December 2016 – APSEEE

Month: December 2016

Switchgear | Protection system

Switchgear

Protection system is required for power system. An electrical energy becomes a part and parcel of over daily life. Electrical energy generate in bulk to meet the demands of consumers. The energy is required for lighting, heating, industrial electrical machinery and many other purposes. In present days, we totally rely on electrical energy. So it is desirable to protect the power system from damage during fault conditions such as in short circuit conditions. The protection is achieved by an apparatus known as switchgear. A switchgear consist of switch, fuse, circuit breaker, relays etc.



During normal operation, the protecting device or switchgear allow the switch on or off the electrical equipment machinery, transmission lines, generators etc. When faults occur on any part of power system the value of current is raised from the current flowing during the normal condition. The switchgear detects this increased fault current and disconnect the unhealthy section from the power system. In this way, the switchgear protects power system from adverse effects of faults and maintains the continuity of supply.

What is switchgear? or definition of switchgear

Switchgear usually consists of switches, fuses circuit breakers and relays used for switch on or off, control the equipment, transmission lines, protect the system from faults.

The switchgear equipment essentially used for switch on or off and interrupting currents either under normal or abnormal conditions. The simple switch with fuse is a simplest form of switchgear used fro protection of domestic load and offices. For higher rating circuit, a high-rupturing capacity fuse is used in conjunction with switch. For very high rating circuits, this protection system is not advisable. In this case automatically operated circuit breakers are used to protect the electrical circuits.

Simple switchgear show in figure. The circuit consists of trip coil, battery, relay coil current transformer and circuit breaker. Under normal operating conditions, the contacts remain closed and the circuit breaker carries the load current. The protective relay is connected in the secondary of a current transformer. When fault occurs on a system heavy current flows through the current transformer which increase the secondary current. This current when flows through relay coil, it closes the trip coil circuit. A battery is connected in series with trip coil and current start flowing through the trip coil and trip coil gets energized and disconnect the unhealthy section from the healthy section.

Switchgear, Protection System

Basic requirements of switchgear

Reliability

It should be reliable. Reliability means that the switchgear must be ready to operate, correctly at all times and under all conditions of a fault and abnormal conditions of the power system fro which it has been designed.

Sensitivity

It should have sensitive. Sensitivity means the protection system sense the minimum value of fault quantity when it occurs in a power system. If the sensitivity is increased the cost is also increased.

Selectivity and Discrimination

Selectivity means switchgear should be such it select correctly that part of the system in which fault is occurred and disconnect or isolate the faulty part without disturbing the rest of the system. Discrimination means it must be able to discriminate healthy and unhealthy part of the power system.

Quick Operation

The switchgear would isolate the unhealthy section as quick as possible. The speed of operation of switchgear reduces the amount of damaged incurred.

Switchgear Equipment

Large number of equipment assembled in switchgear which used for switching and interrupting currents under normal and abnormal conditions. The equipment used in switchgear is switches, fuses, circuit breakers, relays and other equipment.

Switches

The switch is a device which is used for make or breaks an electrical circuit. The switch is used in low voltage and small current circuits. When it is used for heavy current and high voltage circuits on the breaking time of circuit, arc is produced between two contacts of the switch. For large power operations circuit breakers are used. The switches can be used under full load or no load conditions but it cannot interrupt the fault current. The switches may be divided into following types

  • Air-Breaker switch
  • Oil switches

Fuses

A fuse is a short piece of wire which melts when excessive current flows through it. Fuse is always connected in series with the circuit to be protected. Under normal conditions, it carry normal current without overheating. When faults occurs on the system excessive current start flows through the fuse and it melts and disconnect the faulty section from the healthy section.

Circuit Breakers

A circuit breaker is a protecting device which can make or break the circuit under no-load, full-load and fault conditions. It is so designed that it can be operated manually under normal conditions and automatically under fault conditions.

Relays

Relays are the device which sense the faults and send signal to trip coil or circuit breakers.



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Instrument Transformers

Instrument Transformers

Instrument transformers are used in with conjunction with ammeter and voltmeter to extend the range of meters. In dc circuit shunt and multipliers are used to extend the range of measuring instruments. Shunt is used to extend the range of ammeter whereas multiplier is used to extend the range of voltmeters. But in case of AC circuits instruments, it is not convenient to use shunt and multiplier for extend the range of ammeter and voltmeter. When it is necessary to measure high values of current voltage, use standard low range instruments in conjunction with specially designed and constructed accurate ration transformers. These types of transformer are called the instrument transformer. This instrument transformers are divided into two types.

