Transformer - Electrical Power

8/13/2019 Transformer - Electrical Power

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TRANSMISSION OF POWER

Types of transformersA transformer is a device that transfers electrical energy from one circuit into the other circuit that arenot directly connected by any physical link or connection but uses magnetic energy as the link between

the two circuits. It allows the energy to cross theformerin the form of magnetic energy which is theninduced into the other circuit by flow of magnetic flux. The changing flux then produces electrical

energy in the secondary circuit.

From the basic principles a wire carrying current produces magnetic field around it. A field is the areawhere the magnetic field is exists, just like a field of rice is the area where rice is planted or exists.

INDUCTANCEWhen a current flows in a coil magnetic flux is generated around the coil. If a former (iron core) is used

at the centre of the coil then most of magnetic flux tends to travel along the forming material and isgreater that when there is none. When this flux cuts the conductors as it circulates around the coil,electrical current is generated on the coil that is caused by the same flux. Hence e.m.f. is induced onthe coil by the circulating flux.

The induced emf is proportional to:1. The rate of change in flux (flux that is cutting the coil)2. Number of turns of coil

The emf is in opposite direction to the changing current from the supply. This emf is called back emf,E = - (change in flux)/t x N

Nt

E

=12

Self inductanceis the when a change of current from the supply produces a flux in a coil and that

changing flux cuts the same coil producing an opposing emf (voltage) on the same coil.

How much a coil induces emf depends on the coils inductance =I

N where I = current, N = number

of turns, = flux. Also emf induced can be expressed as Nt

IILE

=

)( 12or

Since

Field

Magnetic flux Conductor carrying electric current

Area affectedby magnetismis called themagnetic field

t

-m

+m

The full period = t = 1/fThe period = t = 1/(2f)Since flux changes from +mto mat a rate=1/2fTherefore average rate of change of flux =2m/

(1/2f) = 4fmwb per secRms value = 1.11 of Peak henceAverage flux = 1.114fmper turnConsidering all N turns in a coil emf induceE =1.114fmN


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Transformers are made out of coils of wire wound on aformingmaterial on which the coils are wound.

Mutual inductanceMutual inductance is when the flux produced by another coil cuts the second coil producing inducedemf on this second coil as shown below:

Mutual inductance occurs between two different coils where a changing current in one coil (primaryside) causes a change of current in the other coil (secondary side). The secondary side will alsocirculate a flux that links with the primary side again hence mutual inductance (between two sides).

Nt

E

=12

Nt

IIME

=

)( 12where M = mutual inductance

The circuits are made of coils, and when the coil carries current on the primary side energy istransferred across the former as magnetic energy hence the device is call transformer. Transmeaning

across andformermeaning the forming material.

TRANSFORMERThe relationship of primary and secondary voltage can be expressed as follows

p

s

s

p

s

p

I

I

N

N

V

V==

Transformer construction

Materials

Laminated core is made out of steel sheet laminations as the former material because it has goodmagnetic properties, that is, high permeability to magnetic flux and low hysterisis loss due tolaminations.

Describe the construction of Transformers

1. Core typeIn a core type transformer the coil has a core at its centre that it surrounds considerably.

a. Coils are form-wound and are cylindrical by type i.e. forming a cylinder around the core.b. The coil may be circular or oval or rectangular around the core

Secondary

Primary

Magnetic energy circulating

Mutual induced emf

Rectangular coilaround the core

Oval coil aroundthe core

Laminated, rectangular core

Low voltage coil (LV)nearer to the core


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For smaller core type transformers rectangular core, cylindrical coils with circular or rectangular formare used for large core type transformers; round or circular cylindrical coils are used because they havehigh mechanical strength.Layers are insulated by paper, cloth or mica board or cooling ducts.

2. Shell typeIn a shell type transformer the coils have shell (core) that surrounds the coil considerably.

It uses multi-layered disc types of windings. These coils are wound in the form of pancakes. They areinsulated from each other by paper.

3. Berry type transformerIt is basically a shell type transformer but has a different arrangement of core. It has a cylindricalstructure

The transformer core consists of laminations arranged in groups which radiate out from the centre asshown above. It also uses multi-layered disc types of windings.

