What is corona

Here present article about what is corona?? And factors affecting corona and advantage and disadvantage of corana.

➼ what is corona??

• It has been observed that,if the spacing between two conductrs is less than their diameters ,and if the applied potential between them gradually increased,a violet glow occurs round the conductor,along with a hissing sound,at a certain value of the potential.This phenomenon is known as Corona.
• The phenomenon of hissing noise,faint violet glow and production of ozone gas surrounding the overhead lines,due to ionization of air is called corona.
• If the conductors are uniform and smooth,the corona glow will be uniform throughout the length otherwise the rough points will appear brighter.
• In case of d.c.system,the glow due to corona will be different in the two conductors.
• The +Ve conductor has smooth and bright glow whereas the negative conductor has spotty glow.

Theory of corona formation:

• Phenomenon of corona is Ionization of air surrounding the power conductor. Due to cosmic rays, free electrons are normally present in free space.
• When the potential difference between the conductors to be large as compared to diameter of conductor, free electrons move with certain velocity depends on strength of field. Means greater the applied voltage, greater the potential gradient, more is velocity of free electrons.
• Due to large velocity of electrons, electrons have collision with the molecules of air, and dislodge the electrons from these molecules. This produces another ion and one or more free electrons. This process being commulative, will lead to an avalanche of electrons.
•  Eventually there will be ionization of air surrounding the conductor.
• If spacing of conductor is less than 15 times diameter of conductor, flashover will take place between the conductor and either corona is formed or spark takes place between the conductors.

Facts about corona:
(i)  The spacing between the conductors should be very small as compared to their diameters.
(ii)  The phenomenon occurs above a critical voltage, which has the maxi. value of 30 KV (or 21.0 KV rms) per cm. :
(iii) It is accompanied by a hissing sound along with violet glow discharges, Vibration of conductors, formation of ozone gas, and noise.
(iv) Its glow is brightest at rough or dirty surfaces of the conductor.
(v)  It causes power loss and radio interference.
(vi) It introduces harmonic currents.
(vii)It causes ionisation of air between the conductors.
(viii)It causes much damage to lines/equipments, etc.

Phenomenon of Corona :

• For an over head transmission system atomospheric air behaves practically like perfect insulator when potential difference between conductors is small. If we increase the voltage, there is a corresponding increase in the electric field intensity
• If air is subjected to a uniform electric field intensity of peak value less 30kV/cm, flow current between conductors is negligible. But when electric intensity increases and reaches this critical value of 30kV/cm air ionizes come conducting.

This breakdown accompanied by following phenomenon.

1. Hissing Noise is produced.
2. A faint violet glow appears which is visible in the dark.
3.  Production of Ozone gas.
4.  Radio interference occurs
5.  There is a loss of power.

 If all these phenomenon occurs then we say corona produced in a transmission line.

Factors affecting corona

The phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which corona depends :
1. Atmosphere :

 As corona is formed due to ionization of the air surrounding the conditions of the conductor, it is affected by physical state of atmosphere. In the stormy Weather the number of loss is more than normal and as such corona occurs at much less voltage as compared with fair weather.

2. Conductor size:
The corona effect depends upon the shape and condition 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 stranded conductor has irregular surface and hence give to more corona that a solid conductor Corona losses.
3. Spacing between conductor:
If the spacing between the conductor is made very large compared to their diameter,there may not be corona effect.It is because large distance between conductor reduces the electrostatic stress at the conductor surface thus avoiding corona formation.But the cost of installation is increased.
4.Line voltage:
The line voltage greatly affected corona.If it is low there is no change in condition of air surrounding the conductors and hence no corona is formed. However if the line voltage has such a value that electostatic stress developed at the conductor surface make the air around the conductor conducting, then corona formed.
5.Effect of Frequency:
Corona loss is directly proportional to system frequency.
6.Effect of Load Current:
The flow of the load current increases the temperature of the conductor.Thus,it prevents deposition of snow on the conductor's surface.This reduce corona loss.

 Advantages of Corona:

1. Due to the formation of corona,the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased.The increased diameter decreases the electostatic stresses between the conductors.

1. Corona reduces electro-static stresses between the conductors.This reduces the probability of flash over and improves the system performance.

1. Effects of transients produced by lightning surges and other causes are reduced. Charge induced on the line by surges will be partially dissipated as a corona loss.In this way it acts as a safety valve.

Disadvantages of Corona:

The disadvantages are more than the advantages of corona:

1. Corona causes power loss in the line and reduces efficiency.
2. corona currents are non sinusoidal, which produce non- sinusoidal voltage drop. this may produce ratio interference in nearby communication lines.
3. harmonics are produced in the line
4. during the formation of corona, ozone gas is formed. ths gas reacts with the material of the conductors and causes corrosion. 