  • Current transformer (CT)
  • Potential transformer (PT)

Current transformers are employed for measuring large current. Potential transformers are employed for measuring high alternating voltages.



Current Transformer

The current transformer is used in conjunction with current measuring device (such as ammeter, wattmeter, energy meter etc.). Current transformer is used with low-range ammeters to measure currents in high voltage alternating current circuit. It primary winding consist of one or more turns of thick wire connected in series with the line whose current is to be measured.

The current transformer is basically a step up transformer. The secondary winding of current transformer consists of a large number of turns of fine wire. The ratio of primary to secondary current is inversely proportional to the ration of primary to secondary turns. The current transformer has lead impedance or burden on the secondary is very small, so the current transformer always operates on short circuit conditions. The current in secondary depends upon the current flowing in the primary winding but in case of power transformer it depends upon load impedance. Following aspects the current transformer is differ from the power transformer. The3 amount of power which is handled by current transformer (CT) is very small. From the constructional point of view, current transformer (CT) comprises a high permeable laminated steel core.

Instrument Transformers Current transformer

The fine wire of secondary winding with more turns wound on this core. The secondary is usually designed to carry a current of 5A. The current transformer has primary to secondary current ratio of 100:5.

The clamp on meter is the type of current transformer.

Potential Transformer

This is another type of Instrument transformers. The potential transformer is step down transformer. Potential transformers are used in conjunction with standard low range voltmeter. The potential transformer (PT) operates on the same principle as a power transformer. The input voltage of the potential transformer may be high as 1,38,00V or higher and the output voltage of potential transformers has been standardized at 110V no matter what is the input voltage. The small voltage is chosen for output for reason of safety and reduces the necessary insulation of windings and terminals of the instrument which is connected to the potential transformer (PT). Sometimes PT is called voltage transformer.

In case of step down power transformer, the input winding has high voltage and low value of current flows through these winding. Whereas the output winding carry large current relatively at low voltage. But in case of potential transformer currents in both windings are comparatively small. This is because the load on potential transformer is an instrument having a high resistance. So that very small current is required to operate the potential coil of the instruments. So there is not required to construct potential transformer for heavy currents.

The potential transformers cooling are required. Voltage upto 5KV or 5000V type potential transformer (PT). Above 13,800 volts potential transformer always oil immersed type. The output voltage of potential transformer is required to operate instruments or relays etc. So the power required for PT is very small, their ratings are usually of 40 to 100W. The primary and secondary winding of the potential transformer protected from overload by fuses.

Instrument Transformers potential transformer

Instrument Transformers are used in industries, substations, power generating stations.



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Transformer

Electrical energy is generated for away from the load centre due to some reasons such as to get sufficient water head for hydro-electric power plant, availability of water for thermal power plant, availability of water for thermal power plant etc. Then the energy is to be transmitted at considerable distance for town, cities etc. Transmission of electrical energy at high voltage is economical, therefore some means are required for step up and step down the voltage. Step up is required at power generating station and step down is required at the load centre where it is to be used electrical machine used for this purpose is known as “Transformer”.



Electrical Transformer is a static electrical device, which transfer electrical energy from one circuit to another circuit at same frequency but voltage level is usually changed. Transformer consists of two windings (namely primary winding and secondary winding) and a magnetic core which is common for both primary winding and secondary winding. The primary and secondary winding is magnetically coupled. But electrically isolated.

The side which fed by source is called primary side and where load is connected is called secondary side of the transformer. There is no rotating part in the transformer that is why the transformer is most efficient electrical machine or device.

STEP UP AND STEP DOWN TRANSFORMER

When the transformer increases the voltage i.e. it output voltage is higher than the input voltage then it is called step up transformer and when it decrease the voltage i.e. its voltage on the secondary side is less than the voltage on primary side is called a step down transformer.

V1 = Primary voltage

V2 = Secondary voltage

V1> V2  step down transformer

V1< V2    step up transformer

Working Principle of Transformer

The working principle of transformer is mutual induction. Whenever a alternating current flows through a one winding of a transformer alternating flux is set up which links with the another winding and emf is induced.

