Is corona discharge the main problem in transmission?

              Sometimes while crossing the transmission lines can we  heard some hissing cracking noises with sparkles with a micro arc, sometimes we can also see a salty deposits on the conductor when they are intact with environment for a prolonged period of time well in most of their minds has risen a question that what is the reason of that and mainly what is that.
What is corona:
                 Corona, also known as partial discharge, is a type of localized emission. The localized electric field near a conductor can be highly concentrated to ionize air close to the conductors. This can result in a partial discharge of electrical energy called a corona discharge, or corona. It is also defined as "It is a luminous discharge due to ionization of the air surrounding a conductor around which exists a voltage gradient exceeding a certain critical value."



                   Actually, corona can appear in solid, liquid or gaseous insulating materials, and its occurrence is usually associated with the initial phases of electrical failure of the insulation(i.e if the conductor is not perfectly insulated there might be a leakage of the electric charges). In solids, the occurrence of corona generally results in deterioration of the material(i.e the surface area of the conductor becomes small resulting in corrosion or weakening of the material), while in liquids and gases, removal of the electrical over stress eliminates the discharge, and the material generally recovers its original insulating properties.

PRODUCTION OF CORONA:
                   Actually the corona is noticed at the surface of the conductor which is formed due to the avalanche of free electrons around it which occurs when the electric field intensity at the conductor's surface exceeds a minimum value.
                   In nature the free electrons are naturally produced due to the presence of radioactive elements present in the earth's core.  And also it is formed due to the cosmic shower from the outer space. The free electrons will be energized during the positive cycle of the a.c wave.  And thus the free electrons acquire sufficient energy due to the high electric field intensity of the conductor. These electrons as the name says it has negative charge are get attracted towards the positive charge which occurs at the positive half cycle and the negative half cycle repels the electrons.  The velocity thus attained by the electrons due to the field intensity on the conductor is not too great so the collision of the electrons as followed by the repulsion collide with the gas molecules of the air which is perfectly elastic collision. (i.e) the electron bounces off the air molecule with no transfer of any kind of energy to the atmosphere.  But if the intensity of the electric field exceeds a critical value, the free electrons acquire very high energy such that it can ionize the molecules on the atmosphere.  This phenomenon is called as ionization and the molecule will miss an electron and becomes a positive ion.  The initial electron which lose the energy are accelerated by the electric field and at the next collision, each is capable of ionizing an air molecule.  All this time the electrons are advancing toward the positive electrode after each collision thus creating an avalanche of electrons.  Each avalanche is created or initiated by a single free electron which finds itself in an intense electrostatic field.

TYPES OF CORONA:

  • Visual corona-which appears as a violet colored light coming from the regions of electrical overstress when the discharge line is viewed in the dark.
  • Audible corona- At about 25 kV rms/cm, negative polarity “brush” discharges occur. which appears as a hissing or frying sound whenever the discharge line  is energized above the corona threshold voltage. The sound waves are produced by the disturbances set up in air in the because of the discharge, possibly by the movement of the positive ions as they are suddenly created in an intense electric field. 
  • Radio influence or RI-At a gradient of around 30 kVrms/cm positive polarity plume corona is generated. These avalanches, being electrons in motion, actually constitute electric currents, and as such, produce both magnetic and electrostatic fields in the vicinity. Being formed very suddenly and being of short duration, these magnetic and electrostatic fields can induce high frequency voltage pulses in nearby radio antennas, and hence may cause RI.
  • A glow discharge occurs at a gradient of approximately 20 kV rms/cm. Glow discharge is a light glow off sharp points that does not generate objectionable RIV/TVI or cause any audible noise.
CORONA DETECTION:
            Corona can be visible in the form of light, typically a purple glow, as corona generally consists of micro arcs.  Darkening the environment can help to visualize the corona.  We once attached a camera to a viewing window in a vacuum chamber to confirm that corona was indeed occurring, and thereby confirming our assumptions.
            You can often hear corona hissing or cracking.  Thus, stethoscopes or ultrasonic detectors (assuming you can place them in a safe location) can be used to find corona.  In addition, you can sometimes smell the presence of ozone that was produced by the corona

              The corona discharges in insulation systems result in voltage transients.  These pulses are superimposed on the applied voltage and may be detected, which is required for the detectors to detect the corona discharge.
               It is important that the voltage source and the coupling capacitor exhibit low noise so as not to obscure the corona.  In its simplest form the pulse detection network is a resistor monitored by an oscilloscope

The Formation of Brush and Plume Discharges:

Corona Effects:
 The presence of corona can reduce the reliability of a system by degrading insulation.  While corona is a low energy process, over long periods of time, it can substantially degrade insulators, causing a system to fail due to dielectric breakdown.  The effects of corona are cumulative and permanent, and failure can occur without warning.  Corona causes:  

  • Light  
  • Voltage Source  
  • Coupling Capacitor  
  • Pulse Detection Network  
  •  Ultraviolet radiation 
  • Sound (hissing, or cracking as caused by explosive gas expansions) • Ozone • Nitric and various other acids 
  • Salts, sometimes seen as white powder deposits 
  • Other chemicals, depending on the insulator material 
  • Mechanical erosion of surfaces by ion bombardment • Heat (although generally very little, and primarily in the insulator) 
  •  Carbon deposits, thereby creating a path for severe arcing  
CORONA CALCULATIONS:
          
            The following corona calculations are from Dielectric Phenomena in High Voltage Engineering, F.W. Peek, 1929  
For Concentric Cylinders in Air:
  • Corona will not form when RO / RI < 2.718.  (Arcing will occur instead when the voltage is too high.)  
For Parallel Wires in Air: 
 • Corona will not form when X / r < 5.85.  (Arcing will occur instead when the voltage is too high.)   
For Equal Spheres in Air:
  • Corona will not form when X / R < 2.04.  (Arcing will occur instead when the voltage is too high.) • Arcing difficult to avoid when X / R < 8   
Where
 • RO = Radius of outer concentric sphere 
• RI = Radius of inner concentric sphere
 • R = Sphere radius
 •  r = wire radius  
 • X = Distance between wires or between spheres 

Disadvantages of corona discharge:
The undesirable effects of the corona are:
1. A faint glow appears around the conductor which is visible in the dark.
2. There is an audio noise.
3. There is the tendency in the conductor to vibrate.
4. Corrosion due to ozone and oxides of nitride formation.
5. There is the loss of power.
6. There is radio and television interference
7. Harmonic current flows resulting from corona formation.
PREVENTION:
           Corona can be avoided by minimizing the voltage stress and electric field gradient.  This is accomplished by using utilizing good high voltage design practices, i.e., maximizing the distance between conductors that have large voltage differentials, using conductors with large radii, and avoiding parts that have sharp points or sharp edges.  
          Corona inception voltage can sometimes be increased by using a surface treatment, such as a semiconductor layer, high voltage putty or corona dope.  
          Also, use a good, homogeneous insulator, such as properly prepared silicone and epoxy potting materials work well.  If you are limited to using air as your insulator, then you are left with structure of the conductor.   
          Finally, ensure that steps are taken to reduce or eliminate unwanted voltage transients, which can cause corona to start.


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sudhan Hunt
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7 January 2017 at 08:03 ×

Wow Gud..very clear

Congrats bro sudhan Hunt you got PERTAMAX...! hehehehe...
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