November 2016 – APSEEE

Month: November 2016

Power Factor

The electrical energy is generated, transmitted and distributed in the form of alternating current. In alternating current the direction and magnitude of quantity is varied with regularly intervals. It means AC current has phase angle. The lag or lead of current in any electrical circuit from voltage make a angle between voltage and current is called power factor. There are three types of circuit resistive circuit, inductive circuit and capacitive circuit. In resistive circuit current and voltage always in phase. In case of inductive e circuit current lags behind the voltage and in case of capacitive circuit currents lead the voltage. Where current in phase with voltage is called unity power factor. Current lags behind the voltage called zero lagging power factors. Current leads the voltage is called leading power factor. Most of the loads are inductive in nature and hence have low lagging power factor. Low power factor causing increase in current which increase in loss. In this article we will discuss about concept, causes, disadvantages and methods of improvement of power factor.




Concept of Power Factor

Most of loads are inductive loads (such as induction motors). The current taken by inductive motors or inductive circuit consists of two components. One is called magnetizing component active components transformed energy into useful work and magnetizing component which is often termed as idle components. Magnetizing component is called wattles component. Active component being in phase with the voltage accounts for the useful work done and magnetizing component being in quadrature with the voltage does not do any work in circuit and is mainly responsible for the creation of magnetism. More magnetizing current causing low power factor.

What is Power Factor?

The cosine of the angle between voltage and current in an ac circuit is called power factor.

POWER TRIANGLE

The analysis of power factor can also be made in terms of power drawn by the ac circuit. Draw a triangle which is called power triangle.

1

 OA = VI cosф and represents the active power in watts or KW and denoted by P.

AB = VI sinф and represents the reactive power in VAR or KVAR and it is denoted by θ.

OB = VI and represents the reactive power in VA or KVA. It is denoted by S.

 

The following points may be noted from the power triangle.

  1. The apparent power consists of two components

Active power and Reactive power

Both are mutually perpendicular to each other.

(Apparent Power)2 = (active power)2 + (reactive power)

  • Power factor

    power factor

From above equation power factor may be defined as it is the ratio of active power to the apparent power.

  • It is also observed from the power triangle the reactive power is responsible for the low power factor. So, if the reactive power is more low the power factor of the circuit.

  • The reactive power is neither consumed in the circuit nor it does any useful work. A wattmeter does not measure reactive power.

Causes of low Power Factor

  1. Induction motors are extensively used in industries. Induction motor is used for variety of purposes. At full load 3-phase induction motor operates at a power factor of around 0.8 lagging. At light load the value of power factor is 0.2 to 0.8 which is considered very poor. The power factor of a single phase induction motors is about 0.6 lagging.

  2. Are lamps, electric discharge lamps and industrial heating furnaces operate at low power factor in the system.

  3. The transformers draw a magnetizing current from the supply system. This current is at a power factor of zero lagging.

DISADVANTAGES OF LOW POWER FACTOR

Power consumed by the circuit given by the relation

single phase power factor

Low power factor has following disadvantages.

  • Large KVA rating of equipments

The electrical machinery is always rated in KVA. The KVA rating of the machine is inversely proportional to the power factor. If the power factor of the load is low the drawn by the machine will be high and vice versa and large KVA rating of the machine is required.

  • Greater conductor size

To transmit or distribute a certain amount of power at constant voltage, the conductor will have to carry more current at low power factor.

  • Large copper losses

If the conductor will carry large current the loss will be also high because the power losses are directly proportional to the square of load current.

  • Poor voltage regulation

The large current at low power factor causes greater voltage drops in electrical machine (such as alternators, transformer etc.) Which drop the voltage at the supply end which results in poor voltage regulation. In order to keep the supply end voltage with in permissible limits, extra equipment is required.

To Avoid these limitations power factor correction is necessary.

Effects of low Power factor

  1. To meet the load requirement at a low power factor, the capacity of power plant and substation equipment has to be more than that which would be necessary if the load were demanded at unity power factor.

  2. At low power factor machine will draw large current hence higher energy losses.

  3. Law power factor causes the voltage regulation to be poor.

ADVANTAGES OF POWER FACTOR IMPROVEMENT

  1. By improving power factor circuit current is reduced.

  2. Improve the voltage regulation.

  3. By improving power factor copper losses is reduced due to decreasing line current.

  4. Reduction in investment in the system facilities per KW of the load supplied.

Power Factor Improvement

Low power factor has many disadvantages such large current required, large KVA rating more copper loss etc. Most of the load is inductive in nature. Therefore, it takes lagging currents. In order to improve the power factor, some device taking leading power should by connect in parallel with the load. Power factor formula is used to calculate the required KVAR to improve it. Power factor calculations are required to know the exact valve of equipment required for improve the low PF.  Power factor meter is used in industries to monitoring  the PF.