Operation or Working

The alternating supply is given to the primary side of a transformer, which cause of alternating current start flowing through the primary winding and flux is set up in the primary winding.In the core, this flux linked with primary as well as secondary winding. The e.m.f induced in primary winding is called self induced e.m.f. The direction of induced e.m.f in primary winding is opposite to the applied voltage. The alternating flux when links with the secondary winding, an e.m.f is induced in it is called mutual induced e.m.f. The direction of induced e.m.f also opposite to the direction of applied voltage V1 according to the lenz’s law. There is no electrical connection between primary winding and secondary winding but electrical energy is transferred through magnetic flux which links both windings So that both primary and secondary winding are magnetically coupled not electrically. The induced e.m.f in the primary and secondary depends upon the rate of change of flux linkages.

working of transformer

Construction of a Transformer

The conventional transformer consists of two windings (namely primary winding and secondary winding) and a magnetic core. The two windings are insulated from each other and magnetic core. The magnetic core provides a continuous magnetic for flux which links both primary and secondary winding. The core is made from laminated silicon steel. The laminations are used for reducing eddy current loss. The laminations are insulated from each other by a light coating of varnish. The thickness of laminations varies from 0.35mm for a frequency of 50Hz to0.5mm for a frequency of 25Hz. The silicon steel is used for making the transformer core in order to reduce hysteresis losses

According to core construction and the manner in which the windings are placed. The transformer is divided into two types.

1 Core type

2 Shell type

Core type Transformer

In core type construction the primary and secondary winding are wound around two legs of limbs of a rectangular magnetic core. The primary and secondary winding is not wound  a separately limb, but interleaved. To construct the core type transformer of L-shape strips.

core type transformer

To avoid high reluctance at the joints where laminations are butted against each other the alternate layers are stacked differently to avoid continuously joints.

The low voltage winding always placed near the core in order to reduce amount of insulation materials required.

Shell type Transformer

In this type of construction of transformer have three limbs or legs. Both primary and secondary windings are wound on the central limb. The entire flux passes through the central part of the core but outside it is divided into two and passes through other two limbs. The low voltage winding is placed near to the core in order to reduce the amount of insulation materials required. In this arrangement the leakage flux is reduced to very small value.

EMF Equation of  Transformer

When the sinusoidal voltage is applied to the primary winding of a transformer, a sinusoidal magnetic flux is set up in high permeability magnetic core. This flux links with both primary and secondary winding.

N1 = Number of primary turns

N2 = Number of secondary turns

Фm = Maximum value of flux in weber, wb

f = Supply frequency in Hz.

emf equation of transformer

r.m.s value of induced e.m.f in primary winding

E r.m.s = 4.44 N1 фm volts

r.m.s value of induced e.m.f in secondary winding

E r.m.s = 4.44 N1 фmax volts.

Transformation Ratio

The ratio of secondary voltage to primary voltage is called transformation ratio of the transformer. It is denoted by K.

4

Transformer on NO Load

A transformer is said to be on no load when secondary winding is open circuited. The current I2 flows through secondary winding are zero. In case of ideal transformer the current draws by primary winding will be zero. In actual transformer, small primary current flows through the primary winding to compensate losses. Such as copper losses and iron losses. The value of this current is 2% to 6%. The primary current is called no load current I0 or exciting current. The exciting current has two components

transformer on no load

  • Active or working components
  • Reactive or magnetizing component

Active Component or Working Component

Active component is in phase with the applied voltage V1. Its function is to overcome the iron losses (eddy current loss and hysteresis loss) in the core of a transformer and small copper loss I2 R in the primary winding. Active or working component is denoted by Iw

Reactive Component or Magnetizing Component

Reactive component is in quadrature with applied voltage V1. It produces necessary flux in the core of a transformer. Magnetizing component in phase with the flux ф but lags behind the voltage by π/2. This component of I0 is denoted by Im.



Transformer Test

Open circuit and short circuits are performed to determine the transformer circuit parameter (RO and XO), efficiency and voltage regulation. Without actually loading the transformer, there tests are very convenient and economical.

Open Circuit Test

Open circuit test is conduct to determine the core loss, Pi and no-load current IO. Open circuit test is always performed on low voltage side and secondary side is kept open circuited. To measure no load current, no load input power and applied voltage instruments are connected in primary side are ammeter, voltmeter and wattmeter normal rated voltage V1 is applied to the primary winding. Small no load current IO start flows through the winding. The value of this primary current is very small, usually 2% to 10% of full load current. Copper loss is very small in primary and nil in secondary. The wattmeter reading indicates the core loss under no-load conditions.

poen circiut transformer test

The open circuit test is also called no-load test.

