Transformer Brochure Diagnosis Testing

5/24/2018 Transformer Brochure Diagnosis Testing

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Diagnostic Testing Solutions for

Power Transformers


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testing during manufacturingfactory acceptance testing

commissioning acceptance testing

Prevention is better than cure - know more abou

manufacturing commissioning

Taking the right action at the right time

> periodic testing

> testing after an event - relocation, protectio

Keep your transformer

with testing

mechanical impacts>transportation event>post fault event>seismic activity event>etc.

maintain OLTC>corroded contacts>diverter switch>motor & brake

transformercondition

100 %

and subsequent prevent


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the condition of your transformer

replacementoperation

or warning, overcurrent, overvoltage, earthquake ...

good condition

factors causing deterioration

aging>overloading>overheating>moisture

protection problems>protection underfunction>protection failure

Processing of insulation>degassing of fluid>retrofilling>drying of transformer>passivators or inhibitors

replace parts>bushings>surge arresters>gaskets>pumps, fans, etc.

actions

transformer life expectancy


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Transformer parts and their possible faults

Part

Bushings

Bushing CTs

Insulation ma

Leads

OLTC

Windings

Core

Surge arrester


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Transformerdiagnostic set:see pages 6-21

Dielectric responseanalysis instrument:see pages 22-23

Frequency responseanalysis instrument:see pages 24-25

Partial dischargeanalysis system:see pages 26-29

Capacitance,dissipationfactor/

powerfactorat50Hzor

60Hz

Shortcircuitimpedance/

leakagereactance

Transformerratio

Excitingcurrent

DCwindingresistance

Powerfactor/dissipation

factor

Tipuptest

Variablefrequencypower

factor/

dissipationfactor

Frequencyresponseofstra

ylosses

Dynamicresistance

Watt-lossandcurrent

measurement

Dielectricresponseanalysis

Frequencyresponseanaly

sis

Partialdischargeanalysis

Currenttransformeranalysis

Current transformertesting instrument:see CT Analyzer brochure

Faults detectable MeasurementPartial breakdown between capacitive graded layers,cracks in resin-bonded insulation x x x x

Aging and moisture x x xOpen or compromised measuring tap connection x xPartial discharges in insulation x xLoss of oil in an oil-filled bushing xCurrent ratio or phase error considering burden, excessive residualmagnetism, non-compliance to relevant IEEE or IEC standard x

Moisture in solid insulation x1 x1 xAging, moisture, contamination of insulation fluids x x xPartial discharges x xContact problems x xMechanical deformation xContact problems in tap selector and at diverter switch x x x x

Open circuit, shorted turns, or high resistance connections in the OLTCpreventative autotransformer, series autotransformer or series transformer x x x

Contact problems in the DETC x x xShort circuits between windings or between turns x x x xStrand-to-strand short-circuits x xOpen circuits in parallel strands x x xShort circuit to ground x x xMechanical deformation x x x xContact problems, open circuits x xMechanical deformation x2 x2 xFloating core ground x x xShorted core laminates x x

Deterioration and aging x

Notes:1) Power factor / dissipation factor measurements at 50 Hz or 60 Hz can detect high moisture contents, but have a blind spot for low moisture contents. Measuring power factor / dissipation factor at

lower frequencies, such as 15 Hz, improves sensitivity. The most sensitive method to determine moisture i n solid insulation is dielectric response analysis.2) If the core ground can be opened.


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All in one: the multi-functional transformer test

Leakage reactance/short-circuit impedance measurement instrument

DC winding resistance measurement instrument

winding resistanceI

TR

VRLVRHV

mechanical?

TR

V

I

Frequency response of stray losses measurement instrument

winding strands

TR

V

I

+ more substation diagnostics

> Ground impedance measurement> Line impedance and ground factormeasurement

> Resistance measurement> Primary relay testing

Measurement

V, I, P, Q, S

Cp: 1 pF - 3 F

dissipation factor / power factor

resistance: 0.5 - 20 k

Output

12 kV

800 AAC

400 ADC

15 - 400 Hz

Precision

output signal digitally generated

Cpmeasurement: < 0.05 % error

independent from mains qualityin wave form and frequency

W

CP

CP


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set CPC 100 / CP TD1

IR ICV

I

Insulation

+ more substation diagnostics

> Current transformer testing> Voltage transformer testing

Power factor / dissipation factor measurement instrument

Transformer ratio measurement instrument

Transformer exciting current test instrument

core

I

TR

NP/NS

TR

VP VS

High voltage cable

20 m / 65 feet

double screen

insulation supervision

Trolley

to conveniently transport:

CPC 100,CP TD1,

measurement cable,high voltage cable

of units

29 kg / 65 lbs

26 kg / 56 lbs

Power supply

110 - 240 V

50 - 60 Hz

16 A


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Surge arresters Bushings OLTC Leads Insulation materials Windings Core

Capacitance and power factor / dissipationfactor (PF / DF) measurements are performedto investigate the condition of bushings as wellas the transformer overall insulation. Aging anddecomposition of the insulation, or the ingressof water, increase the energy that is turned intoheat in the insulation. The level of this dissipationis measured by the PF / DF.

