six units of m

8/7/2019 SIX UNITS OF M

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Definition of single phase transformer

A Transformer is a device that transfers electrical energy from one circuit toanother through inductively coupled conductorsthe transformer's coils. A varyingcurrent in the first or primary winding creates a varying magnetic flux in the

transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or"voltage " in the secondary winding. This effect is called mutual induction .

If a load is connected to the secondary, an electric current will flow in thesecondary winding and electrical energy will be transferred from the primarycircuit through the transformer to the load. In an ideal transformer, the inducedvoltage in the secondary winding ( VS) is in proportion to the primary voltage ( VP),and is given by the ratio of the number of turns in the secondary ( NS ) to thenumber of turns in the primary ( NP) as follows:

Basic principlesThe transformer is based on two principles: firstly, that an electric current

can produce a magnetic field (electromagnetism ), and, secondly that a changingmagnetic field within a coil of wire induces a voltage across the ends of the coil(electromagnetic induction ). Changing the current in the primary coil changes themagnetic flux that is developed. The changing magnetic flux induces a voltage inthe secondary coil.

An ideal transformer is shown in the adjacent figure. Current passing throughthe primary coil creates a magnetic field . The primary and secondary coils arewrapped around a core of very high magnetic permeability , such as iron , so thatmost of the magnetic flux passes through both the primary and secondary coils.
http://wiki/Electrical_energyhttp://wiki/Electrical_networkhttp://wiki/Inductive_couplinghttp://wiki/Electric_currenthttp://wiki/Magnetic_fluxhttp://wiki/Magnetic_fieldhttp://wiki/Electromagnetic_inductionhttp://wiki/Electromotive_forcehttp://wiki/Volthttp://wiki/Mutual_inductionhttp://wiki/Electrical_loadhttp://wiki/Electric_currenthttp://wiki/Magnetic_fieldhttp://wiki/Electromagnetismhttp://wiki/Electromagnetic_inductionhttp://wiki/Magnetic_fieldhttp://wiki/Magnetic_corehttp://wiki/Permeability_(electromagnetism)http://wiki/Ironhttp://wiki/Electrical_energyhttp://wiki/Electrical_networkhttp://wiki/Inductive_couplinghttp://wiki/Electric_currenthttp://wiki/Magnetic_fluxhttp://wiki/Magnetic_fieldhttp://wiki/Electromagnetic_inductionhttp://wiki/Electromotive_forcehttp://wiki/Volthttp://wiki/Mutual_inductionhttp://wiki/Electrical_loadhttp://wiki/Electric_currenthttp://wiki/Magnetic_fieldhttp://wiki/Electromagnetismhttp://wiki/Electromagnetic_inductionhttp://wiki/Magnetic_fieldhttp://wiki/Magnetic_corehttp://wiki/Permeability_(electromagnetism)http://wiki/Iron


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The repairing transformer in the M.P.E.B.

1. Instrument transformer

2. Poly phase transformer

3. Leakage transformer

4. Auto transformer

5. Audio transformer

6. Resonant transformer

Types of transformer

1. Instrument transformer:

Instrument transformers are used for measuring voltage and current inelectrical power systems, and for power system protection and control. Where avoltage or current is too large to be conveniently used by an instrument, it can bescaled down to a standardized, low value. Instrument transformers isolatemeasurement, protection and control circuitry from the high currents or voltages

present on the circuits being measured or controlled.

Fig: Instrument transformer
http://wiki/Power_system_protectionhttp://wiki/Power_system_protection


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2. Poly phase transformer:

It supplies, a bank of three individual single-phase transformers can be used,or all three phases can be incorporated as a single three-phase transformer. In thiscase, the magnetic circuits are connected together, the core thus containing athree-phase flow of flux. [46] A number of winding configurations are possible,giving rise to different attributes and phase shifts .[47] One particular poly phaseconfiguration is the zigzag transformer , used for grounding and in the suppressionof harmonic currents

3. Auto transformer:An autotransformer has a single winding with two end terminals, and one

or more terminals at intermediate tap points. The primary voltage is applied acrosstwo of the terminals, and the secondary voltage taken from two terminals, almostalways having one terminal in common with the primary voltage. The primary andsecondary circuits therefore have a number of windings turns in common. [43] Since the volts-per-turn is the same in both windings, each develops a voltage inproportion to its number of turns.

