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Transformer BasicsTransformer Basics

Good Transformer

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A Not Good Transformer

Classic teaching model

The way they are actually built

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Core formed 3 phase 3 legged transformer

Shell formed Transformer

Rh Xh

Rm Xm

XLRL

Ideal TransformerTransformer Model

Simplified to this as Rm and Xm are high compared to Winding impedance

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Which evolves to this in Load flows and fault studies as no load losses don’t contribute to fault currents and Rw is small as we want low loss transformers

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No 2 Copper Loss=52410W

No 3 Core Loss= 21880W

No 6 Exciting current=

0.42%

No 8 impedance= 7.79%.

%R=52.410/12000*100=.437%

.437%

%RC=1*12000/21.880*100=54844%

54844%

%X=(7.79**2-0.437**2))**1/2=7.78%

7.78%

%XC=1*100/(0.0042-(21.88/12000))

42075%

.42%

21.88/12000*100=.1823%

The turns ratio is determined by the voltage ratio but you have to be careful as we have to consider the transformer connection delta delta, delta wye, wye wye. More to come on this

Transformer designTransformer design

EEpp== primary voltageprimary voltageNNpp== number of turns in the number of turns in the primaryprimaryF =F = frequency in cycles per frequency in cycles per secondsecondA =A = cross sectional area of cross sectional area of magnetic core steelmagnetic core steelB =B = flux densityflux densityEEs s == secondary voltagesecondary voltageNNs s == number of turns in the number of turns in the primaryprimary

p p-8E = 4.44 x ( f A B N ) x 10 volts• • •

s s-8E = 4.44 x ( f A B N ) x 10 volts• • •

NN =

EE

s

p

s

p

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What is Transformer NoiseWhat is Transformer Noise

We have have heard the humming of a transformerWe have have heard the humming of a transformerIt is cause by the alternating current cause the core It is cause by the alternating current cause the core steel to expand and contract 2 times during one steel to expand and contract 2 times during one cycle of AC.cycle of AC.Therefore the fundamental frequency of the noise Therefore the fundamental frequency of the noise at 60 Hz is 120 Hz.at 60 Hz is 120 Hz.Because the steel is not well confined in some Because the steel is not well confined in some areas you can get multiple vibrations producing areas you can get multiple vibrations producing 3,5,7 harmonic also.3,5,7 harmonic also.Fans and pumps also produce transformer noise. Fans and pumps also produce transformer noise.

Transformer CoolingTransformer Cooling

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Cooling Methods & DescriptionsCooling Methods & DescriptionsRadiators Radiators –– open airopen air OAOAFans Fans –– forced air forced air FAFAPumps Pumps –– forced oilforced oil FOAFOA–– NonNon--directed oil flowdirected oil flow–– Directed oil flowDirected oil flow

Coolers Coolers –– forced oilforced oil–– Oil to airOil to air FOAFOA–– Oil to waterOil to water FOWFOW

Dry TypeDry Type–– Open airOpen air AAAA–– Forced airForced air FAFA

Radiators Radiators –– Open Air CoolingOpen Air Cooling

Tube TypeTube TypePlate TypePlate TypePermanently AttachedPermanently AttachedRemovableRemovableBanked Banked RemoteRemote

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RadiatorsRadiators

Fans Fans –– Forced Air CoolingForced Air Cooling

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Pumps Pumps –– Forced Oil CoolingForced Oil Cooling

Directed Oil Directed Oil FlowFlow

CoreCore--form form TransformerTransformer

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Oil Flow in CoreOil Flow in Core--form Windingsform WindingsNon-Directed Directed

Directed Oil Directed Oil Flow Flow

ShellShell--form form TransformerTransformer

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Coolers Coolers –– Forced Oil CoolingForced Oil Cooling

Special Cooling SituationsSpecial Cooling Situations

Water Cooling Water Cooling –– oiloil--water heat exchangers water heat exchangers are used to cool the transformer. Typically are used to cool the transformer. Typically used in industrial applications such as arc used in industrial applications such as arc furnace transformers. furnace transformers. Dry Type Dry Type –– air is used to cool the windings. air is used to cool the windings. Typically used in industrial or commercial Typically used in industrial or commercial applications where oil filled units are not applications where oil filled units are not permitted. permitted.