On surge arresters, currents and watt losses ofidentical units can be compared. Deviations mayindicate aging effects, poor contacts or open

circuits between elements.

Capacitance values of bushings show if therehave been breakdowns between capacitivelayers. For resin bonded paper bushings, cracksinto which oil has leaked, can also change thevalue of the capacitance.

A rise in capacitance of more than 10 % isnormally considered to be dangerous, since itindicates that a part of the insulation distance isalready compromised and the dielectric stress tothe remaining insulation is too high.

Preventing bushings from exploding

Increased heat dissipation accelerates theaging of the insulation. If an aged insulationcan no longer withstand the electrical stress,bushings explode.

Better understanding of losses

At line frequency, conductive losses can berepresented with a parallel equivalent circuit.Polarization losses can be represented by a seriesequivalent circuit consisting of an ideal capacitorand a resistor.

Increased losses may pass a test at linefrequency unnoticed, leaving the tester unawareof an insulation in distress. Measuring the DF /PF over a broad frequency range helps to betterunderstand both types of losses.

Measuring capacitance & power factor / dissipatio

Typical loss shapes in 15 - 400 Hz range

conductive lossespolarization losses

superposition of both effects

equivalent circuits *)

typical loss shapes

Damaged T

Capacitive

center cond

f f

f


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factor

High voltage is applied to the insulation tobe tested, i.e. the bushing tip, and a low lossreference capacitor (integrated in CP TD1) isconnected in parallel. The currents flowing throughthe insulation and through the reference capacitorare measured and the time difference betweentheir zero crossings is determined. The loss angle is then calculated from this time difference. Thetangent of this angle is the dissipation factor. Thecosine of the angle between voltage and current isthe power factor. Results are compared with valuesgiven in IEEE C57.10.01 and IEC 60137, and can

be compared with a base measurement, anotherphase, or a sister transformer.

If values deviate more than indicated by thestandards, then dielectric response analysiscan be performed to check for increasedmoisture.

Chemical tests can be performed to verifythe quality of the insulation fluid (DGA,dielectric breakdown strength, interfacialtension, etc.) Measuring the power factor /dissipation factor of the insulation fluid canalso be done with a CPC 100 accessory, theCP TC12 oil test cell.

bushing explosion How does it work? How can results be confirmed?

in bushings

CA

grounded layerand tap electrode

on flange

OIP bushing: PF / DF tip up test

Type RIP OIP RBP

Insulation

Resin

impregnated

paper

Oilimpregnated

paper

Resin

bonded

paper

IEC 60137 < 0.70 < 0.70 < 1.50

IEEE C57.10.01 < 0.85 < 0.50 < 2.00

Typical newvalues

0.3 - 0.4 0.2 - 0.4 0.5 - 0.6

*) at 50 / 60 Hz and 20 C

Dissipation factor / power factor in % *)

OIP bushing: PF / DF variable frequency test


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Measuring capacitance & power factor / dissipatio

Power factor / dissipation factor (PF / DF)

measurement indicates the condition of theliquid and solid insulation within a transformer.

Power and accuracy

The CPC 100 / CP TD1 can measure capacitanceand PF / DF (tan ) in laboratories, test fieldsand on site.

A powerful test voltage source (12 kV, 100 mAcontinuous, 300 mA short-term load current)with variable frequency (15 - 400 Hz), combinedwith high accuracy measuring inputs allows fast,effective and accurate measurements.

Prepared test procedures can guide the userthrough the testing process and offer a basis forcomprehensive reporting.

Modular equipment

The modular equipment (CPC 100: 29 kg /65 lbs, CP TD1: 26 kg / 56 lbs) can be easilytransported thanks to its sturdy cases, whichcan also be used to place the instruments ontothem for working at a comfortable height, asshown on page 12.

For convenient transport or mobile use such asin test fields or in substations/power plants, theinstruments can be mounted onto a trolley.

The CPC 100 is used to control the test, i.e.:

> entering the voltage and frequency valueswhere C and cos j/ tan shall be measured

> starting and stopping the test

> supervising the measurement progress andintermediate results

> storing results on flash disk and

USB memory stickThe CP TD1 includes

> a high voltage step-up transformer

> a reference capacitor (pressurized gas type)

> the unit to measure and compare currents inamplitude and phase

Power factor / dissipation factor tip up results

Energize HV to measure CH+ CHL| CH| CHL, then energize LV to m

CL

IN A


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factor

Your Benefits

> perfect digitally generated sine wavetest signal that is independent frompower quality and line frequency

> laboratory precision for on-site use:< 0.05 % error for capacitance Cp

> portability (CPC 100: 29 kg / 65 lbs,CP TD1: 26 kg / 56 lbs)

> mobility through the use of a

specialized trolley> ruggedness and ergonomic design:transport cases with wheels bringequipment to appropriate workingheight (see page 12)

> automatic tests at different voltages

> automatic tests at different frequencies:early detection of insulation stress dueto the improved sensitivity provided bymeasurements made in the range of15 - 400 Hz

