What is Corona Effect in Transmission Line?

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What is Corona Effect in Transmission Line?

When the two conductors of a transmission line are connected to an alternating potential difference, whose spacing is large in comparison with their diameter, then the atmosphere air surrounding the two conductors is subjected to electrostatic stresses. When this potential difference is low, there is no change in the condition of atmosphere air around the conductors.

When the potential is gradually increased and reaches a value called Critical Disruptive Voltage, the air between the conductor starts ionization (starts producing ions) and a faint luminous glow of violet color appears because of the electrostatic stresses on the surrounding air. This phenomenon is called Visual Corona and is accomplished by the production of ozone gas, radio interference, and hissing noise.

Corona Effect in Transmission Lines

If the potential difference is further increased, the glow and noise will increase rapidly and result in the breakdown of the air insulation that leads to spark over. The whole phenomenon or process of appearance of hissing noise, the violet glow, and the production of ozone gas is known as Corona Effect in transmission lines.

Corona can be simply summarized as,
  • A violet glow is observed around the conductor.
  • It produces a hissing noise.
  • It produces ozone gas.
  • The glow is maximum over rough and dirty surfaces of the conductor.
  • It is accomplished by a power loss.
  • Under corona conditions, the charging current in the line gets increased.
  • Interference will occur in the radio and communication lines.

Power Loss Due to Corona :

Whenever corona occurs, the space around the conductor gets ionized and hence produces ions. These ions move around the conductor. In order to maintain the motion, the ions require some energy, which they take from the supply. This additional energy taken by the ions, dissipated as heat, sound, and light is called corona loss. The power loss due to corona under fair weather conditions is given by Peek's formula i.e.,

Power loss due to Corona Effect in Transmission Lines

Where,
  • f = Supply frequency in Hz
  • Vph = Phase to neutral voltage (RMS)
  • Vc = Critical disruptive voltage (RMS) per phase
  • δ = Air density factor
  • d = Spacing between the conductors
  • r = Radius of conductors.

Factors Affecting Corona in Transmission Lines :

The factors affecting the corona effect in transmission lines are categorized as follows,
  • Electrical factors
    • Effect of frequency
    • Effect of line voltage
    • Effect of load current.
  • Atmospheric factors
    • Effect of temperature and pressure
    • Effect of dust, rain, snow, and hail.
  • Factors related to conductor
    • Conductor size
    • Spacing between conductors.

Electrical Factors :

  • Effect of Frequency: From Peek's formula, power loss due to corona is,
    Pc = (f + 25)(V - Vd) √(r/d) × 10-5 kW/km/ph
    The above formula shows that the power loss is a function of frequency. The loss is proportional to system frequency and hence corona losses increase with an increase in supply frequency. Corona loss is high for ac supply when compared to corona loss of dc supply due to the presence of third harmonic components in ac supply.
  • Effect of Line Voltage: The supply voltage in a transmission line greatly affects corona loss. As the supply voltage increases and reaches a point called critical disruptive voltage, the corona is formed and corona loss occurs. This corona loss increases if the line voltage is increased further.
  • Effect of Load Current: Load current has a slight effect on the corona losses due to the fact that an increase in load current increases the temperature of the conductor which prevents the deposition of snow leading to fewer corona losses. However, this factor is least considered during the rainy season.

Atmospheric Factors :

  • Effect of Temperature and Pressure: From the formula of critical disruptive voltage, we have,
    Vd = 21.1 mo δr ln(d/r) kV
    And from Peek's formula, we can obtain the relation,
    loss ∝ (V - Vd)2
    From the above two equations, we can infer that corona loss will be high for lower values of air density factor δ because lower values of δ in the first equation will result in lower critical disruptive voltage Vd, and Vd in the second equation will result in a higher value of corona loss. For this reason, corona loss in hilly areas is more than in plain areas.
  • Effect of Dust, Rain, Snow, and Hail: Due to the deposition of dust particles on the surface of conductors, the critical disruptive voltage Vd decreases which causes more corona loss. The case is the same in bad atmospheric conditions like rain and snow. Therefore, corona losses are more in bad weather conditions when compared to fair weather.

Factors Related to Conductors :

  • Conductor Size: From the equation of Peek's formula we can obtain the relations,
    loss ∝ √(r/d) and loss ∝ (V - Vd)2
    From the first equation, corona loss is directly proportional to the square root of the radius of the conductor which means corona loss increase with an increase in the size of the conductor. But, from the formula of critical disruptive voltage, Vd is directly proportional to the size of the conductor, and the relation of loss ∝ (V - Vd)2 shows that corona loss decreases with an increase in Vd. In practice, the effect of the second case is found to be more on corona losses than the first one. Hence, corona losses will be less for the larger diameter of the conductor.
  • Spacing between Conductors: From the relation, loss ∝ √(r/d), we can see that the loss is inversely proportional to the square root of the distance between conductors. Thus, corona losses will be less if the spacing between conductors is more.

Methods of Reducing Corona Effect :

The corona effect can be reduced by the following methods,

By Increasing Conductor Size :

When the size of the conductor has increased the voltage at which corona occurs and appears also increases. As the size of the conductor is increased, the electric field intensity reduces which in turn reduces the effect of the corona.

However, an increase in the size of the conductor will increase the cost of conductors, mechanical stress on the insulator, etc. Thus, we cannot increase the size of the conductor to a large value in order to avoid the early occurrence of the corona. Hence, hollow conductors are employed for reducing the corona in EHV lines.

By Increasing Spacing between the Conductors :

By increasing the spacing between the conductors of the transmission line, the electrostatic stress between the two conductors decreases, and hence the air between the conductors gets ionized at a higher voltage i.e., the value of Vd and Vc increases and the effect of the corona is eliminated.

However, the increase in spacing between the conductors is limited, due to the increased size of the tower and the land occupied by the tower which in turn leads to an increase in the cost. Hence, an optimum value of spacing should be chosen, so that the critical breakdown voltage increases, and the effect of the corona is reduced.

By using Bundled Conductors :

For the same power rating, the use of bundled conductors increases the diameter of conductor compared to the normal conductor. An increase in the diameter of the conductor decreases the electric field intensity which in turn decreases the effect of the corona.

Advantages of Corona Effect :

  • The electrostatic stress between the conductors is reduced due to the formation of corona which in turn reduces the probability of flashover, improving the performance of the system. This process takes place when the air surrounding the conductor conducts on the formation of the corona. The conductive nature of the air increases the diameter of the conductor which in turn reduces the electrostatic stress or the maximum potential gradient between the conductors.
  • Corona reduces the effects of transient produced due to lighting or switching by partially dissipating as a corona loss.

Disadvantages of Corona Effect :

  • Corona affects the transmission efficiency of the line as there is a definite loss of energy due to corona.
  • The voltage drop is non-sinusoidal since the current drawn by the line due to the corona is non-sinusoidal.
  • Electromagnetic and electrostatic effects may cause some inductive interference with the neighboring communication circuits or lines.
  • Ozone gas is produced by corona which chemically reacts with the conductor to cause corrosion.
  • The effective capacitance of the conductor increases.

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