If you can't explain it simply, then you don't know it well enough. — Albert Einstein

Corona discharge - its effects and methods of reducing it

Electrical power transmission generally deals with very high voltages as a bulk amount of electrical energy has to be transmitted from generating stations to load centers. At this much high voltages, an effect called as corona effect is introduced. As this corona effect results in loss of electrical energy, it is also called as corona discharge.

What is Corona effect in transmission line?

Have you ever heard a hissing noise when standing below a high voltage transmission line? That hissing noise is due to the corona discharge. Corona discharge is usually accompanied by a hissing or cracking audible noise, visual violet glow, production of ozone gas around the conductor, power loss and radio interference.
When a potential difference is applied between two conductors, a potential gradient (or electric field) is set up in the air. This potential gradient is maximum at the surfaces of the conductors. Under the influence of this potential gradient, existing free electrons in the air acquire greater velocities. Some free electrons are always present in the air due to cosmic rays, UV radiations etc.. Greater the applied voltage, greater the potential gradient and, hence, greater the velocity of free electrons.
When the potential gradient at conductor surfaces is large enough (about 30 kV/cm), existing free electrons strike neutral air molecules with enough velocity to dislodge one or more electrons from it. Hence, cumulative ionization of the air near the conductor surfaces occurs. Ionized air is partially conductive. Electric discharge occurs due to the ionized air which results in corona. And if the conductors are close enough to each other, the air insulation breaks down and electric discharge occurs through a spark.
The minimum phase-neutral voltage at which corona starts to occur is called as critical disruptive voltage. And, the minimum phase-neutral voltage at which visual corona glow appears all along the conductors is called as visual critical voltage.

Factors affecting corona

  • Atmosphere: As it is already explained that the corona forms due to ionization of the air. There are always some free electrons in the air (which means air is pre-ionized to a little extent). However, in stormy weather, the number of free electrons is more than that in normal conditions. In such case, corona occurs at much lesser voltage.
  • Conductor size: Corona discharge also depends on the shape and size of the conductors. Irregularities on the conductor surface concentrate the electric field at locations, resulting in corona at these spots. Thus, a stranded conductor gives rise to more corona than a solid conductor with a smooth surface. Also, conductors having large diameter have lower electric field gradient at the surface. Hence, conductors having large diameter produce lower corona than small-diameter conductors.
  • Spacing between the conductors: Larger distance between the conductors reduces the electric stresses between them. And, hence, larger the distance between conductors, lesser the corona formation.
  • Line voltage: As it is already explained, lesser the line voltage, lesser the ionization of surrounding air. Corona discharge starts to occur when the voltage becomes greater than a minimum critical voltage called as critical disruptive voltage.

How to reduce the corona discharge?

Corona discharge is always accompanied by power loss (which is dissipated in the form of sound, light, heat and chemical action). Though it accounts for a small percentage of total losses, power loss due to corona becomes significant in foul or wet weather conditions. Corona discharge can be reduced by the following methods:
  • By increasing the conductor size: As explained above, larger the diameter of the conductor, lesser the corona discharge.
  • By increasing the distance between conductors: Larger the conductor spacing, lesser the corona.
  • Using bundled conductors: Using a bundled conductor increases the effective diameter of the conductor. This results in 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 equipments such as at the bushings of a very high voltage transformer (Corona discharge also occurs in high voltage equipment). A corona ring is electrically connected to the high voltage conductor, encircling the points where corona discharge may occur. This significantly reduces the potential gradient at the surface of the conductor, as the ring distributes the charge across a wider area due to its smooth round shape. Corona rings are employed at the top and bottom of the string insulators of a high voltage transmission line.
corona rings to reduce corona discharge
Corona Rings
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Types of Conductors used in Overhead Power Lines

A conductor is one of the most important components of overhead lines. Selecting a proper type of conductor for overhead lines is as important as selecting economic conductor size and economic transmission voltage. A good conductor should have the following properties:
  • high electrical conductivity
  • high tensile strength in order to withstand mechanical stresses
  • relatively lower cost without compromising much of other properties
  • lower weight per unit volume