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New Cooling DescriptionsNew Cooling Descriptions

New Cooling Class DesignationsNew Cooling Class Designations

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Temperature Rise LimitsTemperature Rise Limits

System Temp. RiseSystem Temp. Rise ((°°C)C) 5555 6565

Average Winding Rise (Average Winding Rise (°°C)C) 5555 6565Top Oil Rise (Top Oil Rise (°°C)C) 5555 6565Hot Spot RiseHot Spot Rise ((°°C) C) 6565 8080

Transformer ConstructionTransformer Construction

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2 Basic Types of Construction2 Basic Types of Construction

CoreCore--formformShellShell--formform

CoreCore--form Transformerform Transformer

A transformer whose core typically consists of A transformer whose core typically consists of two or more vertical legs surrounded by two or more vertical legs surrounded by concentric cylindrical windings and connected by concentric cylindrical windings and connected by common yokes at the top and bottom.common yokes at the top and bottom.

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CoreCore--form Coilsform Coils

ConcentricConcentric--Cylindrical Cylindrical

CoreCore--form Coilsform Coils

ConcentricConcentric--RectangularRectangular

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CoreCore--form Coresform CoresVertical legs with yokes at top and bottomVertical legs with yokes at top and bottom

Rectangular CrossRectangular Cross--sectionsection Circular CrossCircular Cross--sectionsection

CoreCore--form Coresform CoresStacked vs. WoundStacked vs. Wound

Stacked CoreStacked Core Wound CoreWound Core

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CoreCore--form form Transformer Transformer AssemblyAssembly33--PhasePhase

Complete 3Complete 3--Phase Core & CoilsPhase Core & Coils

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33--Phase CorePhase Core--form Transformersform Transformers

Common 3Common 3--Phase ConfigurationsPhase Configurations

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SingleSingle--Phase CorePhase Core--form Transformerform Transformer

SingleSingle--Phase Core ConfigurationsPhase Core Configurations

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SingleSingle--phase Corephase Core--form Schematicform Schematic

ShellShell--form Transformerform Transformer

A transformer whose core typically consists A transformer whose core typically consists of multiple horizontal legs and interof multiple horizontal legs and inter--connecting yokes that surround the axially connecting yokes that surround the axially interleaved groups of pancake coils. interleaved groups of pancake coils.

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ShellShell--form Coilform Coil

RectangularRectangular--PancakePancakeCan also be Can also be CircularCircular

ShellShell--form Coilsform Coils

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ShellShell--form Coils and Insulationform Coils and Insulation

ShellShell--form Phase Package form Phase Package Being Lowered into TankBeing Lowered into Tank

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ShellShell--form form Transformer Transformer AssemblyAssembly33--PhasePhase

ShellShell--form Core Stackingform Core Stacking

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Complete 3Complete 3--Phase Core & CoilsPhase Core & Coils

CoreCore--form form vs. vs.

ShellShell--form form ComparisonComparison

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33--Phase Core vs. Shell ComparisonPhase Core vs. Shell Comparison

SingleSingle--Phase Phase CoreCore--form / form / ShellShell--form form

ComparisonComparison

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Transformer Winding InsulationTransformer Winding Insulation

Oil Oil –– used for both insulation and coolingused for both insulation and coolingPaper Paper –– used to insulate strandused to insulate strand--toto--strand and strand and turnturn--toto--turnturnPressboard Pressboard –– used for insulation and used for insulation and mechanical supportmechanical support

Core Insulation & GroundingCore Insulation & Grounding

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Transformer ApplicationTransformer Application

Transformer ApplicationTransformer Application

RegulationRegulationAutotransformersAutotransformersTertiary WindingsTertiary WindingsLoadingLoading

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Voltage Regulation DefinitionVoltage Regulation Definition

Regulation is defined as the change in Regulation is defined as the change in secondary (output) voltage, expressed in secondary (output) voltage, expressed in % of rated secondary voltage, which % of rated secondary voltage, which occurs when the rated occurs when the rated kVAkVA output at a output at a specified power factor is reduced to zero, specified power factor is reduced to zero, with the primary impressed terminal with the primary impressed terminal voltage maintained constant. voltage maintained constant.