Fig: Poly phase transformer

Fig: Auto transformer
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4. Leakage transformer:A leakage transformer, also called a stray-field transformer, has a higher

leakage inductance than other transformers, sometimes increased by a magneticbypass or shunt in its core between primary and secondary, which is sometimesadjustable with a set screw. This provides a transformer with an inherent currentlimitation due to the loose coupling between its primary and the secondarywindings. The output and input currents are low enough to prevent thermaloverload under all load conditionseven if the secondary is shorted. Leakagetransformers are used for arc welding and high voltage discharge lamps (neon andcold cathode fluorescent lamps , which are series-connected up to 7.5 kV AC).

5. Resonant transformer:

A resonant transformer is a kind of leakage transformer. It uses theleakage inductance of its secondary windings in combination with externalcapacitors, to create one or more resonant circuits . Resonant transformers such asthe Tesla coil can generate very high voltages, and are able to provide muchhigher current than electrostatic high-voltage generation machines such as theVan de Graaff generator .[49] One of the applications of the resonant transformer isfor the CCFL inverter . Another application of the resonant transformer is to couplebetween stages of a superheterodyne receiver , where the selectivity of thereceiver is provided by tuned transformers in the intermediate-frequencyamplifiers. [50]

Fig: Leakage transformer
http://wiki/Leakage_inductancehttp://wiki/Arc_weldinghttp://wiki/Cold_cathode_fluorescent_lamphttp://wiki/Electrical_resonancehttp://wiki/Leakage_inductancehttp://wiki/Resonant_circuithttp://wiki/Tesla_coilhttp://wiki/Van_de_Graaff_generatorhttp://wiki/Van_de_Graaff_generatorhttp://wiki/Van_de_Graaff_generatorhttp://opt/scribd/conversion/tmp/scratch6177/ckt.htmhttp://wiki/CCFL_inverterhttp://wiki/Superheterodyne_receiverhttp://wiki/Superheterodyne_receiverhttp://wiki/Superheterodyne_receiverhttp://opt/scribd/conversion/tmp/scratch6177/ckt.htmhttp://wiki/Leakage_inductancehttp://wiki/Arc_weldinghttp://wiki/Cold_cathode_fluorescent_lamphttp://wiki/Electrical_resonancehttp://wiki/Leakage_inductancehttp://wiki/Resonant_circuithttp://wiki/Tesla_coilhttp://wiki/Van_de_Graaff_generatorhttp://wiki/Van_de_Graaff_generatorhttp://wiki/Van_de_Graaff_generatorhttp://opt/scribd/conversion/tmp/scratch6177/ckt.htmhttp://wiki/CCFL_inverterhttp://wiki/Superheterodyne_receiverhttp://wiki/Superheterodyne_receiverhttp://opt/scribd/conversion/tmp/scratch6177/ckt.htm


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7. Audio transformer:

Audio transformers are those specifically designed for use in audio

circuits. They can be used to block radio frequency interference or the DCcomponent of an audio signal, to split or combine audio signals, or to provideimpedance matching between high and low impedance circuits, such as betweena high impedance tube (valve) amplifier output and a low impedance loudspeaker ,or between a high impedance instrument output and the low impedance input of amixing console .Such transformers were originally designed to connect differenttelephone systems to one another while keeping their respective power suppliesisolated, and are still commonly used to interconnect professional audio systemsor system components.

8. 3 Phase Electrical Power Transformer:

Three phase transformers are used throughout industry to changevalues of three phase voltage and current. A 3 phase transformer, there is a three-legged iron core as shown below. Each leg has a respective primary and secondarywinding. Most power is distributed in the form of three-phase AC.. Basically, the powercompany generators produce electricity by rotating (3) coils or windings through amagnetic field within the generator. These coils or windings are spaced 120 degrees

apart. As they rotate through the magnetic field they generate power which is then sentout on three (3) lines as in three-phase power. 3 phase transformers must have (3) coilsor windings connected in the proper sequence in order to match the incoming power andtherefore transform the power company voltage to the level of voltage we need andmaintain the proper phasing or polarity.