Power Factor Improvement Equipment

The power factor can be improved by using following equipments.

Power factor improvement using shunt capacitors

Shunt capacitors are uses in rating from 15 KVAR to 10000 KVAR. The power factor can be improved by connecting capacitors in parallel with the equipment which is operating at lagging power factor. The capacitors draws a leading current which neutralizes the lagging reactive component of load current and improve the power factor of the load. Shunt capacitor is also known as static capacitors. Shunt capacitor are invariably used for power factor improvement in industries.

 capacitor power factorcapacitive satic power factor

 Power Factor Improvement Using Synchronous Condenser

When the KVAR requirement is small, shunt or static capacitor is used. However when requirement exceeds 10,000 KVAR. It is generally more economical to use synchronous condensers. A synchronous condenser is a overs exited synchronous machines runs at no load. A synchronous motor takes leading current when over excited. Therefore, behaves as a capacitor. A synchronous condenser is connected in parallel with the equipment whose power factor is to be improved and it take leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved by using synchronous condenser.

power factor synchronous condenser

  By use of synchronous condenser a finer control is possible than by use of static capacitors.

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How to Reverse the Direction of Rotation of Induction Motors?

Reversal of direction of rotation three phase squirrel cage induction motor

The direction of rotor of three phase induction motors can be reversed by interchanging any two of the three supply terminals.

resverse the direction of motor in clockwisereverse the direction of rotation of anticlockwisw




In above diagram assume the motor is rotated in clock wise direction if we exchange any two of three supply terminals the direction of rotation of motor will be changed. In other words by changing the phase sequence of two phases the direction of rotation of motor will be reversed. It is shown in second figure.

Reverse the  Direction of Split Phase and Capacitor Motors




The direction of rotation of split phase and capacitor motors can be reversed by reversing the connections of either the main (or running) winding or the auxiliary (or starting) winding. For this purpose, four leads are brought outside the frame.

reverse the direction of rotation in clock wise in single phase induction motors

reverse direction

Reverse the Direction of Rotation of Universal Motor

The direction of rotation of universal motor can be reversed by reversing the direction of the current flow in either the armature or the field winding.

reverse the direction of universal motor

Reverse the Direction of Rotation of Shaded Pole Motor

The direction of rotation of motor depends upon the position of copper rings. Unless the machine is constructed so that copper rings cannot be shifted to the other side of the pole. Hence, the direction of rotation of shaded pole motor cannot be reversed.




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Semiconductor Diodes | PN junction Diode | Light emitting diode | Varactor Diode | Types of Diode

INTRODUCTION

Semiconductor diodes is considered very important component for construction of electronic circuits. we know that domestic supply is in AC nature, So convert AC into DC Semiconductor Diodes are used and for various function in electronic circuits  special purpose  are used, such as Varactor diode, Light emitting diode, Photo diode, tunnel diode etc. In this article we will discuss about semiconductor diode. A semiconductor diode made by using P-type and N-type semiconductor materials. Therefore, it is also known as PN junction diode.




SEMICONDUCTOR DIODE

The semiconductor diodes is a two terminal device, Positive terminal is taken from P-type material of diode and Negative terminal is taken out from N-type material of diode.

The positive terminal of semiconductor diodes is known as the “Anode”. The negative terminal of semiconductor diodes is known as the “Cathode”.

Semiconductor diodes also known as a crystal diode because it is developed out of crystal like germanium or silicon. A semiconductor diodes operated only when it is forward biased i.e. P-side of diode connected to the positive terminal of the battery and N-side of diode connected to the negative terminal of the supply. When N-side of diode is connected to the positive terminal P-side is connected to the positive terminal , then diode does not conduct. Hence, it is reversed biased.

Symbol of diode

VI CHARACTERISTICS OF SEMI-CONDUCTOR DIODE

The V-I characteristics of a semiconductor diodes is the curve drawn between the voltage and current of the circuit. To draw the curve, we have a circuit which consists of a resistance, a semi-conductor diode, voltmeter, ammeter and a battery. A resistor R is connected in series with semiconductor diode which controls the forward current from increasing the permitted value. The characteristics are explained under four regions.

fig-2-copy

Zero external voltage region.

