Electrical Corona Effect
Electrical corona effect occurs due to ionization of surrounding air around the conductor. The ionization of air results in distortion of electric field resulting in producing tiny electrical discharge. This electrical discharge is referred as corona effect. Engineering companies are trying to reduce corona effect because in addition to low humming noise it results in power loss. Corona causes:
- Humming sound
- Ultraviolet radiation
- Nitric and various other acids
- Salts, sometimes seen as white powder deposits
- Mechanical erosion of surfaces by ion bombardment
- Other chemicals, depending on the insulator material
- Carbon deposits, thereby creating a path for severe arcing
Corona usually occurs in all type of transmission lines but it becomes more prominent in case of high voltages.
Formation of electrical corona effect
Air is not a perfect insulator, means under condition it still have free electrons and ions. As the electric field intensity creates between adjacent conductors, those electrons and ions gets energized. Thus, experiences stress upon them. Because of this effect, the electrons and ions accelerates and starts moving. These ions/electrons during their motion collides with other uncharged molecules leaving them energized. Thus, the number of charged particles goes on increasing resulting in increase of air conduction between adjacent conductors until a point reaches when an arc forms between those conductors.
Affecting factors of electrical corona effect
As mentioned above, line voltage is the main determining factor for corona effect in transmission lines. At low level of voltage, there is no electrical discharge hence no corona effect. In case of high voltages, corona effects increases due to high air ionization. The following are the few major affecting factors for corona effect.
1) Line Voltage
The line voltage directly impacts corona and the corona loss. For lower voltages, corona loss may be ignored but it gets prominent in case of high voltage lines resulting in high corona loss due to high electric field intensity.
2) Surface Condition
Corona effect depends on surface condition of conductors. Rough and uneven surface gives more corona loss because break down voltage decreases due to uneven surface. The disruptive voltage value is less and corona effect is dominant. The surface of stranded conductor has an uneven surface thus has more corona effect than solid conductors.
3) Atmospheric Condition
The atmospheric pleasure and temperature adversely control the presence of corona. The expression of corona loss
implies that it is a function of the air density correction factor value of δ. The lower the value of δ the higher the
loss. The pressure and temperature together decide the value of air density which affects critical disruptive voltage
and the corona loss. The lower the value of critical disruptive voltage the higher the corona loss. For lower pressure
and higher temperatures, the value of critical disruptive voltage will be small and corona effect and loss is dominant. Hence in mountain areas the corona loss is high. Corona is affected by the physical state of the atmosphere. In stormy weather conditions, dusty and rainy conditions, number of free electrons and ions are more hence disruptive voltage is lower. This increases the corona loss considerably.
4) Supply Frequency
Corona loss is affected by high frequency. Peek’s equation states that, higher the frequency higher will be the corona loss. Corona loss in dc lines is lower as compared to loss in ac lines.
5) Load Current
Corona loss decreases with the increase in load current. Due to increase in current, the temperature of conductor arises that do not allow moisture surrounding it. Thus minimizing air conductivity round the conductor that result in reducing corona formation.
6) Spacing Between Conductors
Corona loss occurs when the distance between conductors is more as compared to the diameter of conductor. The dielectric medium between the conductor decreases when the distance between conductor extend beyond a certain limit hence reduces corona loss.
7) Conductor Profile
The structure of conductor also effect corona up to certain extend. The electric field uniformly distributes over the cylindrical shaped conductor that reduces corona loss as compared to other shaped conductors.
Effect of corona
1) Audible Noise
Due to corona, EHV lines generates audible noise which is specially high during foul weather.
2) Radio Interface
Corona interference to radio broadcast in the range of 0.5 MHz to 1.6 MHz. In addition to corona generated on line conductors, there are spark discharges from broken insulators and loose wires which interfere with TV reception in the 80 – 20 MHz range. Corona on conductors also causes interference to Carrier Communication and Signaling in the frequency range 30-500 KHz.
Methods to reduce corona effect
Corona affects negatively on the efficiency of transmission lines. Therefore, it is mandatory to reduce it. The following techniques can be applied to reduce corona effect:
- By increasing conductor diameter, corona effect can be reduced effectively.
- By using corona rings, to distribute charge across a wider area, thereby reducing the electric field and the
resulting corona discharges
- By increasing spacing between conductors, corona discharge effect can be reduced.
- Due to air ionization around the conductor, the virtual diameter of conductor increases hence reduces electrostatic stresses between adjacent conductors.
- Corona reduces surge transient effect.
- Corona results in power loss.
- Due to ozone formation round the conductor, conductor becomes corrosive.
- Corona results in radio and television signal interface.