TRANSFORMER, PRINCIPLE, OPERATION AND MAINTENANCEPRESENTED BY: ENGR. M. S. AMINU

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Transformer ClassificationsPower Transformers

Generator Unit Transformer Transmission TransformersSub-Transmission Subtraction TransformersDistribution TransformerInstruments Transformers

Current TransformersVoltage Transformers

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1.1 Definition:Transformers :transfer electrical energy at system voltage to electrical energy at the required voltage or higher voltage.1.2.1 Oil immersed transformers :Oil immersed transformers have their cores & windings immersed in mineral oil.

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1.2.3 Dry type transformers:Dry type transformers have no insulating & cooling fluid they are designed according to the type of insulation of the windings into varnish insulated & silicon insulated dry type transformers.Self Cooling :Distribution transformers are mainly manufactured with self cooling , with this kind of cooling the heat generated is dissipated by the natural air flow and by radiation .With forced air cooling the cooling air is circulated by fans . subsequent addition of fans to the transformer is only possible after consulting the manufacturer . The limits of temperature rise permitted for various insulating materials are specified in ( VDE-0532 ) and these are based on the following limits of cooling air temperature : -Maximum temperature of air 40h C .Daily mean temperature of the air 30h C .Yearly mean temperature of the air 20h C .The temperature of the ambient air is measured at a distance of 1-2 m from the transformer .

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1.2.4 Instrument transformers :- Current transformers- Voltage transformersCurrent transformers:

Current transformer is transformer with small rate power (burden) , whose primary windings are in series with the line circuit , and secondary windings are connected to measuring instruments , electricity meters relays or control devices , current transformers isolate the measuring of protection circuits from the primary voltage and also protect the apparatus corresponding to the over current response of the current .1.2.5 Voltage transformers : Voltage transformers are also of small power rate and operate at almost no load . they isolate the primary high voltage from the connected measuring or protected circuits.

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Transformer major Parts The magnetic circuitThe windings (primary and secondary, and tertiary)Solid insulation materials of the windings and coreTank enclosure

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Major Parts of Transformer (Contd)Bushings and leadsCoolants (Insulants) and cooling arrangementTaps and tap-changing arrangementProtective gear, circuit breakers, protective relaysOther auxiliary equipment

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Magnetic CircuitThe core, a magnetic circuit with a clamping structure, is the part of the transformer in which a magnetic field oscillatesThe metallic composition of the core is a special high grade silicon sheet steel.A typical sheet of steel is 0.014 inches (0.3mm) thick.

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Magnetic Circuit (Contd)These sheets are laminated into sections that are several inches wide.These core laminations help reduce eddy currents or currents induced in the iron parts of the unit.Each lamination in turn is coated with insulating material. This coating helps prevent magnetic losses and reduces heating losses

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WindingsThe primary and secondary windings include clamping arrangements.There are four types of coils or windings used on core-type transformers, these having the following designations:

Spiral typeCrossover typeHelical typeContinuous disc type

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Tank Enclosure and Cooling surfacesThe tank contains the transformer together with cooling surface and coolant.It serves as a surface to radiate heat to the surrounding air.For self-cooled transformers, the main types of tanks in modern use are.

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Tank TypesPlain sheet steelBoiler plate with external cooling tubesRadiator tanksTanks with separate coolers

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BushingsBushings are usually made of porcelainThe inside of the bushing may be oil, paper, epoxy, or fiber glass.Bushings serve to insulate the primary and secondary windings from the tank (ground).

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Insulants or CoolantsAir, gas, oil, or synthetic liquid may be used.

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Tap-Changing ArrangementTappings can be used for the following purposes.

Primary tappings to vary the primary voltage Secondary tappings to vary the secondary voltagePrimary tappings to compensate for variations in the primary voltage

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TRANSFORMERS

Definition : Transformers can be defined as a static electric machine which converts electric energy from one potential to another at the same frequency .

It can also be defined as consists of two electric circuits linked by a common variable flux.

