91. The effect of Corona in transmission line is
(a) partial breakdown of air,
(b) complete breakdown of air.
(c) sparking between lines.
Answer: (b) complete breakdown of air.
Explanation:
- When an 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.
- When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called corona.
- The phenomenon of corona is accompanied by a hissing sound, production of ozone, power loss and radio interference.
- If the voltage raised higher, the luminous envelope becomes larger and higher following with greater sound and the power loss.
- If the applied voltage is further increased to breakdown value, a flash over will occur between the conductors due to breakdown of air insulation.
92. Which of the following statements is true regarding
(a) Corona takes place at voltage lower than breakdown voltage.
(b) Corona takes place at a voltage higher than breakdown voltage.
(c) Corona is a current phenomenon.
(d) Corona increases the transmission line efficiency.
Answer: (a) Corona takes place at voltage lower than breakdown voltage.
Explanation:
Corona Effect: The phenomenon of ionization of surrounding air around the conductor due to which luminous glow with hissing noise is rise is known as the corona effect.
The disadvantage of Corona:
- Corona causes power loss, due to this power loss, the transmission efficiency reduces.
- Ozone is produced due to Corona. This Ozone may cause degradation of the conductor due to chemical action.
- Due to corona, unwanted signals are generated, these noise signals may interfere with the TV or Radio Signal.
- The corona effect produces the non-sinusoidal signal thus the non-sinusoidal voltage drops occur in the line.
- Corona discharge can be observed as a bluish glow.
Additional Information
- Corona discharge can be reduced by the following methods:
- By increasing the conductor size: The larger the diameter of the conductor, the lesser the corona discharge.
- By increasing the distance between conductors: The larger the conductor spacing, the lesser the corona.
- Using bundled conductors: Using a bundled conductor increases the effective diameter of the conductor. This results in the reduction of the corona discharge.
- Using corona rings: The electric field is greater where the conductor curvature is sharp. Therefore, corona discharge occurs first at the sharp points, edges, and corners. To mitigate this, corona rings are employed at the terminals of very high voltage equipment such as at the bushings of a very high voltage transformer.
93. Which of the following statements regarding corona are true?
1. It causes radio interference.
2. It attenuates lightning surges.
3. It amplifies switching surges.
4. It causes power loss.
5. It is more prevalent in the middle conductor of a transmission line employing a flat conductor configuration.
Select the correct answer using the codes given below.
Codes:
(a) 1, 3, 5
(b) 2, 3, 4
(c) 1, 2, 4.5
(d) 2, 3, 4, 5
Answer:
Explanation:
The phenomenon of ionization of air around the conductor due to which luminous glow with hissing noise is rise is known as the corona effect.
Factors affecting the corona:
- Supply voltage of transmission lines
- Operating frequency of transmission lines
- Spacing between the conductors
- The radius of the conductor
- Condition of conductor surface
- Air Density Factor
Disadvantages of corona discharge:
- The glow appears across the conductor which shows the power loss occurs on it.
- The audio noise occurs because of the corona effect which causes the power loss on the conductor.
- The vibration of the conductor occurs because of the corona effect.
- The corona effect generates the ozone because of which the conductor becomes corrosive.
- Corona power loss reduces the efficiency of the line.
- The radio and TV interference occurs on the line because of the corona effect.
- Corona effect is more prevalent in the middle conductor in a flat conductor configuration.
Important Points:
- Disruptive voltage is the minimum voltage at which the breakdown of air occurs, and corona starts.
- Visual critical voltage is the minimum voltage at which visible corona begins.
- One of the advantages of the corona effect is that it reduces the magnitude of lightning and switching surges
94. Corona is accompanied by
(a) violet visible discharge in darkness
(b) hissing sound.
(c) vibration.
(d) power loss.
(e) radio-interference.
(f) ozone.
(g) all of the above.
Answer: (g) all of the above.
Explanation:
Corona: The phenomenon of violet glow, hissing noise, and production of ozone gas in an overhead transmission line is known as the corona.
Factors Affecting Corona:
Atmosphere:
- As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere.
- In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.
Conductor size:
- The corona effect depends upon the shape and conditions of the conductors.
- Corona loss varied along with the change in the diameter of the conductor.
- 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.
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 a larger distance between conductors reduces the electrostatic stresses 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 formed.
- 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 corona is formed.
Mistake Points
Note that, Corona loss is also dependent on the climate condition.
Additional Information
Advantages of Corona:
- 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 stresses between the conductors.
- Corona reduces the effects of transients produced by surges.
Disadvantages:
- Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
- Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
- The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.
Important Points
- Critical disruptive voltage: It is the minimum phase-neutral voltage at which corona occurs.
- In order that corona is formed, the value of g must be made equal to the breakdown strength of air.
- The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.).
- Visual critical voltage: It is the minimum phase-neutral voltage at which corona glow appears all along the line conductors.
Methods of Reducing Corona Effect:
- It has been seen that intense corona effects are observed at a working voltage of 33 kV or above.
