7. luwendran moodley

7/28/2019 7. Luwendran Moodley

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EFFECETIVE TRANSFORMER CONDITION ASSESSMENT

Luwendran Moodley, Doble Engineering Africa

Abstract

.

This paper details a novel approach to transformercondition assessment. This method has provenitself for many years in very large utilitiesthroughout the world. The implementation of thistwo phase approach is discussed in great detail.

1. Introduction

The catastrophic failure and poor performance oftransformers is becoming a constant grim reality inthe life of Maintenance Engineers and AssetManagers in South Africa. Most of which are leftpowerless in this struggle due to financial

constraints imposed upon them when it comes tothe huge capital investment that is required toreplace a power transformer and the everdwindling shortage of highly technical staff.Further to this is the ever increasing delivery timefor power transformers. So the message is clearMake do with what you got and implement a longterm replacement exercise. The question still inthe minds of Maintenance Engineers and AssetManagers is How do I do this if I do not know thecondition of my transformer? The answer iseffective condition assessment

2. Transformer Condition Assessment

What we know about transformers is that their lifeexpectancy can vary from a few cycles (ms) tomore than fifty years. This fact is interesting butnot very useful to an engineer responsible for agiven network.What we need to know is the life expectancy of aparticular transformer in a given network. This factis interesting and very useful. This is the essencesof condition assessment.Effective condition assessment is not just testing atransformer and reproducing the test results nor is

it diagnosing the cause of a failure after thetransformer has failed. Cigre Working Group onLife Management Techniques for PowerTransformers has defined condition assessmentas A comprehensive assessment of the conditionof a transformer taking into account all relevantinformation eg. Design information, servicehistory, operational problems, and results ofcondition monitoring and other chemical andelectrical tests. This is an excellent definition that

encompasses all aspects of the transformers life.This model has been successfully implemented in

a number of utilities world wide. However, caneffective condition assessment be implemented inutilities with little to no information?By using Dobles two phase process for conditionassessment utilities with little documentedinformation can enjoy the benefits of acomprehensive condition assessment on all typesof transformers on the network.

3. Dobles Condition Assessment

Dobles condition assessment program is a twophase process. Both steps include proprietary risk

scoring system and combine analysis of individualunits and FMMAA analysis (family /make/ model/application/ age) of similar designs with similaroperating conditions and age. FMMAA analysis isbased on existing Dobles equipment performancedatabase with test results and equipment failureand trouble data collected from Dobles customersover more than 40 years.

3.1 Phase OneThis phase is applied to all units in the networkand does not require the units to be removed fromservice. Phase one of assessment is a scanning

approach and is more appropriate as a low costassessment and step to provide initial riskassessment and ranking of transformers in anetwork. This should identify the group of unitsthat are in a sound condition and at a low risk dueto their technical condition. The remainder, thoseidentified as higher risk can then be selected formore detailed Phase Two investigation, asidentified in the following section, 3.2.The first step is essentially a review of availableinformation. These include as much as possible ofthe following:

3.1.1 Step 1: Basic nameplate information fromtransformer and tapchanger.

All information related to the transformersmanufacturer, vintage, serial number design,ratings, BIL, fault level, impedance, coolingsystem etc must be captured. From thisinformation design related issues withtransformers, service advisories frommanufacturers, reports of failure on similardesigns, pattern of failure on similar designs can

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be identified. The above can be obtained fromDobles database with test results (25 millionresults) and equipment failure data collected forover 40 years.

3.1.2 Step 2: External visual inspection.A visual inspection is conducted on the following:

Plinth check for cracks or deterioration,

anchor bolts missing or rusty, evidence ofoil leaks, ground leads or connectorsoxidized/tight etc.