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Theory of operation :

The primary coil of the transformer is connected to a supply of sine wave voltage . an alternating sine wave current will flow in the primary . thus the primary m.m.f ( N.I ) will produce a common flux ( g ) which is also alternating and in phase with the current according to Faradays law the common flux interesting two coils will induce in them an alternating e.m.f ( e1 , e2 ) .

e1is an e.m.f of self induction

e2is an e.m.f of Mutual induction

from 1,2 the transformation ratio

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Insulation SystemInsulation system is made up of

Liquid coolantSolid insulating materialsThe life of the transformer depends primarily on the life of this insulation

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Insulation SystemInsulation system consists of

The liquid insulants together with The solid insulating materials around the windings The insulation system isolates the transformer windings from each other and from the ground.

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Insulation System (Contd) The most widely used transformer insulation systems continue to use two basic items:

A liquid insulation (transformer mineral oil askarel or sillcone, of which more than 90% are oil-filled) and Solid insulation (Kraft paper, cellulose products, PVC, PE and XLPE.

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Liquid InsulantsThe insulating fluid serves three primary purposes:

Provides dielectric strength Provides sufficient cooling Protects the insulation system.

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Solid Insulating Materials The solid material insulates the winding because it possesses two distinct properties.

It has the ability to withstand both electrical and mechanical stresses due to the voltages used. It is such a poor conductor that a negligibly small current can flow through it and leak away. In other words, a good insulator will neither allow current to break through it nor to steal through it.

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Solid Insulting materials (Contd)Consequently, a practical insulation system must contain material that performs the four major function listed below:

The ability to withstand the relatively high voltages encountered in normal service (dielectric strength). This voltage includes both impulses and transient surges The ability to withstand the mechanical and thermal (heat) stresses which accompany a short circuit.

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Solid Insulating Materials (Contd.)The ability to prevent excessive heat accumulation (heat transfer). The ability to maintain desired characteristics for an acceptable service life period, given proper maintenance.

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Cooling in Transformers No-load losses and load losses are the two significant sources of heating considered in thermal modeling of power transformers. Load losses are the more significant source of transformer heating, consisting of copper loss due to the winding resistance and stray load loss due to eddy currents in other structural parts of the transformer.

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Cooling In Transformers (Contd)The basic method for cooling transformers is transferring heat from the core and windings to the insulating coolant such as oil. The wasted energy in the form of heat generated in the transformers due to the foregoing iron and copper losses must be carried away to prevent excessive temperature rise and injury to the insulation about the conductors.

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Cooling In Transformers (Contd) Cooling may be by

Natural circulation Forced cooling using of fans, pumps, etc.

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Cooling System

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Cooling System

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Cooling Arrangements and Designations Both the IEEE and the IEC established standard designations for the various cooling modes of transformers The designation completely describes the cooling method for the transformer, and the cooling method impacts the response of the transformer insulating oil to overload conditions.

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Cooling Arrangements and Designation (Contd).

DRY-TYPE TRANSFORMERSNatural Cooling: type AN Forced Cooling: type AF

OIL-IMMERSED TRANSFORMERSOil CirculationCooling MethodIEC AbbreviationNatural Thermal Head only Air naturalONANAir blastONAFForced Oil Circulation by Pumps Air natural OFANAir blastOFAFWater forcedOFWF

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Transformer Loading And Temperature Limits Design standards express temperature limits for transformers in rise above ambient temperature. The use of ambient temperature as a base ensure a transformer has adequate thermal capacity, independent of daily environmental conditions.

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Transformer Loading And Temperature Limits (Contd)The useful life of a transformer is dependent on the life of its insulation, which ages rapidly at elevated temperatures To ensure a reasonable expectancy of life, it is essential that the transformer is loaded according to the prevailing ambient temperature and also to the temperature of the windings before loading.

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Transformer Loading And Temperature Limits (Contd)

Type Insulation ClassTemperature Rise (oC)ANA60AFB80ANC150ONANA65ONAFA65OFANA65OFAFA65OFWFA65

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Tap Changing FacilityOne of the simplest and most inexpensive methods of providing for adjustments in supply voltages is to arrange tapings on transformer windings Tapings are usually provided for the following purposes:

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Purposes of Tap ChangersFor maintaining the secondary voltage constant with a varying primary voltage.For varying the secondary voltage.

