Task 1: Describe the construction and properties of an overhead line.Outcome P1.1

Part 1

b). Define the term “Corona”, with regards to overhead power lines, and explain its effect on line efficiency. Explain also how corona levels can be reduced in practice.

When the word corona is usually mentioned, we typically think of our “Sun”, with the visible glow that can be seen during a solar eclipse. The pinkie-purple glow that surrounds is only seen in 2D, but in reality it is in 3D and covers the entire solar globe. The cause of the corona glow is due to highly ionise Iron (Fe) with reduced electron count (rather than the standard 26 electrons)

Corona, with regards to power transmission cables can also be visualised in a similar way, although we cannot always see it, its presence can be detected. If a power cable were able to seen in 2D whilst “live”, the halo effect would be visible. However, it’s a cable in 3D, therefore it would have to be visualised as a tube or pipe that evenly surrounds it.

An Image of Corona discharges on bundled conductors at 400 kV line-to-ground.

Corona is defined as;

“The radiant luminous discharge due to ionisation of the air in the vicinity of a conductor when the voltage gradient exceeds a certain critical value.”

“The alternating potential between two parallel conductors’ increases beyond a certain limit, a point is reached when a pale violet glow appears on the conductor surface, and accompanied by a hissing sound.”

“When the potential gradient in a particular conductor attains a critical value of 30KV/cm or more, the air in the immediate vicinity of the conductors becomes conducting and a hissing sound is heard along with vibration of the conductor.”

“If the spacing between two conductors is less than their diameters, and if the applied potential between them is gradually increased, a violet glow occurs round the conductors, along with a hissing sound, at a certain value of potential. This phenomenon is known as Corona”

The Manifestations of Corona

The discharges which are produced may be observed in three different ways;

1. Perhaps the best known manifestation is “visual corona” which appears as a violet coloured light coming from the regions of electrical overstress when viewed in the dark.

2. The second manifestation of this discharge is “audible corona”, which appears as a hissing or frying sound whenever the specimen is energised above the corona threshold voltage. The sound waves are produced by the disturbances set up in air in the vicinity of the discharge, possibly by the movement of the positive ions as they are suddenly created in an intense electric field.

3. The third, and perhaps most serious manifestation of this discharge from the point of view of the power company, is the electrical effect which causes radio influence or RI. This is as a result of electrons in motion, and comprise of electric currents. Thus, both magnetic and electrostatic fields in the vicinity are formed. As they are created very suddenly and being of short duration, these magnetic and electrostatic fields can induce high frequency voltage pulses in nearby radio antennas, and hence may cause RI. The generated radio noise is usually called the “radio influence voltage” or RIV of the corona.

The Cause and Nature of Corona

There are always a few free electrons in the air as a result of traces of radioactive materials in the earth’s crust and cosmic ray bombardment of the earth from outer space. As the conductor becomes energised on each half cycle of the AC voltage wave, the electrons in the air near its surface are accelerated by the electrostatic field. These electrons, having a natural negative charge, are accelerated toward the conductor on its positive half cycle and away from the conductor on its negative half cycle.

A Graph to illustrate basis negative corona modes and their regions in typical spacing gap.The velocity attained by a free electron is dependent upon the intensity of the electric field. If the intensity of the electric field is not too great, the collision between an electron and an air molecule such as oxygen (O2) or nitrogen (N2) bounces off the air molecule with no transfer of energy to it.

On the other hand, if the intensity of the electric field exceeds a certain critical value, any free electrons in this field will acquire a sufficient velocity so that during its collision with the air molecule there is sufficient energy to knock one of the outer orbit electrons clear out of one of

the two atoms of the air molecule. This is the phenomenon known as ionisation, and the molecule with the missing electron becomes a positive ion.

The initial electron, which lost most of its velocity in the collision, and the electron knocked out of the air molecule, which also has a low velocity, are both accelerated by the electric field, and at the next collision, each electron is capable of ionizing an air molecule. After the second collision, there are now four electrons to precede, and so on, the number of electrons doubling after each collision. All this time, the electrons are all advancing toward the positive electrode and after many collisions, their number has grown enormously. This is the process by which the so-called electron avalanche is built up, each avalanche being initiated by a single free electron which finds itself in an intense electrostatic field.

Factors Affecting Corona

As a rule of thumb, if the ratio of spacing between conductors to the radius of the conductor is less than 15, flashover will take place between the conductors before corona occurs. In reality, this is unlikely, as in overhead lines the ratio is usually far greater than 15. At a given voltage level, the factors affecting corona include;

Line configuration and Spacing between the conductors. Conductor type and Line Voltage Condition of the conductor surface. (Rough, smooth, uneven, pitted etc.) Weather-Atmosphere

In the horizontal plain, (as in point to point line) the generated magnetic field near the middle conductor is larger than the field near the outside conductors. Therefore, the disruptive critical voltage is lower for the middle conductor, causing larger corona loss than the ones for the two other conductors. If the conductors are not evenly spaced, the surface gradients of the conductors, and therefore the corona losses are not equal.