Tank - Paint peeling and rust, signs ofinternal deformation or overheating, oilleaks, loose or missing nuts, bolts, orwashers, record liquid level in main tankor any conservator tank, inspect liquidlevel gauges and wiring, inspect pressurerelay and pressure relief device andwiring etc

Cooling system - Paint peeling and rust,oil leaks, inspects pumps and wiring,inspect fans and wiring, inspect radiatorsfor cleanliness, etc

Temperature reading - Recordtemperatures, record position ofmaximum pointers, inspect temperaturesensors and wiring, etc

Marshalling kiosk - Inspect external forpaint peeling and rust, inspect interior forwater ingress and rust, heater operating,inspect breakers, contactors, terminals,wiring, etc

Tapchanger - Paint peeling and rust,signs of internal deformation oroverheating, oil leaks, loose or missing

nuts, bolts, or washers, record liquid levelinspect pressure relay and pressure reliefdevice and wiring, record number ofoperations, inspect tap changermechanism, etc

Bushing - Chipped or broken sheds, oilleaks, oil levels, Inspect connections, etc

Surge Arrester - Chipped or brokensheds, inspect connections, etc

3.1.3 Step 3: Review of all availabledocumentation

Factory test report - Used to comparewith current test results and operatingability

Purchasing specification - Used tocompare to current manufacturingstandards

Tests results (electrical and oil) - Currentdata can be compared to Doble databasefor industry norms

Failure reports - Indicates the rate ofaging, availability and performance

Maintenance practices - What are youdoing?

Major modifications or rebuild - Indicatesthe rate of aging generally expected

Substation fault level - Changes in faultrating

Loading - Used to calculate loss of life

3.1.4 Step 4: Additional non invasive tests(i) Oil tests (main tank)

A sample would be taken and analyzed with thestandard methods. The table below gives a fewstandard oil tests.

Dissolved Gas(DGA)

Detection of incepted faults IEEE, IEC etc

Furfuraldehyde(FFA)

Paper insulation degradation Chendong relation to DP

Moisture in oil Insulation dryness

Breakdown Voltage Dielectric integrity

Acidity Ageing and sludgeInterfacial Tension Ageing, sludge and contaminatio

(ii) Doble DGA Scoring SystemDoble has developed an algorithm to mimic thekey gas response and gives a single number totrack the change in pattern. This method uses thekey gas method to present DGA used by IEEEmethod. The relative proportions of the sicombustible gases CO, H2, CH4, C2H4, C2H6and C2H2 are displayed as a bar chart to illustratethe gas signature. The novel aspect of theapproach proposed here is that this method is

used to investigate and illustrate the cleardifference that exists between normal andabnormal results. By contrast, in the IEEE Guidefour examples of faults are given, but there is noguidance on what a normal result would look like.The DGA score reflects the seriousness of thesignature. DGA results for normal transformerswould be expected to return a score of no morethan about 30, whereas a core circulating currentwould rate about 60 and more serious problemswould score around 100.

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Figure 1: DGA signatures for faulty transformers

A Core bolt faultB Core and frame to earth circulating currentsC Winding inter-stand faultD Winding shorted turnsE Winding phase to earth faultF Winding tracking faultG Winding clamping bolt sparking fault

Figure 2: DGA score for core earth fault

(iii) Infra Red ScanInfra-red will indicate external joint issues, bushingtap problems, oil levels in bushings and radiators,blockages in radiators, fan function- it can alsoindicate tank heating from stray flux, or frame tankcirculating current. The figure below illustrates aninternal tank hot spot.

akenduceaces aen

trumhas

ofulty

erge

Figure 4: UHR scan

0

20

40

60

80

100

Transformer

PercentageTCG

A B C D E F G

CO H2 CH4 C2H4 C2H6 C2H2

(iv) UHF RIF ScanUHF interference surveys have been undertfor the last 20 years in UK. Corona will prointerference up to a few 10s of MHz, and surfdischarge in contamination on bushings haspectrum extending to 200MHz. However, whinternal partial discharge occurs the specextends to 1GHz. Scanning 300-600MHzproven effective in identifying a rangesubstation faults, including discharge at fabushing taps and within the main tank itself. Thfigure below illustrates a UHF scan with dischaactivity on one tap position only.

