode(A), Cathode(K) and Gate(G). The Gate terminal is used in ON process. It can be split into two sections of NPN and PNP as shown below,

It has three junctions J1, J2 and J3. The anode and cathode are connected to the main power circuit. The gate terminal carries a low level gate current in the direction of gate to cathode. Normally, the gate terminal is provided at the P layer near the cathode as shown in above figure. This is known as cathode gate.