APSEEE – Page 10

Intrinsic and Extrinsic Semiconductor Materials

INTRINSIC AND EXTRINSIC SEMICONDUCTORS

INTRINSIC SEMICONDUCTORS

The semiconductor available in an extremely pure form is known as intrinsic semiconductor. We know that a semiconductor at absolute zero temperature behave as an insulator because at absolute zero temperature valance band is completely filled and conduction band is completely empty. When some thermal energy is given to in covalent bond is broken and electrons cross over the conduction band and a vacancy leave behind is equal to the number electrons enters in the conduction band. The electrons reaching at the conduction band are free to move at random and similarly holes created also move at a random in semiconductor.




WHAT IS HOLES?

A vacancy left behind in the valence band due to entering of electrons in the conduction band. Holes in created in the place of electrons in in valence band when electron leave the valence band and enters in the conduction band. The charge on hole is positive charge. Holes are created when some external energy such as heat is given the semiconductors.

WHAT IS RECOMBINATION OF ELECTRON HOLE?

When some heat energy supplied to the semiconductor. The electrons in the valence band move away and enter in the conduction band. There is a vacancy left in the valence band. This vacancy is called holes. Electrons and holes are move randomly with in the crystal lattice, there is a possibility of an electrons meeting a hole when a free electron approaches the hole it gets attracted and falls into a hole. This merging of a free electrons and a hole is called recombination.




EXTRINSIC SEMICONDUCTOR

Intrinsic semiconductor is not usefu8l for making devices or components because it has little current conduction capability at ordinary room temperature. To improve the current conduction capability of the semiconductor small amount added in the pure or intrinsic semiconductor. The obtained semiconductor is called extrinsic or impure semiconductor. The above explanation tells why impurity is added in the intrinsic semiconductor?

DOPING: – The process of adding impurity in a pure semiconductor is called Doping. One impurity atom is added to 108 atoms of a semiconductor. The purpose of adding impurity in the semiconductor crystal is to increase the number of free electrons or holes to make it conductive. There are two types of impurity which is added to a pure semiconductor crystal. One is called pentavalent impurity and trivalent impurity. When pentavalent impurity is added to a pure semiconductor a large number of free electrons will exist in it. When a trivalent impurity is added to a pure semiconductor, a large number of holes will be generated. Accordingly the impurity added to a semiconductor, the extrinsic semiconductor my be classified as:-

N – TYPE EXTRINSIC SEMICONDUCTOR

N – Type semiconductor is obtained by added pentavalent impurity to pure semiconductors. Arsenic, antimony, bismuth or phosphorus are the examples of Pentavalent impurity. Penta means five. It means the atom has five electrons are present in the valance shell of the atom. When pentavalent impurity is added in semiconductor, four electrons is pentavalent atom share with four electrons of semiconductor. One spare valence shell electron is produce for each impurity atom added. Each spare electron so produced enters the conduction band of a pure semiconductor as free electrons. This free electron improves the conductivity of the material. We have seen there is large number of free electrons available in semiconductor. The charge on electron is negative. That is why this is called N-type extrinsic semiconductor because N is stands for negative. Pentavalent impurity is also called donar impurity.

n type semiconductor materials

 

P-TYPE EXTRINSIC SEMICONDUCTORS

P type semiconductor is obtained by adding trivalent impurity to pure semiconductors. Boron, Gallium, Indium or Aluminium are the examples of trivalent impurity. Tri mean three. It means the atom has three electrons are present in the valence shell of the atom. When a trivalent impurity is added to a semiconductor atoms. These impurity atoms from covalent bonds with four surrounding intrinsic semiconductors atoms but one bond is left incomplete and it incomplete bond creates a hole in simple words. One electron is less to make a bond. This missing electron is called holes. Small amount of trivalent impurity provides millions of holes in the semiconductors. Holes have a positive charge that is why it is called P-type extrinsic semiconductor P is stands for positive charge. This type of impurity is called acceptor impurity.

intrinsic semiconductor



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Rectifiers

Rectifiers

We know that electrical energy is generated at power plants in the form of ac due to many reasons such as ac system is economical but for the operation electronic devices or circuits, dc supply is required cells and batteries cannot meet the demands. So there is conversion of ac into dc is necessary. In Article we will study about, Half Wave Rectifiers and Full wave rectifiers.

What is Rectifier?

Rectifier is a electronic device which converts alternating current (ac) into direct current (dc).




According to their operation rectifiers divided into two types

  • Half wave rectifier
  • Full wave rectifier

Full wave rectifier further divided into two types

  • Centre-tap full wave rectifier
  • Full wave bridge rectifier

Half wave rectifiers

In half wave rectifier only one half of the supply of the alternating current is conducts. At output we have only one half cycle the other half of the supply is skipped.

Construction of Half wave rectifier

In half wave rectifier a semiconductor diode is connected in series with load RL. A transformer is used. Basically step down transformer is used. Secondary side diode and load is connected and Primary side of transformer is connected to the ac source.

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Operation or working

When primary side of the transformer connected to ac source of induced in secondary side.

  • During positive half cycle of the supply end B become negative. At this time diode D become forward biased and starts conduct and current start flowing through the load RL.



  • Path of current during positive half cycle is A – Load – B.
  • During negative half cycle of the supply end A of the transformer become negative and end B of the transformer become positive. At this time, the diode D become reverse biased and it does not conduct current and diode remains always in non-conducting state.

Output wave form

wave1

Centre-tap full wave rectifier

In full wave rectifier both half cycle of the alternating supply conducts.

Construction

In such type of rectifier two diodes are used. These diodes are D1 and D2 a centre tapped transformer is used. The secondary of the transformer is centre tapped. The load RL is connected in such a way that it carry the current in one direction.

full wave centre-tapped rectifier

Operation or working

The ac supply is connected to the transformer. During positive half of the supply end A of the secondary winding become positive and end B become negative. At this time D1 become forward biased and diode D2 become reverse biased. The diode D1 conduct while diode D2 does not. The current through D1 and load RL. During negative half of the supply end A of the secondary winding become negative and end B of the secondary winding become positive. At this time, diode D1 become reverse biased and diode D2 become forward biased. Now diode D2 conducts while diode D1 does not. The start flowing through diode D2 and load RL. The direction of flow of current is same, so the output is unidirectional. In this rectifier when one diode is conducts the other diode remains off condition means does not conduct.

Output wave form

wave form of full wave centre apped rectifier

Full wave bridge rectifier

In this rectifier four diodes are used. This type of circuit also conduct both half cycle of the alternating current.

Construction of full wave bridge rectifier

In such type of circuits the four diode (D1 D2 D3 and D4) connected in such a way the two diodes are conduct during positive half cycle of the supply and other two diodes are conduct negative half cycle of the supply. In this rectifier ordinary two winding transformer is used.

circuit of full wave bridge rectifier

Operation or working 

Ac supply is connected to the primary side of the transformer. By mutual induction e.m.f is induced in secondary winding of the transformer.