Method of reducing corona

1. the corona can be reduced by increasing diameters of the wires. this is the reasin that aluminium conductor steel reainforced(ACSR) of large cross section are used for transmission lines
2. the spacing between the parallel conductors can be increased,  but this has a limit as it will increase cost of supports, cross arms, etc.

Basic definations of transmission line

Here present the article about the basic definations of  transmission line

• Alternating Current (AC) - Electric current in which the direction of the current's flow is reversed or alternated at 50Hz in the india.
• Conductors (Power Lines) – Metal cables used for carrying electric current.
• Corona – Electrical breakdown of the air near high voltage conductors into charged particles.
• Current – The flow of electricity or the movement of electrons through a conductor typically measured in watts.
• Direct Current (DC) – Electric current flows continuously in the same direction as contrasted with alternating current.
•  Distribution Line – A line that carries electricity at lower voltages of 12kV to 44kV and is used to distribute power drawn from high-voltage transmission systems to end-use customers.
• Electric & Magnetic Fields (EMF) – Invisible areas of energy, often referred to as radiation, that are associated with the use of electric power. EMFs fall into one of two radioactive categories – non-ionizing (low-level of radiation) or ionizing (high-level of radiation).
• Electric Load – Electricity consumers, such as residences, businesses, and government centers that use electricity.
• Electric Power Transmission – The process by which large amounts of electricity produced are transported over long distances for eventual use by consumers.
• Energy – The amount of work that can be done by electricity, typically measured in kilowatt-hours (kWh) or megawatt- hours (MWh).
• Foundation – System that transfers to the ground the various dead and live loads of the transmission structure and conductors.
•  Generation – The production of electric energy. Fossil fuels, wind turbines, solar panels, and other technologies are used to generate electricity.
• Insulators – Used to contain, separate, or support electrical conductors.
• Interconnection – Points on a grid or network where two or more transmission lines are connected at a substation or switching station, or where one stage of the energy supply chain meets the next.
•  Load Center – A particular geographical area where energy is used. Most commonly refers to an area within a utility’s service territory where energy demand is highest (i.e., cities, major industrial areas, etc.).
•  National Electrical Safety Code (NESC) – The NESC is the U.S. standard of the safe installation, operation, and maintenance of electric power systems.
• Power – Rate at which electricity does work. Measured in watts or kilowatts (kW) or megawatts (MW).
• Rights-of-Way (ROW) – A legal land right, easement, set a side for the transmission line structure and conductors needed for clearances and maintenance activities.
• Shield and Ground Wire – Wires used primarily for protection from lightning strikes and corresponding surges. 
• Substation – A part of an electrical transmission system that transforms voltage from high to low, or the reverse.
• Switching Station – A part of an electrical transmission system that ties two or more electric circuits together through switches, to permit a circuit to be disconnect, or to change the electric connection between circuits. 
• Transmission Line – A line that carries electricity at voltages of 69kV or greater and is used to transmit electric power over relatively long distances, usually from a central generating station to main substations.
• Transmission Structures – Used to keep high-voltage conductors (power lines) separated from their surroundings and from each other.
• Voltage – Electric “pressure” measured in volts. Power systems are typically measured in 1,000s volts or kV.
•  Watt – Unit of electrical power. 1MW is one million watts.

Types of interconnection power system and advantage and disadvantage

Here present the knowledge about the types of interconnection power system and advantage and disadvantage

There are mainly three types of interconnection.

1. AC interconnection
2. DC interconnection
3. Hybrid AC/DC interconnection

(1) AC interconnection

This figure show the AC interconnection scheme, two systems are directly connected to each other with converting station.

(2) DC interconnection

This figure shows the DC interconnection system. Both the system A and B having no converting station to change the voltage level and control the voltage. Transmission of large power blocks over long distances is however utilize by HVDC at the same time can strengthen the interconnection to avoid dynamic problem.

(3)Hybrid AC/DC interconnection

The hybrid interconnection system. Both AC and DC power is transmitted over long distance. The hybrid interconnection could therefore offer the best possibility for system interconnections in the case of continental networks, consisting of a number of small system.

➡Advantages of interconnected power system:-

(1) exchange of peak loads:

• An important advantage of interconnected power system is that the peak load of the power station can be exchanged.
• If the load curve of a power station shows a peak demand that is greater than the rated capacity of the plant,then the excess load can be shared by other stations interconnected with it.

(2) Use of Older Plants:

• The interconnected system makes it possible to use the older and less efficient plants to carry peak loads of short durations.
• Although such plants may be inadequate when used alone,yet they have sufficient capacity to carry short peaks of loads when interconnected with other modern plants.
• Therefore,interconnected system gives a direct key to the use of obsolete plants. 