Basic Transformer construction features

Basic Transformer construction features

Item Construction Comment

Coils At least two separate coils,linked by magnetic flux

Coils circuits are not electrically continuous butmutual inductances cause magnetic energy to be

transferred between the two circuits hence generatingelectrical energy on the other winding.

Core Made of laminated steelsheets. It has high siliconcontent, heated treated toproduce a high permeabilityand low hysteresis sheet.

The steel laminations must provide continuousmagnetic paths and hence must have minimum air gapbetween the laminations. Silicon in steel sheetsreduces hysteresis and hence increases magneticpermeability of the steel.

Insulation Strips of laminates areinsulated by vanish.

Coils are electrically insulated as well as the core andtheir insulation type depends on the voltage ratings ofthe machine

Laminations The sheets of steel arelaminated together to form a

rigid core or formermaterial. Laminates are

insulated by core platevanish or oxide layer on thesurface

The use of lamination instead of a continuous sheet isto break the eddy current circuits into smaller isolated

circuits and hence effectively reduce eddy currentlosses. Varying magnetic field cutting the core causes

electric current called eddy current which is the flowof electric current on the core material; this causesheating up of the core and energy is wasted as well

LV

HV

HV

LV

Insulation

Cylindrical coils

Magnetic core laminations

Circular laminated core

Insulation layer

LV coil (easy to insulate)

HV


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resulting in poor efficiency of the machine.

Assembly

housing

Suitable container for

assembly and coils

Assembly orContainerinsulation

Core and windings areinsulated from the frame ofthe container

A suitable material is chosen according to the size andrating of the device.

Bushing Porcelain, oil filled orcapacitor type

For insulation and act as guide duct through whichterminal leads are laid or drawn out.

Coolingsystem

For big devices a coolant isused , which is normally oil

The arrangement of allows oil coolant to circulate andremove heat from the machine by convection and thenexchange hot temp from the machine with temp of theair or environment or other heat exchange material.

Windings types

Cylindrical helical layersMostly used in core transformers

These windings are arranged into a helical structure

1. LV is normally at the centre2. HV is normally on the outer helix3. Insulation separates the windings

For currents up to 50A the bobbin type of winding method can be used just like thread of a sowing

machine. For currents above 50A spiral windings are preferred.

Disc windingThese are mostly used on shell type of transformersThe windings are wound to form a shape like a pancake

The wires for each winding are wound around the former expanding outward from the centre forming aspiral discDisk windings of spiral type are used advantageously at ratings between 600 to 1200A, since

continuous disc type tend to be too expensive and is limited to 600AInterleaved disc windings

Higher machine rating of 66kV and above use double layer spiral interleaved type. Interleaving createsuniformity in charge distribution between windings across the whole transformer (protective effect)and also reduces copper losses. Interleaving is made by winding multilayer coils of different windingsover each other in order to evenly distribute charge. Melinex can be used as insulation instead of paperbetween the coils.

Layered windings

This allows several windings to be arranged in a single formerFor disk layer

LV layer

HV layer

Insulation

One layer in ahelical typeLV winding

One layer in ahelical type- HV winding

Transformer laminations that make upthe core frame

One layer in a helical type

One layer in a Disc type


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Sandwich Style Winding Construction

(a)Currency Dividing (b)Voltage Dividing (c)Construction of Winding

Laminations1. Laminations are made of steel + 3 to 4% of silicon2. Steel can have high flux density of 30% within the coreProperties of steel

1. It has a simple chemical composition which allows it to be processed easily though it isexpensive to get the minerals2. It has high permeability3. lower losses at a given flux density4. Laminations reduce the losses due to eddy currents.

Insulating oil characteristics

1. Must be free from organic acids and corrosives e.g. sulphur2. Must be of low viscosity to enable efficient heat transfer3. Must have high dielectric strength4. It must not sledge5. It must have good resistance to emulsion that is it must have an ability to throw away water

Comparison of Transformer types

Core type Shell type

More turns area Less turns limited by surrounding coreLess cross section- less magnetic path than Shell Shorter length of turns

The manufacturer then chooses the kind he wants based on these characteristics and cost but generallythey are almost always comparable.

NO LOAD PHASOR DIAGRAM OF TRANSFORMER

Consider a transformer shown below:

- Alternating voltage on the primary induces alternating flux in the core thatflows or circulates around the core.