  • During positive half cycle of the ac supply end terminal A of the secondary winding become positive and  B become negative. At this time, diode D1 and D3 conduct while diode D2 and D4 does not. The current will flow through D1, load and D2. The current flows in one direction in same direction as the positive half cycle of the supply.
  • During negative half cycle of the ac supply terminal B of the secondary winding become positive and  A become negative. At this time, diode D2 and D4 conduct while diode D1 and D3 does not. The current will flow through D2, load and D4. The current flows in one direction in same direction as the positive half cycle of the supply.

Output wave form

wave form of full wave bridge rectifier




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Transmission Line Supports | Wooden Poles | RCC Poles | Steel Towers

MECHANICAL DESIGN OF OVERHEAD LINES

LINE SUPPORTS

Line supports are used to support the conductors of transmission lines and distribution lines. Line support must be capable of carrying the load due to insulators and conductors. The line supports may be wood, steel and reinforced concrete poles and steel towers.

The lines support should have the following properties.

  • The line supports should be high mechanical strength to withstand the weight of the conductors and wind load etc.




  • The line supports should be light in weight without the loss of mechanical strength.
  • The line supports should be cheap in cost.
  • The line supports should be longer life.
  • The maintenance cost of line support should be low.

WOODEN POLES

Wooden poles are cheapest in cost, easily available, provide insulating properties. These are made of seasoned wood (sal or chir). The use of wooden poles are limited to low voltage upto (22kv) and suitable for lines of relatively shorter spans upto 50 meters. These are extensively used for the distribution purposes in rural area. The cost of wooden poles is low. To present decay owing to snow and rain, the wooden poles are protected by Aluminium, zinc or cement cap at the top. The wooden poles usually tend to rot below the ground level, causing foundation failure. In order to prevent this, the portion of poles below the ground level treated with creosote oil or any preservative compound. The wooden poles well impregnated with creosote oil or any preservative have life from 25 top 30 years. Double pole structures of ‘A’ or ‘H’ type are often used to obtain a higher transverse strength than that could be economically provided by poles in normally is 10m to 12m.

The disadvantages of wooden poles are given below

  • The life of wooden poles is comparatively smaller than other poles.
  • The wooden poles cannot be used above the voltage rating higher than 20kv.
  • The mechanical strength of wooden is less.
  • Wooden poles are required periodic maintenance.




STEEL POLES

The steel poles are generally used in place of wooden poles. The steel poles has longer life, permit longer span and it possess greater mechanical strength. The span between two poles is 50 to 80 metre in case of steel poles. The cost of steel poles is higher than the cost of wooden poles. Steel poles are generally used for distribution purposes in cities. The life of steel poles is increased by regular painting. The steel poles are three types (i) tabular poles (ii) rail poles and (iii) rolled steel joist. The tabular poles rolled steel joist. The tabular poles are of round cross-section, the rail poles are of the shape of track and rolled steel joist are I cross-section. The rail poles are used in railways for electrification purposes. The average life of steel poles is more than 40 years.

Steel Poles

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ADVANTAGES OF STEEL POLES

  • The weight of steel poles is lighter than wooden poles.
  • Steel poles are easy to install.
  • It does not require special equipment for its erection.
  • Steel poles gives good appearance.

RCC POLES

These poles are made of reinforced cement concrete and usually it is known as concrete poles. Concrete poles are extensively used for low voltage and high voltage distribution lines upto 33kv. In recent years RCC poles becomes most popular as a line supports. RCC poles have greater mechanical strength, longer life and permit longer span than steel poles. RCC poles give good appearance, require very little maintenance, have good insulating properties than RCC poles and resistance against chemical actions. RCC poles can be used for longer span between 80m to 200m. Such poles are most suitable for work logged situations where other types of poles like wooden poles and steel poles will not be at all suitable. The disadvantage of these poles, is the high cost of transportation because these poles are very bulky and heavy. Therefore, RCC poles are manufactured at site itself to avoid heavy cost of transportation.

STEEL TOWERS

For long distance transmission lines at higher voltage steel towers are used. Steel tower have greater mechanical strength. Due to this property steel tower can permit the used of longer span. Steel tower can withstand most severe climatic conditions steel tower have longer life comparatively wooden poles and concrete poles. Steel towers also known as lattice steel tower. There are two types of lattice steel towers. (i) Narrow base lattice steel towers and (ii) broad base lattice steel towers are used for transmission at 11kv and 33kv and broad base lattice steel towers are used for transmission purposes at 66kv and above. The cost of steel tower very high but for longer spans steel towers is more economical. Lightning troubles is minimized by using lightning conductors at the top of the steel towers. Tower footings are usually grounded by driving rods into the earth. For protection against periodically painted. However, at a moderate addition cost, double circuit steel towers can be used because the double circuit steel towers give the insurance against discontinuity of supply. In the event of breakdown to one circuit it is possible to carry out repairs while maintaining the continuity of supply on the other circuit.

tower w
                                                        Double Circuit Tower

tower-2-copy_2                                                                Single Circuit Tower



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Semiconductor Physics and Materials

Semiconductor Physics and Materials

Semiconductor materials are those materials their conductivity lies in between the conductivity of conductors and insulators Germanium, Silicon, Carbon etc. are the semiconductor materials. Due to some useful properties of semiconductors they are extensively used in electronic circuits. Before the invention of semiconductors as an electronic components tube devices are used in the circuits, but now these days semiconductors components are extensively used because of their many advantages such as compact in size, cheap in cost, less maintenance etc. Now in these days the semiconductor devices occupies most important place in our daily life. Before studying the semiconducting materials we should about the behavior of the semiconductors.

What is an Atom?

An atom is tiny part of any substance which cannot be further subdivided. According to the BOHR’s ATOMIC MODEL (Neil’s Bohr is a Danish Physicist) an atom consists of a nucleus which contains neutrons and protons. Proton has positive charge whereas neutron has no charge. Extra nucleus contains electrons. The electrons revolve around the nucleus in different orbits. The charge on electrons is negative. An atom is electrically neutral because the number of electrons in an atom is equal to number of protons. The nucleus has a positive charge which attracts the electrons due to centrifugal force the electrons never stick on the protons.




IMG-20160818-WA0018

The electrons in the valence shell decide the behavior of a materials, whether the materials are conductors or semiconductors or insulators.

INSULATORS, SEMICONDUCTORS AND CONDUCTORS AND ENERGY BANDS

INSULATORS

Insulators are those materials which do not allow the passage of electric current through them. The valance band of the insulators is full, but conduction band is empty. So the forbidden energy gap is very large so there is not possibility of flow of electric current through an insulator, because the large amount of energy required pushing the valence electrons to the conduction band.

IMG-20160818-WA0016

SEMICONDUCTORS

Semiconductors are those materials whose conductivity lies in between conductor and insulators. In case of semiconductors the forbidden energy gap is more in case of conductor and less in case of insulator. It means valence electrons required least energy to enter the conduction band. Germanium, silicon carbon are the examples of semiconductor materials. Under normal condition, do not conduct current and behave as an insulator. When some energy (Heat of electric field) given to the valence electrons and some electrons cross over to the conduction band.

