We often notice a faint luminous glow in transmission during the night, accompanied with a sort of hissing noise, UV emissions with RF interference. This luminescent light is especially prevalent in lines operating beyond 100KV. It is a phenomenon called corona and arises because of the ionisation of air. You can recollect that the conductor of overhead line is bare, having only air as the insulator. At a particular operating voltage and electric field strength, air retains its insulation property almost perfectly. But at a specific breakdown strength, ionisation sets in between the conductors. Below 30kv/cm, the air still maintains its insulation property, but beyond this, ionisation persists.
Wikipedia Creative Commons: Corona discharge on the insulator ring of 500 kV overhead power line.
Air is not a perfect insulator. It is made up of small number of electrons and ions. These electrons and ions come into being as a result of ultraviolet radiations from the sun, radioactivity of the soil, cosmic rays. When an electric potential is set up in the air, let us say between two large parallel conducting planes, the electric field will propel the electrons and ion, and a small current will flow between the conducting planes by convection.
At the electric field intensity of 30KV/cm, the current is minimal and so is the electric arc. But beyond 30kv/cm, the electric field intensity reaches a specific value called critical value. At this critical value, the ions has achieved a reasonable high velocity that it can knock off one or more electrons from the neutral molecule. This produces a new electron and another positive ion which is also at the same velocity. They move at the same speed and join in dislodging more electrons and positive ions.
The cumulative effect is the result of ion avalanche. If the field is uniform all around as in the case of parallel plate electrodes, the condition necessary to initiate such avalanche are reached precisely at one time throughout the air. In such case, a complete electric breakdown occurs and the arc is sustained between the electrodes
In the case of a cylindrical conductor, the electric field in the area around the conductor is not uniform. The intensity is highest at the surface of the conductor and decreases in inverse proportion as you approach the center of the conductor. Because of the non-uniformity of the field, the ionization will only affect the layer where the electric field strength is 30KV/cm which is at the surface of the conductor and there would be no ripple effect. Due to this non-uniformity, there would be no complete breakdown of the gap but nonetheless there would be a region of sustained ionization in the vicinity of the conductor.
The ionization is followed by a luminous glow, a hissing noise and the production of ozone. If the conductors are rough and dirty, the spiky and dirty parts will produce a local region of ionization and would be the brightest as compared to the smoother parts. Corona is prevalent in power line of 100KV and above .The ion produced by the corona forms a space charge which are set in motion by the ac field.
The energy necessary for moving the space charge around is derived from the conductor resulting in an energy loss known as corona loss. This energy is given out in the form of heat, light ,sound and chemical action. The rate of ionization is not uniform in overhead lines but varies and fluctuates which invariably induces similar changes of the electric field.
Some of the disadvantages of corona include:
- Corona leads to power loss.
- Production of ozone could lead to corrosion of the overhead line conductors as a result of chemical action.
- There is non-sinusoidal current flow due to corona which in turn leads to non-sinusoidal voltage drop, resulting in inductive interference with communication line in the vicinity.
How to Reduce Corona Effect
- The use of bundled conductor reduces corona which entails the use of more than one conductor per circuit, which offers an increase in the diameter of the conductor.
- Deploying Aluminum conductor steel reinforced (ACSR). The steel reinforcement gives it a more substantial radius as compared to a standard copper conductor.
- Employing smooth conductors while avoiding the use of stranded and rough surface conductors. The uneven surface lowers the breakdown strength at which corona occurs as a result of the field distortion as a result of a local area of ionisation between any two protruding roughened surface within the same conductor.
Some of the advantages of corona include:
- It clamps down the effect of travelling waves as a result of lightning. Lightning induces charges on the line. Due to the corona, there is consequent neutralisation and dissipation of the charge as corona discharge.
- Another advantage of corona is that it increases the virtual diameter of the conductor because the surrounding air conducts thereby reducing the electrostatic stress between the conductors.
PRACTICAL CONSIDERATION:
Corona loss is higher during bad or stormy weather owing to the presence of conductive electron and ion per unit volume of air. This effect increases the corona loss thus decreasing the efficiency of the line.
In hilly areas or regions at high altitude, corona loss is more pronounced due to the decreased value of air density at high altitudes. There is need to take into account during route profiling and stringing of the overhead transmission line, the type of terrain involved. It will help in choosing the conductor type that is best suited for the topography involved. A bundled conductor or aluminium conductor steel reinforced conductors (ACSR) is suitable for hilly areas or at high altitudes. They offer a more substantial radius while lessening the surface field intensity. This practice leads to reduced corona loss.
Flickr Attribution 2.0 Generic CC: A well-spaced conductor on a transmission line could help fix the corona
To reduce the critical intensity, one could increase the spacing between the conductors. Increasing the spacing between the conductors involves using more extended cross arms leading to increased tower height. This practice sets a limitation to the maximum transmission voltage, increased tower height with a commensurate increase in cost.
The non-sinusoidal current that flows between the conductor leads to non-sinusoidal voltage drop with consequent production of large third harmonic which interferes with neighbouring communication circuit line in the vicinity. The use of same support for both the power line and communication line (as in power line carrier), can induce an appreciable voltage in the telephone receiver and care must be taken to avert that. This interference can be avoided by using different structures to support the high voltage line away from the vicinity of such communication circuit.
REFERENCES
- B.R.GUPTA,’’Power System Analysis and Design’’.(2005).S. Chand & Company Ltd.Ram Nagar, New Delhi-110055.
- Kothari D.P. and Nagrath I.J. ‘’Modern Power System Analysis.(2002)’.’Tata McGraw Hill Education Private Limited, New Delhi.
- S.L. Uppal and S. Rao. ‘’Electrical Power Systems’’.(2009). Romesh Chamber,2-B. Nath Market, Nai Sarak Delhi-110006 (India).
- http://www.brainkart.com/article/Inductive-Interference-with-Neighbouring-Communication-Circuits_12363/
- https://www.slideshare.net/vishalgohel12195/disadvantages-of-corona-radio-interference-inductive-interference-between-power-and-communication-lines