Concept of Corona MCQ Quiz in தமிழ் - Objective Question with Answer for Concept of Corona - இலவச PDF ஐப் பதிவிறக்கவும்

The Corona decreases with increasing the diameter of the conductor. A stranded conductor exhibits more corona than a solid conductor, the shape of cross – section is a series of across of circles each of much smaller diameter than the conductor as a whole

Corona Effect:

• 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.

Corona loss:

The power dissipated in the system due to corona discharges is called corona loss. Accurate estimation of corona loss is difficult because of its variable nature. It has been found that the corona loss under fair weather conditions is less than under foul weather conditions.

The corona loss under appropriate weather conditions is given below by the Peek’s formula,

\({P_C} = \frac{{244}}{\delta }\left( {f + 25} \right){\left( {{E_n} - {E_0}} \right)^2}\sqrt {\frac{r}{D}} \times {10^{ - 5}}\frac{{kW}}{{km}}/phase\)

Where Pc – corona power loss

f – Frequency of supply in Hz

δ – Air density factor

En – r.m.s phase voltage in kV

Eo – disruptive critical voltage per phase in kV

r – Radius of the conductor in meters

D – Spacing between conductors in meters

Corona is affected by the following factors:

• The physical state of atmosphere: Corona is formed due to the ionization of air surrounding the conductors. The number of ions is more than normal in the stormy weather.
• The irregular and rough surface cause more corona loss because unevenness of the surface decreases the value of breakdown voltage.
• A larger distance between conductors reduces the electrostatic stresses at the conductor surfaces. It helps in avoiding the formation of the corona.
• The operating voltage
• Air density factor

 

Corona loss is independent of the height of the conductor.

Corona Formation:

• The ionization of air surrounding the high voltage transmission lines causing the conductors to glow, producing a hissing noise, is called Corona Discharge or Corona Effect.
• In transmission lines, conductors are surrounded by the air.
• Air acts as a dielectric medium. When the voltage of air surrounding the conductor exceeds the value of 30kV/cm, the charging current starts to flow through the air, which causes the air to be ionized.
• The ionized air act as a virtual conductor, producing a hissing sound with a luminous violet glow known as the corona.
   

Corona in EHV transmission line:

• Pulseless corona is one of the types of corona discharge that takes place in EHV transmission line conductors.
• The corona current pulses generated by corona discharge are the sources of radio interference from transmission lines.

Concept:

Corona loss (PL)

\({P_L} = 241 \times {10^{ - 5}}\left( {\frac{{f + 25}}{\delta }} \right)\left( {\sqrt {\frac{r}{d}} } \right){\left( {{V_o} - {V_C}} \right)^2}kW/km\)

∴ Corona loss, PL ∝  (f + 25)

Where, 

f = Supply frequency in Hz

r = Radius of conductor

d = distance between two adjacent conductors

Vo = Operating voltage in kV

VC = Critical disrputive voltage in kV

δ = Air density correction factor

Calculation:

Given-

f1 = 50 Hz, f2 = 25 Hz, PL1 = 1 kW / km / phase

\(\begin{array}{l} \Rightarrow \frac{{1}}{{{{\left( {{P_{loss}}} \right)}_{{{60}_{Hz}}}}}} = \frac{{\left( {50 + 25} \right)}}{{\left( {25 + 25} \right)}}\\ \Rightarrow {\left( {{P_{loss}}} \right)_{{{60}_{Hz}}}} = \frac{{\left( {50} \right)\left( {1} \right)}}{{75}} = 0.667\ kW/ph/kM \end{array}\)

Corona:

• This phenomenon of electric discharge occurring in high voltage transmission lines due to complete breakdown of air is known as the corona effect.
• Corona appears in the transmission line when the surface voltage gradient at the line conductor reaches the breakdown voltage of the air.
• The value of the voltage gradient is 30 kV/cm (max) or 21.2 kV/cm (RMS).
• The phenomenon of the corona is accompanied by a hissing sound, production of ozone, power loss, and radio interference.