IMG-20160818-WA0020

CONDUCTORS

Conductors are those materials which allow the passage of current through them. The valance band of these materials overlap the conduction band due to this overlapping a large numbers of free electrons are available for conduction.

PROPERTIES OF SEMICONDUCTORS

  1. The resistivity of the semiconductors is less an insulators but more than a conductor. It means semiconductors neither a perfect insulator nor perfect conductor.

  2. The semiconducting materials have negative temperature coefficient of resistance. So that with increase in temperature on semiconductor the resistance is decreased and vice versa.

  3. When a suitable metallic impurity is added in the semiconducting materials, it changes the current conducting properties of the semiconductor appreciably.

COMMONLY USED SEMICONDUCTOR MATERIALS

There are large numbers of semiconductor materials are available such as, carbon, germanium, silicon etc. The minimum energy required for breaking the covalent bond in these materials is 7, 1.12, .75 etc. Carbon materials required large energy to break the covalent bond. In other words large energy is required to make conducting materials. Therefore, germanium and silicon are considered to be the most suitable semiconductor materials.

SILICON

The atomic number of silicon is 14. The silicon materials are obtained from silicon dioxide. To get pure silicon materials the silicon oxide is chemically treated. The number of electrons in the first orbit, second and third orbit is 2, 8 and 4 respectively. Silicon material is tetravalent because in valence shell of silicon has four electrons.

IMG-20160818-WA0019




GERMANIUM

It is an earth element recovered from the ash of certain coals or from the flue dust of zinc smelters. It was discovered in 1886. It is obtained from germanium oxide. Germanium oxide is chemically treated and it is then reduced to pure germanium. The atomic number is 32. Therefore it has 32 protons in the nucleus and 32 electrons in the different orbits. The number of electrons in the first, second, third and fourth orbit are 2, 8, 18 and 4 respectively. Four electrons in last orbit show the germanium is semiconductor.

IMG-20160818-WA0021

COMPARISON BETWEEN SILICON AND GERMANIUM

Silicon and germanium are must common semiconductors which are used to fabricate the semiconductor devices or components. But we prefer silicon over germanium. The question is here why silicon semiconductor preffered over germanium? The answer is, In case of germanium the forbidden energy gap between valance band and conduction band is less as comparatively silicon. So less energy is required to germanium to change its conducting properties because many free electrons are available at room temperature. On the other hand silicon has no free electrons at room temperature. For this reason we considered silicon as a best semiconductor for fabrications of electronic devices or components.

 




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Electronics Component | Active Component | Passive Component

To perform a particular function, electronic circuits are designed by connecting suitable components. Theses components are active components and passive components. All the electronic circuits are contains these components. Both components are important to design a electronic circuits.

ACTIVE COMPONENTS

Active components are those components which are capable of amplifying or processing an electrical signal. There are two types of active components which are called tube devices and semiconductor devices. Vacuum diode, vacuum triode, vacuum pentode, thyratron etc are the example of tube devices. Where as PN junction diode, Zener diode, photo diode bipolar junction transistor (BJT), field effect transistor or FET, Unijunction transistor or UJT, silicon controlled rectifier SCR or thyristor, metal oxide semiconductor field effect transistor MOSFET, TRIAC, DIAC etc are belongs to semiconductor devices family. In above components tube devices are not used in present days because these components are bulky in size and high in cost.

PN JUNCTION DIODE

A PN junction diode is known as semiconductor diode. It has two terminals. Semiconductor diodes are used as a rectifier. It is also known as crystal diode.

BIPOLAR JUNCTION TRANSISTOR




Transistor is a three terminal three layer (PNP or NPN) semiconductor device. Transistor used as an amplifier because it can process the electrical signal. Bipolar junction transistor is so called because current conduction in transistor due to both charge carriers (electrons and holes). BJT is a current controlled devices.

FIELD EFFECT TRANSISTOR (FET)

Field effect transistor is a three terminal semiconductor device like bipolar junction transistor. The operation of FET depends upon the flow of majority carriers (either by electrons or by holes). A FET is a voltage controlled devices in which output current is controlled by the input voltage.

SILICON CONTROLLED RECTIFIER (SCR)

Silicon controlled rectifier is a three terminal (namely Anode, Cathode and Gate) four layer, three junction semiconductor device which is used for rectification, inversion purpose etc. SCR is used where large power has to be handled.

PASSIVE COMPONENTS

Passive components are those components which are not capable of amplifying or processing an electrical signal. The example of passive components is Resistors, inductors and capacitors we can not design electronic circuits without the use of passive components. In electronic circuit these components are as important as active.

RESISTORS

Resistors are the components which are used to limit the electric current in a circuit. It is also used to divide the voltage in the electronic circuits. The ability to restrict the electric current in a circuit is called resistance. The unit of resistance is Ohm.

TYPES OF RESISTORS

According to the operating conditions, the resistors may be divided into two types:-

Fixed resistors and variable resistors.

Fixed resistors may be classified as carbon composition resistors and wire wound resistors.

CARBON COMPOSITION RESISTORS

Carbon resistors are used in electronic circuits with lower power rating. It is made of mixture of carbon and clay. The value of resistors depends upon the properties of carbon and clay. The resistor element is enclosed in a plastic case, for insulation (to avoid leakage of current) and for providing necessary mechanical strength to the resistor element. This type of resistors mostly used in electronic circuits. The value of carbon resistor ranging from few Ohm to mega Ohm. The power rating of carbon resistor is generally ¼ W, 1/2W, 1W and 2W. The cost of this type of resistor is very low.

WIRE WOUND RESISTORS

The material used for wire is constantan ( 60% copper 40% nickel) and manganin. The materials should have high resistivity and low temperature coefficient of resistance. The wire wound resistors is coated with an insulating material such as baked enamel.




RESISTOR COLOR CODING

The size of carbon composition resistors is very small. So it is very difficult to print the value of resistance on their body. To overcome this difficulty color bands are printed on the body of the resistors. The method to representing the value of resistance is called color coding. There are four bands are printed on the resistors. The color band always read left to right from the end that has the band nearer to it. The first two band represent the digit. Third band represent the multiplier (the value which is multiplied by the significant digits). The fourth band shows the tolerance of the resistor. If there is no color in fourth band, then it shows in fourth band, then it shows that the resistor has ±20% tolerance.

IMG-20160818-WA0017

resistor

INDUCTORS

Inductor is a device or component which opposes the any change of current in the circuit is called an inductor. The property of a coil by virtue which it oppose change the magnitude and direction of current flowing through the circuit is called inductance. Inductor offers high impedance to ac but low impedance to dc. The function of inductor in electronic circuit is to block ac component but to pass dc component. There are two types of inductors fixed inductors and variable inductors. Filter chokes, iron core chokes and Radio-frequency inductors are the example of inductors.

