CORONA

                                         CORONA

WHAT IS CORONA?

WHEN THE ALTERNATING POTENTIAL DIFFERENCE IS APPLIED ACROSS TWO CONDUCTORS WHOSE SPACING IS LARGE AS COMPARED TO THEIR DIAMETERS, THERE IS NO APPARENT CHANGE IN THE CONDITION OF ATMOSPHERIC AIR SURROUNDING THE WIRES IF THE APPLIED VOLTAGE IS LOW. HOWEVER, WHEN THE APPLIED VOLTAGE EXCEEDS A CERTAIN VALUE, THEN IS CALLED A CRITICAL DISRUPTIVE VOLTAGE, THE CONDUCTORS ARE SURROUNDED BY A FAINT VIOLET GLOW CALLED CORONA.

# CORONA

WHEN THE PHENOMENON OF CORONA IS ACCOMPANIED BY A 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 ARE THE SOUND, THE POWER LOSS, AND THE RADIO NOISE. IF THE APPLIED VOLTAGE IS INCREASED TO THE BREAKDOWN VALUE, A FLASH-OVER WILL OCCUR BETWEEN THE CONDUCTORS DUE TO THE BREAKDOWN OF AIR INSULATION.

THE PHENOMENON OF VIOLET GLOW, HISSING NOISE, AND PRODUCTION OF OZONE GAS IN AN OVERHEAD TRANSMISSION LINE KNOWN AS CORONA.

IF THE CONDUCTORS ARE POLISHED AND SMOOTH, THE CORONA GLOW WILL BE UNIFORM THROUGHOUT THE LENGTH OF THE CONDUCTORS, OTHERWISE, THE ROUGH POINTS WILL APPEAR BRIGHTER, WITH DC VOLTAGE, THERE IS A DIFFERENCE IN THE APPEARANCE OF THE TWO WIRES. THE POSITIVE WIRE HAS A UNIFORM GLOW ABOUT IT, WHILE THE NEGATIVE CONDUCTOR HAS A SPOTTY GLOW.

FORMATION OF CORONA

WHEN SOME ISOLATION IS ALWAYS PRESENT IN THE AIR DUE TO COSMIC RAYS, ULTRA-VIOLET RADIATIONS, AND RADIOACTIVITY. THEREFORE, UNDER NORMAL CONDITIONS, THE AIR AROUND THE CONDUCTORS CONTAINS SOME IONISED PARTICLES(i.e., FREE ELECTRONS AND POSITIVE IONS) AND NEUTRAL MOLECULES. WHEN POTENTIAL DIFFERENCE IS APPLIED BETWEEN THE CONDUCTORS, A POTENTIAL GRADIENT IS SET UP IN THE AIR WHICH WILL HAVE MAXIMUM VALUE AT THE CONDUCTOR SURFACES. UNDER THE INFLUENCE OF A POTENTIAL GRADIENT, THE EXISTING FREE ELECTRONS ACQUIRE GREATER VELOCITIES. THE GREATER THE APPLIED VOLTAGE, THE GREATER THE POTENTIAL GRADIENT, AND MORE IS THE VELOCITY OF FREE ELECTRONS.

FACTORS AFFECTING CORONA

THE PHENOMENON OF CORONA IS AFFECTED BY THE PHYSICAL STATE OF THE ATMOSPHERE AS WELL AS BY THE CONDITIONS OF THE LINE. THE FOLLOWING ARE THE FACTORS UPON WHICH CORONA DEPENDS:-

1. ATMOSPHERE
2. SPACING BETWEEN CONDUCTORS
3. LINE VOLTAGE
4. CONDUCTOR SIZE

# ATMOSPHERE

CORONA IS FORMED DUE TO THE IONISATION OF AIR SURROUNDING THE CONDUCTORS, THEREFORE, IT IS AFFECTED BY THE PHYSICAL STATE OF THE ATMOSPHERE. IN THE STORMY WEATHER, THE NUMBER OF IONS IS MORE THAN NORMAL, AND AS SUCH CORONA AT MUCH LESS VOLTAGE AS COMPARED WITH FAIR WEATHER.

