review of transformer basics adul

UET LAHORE.FARHAN MAHMOOD

Review of Transformer Basics

UET, LAHORE

Transformer is a static electromagneticmachine with two or more windings, linkedwith common magnetic field.

The transformer performs the followingfunctions,

• Changes the voltage level• Changes the current level• Changes the impedance• Can provide isolation• Filters dc from a waveform that is a mixture of

ac and dc

FARHAN MAHMOOD

Transformer Definition Revisited

UET, LAHORE

Transformers are classified into three categories,

(a) Position of windings with respect to core

Core type Shell type

(b) Transformation ratio

Step up Step down

(c) Type of service

Power Distribution

Types of Transformers

FARHAN MAHMOOD

UET, LAHOREFARHAN MAHMOOD

Position of Winding with respect to Core

1. CORE TYPE TRANSFORMER The windings of a core type transformer surround a

considerable part of the core.

A single phase core type transformer consists of amagnetic frame with two limbs, upper and bottom yokeforming the magnetic circuit.

A 3-phase core type transformer consists of aminimum three limbs.

Core Type

2. SHELL TYPE TRANSFORMER The core of a shell type transformer surround a

considerable part of the windings.

The windings are enclosed by two empty parts of the coregiving transformer an appearance of a shell.

A single-phase shell type transformer consists of amagnetic frame with a central limb and two side limbs,completing the path of magnetic flux.


Shell Type

FARHAN MAHMOOD UET, LAHORE



COMPARISON BETWEEN CORE TYPE & SHELL TYPE TRANSFORMER

Construction: Core type transformers can be assembledand dismantled more easily as compared to shell type.

Repair: In core type transformers, repair can be done moreeasily as compared to shell type, because more portion ofthe windings is accessible.

Cooling: In core type transformers, cooling of winding isbetter, because windings encloses the core whereas inshell type cooling of core is better.

Leakage Reactance: Leakage Reactance of shell typetransformer is smaller as compared to core type, becauseof better flux linkage between low voltage and high voltagewinding.



COMPARISON BETWEEN CORE TYPE & SHELL TYPE TRANSFORMER

Mechanical Forces: Shell type transformers are morerobust mechanically compared to core type, because ofbetter support to windings against electromagneticforces (windings enclosed by core).

Application: Shell type transformers are used for lowvoltages and large output. On the other hand, core typetransformers are used for high voltage andcomparatively smaller output.

CONCLUSION: Core type transformersare more commonly used because ofmuch better heat dissipation facilities.

UET, LAHORE

1. STEP UP TRANSFORMER It transforms low voltage, high current a.c system into

high voltage, low current a.c system.

The secondary winding consists of large number ofturns and primary winding consists of few turns.

2. STEP DOWN TRANSFORMER It transforms high voltage, low current a.c system into

low voltage, high current a.c system.

The secondary winding consists of few turns andprimary winding consists of large number of turns.

FARHAN MAHMOOD

Transformation Ratio

UET, LAHORE

1. POWER TRANSFORMER

These transformers are above 500 kVA and are used ingenerating stations.

They are not operated for 24 hours a day.

They are designed to have maximum efficiency at or nearfull load (nearly 80% of full load).

Iron losses are 5-6 times as compared with distributiontransformer.

2. DISTRIBUTION TRANSFORMER

These transformers are upto 500 kVA and are used atdistribution level.

They are operated for 24 hours a day.

They are designed to have maximum efficiency at or near50% of full load.

FARHAN MAHMOOD

Type of Service

Construction


The transformers mainly consists of following circuits.1. MAGNETIC CIRCUIT

The magnetic circuit consists of limbs, yoke and clamping circuit(providing the path for flux).

2. ELECTRIC CIRCUITThe electric circuit consists of low voltage & high voltagewindings and sometimes a tertiary winding.

3. DIELECTRIC CIRCUITThe dielectric circuit consists of insulations in different formsand different places in transformer i.e. core to LV, LV to HV etc.