> optional measurement bandwidthreduction to 5 Hz and averagingof up to 20 results for precisemeasurements despite strongelectromagnetic interference

> temperature correction accordingto type of insulation and relevantstandard

> internal recalibration of electroniccircuits of the CP TD1 with eachmeasurement

> automatic reporting of capacitanceCp, DF (tan ), PF (cos j), power(active, reactive, apparent), impedance(absolute value, phase, inductivity,resistance, Q)

> automatic assessment if referencevalues for capacitance and powerfactor / dissipation factor are known

> less wiring effort through twomeasurement inputs (IN A, IN B) thatcan be used to measure for example

the capacitance of a bushing at thesame time as the main insulation

CL+ CHL| CL| CHL- thanks to internal switching logic with guard

HV

CHL

CH

Power factor / dissipation factor variable frequency results


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Measuring ratio & exciting (no-load) current

Ratio measurement with the CPC 100

The CPC 100 measures the transformer ratio byapplying a high voltage at the HV winding ofone transformer leg. In amplitude and phase, itmeasures the applied voltage and the voltage atthe LV winding, as well as the exciting (no-load)current. The deviation from rated values isdisplayed as a percentage.

Measuring ratio per tap

The CPC 100 measures ratio and excitationcurrent at each tap position. Each time theuser operates the tap changer, the CPC 100automatically starts a new measurement andmeasures and displays ratio, phase angle, andfor each tap, the deviation from nominal ratio isdisplayed as a percentage.

For automatically measuring winding resistance

and ratio of all phases and all taps, see page 16.

The measurement is performed for assessingpossible winding damage, such as turn-to-turnshort circuits, comparing the measured ratio andmagnetizing currents to specifications, factorymeasurement results, and/or across phases.

In the factory, this measurement is performed toverify that ratio and the vector group is correct.

Setup for automatically measuring ratio and resistance per ta

Surge arresters Bushings OLTC Leads Insulation materials Windings Core

VP

Pri

V


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page 16)

Your Benefits

> powerful AC voltage source,controllable from 0 to 2000 V

> a perfect digitally generated

sine wave test signal that isindependent from the qualityof the mains wave form

> convenient and quick testing byautomatic detection of tap changeroperation as trigger for the nexttap measurement

> exciting current measurementin amplitude and phase

> variable frequency formeasurements outside mainsfrequency for noise suppression,if selected by the user

> accuracy and safety

> automatic reporting of measuredvoltage values and phase angles,measured ratio and deviation asa percentage, exciting current inamplitude and phase

> tabular and graphical resultrepresentation for every tap

test setup How does it work? How can results be confirmed?

The winding ratio between primary andsecondary windings is measured for eachtransformer leg, applying high voltage at theHV side and measuring on the LV side. The ratioof these voltages, equalling the turns ratio, iscalculated. Results are compared with name-plate values and across phases.

The exciting current is the corresponding currentflowing in the HV winding if the LV winding isopen. Results are compared with a referencemeasurement, or a measurement performed on

a sister transformer; in three phase transformers,the two outer phases can also be compared.

With the turns-ratio test, shorted turns canbe detected. If a problem is suspected froma DGA, a dissipation factor test, or a relaytrip, a turns-ratio test can be performed torule out / verify if turns are shorted.

If the exciting current test shows deviations,and DC winding resistance and ratio test donot show errors, then the cause may be acore failure or unsymmetrical residual flux.

CPC 100 TRRatio test card

Exciting current [mA] per tap

TR

VS

NP/ NS

Watt losses [W] per tap


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Winding resistance per tap

Measuring DC winding resistance and OLTC

Measuring resistance with the CPC 100

The CPC 100 injects DC current into thewinding, measures current and voltage and thencalculates and displays the resistance. When theresistance value is stable, the CPC 100 makes thefinal measurement, and reduces the test currentto zero to discharge the energy saved in the

winding. When it is safe to remove test leads,the CPC 100 illuminates its green safety light.

Tapped windings and OLTC

In semi-automatic mode, the CPC 100 measuresthe resistance of each subsequent tap position.Each time the user operates the OLTC, theCPC 100 waits until the values stabilize, andthen measures and displays the windingresistance at this tap position. When all tapshave been measured, the CPC 100 dischargesthe inductive energy stored in the winding and

indicates when this process is completed. Forautomatically measuring static and dynamicwinding resistance and ratio of all phases andall taps, see page 16.

Dynamic resistance measurement

The OLTC has to switch from one tap positionto another without interrupting the loadcurrent. When switching the tap changer duringwinding resistance measurement, the DC currenttemporarily decreases. This current decreaseshould be measured and compared across

taps, as recommeded in the Cigr TransformerMaintenance Guide 445.

Winding resistance measurements areperformed for assessing possible windingdamage. It is also used to check the On-LoadTap Changer (OLTC) - to know when to cleanor replace OLTC contacts, or to know whento replace or refurbish the OLTC itself, whichhas a shorter life span than the active part ofthe transformer.