- Assuming that all flux travel along the core and is not lost to the spacebetween the primary and secondary circuits

o Then flux circulates on the secondary and primary equally- Say each turn of wire generate an emf = ewhen cut by flux then

- Emf induced on the Secondary/ Emf induced on the Primary= E1/E2= (Nsx e)/ (Nsx e)

E2E1

Flux

Vp Vs


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E1= Nsx e since the total induced voltage is the sum all winding

emfs represented each by e

- with secondary open then its terminal voltage is the same as induced emf- IPis very small since Isis small with an open circuited secondary- Vpis practically = emf induced in primary, E1

- Therefore Vs/Vp= Ns/Np- Since a transformer efficiency is almost 100% then

o IpVpx p.f. (Primary) = IsVsx p.f. (Secondary)- Since power factors are equal at full load then

o IP/Is=Vs/Vp

Phasor diagram of transformer

- A phasor diagram shows the relation in terms of direction and size ofelectrical/magnetic quantities under consideration

- This is important to determine mathematically the output of the voltages and

currents and analysis of internal properties of electrical quantities in differentaspects

Drawing the phasor diagram

Step 1: choosing the reference quantity

- In most cases it is recommended to choose the quantity that is most commonor occurs in such a way that all electrical /magnetic quantities can be

compared with its position easily

- Any quantity can be used as a reference in principle though- In a transformer we will use flux since it the linking quantity between the

primary and secondary

- a reference phasor is normally drawn horizontally along the x-axis though intruth it can be drawn anywhere just to act as a reference or positional

comparison with others

- in our case we will draw the flux horizontally as shown below- when the Vacsupply is applied at the input, it causes alternating flux to flow

in the core

- the flux cuts the primary and secondary windings of the transformersimultaneously as it circulate hence generating self-induced emf in N1turns

and mutually induced emf on the secondary winding N2

- The induced emf E1and E2depend for their magnitude on the number ofturns or N of the corresponding side of the transformer

- The induced emf is lags behind the flux that produced it by 90 because fluxflows first before emf is inducedThe phasor diagram is shown below


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Example:

Given Np=480, Ns=90, Vp=2200 at 50Hz, Iom=1.19A and core loss =447W finda. m ( use E1=4.44 Npf m you get m=0.0206 Wb )b. Vs (use Vs= VpNp/Nsyou get Vs= 412.5V )c. Ip(find Io1 from Io1Vp=core loss then find Ip= Ioat no load=(I

2o1+ I

2om)

d. No load phase angle

PHASOR DIAGRAM OF A LOADED TRANSFORMER

V2is the same as E2induced on the secondary

V1is the equal but opposite phase with E1according to our assumption

If there turns ration are the same then E1= E2. If the transformer is loaded by a load

with a lagging power factor such as lights or an induction motor as shown below:

Then

Therefore Islags Vsby angle 2as shown below

E2E1

Flux

Vp Vs P = IV cos 2

V1

oIo1

Im

Io

Io consists of two components Io1, the active or power component

supplying hysterisis and eddy currentlosses in the core and negligible I

2R loss

in the primary winding Iomproducing the required flux that is

responsible for transferring emf to thesecondary side

Iomis in phase with the flux while Io1is inphase with the applied voltage V1. the phaseangle between then is therefore 90Io1V =core power loss (heat generated on thetransformer core). Io1is however very smallcompared to Iomhence no load power factoris very small.No load current = Io= (I

2o1+ I

2om)

Power factor = cos o= Io1/ IoE1E2& V2


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Example:A single phase transformer has 1000 turns on primary and 200 on the secondary. The no load current is3A at a power factor of 0.2 lagging. Calculate the primary current and power factor when the secondary

is 280 A at a power factor of 0.8 lagging.Assume voltage drop in the windings isnegligible

Since I2 N1= I2N2in magnitude thenI2(primary side current) =I2x(N2/N1) = 56A

Cos 2= 0.8 therefore sin 2= 0.6

Cos 0= 0.2 and sin o= 0.98Consider the active component of I1I1Cos 1= I2Cos 2+IoCos o

=45.4

The reactive component of I1isI1sin 1= I2sin 2+Iosin o

=36.54AHenceI1 = (45.4)

2+ (36.45)