(3)Ensures Economical Operation:

• The interconnected system makes the operation of concerned power stations quite economical.
• It is because sharing of load among the stations is arranged in such a way that more efficient stations work continuously throughout the year at a high load factor and the less efficient plants work for peak load hours only.

(4)Increases Diversity Factor:

• The load curves of different interconnected stations are generally different.
• The result is that the maximum demand on the system is much reduced as compared to the sum of individual maximum demands on different stations.
• In other words,the diversity factor of the system is improved,thereby increasing the effective capacity of the system.

(5)Reduces Plant Reserve Capacity:

• Every power station is required to have a standby unit for emergencies.
• However,when several power stations are connected in parallel,the reserve capacity of the system is much reduced.
• This increases the efficiency of the system.

(6)Increases Reliability of Supply:

• The interconnected system increases the reliability of supply.
• If a major breakdown occurs in one station,continuity of supply can be maintained by other healthy stations.

➡Disadvantage of interconnected power system

(1)Synchronizing Problem:

• The generators of all the interconnected generating stations must operate the same at frequency and in a synchronized manner.
• During the heavy load condition some generators go out of step due to this synchronization breakup.
• And finally complete system goes out of step is also called black out condition.

(2)Expensive Tie Lines:

• In the interconnected power system to control the system reactive power,tie lines are used.
• Expense of tie lines for construction of interconnecting transmission line between generating stations.

(3)Expensive Circuit Breaker:

• An interconnected system use circuit breaker to isolate the faulty part from the healthy part during the fault condition.
• For high voltage power system,higher rating circuit breaker is used.
• During the large flow of current on transmission line under faulty condition with a consequent increase in capacity of circuit breakers therefore expensive circuit breaker are required with interconnected power system.
• Hence cost of circuit breakers increase.

4.Metering and Instrumentation:

• Adequate metering and instrumentation is needed at different levels.
• It is necessary to install sequential recorders,disturbance recorder,time of day meters etc.

Basic concept of interconnected power system

Basic concept of interconnected power system

Here Present the knowledge about the basic concept of interconnected power system. 

Definition: the connection of number of power generating station in parallel is know as interconnected power system.

• The main requirement of any power system is to give an uninterrupted power supply.
• In some parts of the contry there are resources of generating power but their requirements is low where as in other parts power requirements is large as compared to available resources of power.
• These above difficulties can be reduced by interconnecting different power station in parallel.
• Inter Connected power system is basically an interconnection of the buses of more than one generating station.
• It is also defined as a distribution system with multiple available power sources that can loop throughout the network.
• If one source goes down, a different source can be activated to maintain service.

• This figure shows the interconnected power system having two power generating station G1 and G2 supplying current I1 and I2 to the load through transmission line 1 and 2 in addition to their local load.
• The power generating stations are also interconnected by transmission line, which is carrying the current I1.
• In both transmission line deliver equal power and the system operates at the same terminal voltage, it is neccessary that the active components of the line currents of the line currents I1 and I2 are equal.

line insulators.

Here present the how the work of line insulator and types of insulators.

•   The insulators for overhead lines provide insulation to the power conductor from the ground.
•  The insulators are connected to the cross arm of the support structure and the power conductor passes through the clamp of insulator.
•  These insulator are mainly made of either glazed porcelain or toughened glass. 
• The materials used for porcelain art silica 20 % feldspar 30 % and clay 50 %.
•  It is desirable that porcelain to be used for insulators should be air free and non porous.
•  The dielectric strength of porcelain should be 15 to 17 kV for every one tenth inch thickness.
• As porcelain is mechanically strong, less affected by temperature and has minimum  leakage problem hence suitable for overhead transmission line.  
• Toughened glass is also sometimes used for insulators  because it has higher dielectric strength (35 kV for one tenth  inch thickness) which makes it possible to make  use of single piece construction whatever be the operating voltage.
• As the line insulators are exposed to atmosphere, dust and dirt deposits on the surface of the insulator which gradually increase the leakage current. 
• In order to avoid breakdown due to leakage current, clearance between two ends of the insulator should be sufficient. 
• The shortest distance between the two conducting parts along the surface of the insulating material is known as creepage distance.

Types of insulators:

• Basically there are five types of insulators used for overhead transmission line.

1. Pin type
2. Suspension type 
3. Strain type 
4. Shackle insulator
5. Stay strain insulator

1. Pin insulator

• this type of insulator is commonly employed for operating voltage up to above 25 KV.
• Pin type insulator consists of a single or multiple shells adapted to be mounted on a single spindle to be fixed to the cross arm of the supporting structure.
• For supporting the insulator steel bolt or pin secured to a cross arm on the transmission pole is used.
• There is groove on the upper end of the insulator for housing the conductor.
• The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.
• Beyond operating voltage of 33 KV the pin type insulator becomes too bulky and hence uneconomical.