CAPACITORS

Capacitor is a passive component. Capacitor consist of two plates and electrodes which are separated by an insulating material. The insulating material is basically a dielectric material. The function of a capacitor is to store electric charge. The ability of store electric charge in a capacitor is called its capacitance. Capacitance is the ratio of charge per unit voltage or potential difference. The unit of capacitance is farad (F). A capacitor is component which offers low impedance to ac but very high impedance to dc another may we can say that capacitor block the dc signal but pass the ac signal. Capacitor is main components which are used in electronic circuits. It is used in coupled circuit, by passing and filter circuits. The capacitors may be fixed and variable.

TYPES OF CAPACITORS

The most commonly used dielectrics are air, mica, paper etc. A capacitor is generally named after the dielectric used for example air capacitor mica capacitor, etc. The capacitor may be fixed value capacitor and variable capacitor. The different types of capacitor used in electronic circuits are paper capacitors, Mica capacitors, ceramic capacitor, electrolytic capacitors, Air capacitors etc.

PAPER CAPACITOR

Paper capacitor is most commonly used capacitor in electronic circuits. Paper capacitor consist of two electrode (Aluminum or tin) separated by paper impregnated with dielectric such as wax or oil. Paper capacitors are in available in wide range of capacitance values and voltage ratings.

MICA CAPACITORS

These capacitors are made of sheet of metals which are separated by dielectric medium such as mica sheets. These types of capacitors are mostly used in rf tuned circuits.

CERAMIC CAPACITORS

In these capacitors two electrodes or plates are separated by a ceramic materials titanium oxide or other silicates are used to obtain very high value of dielectric constant of ceramic material. Ceramic capacitors are available in different shapes such as disc type ceramic capacitors tubular ceramic capacitors etc.

ELECTROLYTIC CAPACITORS

In these capacitors electrolytic medium is used as a dielectric medium. The electrolyte may be borax, phosphate etc. These capacitors are used in filter circuits to reduce the ripples from the rectified output in rectifier circuit. These capacitors must be connected in the circuit as per polarity marked on the capacitor.




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Fuse and Types of Fuses

INTRODUCTION

Electrical Fuse is simple and cheapest device which are used for interrupting an electrical circuit under abnormal conditions (such as short circuit conditions, overload conditions etc). Fuses are used upto 66kv called high voltage fuse and low voltage fuse upto 400V. In this article, we discuss about definition of fuse, working principle and fuse, fuse element materials and types of fuse.




Definition of Fuse

What is fuse?

A fuse is small piece of wire, inserted in the circuit which melts when the value of current plows more than the permissible limit through it and thus break the circuit. Fuse is always connected in series with the circuit.

The fuse element is usually made of materials having low melting point, high conductivity etc. The action of a fuse is based on the heating effect of the electric current when flows through a fuse element. When the current flows through a fuse is in safe value, the heat developed in fuse element rapidly dissipated into the surrounding air and therefore fuse element remains at a temperature below its melting point. However when fault occurs in the electric circuit, the current exceeds the safe value or limiting value. The heat is generated due to this excessive current cannot be dissipated fast enough and fusible element gets heated, melts and breaks the circuit. If the value of current is large enough, the more rapidly the fuse will blow i.e. the fuse has inverse time current characteristics.

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ADVANTAGES AND DISADVANTAGES OF FUSE

ADVANTAGES

It is cheapest and simple form of protection.

It needs no maintenance.

It interrupts very large short circuit current without any noise, flame gas or smoke.

It operation is inherently completely. It means there is no required any more equipment for automatic action.

Its inverse time current characteristics enable. Its use for over load protection.

DISADVANTAGES

The fuse has following disadvantages: –

Considerable time is waste n rewiring or replacing a fuse after operation.

The current time characteristic of a fuse cannot always be correlated with that of the protected apparatus.

DESIRABLE CHARACTERISTICS OF FUSE ELEMENT

The fuse elements should have low melting point like tin and lead.

It should have high conductivity like silver and copper.

It should be free from deterioration due to oxidation. Silver material has such properties.

The fuse element should be cheap in cost like lead and tin.




FUSE ELEMENT MATERIALS

The materials used for fuse elements must be of low melting point, low ohmic loss, high conductivity, low cost and free from deterioration. A low melting point is available with a high specific resistance metal. The most commonly used materials for fuse element are lead, tin, copper, zinc and silver. For smaller value of currents tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For exceeding the current 15A this alloy is not used as the diameter of the wire will be lager so that beyond 15A rating circuits Copper wire fuses are employed. Zinc fuse is good if a fuse with considerable time-lag is required i.e. One which does not melt very quickly with a small overload.

The present trend is to use silver despite. Its high lost due to the following advantages.

It does not oxidized. It means it is free from oxidation.

It does not deteriorate when used in dry air. There is no effect of moist on the silver fuse element, when the silver surface is attacked and a layer of silver sulfide is formed at the surface of a fuse element which shield the silver fuse element from further attack.

The conductivity of silver fuse element is very high. Therefore, for a given rating of fuse element, the area of cross section of silver metal is required is smaller than that of other materials.

The conductivity of silver does not deteriorate with oxidation so that life of silver fuse element is long.

Owing to its high conductivity the mass of molten fuse element to be handled is minimum due to this property the operating speed is fast.

IMPORTANT TERMS OR DEFINITIONS

CURRENT RATING

It is the r.m.s value of current which the fuse element can carry without overheating. It depends upon the temperature rise of contacts of the fuse holder, fuse material and the surrounding area of the fuse.

FUSING CURRENT

It is defined as the minimum value of current at which the fuse element melts. It means the value of fusing current is always more than the value of rated current of the fuse element.

Approximated value of fusing current is given by

I = kd3/2

I = current

K is constant

d is diameter of the fuse element

The fusing current is depends upon the following factors: –

The fusing current the fuse element depend upon the type of materials used for making fuse element.

Cross-section of area of the fuse element whether it is round or rectangular.

Length the shorter length of the fuse element the grater the current can conduct heat easily.

Diameter of fuse wire.

Type of enclosure used.

FUSING FACTOR

It is the ratio of minimum fusing current to the current rating of the fusing element is called fusing factor. It is always greater than unity.

For a semi-enclosed or rewirable fuse which employs copper wire as the fuse element, fusing factor is equal to 2.0 and for cartridge fuses fusing factor is equal to 1.45.

BREAKING CAPACITY

Breaking capacity of a fuse is the r.m.s value of the AC component of the maximum prospective current with at rated system voltage.

PRE-ARCING TIME

When a fault occurs, the fault current rises sharply and heat is generated in the fuse element. At the fault current attains it cut-off value the fuse melts and an arc is initiated.

ARCING TIME

This is the time between the instant of arc initiation and the instant of arc being extinguished.

TOTAL OPERATING TIME

The sum of Pre-arcing time and arcing time is called total operating time.