2= 3398

I1 ==58.3ATan 1= 36.54/45.4 =0.8051=39Pf = cos 39 = 0.78 lagging

Equivalent circuits for load conditionsA transformer has winding losses whose value is represented I

2R

The transformer when loaded draws enough current to cause significant flux line around the

winding. This flux has two possible paths either it is directed along the core to the secondaryside of the transformer or it escapes into the air and flows back to the same coil cutting thecoil generating hence creating self induced emf.The transformer on the input therefore has two voltage drops IR component and IX (selfinduced emf)

R2R1

V1 V2

Leakage flux causesself inductance

o

1

I 2

I2

2

Io

I2is the secondary current and lags V2by anangle 2.I2is the component that flows in the primary asa result of the load current I2in other words theprimary side draws extra current tocompensate for the requirement of the

secondary side. While I1cause flux (magnetizing), I2flows converts the flux intoelectrical (demagnetizing) hence tocompensate for the conversion I2has to bedrawn from the primary -- I2is called I2referred to the primaryIo is the no load current of the transformer.Therefore total current drawn by thetransformer is I1 = phasor sum of I2and IoThe primary side phase angle is given by 1which is the phase difference between V1andI1. typically Iois very small compared to othervalues o current than it is shown on the graph

I 1

E1E2& V2

V1


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self inductance causes a voltage drop on the winding represented by reactance X

If the self induced emf = el1then x1= (el1)/ I1for secondary x2= (el2)/ I2

Point to note

1. the leakage flux links only one side and does not contribute to emf on theother coil

2. V1has to supply I1X1and I1R1similarly E2supplies I2X2+ I2R2Equivalent circuit

COOLING OF TRANSFORMEROil filled self cooled systemUsed on small and medium size transformers

Characteristics are:1. Welded oil tight steel tank2. High quality insulating oil3. Smooth tank

Oil filled water cooled system

For bigger machines

3-TRANSFORMERS AND STANDARD DIAGRAMS

Purpose of a 3-phase transformer

1. step up generated voltage of 13.2kV to 110, 132, 275, 400, 750kV for transmission2. step down the voltage that has been transmitted down to 6.6kV, 4,6kV, 2.3kV and 440, 220,

110V for consumers

Construction3-phase transformers can either be a single unit or three separate transformers for each phase.Nowadays a single unit is preferred because is convenient; occupies less space, less weighty, less cost,though when it becomes partially faulty in most cases the whole unit is removed such that service isseverely affected unlike the case of single transformers.

TypesThere are two basic types like single transformers namely Core and Shell types.

E2E1

R2R1

X1 X2

V1 V2

Transformer

Water cooled systemWater runs in a system

of tubes to take awayheat from the oil

Oil filled tank

TransformerOil filledtank


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Core Type

Winding arrangements

Shell typeThe structure is similar to three adjacent transformers, but there is some serving on sharing the samecore material and size reduction. Yet 3 separate transformers have an advantage in that if one becomes

faulty the system can work at reduced load in an open delta configuration (to be considered later).

STANDARD CONNECTIONS

Y-Y or Star-Star connection

PARALLEL TRANSFORMER OPERATIONIt normally required to supply load that is more than one transformer can supply. In such cases two

transformers are used.

Coils

Core

The sum of the fluxes is equal tozero. Since they are 120 thereis no flux at the centre leg.

Coils

Core

Practical Core transformer wound with cylindrical coilsaround a core legs that runs through the centre of each coil

Core

Secondary

Practical Shell transformer multiplayer disccoils wound around core legs that runs throughthe centre of each coil

Coils

A C

a b c

B B

A

C

Primary sidephase 120 apart

ac

Secondary sidephase 120 apart

b


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Connecting transformers in parallelOn connecting two or more transformers in parallel, terminals of similar polarity must be joinedtogether to the same bus bars as shown above. Otherwise if the polarities are reversed on the secondaryside then the transformer will draw current on no load due to a complete circuit path betweensecondary terminals. This is a will produce a dead short circuit at the secondary side.