Forward biasing region.

Reverse biasing region.

Breakdown region.

ZERO EXTERNAL VOLTAGE

When external voltage is not applied to the circuit i.e. the switch K is open (fig-2) then no current flow through the circuit.

FORWARD BIASING REGION

In forward biasing region, P-side of diode is connected to the positive terminal of the battery and N-side of diode is connected to negative terminal of the battery. When switch K of the circuit (fig-2) is closed, then the PN junction is forward biased. At forward bias, the current increased slightly till the barrier potential is wiped off.

fig-2-copy

V-I characteristics in the forward region reveals that the current is negligibly small for voltage smaller than 0.5v. This value is known as “cutin voltage”. The current slowly rised because applied voltage is used to overcome the potential barrier (0.7v for si) of PN junction. When the potential barrier is eliminated and supply voltage is increased further. Then circuit current rises very sharply. This curve is linear.




The forward voltage 0.7v for silicon diode at which current through semi-conductor diode start rising is known as “knee voltage”. If forward current increased more than the rated value, then diode can be damaged.

vi characteristics of semiconductor diode

REVERSE BIASING REGION

In reverse biasing region, P-side of diode is connected to negative terminal and N-side of diode is connected to the positive terminal of the battery. Then PN junction act as reverse biased (fig 4). Under such condition, potential barrier increased at the junction and no current flow through the circuit because junction resistance becomes very high. In actual, very small current flow through the circuit (in order of μA) due to minority carriers. 

reverse bias circuit

The reverse current increase slowly with increase is reverse bias voltage. For silicon diodes, the maximum value of reverse current is 1 μA.

BREAKDOWN REGION

The breakdown region occurs with when reverse voltage increased continuously and exceeded the threshold voltage level that is enough to the particular diode, called as Breakdown Region.

At this stage, kinetic energy of electrons increased that they knock out electrons from semiconductor bonds, so breakdown region occurs at this stage. This may destroy the junction.

TYPES OF DIODES

As we know semiconductor diodes are mainly used for rectification but there are different types of diode which are used in non-rectifier applications. There diodes are given below.

Zener diode

Light emitting diode (LED)

Schottky diode

Varactor diode

Tunnel diode

ZENER DIODE

Zener diode is a special purpose diode which is used to operate in the breakdown region. But normal diode can not worked in breakdown region because this can be damaged them.

Zener diode have highly doped PN junction and it permits current to flow in the reverse direction. When its Zener voltage reached. It s symbol is shown in the fig 5 below. Its symbol is similar to ordinary diode except that its bar turned in Z-shape.

fig-5

VI-CHARACTERISTICS OF ZENER DIODE

The V-I characteristics of the Zener diode are similar to the ordinary diode. Except that it has sharp breakdown voltage which is known as Zener voltage (vZ). This voltage us almost constant over the operating region.

Zener diode can also operated in the forward region like an ordinary diode and used for rectification purpose.

During the operation, it will not damaged as long as external circuit controls the current flowing through it below the maximum rating current of Zener i.e. IZM. The characteristics of Zener diode as shown in figure.

symbol of zener diode

ADVANTAGES

It controls the reverse current flowing through it.

It has small size.

It is compatible with many electronic circuits. However, Zener diodes are preffered method to regulate voltage.




DISADVANTAGES

It cannot be used for rectification because of high cost.

It has poor regulation ration than the transistor.

APPLICATION OF ZENER DIODE

Some important application is given below.

Voltage regulation

Voltage regulation is also known as Zener diode shunt regulator or voltage stabilizer. It has ability to maintain a constant output voltage even when the input voltage or load current has variation. Fig shows the circuit arrangement of voltage regulation.

fig-7

As shown in the fig , the series resistance RS is used to limit the reverse current flow thorough the diode. The Zener diode reversely connected across the load resistance RL across which constant voltage is required. To get the constant voltage, we used series resistance RS which absorb the variation of output voltage.

When the value of vin is less than the Zener voltage VZ , then no current flow through the diode and same voltage appear across RL. When Vin voltage is more than VZ, then Zener diode conducted and large current flow through diode. Thus, the input voltage excess of VZ is absorbed by RS and constant voltage VOUT is maintained across RL.

ADVANTAGES OF ZENER VOLTAGE REGULATOR

They are smaller in size and lighter in weight.

They are also simple and cheaper.

They have longer life.

DISADVANTAGES

Their efficiency is low.

Output voltage varies slightly due to impedance.