Short Circuit Test

Short circuit test is carried out to determine copper losses. Copper losses are required for the calculations efficiency of the transformer. Short circuit test is usually carried out on the high voltage side of the transformer and low voltage side is short circuited by a thick strip. A variable voltage is applied across the low voltage side (say primary winding). The input voltage is gradually increased till at voltage VSC, full load current II flows in the primary. The test is performed low voltage so that the value of core losses is very small with the result the wattmeter reading show the full load copper loss or I2 R losses for the whole transformer. Under short-circuit condition, there is no output from the transformer. Therefore all input is dissipated in loss and this loss is almost copper loss because the value of iron loss or core loss is very small. It may be neglected for performing short circuit test the instruments such as ammeter, voltmeter and wattmeter are connected on high-voltage side.

short circuit transformer test

Losses and Efficiency

There are two types of losses:-

  1. Copper losses
  2. Core or iron losses.
  3. Copper losses:-

The losses occurs in their ohmic resistance is called copper loss. The value of copper losses depends on the value of current which flows through the windings. Copper losses can be determined by short circuit test. Copper loss is directly proportional to the square of load current.

Copper losses in primary winding, PQ = I2 R1

Copper losses in secondary winding PC = I12 R2

Total copper losses PC = I12 + I12 R2

Core or Iron loses

Core or iron losses consists hysteresis losses and eddy current losses. The core or iron losses caused by the alternating flux in the core. To determine core or iron losses open circuit test is carried out.

Hysteresis losses = khf B106m watt/m2

Eddy current loss = kef2B2m t2 watts/m2

Efficiency of a Transformer

The efficiency of a transformer is defined as the ratio of output power to input power.

 

The efficiency of a transformer is very high.

All Day Efficiency

The ratio of output in kWh to the input in kWh of a transformer over a 24-hour period is called all day efficiency




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Autotransformer | Construction Advantage and Application

Working or Operating Principle and Construction of Autotransformer

The operating principle of autotransformer is same as two winding transformer. It  is one winding transformer i.e. the primary and secondary windings are inter-related.The construction of autotransformer is totally differ from conventional two winding transformer. It is a single winding transformer which is common for primary and secondary side.  There is no electrical isolation between primary and secondary windings. In a conventional two winding transformer, the primary and secondary windings are completely insulated from each other but are magnetically coupled by a common core. But in autotransformer primary and secondary windings are connected electrically as well as magnetically. Its working is same as conventional two winding transformer.



Show the connections of step up autotransformer and step down autotransformer. In case of step up transformer, winding PQ having N1 turns is the primary winding and winding QR having N2 turns is the secondary winding. In case of step down transformer winding QR having N1 turns is primary winding and winding PQ having N2 turns is the secondary winding. In case of this type of transformer the power transferred from primary to secondary conductively as well as inductively (transformer action). In an autotransformer primary and secondary voltages related in the same way as in a conventional two winding transformer. Diagram of auto transformer is given below

autotransformer construction

The autotransformer looks like a resistance type potential divider. But its operation it quite different from it. The autotransformer can be stepped up or stepped down the voltage level. But in case of potential divider it is not possible. In case of potential divider power loss occurs whereas it is not so in case of autotransformer. Voltage regulation also poor in case of a potential divider. Less conductor material is required in case of auto transformer as compared to two winding transformer. The cross sectional area of conductor is proportional to the current to be carried and length is proportional to number of turns. Here only one winding is used in autotransformer so that length is required less and hence saving in conductor material. An auto transformer used as a variac.

Advantages

The reduction in conductor material also reduces the core length. It means less core size is required in case of auto transformer.

It is cheapest than two winding transformer. In case of auto transformer saving in both conductor and core material which reduce the cost.

The losses are low in case of auto transformer as comparatively two winding transformer. Less losses results in high efficiency.

Applications

It is used as a starter for induction motor.

It is used in electrical testing laboratory.

It is used as booster to rises the voltage level.

It is used in locomotives for control equipment.

It is used as furnace transformers for getting a suitable supply from normal 230V supply.