- The corona effects can be reduced by the following methods:
- 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 ACSR conductors that 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.
95. Corona effect can be detected by
(a) presence of ozone detected by odour.
(b) hissing sound.
(c) faint luminous glow of bluish colour.
(d) all of the above.
Answer: (d) all of the above.
Explanation:
- When an 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.
- When the applied voltage exceeds a certain value (critical disruptive voltage), the conductors are surrounded by a faint violet glow called corona.
- It can be detected by a hissing sound, faint luminous, and presence of ozone which is detected by odour.
- The presence of ozone is harmful because it corrodes the conductor.
96. Corona loss increases with
(a) decrease in conductor size and increase in supply frequency.
(b) increase in both conductor size and supply frequency.
(c) decrease in both conductor size and supply frequency.
(d) increase in conductor size and decrease in supply frequency.
Answer: (a) decrease in conductor size and increase in supply frequency.
Explanation:
- Frequency of supply: Corona loss increases as the supply frequency increases
- Air Pressure: In hilly areas, the corona effect is more dominant due to reduced pressure
- By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.
- By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.
97. The good effect of corona on overhead lines is to
(a) increase the line carrying capacity due to conducting ionized air envelope around the conductor.
(b) increase the power factor due to corona loss.
(c) reduce the radio interference from the conductor.
(d) reduce the steepness of surge fronts.
Answer: (d) reduce the steepness of surge fronts.
Explanation:
Advantages of Corona Effects:
- Due to corona across the conductor, the sheath of air surrounding the conductor becomes conductive which rises the conductor diameter virtually.
- This virtual increase in the conductor diameter reduces the maximum potential gradient or maximum electrostatic stress. Thus, the probability of flashover is reduced.
- Effects of transients produced by lightning or electrical surges are also reduced due to the corona effect.
- As, the charges induced on the line by surge or other causes, will be partially dissipated as a corona loss.
- In this way, corona protects the transmission lines by reducing the effect of transients that are produced by voltage surges.
98. Compared with a solid conductor of the same radius, corona appears on a stranded conductor at a lower voltage, because stranding
(a) assists ionization.
(b) makes the current flow spirally about the axis of the conductor.
(c) produces oblique sections to a plane perpendicular to a axis of the conductor.
(d) produces surfaces of smaller radius.
Answer:
Explanation:
- Basic, reason of using stranded conductor is to make the conductor flexible. If we use a single solid conductor.
- It does not have sufficient flexibility and it is difficult to coil a solid conductor.
- Hence, it becomes difficult to transport a single solid conductor of long length over the distance.
Factors Affecting Corona:
Atmosphere:
- As corona is formed due to the ionization of air surrounding the conductors, therefore, it is affected by the physical state of the atmosphere.
- In stormy weather, the number of ions is more than normal, and as such corona occurs at much less voltage as compared with fair weather.
Conductor size:
- The corona effect depends upon the shape and conditions of the conductors.
- Corona loss varied along with the change in the diameter of the conductor.
- 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.
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 a larger distance between conductors reduces the electrostatic stresses 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 formed.
- 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 corona is formed.
99. Corona loss can be reduced by the use of hollow conductors because
(a) the current density is reduced.
(b) the eddy current in the conductor is eliminated.
(c) for a given cross section, the radius of the conductor is increased.
(d) of better ventilation in the conductor.
Answer: (c) for a given cross section, the radius of the conductor is increased.
Explanation:
The corona loss can be reduced by using:
- Conductors with large diameters: The voltage at which the corona occurs can be increased by increasing the size of the conductor and hence, the corona loss can be reduced.
- Hollow conductors: These are used to increase the effective diameter of the conductor without using any additional material. Since corona loss is inversely proportional to the diameter of the conductor, corona loss decreases with an increase in the diameter. For reducing corona loss, we need to increase the diameter of the conductor.
- Making conductors have increased diameter reduces the intensity of the electric field at the surface of the conductor, which reduces corona.
- Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminium conductors (ACSR).
- Bundled conductors: These are made up of two or more sub-conductors and are used as single-phase conductors. When using two or more sub-conductors as one conductor, the effective diameter of the conductor increases, resulting in reduced corona loss.
100. Bundled conductors reduce
(a) surface electric stress of conductor.
(b) increases the line reactance.
(c) decreases the line capacitance.
Answer:
Explanation:
Bundled conductor:
- Bundled conductor are those conductors which form from two or more stranded conductors, bundled together to get more current carrying capacity.
- By using bundle conductors instead of the single conductor in the transmission line increases the GMR of the conductors.
- Hence, by using bundle conductors GMR is increased, so inductance L decreases, and capacitance C increases.
Advantages of Bundled Conductors:
- Bundling of conductors leads to a reduction in line inductance.
- Bundling of conductors leads to increases in capacitance.
- An important advantage of bundled conductors is its ability to reduce corona discharge
- Reduction in the formation of corona discharge leads to less power loss and hence improved transmission efficiency of the line.
- Reduction in communication line interference.
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