The disruptive critical voltage can be determined from;

Also, the conductor height affects the corona loss, that is, the greater the height, the smaller the corona loss.

The corona loss is also proportional to frequency, hence the higher the frequency, the higher the losses. However in a DC set-up corona loss is far less than one for AC set-up.

The irregularity of the conductor surface, in terms of scratches, raised strands; die burrs, die grease, and particles of dust and dirt that clog the conductor can significantly increase corona loss. The smoother a surface, of a given cylindrical conductor, the higher the disruptive voltage.

For the same diameter, a stranded conductor is usually satisfactory for about 80%-85% of the voltage of a smooth conductor. The larger the diameter, the less likelihood of corona. Therefore, the use of bundled conductors increases the effective diameter by reducing the electric stress at the conductor surfaces. This is the reason that Aluminium Conductors Steel Reinforced (ACSR) of large cross- section is used for transmission lines. Thus, the spacing

between the parallel conductors can be increased, but this has limit as it will increase cost of support, cross arms etc.

Corona is caused by the ionization of air surrounding the conductors. A stormy weather has more ions and, therefore, gives rise to more corona as compared to a fair weather. The breakdown strength of air varies with atmospheric conditions and is thus directly proportional to the density of air. The air-density factor is defined as;

Where δ = air densityp = barometric pressure in centimetres of mercuryt = ambient temperature in degrees Celsius

And hence barometric pressure is a function of attitude. The below chart is a standard barometric pressure as a function of altitude.

All different types of weather conditions affect corona. Wet conditions, all lower voltage and thus increase corona. (Includes, rain, snow, frost, sleet, and fog.) Rain affects corona loss more than any other factor. E.g. it may cause the corona loss to be produced on a conductor at voltages as low as 65% of the voltage at which the same loss takes place during fair weather. Heavy winds have no effect on the disruptive critical voltage or on the loss, however the presence of smoke or ash, lowers the critical voltage and increase the loss.

Corona in fair weather may be negligible up to a voltage close to the disruptive critical voltage for a particular conductor. Above this voltage, the impacts of corona increase very quickly. I n designing a transmission line, its operating parameters should be to operate at just below the disruptive critical voltage in fair weather, so that corona only takes place during poor adverse atmospheric conditions.

A graph to illustrate Fair weather Corona-Loss curves for standard conductors. Air density, δ = 1

A graph to illustrate wet weather Corona-Loss curves for standard conductor.

Peeks Equation

According to FW Peek, the fair weather corona loss per phase or conductor can be calculated from;

Where f = frequencyV = line to neutral operating voltage in kilovoltsVo = disruptive voltage in kilovolts. (shown on page 3)

(The wet weather corona can be calculated by multiplying Vo by 0.88.)

The correct result can only be if; the frequency is between 25 and 120 Hz, the conductor radius is greater than 0.25cm, the ratio of V to Vo is greater than 1.8

Peek has a variation calculation for Vo, by including a smoothness/roughness factor.

Peek further expanded the calculation for Vo, by including, that in the event that the potential difference (or critical gradient) is further increased; a second point is reached at which a weak luminous glow of colour can be seen around each conductor. peek called this the visual critical voltage.

(as before, these equations are for fair weather, for wet weather, multiply the fair weather voltages by 0.8)

Graphs to illustrate the affect of altitude on corona loss

Graph to illustrate radio influence and corona loss measurement made on a experimental test line by J.S. Carroll, L.H.Brown, and D.P. Dinapoli.

Key Points Factors about Corona;

Corona is partial discharge, which is an electrical discharge that does not bridge the entire space between the two electrodes.

Ionisation of air between conductors occurs. The air between the conductor becomes conducting The minimum voltage at which the corona occurs is called Disruptive Critical Voltage. A faint glow appears around the conductors which is visible in dark There is acoustical noise. (Frying Egg like ) There is a tendency in the conductors to vibrate. Ozone and oxides of nitrogen are produced. There is a loss of power. There is radio interference.

Explain also how corona levels can be reduced in practice.

Corona levels are reduced in three stages;1. At the design stage, before construction.2. Post construction, when issues manifest themselves.3. Continuing preventative and casualty maintenance.

During the design stage, all the points I have mentioned earlier are factored in. Is the line to be constructed fit for purpose?Therefore to minimise corona, the following needs to be known and designed into the project.

Ultimate voltage carrying capacity. Tower design. Tower arm dimensions and insulators. Height of tower arms and conductors. Spacing between towers. Spacing between conductors. Other distribution or transmission networks nearby/crossing. (Overhead and

underground) Underground pipes.(oil, gas water) Transformer sub-station design and capacity. Terrain and land geology. Drainage. Altitude. Longitude especially in the polar regions. (Aurora means more free ions) Climate and local weather patterns. Cable design, type, dimensions and manufactured material. Earth bonding. Harmonic vibration due to wind. Material Creep. Span (Basic, wind, weight)

Post construction;

Conductor spacers. Conductor bundles. Dampers. Clamps. Increasing insulators. Corona Rings. Corona Ball.