4. TANK & ACCESSORIESTank & accessories, such as cooling devices, conservator,breather, bushings, protective devices.

UET, LAHORE

Core Windings Transformer Tank/Tank Cover Conservator Breather Cooling Arrangements Tap changer Bushings Accessories

FARHAN MAHMOOD

Various Parts Of Transformers

UET, LAHORE

FIGURE SHOWING DIFFERENT PARTS OF A TRANSFORMER

FARHAN MAHMOOD

Constructional Features of a Transformer

UET, LAHORE

• The function of the core is toguide the magnetic fluxcreated by currents that flowin the windings.

• It is magnetic link betweenthe primary and secondarywindings.

• It is made of high quality thinelectrical silicon steel sheetlaminations to reducehysterisis loss. The core islaminated to reduce eddycurrent loss.

FARHAN MAHMOOD

CORE

UET, LAHORE

Single-phase Transformers

Three-phase Transformers

•• TheThe partpart ofof thethemagneticmagnetic circuitcircuitwhichwhich carrycarry thethetransformertransformer windingswindingsareare calledcalled limbslimbs ororlegslegs..

•• ThoseThose partsparts whichwhichconnectconnect thethe legslegs andandserveserve forfor closingclosing thethemagneticmagnetic circuitcircuit arearecalledcalled yokesyokes..

FARHAN MAHMOOD

Parts of CoreParts of CoreP3

P3 PKK1758, 2/19/2004

UET, LAHORE

Windings

• Transformer Windings form the electrical circuit of atransformer. They are made from high conductivitycopper with several layers of insulating paper of highdielectric strength.

• Among individual coils, there are cooling ductsthrough which oil flows and carries away heatgenerated by losses in windings.

• The windings must be electrically and mechanicallystrong to withstand both over-voltages under transientsurges and mechanical stresses during short circuitand should not attain temperatures beyond the limitunder rated and overload conditions.

FARHAN MAHMOOD

UET, LAHORE

Windings

LV Winding HV Winding

FARHAN MAHMOOD

UET, LAHORE

Windings

Tapping Winding

Complete Winding Assembly

FARHAN MAHMOOD

P1P2

P1 PKK1758, 2/19/2004

P2 PKK1758, 2/19/2004

Dielectric Circuit


• The dielectric circuit consists of insulationused at different places in the transformer toinsulate the conducting parts. All theconducting and magnetic parts of thetransformer must be well insulated from eachother. The insulation can be classified as,

1. MAJOR INSULATION

2. MINOR INSULATION

Dielectric Circuit


MAJOR INSULATION

• It includes the insulation between core and lowvoltage winding. The insulation between core and lowvoltage winding is provided by an insulating cylinderof press board (or Synthetic Resin Bonded Papercylinders in case of low power transformers). Similarinsulation is used between low voltage and highvoltage windings and between high voltage windingsof different phases.

• Mineral oil is used in transformer to improve thedielectric strength of the insulation. In addition, it alsodissipates the heat from core and windings.

Dielectric Circuit


MINOR INSULATION

• It includes the insulation used between the elementsof a given winding i.e. conductor insulation,insulation between turns, layers and coils.

• The insulation of the conductors may be of paper,cotton or glass tape.

Transformer Tank


• For cooling purposes, the transformer is required tobe immersed in oil inside the tank.

• Transformer tank is a housing, filled with insulatingoil, for placing core and coil assembly.

• The function of transformer tank is to provide aprotective cover to the core, windings and otherinternal parts including transformer oil. The tankalso provides external surface for dissipating heat.

• Tanks for small transformers are fabricated fromwelded mild steel sheets. The lids of thesetransformer tanks can be of cast iron.

• A number of accessories are provided on the coverof the tank.

Transformer Tank


UET, LAHORE

• Conservator is an airtight cylindrical metal drumwhich is supported horizontally on thetransformer lid. The conservator and main tankare connected by a pipe.

• Under increasing load, the oil expands andsimilarly, oil contracts under decreasing load.

• Small transformers are sometimes not providedwith conservator. So, the oil level has to be keptsome distance below the top cover to providespace for oil expansion under temperature rise.