Conductor Materials

Copper was the preferred material for overhead conductors in earlier days, but, aluminium has replaced copper because of the much lower cost and lighter weight of the aluminium conductor compared with a copper conductor of the same resistance. Following are some materials that are considered to be good conductors.
  • Copper: Copper has a high conductivity and greater tensile strength. So, copper in hard drawn stranded form is a great option for overhead lines. Copper has a high current density which means more current carrying capacity per unit cross-sectional area. Therefore, copper conductors have relatively smaller cross-sectional area. Also, copper is durable and has high scrap value. However, due to its higher cost and non-availability, copper is rarely used for overhead power lines.
  • Aluminium: Aluminium has about 60% of the conductivity of copper; that means, for the same resistance, the diameter of an aluminium conductor is about 1.26 times than that of a copper conductor. However, an aluminium conductor has almost half the weight of an equivalent copper conductor. Also, tensile strength of aluminium is less than that of copper. Considering combined factors of cost, conductivity, tensile strength, weight etc., aluminium has an edge over copper. Therefore, aluminium is being widely used for overhead conductors.
  • Cadmium-copper: Cadmium-copper alloys contain approximately 98 to 99% of copper and up to 1.5% of cadmium. Addition of about 1% of cadmium to copper increases the tensile strength by up to 50% and the conductivity is reduced only by about 15%. Therefore, cadmium-copper conductors can be useful for exceptionally long spans. However, due to high cost of cadmium, such conductors may be uneconomical in many cases.
  • Other materials: There are many other metals and alloys that conduct electricity. Silver is more conductive than copper, but due to its high cost, it is not practical in most of the cases. Galvanised steel may also be used as a conductor. Although steel has very high tensile strength, steel conductors are not suitable for transmitting power efficiently due to the poor conductivity and high resistance of steel. High strength alloys such as phosphor-bronze may also be used sometimes at extreme conditions.

Types of Conductors

As it is already mentioned above, aluminium conductors have an edge over copper conductors considering combined factors of cost, conductivity, tensile strength, weight etc. Aluminium conductors have completely replaced copper conductors in overhead power lines because of their lower cost and lower weight. Though an aluminium conductor has larger diameter than that of a copper conductor of same resistance, this is actually an advantage when 'Corona' is taken into consideration. Corona reduces considerably with increase in the conductor diameter. Following are four common types of overhead conductors that are used for overhead transmission and distribution to carry generated power from generating stations to the end users.
Generally, all types of conductors are in stranded form in order to increase the flexibility. Solid wires, except for very small cross sectional area, are very difficult to handle and, also, they tend to crystallize at the point of support because of swinging in winds.
  1. AAC : All Aluminium Conductor
  2. AAAC : All Aluminium Alloy Conductor
  3. ACSR : Aluminium Conductor, Steel Reinforced
  4. ACAR : Aluminium Conductor, Alloy Reinforced

AAC : All Aluminium Conductor

This type is sometimes also referred as ASC (Aluminium Stranded Conductor). It is made up of strands of EC grade or Electrical Conductor grade aluminium. AAC conductor has conductivity about 61% IACS (International Annealed Copper Standard). Despite having a good conductivity, because of its relatively poor strength, AAC has limited use in transmission and rural distribution lines. However, AAC can be seen in urban areas for distribution where spans are usually short but higher conductivity is required.

AAAC : All Aluminium Alloy Conductor

These conductors are made from aluminium alloy 6201 which is a high strength Aluminium-Magnesium-Silicon alloy. This alloy conductor offers good electrical conductivity (about 52.5% IACS) with better mechanical strength. Because of AAAC's lighter weight as compared to ACSR of equal strength and current capacity, AAAC may be used for distribution purposes. However, it is not usually preferred for transmission. Also, AAAC conductors can be employed in coastal areas because of their excellent corrosion resistance.

