Skin Effect of Conductors In Transmission Lines

Skin Effect of Conductors In Transmission Lines

When a conductor is carrying steady direct current (d.c.), this current is uniformly distributed over the whole X-section of the conductor. However, an alternating current flowing through the conductor does not distribute uniformly, rather it has the tendency to concentrate near the surface of the conductor as shown in Fig. This is known as skin effect in transmission lines.

The tendency of alternating current to concentrate near the surface of a conductor is known as skin effect.

                                

Due to skin effect, the effective area of cross-section of the conductor through current flows is reduced. Consequently, the resistance of the conductor is slightly increased when carrying an alternating current. The cause of skin effect can be easily explained. A solid conductor may be thought to be consisting of a large number of strands, each carrying a small part of the current. The inductance of each strand will vary according to its position. Thus, the strands near the center are surrounded by a greater magnetic flux and hence have larger inductance than that near the surface. The high reactance of inner strands causes the alternating current to flow near the surface of conductor. This crowding of current near the conductor surface is the skin effect.

Skin effect depends upon the following factors :

(i) Nature of material

(ii) Diameter of wire − increases with the diameter of wire.

(iii) Frequency − increases with the increase in frequency.

(iv) Shape of wire − less for stranded conductor than the solid conductor.

It may be noted that skin effect is negligible when the supply frequency is low (< 50 Hz) and conductor diameter is small (< 1cm).

Read More

Type of Electrical Poles : Overhead Line Supports

Type of Electrical Poles : Overhead Line Supports 

Overhead transmission lines requires some support to carry the conductors.The supporting structures for overhead transmission line conductors are various types of poles and towers called line supports or electrical poles.

Properties of Line Supports :-

(i) Line Supports should have high mechanical strength to withstand the weight of conductors and wind loads etc.

(ii) Light in weight without the loss of mechanical strength.

(iii) Poles should be cheaper in cost and eco-friendly .

(iv)High life time because frequent replacement of poles may economical .

(v) Easy accessibility of conductors for maintenance.

Various types of poles:-

We have different types of electrical poles to carry various types of conductors,we can't use any one of the below line support for all types of conductors. 

1.Wooden poles
2.Steel poles
3.R.C.C. poles 
4.Lattice steel towers.

1. Wooden poles:-

Wooden poles are made of wood (sal or chir) and are suitable for lines short distance (up to 50m) and less area lines.wooden pole supports are cheap in cost,easily available, provide natural insulating properties and moreover economical.The wooden poles generally tend to rot below the ground level, causing foundation failure. In order to prevent this, the portion of the pole below the ground level is impregnated with preservative compounds like creosote oil.but now a days these poles are out of use,they are replaced by R.C.C poles because of their long life.

Disadvantages of wooden supports are :

(i) tendency to rot below the ground level
(ii) comparatively smaller life (20-25 years)
(iii) cannot be used for voltages higher than 20 kV
(iv) less mechanical strength and 
(v) require periodical inspection.

2. Steel poles:

The steel poles are used as a replacement for wooden poles.Because steel poles possess greater mechanical strength, longer life than wooden poles and able to withstand longer spans.These poles are generally used for distribution purposes in the cities. This type of supports need to be galvanised or painted in

order to prolong its life.

The steel poles are of three types viz., 

(i) rail poles

(ii) tubular poles and

(iii) rolled steel joints.

3.RCC poles: 

RCC stands for reinforced concrete.These poles have become very popular as line supports in recent years.They have greater mechanical strength, longer life and permit longer spans than steel poles. Moreover, they give good outlook, require little maintenance and have good insulating properties.Expect high weight and manufacturing cost they are very good alternative for all types of line supports.

4.Steel towers: 

In practice, wooden, steel and reinforced concrete poles are used for distribution

purposes at low voltages, say upto 11 kV.To transmit higher voltages than 11kv we use steel tower.Steel towers have greater mechanical strength, longer life, can withstand most severe climatic conditions and permit the use of longer spans and we can increase or decrease length of the pole by assembling some more cross arms.these are more efficient but requires more space to establish.

These are the different types of electric poles which we can see in daily observations.

Read More

String Efficiency & Improving Methods

String Efficiency:

The total voltage applied across the string of suspension type insulators is not equally distributed across all discs.In this distribution of voltage, disc nearest to the conductor will be at higher potential than the other discs. This unequal potential distribution is undesirable and is usually expressed in terms of string efficiency

What Is String Efficiency?

"The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency."

i.e.,String efficiency =Voltage across the string/(n ×Voltage across disc nearest to conductor)

Where n = number of discs in the string.