Figure 3: IR scan of a hot spot

28 /1 0/9 5 15 /0 5/96 0 1/1 2/96 1 9/06/97 0 5/01 /9 8 24/07 /9 8 09 /0 2/9 9

0

20

40

60

80

100

120

140

DGA

Score

below.

Scor

3.1.5 Step 5: Consultation with staffConsultation with all staff involved in the lifemanagement of transformers forms an integralpart of this process in that this is a great source ofinformation that has not been documented.

3.1.6 Assessment of Technical Condition

Once all the information has been gathered andthe additional non invasive tests performed thetransformers can then be scored based on itscondition. The Doble scoring system is given

Condition Definition

New No damage 1

Normal ageing Reasonable for age 3

AgedSome ageing in need ofsome monitoring

10

SuspectIdentified ageing, significantrisk for failure

30

Unacceptable Unacceptable ageing 100

Transformers condition is further divided in design,dielectric, thermal and mechanical and scored to aDoble scoring system. A typical assessment of the

ch s given bte nical condition i elow.

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All units have been assessed in terms of designgroups with problems, overall condition, thermaland dielectric condition. Also included in theassessment is a score for the design group, tapchanger, bushings and surge arresters. Eachaspect has its own score - a number betwand 100. Even with summation any aspect w100 score will be carried through and

recognized. The results are assessed usinsum of the numerical scoring system and it is this

ronment, staff and third partiesll the transformers that fall in the above category

hase two of the

3.2 PhaThis nits that have beenidenThe Phaendanalysis

onse Analysis

reactance

Exciting current

3.2.Once a line tests are performed thetech a beresc is shown below.

Un eighting Risk

een 1ith aeasily

g this

sum that determines the position in the leaguetable and summarized using a red- green colourtraffic light code. It should be emphasized that thescore is not permanent its a live document,reviewed each month as new evidence ispresented.

3.1.7 Outcomes of Phase OneOnce this process is completed the following ismade evident:

Establishment of an asset register

Design weakness High risk transformers in terms of the

dielectric and thermal condition

High risk transformers in terms of theenvi

Awould then be considered for pcondition assessment process.

se Twophase is applied only to u

dtifie as high risk from Phase One.se Two process is shown in Figure 2 (see

of paper.) This phase is a comprehensiveof the transformer and requires off line

testing. The standard off line tests are as follows:

Tan and Capacitance - windings andbushings

Sweep Frequency Resp

Leakage

Insulation Resistance

Winding resistance

Ratio testNote: Explanation on the above electrical tests isgiven in the Appendix A.

Designs are ratediew

database.

ers

is divided indielectric,

1 Rescoring the Technical Conditionll the off

nic l condition of each transformer canored with greater detail. This

it Level Condition W

Th 0.8 8ermal 10

Die 0 0.8 24lectric 3

Mechanical 3 0.6 1.8

Core 10 0.4 4

Oil 3 0.8 2.4

Tank 3 0.2 2

Bushings 3 0.8 2.4

Tapchanger 3 0.6 1.8

The rescoring now includes the mechanicalfinal scoring

ow inbe assigneevel can

condition of the transformer. With thefor the condition of the transformer n place aweighting for each unit level can d.From this a risk of each unit l bedetermined. A total risk of each tra canthen be calculated.

3.2.2 Outcomes of Phase TwoOnce the rescoring has been completed thefollowing is made evident:

The transformers risk

below.

nsformer

High risk transformers in terms of thedielectric, thermal and mechanicalcondition.

More accurate overall condition as aresult of the off line tests

An action plan in terms of units thatrequire replacement, repair and monitor

The results of phase two are merely added to theexisting assessment. A typical layout is shown

after revDoble

of

Transformcondition

thermal

and scored

This example is

pulationhere oilta ispplema

asonabount of

ectricalst data

based on a

pow testdasuby

en dte

re leamelte .