• When the oil expands, the air is expelled out andunder contraction of oil, air is drawn inside. Thisprocess is called breathing.

Conservator

FARHAN MAHMOOD

UET, LAHORE

• Thus the oil is in contact with air. Exposure to airspeeds up the ageing of the oil due to increasedsludge and acid formation through moistureabsorption and oxidation.

• The function of conservator is to keep the main tankof the transformer completely filled with oil in allcircumstances while the conservator is alwayspartly filled with oil.

• Conservator takes up expansion and contraction oftransformer oil and keeps the main tank full of oil.

• This reduces the rate of oxidation of oil becauseless oil surface is exposed to air.

Conservator

FARHAN MAHMOOD

Conservator

• In this way, the sludgeformation is considerablyreduced and whateversludge is formed, settlesto the bottom of theconservator into a sludgepan from where it isperiodically removed bymeans of drain tap.

• Normally, the capacity ofconservator should beapproximately 10 to 12%of the volume of oil of themain tank.


90 MVA, 132/33 kV Transformer

UET, LAHORE

• When oil expands air isexpelled out. When oilcontracts air is drawn in.

• Air entering thetransformer is passedthrough a breatherwhich absorbs moisturefrom the air.

• Dehydrating materialused is silica gel crystalimpregnated with cobaltchloride.

• Material is blue whendry and whitish pinkwhen damp.

FARHAN MAHMOOD

Breather

UET, LAHORE

• Bushings are the means of connectingtransformer with the network / system inwhich it is placed.

• A bushing is necessary when a conductor istaken out through a metallic tank. One end ofthe bushing goes into transformer tank whilethe other end is in the outside air.

• Bushing consists of conducting rod, aporcelain cylinder for isolating the conductingpart.

FARHAN MAHMOOD

Bushings

UET, LAHOREFARHAN MAHMOOD

Bushings

UET, LAHORE

Tap Changers

• Normally it is used to provide adjustment insystem voltage variation so as we getrequired output voltage on secondary loadside.

• Generally there are two types of tap changer.– Off load Tap changer– On load Tap changer

• The selection of tap changer depends oncurrent, voltage and no. of tappings required.

• Tap changer is normally connected to highvoltage winding to provide wide range ofvoltage adjustment.

FARHAN MAHMOOD

UET, LAHORE

Tap Changers

FARHAN MAHMOOD

UET, LAHORE

Accessories

FARHAN MAHMOOD

UET, LAHORE

• Temperature indicatorsare precisioninstruments, speciallydesigned for theprotection oftransformers.

• Normally two separateinstruments are usedfor indicating oil andwinding temperatures.

FARHAN MAHMOOD

Temperature Indicators

UET, LAHORE

Temperature IndicatorsTemperature indicators perform followingfunctions,

• Indicate maximum oil temperature andmaximum or hottest spot temperature ofwinding.

• Operate an alarm or a trip circuit at a pre-determined temperature.

• Switch on the cooling equipment when thewinding attains a preset high temperatureand switch it off when the temperaturedrops by an established differential (so asto avoid too frequent on and off operationof the switch.

FARHAN MAHMOOD

UET, LAHORE

Oil Level Indicator

• Oil Level indicator isused to monitor the levelof oil in the transformer .

• All power transformersare fitted with amagnetic oil level gaugewhich also incorporatesa mercury switch. Theswitch closes andactuates an audiblealarm in the event of oillevel dropping to nearempty position in theconservator.

FARHAN MAHMOOD

UET, LAHORE

Buchholz Relay

• Buchholz relay is aprotection device forthe detection ofincipient faults such asinter turn faults, corefaults due to core boltinsulation failure whichgives rise to shortcircuit and subsequentarcing.

FARHAN MAHMOOD

UET, LAHORE.

Pressure Relief Device

• It is used to preventthe rapid build up ofpressure within thetransformer tank thatmight cause anexplosion hazard.They are designed /selected to operateand discharge anypressure greater thantheir set value to theatmosphere.

FARHAN MAHMOOD

review of transformer basics adul

Documents