String efficiency is an important factor is transmission line designing.since it decides the potential distribution along the string.to get uniform distribution string efficiency should be high.Thus 100% string efficiency is an ideal case for which the volatge across each disc will be exactly the same.that gives easy calculations to no.of discs to be added.but it is impossible to achieve 100% string efficiency,yet efforts should be made to improve it as close to this value as possible.

Mathematical expression for String Efficiency:-

Above Fig. shows the equivalent circuit for a 3-disc string. Let us suppose that self capacitance of each disc is C. Let us further assume that shunt capacitance C1 is some fraction K of selfcapacitance i.e., C1 = KC. Starting from the cross arm or tower, the voltage across each unit is V1,V2 and V3 respectively as shown. 
Applying Kirchhoff’s current law to node A, we get,

I2 = I1 + i1
or V2ω C* = V1ω C + V1ω C1
or V2ω C = V1ω C + V1ω K C
∴ V2 = V1 (1 + K) ...(i)

Applying Kirchhoff’s current law to node B, we get
I3 = I2 + i2
or V3 ω C = V2ω C + (V1 + V2) ω C1 
or V3 ω C = V2ω C + (V1 + V2) ω K C
or V3 = V2 + (V1 + V2)K
= KV1 + V2 (1 + K)
= KV1 + V1 (1 + K)2   since [ V2 = V1 (1 + K)]
= V1 [K + (1 + K)²]
∴ V3 = V1[1 + 3K + K²] ...(ii)
Voltage between conductor and earth (i.e., tower) is
V = V1 + V2 + V3
= V1 + V1(1 + K) + V1 (1 + 3K + K²)
= V1 (3 + 4K + K²)
∴ V = V1(1 + K) (3 + K) ...(iii)
From expressions (i), (ii) and (iii), we get,
 V1/1=V2/(1+K)=V3/(1 + 3K + K²)=V/(1+K)(3=K)

∴Voltage across top unit, V1 = V/(1 + K)(3 + K)
Voltage across second unit from top, V2 = V1 (1 + K)

Voltage across third unit from top, V3 = V1 (1 + 3K + K²)
%age String efficiency =Voltage across the string*100/(n ×Voltage across disc nearest to conductor)
=V*100/3V3

Methods of Improving String Efficiency:

By using longer cross-arms:-

From above observation we can say The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance.For the low the value of K, the string efficiency will be high and we get uniform voltage distribution.value of K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used.

By grading the insulators:-

In this method, different dimensions of  insulators are chosen which have different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required.

By using a guard ring:-

The potential across each unit in a string can be equalized by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units.

Related:
string efficiency in power system
string efficiency formula
string efficiency of suspension insulators
string efficiency in power system pdf
string efficiency transmission line
string efficiency of insulators ppt
string efficiency ppt

Read More

Types of Insulators In Overhead Transmission Lines

Types of Insulators In Overhead Transmission Lines

Transmission line insulators separate, contain and suspend transmission line conductors.they protects short circuit path between earth and transmission line.There are several types of insulators In overhead transmission lines.but the most know are pin type, suspension type, strain insulator,shackle insulator and Stay Insulator.

Types of electrical insulators in transmission lines:

1. Pin type insulators:

The name "pin" came from constructional design of it.Pin type insulators are used for the transmission of lower voltages.transmission range of pin type is up to 33 kV (kilovolts).Pin type insulators are secured with steel or lead bolts onto transmission poles. These are typically used for straight-running transmission lines.There is a 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.we can use this type of insulator beyond 33kv,but that becomes bulky in size & costly. 

2.Suspension type insulators :

Suspension type insulators are used to transmit above 33kv.because above 33kv using of pin type is uneconomical.In suspension type insulator numbers of insulators are connected in series to form a string and the conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.Each insulator of a suspension string is called disc.

Advantages of Suspension Type insulators :

(i)Suspension type insulators are very economical voltages beyond 33 kV.

(ii) Each unit or disc of suspension type insulator is designed for low voltage,usually 11 kV.As per requirement of operating voltage we can add no.of discs in series. 

(iii) If any one disc is damaged, the whole string does not become useless because the damaged disc can be replaced by the sound one.

(iv) The suspension arrangement provides greater flexibility to the line. The connection at the cross arm is such that insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum.

(v) The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning.

3.Strain Type insulators:

Strain type insulators are simply horizontally suspended suspension insulators.When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great  tensile load of conductor or strain.The discs of strain insulators are used in the vertical plane. When the tension in lines is exceedingly high, at long river spans, two or more strings are used in parallel.voltage rating almost equal to suspension type.