Fig: 3 Phase Electrical Power Transformer
http://wiki/Valve_amplifierhttp://wiki/Loudspeakerhttp://wiki/Mixing_consolehttp://wiki/Professional_audiohttp://www.temcotransformer.com/3-phase-transformers.htmlhttp://wiki/Valve_amplifierhttp://wiki/Loudspeakerhttp://wiki/Mixing_consolehttp://wiki/Professional_audiohttp://www.temcotransformer.com/3-phase-transformers.html


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Testing of Transformers

1. Winding resistance test

2. Polarity test

3. Load test

4. Open circuit test

5. Short circuit test

1. Winding resistance test:

This is nothing but the resistance measurement of the windings byapplying a small d.c voltage to the winding and measuring the current through thesame. The ratio gives the winding resistance, more commonly feasible with highvoltage windings. For low voltage windings a resistance-bridge method can be

used. From the d.c resistance one can get the a.c. resistance by applying skineffect corrections.

2. Polarity Test:

This is needed for identifying the primary and secondary phasorpolarities. It is a must for poly phase connections. Both a.c. and d.c methods canbe used for detecting the polarities of the induced emfs. The dot method discussedearlier is used to indicate the polarities. The transformer is connected to a lowvoltage a.c. source with the connections made as shown in the fig. 18(a). A supply

voltage Vs is applied to the primary and the readings of the voltmeters V1, V2 andV3 are noted. V1: V2 gives the turns ratio. If V3 reads V1V2 then assumed dotlocations are correct (for the connection shown). The beginning and end of theprimary and secondary may then be marked by A1 A2 and a1 a2 respectively. If the voltage rises from A1 to A2 in the primary, at any instant it does so from a1 toa2 in the secondary. If more secondary terminals are present due to taps takenfrom the windings they can be labeled as a3, a4, a5, a6. It is the voltage rising


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from smaller number towards larger ones in each winding. The same thing holdswell if more secondaries are present.

The D.C. method of testing the polarity. When the switch S is closed if thesecondary voltage shows a positive reading, with a moving coil meter, the

assumed polarity is correct. If the meter kicks back the assumed polarity is wrong.

3. Load Test:

Load Test helps to determine the total loss that takes place, when thetransformer is loaded. Unlike the tests described previously, in the present casenominal voltage is applied across the primary and rated current is drown from thesecondary. Load test is used mainly

1. to determine the rated load of the machine and the temperature rise

2. to determine the voltage regulation and efficiency of the transformer.

Rated load is determined by loading the transformer on a continuousbasis and observing the steady state temperature rise. The losses that aregenerated inside the transformer on load appear as heat. This heats thetransformer and the temperature of the transformer increases. The insulation of the transformer is the one to get affected by this rise in the temperature. Bothpaper and oil which are used for insulation in the transformer start get-tingdegenerated and get decomposed. If the flash point of the oil is reached thetransformer goes up in flames. Hence to have a reasonable life expectancy theloading of the transformer must be limited to that value which gives the maximumtemperature rise tolerated by the insulation. This aspect of temperature risecannot be guessed from the electrical equivalent circuit. Further, the losses likedielectric losses and stray load losses are not modeled in the equivalent circuit andthe actual loss under load condition will be in error to that extent. Many externalmeans of removal of heat from the transformer in the form of different coolingmethods give rise to different values for temperature rise of insulation. Hencethese permit different levels of loading for the same transformer. Hence the onlysure way of ascertaining the rating is by conducting a load test. It is rather easy to

load a transformer of small ratings. As the rating increases it becomes difficult tofind a load that can absorb the requisite power and a source to feed the necessarycurrent. As the transformers come in varied transformation ratios, in many cases itbecomes extremely difficult to get suitable load impedance. Further, thetemperature rise of the transformer is due to the losses that take place inside thetransformer. The efficiency of the transformer is above 99% even in modest sizeswhich means 1 percent of power handled by the transformer actually goes to heat


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up the machine. The remaining 99% of the power has to be dissipated in a loadimpedance external to the machine. This is very wasteful in terms of energy also. (If the load is of unity power factor) Thus the actual loading of the transformer isseldom resorted to. Equivalent loss methods of loading and Phantom loading arecommonly used in the case of transformers. The load is applied and held constanttill the temperature rise of transformer reaches a steady value. If the final steadytemperature rise is lower than the maximum permissible value, then load can beincreased else it is decreased. That load current which gives the maximumpermissible temperature rise is declared as the nominal or rated load current andthe volt amperes are computed using the same

4. Open-circuit or No-load Test:

This test is performed to determine core or iron loss, P i and no-loadcurrent I 0. This test is helpful in determination of magnetizing component I menergy component I e and so no-load resistance R 0 being given as V 1/Ie and no-loadreactance given as V 1/Im. In this test secondary (usually high voltage) winding is leftopen, all metering instruments (ammeter, voltmeter and wattmeter) are connectedon primary side and normal rated voltage is applied to the primary (low voltage)winding, as illustrated below

Iron loss P 1 = Input power on no-load W 0 watts (wattmeter reading)

No-load current = 0 amperes (ammeter reading)

Angle of lag, 0 = cos -1 Wo/V1Io

Ie = I 0 cos 0 and I m = I 2o - I2e

Caution: Since no load current I0 is very small, therefore, pressure coilsof watt meter and the volt meter should be connected such that the current taken


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by them should not flow through the current taken by them should not flowthrough the current coil of the watt meter.