TYPES OF FUSES

A fuse unit usually consists of a metal fuse element, a set of contacts between which it is fixed and a body to support the fuse element and isolate them.

How does a Fuse works?

Fuse is the simplest current interrupting device for protection of the electrical circuit against short circuit or overloads which results excessive current flow through the electrical circuit. Many types of fuses also incorporate means for extinguishing the arc that appears when the fuse element melts.

Different Fuse Types are given below

Low voltages fuses

High voltages

LOW VOLTAGE FUSES

Low voltage fuses are further divided into three types namely semi-closed or rewirable type and cartridge type.

REWIRABLE FUSES

Rewirable fuse is most commonly used fuse in house wiring and in small current circuits. It is simplest form of protection available. Semi-enclosed or Rewirable fuse is also sometimes called kit-kat type fuse. It consist a base and a fuse carrier. The fuse element is placed in fuse holder. Both are made of porcelain material.

When fault occurs, the value of current flowing through fuse element is increased which generated heat in the fuse element and fuse element melts.

CARTRIDGE FUSE

In case of cartridge fuse the fuse element is totally enclosed in a enclosed container and is provided with metal contacts on both sides. The cartridge fuse divided into two types (i) D-type and (ii) Link type.




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SUBSTATIONS

substation

 INTRODUCTION

The electrical power is generated at generating station (such as hydro electric power plant, thermal power plant and nuclear power pant) which is located far away from the load centre. Electrical power is transmitted through transmission lines and distributed through distribution lines. For economical reasons, voltage is transmitted at high voltage and distributed at low voltage 400/230 volts. These voltage transformations are carried out at substation, located at suitable places. Thus, it form the most important part of power system. In this article, we will study about the its various types and  various function.




SUBSTATION

It is an assembly of various apparatus (such as, circuit breaker, protective relays etc) which are installed to control transmission and distribution of the electric power.

The various function are given below

It is used to transformation of voltage level from higher level to lower level or vice-versa.

The function of substations is switch ON and OFF the power lines.

In some cases these required to convert DC power into AC power or vice-versa. For some electric traction system DC convert AC power into DC power. In electrolysis process DC power is used.

We can change the frequency from higher level or lower level to higher at the substations.

To improve power factor by installing capacitor banks at the receiving end of the line.

CLASSIFICATION OF SUBSTATION

These are classified in several ways. However, the most important ways of classifying the substations are given below

  1. Service requirement
  2. Operating voltage

ACCORDING TO THE SERVICE REQUIREMENT

TRANSFORMER SUBSTATION

The substations where voltage level is changed is called transformer substation. These substations receive electric power at some voltage level and deliver it at some other voltage. The main component in this substation is transformer. Maximum number of substations in power system is this type of substations.

SWITCHING SUBSTATION

In switching substation voltage level is not changed. It means incoming line voltage and outgoing line voltage is same. It performs only switching operation connect or disconnect the power lines.

POWER FACTOR CORRECTING SUBSTATION

In these types of substation power factor is improved. Such substation is located at the receiving end of the transmission lines. These substations are equipped with synchronous condensers and capacitor banks. But usually synchronous condenser is used.

CONVERTING SUBSTATION

In these types of substations are installed to change characteristics of the electrical power (such as converting ac into dc or vice versa). We know that electrical energy is generated in ac form. But most of application dc power is required such in electric traction system, in electrolysis processes. These substations receive ac power and converted into dc power. In these types of substations frequency level is also changed. We can change frequency from higher level to lower level and vice versa.




CLASSIFICATION ACCORDING TO THE OPERATING VOLTAGE

HIGH VOLTAGE SUBSTATION

These are used for the voltage from 11kv to 66kv. These types of substations are basically located in industries or near the consumers.

EXTRA HIGH VOLTAGE SUBSTATION

The operating voltage in this substation is varies from 132kv to 400kv. The electric power is generated at low voltage but transmitted at high voltage to reduce transmission losses. But electric power sends to consumer at low voltage. The of voltage is changed in EHV substations. These substations also located at generating station.

ACCORDING TO THE DESIGN

INDOOR SUBSTATION

In this type of substation, the apparatus is installed with in the substation building, so that why these substations are called indoor substation. These substations are designed for voltage upto 11kv but can be erected for 33kv. These are located where the surrounding atmosphere is contaminated with such impurities which may damage the  equipment. These substations are installed in industries. The extension is not possible. This is the limitation of indoor substations.

The auxiliaries of the indoor type of substations are

STORAGE BATTERIES

Storage batteries are used to operate the relays in substations. Batteries are heart of the substations.

Fire extinguishers.

Emergency lighting in substations in case of failure of supply.

OUTDOOR SUBSTATION

For the voltage above 66kv, outdoor substation is used. In outdoor substations, equipment is installed outdoor. It is because for high voltages, the spacing between conductors and the space required for the switches, circuit breakers and other equipment becomes so great that it is not economical to install the equipment indoor. The outdoor substations are divided into two types.

Pole Mounted Substation

Foundation Mounted Substation

POLE MOUNTED SUBSTATION

These types of substations are erected for mounting distribution transformer of capacity about upto 200KVA. The substation is located near the consumers and are cheapest, simple and small in size. All  equipment are mounted on the supporting structures and these equipment are outdoor types. The H type pole is used for supporting structure. In early days, the supporting structure is made from wood but now RCC structure is used. Triple pole mechanically operated (TPMO) switch is used for switch ON and OFF of HT transmission line. HT fuse is installed for protection purposes. Lighting arrester are also used to protect the transformer against the surge. Earthling is used to protect the maintenance cost of such transformer is low.

FOUNDATION MOUNTED SUBSTATION

These substations are built in open place and in this type of substations all the equipment is assemble into one unit enclosed by a fence from the point of view of safety. These substations on above 200KVA. To select the site for these substations, there should be good access for heavy transportation. The extension is possible in foundation mounted substations.

ADVANTAGES OF OUTDOOR SUBSTATION OVER INDOOR SUBSTATION

Fault can be located easier than indoor substations.

The extension of installation is easier, if required.

Building materials required for the outdoor substation is smaller than indoor substation.

Outdoor substation can be easily repaired.

The construction work required is comparatively smaller and cost of the switch gear installation is low.

DISADVANTAGES OF OUTDOOR OVER INDOOR SUBSTATION

The space required for the outdoor substation is more.

In case of outdoor substations more protection devices are required for the protection against lightning surges.

The length of control cables required is more.

UNDERGROUND SUBSTATION

In sophisticated areas or thickly populated cities, there is scarcity of land as well as the prices of land are very high these are installed. The equipment are placed underground. The design of requires more careful considerations as compared to other types of substations.

EQUIPMENT IN A TRANSFORMER SUBSTATION

BUS BARS

Bus Bar is a conductor carrying an electric current to which many connections may be made. It is a bar of conducting material such as Aluminum, copper etc. The Bus Bars are usually of Aluminum.