Conditions for connecting transformers in parallelThough the windings may be connected with correct polarity in parallel this is not enough and certain

conditions must be fulfilled for the transformer to work properly and these are listed below.1. Same voltage ratio (same transformation ratio) the voltage rating of primary to secondary of

one transformer must be identical on both transformers.2. Same phase displacement same ratio of X/R to avoid circulation of currents and operation at

different power factors3. Same voltage regulation voltage regulation is the change in voltage expressed relative to the

normal no load voltage usually, that occurs when the transformer is loaded. Therefore bothtransformers need to have the same voltage regulation to keep the same voltage at their output

even if they are carrying loads4. Transformers should be connected with correct polarities to avoid short circuits due to

conflicting phasesIf the transformers are slightly different in their voltage ratio then a secondary current will flow even inthe absence of load current. Hence when loaded the transformers cannot carry their expected full loadwithout one transformer over heating.If the ratio of R/X are not exactly the same then the impedance triangles are not the same hencetransformers would operate at different power factors. The transformers will not share the load inproportion to their kVA ratings.

Auto transformerIt is a transformer with one winding which is shared between the primary and the secondary circuits.The primary and the secondary are not electrically isolated but energy is transferred magnetically tolink the two sides such that its behavior is similar to two-winding transformers. It is used mainly wherethe voltage ratio is not far from 1.

Sketch of an auto-transformer

Step-down

AB primary winding

BC secondary winding

I2= I1+I3

kI

I

N

N

V

V===

2

1

1

2

1

2

B

A

C

V1

Load

I1

I1I2

I2

V2I3

A

a b

B PRIMARY BUSBARS

SECONDARY BUSBARS


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Step-up autotransformer

Advantages of an auto transformer1. uses less copper2. higher efficiency that an identically rated two-winding transformer3. Occupies less space4. superior voltage regulation

Applications of an auto transformer1. High voltage distribution cables

Transmission cables have losses and hence the terminal voltage falls or is lower at the terminal end. Inorder to distribute the correct voltage to the customers a booster transformer is required. The auto

transformer is ideal for this application because it has high efficiency and its transformation ratio isclose to unity it can increase the voltage by a small amount which is difficult to achieve with anordinary transformer.

Under these conditions the auto -transformer is used such that it has additive polarity to the line voltageand the total voltage at the output is VL+ Vawhere Vais the voltage from the auto transformer.

Voltage Control or RegulationThe power supply system must have limited variations to safeguard the interest of consumers and their

equipment. Country laws therefore enforce regulations on voltage variations to be within the limits of6% of the stipulated (specified or rated) terminal voltage at the consumer side.

Causes of variationThe causes of voltage variations are:

1. Increase of load on the system the load may increase due toa. alternator synchronous impedanceb. transmission line voltage dropc. transformer impedance

d. feeders and distributors2. Decrease on the load of the power supply system caused by the reverse of the conditions

stated above

Effect of changes of consumer voltage levels1. reduction of lighting system output on decrease of terminal voltage and fall rated life span of

the lighting system2. Erratic operation of the induction motors incase of over-voltage results in - saturated

magnetic flux, large magnetizing current, overheating and low power factor (decrease inefficiency). In case of low voltage reduction of starting torque of motors.

3. Distribution transformers may over-heat due to wide variations of voltage and hence have lowoutput.

Solution

To solve the above problem some voltage control equipment is put in place.The equipment is distributed (placed in many different places on the network) because:

B

A

C

V1

I1

I2

I2

V2Load

I3

I1

B

A

C

V1

I1

I2

I2

V2Load

I3

I1


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1. The power supply system itself is very extensive (distributed) and hence there is considerablevoltage drop in the transmission and distribution

2. Various circuits of the power supply are dissimilar in terms of their load characteristicsThe voltage control equipment is placed at

i. generating stationsii. transmitting stations

iii. feeders if the voltage exceeds the limits allowedThe methods of voltage control

1. Excitation control - used at generation stage only. The load current is varied according to theload sensed at the output of the alternator

2. tap changing transformers used at transmission and distribution3. Auto transformer- used at transmission and distribution4. Booster transformer- used at transmission and distribution5. Induction regulators- used at transmission and distribution6. synchronous condenser forcontrol of transmission line voltage

All the above methods try to maintain a constant voltage output at the consumers side despitechanges in the loading of the system.

Transformer tap changing methodSince excitation control is for works in generation and for short distances (otherwise wide current

adjustments would be required if it were to work to serve interests of long transmission lines.) othermethods are used.

