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02/04/2022

Corona Loss in the Transmission Line

 

Corona

  • When alternating voltage across transmission is raised beyond a certain level, the conductors are surrounded by pale violet glow and smell of ozone gas. 
  • It is called as Corona effect on the transmission line.

Critical disruptive voltage

  • It is minimum phase to neutral voltage required to start ionization of air.
  • Let us consider that distance between two conductors are d cm and its radii r cm. 
  • The potential gradient at the conductor surface is given by

        g = V / [ r Loge ( d / r ) ]

        V = go  r Loge ( d / r )

  • The effect of air density factor as well as surface of the conductor is considered for critical disruptive voltage.

        VC = m0go δ r Loge ( d / r )

        Where VC = Phase to neutral voltage

                       r = Radius of conductor

                      d = Diameter of conductor

                    g0 = Breakdown ( Dielectric ) strength of air at 76 cm of

 mercury at 25 degree centigrade

                       = 30 kV / cm ( Max )

                       = 21.2 kV / cm ( RMS )

m0 = Irregular factor

    = 1 for polished conductor

    = 0.98 to 0.92 for dirty conductor

    = 0.87 to 0.8 for stranded conductor

Air density factor

  • Under the standard condition of 76 cm of Hg and 25 degree centigrade, the air density factor is given by
  • Air density factor δ = 3.92b / 273 + t
  • Where b = atmospheric pressure of b cm
                   t = Temperature in centigrade
  • The breakdown strength of air is directly proportional to air density factor. The air density factor under standard condition is taken as unity.
  • Line voltage = √ 3 × VC

Visual critical voltage

  • It is minimum phase to neutral voltage at which corona glow starts on the conductor. The visual critical voltage is higher than the disruptive critical voltage.
  • VV = m0go δr { 1 + 0.3 / √ δr } Loge ( d / r )

       Where

       m0 = Irregular factor

        g0 = Breakdown ( Dielectric ) strength of air

          r = Radius of conductor

         d = Diameter of conductor

         δ = Air density factor

Power loss due to corona

  • Under the normal weather condition, according to Peek’s Formula, The power loss due to corona is given by
  • P = 242.2 [( f + 25 ) / δ ] √ r / d ( V – VC )2 × 10 – 5 kW / km / Phase

       Where

       f = Supply frequency

       V = Phase to neutral voltage ( RMS )

       VC = Critical disruptive voltage ( RMS ) per phase

          δ = Air density factor

  •  Power loss due to corona α ( V – VC )2
  • The power loss increases due to bad weather condition.

Advantages and Disadvantages of Corona

Advantages

  • It reduces transient produced by surges.
  • Due to corona, virtual diameter of conductor increases which reduces electro – static stress between conductors.

Disadvantages

  • Power loss
  • It reduces the transmission line efficiency.
  • Harmonic current
  • Due to corona, non – sinusoidal current flows in the transmission line. It may cause inductive interference to nearby communication lines.
  • Radio interference

Factors affecting corona

Line voltage

  • If the voltage is low, there is no change in surrounding condition of conductors. 
  • As voltage increases beyond certain limit, corona is formed due air around conductor becomes conducting.

Spacing between conductors

  • As the space between conductor increases, the electro-static stress on the conductor surface reduces which reduces effect of corona formation.

Diameter of conductor

  • The corona loss depends upon shape and size of conductor. 
  • It increases for rough and irregular surface of conductor as compared to polished surface. 
  • The stranded conductor has irregular surface as compared to solid conductor, therefore corona loss reduces on the stranded conductor.

Atmosphere condition

  •  The corona is formed due to ionization of air surrounding conductor.
  •  The number of ions more in the stormy weather, therefore corona occurs at much less voltage as compared to standard weather condition.

The corona effect is reduced by

  • Increasing space between conductors
  • Increasing diameter of conductor : Large diameter, Hollow conductor and Bundle Conductors. 

 

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Corona loss in HVDC Transmission lines

  • The negative charged particle near the conductor surface moves from negative conductor to positive conductor continuously. 
  • There is a electric field distribution between positive conductor and negative conductor.  
  • The corona loss in the HVDC transmission line is less due to √ 2 factor ( Maximum voltage in the ac transmission line = √ 2 × RMS ac voltage ) in the voltage waveform. 
  • The corona loss in the DC transmission line does not affect as much as that of AC transmission line in the bad weather condition. 
  • The corona loss is minimum in the monopole lines. 

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