In the factory, this measurement is performed tocalculate the I2R component of conductor lossesand to calculate winding temperature at the end

of a temperature test.

Table in CPC 100 TRTapCheck test card

Surge arresters Bushings OLTC Leads Insulation materials Windings Core Burn-off

Switc


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Slope per tap

Ripple per tap

Your Benefits

> convenient and quick testing by usingOLTC operation as a trigger for thenext tap measurement

> additional condition assessment of theindividual OLTC taps through dynamicresistance measurement, recordedas a part of classical resistancemeasurement, without extra effort

> high accuracy and safe testing throughthe use of a 4-wire connection. TheCPC 100 visually indicates when itis safe to remove test leads, evenif its power supply is interruptedduring testing. If the test leads areremoved or interrupted accidentally,

the test current will flow through thevoltage path, preventing dangerousovervoltages. If the CP SA1 accessoryis in use during such an accidentalinterruption of test leads, damage tothe CPC 100 will be prevented.

> automatically created report showingthe test duration, the resistancevalue at measurement and referencetemperature, etc.

> tabular and graphical results are

produced for every tap for easyvisual comparison

How does it work? How can results be confirmed?

To measure the winding resistance, thewinding under test must first be loaded withenergy (E=1/2*L*I2) until the inductance ofthe winding is saturated. Then the resistancecan be determined by measuring DC currentand DC voltage. For tapped windings, thisshould be done for every tap position, hencetesting the OLTC and the winding together.Results should be compared to a referencemeasurement, across phases, or with a sistertransformer. In order to compare measurements,the resistance values have to be re-calculated,

to reflect different temperatures duringthe measurements.

Results should not differ more than 1 %compared to the reference measurement.Differences between phases are usually lessthan 2 - 3 %.

Transformer turns ratio or frequencyresponse analysis can be used to confirmcontact problems. In both cases, hot spotsin the transformer will result in a DGAindicating increased heat. However, gassignatures are not unique and thus do notallow for the identification of the root cause.

diverter switch

process


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Automatically measuring ratio & winding resistance

Using the CP SB1 accessory, the CPC 100 can

automatically> measure ratio and the exciting current of allof the taps and all phases

> confirm the vector group

> measure static and dynamic windingresistance of all of the taps and all phases

This accessory helps to save a lot of time aswiring is only necessary once. With the samecabling, both ratio and resistance measurementscan be performed.

Through the CP SB1, the CPC 100 is connected

to all phases of a transformer. The up and downcommand inputs of the OLTC are also connectedand controlled by the CPC 100 and the CP SB1.

Ratio measurement

The CPC 100 only requires the user to enterratio and the vector group to measure theratio and the exciting current for each tap ofeach phase automatically. For each tap, resultsare compared to the specified ratio and thedeviations are displayed.

Winding resistance measurement

With the CP SB1, the CPC 100 injects DC currentinto each tap of each winding. The CPC 100then waits for the current to stabilize andmeasures the resistance value, as well as thedata describing the switching process (dynamicresistance measurement).

The tap changer is then operated automaticallyuntil the measurement on one transformerphase is finished. Between measuring thedifferent phases, the energy stored in the

windings is quickly discharged. When thewindings are fully discharged, the CPC 100 /CP SB1 automatically switches to the next phase.

At the end of the measurement, the last windingis discharged and the operator is notified visuallythat it is safe to remove the wiring.

AC, DC,

LV

Faster Safer


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of all taps and all phases

control

Your Benefits

> several times faster than conventionalwiring technique:- minimum wiring - only once for all

connections- automatic discharging of the

windings between measurements- automatic tap changer operation

> increased safety: no repeated climbing

up and down the transformer

> simple workflow: a single, automaticmeasurement for determining ratioand exciting current, as well as staticand dynamic winding resistance

> prevention of wiring errors: prior tothe measurement, wiring plausibility isautomatically checked

> comprehensive automatic reporting forall phases and taps

Measurement with switchbox

Switchbox connected to CPC 100 / CP TD1

OLTC HV


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Measuring short circuit impedance / leakage reactanc

Numerous incidents exist of asset managersinvestigating the reason why their transformer isgassing even though all standard electrical testsshow acceptable results. This illustrates thattheir tools do not cover all trouble and failurepossibilities.

The Frequency Response of Stray Losses of each

phase will be nearly identical if all phases are ingood condition. An increase in frequency willresult in an increase in impedance as the skineffect becomes more pronounced.

Just like measuring leakage reactance or shortcircuit impedance at power system frequency,the CPC 100 measures leakage reactance, orshort circuit impedance, across a frequencyrange of 15 - 400 Hz, as defined by the user.

It applies AC voltage to the high voltagewinding, with the low voltage winding short-

circuited. It then measures the load currentin amplitude and phase and calculates theimpedance. The measurement is performed foreach transformer phase. The user then comparesresults across phases and / or over time.

The measurement is performed for assessingpossible damage/displacement of windings.Measurements are compared over time or bycomparing phases.