ACSR : Aluminium Conductor, Steel Reinforced

ACSR conductor (Aluminium Conductor Steel Reinforced)
ACSR consists of a solid or stranded steel core with one or more layers of high purity aluminium (aluminium 1350) wires wrapped in spiral. The core wires may be zinc coated (galvanized) steel or aluminium coated (aluminized) steel. Galvanization or aluminization coatings are thin and are applied to protect the steel from corrosion. The central steel core provides additional mechanical strength and, hence, sag is significantly less than all other aluminium conductors. ACSR conductors are available in a wide range of steel content - from 6% to 40%. ACSR with higher steel content is selected where higher mechanical strength is required, such as river crossing. ASCR conductors are very widely used for all transmission and distribution purposes.

Aluminium Conductor, Alloy Reinforced

ACAR conductor is formed by wrapping strands of high purity aluminium (aluminium 1350) on high strength Aluminum-Magnesium-Silicon alloy (6201 aluminium alloy) core. ACAR has better electrical as well as mechanical properties than equivalent ACSR conductors. ACAR conductors may be used in overhead transmission as well as distribution lines.

Bundled Conductors

Bundled Conductors
Transmission at extra high voltages (say above 220 kV) poses some problems such as significant corona loss and excessive interference with nearby communication lines when only one conductor per phase is used. This is because, at EHV level, the electric field gradient at the surface of a single conductor is high enough to ionize the surrounding air which causes corona loss and interference problems. The electric field gradient can be reduced significantly by employing two or more conductors per phase in close proximity. Two or more conductors per phase are connected at intervals by spacers and are called as bundled conductors. The image at right shows two conductors in bundled form per phase. Number of conductors in a bundled conductor is greater for higher voltages.
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Types of Electrical Loads

An electrical load is a device or an electrical component that consumes electrical energy and convert it into another form of energy. Electric lamps, air conditioners, motors, resistors etc. are some of the examples of electrical loads. They can be classified according to various different factors. Some popular classifications of electrical loads are as follows.

Resistive, Capacitive, Inductive

Electrical loads can be classified according to their nature as Resistive, Capacitive, Inductive and combinations of these.

Resistive Load

  • Two common examples of resistive loads are incandescent lamps and electric heaters.
  • Resistive loads consume electrical power in such a manner that the current wave remains in phase with the voltage wave. That means, power factor for a resistive load is unity.

Capacitive Load

  • A capacitive load causes the current wave to lead the voltage wave. Thus, power factor of a capacitive load is leading.
  • Examples of capacitive loads are: capacitor banks, buried cables, capacitors used in various circuits such as motor starters etc.

Inductive Load

  • An inductive load causes the current wave to lag the voltage wave. Thus, power factor of an inductive load is lagging.
  • Examples of inductive load include transformers, motors, coils etc.

Combination Loads

  • Most of the loads are not purely resistive or purely capacitive or purely inductive. Many practical loads make use of various combinations of resistors, capacitors and inductors. Power factor of such loads is less than unity and either lagging or leading.
  • Examples: Single phase motors often use capacitors to aid the motor during starting and running, tuning circuits or filter circuits etc.

Types of loads in power system

Domestic load / residential load

Domestic load consists of lights, fans, home electric appliances (including TV, AC, refrigerators, heaters etc.), small motors for pumping water etc. Most of the domestic loads are connected for only some hours during a day. For example, lighting load is connected for few hours during night time.

Commercial load

Commercial load consists of electrical loads that are meant to be used commercially, such as in restaurants, shops, malls etc. This type of load occurs for more hours during the day as compared to the domestic load.

Industrial load

Industrial load consists of load demand by various industries. It includes all electrical loads used in industries along with the employed machinery. Industrial loads may be connected during the whole day.

Municipal load

This type of load consists of street lighting, water supply and drainage systems etc. Street lighting is practically constant during the night hours. Water may be pumped to overhead storage tanks during the off-peak hours to improve the load factor of the system.

Irrigation load

Motors and pumps used in irrigation systems to supply the water for farming come under this category. Generally, irrigation loads are supplied during off-peak or night hours.