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Single Phase Induction Motors

Single Phase Induction Motors

AC motor is divided into two types three phase induction motor and single phase induction motor. Three phase induction motor are employed for industrial purposes for bulk power conversion from electrical to mechanical. But for small power conversions. Single phase motors are mostly used. Single phase induction motors are available in large number of types to perform wide variety of useful services in home, offices, factories, workshops etc. Almost in all domestic appliances such as fans, mixers, washing machines etc single phase induction motors are employed. In this article we will discuss about working principle of single phase induction motors, types of motors and application of single phase induction motors.



Nature of field produced in single phase induction motors

The field produced in single phase induction motor can be explained by using double revolving field theory.

Double field revolving theory

This theory is based on the phenomenon that a single phase induction motor is not self starting but once rotated in one direction (either clockwise or anticlockwise). It will continue to rotate in that direction. This theory explain that pulsating field produced in single phase motor can be resolved into two components of half the magnitude and each rotating at synchronous speed in apposite directions. The two fields have same strength which are revolving with same speed. One in clockwise direction and other in anticlockwise direction. This is answer of question which may arise readers “why single phase induction motors are not self staring?

double field revolving theory single phase inductuion motor

Types of single phase induction motor

Different types of single phase induction motors are used for different purpose but in this article will focused only split phase induction motor and capacitor motors. According to their construction these are classified as

  • Split phase motors
  • Capacitor motors

The capacitor motors are further divided into following types

  1. Capacitor start motor
  2. Capacitor run motor
  3. Capacitor start capacitor runs motor

Split phase motor

Constructional features

The outer frame and stator of single phase split phase induction motor is similar to the three phase induction motor. But in case of split phase motor. It uses an additional winding called starting winding in addition to main winding. These windings are placed in the stator slots. Both the winding are connected in parallel. The starting or auxiliary winding has greater resistance and has small cross sectional area. Where as running or main winding has high inductance and has larger cross sectional area than starting winding. Both windings carry different current with different phase angle. The purpose is to get different currents sufficiently displaced from each other so that torque is developed at start.

Working of split phase motor

According to construction the motor has two windings. One winding has larger reactance and other winding has greater resistance. The running winding has larger reactance and current flows through running winding Im lags the supply voltage by greater angle фm.

split phase single phase induction motors

The current in the starting winding IS lags the supply voltage by lesser angle фS. The currents plowing through running winding and starting or auxiliary winding have a phase difference. This makes the single phase motor to two phase motor. Thus, a revolving field is set up in the stator and a staring torque is developed in the rotor and rotor picks up a speed. A centrifugal switch is connected in series with the staring winding. When the motor attains a speed about 75 % of synchronous speed there is sufficient centrifugal force to disconnect the starting winding from main supply if the switch is not opened the auxiliary winding over heated and may damage because it is made of thin wire.

Applications

The starting torque of split phase motor is poor. So this motor is used in washing machine e, fans, grinders etc.



Capacitor motors

It is also a split phase motor. In capacitor motors, a capacitor is connected in series with the starting winding. These motors have high starting torque because the angular displacement between starting current IS and main current Im can be made nearly 900. The capacitor in the auxiliary or staring winding may be connected permanently or temporarily. The types of capacitor motor named below

  • Capacitor start motors
  • Capacitor run motors
  • Capacitor start and capacitor run motors

Capacitor start motor

In capacitor start motors capacitor is connected in series with centrifugal switch and starting winding. The value of capacitor selected in such a way that the motor will give high starting torque. Capacitor employed is of electrolytic type and short time duty rating. When the motor attains the speed of about 75% of synchronous speed staring winding is cut off.

single phase induction motor capacitor start

Applications

The capacitor start motor is used in refrigerators

Capacitor run motors

These types of motors capacitor is permanently connected in the starting winding. Paper capacitor is used in capacitor run motors. Because the electrolytic capacitor is designed only for short time rating and hence can not be permanently connected in the winding. This motor has high power factor about unity.

single phase induction motor

Capacitor start and capacitor run motors

In this motors, two capacitors are used. One for starting purpose and other for running purpose. Electrolytic capacitor is used for starting purpose because it automatically disconnected by centrifugal switch when motor attains 75% of synchronous speed. Paper capacitor is used which remains in the circuit of auxiliary or starting winding during running conditions. Starting capacitor has higher value than the value of running capacitor.

capacitor start capacitor run motor



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Overhead Line Insulator | Pin Type insulator | Suspension type insulator | Disc Type insulator

Overhead line insulator is most important item. The overhead line conductors are naked and not covered with any insulating material. The line conductors are not directly connected to the line supports. These line conductors are firstly secured to the supporting structures by means of insulating fixtures, called insulators. Insulators avoid the current to flow through the overhead structure. Insulators are equipped with the cross-arms and the line conductor is attached to the insulators. The line conductor property insulated from supports.