In case of a short-circuit, forces work towardsthe core for the inner winding and away fromthe core for the outer winding. If these forcesaffect the placement of windings, the leakageflux will change. In particular, short circuitsbetween parallel strands of ContinuouslyTransposed Conductors (CTCs), and local

overheating due to excessive eddy current losseslinked by the stray flux can be detected.

Measuring frequency response of stray loss

Surge arresters Bushings OLTC Leads Insulation materials Windings Core Re


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& frequency response of stray losses

Your Benefits

> a perfect digitally generatedsine wave test signal that isindependent from power quality

> additional diagnostic informationthrough the measurement of theleakage reactance or short circuitimpedance at several frequencies

> variable frequency formeasurements outside mainsfrequency for noise suppression,if selected by the user

> accuracy and safety

> automatic reporting of allmeasured values

> display of result as Z and ,R and X, or R and L

> graphical results representation

overheating How does it work? How can results be confirmed?

An AC source is connected to each phase of theHV winding with the corresponding LV windingshorted. The current and the voltage acrossthe HV winding are measured in amplitudeand phase, and the short circuit impedanceis calculated. Short-circuit impedancemeasurements should ideally be performed overa range of frequencies, commonly known asFrequency Response of Stray Losses. Here, theAC source features variable frequency. Aftersource current and voltage have been measuredacross the HV winding, the stray losses are

represented by the inductive part of the shortcircuit impedance at higher frequencies.

Leakage reactance: deviations of more than1 % ought to be investigated with othertests such as FRA. Differences betweenphases are usually less than 2 %. Deviationsof more than 3 % are considered significant.Frequency response of stray losses (FRSL) resultscan be cross-checked with PD measurement,FRA, and DGA. If parallel strands are shorted,higher losses in the stray channel willcause high internal temperatures, normallyindicated by a DGA. The gas signatureis not unique and does not provide the

identification of the root cause, however.FRSL is unique in this respect.

HV

Leakage flux

HVLV LV

Leakage flux force direction

FRSL test results with faulty phase C


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CPC 100 / CP TD1 - operation according to individu

Reporting

Performed tests can be saved and arethe basis for comprehensive reports.

For customized reporting, all databelonging to the measurement,including settings, results, andadministrative information such asdate & time, filename, etc. can alsobe imported to MS Excel. OMICRONprovides templates containing typicaltest procedures for power system

apparatus, providing guidance duringthe measurement and convenientlyand quickly producing comprehensiveresult representations in MS Excel. Testreports can automatically be enteredinto customer-specific sheets andfurther content, for example companylogos, can be added.

Test preparation on PC

Tests can also be prepared in theoffice on a PC or laptop - without the

CPC 100, with which the test will laterbe executed at site, step by step.

Manual front panel operation

Operating CPC 100 / CP TD1 manuallyprovides results with minimal training perfect for users operating the devicesoccasionally. Operating directly throughthe device, the user just selects theoutput to be used, the measurementto be made and performs it by pressingthe green button. Users can measureexactly the way they consider best byusing the device in this way.

Front panel operationsupported by test cards

Dedicated test cards help whenperforming frequent applicationsconveniently and efficiently. Thecards contain predefined procedures,dedicated to specific applications (forexample power factor / dissipationfactor, winding resistance and tapchanger test, ratio measurement, etc.).

Several test cards can be combinedto form an entire test plan for apower system apparatus (e.g. a power

transformer), guiding the user throughthe measurement.

Test cards dedicated to specific tests

Result representation in MS Excel

Directly setting output values

Result representation on PC / laptop


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needs

Primary Test Manager (PTM)

Primary Test Manager (PTM) softwaresupports the users workflow duringdiagnostic testing. The user can defineand manage test objects, create test plans,perform measurements, and generatereports. PTM manages the entire workflowduring testing, guiding the user throughthe process step-by-step.

1. Asset managementPTM supports the administration of

asset data of power transformers:general identifying characteristics likelocation, manufacturer, productiondate, serial numbers, etc. can beentered just like electrical data suchas number of windings, voltage andpower ratings, vector group, etc.

2. Dynamic test plan generationBased on the electrical data of theapparatus (such as vector groupor bushing type), PTM generates aplan of diagnostic measurements to

be performed in accordance withindustry standards, thus saving time

and reducing the risk of errors.The test procedure can easily beadapted by selecting / de-selectingelements.

3. Guidance through testingDuring the measurement, PTMenables the user to directly controlthe test instrument from a PC orlaptop. Clear connection schemeshelp the user to make correctconnections and to avoid errors.The test progress is visible in the

test table throughout the test.4. Reporting

After the tests, reports can begenerated at any time for anyof the measurements madepreviously. The report contentis flexible and customizable.Customer specific report forms canbe generated and other elementscan be added, such as companylogos.

PC control and application management with PTM

Primary Test Manager main screen

1. Asset management

2. Dynamic test plan generation

3. Guidance through testing

4. Result representation in PTM


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Dielectric response analysis of power transformers an

Displaying the dissipation factor over a widefrequency range provides insight into thespecific properties of the oil, the geometryof the solid insulation in the form of spacersand barriers, and the condition of the solidinsulation itself. This is the only method thatcan - non-invasively - directly measure the actual

moisture content in the solid insulation.The method is scientifically approved byCIGR. Aging threshold values as defined inIEC 60422 allow for an automatic insulationcondition assessment and correspondingrecommendations for further actions such astransformer drying.