Consumers terminal voltages are reduced on account of impedance drops, and this necessitates tap-changing facilities to effect a slight change in the turn ratio. Seasonal (5-10%), daily (3-5%) and short-period (1-2%) adjustments are needed in accordance with the corresponding variations of load.

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Purposes of Tap Changers (Contd)For providing an auxiliary secondary voltage for a special purpose, such as lightingFor providing a reduced voltage for starting rotating machinery Control of active and reactive power flow in the power system networkFor providing a neutral either for earthing, or for dealing with out-of-unbalance current in single-phase, three-wire circuits, in three-phase four-wire circuits, etc.

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Tap Changing ModesOff Load Tap Changing On-Load Tap Changing.

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Transformer Connections and Vectors The main connections to be considered are:Primary SecondaryY-Y, Y-Delta, Delta-Y, Delta- DeltaY connections provide the opportunity for multiple voltages, while Delta connections enjoy a higher level of reliability (if one winding fails open, the other two can still maintain full line voltages to the load).

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Transformer Connections (Contd).Probably the most important aspect of connecting three sets of primary and secondary windings together to form a three-phase transformer bank is proper winding phasing The dots are used to denote polarity of windings.

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Transformer MaintenanceThree kinds of maintenance are normally recognized:

Unscheduled maintenance- leads to inevitable breakdown.Ordinary maintenance repairs, adjustment and replacement of parts shown to be necessary by visual inspections made at irregular intervals:Scheduled maintenance regularly scheduled inspections and periodic dismantling or testing of equipment to check every detail likely to cause trouble.

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Transformer Maintenance (Contd)The possibility of a fault occurring in a transformer is NOT something remote. In the light of this, the need to undertake protective maintenance is obvious.There are three types of protective maintenance.

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Protective MaintenancePredictive maintenance Preventive maintenance Corrective maintenance

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Predictive Maintenance Predictive maintenance involves more frequent monitoring (inspection and testing) of critical equipment by location, function and by operating environment.

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Preventive Maintenance Preventive maintenance of transformers is an integral part of an annual maintenance program.It may involve monthly inspection, annual energized testing of equipment (oil testing, gas-in-oil analysis, infrared inspection), and de-energized biennial or triennial dismantling or testing of equipment

To check every detail likely to cause trouble (electrical insulation test, switchgear, and so forth)

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Corrective MaintenanceCorrective maintenance is concerned with units which have shown some definite warning signals (such as cloudy oil or unusual odour).When equipment performance begins to tail off and the deterioration so recognized, corrective maintenance is carried out to pinpoint the causes.

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Important Insulation Oil TestsDielectric Breakdown Test Neutralization Number/Acidity Test Moisture Content Test Interfacial Tension Test Oil Power Factor.

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Insulation Oil Tests (contd)Oil Colour Test Visual Examination Specific Gravity Test Sediment in Transformer Oil Test Dissolved Gas Analysis.

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Dielectric Breakdown TestDielectric strength is a measure of the electrical strength of a material as an insulator.Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as volts per unit thickness.The higher the dielectric strength of a material the better its quality as an insulator. Instrumentation available now for dielectric breakdown test is quite compact and efficient.

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Dielectric Breakdown Test (Contd)The primary tests that measures insulation quality are ASTM D-877 and D-1816, D-924 The dielectric breakdown potential tests use two electrodes of fixed geometry and a specified separation.

D-877D-1816Cap 10Cap 10

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Dielectric Breakdown Test (Contd)Voltage is applied and uniformly increased at a rate of approximately 3 kV/s (rms value) until breakdown occurs. When the current arcs across the gap, the voltage recorded at that instant is the dielectric breakdown strength of the insulating liquid.For new oils, this is considered to be 35 kV or above. Used oils would not be acceptable below 25kV.