The insulator should have following properties

High mechanical strength in order to bear conductor, wind force etc.

High electrical resistance of insulator material in order to prevent leakage current to earth.

High relative permittivity so as to provide high dielectric strength.

The insulating material should be non-porous and free from impurities.

Insulator should not crack when subjected to high temperature in summer and low temperature during winter.

Insulator Materials

Porcelain

Porcelain is the mixture of kaolin, feldspar and quartz. Porcelain is mechanically stronger than glass. The porcelain insulator is glazed with some material in order to prevent the dirt and dust on the surface of insulator. If this insulating material is manufactured at lower temperature, the mechanical properties of the porcelain are improved but the material remains porus. Due to poursity material gets deteriorate easily when is expose to atmosphere. If this material is manufactured at high temperature, the porosity is reduced but material becomes brittle. The porosity reduced the insulating properties of material and also dielectric strength of the insulating material. To overcome all problems the porcelain is manufactured at suitable temperature kiln is designed. The porcelain is one of the mast important insulators is consider among the insulators. The porcelain insulator is used in transmission line, distribution lines and in substations.

Glass

Glass is another type of insulator used in power system. Glass gives high resistivity and high dielectric strength. It is cheaper than porcelain insulator. The glass insulators have simpler design. Glass is quite homogeneous material and can with stand higher compressive stresses as comparatively porcelain. Due to all these advantageous glass insulator is not preferred because moisture more readily condenses on its surface and facilitates the accumulation of dirt and dust deposits and it reduces the dielectric strength of the insulator. This results insulator gets easily damaged.

Types of overhead insulators

Pin type insulators

Pin type insulators are used for supporting the line conductors. It is screwed on to galvanized steel bolt which in turn is installed on the cross-arm of the pole. There is groove on the upper end of the insulator for housing the conductor. The conductor which is placed in groove tied with an annealed wire of the same as that of conductor.

PIN TYPE OVERHEAD LINE INSULATOR SHED

These insulators may have one, two or three rain sheds. These rain sheds are so designed that during rain it provides sufficient dry space inner sheds. These insulators are designed upto 50KV because beyond this voltage they become uneconomical. Generally these insulators are not used beyond 33KV. These types of insulators are used on RCC poles and steel poles. Above 33KV, Suspension type insulators are used. The design of pin type insulator beyond 33KV increases the cost as well as size.

PIN TYPE OVERHEAD LINE INSULATOR

Suspension type insulator

The cost of pin type insulator is increases as the working voltage is used upto 33KV. Beyond this value the pin type insulator becomes uneconomical. For high voltage suspension type insulators are used. Suspension type insulators consist of a number of porcelain discs connected in series by metallic links in the form of a string. The suspension insulator hangs from the cross arms and conductor is attached to the last disc. By increasing no of disc in string the working voltage will increase.

SUSPENSION TYPE OVERHEAD LINE INSULATOR

The suspension type insulators have the following advantages over pin type insulator

Beyond 33KV the suspension type insulators are usually cheaper in cost.

Each disc of suspension insulator is designed for 18KV, so by connecting number of such discs in series a string of insulator can be designed for any system voltage.

In case of failure of any disc, damaged disc can be changed or replaced easily.

Suspension insulator flexible. Therefore, it is free to swing in any direction.



By the use of suspension type insulators the line conductors are less affected by lightning.

Strain insulators

At the dead ends, on sharp turns or at the river crossing the line is subjected greater tension. The insulator employed to relieve the line of excessive tension, strain insulators are used. For low voltage line, shackle insulator is used but for high voltage line strain insulators consisting of an assembly of suspension type insulators are used. The strain insulator must have good dielectric properties. When the tension in exceedingly high at Long River spans, two or more strings are used in parallel.

STRAIN TYPE OVERHEAD LINE INSULATOR

Shackle insulator

Shackle insulators are used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. The surface can be easily cleaned and it will not crack, when subjected to temperature changes.