An illustration of a 230kV - 2 Conductor Spacer (Courtesy - Burndy, FCI Inc.)

An illustration of 550kV - 4 Conductor Spacer Damper (Courtesy - Slacan, Tridem Inc.)

An illustration of 550kV - 4 Conductor Spacer Damper (Courtesy - Burndy, FCI Inc.)

Continuing preventative and casualty maintenance;

Cut and clean trees, buses, creepers along corridors of transmission lines. Check transmission line conductors, jumpers for their soundness. If found broken, repair

them by using repair sleeves, if more than ten percent strands are found broken replace the conductor

Check for soundness of reactors and capacitors banks connected to transmission lines Check looseness of jumper connections, insulator tie points, etc. Check all insulators, if cracks are found, replace them. Check earth wire for its adequate mechanical strength. Maintain proper sag tension, earth clearance of line conductors. Check across arms and cross arm fixtures for their mechanical strength Paint supporting structures with anti corrosive paints. See that all clamps, nuts and bolt are intact and in good condition Check tower footings and protect the same from land sliding, soil erosion etc. Check conditions of vibration dampers, replace, them, if required Adjust arcing horn gaps to proper value. Check soundness of conductor spacers. Check earth resistance at every tower footing Clean bird nests, darts, chemicals deposited on insulators, cross arms and structures. Determine load on each phase in regular interval and balance the load in three phases

by shifting the load from the heavily loaded phase to lightly loaded phase. Check voltage regulation at consumer premises. If voltage dip is more than allowed

value, necessary arrangements should be made to reduce voltage drop by installing additional distribution transformer (s).

Wherever possible provide LT less service to small groups of consumers by installing 3 to 5 kva, 11/0.4 kv transformers mounted on HT (11 kv) line supports.

The conventional overhead weather proof domestic / commercial service connection wires should be replaced by under-ground cables having conductor material same as that of LT distribution line conductors wherever feasible.

Check vertically of line supports and associated structures. Check tightness of stay wires and soundness of stay insulators. Replace cracked, broken

line insulators.

Maintain ground, structure and building clearance of line conductors and other live parts.

Clean bird nests, darts and chemicals from cross arms and insulators Check mechanical soundness of conductors, earth wires, C.I. Blocks, bolts, clamps, nuts, supports,

cross arms etc. Paint all metallic parts with anti-corrosive paints. Check alignment and soundness of AB switches. Check proper functioning of horn gaps and lighting arresters. Check for adequate dielectric strength (BDV) of distribution transformers oil. Check HT and LT fuse carriers, replace them if found defective.

There are also maintenance contractors that provide specialist overhead camera work on aircraft, which can work in daylight.

Photographs overhead camera work

This type of detection system can provides a simultaneous video image of an infrared spot and corona. This information assists the user in determining the location of the hot spot and/or corona and also, by a close examination, it’s possible cause. Since the video provides a moving image, it is possible to determine the seriousness of the hotspot (i.e. how the pattern develops over time), the characteristics of the corona source and to identify the type in question.

References.

http://www.tpub.com/content/nasa2000/NASA-2000-tm210077/NASA-2000-tm2100770010.htm

http://encyclopedia2.thefreedictionary.com/Corona+Losses

http://en.wikipedia.org/wiki/Corona_discharge

http://en.wikipedia.org/wiki/Peek's_law

http://www.allinterview.com/showanswers/117212.html

Development of Corona Loss Formula. A.I.E.E Transactions Vol. 52 W.S Peterson

Corona Loss Measurements on a 220 KV, 60 Cycle, Three-Phase Experimental Line. A.I.E.E. Transactions Vol. 50, 1931. J.S. Carroll, L.H.Brown, D.P. Dinapoli.

Corona Loses from Conductors 1.4-inch Diameter. A.I.E.E. Transactions Vol. 53, 1934.J.S.Caroll, B.Cozzens, T.M.Blakelee.

Corona Losses at 230 KV with one conductor grounded. A.I.E.E. Transactions Vol. 54, 1935.J.S.Caroll, D.M.Simmons.

Empirical Method of Calculating Corona Loss from High-Voltage Transmission Lines. A.I.E.E. Transactions Vol. 56, 1937.J.S.Caroll, M.M.Rockwell.

What is Corona?Hubbell Power Systems Bulletin EU 1234-H

Suppressing Corona on HV Transmission linesGene S. Ng.

Dielectric Phenomena in High Voltage EngineeringF.W.Peek.

Mirotech Limited

A.I.E.E. stands for The American Institute of Electrical Engineers