# SPACING BETWEEN CONDUCTORS

IF THE SPACING BETWEEN THE CONDUCTORS IS MADE VERY LARGE AS COMPARED TO THEIR DIAMETERS, THERE MAY NOT BE ANY CORONA  EFFECT. IT IS BECAUSE THE LARGE DISTANCE BETWEEN CONDUCTORS REDUCES THE ELECTROSTATICS AT THE CONDUCTOR SURFACE, THUS AVOIDING CORONA FORMATION.

# LINE VOLTAGE

THE LINE VOLTAGE GREATLY AFFECTS CORONA. IF IT IS LOW, THERE IS NO CHANGE IN THE CONDITION OF AIR SURROUNDING THE CONDUCTORS AND HENCE NO CORONA IS FOUND. HOWEVER, IF THE LINE VOLTAGE HAS SUCH A VALUE THAT ELECTROSTATIC STRESSES DEVELOPED AT THE CONDUCTOR SURFACE MAKE THE AIR AROUND THE CONDUCTOR CONDUCTING, THEN CORNA IS FORMED.

# CONDUCTOR SIZE

THE CORONA EFFECT DEPENDS UPON THE SHAPE AND CONDITIONS OF THE CONDUCTORS. THE ROUGH AND IRREGULAR SURFACE WILL GIVE RISE TO MORE CORONA BECAUSE UNEVENNESS OF THE SURFACE DECREASES THE VALUE OF BREAKDOWN VOLTAGE. THUS A STRANDED CONDUCTOR HAS AN IRREGULAR SURFACE AND HENCE GIVES RISE TO MORE CORONA THAN A SOLID CONDUCTOR.

ADVANTAGES AND DISADVANTAGES OF CORONA

ADVANTAGES

1. CORONA REDUCES THE EFFECTS OF TRANSIENTS PRODUCED BY SURGES.

2. DUE TO CORONA FORMATION, THE AIR SURROUNDING THE CONDUCTOR BECOMES CONDUCTING AND HENCE VIRTUAL DIAMETER OF THE CONDUCTOR IS INCREASED. THE INCREASED DIAMETER REDUCES THE ELECTROSTATIC STRESS BETWEEN THE CONDUCTORS.

DISADVANTAGES

1. CORONA IS ACCOMPANIED BY A LOSS OF ENERGY. THIS AFFECTS THE TRANSMISSION EFFICIENCY OF THE LINE.

2. OZONE IS PRODUCED BY CORONA AND MAY CAUSE CORROSION OF THE CONDUCTOR DUE TO CHEMICAL ACTION.

METHODS OF REDUCING CORONA EFFECT

1. BY INCREASING CONDUCTOR SIZE

2. BY INCREASING CONDUCTOR SPACING

# BY INCREASING CONDUCTOR SIZE

BY INCREASING CONDUCTOR SIZE, THE VOLTAGE AT WHICH CORONA OCCURS IS RAISED AND HENCE CORONA EFFECTS ARE CONSIDERABLY REDUCED. THIS IS ONE OF THE REASONS THAT CONDUCTORS WHICH HAVE A LARGER CROSS-SECTIONAL AREA ARE USED IN TRANSMISSION LINES.

# BY INCREASING CONDUCTOR SPACING

BY INCREASING THE SPACING BETWEEN CONDUCTORS, THE VOLTAGE AT WHICH CORONA OCCURS IS RAISED, AND HENCE CORONA EFFECTS CAN BE ELIMINATED. HOWEVER, SPACING CANNOT BE INCREASED TOO MUCH OTHERWISE THE COST OF SUPPORTING STRUCTURE MAY INCREASE TO A CONSIDERABLE EXTENT.