Transmission and Distribution Objective Questions with Easy Explanation Part-11

 101. Corona loss in a transmission line is dependent on

(a) diameter of the conductor. 

(h) material of the conductor.

(c) height of the conductor.

 

Answer: (a) diameter of the conductor. 

 

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.

 

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.
  • Consider two conductors of radius r cm and spaced d cm apart.
  • 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.

 

102. Corona occurs between two transmission conductors when they 

(a) have high potential differences. 

(b) air closely spaced.

(c) carry dc power.     

(d) both (a) and (b).

 

Answer: (d) both (a) and (b).

 

Explanation:

  • This phenomenon occurs when the electrostatic field across the transmission line conductors produces the condition of potential gradient.
  • The air gets ionized when the potential gradient at the conductor surface reaches the value of 30kV/cm at normal pressure and temperature.
  • If the voltage applied between two lines separated by some distance is greater than a certain value then we can see sparks or violet glow appearing.
  • 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, called critical disruptive voltage, the conductors are surrounded by a faint violet glow called Corona.
  • The phenomenon of the 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 the production of ozone gas in an overhead transmission line is known as the 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 d.c. 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.

 

103. In humid weather, the corona occurs at a voltage 

(a) much less than that needed in fair weather. 

(b) much higher than that needed in fair weather. 

(n) equal to that needed in fair weather.

(d) none of the above.

 

Answer: (a) much less than that needed in fair weather. 

 

Explanation:

  • The phenomenon of violet glow, hissing noise, and the production of ozone gas in an overhead transmission line is known as the corona.
  • The voltage at which the self-sustained discharge will be initiated is called the critical disruptive voltage.
  • Effect of atmospheric factors on corona occurrence:
  • If the critical disruptive voltage is less that means the corona initiation takes place at low voltage. So, any factor that is affecting the critical disruptive voltage that can affect the occurrence of the corona.
  • From the formula of critical disruptive voltage, we can observe that it is directly proportional to the air density factor (δ ) that is if 'δ ' increases as a result Vd increases so that the occurrence of corona will take place at higher voltages, so we can reduce the occurrence corona by increasing air density factor δ.
  • The air density factor depends on both temperature and pressure, In the hilly area, the falling of pressure is very high when compared to the falling of temperature. So 'δ' will be less in a hilly area and the chances of occurrence of the corona are more.
  • Deposition of dust, ice, snow on the surface of the conductor, which will reduce the Vd.

 

Important Points

  • Humidity is the addition of water vapor to air that reduces the density of the air (δ), which may at first appear counter-intuitive.
  • This occurs because the molar mass of water is less than the molar mass of dry air.
  • And during the humid weather ion concentration in air will decrease.
  • As the value of the air density factor is reduced to the humid weather condition as a result there are maximum chances of the occurrence of the corona. 

 

104. The effect of the corona in power system is

(a) increased energy loss. 

(b) increased reactance. 

(c) increased inductance.       

(d) all of the above

 

Answer: (a) increased energy loss. 

 

Explanation:

  • There is a hissing noise with violet glow phenomenon termed as corona effect which is commonly observed in high voltage transmission lines. The corona effects leads to high voltage drop and energy loss along with release of ozone gas.
  • 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.

 

105. The charging current in a transmission line increases due to corona effect because corona increases

(a) line current.          

(b) effective line voltage

(c) power loss in lines. 

(d) the effective conductor diameter.

 

Answer: (d) the effective conductor diameter.

 

Explanation:

Ferranti Effect: At no load (or) at light load, the voltage at the receiving end of the transmission line is more than the sending voltage. It is known as the Ferranti effect. It is due to the charging current of the line.

Skin Effect: The tendency of alternating current to concentrate near the surface of the conductor is known as skin effect. The skin effect depends on the following factors.

a) Frequency

b) Diameter of the conductor

c) Shape of the conductor

The skin effect increases with the increase of cross section, permeability and supply frequency. It reduces with the increase in resistivity of the conductor material.

Proximity Effect: The alternating flux in a conductor is caused by the current of the other nearby conductor. This flux produces a circulating current or eddy current in the conductor which results in an apparent increase in the resistance of the wire. Thus, more power losses in the windings. This phenomenon is called the proximity effect.

 

Corona Effect:

  • 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 discharging current in a transmission line increases due to corona effect because corona increases the effective diameter. So that the capacitance will be increases and inductance will be reduced.

 

106. The chances of occurrence of corona are maximum during

(a) humid weather.     

(b) dry weather.

(c) winter.      

(d) hot summer.

 

Answer: (a) humid weather.

 

Explanation:

Effect of atmospheric factors on corona occurrence:

  • If the critical disruptive voltage is less that means the corona initiation takes place at low voltage. So, any factor that is affecting the critical disruptive voltage that can affect the occurrence of the corona.
  • From the formula of critical disruptive voltage, we can observe that it is directly proportional to the air density factor (δ ) that is if 'δ ' increases as a result Vd increases so that the occurrence of corona will take place at higher voltages, so we can reduce the occurrence corona by increasing air density factor δ.
  • The air density factor depends on both temperature and pressure, In the hilly area, the falling of pressure is very high when compared to the falling of temperature. So 'δ' will be less in a hilly area and the chances of occurrence of the corona are more.
  • Deposition of dust, ice, snow on the surface of the conductor, which will reduce the Vd.