Regulation FormulaRegulation Formula% Regulation % Regulation = 100 x (= 100 x (VpVp –– Vs) / Vs Vs) / Vs ((EqEq. 1). 1)

= = pRpR + + qXqX + (+ (pXpX –– qR)qR)22 / 200/ 200 ((EqEq. 2). 2)

VpVp = primary voltage (per unit of rated)= primary voltage (per unit of rated)IpIp = primary current (per unit of rated)= primary current (per unit of rated)Vs = secondary voltage (per unit of rated)Vs = secondary voltage (per unit of rated)Is = secondary current (per unit of rated)Is = secondary current (per unit of rated)IrIr = rated secondary current at the operating tap position = 1.0 = rated secondary current at the operating tap position = 1.0 pupup = per unit power factor = p = per unit power factor = coscosθθq = per unit reactive factor = sinq = per unit reactive factor = sinθθ; negative for leading pf; negative for leading pfZ = % impedance at rated Z = % impedance at rated kVAkVAR = % resistance = 100 x kW Load Loss at rated R = % resistance = 100 x kW Load Loss at rated kVAkVA / rated / rated kVAkVAX = % reactance = sqrt[(Z)X = % reactance = sqrt[(Z)22 –– (R)(R)22]]θθ = angle between secondary voltage current = = angle between secondary voltage current = θθVs Vs -- θθIs, in degreesIs, in degrees

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Regulation Vector DiagramRegulation Vector Diagram

Vp

sVsI R

I s

XI s

ZsI

p = p = coscos ØØss q = sin q = sin ØØss

Regulation ExampleRegulation ExampleTransformer Type:Transformer Type: Substation StepSubstation Step--DownDownkVAkVA Ratings @ 55Ratings @ 55°°C RiseC Rise 40000 / 53333 / 6666740000 / 53333 / 66667

@ 65@ 65°°C RiseC Rise 44800 / 59733 / 7466744800 / 59733 / 74667Cooling Class:Cooling Class: OA / FA / FAOA / FA / FATemperature Rise:Temperature Rise: 55 / 6555 / 65°°CC

Primary Volts & Connection:Primary Volts & Connection: 230000 Delta230000 DeltaSecondary Volts & Connection:Secondary Volts & Connection: 67000 67000 GrdYGrdY with LTCwith LTCCore Loss Core Loss @ 100% Voltage@ 100% Voltage 23.7 kW23.7 kWLoad Loss Load Loss @ 40000 @ 40000 kVAkVA 94.9 kW94.9 kW

@ 74667 @ 74667 kVAkVA 331 kW331 kWImpedance Impedance @ 40000 @ 40000 kVAkVA 9.5%9.5%

@ 74667 @ 74667 kVAkVA 17.75%17.75%Resistance Resistance @ 40000 @ 40000 kVAkVA 0.237%0.237%

@ 74667 @ 74667 kVAkVA 0.443%0.443%Rated Amps @ 74667 Rated Amps @ 74667 kVAkVA Primary:Primary: 187187

Secondary:Secondary: 643643

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Regulation CalculationsRegulation CalculationsQuantityQuantity PrimaryPrimary SecondarySecondary with LTCwith LTCVoltage DropVoltage Drop 10%10% 10%10%Load Power FactorLoad Power Factor 91.5%, lag91.5%, lag 91.5%, lag91.5%, lagTap PositionTap Position RatedRated RatedRated 11% Boost11% Boost

Rated VoltsRated Volts 230000230000 6700067000 7443074430Measured VoltsMeasured Volts 230000230000 6030060300 6700067000Per unit MeasuredPer unit Measured 1.001.00 0.900.90 0.090.09

Rated AmpsRated Amps 187187 643643 579579Measured AmpsMeasured Amps 208208 715715 643643Per unit MeasuredPer unit Measured 1.1111.111 1.1111.111 1.1111.111

Rated Rated kVAkVA 7466774667 7466774667 7466774667Measured Measured kVAkVA 8296082960 7466774667 7466774667Per unit MeasuredPer unit Measured 1.1111.111 1.001.00 1.001.00

Effective Effective kVAkVA 8296082960 8296082960Per unit EffectivePer unit Effective 1.1111.111 1.1111.111

Consequences of RegulationConsequences of Regulation1.1. Rated voltage is not maintained on the secondary of the Rated voltage is not maintained on the secondary of the

transformer transformer 2.2. The measured The measured kVAkVA on the primary (input) terminals will on the primary (input) terminals will

not equal the measured not equal the measured kVAkVA on the secondary (output) on the secondary (output) terminals. terminals.

3.3. The measured The measured kVAkVA on the secondary terminals will not on the secondary terminals will not equal the effective equal the effective kVAkVA load on the transformer. load on the transformer.