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Effect of Moisture on Dielectric BreakdownIt is commonly acceptable that water that is dissolved in the oil has a minimum effect on the dielectric breakdown voltage.Free water, on the other hand significantly lowers the dielectric strength of the oil. The variation of dielectric breakdown potential with moisture is shown in Fig. Below.

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Effect of Moisture on dielectric Breakdown Diagram.

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Moisture content TestMineral oil is hydrophobic and the amount of water that will dissolve in oil is very small.The solubility of water in oil is dependent on temperature as shown in Fig. Below.

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Moisture Content Test Effect of Temperature.

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Moisture Content Test The amount of moisture that can be dissolved in oil increases rapidly as the oil temperature increases as shown in Fig above Therefore insulating oil purified at too high a temperature may lose a large percentage of its dielectric strength on cooling, because the dissolved moisture is then changed to an emulsion unless vacuum dehydration is used as the purification process.

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Moisture Content Test (Contd)It is commonly accepted that water that is dissolved in the oil has a minimum effect on the dielectric breakdown voltage. Free water, on the other hand significantly lowers the dielectric strength of the oil. The variation with moisture is shown in Fig below.

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Moisture content Test.The maximum allowable moisture in oil for different voltage levels are as shown in table below:

Voltage level (kV)Maximum moisture530153035256920> 13815

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Oil power Factor TestThe power factor indicates the dielectric loss of the liquid and thus its dielectric heating.The power factor test is used as an acceptance and preventive maintenance test for insulting liquid.Liquid power factor testing in the field is usually done with portable, direct-reading power factor measuring test.

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Other Transformer TestsTurns and Polarity TestsInsulation power Factor Core Excitation Current Polarization Index

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Other Transformer Tests (Contd)Insulation (AC) resistance D.C. Winding Resistance D.C. Over-potential Core Ground Ground Resistance.

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Turns Ratio And Polarity TestsRatio and polarity tests are carried out on every transformer to ensure that the turns ratio of the windings is correct and also the tapping on any of windings have been made at the correct position.The turns ratio test is primarily used as an acceptance test. It is also useful as a tool for investigating problems, as well as an integral part of a routine maintenance program.

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Turns Ratio And Polarity Tests (Contd)The turns ration does not tell how many turns of wire are on the primary or secondary coil, but only gives the ratio of the primary to secondary turns.During the manufacture of new transformers, the turn ratio test is performed on all tap positions to verify that the internal connections are correct and that there are no short circuited turns

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Power Factor Tests (Contd)Cellulosic insulation forms an effective capacitance networkIn each capacitance are dielectric losses which can be conveniently represented by a resistor in series with a capacitor.The insulation power factor is commonly defined as the ratio of the resistance R to the impedance Z of this combination and can be measured by applying a voltage across this capacitance and measuring the amperes and watt loss and then calculating the power factor.

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Turn Ratio And Polarity Tests (Contd)During routine maintenance tests, the turns ratio test could be performed to identify short circuited turns, incorrect tap settings, errors in turn count, mislabeled terminals, and failure in tap changers.

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Power Factor Tests Insulation power factor should not be confused with system power factor in an AC network.Insulation power factor provides an indication of the quality of the insulation Any winding in a transformer is separated from all other winding and ground potential by solid insulation

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Power Factor Tests (Contd) This measurable dielectric loss will develop heat in the insulation during transformer operation (in the equivalent resistor) and his heat, along with moisture and other factors can cause deterioration of the insulation.

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Core Excitation Current TestExcitation current measurement is used for

Field detection of shorted turns and Heavy core damage such as shorted laminations or core bolt insulation breakdown.

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Polarization Index (PI)The polarization index is obtained from the dielectric absorption data.Polarization index is the ratio of the 10 minute resistance to the 1 minute resistance valueIt is dimensionless and often used in dielectric evaluation.

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Polarization Index (PI) Guide

Polarization index Guide for Evaluation of Transformer Condition.Polarization IndexConditionLess than 1.0Dangerous1.0-1.1Poor1.1 1.25Questionable1.25 2.0FairAbove 2.0Good.

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