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be effectively introduced by using this tapproach. This method of condition as

can be implemented irrespective of theinformation. It allows utilities to finanswers to the following situations:

When to have maintenance outa es

How to respond to a protection trip

To know capability to increasetransformer rating

when to replace (5, 10, 15sformers

RE R

[1] Wils hnical condition inutilit othe 2Clients, Boston, MA

s,oston, MA

apworth, Asset Health Reviewto Manage Remaining Life IEEE PES Power

4. Conclusion

Transformer condition assessment program canwo phasesessment

amount ofally have

g

To knowyears) tran

An added advantage is that this method forces theutilities to make the bold move to condition basedmaintenance. A further advantage is the riskassessment and residual life can finally be

achieved through sound engineering principles.

FE ENCES

on A, "Impact of teczati n of power transformers Proceedings of

002 International Conference of Doble

[2] J.A Lapworth, A scoring system forintegrating dissolved gas analysis results into alife management process for powertransformers Proceedings of the 2002International Conference of Doble ClientB[3] Wilson A, JA L

Africa, Johannesburg, South Africa, 2007

[4] CIGRE Working Group 12.18, LifeManagement of Power Transformers

Most of thepopulationare OK!

One has a fault,one is ageand ano

dther

might have a

fault.

No need for actionf

s to

be replaced.The sistertransformerneeds to be tested.

plans for most othe population.

The faultytransformerneed

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APPENDIXElectrical Testing of Transformers

Tan and Capacitance - WindingsTan (dissipation factor) is merely the tangent ofthe loss angle that is created by the capacitive andresistive current that is present in a dielectricmedium. Measurements are typically madebetween the high voltage winding to ground,between the high voltage and low voltage windingand between the low voltage winding to ground.This measurement method allows assessment ofindividual winding hence focusing on the area ofdeterioration. As Tan is dependent ontemperature the measure values are normalizedto 20

0C by applying a correct factor.

Tan is an evaluation of the quality of theinsulation and is size independent. Tan hasproven to be effect in detection of the followingproblems in transformers of all sizes:

Moisture;

Carbonisation of insulation;

Contamination of oil by dissolvedmaterials or conducting particles; and

Improperly grounded core.Winding capacitance on the other hand evaluatesphysical makeup of insulation system which is sizedependent. Capacitance measurements haveproven over the year to be very effective ifreference results are available. Changes in theregion of 10% would normally indicate extremewinding movement. However, this method ofdetecting winding movement is not as effective asSweep Frequency Response Analysis.

Tan and Capacitance BushingsIf bushings are equipped with a test tap twomeasurements can be performed which are a C1and a C2. The C1 test measures the condition ofthe main bushing insulation to the test tap. The C2test measures the condition of the test tapinsulation to ground and core Insulation betweentapped layer and bushing ground sleeve.The C1 and C2 tests are effective in identifying thefollowing defects:

Moisture ingress;

Carbonisation of insulation;

Short circuited condenser layers;

Contamination of oil by dissolvedmaterials or conducting particles; and

Open circuits such as break in the bandbetween the ground and mounting flange.

Exciting CurrentsThe exciting current is, for practical purposes thecurrent that flows when the winding of atransformer is energized under no-load conditions.

The exciting current creates a magnetic flux in thecore, and the flux in turn induces a voltage in theenergized winding that opposes the appliedvoltage. Consequently, the exciting current issmall, usually only a few percent of the rated loadcurrent of the winding. The exciting current of atransformer is made of three components:(a) A magnetizing part (Im) required to build themagnetic field in the transformer core. It is oftenreferred to as the magnetizing current.(b) A resistive part (Ir) required to supply all thelosses in the transformer at no load.(c) A capacitive part (Ic) required to build theelectrical field in the insulation of the transformer.This is a single-phase test that was introduced inNorth America as a diagnostic tool in 1967 andtoday is part of standard insulation tests in thefield. The single-phase exciting-current test isuseful in locating problems such as defects in themagnetic core structure, failures in the turn-to-turninsulation, or problems in the tap-changing device.These conditions result in a change of the

effective reluctance of the magnetic circuit, whichconsequently affects the current required to forcea given flux through the core.The diagnostic analysis of exciting current testresults is based largely on pattern recognition.