4.4. The effective per unit The effective per unit kVAkVA load on the transformer will load on the transformer will equal the per unit secondary current. equal the per unit secondary current.

5.5. The The kVAkVA loading capabilities derived from loading loading capabilities derived from loading programs cannot be applied to the measurements on the programs cannot be applied to the measurements on the secondary terminals. secondary terminals.

6.6. The application of LTC switches to compensate for the The application of LTC switches to compensate for the secondary voltage drop does not negate the conclusions secondary voltage drop does not negate the conclusions listed above. listed above.

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Remedies for RegulationRemedies for Regulation

Use Load Tap ChangersUse Load Tap ChangersImprove the Load Power FactorImprove the Load Power FactorUse Lower Impedance TransformersUse Lower Impedance Transformers

AutotransformersAutotransformers

A transformer A transformer in which at in which at least two least two windings have windings have a common a common section.section.

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Autotransformer PropertiesAutotransformer Properties

Lower cost than 2Lower cost than 2--winding transformerswinding transformers33--Phase units or banks must be YPhase units or banks must be Y--connectedconnectedNo phase shift from primary to secondaryNo phase shift from primary to secondaryNo electrical isolation between windingsNo electrical isolation between windings

Equivalent Equivalent kVAkVA CalculationCalculation

Equivalent Equivalent kVAkVA = Rated = Rated kVAkVA x ((HV x ((HV –– LV) / HV)LV) / HV)= 2400 = 2400 kVAkVA x ((240V x ((240V –– 200V) / 240V)200V) / 240V)= 2400 = 2400 kVAkVA x 0.167x 0.167= 400 = 400 kVAkVA

= Rated = Rated kVAkVA x (series / (series + common))x (series / (series + common))= 2400 = 2400 kVAkVA x (40V / 240V)x (40V / 240V)= 2400 = 2400 kVAkVA x 0.167x 0.167= 400 = 400 kVAkVA

Autotransformer CoAutotransformer Co--ratio = series / (series + common)ratio = series / (series + common)

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Tertiary WindingsTertiary WindingsAn additional winding in a transformer that can be used for one An additional winding in a transformer that can be used for one

or more of the following purposes: or more of the following purposes: To stabilize voltages to neutral during unbalanced system To stabilize voltages to neutral during unbalanced system conditions, when delta connectedconditions, when delta connectedTo reduce the effects of 3To reduce the effects of 3rdrd harmonic currents and voltages, harmonic currents and voltages, when delta connectedwhen delta connectedTo reduce the zeroTo reduce the zero--sequence impedance of the transformersequence impedance of the transformerTo carry load at a 3To carry load at a 3rdrd voltagevoltageTo apply power factor correction capacitors or reactorsTo apply power factor correction capacitors or reactorsTo provide a location for detecting zeroTo provide a location for detecting zero--sequence currents for sequence currents for relaying purposesrelaying purposes

Transformer LoadingTransformer Loading

Windings & OilWindings & OilTanks, Internal Structures and ShieldingTanks, Internal Structures and ShieldingCables and Buss BarsCables and Buss BarsBushingsBushingsTap ChangersTap ChangersAuxiliary TransformersAuxiliary TransformersCurrent TransformersCurrent Transformers

Limited by temperature rises in the following components:Limited by temperature rises in the following components:

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Temperature Rise LimitsTemperature Rise Limits

System Temp. RiseSystem Temp. Rise 5555 6565Average Winding RiseAverage Winding Rise 5555 6565Top Oil RiseTop Oil Rise 5555 6565Conductor Hot Spot RiseConductor Hot Spot Rise 6565 8080Metallic Hot Spot RiseMetallic Hot Spot Rise ------ 100100

All Temperatures are in Degrees CAll Temperatures are in Degrees C

Ambient Temperature LimitsAmbient Temperature Limits

Average Over 24 Hours Average Over 24 Hours = 30= 30°°CCMaximumMaximum = 40= 40°°CC

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Other Causes of Transformer HeatingOther Causes of Transformer Heating

High Ambient TemperaturesHigh Ambient TemperaturesOverOver--excitationexcitationGeomagnetic CurrentsGeomagnetic CurrentsHarmonicsHarmonicsLoss of CoolingLoss of Cooling

Other Transformer Loading LimitationsOther Transformer Loading Limitations

Insulation LossInsulation Loss--ofof--LifeLifeBubble Formation in the InsulationBubble Formation in the Insulation