4.Shackle Type Insulators:

Shackle type insulators, similar to strain type insulators, are used on sharp curves, end poles and in section poles.These insulators are single, round porcelain parts that are mounted horizontally or vertically.The shackle insulator or spool insulator is usually used in low voltage distribution system.They can be directly fixed to the pole with a bolt or to the cross arm.

5.Stay insulators :

The insulator used in the stay wire is called as the stay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the guy-wire will not fall to the ground.

This Is Complete Information On Types of Electrical Insulators ; Insulators In Overhead Transmission Lines.

Tags: types of electrical insulators ppt,types of electrical insulators pdf,types of electrical wire,function electrical insulators,benefits electrical insulators,invented electrical insulators,insulators electrical electronic applications,what is an insulator of electricity

Read More

Hydro-electric Power Station:Arrangement & Working

Definition: A power generating station which utilises the potential energy of water at a high level for the generation of electrical energy by converting potential energy to kinetic energy is known as a hydro-electric power station.

Arrangement of Hydro-electric Power Station & Working:

Hydro-electric Power Plant Consists of basic sections like:
1) Dam
2) Reservoir
3) Control Gate
4) Penstock
5) Turbines and generators
6) Power House

Dam:- A dam is a barrier which reserves the water and creates water head.reserved water will be at some height this height results in potential energy (E=mgh)of water.where h is height of the water that depends on height & storage capacity of the dam.

Reservoir:-The part of river where water will be stored is called Reservoir.

Control Gate&Penstock:Penstocks are generally made of reinforced concrete or steel to transport water from reservoir to turbine with less friction losses.control gate is used to control over the water travelling in penstock.

Turbines and generators:-Water turbines are used to convert the energy of falling water into mechanical energy.generators to produce electrical energy from rotating shaft of turbine.

Power House:-At the power house generated power from alternator will be stepped up and supplied to transmission lines.

Typical Hydro Electric Power Station Block Diagram

Choice of Site for Hydro-electric Power Stations:-

The following points should be taken into account while selecting the site for a hydro-electric power station :

(i) Availability of water & Storage of water. Since the primary requirement of a hydro-electric power station is the availability of huge quantity of water, such plants should be built at a place (e.g., river,canal) where adequate water is available at a good head not only the amount of water,storage of water at heights also important to convert potential energy of water into kinetic energy then into electrical energy.

(ii) Capital Cost and type of land. The land for the construction of the hydel power plant should be adequate to withstand the weight of heavy electrical equipment like alternators,turbines,transformers and cost of the land should be less to get profits.

(iii) Transportation facilities. The place selected for a hydro-electric (hydel power)plant should be accessible by rail and road so that necessary equipment and machinery could be easily transported.

Ideal choice of Hydro-electric Power Stations is a site at  river in hilly areas where dam can be conveniently built and large reservoirs can be obtained.

Advantages
(i) It requires no fuel as water is used for the generation of electrical energy.
(ii) It is quite neat and clean as no smoke or ash is produced.
(iii) It requires very small running charges because water is the source of energy which is available free of cost.
(iv) It is comparatively simple in construction and requires less maintenance.
(v) It does not require a long starting time like a steam power station. In fact, such plants can be put into service instantly.
(vi) It is robust and has a longer life.
(vii) Such plants serve many purposes. In addition to the generation of electrical energy, they also help in irrigation and controlling floods.
(viii) Although such plants require the attention of highly skilled persons at the time of construction, yet for operation, a few experienced persons may do the job well.
Disadvantages
(i) It involves high capital cost due to construction of dam.
(ii) There is uncertainty about the availability of huge amount of water due to dependence on weather conditions.
(iii) Skilled and experienced hands are required to build the plant.
(iv) It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.


Reference:Google Books,Hydro Power Plant PDFs Download

Searches related:Hydro-electric Power Station
hydroelectric power station in india
hydroelectric power station advantages and disadvantages
how does a hydroelectric power station work
hydroelectric power station pdf
hydroelectric power station wikipedia
hydroelectric power station definition
thermal power station
hydroelectric power station energy conversion

Read More

Thermal Power Generation Plant or Steam Power Station

Thermal Power Generation Plant or Steam Power Station :

Thermal power generation plant or Steam Power Station is the most conventional source of electric power over the world.We can define thermal power station as "A generating station which converts heat energy of coal combustion into electrical energy through steam as intermediate energy form is known as a steam power station."