Traction load

Electric railways, tram cars etc. come under traction loads. This type of loads reaches its peak during morning and evening hours.

Some other classifications of electrical loads

According to load nature

  • Linear loads
  • Non-linear loads

According to phases

  • Single phase loads
  • Three phase loads

According to importance

  • Vital electrical loads (e.g. required for life safety)
  • Essential electrical loads
  • Non-essential / normal electrical loads
Electrical loads may also be classified in may different manners, such as according to their functions.
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The Future of Solar Energy in India

India, with its booming economy and humongous population of over 1 billion, has always faced shortage of energy. Even though the country is among the largest producers of electricity in the world, it is hardly ever able to meet the electricity requirements of its ever-so-rapidly increasing population. At present, almost 53% of India’s energy requirements are met with coal; going by the predictions, the coal reserves of the country will not last beyond 2050. [coal power plant]. It is common knowledge that over 72% of the population of this third world country still resides in villages, with only about half of its rural population getting access to electricity. It is high time India moved to renewable ways to feed its population its fair-share of electricity.

Solar energy has emerged as the most viable and environment-friendly option for India to cater to the energy requirements of one and all—including the 50% of its rural inhabitants who still live without electricity. A typical solar system is very easy to set up and just entails installing solar panels correctly in order for it to work. Quite a few people were already aware of its benefits and were really quick at setting their properties up with solar systems; in fact, the utilization of solar energy in India is nothing new and has existed in select locations for quite some time now. However, it has yet to pick up steady momentum.
the future of solar energy in India
The future of solar energy in India is as bright as that of the sun the solar systems derive power from. A brief overview of why India will definitely turn to solar power sooner or later is as follows:

1. Geographical Advantage
geography wise installed solar capacity in India
How long can India ignore the looming threat to its fossil fuel reserves? The geographical location of India is such that it can not only produce enough energy to meet its own requirements, but also produce enough energy for the entire world! Because it falls in the tropical region, it receives generous amounts of solar radiation all through the year amounting to nearly 3,000 long hours of sunshine. In India, there are top five states which have highest renewable energy capacity where solar modules are able to produce ample amounts of electricity even on overcast days.

2. Upcoming Solar Projects in India
The states of Andhra Pradesh, Gujarat, Madhya Pradesh, Rajasthan, Punjab, Haryana and Maharashtra have incredulous amounts of potential to tap solar energy, owing to their strategic location. At present, the Thar region in Rajasthan is home to some of the best solar projects of the country, generating close to 2,100 GW power. Gujarat houses one of the largest solar power plants in India. Last year saw Indian government has been approved of a master plan envisaging the upgradation of 50 of India’s cities to blossoming solar cities.

3. How to Use Solar Energy in Multiple Applications Around You?
One more reason as to why the future of India’s electricity lies inevitably in harnessing solar energy is because of the number of ways in which the radiation of the sun can be put to use—from solar panels that are the backbone of any solar system, solar inverters, solar street lights, solar UPSs, solar fans, solar lanterns, solar cables, solar mobile chargers, solar power conditioning units, solar home systems, solar road safety equipment and solar fencing to even solar CCTV cameras!
solar energy powered appliances around you
4. Highly Advantageous at Cheap Cost
It is true that solar panels and solar systems are slightly expensive to purchase, to begin with. It is, however, also true that solar systems once set up help save money, from the moment on! Solar panels usually have a lifespan of around 25 years and are definitely worth the investment in every respect. The use of solar energy to power electrical appliances eliminates any dependency whatsoever on the constant supply of electricity to any place. Solar power is also good riddance of hefty monthly electricity bills for the common man.