OMICRONs DIRANA can measure dielectricresponse over an extremely wide frequencyrange (10 Hz - 5 kHz). It minimizes testing timeby combining frequency domain spectroscopy(FDS) at high frequencies and polarization and

depolarization current measurement (PDC)at low frequencies. DIRANA also displays thepolarization index (PI) based on FDS/PDCmeasurement. It thus replaces measuringinsulation resistance, delivering the sameinformation, but being more accurate formoisture determination. Testing time is furtherminimized by simulaneously measuring throughtwo channels, and the application of anintelligent curve recognition. Measurements areended automatically as soon as the typical shapeof the curve, including the hump, indicates thatall relevant points have been measured.

Dielectric response analysis is used to assess thewater content of the solid insulation (cellulose)and thus periodically monitor its condition.

Knowing the water content is important for thecondition assessment of transformer bushingsand the transformer in its entirety.

In the factory, this measurement is used at theend of production to control the drying procressand to assure low moisture after drying.

Verifying the insulation condition with DIRA

Surge arresters Bushings OLTC Leads Insulation materials Windings Core Typical sha

issipation

actor

low

high

0,001

0,01

0,1

1

0,001 Hz


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limited

comprehensive

comprehensive

bushings

DIRANA and accessories in sturdy case

Your Benefits

> insulation condition assessmentconcerning moisture / aging ofcellulose and oil conductivity

> automatic result evaluation accordingto IEC 60422 (dry, moderately wet,wet, extremely wet), indicating iffurther actions are required

> completely non-invasive measurement

> minimum down-time: a measurementcan be performed directly after thetransformer has been shut down, asequilibrium is not required

> fast measurement through an intelligentcombination of methods FDS and PDC,

simultaneous measurement with twoinput channels and forecast algorithm

> automatic compensation of theinfluence of conductive agingby-products avoiding overestimation ofmoisture content

> predefined tests for all transformertypes and bushings

> step-by-step software guidance

> also measure the insulation condition

of cables, generators, motors andinstrument transformers

dielectric response How does it work? How can results be confirmed?

If the dissipation factor of a transformer isplotted against a wide frequency range, theresulting dielectric response curve containsinformation on the insulation condition.

The very low and the high sections containinformation on moisture and aging in the solidinsulation, while the position of the slope in themid range frequencies indicates the conductivityof the liquid insulation.

This curve is compared to model curves to

evaluate aging, particularly for assessing themoisture content in the insulation.

There are no other non-invasive ways toassess moisture in a transformer; dielectricresponse analysis is unique in this respect.

The Karl Fischer titration method candetermine moisture content in oil or inpaper, but has several disadvantages.

For instance, to determine moisture inpaper, the method requires opening thetransformer and taking a paper sample.During the process, the insulation itself is

being damaged and the sample takes upnew moisture.Frequency

Moisture and agingof cellulose

Oil conductivity

Insulation Geometry

low

high

low

high

Hz 1000 Hz

~ 6.0 hFDS

~ 2.9 hDIRANA

~ 5.5 hPDC

durationf range


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Sweep Frequency Response Analysis

Frequency response analysis (FRA) is based onthe comparison of a reference test with anactual test, where the reference test is usually afingerprint that has been previously measured.When such a fingerprint is not available, anotherphase or a sister transformer can also be usedfor comparison.

For on site use, FRAnalyzer comes in a ruggedcase which all necessary accessories fit into,including a battery with sufficient powerto complete the comprehensive testing ofany transformer.

FRAnalyzer uses braids for its connections whichallow a high level of reproducibility due to theirtight connection close to the bushing usingclamps and screws.

This technique is recommended in the CIGRbrochure 342 on FRA:

> connection close to the bushing> broad braids minimize test setup interference

Frequency response analysis (FRA) is used toverify the electrical and mechanical integrityof the active part of the transformer (core,leads, windings). FRA is also ideal for furtherdiagnosis, if periodic testing or monitoringidentifies irregularities.

A growing number of utilities also use FRAduring routine testing, since the method candetect a wide range of faults while beingcompletely non-invasive.

FRA is the most sensitive method for detectingmechanical deformations.

Test lead connection on transformer bushing using broad braids, c

clamp connection

Surge arresters Bushings OLTC Leads Insulation materials Windings Core Elements for


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and screws

Your Benefits

> non-invasive detection of:- winding deformations- shorted parallel strands

- winding or interturn short circuits- winding shorted to ground- shorted core laminates- floating core ground- open circuits- contact resistance problems

> excellent reproducibilty throughinnovative connection technique

> powerful and easy-to-use software:- database solution- import of FRA traces from other

vendors (Doble, FRAMIT, FRAX, etc.)- export traces in CIGRE exchange (.xfra)or .csv format

- data export to MS Excel or databaseapplications

- automatic assessment throughproven algorithm

> automatic reporting

> high accuracy and wide dynamic range

> small and lightweight device

> support is available from OMICRON forthe interpretation of results

unique fingerprint How does it work? How can results be confirmed?