INSULATORS

The insulators performed two functions, first function is to support the conductor such as bus bar and other function is to confine the current to the conductors. The most commonly used material for the manufacture of insulators is porcelain. The insulators may be pin type insulator, suspension type insulators, strain type insulators etc.

ISOLATORS

Isolators are the switch which used to disconnect a part of system for maintenance and repair. The isolator is designed to open circuit at no load.

CIRCUIT BREAKER

A circuit breaker is a equipment which can open or close a circuit under normal as well as abnormal conditions. It is designed so those which can open the circuit manually under normal conditions and automatically under fault conditions. Circuit breaker may be air blast circuit breaker, oil circuit breaker vacuum circuit breaker and SF6 circuit breaker.

TRANSFORMER

Transformer is a static device which transfer electrical energy from one circuit to another at different voltage level, frequency remains same. A transformer used in a substation to step up or step down the voltage.

INSTRUMENT TRANSFORMER

The large current and high voltage cannot be measured with ordinary instruments. To measure these quantities at this level transformer are connected with these instruments. These transformers are called instrument transformer. The transformer connected with ammeter to extend the range of ammeter is called current transformer (CT) and transformer which is connected with voltmeter to extend the range of voltmeter is called potential transformer (PT).




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Corona and Corona Loss

corona

Corona is the phenomenon of the ionization of air surrounding the line conductor. When an alternating potential difference is applied between two conductors whose spacing is large as compared to the diameter of the conductors there is no change in the atmospheric air around the conductor, if the applied voltage is low. However potential difference between two conductors is increased and when potential difference or voltage exceeds the certain value the faint luminous  glow of violet color appears surrounding the conductors is called Corona or Corona Effect. The voltage at which corona glow appears is called critical disruptive voltage.




At higher voltage, where corona occurs it also accompanied by hissing noise production of ozone gas. If the voltage is increased again, the luminous envelope become higher, hissing noise is increased and power is also increased. If the applied voltage is increased to the break down value, a flash-over will occur between the conductors

If the shape of conductors is uniform and smooth the corona glow uniform through out of the length of the conductor. If shape of the conductor is not uniform to corona glow will appears brighter at the rough points.

In case of dc voltage the corona glow will be different in the two conductors. Positive conductor will glow brighter and negative conductor will glow with spotty light. The corona occurs at the voltage 100KV or above.

THEORY OF CORONA FORMATION

Some ionization always present in air surrounding the conductor due radio active and cosmic rays. Free electrons normally present in free electron space between the conductors. Neutral molecular also present in the free space. When the alternating potential difference is applied between the two conductors, voltage gradient is set-up in the air which will have maximum value at the conductor surface as the potential between two conductors increased the gradient around the conductor also increased. When the potential gradient at the conductor surface reaches about 30KV per cm max value (or 21.7 r.m.s value) the free electrons will move with certain voltage and collide with a neutral molecule with enough force to dislodge one or more electrons from it, thereby number of free electrons will increase. The process of ionization is thus cumulative. It means the number of free electrons is increased with every collision. If the ration of spacing between the conductors to the radius of the conductor is less than is, flash over will occur between two conductors before the corona phenomena occurs.

DEFINE CORONA

Corona is the phenomenon which is accompanied by violet glow, hissing noise and production of ozone gas.

FACTOR AFFECTING CORONA

ATMOSPHERE

Corona is affected by atmospheric conditions. In different atmospheric conductor the corona is different. Corona is caused due to the air surrounding the conductor. In the stormy weather the number of ions is more than normal atmosphere and as corona occurs at much smaller voltage as compared to fair weather.

CONDUCTOR SHAPE

The corona greatly affected by shape and conditions of the conductor. The rough and irregular surface will give rise to more coronas. Rough and irregular cause it decreases the value of breakdown voltage. A stranded conductor gives rise to more corona than a solid conductor. As the diameter of conductor is increased the corona is decreased.

LINE VOLTAGE

The line voltage greatly effects the corona. At low voltage corona is less. If the line voltage is increased the electrostatic stress is developed at the conductor surface which makes the air around the conductor conducting and corona is formed. It means corona is formed always at high voltage.

SPACING BETWEEN THE CONDUCTORS

By increasing the spacing between the two conductors the corona is reduced. This is due to if the distance between two conductors is increased the electrostatic stress at the conductor surface is reduced which results corona is reduced.

CORONA LOSS

When the electric field is applied to the conductor ions is produced which results in space charge which move round the conductor. These space charge is derived the energy from the supply system to maintain its motion. The space surrounding the conductor is loss. Due to this, additional energy which is required to space charger. Draws from supply system and the necessary energy required by the space charge considered into loss and this loss is called corona loss.

FACTOR EFFECTING CORONA LOSS

SUPPLY FREQUENCY

The corona loss varied directly as the supply frequency.

SUPPLY VOLTAGE OR LINE VOLTAGE




If the line voltage is increased the air around the conductor becomes ionized and greater is the electric field and greater the power loss due to corona.

LOAD CURRENT

When the load current is increased, due to flow of load current which produce I2R loss due to this loss conductor produce heat. Heating of the conductor prevents deposition of dew on the surface of the conductor and it reduces the corona loss.

CONDUCTOR DIAMETER

With increase in the diameter of the conductor, the electric field intensity is decreased. Hence with larger diameter of the conductor resulting in lower of corona power loss.

CONDUCTOR SURFACE

Conductor surface also affect the corona loss. The potential gradient at the surface of a stranded conductor is greater than it’s to the solid conductor. It means breakdown voltage is lower for stranded conductor and corona loss is more.

ADVANTAGES

Due to formation of corona, the air surrounding the conductor becomes conducting and virtual diameter of the conductor is increased. Due to increased virtual diameter electrostatic stress between the conductor decreases.

Another advantage of the corona is it reduces the effects of transients produced by lightning.

Due to corona electrostatic stress is reduce which improves the system performance by reducing the probability of flash over.

DISADVANTAGES

Due to corona, ozone gas in produced which chemically react with the conductor causing corrosion.

Due to corona, loss of energy which is dissipated in the form of light, heat, sound and chemical action.

The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may result radio interference with communication circuits.

METHODS TO REDUCE CORONA EFFECT

The corona effects can be reduced by the following methods

BY INCREASING CONDUCTOR SIZE

By increasing the size of conductor the voltage at which the corona occurs is increased and hence corona effects are reduced. The diameter of conductors can be increased by using hollow conductors and Aluminium conductor steel reinforced (ACSR) ACSR conductor is preferred for transmission lines.

BY INCREASING THE SPACING BETWEEN CONDUCTORS

By increasing the spacing between the conductors the voltage at which corona occurs is raised and hence corona effects can be avoided. However, the spacing cannot be increased too much. It will results in heavier supporting structures. It increases the cost.

IMPORTANT TERMS

CRITICAL DISRUPTIVE VOLTAGE

Critical disruptive voltage is defined as the voltage at which corona occurs. It depends upon the breakdown strength of the air. The critical disruptive voltage also depends on surface conditions of the conductor. The deposition of dust and dirt on its surface is due to irregularities on the surface. It reduce the breakdown voltage.