6. Short-circuit or Impedance Test:

This test is performed to determine the full-load copper loss andequivalent resistance and reactance referred to secondary side. In this test, theterminals of the secondary (usually the low voltage) winding are short circuited,all meters (ammeter, voltmeter and wattmeter) are connected on primary side anda low voltage, usually 5 to 10 % of normal rated primary voltage at normalfrequency is applied to the primary, as shown in fig below. The applied voltage tothe primary, say V s is gradually increased till the ammeter A indicates the full loadcurrent of the side in which it is connected. The reading W s of the wattmeter givestotal copper loss (iron losses being negligible due to very low applied voltageresulting in very small flux linking with the core) at full load. Le the ammeterreading be I s.

Full load copper loss,

Pc= I2s R1 = Ws

Equivalent resistance referred to primary,

R'1 = Ws/I2s

Equivalent impedance referred to primary,

Z'1 = Vs/Is

Equivalent impedance referred to primary,

X'1 = (Z'1)2 - (R'1)2'


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Sub station

An electrical substation is a subsidiary station of an electricity generation,

transmission and distribution system where voltage is transformed from high tolow or the reverse using transformers. ...

Function of substation:

The main functions of sub-station are to receive energy transmitted at high voltagefrom the generating station, reduce to a value appropriate for local distribution andprovide facilities for switching.

Grid

A heavy, rigid electrical conductor (usually uninsulated copper or aluminum) whichserves as an interconnection between power-handling devices (such as switchesand circuit breakers) or as a common connection between several circuits.

Transmission line

A transmission line is the material medium or structure that forms all or part of apath from one place to another for directing the transmission of energy, such aselectromagnetic waves or acoustic waves , as well as electric power transmission .Types of transmission line include wires , coaxial cables , dielectric slabs, strip lines ,optical fibers , electric power lines , and waveguides .

Electric power transmission or "high voltage electric transmission" is thebulk transfer of electrical energy , from generating power plants to substationslocated near to population centers. This is distinct from the local wiring betweenhigh voltage substations and customers, which is typically referred to as electricity distribution . Transmission lines, when interconnected with each other, becomehigh voltage transmission networks. In the US, these are typically referred to as"power grids" or sometimes simply as "the grid". North America has three majorgrids: The Western Interconnection ; The Eastern Interconnection and the Electric Reliability Council of Texas (or ERCOT) grid.

Classification of overhead transmission line

Short transmission line: - its length is up to 50km and voltage is


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Medium transmission line: - its length is about 50 to 150km.and line voltage ismoderately high >20kv20kv


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digger).Much less subject to conductor theft, illegal connections, sabotage, anddamage from armed conflict.

Disadvantages of underground power cables

1. Undergrounding is more expensive, since the cost of burying cables attransmission voltages is several times greater than overhead power lines, and thelife-cycle cost of an underground power cable is two to four times the cost of anoverhead power line. Above ground lines cost around $10 per foot andunderground lines cost in the range of $20 to $40 per foot.

2. Whereas finding and repairing overhead wire breaks can be accomplished inhours, underground repairs can take days or weeks and for this reason redundantlines are run.

3. Underground power cables, due to their proximity to earth, cannot bemaintained live, whereas overhead power cables can be. Operations are moredifficult since the high reactive power of underground cables produces largecharging currents and so makes voltage control more difficult.

Transmission and Distribution supply system.
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PROTECTION SCHEME

circuit breakers switchgear fuses

relays

1. Switch gearThe apparatus used for switching, controlling and protecting the electrical

circuit and equipment is known as switch.

2. FusesA fuse is a short piece of wire or thin strip which melts when excessive

current flows through it for sufficient time. It is inserted in series with the circuit tobe protected.