Ratio TestDobles method uses a high voltage (10 kV) andinvolves measuring the capacitance of thecapacitor by itself and the apparent capacitancewhen it is connected across the low voltagewinding. The ratio of these to capacitance yieldsthe turns ratio of the transformer. The greatest

advantage of this high voltage test method is thathigh resistance areas can be overcome, where aslow voltage test sets might show such an area asan open circuit. Ratio test has been used verysuccessfully over a number of years for thefollowing:

Confirm ratios are within 0.5% ofnameplate data;

Detect short circuited turn-to-turn;

Detect open circuit windings; and

Confirm tap lead connections.

Sweep frequency response Analysis (SFRA)

The loss of mechanical integrity in the form ofwinding deformation and core displacement inpower transformers can be attributed to the largeelectromechanical forces due to fault currents,winding shrinkage causing the release of theclamping pressure and during transformertransportation and relocation. These windingdeformation and core displacement if not detectedearly will typically manifest into a dielectric orthermal fault. This type of fault is irreversible with

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the only remedy been rewinding of the phase or acomplete replacement of the transformer. It istherefore imperative to check the mechanicalintegrity of aging transformers periodically andparticularly after a short circuit event to provideearly warning of impending failure. Hence an earlywarning detection technique of such aphenomena is essential. Frequency responseanalysis is recognized, as been the most sensitivediagnostic tool to detect even minor windingmovement and core displacement.The transformer is considered to be a complexnetwork of RLC components. The contributionsto this complex mesh of RLC circuit are fromthe resistance of the copper winding;inductance of winding coils and capacitancefrom the insulation layers between coils,between winding, between winding and core,between core and tank, between tank andwinding, etc. However, a simplified equivalentcircuit with lumped RLC components asillustrated in Figure 1 can be used to accurately

explain the principle of frequency response.Any form of physical damage to the transformerresults in the changes of this RLC network.These changes are what we are looking for andemploy frequency response to highlight thesesmall changes in the RLC network within thetransformer. Frequency Response is performedby applying a low voltage signal of varyingfrequencies to the transformer windings andmeasuring both the input and output signals.The ratio of these two signals gives therequired response. This ratio is called thetransfer function of the transformer from which

both the magnitude and phase can beobtained. For different frequencies the RLCnetwork offers different impedance paths.Hence, the transfer function at each frequencyis a measure of the effective impedance of theRLC network of the transformer. Anygeometrical deformation changes the RLCnetwork, which in turn changes the transferfunction at different frequencies and hencehighlights the area of concern.

Impedance at different frequencies relate to theresistance, capacitance and inductance of a

transformer. The resistance is related to thephysical construction of the winding (shortedturns, core earth etc.) and results in the verticalshift (dB axis) of the response. The capacitanceand inductance are related to the geometry of thewinding (deformation) and results in a horizontalshift or frequency shift.

At the lower frequency range the capacitance ofthe transformer can be disregarded and the

response is purely inductive. At these frequenciesthe inductance of the magnetic circuit dominates.There is a significant difference in the responsesbetween the outer two phases and the centerphase at this frequency range. This is due to theflux paths of the core. The center phase has twoflux paths of equal reluctance and the outer phasehas two flux paths of different reluctance. As aresult the outer phases has two resonance pointsas compared to the center phase that has just oneresonance point. This also accounts for thedifference in the starting dB values.

At higher frequency ranges the response look veryconfusing and complex as a result of thenumerous resonance points. At this frequencyrange the winding inductance dominates with themagnetic circuit effectively screened. Hence, thewinding responses are less dependent on themagnetic circuit, which makes the measurementmore sensitive to winding deformation. At thehighest frequencies the inductance can be

disregarded and the response is effectivelycapacitive.

Figure 1: Simplified equivalent circuit withlumped RLC components

Figure 2: Frequency Response of a soundtransformer

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7. luwendran moodley

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