SHACKLE OVERHEAD LINE INSULATOR




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Transmission Line Conductor | Copper Aluminium Iron Bundle ACSR

The transmission line conductor is most important thing as insulating materials, line supports, etc. The conductor is used to carry the from sending end (such as generating station) to receiving end. Conductors connect the generating station and grids or substations and substations to load. The conductors are used for transmission lines and distributor lines are stranded in order to increase the flexibility. The solid conductor, except of small sixes, is difficult to handle because of their poor flexibility. Stranded conductors usually have a central wire around which successive layers of 6, 12, 18 and 24 wires. Transmission line theory

Properties of conductor material

The conductor material used for transmit and distribute electrical power should have following properties




High conductivity

The conducting material used for transmit and distribution of electrical power should have higher conductivity. If the conductivity is higher, then smaller is the area of cross-section. It reduces the weight of conductor which results in reduction in projected area for wind pressure and it reduces the cost of supporting structure.

High tensile strength

The tensile strength of transmission line conductor should be high. So that conductor can bear mechanical stresses. Due the this property of conductor less number of towers are required because this conductor with this property can use larger spams. Consequently it reduces the cost of overhead lines.

Low specific gravity

Lower the specific gravity, smaller is the weight of conductor. It the weight of conductor is less, it results reduces cost of supporting structure.

Cost

Cost of transmission line conductor should be low because these conductors are used for long transmission lines. Hence, the cost of conductors also effect the capital outlay invested in transmission line.




Conductor material

The most commonly used conductor materials as a transmission line conductor are followings.

  • Copper
  • Aluminium
  • Aluminium conductor steel reinforced (A.C.S.R)
  • Galvanized steel
  • Cadmium copper

transmission line conductor

Copper

Copper is the best conductor due to its high electrical conductivity and greater tensile strength. Copper is always used in the form of stranded hard drawn copper. Though hard drawn reduces the conductivity of the conductor but it increases the tensile strength considerately.

Copper conductor has high current density. Therefore, lesser the cross-sectional area is required for the given current. It does no t corrode in normal atmosphere and is not subjected to electrolytic troubles. The copper conductor is quite homogeneous, durable and high scrap value. The medium hard-drawn copper conductor is suitable for distribution lines.

Copper is ideal conductor for transmission and distribution lines. But due to its high cost and non-availability. It is rarely used for overhead transmission lines. Now days Aluminium conductor is used for overhead lines. The use of copper conductor only fount machine winding.

Aluminium

Aluminium is cheaper in cost and lighter in weight as compared to copper. But on the same time Aluminium has low conductivity and tensile strength,. The conductivity of Aluminium is 60% to that of cooper. Smaller conductivity means larger diameter of Aluminium conductor is required as compared to the copper. The diameter of Aluminium conductor is about 1.26 times the diameter of copper conductor. The specific gravity of Aluminium is lower than that of copper. It means the Aluminium conductor weight is less than copper so that the supporting structures for Aluminium need not the made so strong as that of copper conductor. Due to light weight of Aluminium, it is liable to greater swings and hence larger cross section is required.

Aluminium conductor steel reinforced (ACSR)

ACSR consists of a core of steel wire surrounded by a number of Aluminium strands. The steel conductor used in centre is galvanized in order to avoiding rusting and electrolytic corrosion. ACSR conductor being of high tensile strength and lighter in weight which produces smaller sag. Therefore, linger spams can be used consequently cost of supporting structure is reduces.

ACSR conductor having following points worth noting

It has larger diameter than other type of conductor of same resistance, with this corona losses are reduces.

The sag with ACSR conductor is much smaller than Aluminium conductor which permits the use of tower having smaller heights.

Corona losses are considerably in case of ACSR conductor.

The ACSR conductor has more skin effect. Hence, the resistance of composite conductor is taken equal to that of a Aluminium alone.

The chemical action of zinc and the Aluminium in atmosphere gets deteriorates the conductor.

Galvanized steel

Steel has very high tensile strength. It is used for extremely long spans because of their very high tensile strength. The use of galvanized steel found suitable in rural areas where cost is most consideration. These are used where a small power is required to handled for a small distance. The resistivity of Galvanized steel is high as compared to copper Aluminium etc. Hence, it is not used in transmission or distribution lines.

The most important application of galvanized steel conductor is as earth wire. It is run at top of towers of transmission lines to protect the main line from lightning. Now a days used of galvanized steel wire is limited to telecommunication lines, earth wires, stay wires and guard wires.

Cadmium copper

The conductor being used in certain cases in copper allayed with cadmium. Cadmium is copper increases tensile strength but at the same time conductivity of cadmium copper is decreased.



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