5. Employment Prospects
The transition to the utilization of solar energy is an imminent and long-impending one. It is only a matter of time before we see an entire solar sector come up. The persistent problem of unemployment in India will definitely also get better and the unemployed youth will be able to see the light of day with the creation of more and more jobs.
number and type of jobs created in India's solar sector
Even though many people have already taken to installing solar panels in their homes and offices for meeting their energy requirements on a daily basis, there is still a long way to go before India becomes a complete solar nation.
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Economic choice of conductor size - Kelvin's law

As economy is one of the most important factors while designing any transmission line, the cost of required conductor material is a considerable part. Thus, it becomes vital to select a proper size of the conductor. The most economic design of a transmission line is for which the total annual cost is minimum. Total annual cost can be divided into two parts, viz. annual charges on capital outlay and running charges. Annual charges on capital outlay include depreciation, interest on the capital cost, maintenance cost etc.. The cost of energy lost during the operation is counted in running charges. Regarding this, there are two important points that must be noted -
  • if the cross-sectional area of the conductor is decreased, the total capital cost of the conductor decreases but the line losses increase (resistance increases with the decrease in the conductor size, hence, I2R loss increases)
  • whereas, if the cross-sectional area of the conductor is increased, the line losses decrease but the total capital cost increases.
Therefore, it is important to find the most economical size of the conductor. Kelvin's law helps in finding this.
[Also read: Economic choice of transmission voltage]

Kelvin's law for finding economic size of a conductor

Let, area of cross-section of conductor = a
annual interest and depreciation on capital cost of the conductor = C1
annual running charges = C2
Now, annual interest and depreciation cost is directly proportional to the area of conductor.
i.e., C1 = K1a
And, annual running charges are inversely proportional to the area of conductor.
C2 = K2/a
Where, K1 and K2 are constants.
Now, Total annual cost = C = C1 + C2
                 C = K1a + K2/a
For C to be minimum, the differentiation of C w.r.t a must be zero. i.e. dC/da = 0.
Therefore,
Kelvin's law for economic choice of conductor size

"The Kelvin's law states that the most economical size of a conductor is that for which annual interest and depreciation on the capital cost of the conductor is equal to the annual cost of energy loss."
From the above derivation, the economical cross-sectional area of a conductor can be calculated as,
a = √(K2/K1)

Graphical illustration of Kelvin's law

As the annual cost of conductor is directly proportional to size of the conductor, it is shown by the straight line C1 in the figure. Annual cost of energy loss is shown by the curve C2. The total annual cost curve is obtained by adding the curve C1 and C2. The lowermost point on total annual cost curve gives the most economical size of the conductor which corresponds to the intersection point of curve C1 and C2. So, here, the most economical area of cross-section of the conductor is represented by ox and the corresponding minimum cost is represented by xy.

Limitations of Kelvin's law

Although Kelvin's law holds good theoretically, there is often considerable difficulty while applying it in practice. The limitations of this law are:
  1. It is quite difficult to estimate the energy loss in the line without actual load curves which are not available at the time of estimation.
  2. Interest and depreciation on the capital cost cannot be determined accurately.
  3. The conductor size determined using this law may not always be practicable one because it may not have sufficient mechanical strength.
  4. This law does not take into account several factors like safe current carrying capacity, corona loss etc.
  5. The economical size of a conductor may cause the voltage drop beyond the acceptable limits.

Modified Kelvin's law

The actual Kelvin's law does not count the cost of supporting structures, erection, insulators etc.. It only accounts for the capital cost of conductor and corresponding interest and depreciation. Also, for underground cables, the cost of insulation and laying is not considered in the actual Kelvin's law. To account for these costs and to get practically fair results, the initial investment needs to be divided into two parts, viz (i) one part which is independent of conductor size and (ii) other part which is directly proportional to the conductor size. For an overhead line, insulator cost is almost constant and the cost of supporting structure and their erection is partly constant and partly proportional to the conductor size. So, according to the modified Kelvin's law, the annual charge on capital outlay is given as, C1 = K0 + K1a. where, K0 is an another constant. The differentiation of total cost C w.r.t. to the area of conductor (a) comes to be same as derived above under the heading Kelvin's law.
The modified statement of Kelvin's law suggests that the most economical conductor size is that for which the annual cost of energy loss is equal to the annual interest and depreciation for that part of capital cost which is proportional to the conductor size.
[Also read: Economics of power generation]
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