VISUAL CRITICAL VOLTAGE

Visual critical voltage is defined as the voltage at which corona glow appears all along the line conductors. At critical disruptive voltage corona phenomenon starts but it is not visible because the charged ions in the air must receive some finite energy to cause further ionization by collisions with molecules and due to this energy is radiate in the form of light energy.

RADIO INTERFERENCE

Radio interference is the effect which is introduced by corona effect. When corona discharge it emits radiations due to this radiations it introduce noise signals in the communication lines, radio and television receives. Radio interference cause power loss in the transmission lines. The rate of radio interference is more for smooth and larger conductors. The amplitude of radio interference level is inversely proportional to the frequency.




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Three Phase Induction Motors

Three Phase induction motors

An induction motor is most widely used ac motor in industrial purposes. More than 90% motors used in industries are poly phase or three phase induction motors.




Advantages of three phase induction motors

  1. It is relative cheap.
  2. It requires less maintenance.
  3. Three phase induction motor has self starting torque.
  4. It has simple and rugged construction.
  5. Three phase induction motor has good speed regulation.
  6. It has sufficiently high efficiency.
  7. It has reasonably good power factor.

Construction of three phase induction motors

The three phase induction motor has two main part called stator and rotor.

Stator

It is the stationary part of the induction motor. The stator consists of following parts.

Outer frame

It is the outer part of a induction machine. The function of the frame is to protect the inner parts of the machine and support the stator core and stator winding. The frame of the motor may be casted or fabricated. For small induction machine the outer frame is casted but for large machine it is fabricated.

Stator core

Stator core is that part of a machine where winding is placed. The core of a induction motor made of laminated silicon steel. Laminations are used to reduce eddy current loss where as silicon steel is used to reduce hysteresis loss. The thin sheets are separated by layer of varnish. The thickness of laminations varies from 0.3mm to 0.5mm.

Stator winding

The stator core carries a three phase windings which connected to the three phase supply. The three phase winding are placed in the stator core and six terminals are brought out. The winding may be connected in star or delta enameled copper is basically used as a windings in induction motor.

Rotor

It is a rotating part of a induction motor. There are two types of rotor used in induction motor.

Squirrel cage rotor

Most of three phase induction motors employed with squirrel cage rotor because of its simplicity and cheapness. A squirrel cage rotor consists of a laminated cylindrical core having circular slots at the outer periphery. In each slot copper, Aluminum bars are placed and their ends are short circuited at end with the help of end rings. Squirrel cage rotor has the advantage of being adaptable to any number of pole pairs. The rotor bars are permanently short circuited. The rotor slots are slightly skewed because of following reasons.

It reduce humming

It reduces the tendency of magnetic locking between stator and rotor.

Slip ring or Phase wound rotor

The motor employing this type of rotor are called slip ring induction motor or phase wound induction motor. A slip ring rotor employed with three phase winding wound for same number of poles as the stator winding. The slip ring rotor is not internally short circuited like squirrel cage rotor. The winding is connected in star or delta on one end and other three terminals are connected the slip rings. These slip rings are mounted on the shaft itself. The external resistance is added in the rotor circuit. This type of rotor is used where high starting torque is required at the starting time.

Principle or operation

When the three phase winding of stator is connected to the three phase supply the current start flowing through the three phase winding and produces a rotating magnetic field which revolve round the stator at synchronous speed (NS – 120f/p). This rotating field sweeps post the rotor conductor and due to relative motion between rotating magnetic field and stationary rotor, an e.m.f induced in the conductors. As the rotor conductors are short circuited the current start flowing through the rotor conductor. The current carrying conductors are placed in the magnetic field which is produced by the stator winding. Consequently, mechanical force acts on the rotor conductors. The torque is produces in the rotor and rotor start rotate in the same direction as the rotating field. According to the lenz’s law, the direction of rotor currents will be such that they lend to oppose the cause producing them. That is why rotor moves always in the direction of stator field. The magnitude of current induced in rotor depends upon the relative motion between rotor speed and rotating magnetic field in the stator.




When relative motion is zero i.e rotor is running at synchronous speed their will be no cutting of flux by the rotor conductors, hence is induced e.m.f in the rotor is zero .Due to no induced e.m.f in the rotor conductor, no current flows hence no torque.

Slip

The difference between the rotating magnetic filed or synchronous speed actual speed of rotor is called slip.

It is expressed as. 1

NS = synchronous speed

N = Actual speed of rotor

Percentage age slip, S%2

Since three phase induction motor always run at less than the synchronous speed that is why the induction is called synchronous motor. In other words, induction motors can never attains synchronous speed.

Significance or importance of slip

Slip plays very important role in the induction motor. Wee know that the induced e.m.f in the rotor is proportion to the difference between the rotor speed and synchronous speed. The difference in the rotor speed and synchronous speed is called slip. More the slip, more will be the induced e.m.f and rotor current resulting in the rotor will develop large torque. This means that the torque is directly proportional to slip.

Conclusion: – The slip is required to develop the necessary torque in the rotor. If the slip is zero no e.m.f induced in the rotor and no current hence the torque develops in the rotor will zero.

EXAMPLE: – A 3-phase, 50Hz pole induction motor runs at 1440 calculate the synchronous speed and slip.

SOLUTION:-

Pole = 4

Frequency = 50

calculations of slip in three phase induction motors

Percentage slip = 4%

Frequency of rotor current

The frequency of rotor currents depends upon the relative motion between rotor and stator field. When the rotor is stationary, the frequency of induced rotor currents is equal to the supply frequency. When the rotor rotates the value of frequency on of rotor currents depends upon slip speed. Difference between synchronous speed of rotating magnetic field of stator and actual rotor speed is called slip speed (NS – N = slip speed)

4

So, frequency of rotor current

fr = S.f

EXAMPLE: – 3-phase 50Hz, 4 pole squirrel cage induction motor runs at 1470 r.p.m. Calculate the value of frequency of rotor current.

SOLUTION:-

Pole = 4

Frequency = 50 Hz

NS = = 1500

5

Frequency of rotor currents, fr = s.f

fr = 0.02*50

f = 1Hz

Rotor current and rotor power factor

When rotating magnetic field cut the stationary rotor conductors an e.m.f is induced in the rotor conductor. Due to this e.m.f current start flowing through the rotor conductor because the rotor conductor are short circuited at the end of rotor in the case of squirrel cage induction motor and externally short circuited in the case of slip ring or phase wound rotor. The induced e.m.f rotor represents as E2 and current I2 at standstill.

I2 is given as

7

Induced e.m.f per phase in rotor = E2

Rotor impedance per phase = Z2

6

R2 = Rotor resistance per phase

X2 = Rotor reactance

Rotor current, I2 =

18

ROTOR POWER FACTOR: –

Power factor of rotor current, cosф2 =

17

AT SLIP, s

Induced e.m.f per phase in the rotor = SE2

Rotor resistance per phase = R2

Rotor Reactance per phase = SX2

Rotor impedance per phase, Z2

Rotor current per phase, I2 =12

TORQUE

Twisting moment of force is called torque. The torque developed in rotor of an induction motor is directly proportional to the (i) Rotor current I2 (ii) Flux per pole ф and (iii) Power factor of the rotor circuit, cos ф2.

