Corona Loss in the Transmission Line

Corona Loss

• 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 Log_e ( d / r ) ]

        V = g_o  r Log_e ( d / r )

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

        V_C = m₀g_o δ r Log_e ( d / r )

        Where V_C = 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 × V_C

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.
• V_V = m₀g_o δr { 1 + 0.3 / √ δr } Log_e ( 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 – V_C )² × 10 ^{– 5} kW / km / Phase

       Where

       f = Supply frequency

       V = Phase to neutral voltage ( RMS )

       V_C = Critical disruptive voltage ( RMS ) per phase

          δ = Air density factor

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

Advantages and Disadvantages of Corona Loss

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 Loss

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.

Methods of Reducing Corona Loss

• 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.