2.  Suspension type of insulator:

• The pin type of insulator are uneconomical beyond 33kV operating voltage. Also the replacement of these insulators is experisive. 
• For these reasons for insulating ovehead lines against high voltages, suspension insulators are used. When it is also called as string insulator when number of units are connected in series.
• These insulators consists of the one or more porcelain units flexibly connected together in series by metal links in the form of a string.
• Upper end of the string is secured to the cross arm of the tower.
• Each disc or units is designed for 11 kv. 

3. Strain type insulators:

• Such insulators are used when there is a dead end of the line or there is corner or sharp curve where line is subjected to greater tension . 
• For low voltages up to 11kV shackle insulators is generally used But for high voltages a string of suspension insulators may become necessary. 
• These strings are placed in the horizontal plane rather than the vertical plane as is done in case of suspension insulators.

4.shackle insulators:

• In early years, the shackle insulators were used as strain insulators.
•  But now a days their application is restricted only for low voltage distribution lines.
• These  insulattors can be used either in a vertical or horizontal position.
•  The shackle insulator can be fixed to the pole with bolt or to the cross arm directly.
• Generally U shape clamp or D clamp is used to position the shackle insulators.
• The  conductor in the groove is fixed with a  soft binding wire.

5. Guy / stay stain insulators:

• Guy insulator are placed to prevent the lower part of the guy from becoming electrically energized by of the upper part of the guy when the conductor snaps and falls on them or conductor due to leakage.
• No guy insulator shall be located less then 3.50 due to leakage.
• A type of insulators are to be used for L.T. line guys.
• One C type of insulator is to be used for 11 kv guys.
• Two C type of insulators are to be used for 33kv.

what is the line components of transmission line

Here present the line components of overhead transmission line.

The line components of overhead transmission line the electric power is transmitted through a bare conductor generally made up of A.C.S.R.(aluminum conductor steel reinforced). This conductor is kept above certain height from the ground to maintain clearance between them to avoid the arcing grounds.

This Fig. Shown the components of the overhead transmission system.

• Conductors: conductors are used to carry electric power from sending end to receiving end. Conductors used for transmission and distribution purpose are made up of copper, aluminum or A.C.S.R.

• line supports: line supports are used to maintain the clearance between conductors and ground line supports are of either wooden, steel, RCC construction depending on the application.
• Insulators: insulator provides very high resistance path between line conductor and earth/support. Also it provides support to line conductor. According to system voltage, pin type, suspension type, shackel type or strain type insulator are used.

• Cross arm: it provides horizontal clearance between the two conductors. It is a mounting accessory of line support which provides platform for insulators.

• Stay sets: stay set provides support to the end pole or coener pole. These are braces of iron wire wound spirally.

• Ground wire: it is run on the top of the pole/tower throughout the length of line. Ground wire protect the transmission line from lightening strokes.
• Lightening arrestors:  L.A. are used to protect the line components from high voltage discharge. One terminal of L.A. is connected to the line and other terminal is earthed. 
• Fuses and isolating switches: these are required to isolate different sections of overhead line.
• Miscellaneous items: such as phase plates, danger plates, anticlimbing wires,(guard wires), 'V' guards etc.

How is the general arrangement of power system

Here Present The general arrangement of power system and reasons for generated electrical energy transmitted over long distance .

It mainy consists of generating stations,substations,transmission network and distribution network.According to the law of conservation, energy can neither be created nor be destroyed, it can be transferred from one form to another form. Therefor power station converts some form of Energy into the electrical energy. It mainly consists of two parts, one is turbine and other is alternator. Turbine converts some form of energy into machanical form and then alternator converts it into electrical form. Generated electricity is in the form of 3 phase supply which is step up in power substation. Then this high voltage electrical energy is transmitted by transmission line over a long distance. Then it is stepped down in distribution substation to utilization level.this step up and step down is done by transformer. Then the electrical energy is distributed to different consumers.

Generated electrical energy has to be transmitted over a long distance owing to following reasons:

• The load on the power syatem is not concentrated but it is widely spread.
• Generating station are created away from the load centers due to some particular site selection criteria.as the case in hydro power station is, it should be located at hilly areas owing to its requirement of huge amount of water and construction of dam.
• In case of thermal power station and nuclear power station the pollution level is very high which is dangerous to human beings.
• Owing to these reasons , the generated electrical energy has to be transmitted to the consumers via some conducting materials.
• If these conductors are located over head with suitable spacing between them and adequate clearance from ground then it is called as overhead transmission system.
• If these conductors are placed underground by properly insulated from each other then the sytem is known as underground transmission system.
• Fig. Shown the typical layout of the power system.