 

Important Points

  • Humidity is the addition of water vapor to air that reduces the density of the air (δ), which may at first appear counter-intuitive.
  • This occurs because the molar mass of water is less than the molar mass of dry air.
  • And during the humid weather ion concentration in air will decrease.
  • As the value of the air density factor is reduced to the humid weather condition as a result there are maximum chances of the occurrence of the corona. 

 

107. Corona is likely to occur maximum in case of

(a) distribution lines. 

(b) transmission lines.

(c) domestic wiring.  

(d) service mains.

 

Answer: (b) transmission lines.

 

Explanation:

If the spacing between the conductors is small, flashover may take place between the conductors without any hissing noise and glow. It is because the distance between the conductors being smaller; there is no time for the glow to occur. 

Following are the leading causes of corona effect in transmission lines:

  • Conductors shape: The corona effect in transmission lines depends upon the shape and conditions of the conductor. The rough and irregular surface will give rise to more corona. It is because the unevenness of the surface decreases the value of breakdown voltage. Similarly, a stranded conductor has an irregular surface and hence gives rise to more corona than a solid conductor having a smooth surface.
  • Atmosphere: Since corona is caused due to ionization of air surrounding the conductor, therefore, it is affected by the physical state of the atmosphere. In the stormy weather, the number of ions is more than normal, and corona occurs 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 radii (r), there may not be any corona effect. It is because larger spacing reduces the electrostatic stresses at the conductor surface.
  • Line voltage: Line voltage significantly affects the corona effect. If it is low, there is no change in the condition of air surrounding the conductor and hence no corona. However, if the line voltage has such a value that electrostatic stresses developed at the conductor surface make the air around the conductor conducting, the corona effect appears.

 

108. Corona is affected by

(a) size of conductor.

(b) shape and surface condition of the conductor.

(c) operating voltage.

(d) all of the above.

 

Answer: (d) all of the above.

 

Explanation:

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.

109. The only advantage of corona is that it

(a) produces a pleasing luminous glow.

(b) makes line current sinusoidal.

(c) works as a safety valve for surges.

(d) ozone gas is produced.

 

Answer: (c) works as a safety valve for surges.

 

Explanation:

  • Due to the formation of the corona, the air surrounding the conductor becomes conducting. It increases the virtual diameter of the conductor. The increased diameter decreases the electrostatic stresses between the conductors.
  • The corona effect reduces electrostatic stresses between the conductors. This reduces the probability of flashover and improves system performance.
  • Corona reduces the effects of transients produced by lightning.

 

The undesirable effects of the corona are:

  • The glow appear across the conductor which shows the power loss occur on it.
  • The audio noise occurs because of the corona effect which causes the power loss on the conductor.
  • The vibration of conductor occurs because of corona effect.
  • The corona effect generates the ozone because of which the conductor becomes corrosive.
  • The corona effect produces the non-sinusoidal signal thus the non-sinusoidal voltage drops occur in the line.
  • The corona power loss reduces the efficency of the line.
  • The radio and TV interference occurs on the line because of corona effect.

 

Minimizing corona:

Corona decreases the efficiency of transmission lines. Therefore, it is necessary to minimize corona.

The following factors may be considered to control corona: 

  • Conductor diameter – For reducing corona loss, this method of increasing conductor diameters is very effective. Diameters of conductors can be increased by using hollow conductors and by using steel-cored aluminum conductors(ACSR) conductors.
  • The voltage of the line – Voltage of transmission lines is fixed by economic considerations. To increase the disruptive voltage the spacing of the conductors is to be increased, but this method has some limitations.
  • Spacing between conductors – If the space between conductors increases, then the voltage drops between them also increases due to increase in inductive reactance.

 

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.


110. Presence of ozone owing to corona

(a) improves the pf.   

(b) reduces the pf.

(c) corrodes the material.      

(d) improves regulation.

 

Answer: (c) corrodes the material.

 

Explanation:

The phenomenon of ionisation of surrounding air around the conductor due to which luminous glow with hissing noise is rise is known as the corona effect.

The following are the factors affecting the corona;

  • Effect of supply voltage –  If the supply voltage is high corona loss is higher in the lines. In low-voltage transmission lines, the corona is negligible, due to the insufficient electric field to maintain ionization.
  • The condition of conductor surface – If the conductor is smooth, the electric field will be more uniform as compared to the rough surface. The roughness of conductor is caused by the deposition of dirt, dust and by scratching, etc. Thus, rough line decreases the corona loss in the transmission lines.
  • Air Density Factor – The corona loss in inversely proportional to air density factor, i.e., corona loss, increase with the decrease in density of air. Transmission lines passing through a hilly area may have higher corona loss than that of similar transmission lines in the plains because in a hilly area the density of air is low.
  • Effect of system voltage – Electric field intensity in the space around the conductors depends on the potential difference between the conductors. If the potential difference is high, electric field intensity is also very high, and hence corona is also high. Corona loss, increase with the increase in the voltage.
  • The spacing between conductors – If the distance between two conductors is much more as compared to the diameter of the conductor than the corona loss occurs in the conductor. If the distance between them is extended beyond certain limits, the dielectric medium between them get decreases and hence the corona loss also reduces.

 

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.


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