Concept:

Corona loss (PL)

\({P_L} = 241 \times {10^{ - 5}}\left( {\frac{{f + 25}}{\delta }} \right)\left( {\sqrt {\frac{r}{d}} } \right){\left( {{V_o} - {V_C}} \right)^2}kW/km\)

∴ Corona loss, P_L ∝  (f + 25)

Where, 

f = Supply frequency in Hz

r = Radius of conductor

d = distance between two adjacent conductors

Vo = Operating voltage in kV

VC = Critical disruptive voltage in kV

δ = Air density correction factor

Calculation:

Given that,

f1 = 50 Hz, f2 = 60 Hz, PL1 = 30 kW

\(\begin{array}{l} \Rightarrow \frac{{30\;kW}}{{{{\left( {{P_{loss}}} \right)}_{{{60}_{Hz}}}}}} = \frac{{\left( {50 + 25} \right)}}{{\left( {60 + 25} \right)}}\\ \Rightarrow {\left( {{P_{loss}}} \right)_{{{60}_{Hz}}}} = \frac{{\left( {85} \right)\left( {30} \right)}}{{75}} = 34 \;kW \end{array}\)

Critical Disruptive Voltage

Critical Disruptive Voltage is defined as the minimum phase to the neutral voltage required for the Corona discharge to start.

When the potential gradient at the surface of the conductor becomes greater than the dielectric strength of air, the phenomenon of corona occurs.

The phenomenon of violet glow, hissing sound, and the production of ozone gas in an overhead transmission line is known as the corona.

Visual critical voltage

Visual critical voltage is the minimum phase-neutral voltage at which the visual corona starts or the appearance of the faint luminous glow of violet color all along the transmission line conductors.

Knee voltage

Knee point voltage, also known as diode cut-in voltage, is the minimum voltage at which the diode begins to conduct heavily and the current begins to increase rapidly in a forward-biased state.

Flashover voltage

The voltage at which an electric discharge occurs between two electrodes that are separated by an insulator is known as the flashover voltage.

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.

If V is the phase-neutral potential, then potential gradient at the conductor surface is given by:

\(g = \frac{V}{{rlo{g_e}\left( {\frac{d}{r}} \right)}}\;volt/cm\)

• 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.).

 

Additional Information

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.

Concept:

The Corona effect is a phenomenon that occurs in high-voltage power systems. It's the ionization of the surrounding air or gas by a high-voltage conductor when the electric field strength exceeds a certain critical value, producing a luminescent discharge and possibly resulting in power loss, audible noise, and interference with communication systems.

Virtual increase in conductor's diameter: This is the correct option. The corona effect creates an ionized layer around the conductor which acts as an increase in the effective diameter. This increase in diameter, or 'virtual diameter', reduces the electric field intensity at the conductor surface and therefore helps reduce the corona effect, including corona loss, noise, and interference.

• A virtual increase in the conductor's weight is not an advantage of the corona effect. In fact, the Corona effect has no impact on the physical weight of the conductor.
• Virtual increase in conductor's voltage: The corona effect doesn't provide a virtual increase in the conductor's voltage. Instead, the voltage might be one of the contributing factors causing the Corona effect.
• Virtual increase in conductor's frequency: The corona effect does not virtually increase the frequency of the power transmission. The frequency is a property of the AC power system and is unrelated to the corona effect.

To summarize, the most correct answer to your question is the second option, a 'virtual increase in the conductor's diameter' which is an advantage of the corona effect. The enhanced effective diameter reduces the electric field intensity, therefore reducing corona-related losses and issues. However, it's important to note that the corona effect itself is generally undesirable in power transmission due to the associated power loss and other problems. The increased effective diameter is not so much an advantage of the corona effect as it is a mitigation of some of its negative impacts.