A low voltage sinusoidal signal with variablefrequency is applied to one terminal of awinding and at the other end of the windingthe response signal is measured. The voltagetransfer function of the winding is determinedas the output / input ratio.

The transfer function of a winding depends onthe resistive, inductive and capacitive elementsof the transformer. Changes in these elementsas a consequence of a failure lead to changes inthe transfer function. Results are represented in

magnitude and phase as in a Bode diagram.

Frequency response analysis can detect awide range of faults. Some of these faultscan be confirmed by other measurements,such as DC winding resistance, frequencyresponse of stray losses, short-circuitimpedance / leakage reactance, excitingcurrent, or ratio measurement.

However, no other method can give asclear an indication as frequency responseanalysis can to determine whether windingshave been deformed, for example by the

mechanical forces resulting from a fault.

Automatic comparison / evaluation of results

Small and lightweight FRAnalyzer


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Partial Discharge Analysis

The OMICRON MPD PD system offers quick andprecise recording of pulses on the three phasesof a transformer using data acquisition fromthree or more channels.

Digital filter

In the MPD 600 the classical analog bandpassfilter has been replaced by a digital filter usinga mathematical algorithm. Digital systemdesign eliminates aging effects and temperaturedrift making measurements comparable andreproducible by perfectly reproducing settings:

> the digital filter can be easily adapted tothe conditions on site by tuning its centerfrequency and bandwith to minimizedisturbances with fixed frequency bands

> calibration values for charge and voltagecan be set directly on the laptop controllingthe test, fully reproducible during the next

measurement

Optical Isolation

Between individual PD acquisition unitsand between the acquisition units and thePC / laptop, fiber-optics are used for thecommunication. Communication with fiber-optics ensures a continuous, disturbance-freetransmission of PD events and test voltage. Theunits are supplied using a battery power supply.This design provides complete galvanic isolationbetween the individual components, minimizingground loops and so reducing interference.

Once initiated, partial discharge (PD) causes aprogressive breakdown of insulating materials byelectrical tree formation.

PD measurements are performed on theinsulation of transformers to determine theinsulations condition and to safely prevent itfrom breaking down.

PD measurement is also part of the factoryacceptance test.

PD analysis on a three phase transformer

C3

void

cond

cond

Surge arresters Bushings OLTC Leads Insulation materials Windings Core Partial disc


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measurement How does it work? How can results be confirmed?

A coupling capacitor is connected in parallelto the capacitances of the measured insulationdistance. Any charge movements within theconnected insulation distance will be reflectedin the charge of the coupling capacitor. Theresulting circulating current of the paralleledcapacitances is measured and interpreted.

Analyzing PD means detecting and evaluatingvery small discharges, while dealing with veryhigh test voltages, often complicated by externaldisturbances.

A chemical dissolved gas analysis (DGA)can also indicate partial discharges. Itis impossible, however, to locate partialdischarges with DGA.

Battery powered acquisition units

The acquisition units are supplied fromrechargeable batteries, which can supplythe units for more than 20 hours. Anotheradvantage of battery power supply is that iteliminates disturbances which would result froma mains power supply.

Noise suppression through gating

Additionally, noise can be eliminated byamplitude / phase gating, dynamic noise gatingor antenna gating. Here, one measurementchannel, which is not connected to theequipment under test, is used as a detectorfor external disturbances. Any pulse pickedup by this unit is considered to be an externaldisturbance and is therefore eliminated onall other units because internal PD cannot bedetected by this antenna channel due to the

shielding effect of tank and graded bushings.

Multi-channel measurement

Measuring simultaneously with several channelswith synchronization accuracy in the range ofnanoseconds has several advantages:

> it minimizes the time for which high voltagehas to be applied to a suspect transformerand speeds up testing

> it allows for real-time de-noising of the datato minimize the influence of disturbances,and helps separating different sources of PDand identifying the type of PD sources

PD acquisition unit

Signals acquired simultaneously by 3 units

C3

insulation

Ccoupling


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Partial Discharge Analysis

Tuning to partial discharges (PD)

When you are listening to the radio, the audiofilter of your receiver filters out all other radiostations, and only plays the one that you arelistening to. The MPD can use two methods fortuning to PD sources to display only what youwant to take a closer look on.

3-Phase Amplitude Relation Diagram

Through cross-coupling, a PD pulse on onephase in a transformer will usually appearon all phases - with different amplitudes.Noise, however, is external and thus produces

amplitudes that are similar on all phases.

By synchronously measuring on all phases thetester can separate pulses by plotting them in the3-Phase Amplitude Relation Diagram (3PARD).

Noise will create a separate cluster in this diagram,usually in the center of the 3PARD. PD, however,the pulses of which are often smaller than thoseof noise, typically form a cluster outside the center.If more than one PD source exists, each of themwill form a separate cluster.