Theory of Thermal Power Station:- 

A steam power station basically works on the Rankine cycle. Steam is produced in the boiler by utilizing the heat of coal combustion. The steam is then expanded in the prime mover (i.e., steam turbine) and is condensed in a condenser to be fed into the boiler again. The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy. This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated.

Schematic Arrangement of Steam Power Station:-

Although steam power station simply involves the conversion of heat of coal combustion into electrical energy, yet it embraces many arrangements for proper working and efficiency.The bituminous coal is used as boiler fuel has volatile matter from 8 to 33 % and ash content 5 to 16 %. To increase the thermal efficiency, the coal is used in the boiler in powder form.

The thermal efficiency of a modern steam power station is about 30%. It means that if 100 calories of heat is supplied by coal combustion, then mechanical energy equivalent of 30 calories will be available at the turbine shaft and rest is lost. It may be important to note that more than 50% of total heat of combustion is lost in the condenser. The other heat losses occur in flue gases, radiation,ash etc.

Schematic Diagram Of Steam Power Station (Thermal Station):-

For Examination Purpose Draw This 

How Thermal Power Generation Plant Will Work?

1. Coal and ash handling plant(Numbers 1,2,8):- 

The coal is transported to the power station by road or rail and is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal strikes, failure of transportation system and general coal shortages. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air. The pulverized coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is necessary for proper burning of coal.

2.Boiler(Number 3):- 

The heat of combustion of coal in the boiler is utilised to convert water into steam at high temperature and pressure. The flue gases from the boiler make their journey through superheater, economiser, air pre-heater and are finally exhausted to atmosphere through the chimney.

(ii) Superheater.

The steam produced in the boiler is wet and is passed through a superheater where it is dried and superheated (i.e., steam temperature increased above that of boiling point of water) by the flue gases on their way to chimney. Superheating provides two principal benefits.

Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of turbine (which would cause blade corrosion) is avoided. The superheated steam from the superheater is fed to steam turbine through the main valve.

(iii) Economiser.

An economiser is essentially a feed water heater and derives heat from the

 flue gases for this purpose. The feed water is fed to the economiser before supplying to the boiler. The economiser extracts a part of heat of flue gases to increase the feed water temperature.

(iv) Air preheater. 

An air preheater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat from flue gases and increases the temperature of air used for coal combustion. The principal benefits of preheating the air are : increased thermal efficiency and increased steam capacity per square metre of boiler surface.

3.Steam turbine. 

The dry and superheated steam from the superheater is fed to the steam
turbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation.

4. Alternator(Number 5). 

The steam turbine is coupled to an alternator. The alternator converts mechanical energy of turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators.

5. Feed water.

The condensate from the condenser is used as feed water to the boiler. Some
water may be lost in the cycle which is suitably made up from external source.The feed water on its way to the boiler is heated by water heaters and economiser.This helps in raising the overall efficiency of the plant.

6. Cooling arrangement. 

In order to improve the efficiency of the plant, the steam exhausted from the turbine is condensed by means of a condenser. Water is drawn from a natural source of supply such as a river, canal or lake and is circulated through the condenser. The circulating water takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the condenser is discharged at a suitable location down the river. In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used. During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled. The cold water from the cooling tower is reused in the condenser.

Choice of Site for Steam Power Stations or Thermal Power Generation Plant :-

(i) Supply of fuel.(should be near to mines to reduce transportation cost)

(ii) Availability of water.(high availability of water is required for boiling & cooling)

(iii) Transportation facilities.

(iv) Cost and type of land.

(vi) Distance from populated area.

(v) Nearness to load centre

Thermal efficiency, 

ηthermal =Heat equivalent of mech. energy transmitted to turbine shaft/Heat of coal combustion

The thermal efficiency of a modern steam power station is about.

Advantages & Disadvantages Of Thermal Plants:-

Advantages

(i) The fuel (i.e., coal) used is quite cheap.

(ii) Less initial cost as compared to other generating stations.

(iii) It can be installed at any place irrespective of the existence of coal. The coal can be transported

to the site of the plant by rail or road.

(iv) It requires less space as compared to the hydroelectric power station.

(v) The cost of generation is lesser than that of the diesel power station.

Disadvantages

(i) It pollutes the atmosphere due to the production of large amount of smoke and fumes.

(ii) It is costlier in running cost as compared to hydroelectric plant.

Searches related:Thermal Power Generation Plant or Steam Power Station,thermal power plants ppt,types of thermal power plants
solar thermal power plants,coal combustion at thermal power plants
moef guidelines for thermal power plants,cea guidelines for thermal power plants,thermal power plants in jhajjar,thermal power plants pollution wikipedia

Read More