T α ф2 I2 cosф2

E2 is directly proportional to the ф2 at standstill

E2 α ф2

T = K E2 I2 Cosф2

STARTING TORQUE: –

E2 = Rotor emf per phase

X2 = Rotor reactance per phase

R2 = Rotor resistance per phase

Z2 = Rotor impedance per phase

Starting torque, T5 = K E2 I2 Cosф2

Put the value of rotor current I2 and rotor power factor Cosф2 in above equation 14

The supply voltage V is constant so that E2 is also constant. The starting torque becomes.

induction motor




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Electric Power Supply System

Electrical energy plays an important role in daily life of human being. It also plays an important role for the economic development of any country. Electrical energy has changed living standard of human life. It brings the revolutionary changed in industries. Electrical energy generated at the power station that are far away from the load centre. These power stations are located far away from the load centre due to economical reasons. The generated electrical energy transmitted through transmission lines and then it is distributed to the different areas. Supply system divided into transmission and distribution.




Any electrical power system has three main components: –

  • Generating station

  • Transmission system

  • Distribution system

GENERATING STATIONS

The generating station is that part of power system where electrical energy is generated. The generating station may be hydro electric power plant, thermal power plants and nuclear power plants.

The electrical power supply system has two components (i) transmission system (ii) Distribution system.

The transmission system is that part of electrical power supply system which used to transmit electrical energy from power station to the load centers. The power transmitted through the transmission lines high voltage to reduce transmission losses. The transmission many advantages such as high transmission efficiency and saving of conductor material. The transmission system divided into two type primary transmission system and secondary transmission n system.

PRIMARY TRANSMISSION SYSTEM

      The electric power transmitted to the receiving station is called primary transmission system. The voltage carried by the primary transmission system may be 132kv, 220kv, 400kv or 765kv. The 3-phase 3 wire system is used primary transmission system.

SECONDARY TRANSMISSION SYSTEM

      The electric power is transmitted from receiving station to the substation is called secondary transmission system. The voltage is stepped down at the receiving station then it transmitted to the substation. 33kv or 66kv is carried in secondary distribution system.

DISTRIBUTION SYSTEM

Distribution system is also main component of an electric supply system. In distribution system electric power is distributed to the various consumers. There are two types of distribution system (i) Primary distribution system (ii) secondary distribution system.

PRIMARY DISTRIBUTION SYSTEM

      At substations the secondary transmission lines are terminated. Here the secondary transmission voltage is stepped up to 11kv. The 3-phase 3-wire system is used for primary distribution system. The bigger consumers are supplied power at 11kv. The 11kv lines are runs along the road sides of the city. These consumers have their own sub-stations to handling power.

SECONDARY DISTRIBUTION SYSTEM

The electric power from primary distribution line is delivered to the distribution sub-stations. These sub stations are located in localities near the consumer premises. At distribution sub-station voltage is stepped down to the 3-phase 4-wire system is used in secondary distribution system. The voltage between any two phases 400v and between any phase and neutral is 230v. Three phase lines is used for industrial load and single phase supply is used for residential lighting load.

NATURE OF SUPPLY CURRENT

There are two types of transmission.

AC transmission and DC transmission. Both systems have their own advantages and disadvantages. Comparison of AC and DC transmission system has given below: –

  1. Dc requires only two conductors. Ac system required three conductor which increase the cost of transmission line.

  1. Three is no inductance, capacitance, phase displacement present in DC system. In AC transmission system these problems are presents.

  2. A DC transmission lines has less corona loss and radio interference. Ac transmission lines have more corona loss.

  3. In DC transmission lines has no skin effect. In AC transmission lines has skin effect therefore current start flowing surface of the conductor instead of flowing through the center of the conductor and it increase the effective resistance of transmission line.

  4. Electric power  cannot be generated at high DC voltage due to commutation problems. There are no such problems in AC system. AC voltage can be generated at 11kv.

  5. Electric potential stress is less on the insulation case of DC transmission system but in case of AC transmission system this stress is high. So it needs more insulation than DC system.

  6. DC transmission system has no dielectric loosed

AC transmission line is no free from dielectric losses.

  1. DC transmission lines have better voltage regulation. AC transmission lines has poor voltage regulation.

In above comparison we arrive at a conclusion, DC transmission system is more superior than AC system, but inspite of all we choose AC transmission system for transmit electric power. Because AC system is easily handled, no extra equipment is required and AC power can be stepped up or stepped down which reduced transmission line losses.

Comparison between overhead and under-ground system.

We can use overhead and underground systems for transmit and distribute electrical power. Each system has their own it’s advantageous and disadvantageous. Comparison between the two electic supply systems is given below: –

  1. SAFETY: –

Public safety is main consideration when we design electric power supply system. Underground system is safer than overhead system.

  1. INITIAL COST: –

The initial cost of underground electric supply system is high due excavation work. The overhead electric supply system is cheaper than underground system.

  1. FAULT LOCATION: –

The fault is n overhead system can be easily locate than underground system.

  1. APPEARANCE: –

Underground electric supply system of distribution or transmission is good looking because no wiring is visible.

  1. VOLTAGE DROP: –

In underground system voltage drop is low because there is less inductance in case under-ground system as compared to over head system. Due to low voltage drop the efficiency of underground transmission and distribution lines is high.




  1. RATIO INTERFERENCE TO COMMUNICATION: –

There is no radio interference with communication circuits in case of under ground system.

  1. MAINTENANCE COST: –

The maintenance cost of underground transmission and distribution lines is very high as compared to overhead system.

  1. WORKING VOLTAGE: –

Underground electric supply system can be operated at limited supply voltage. It can be operated at 66kv or below because of insulation difficulties. But overhead system can be designed for operation up to 765kv.

  1. FLEXIBILITY: –

The extension of line is not possible in case of underground system. But in overhead system extension is possible.

  1. CHANCE OF ACCIDENTS: –

The chance of accidents is more in case of overhead system as compared to underground system.

The distribution system divided into three parts feeders, distributors and service mains.

FEEDERS

Feeders are the conductors which connect the small generating stations or substations to the area where electric power is to be distributed. There are no tappings are taken from the feeders. So that current in the feeders remains same throughout. The main consideration in the design of a feeder is the current carrying capacity.

DISTRIBUTORS

Distributors are the conductor where tappings are taken for supply electric power to the consumers. The current though the distributor is not same throughout. Because tappings are taken at several points. The main consideration in the design of a distributor is the voltage drop. The statutory limit of voltage drop is ±6%.

SERVICE MAINS

Service mains are the conductor that is used to connect the distributor to the consumer premises.

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