When a cluster is selected, the phase-resolved PD

pattern will be shown specifically for this cluster,facilitating pattern recognition, i.e. determiningthe possible cause of a single PD source.

3-Center Frequency Relation Diagram

Another way to separate pulses is 3-CenterFrequency Relation, which requires only onemeasurement channel, for example when thetest object is a single phase transformer.

This method measures with three filters atdifferent measurement frequencies at the same

time. Using spectral differences, distinct internalpulses can be separated from each other and PDcan be discriminated from external noise.

The result of the three measurements is plottedin the 3-Center Frequency Relation Diagram(3CFRD). The unfolding clusters in this diagramcan then be analyzed separately.

Advantages of 3PARD and 3CFRD

> allows the separation of PD activity from noise

> enables the separation of different PD sources

> facilitates pattern recognition

Separated partial discharge Sep

pd cluster

Measurements related to each other in 3PARD or


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Your Benefits

> lightweight

> scalable and modular system

> high speed for the mostcomprehensive testing

> measuring all phases of a transformersimultaneously with nanosecondsynchronicity

> high operator safety through opticalfibers galvanically isolated from thePD acquisition units

> high sensitivity down to pico or evenfemto Coulombs through effectivegating technology

> separating PD sources and noisethrough 3PARD / 3CFRD

> improving the locating of PD andthus assisting the user to make the

right follow-up decisions (e.g. if atransformer can be repaired on-site)

noise

Ultra high frequency PD detection

Within liquid-insulated transformers, PD can alsobe measured using ultra high frequency (UHF)sensors. PD is directly measured from withinthe tank, by flanging UVS 610 sensors directlyonto it, using its natural screening effect. TheUHF 608 accessory converts the signals for theMPD. UHF measurement can also be used totrigger an acoustic PD measurement, or as anadditional gating mechanism - then pulses froman electrical measurement are only accepted if aUHF pulse is also present.

UVS 610 UHF sensor (MPD accessory)

noise cluster


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Power transformer services, training and support

Expertise in transformer diagnostics

OMICRON employs some of the worlds mostrenowned experts in transformer diagnosis.

Among them are members of working groupsconcerned with transformer maintenance anddiagnosis in international standardizationbodies, such as CIGR, the IEEE, or the IEC.

They have performed numerous diagnosticmeasurements on power transformers, often asa result of customer requests.

Moreover, they have published many papers

on power transformer diagnosis, which areavailable in the customer area on the OMICRONwebsite, together with dedicated expert forums,moderated by OMICRON.

Result assessment support

OMICRON experts support customers ininterpreting and assessing results - such aspartial discharge patterns, or FRA fingerprints.

Technical support

High quality technical support teams alsoprovide answers to questions on the use of theequipment, and are the first point of contactshould a functional problem occur. If a repair isnecessary, repair times are short - typically in therange of less than one or two weeks.

Dedicated events

OMICRON hosts the regular DiagnosticMeasurements on Power Transformers

Workshop. There, typically over a hundreddelegates from all over the world share anddiscuss case studies and recent developments intransformer diagnosis.

Themes include best practice experiences andsolutions in transformer testing presented bycustomers and new technological developmentsreported by OMICRON. Informal get togethersaid peer exchange.

Several smaller events on related subjectstargeted on the particularities of specific

geographical regions are also offeredthroughout the year.

Demonstration booth at dedicated event

OMICRON experts evaluating a customers re


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Training courses

OMICRON training courses provide a solidtheoretical and practical background and answera clients individual questions. Training coursesare held at either the customers site, onlinethrough a webinar, or in one of the OMICRONtraining centers wordwide.

Power transformer training topics

> Design, testing and maintenance

> Chemical diagnostic methods

> Diagnostic measurements and residual life

assessment> Condition assessment of HV bushings

> Moisture determination and dielectricdiagnostics

> Frequency response analysis and interpretation

> Partial discharge measurement

> Training courses using OMICRON technology

Your Benefits

> assistance in interpretation andassessment of results

> access to relevant training modules

> dedicated conventions/conferences

> technical assistance in equipment usagefrom our technical support teams

> access to scientific papers on transforme

diagnosis through the customer area onour website

Customer theoretical training

Customer practical training


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OMICRONis an international company serving the electricalpower industry with innovative testing and diagnostic solutions.The application of OMICRON products allows users to assessthe condition of the primary and secondary equipment on theirsystems with complete confidence. Services offered in the area ofconsulting, commissioning, testing, diagnosis, and training makethe product range complete.

Customers in more than 140 countries rely on the companysability to supply leading edge technology of excellent quality.Service centers on all continents provide a broad base ofknowledge and extraordinary customer support. All of thistogether with our strong network of sales partners is whathas made our company a market leader in the electrical powerindustry.

The following publications provide further information on the solutions

described in this brochure:

OMICRONL2206, May 2013Subject to change without noticewww.omicron.at www.omicronusa.com

For more information, additional literature,and detailed contact information of ourworldwide offices please visit our website.

Documents

Transformer Brochure Diagnosis Testing