3. Circuit breakerA circuit breaker is equipment which can open or close a circuit under all

conditions, no load, full load and fault condition. It is also designed that it can beoperated manually or by remote control under normal and abnormal condition. Acircuit breaker is an automatically-operated electrical switch designed to protect anelectrical circuit from damage caused by overload or short circuit .
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TYPES OF CIRCUIT BREAKERS:-

High Voltage Circuit Breakers :There are electric power systems requiring the breaking of higher currents athigher voltages. It is such systems that high-voltage AC circuit breakers are used.

Vacuum Circuit Breaker :The rated current of the device is up to 3000 A. These specialty circuit breakersinterrupt the current by creating and extinguishing the arc in a vacuum container.These devices can only be practically used for voltages up to about 35,000 V,which corresponds to the medium-voltage range of power systems. These circuitbreakers have longer life expectancies between overhaul than do air circuitbreakers.

Air Circuit Breaker :

Sf6Circuitbreaker

Airblast

Circuitbreaker

Oilcircuit

Breaker

Vacuumcircuit

Breaker

Typesof

CircuitBreaker


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The rated current of the device is up to 10,000 A. Some of them are electronicallycontrolled, while others are microprocessor controlled. These circuit breakers areused for main power distribution in large industrial plants.

Air vaccum

Sf6 circuit breakerSf6 gas cylinder

4. Relay:

A relay is a device which detects fault and supplies information to the circuitbreaker for circuit interruption.


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DIFFERENT TYPES OF RELAY:-

1. Solenoid type

2. Attracted armature Type.

3. Electrodynamics Type.

4. Moving coil type

5. Induction type

6. Thermal relay

7. Earth fault relay

Current transformer

Current transformer (CT) is used for measurement of electric currents.Current transformers, together with voltage transformers (VT) (potentialtransformers (PT)), are known as instrument transformers. When current in acircuit is too high to directly apply to measuring instruments, a current transformerproduces a reduced current accurately proportional to the current in the circuit,which can be conveniently connected to measuring and recording instruments. Acurrent transformer also isolates the measuring instruments from what may be


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very high voltage in the monitored circuit. Current transformers are commonlyused in metering and protective relays in the electrical power industry .

Potential transformersIt is a transformer which is used to measure voltage and also it limit the

voltage of the device.

Lightning arresterA lightning arrester is a device used on electrical power systems to protect

the insulation on the system from the damaging effect of lightning . Metal oxide varistors (MOVs) have been used for power system protection since the mid 1970s.The typical lightning arrester also known as surge arrester has a high voltageterminal and a ground terminal. When a lightning surge or switching surge travels
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down the power system to the arrester, the current from the surge is divertedaround the protected insulation in most cases to earth.

Buchholz relay

Buchholz relays have been applied to large power transformers at least sincethe 1940's. The relay was first developed by Max Buchholz (1875-1956) in 1921. Inthe field of electric power distribution and transmission, a Buchholz relay , is asafety device mounted on some oil-filled power transformers and reactors ,equipped with an external overhead oil reservoir called a conservator . TheBuchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment. Depending on the model, the relay has multiplemethods to detect a failing transformer. On a slow accumulation of gas, dueperhaps to slight overload, gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level down. A float switch inthe relay is used to initiate an alarm signal. Depending on design, a second floatmay also serves to detect slow oil leaks. If an arc forms, gas accumulation is rapid,and oil flows rapidly into the conservator. This flow of oil operates a switchattached to a vane located in the path of the moving oil. This switch normally willoperate a circuit breaker to isolate the apparatus before the fault causes additionaldamage. Buchholz relays have a test port to allow the accumulated gas to be

withdrawn for testing. Flammable gas found in the relay indicates some internalfault such as overheating or arcing , whereas air found in the relay may onlyindicate low oil level or a leak.
http://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Reactance_(electronics)http://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Transformer_oilhttp://en.wikipedia.org/wiki/Float_switchhttp://en.wikipedia.org/wiki/Electric_archttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Arcinghttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Reactance_(electronics)http://en.wikipedia.org/wiki/Dielectrichttp://en.wikipedia.org/wiki/Relayhttp://en.wikipedia.org/wiki/Transformer_oilhttp://en.wikipedia.org/wiki/Float_switchhttp://en.wikipedia.org/wiki/Electric_archttp://en.wikipedia.org/wiki/Circuit_breakerhttp://en.wikipedia.org/wiki/Arcing


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Ht meter

Current transformer (CT) is used for measurement of electric currents.Current transformers, together with voltage transformers (VT) (potentialtransformers (PT)), are known as instrument transformers.

six units of m

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