METER PROTECTION

Zener diodes are also used in multimeter to protect the movement of meter against damaged from accidental overloads. In the circuit, Zener diodes are connected in parallel across the meter. Most of current will pass through the Zener diode. Thus protect the meter movement from damage, which have two Zener diodes connected can provide overload protection regardless of the applied polarity.

meter protection zener diode

ZENER DIODE AS A PEAK CLIPPER

Zener diode is also used as a peak clipper which clipping off the input wave. It is used to convert the sine wave into square wave. Peak clipper circuit shown below in fig.

peak input

Zener diode act like a short circuit when forward biased and open circuit when reversed biased till it go into breakdown region at VZ. For positive half cycle of input Zener diode D1 is forward biased and act as short circuit while diode DZ is reversed biased and act as circuit upto Zener voltage VZ. Therefore diode D2 comes into breakdown because of reversed biased and take the output voltage come out at VZ till the input voltage comes below VZ. St this time both the diodes act as short circuit.

When input voltage less than the Zener diode voltage VZ then diode D2 come out of breakdown and act as open circuit. During negative half cycle, the diodes D1 and D2 are forward biased. Then the result of output voltage waveform clipped off an both peak of input.

SWITCHING OPERATION

Zener diode is useful for switching operation, which can produce to change from low current to high current.

LIGHT EMITTING DIODE (LED)

LED operated on the phenomenon of electro luminance which emits the light from a semi-conductor under the effect of electrical field. When a diode is forward biased, the recombination of charge carrier take place as electrons cross from N-region to P-region and recombine with holes. Free electrons are in conduction band and holes in valance band of the energy band. There free electrons give up energy in form of heat and light in fig 11.

symbol; of LED

The light emitting diodes made from different materials. There are made by gallium arsenide phosphide (GaAsp) and gallium phosphide (GaP). With these semi conductor material, the electron give up their energy by emitting photons. ELD emits no light when it is reverse biased.

LED have voltage level from 1.5v to 2.5 v and currents about 10 to 50 mA. Generally, 2V of voltage drop is used while designing any circuit and the brighters of LED depends upon the current of circuit.

APPLICATION OF LED

LED’s are used in digital watches, calculator, multimeter, telephone switch boards and panel indicators. In there systems, we visible light (such as red, green, blue). On the other hand, infrared LED’s used in burglar alarm system and other areas which require invisible radiation.

PHOTO DIODE

A photo diode is a semi conductor device and is a kind of light detector. This is used to convert the light into voltage or current.

When photo diode is reverse biased, a small current flow throught the diode due to minority carriers. These carrier exist because of thermal energy which knock the valance electrons to produce the free electrons and holes.

When light energy falls on the PN junction, it gives energy to valance electrons and light striked on junction control the reverse current in a diode. The magnitude of the reverse current depends upon the intensity of light falling on the diode. Stronger the light larger the reverse current. Symbol and circuit shown in fig 12.

symbol of photo diode

APPLICATION OF PHOTO DIODE

It is used in optical communication devices.

Medical devices

Position sensors

Scanners

VARACTOR DIODE

Varactor diode is also known as varicap diode, variable capacitance diode, variable reactance diode or tuning diode. The symbol of Varactor diode is shown in fig .

fig-13

As shown in fig diode symbol consist of capacitor symbol at cathode side of diode which represents the capacitor characteristics, hence the named variable capacitance diode.

When the junction is reversed biased, the depletion region form a barrier which separates the positive and negative charger of the junction.

When reverse voltage applied, the width of depletion region incr3ease with increase in voltage and junction capacitance decreases as shown in fig.

characteristics of varactor diode

APLICATIONS

As a variable reactor, it’s used in microwave circuits.

It used in television receivers and communication equipments.

It is used as voltage controlled.

TUNNEL DIODES

Tunnel diode is also called as Esaki diode which exhibits negative resistance i.e. when the voltage increased, the current decreased through it. It is highly doped semi conductor device. 

Tunnel diodes are such types of diodes which have breakdown at OV(zero volt). When we increase the doping level of junction, with this heavier doping the forward curve of diodes is also distorted.

When diode is forward biases, then current reached to the maximum value of the peak (IP) at the peak voltage (VP). After with increased the valley voltage (VV).

Current decrease with increase in voltage between the peak and valley point. This is known as negative resistance region.

APPLICATIONS

Tunnel diodes are used in high frequency oscillatory circuit.

Also used as an amplifier.




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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.

<img = "half wave rectifier">

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

untitled

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.




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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.

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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.

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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.

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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.

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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.

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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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