The method of transformer tap changing is used where there is a main transformer; secondary windingsare tapped to provide different primary to secondary voltage ratios. The fig below depicts the idea.

Off load tap changing transformerThe transformer above can be changed to supply voltages tapped from any one of its b legs at a time

according to the load requirement at any given time. B1being the minimum voltage tapping and b5being the maximum voltage tapping at the secondary side. If the load is envisaged to be high a changefrom low to high is done by changing the switch position to a high tapping. Obviously the load must beoff in order to perform the changeover because of the following reasons:-

1. arcing results (large spark) if changeover is made when the load is on line such that there is abreak-before-make (contact broken before making contact with the next stud)

2. If make-before-break contact is made the next winding will be short-circuited and largedamaging current will flow.

So this arrangement has a disadvantage of requiring removal of load before breaking the circuit toadjust to load changes.

On load tap changingThis mechanism avoids interruption of supply.

A

b5

B

PRIMARY

SECONDARYa

b4b3b1 b2

b


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In this arrangement the load is supplied by two secondary coils and on load changeover can beachieved by changing one section of the switch set at a time for a particular voltage ratio. For exampleto change from b1to b2one main switch is switched off , say B0, so that the load is supplied by onesecondary coil at that moment and b1is switched off and changeover to b2is made since there is noload on this coil since B0is off. The switch B0is then closed so that moment both windings are

connected but with different load distribution. The B1 is then broken so that the load is now suppliedonly by the other coil through its b2and then the changeover from b1to b2is made in the usual waydescribed above since the load is off on that coil. Finally B2 isclosed so that the load is distributed

equally on the two secondary coils.This method has a disadvantage of being too

1. Expensive on transformer design and needs too many switches (a double set for each tapchanging).

2. A Voltage surge is created during changeover as there is a change in impedance when onetransformer is removed in circuit and the put back.

Auto-transformer tap changing

In this arrangement center tapped auto transformer or reactor is used as shown on leg b. under normaloperation the main switch B0is closed and this removes the auto-transformer effect since currents intoit would then flow in opposition and there would be no induced voltage across it. To changeovercontacts from say b1 to b2switch B0is open to remove the short circuit across the transformer electrically current now would be passing only through of the reactors coil and hence there is voltageacross the reactor. The switch b2 is then closed the reactor would be in parallel with the coil winding

b1-b2and a local current circulates but has is not much because the reactor has high reactive impedance.The switch b1is then opened and finally switch B0is closed to reaffirm the short circuit.

A

b5

B

PRIMARY

SECONDARYa

b4b3b1 b2

b

B0

A

b5

BPRIMARY

SECONDARYa

b4b3b1 b2

b

b5b4b3b1 b2

B0

B1


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Booster transformer

In this circuit the transformer acts a booster to the line voltage from the main transformer that suppliesthe primary side A-B winding. By controlling the booster transformer voltage trough switches, b, theoutput on a-b line can be increased or decreased. This is used on long transmission lines to compensatefor line losses.Disadvantages: 1. expensive 2. Inefficient because of booster transformer losses

INDUCTION REGULATORSIt is a constant voltage regulator that is similar to an induction motor only difference is that the rotor is

not allowed to rotate continuously. Its displacement can be adjusted or manually automatically by amotor. The primary side winding is wound on the stator and connected across the supply line voltagewhile the secondary winding wound on a rotor and is connected in series with load the diagram belowexplains the idea.

The flux generated by current in A-B winding generates a control voltage on C-D winding that supplies

the load. The magnetic flux that is transformed into electrical energy on the secondary can becontrolled by moving or tilting the rotor by a number of degrees in this way the amount of secondaryvoltage can be controlled. Hence secondary voltage can vary from maximum positive to minimumnegative.

Synchronous condenserThis is a synchronous motor that is specially designed to supply wattles leading kVA to the linedepending on the excitation current. The leading kVA is used to interact with the lagging kVA on the

line either to partly or fully cancel out and hence control the voltage drop on the line. This can then becarefully controlled to keep a constant output at the end of the transmission line as the load varies.

B

AC

D

OutputInput

Stator

Adjustable

Rotor

A

b5

B

PRIMARY

SECONDARYa

b4b3b1 b2

b

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Transformer - Electrical Power