Transformer Condition Monitoring & Health Checkup

MentorMr. Sanjay Thakur

B Chinna RaoGET- ELECTRICAL

ContentsImportanceTransformer FaultsMethods to Determine

Transformer Health & ConditionOil DiagnosticsDissolved Gas AnalysisOrigin of Gases in Transformer OilInterpretation

ImportanceElectrical power systems play a key role in

production and services in both the industrial and commercial sectors and significantly affect the private lives of citizens

A major asset of any power delivery system is the transformer

Transformers represent extensive investment in any power delivery system, and because of the notable effect of a transformer outage on system reliability, careful management of this type of asset is critical

Transformer FaultsThe four main types of transformer

faults are:Arcing (high current break down)Low energy sparking, or partial

dischargesLocalized overheating, or hot spotsGeneral overheating due to

inadequate cooling or sustained overloading

No. Part Faults Detectable

1 Bushings •Partial breakdown between capacitive graded layers,cracks in resin-bonded insulation•Aging and moisture•Open or compromised measuring tap connection•Partial discharges in insulation•Loss of oil in an oil-filled bushing

3 Insulation Materials

•Moisture in solid insulation•Aging, moisture, contamination of insulation fluids•Partial discharges

4 Leads •Contact problems•Mechanical deformation

No. Part Faults Detectable

5 OLTC •Contact problems in tap selector and at diverter switch•Open circuit, shorted turns, or high resistance connections in the OLTCpreventative autotransformer, series autotransformer or series transformer•Contact problems in the DETC

6 Windings •Short circuits between windings or between turns•Strand-to-strand short-circuits• Open circuits in parallel strands•Short circuit to ground•Mechanical deformation•Contact problems, open circuits

7 Core •Mechanical deformation•Floating core ground•Shorted core laminates

8 Surge Arrestor •Deterioration and aging

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Methods to Determine Transformer Health & Condition The following methods are used for

determining the overall health of a transformer:◦Oil Diagnostics◦Dissolved Gas-in-Oil Analysis(DGA)◦Electrical Diagnostics◦Visual Inspection◦Maintenance Record Review◦Operating Record Review

Oil DiagnosticsPerhaps the simplest and least

costly method of quickly assessing the general health of a LPT is to take a sample of the insulating oil

One major advantage of oil testing is that the sample can easily be taken while the unit is on-line, eliminating the need for an outage

S. No.

Test Description

1. Dielectric Breakdown

2. Neutralization Number

3. Interfacial Tension

4. Color

5. Water Content

6. Specific Gravity

7. Visual Examination

8. Power Factor

Dielectric BreakdownThe test involves applying a voltage at a

controlled rate to two electrodes immersed in the insulating fluid

Clean dry oil has an inherently high dielectric strength but this does not necessarily indicates the absence of all contaminates, it may merely indicate that the amount of contaminants present between the electrodes is not large enough to affect the average breakdown voltage of the liquid

TEST METHOD: IEC 156

Neutralization NumberAcids in the oil originate from oil

decomposition/ oxidation productsAn increase in the acidity is an

indication of the rate of deterioration of the oil with SLUDGE as the inevitable by-product of an acid situation which is neglected.

The acidity of oil in a transformer should never be allowed to exceed 0.25mg KOH/g oil

Test Method: ASTM D974

Interfacial TensionThe Interfacial Tension (IFT) measures

the tension at the interface between two liquid (oil and water) which do not mix and is expressed in dyne/cm

Good oil :40 and 50 dynes/cmThe IFT test is a powerful tool for

determining how an insulating oil has performed and how much life is left in the oil before maintenance is required to prevent sludge.

Test Method : ASTM D971

Dissolved Gas-in-Oil Diagnostics

Insulating oils under abnormal electrical or thermal stresses break down to liberate small quantities of gases

The qualitative composition of the breakdown gases is dependent upon the type of fault

By means of DGA, it is possible to distinguish faults such as partial discharge, overheating and arcing in a great variety of oil-filled equipment

Information from the analysis of gases dissolved in insulating oils is valuable in a preventative maintenance program

A number of samples must be taken over a period of time for developing trends

Data from DGA can provide◦Advance warning of developing faults◦A means for conveniently scheduling repairs◦Monitor the rate of fault development

Origin of Gases in T/F Oil PARTIAL DISCHARGE:

◦ fault of low level energy which usually occurs in gas-filled voids surrounded by oil impregnated material

◦ The major gas produced is Hydrogen◦ The minor gas produced is Methane

THERMAL FAULTS:◦ A small amount of decomposition occurs at normal

operating temperatures. As the fault temperature rises, the formation of the degradation gases change from Methane (CH4) to Ethane (C2H6) to Ethylene (C2H4)

◦ Fault at low temperature (<300 oC) produces mainly Methane and Ethane and some Ethylene

◦ Fault at higher temperatures (>300 oC) produces Ethylene. The higher the temperature becomes the greater the production of Ethylene

Origin of Gases in T/F OilARCING is a fault caused by high

energy discharge◦The major gas produced during

arcing is acetylene(C2H2)

◦Heavy arcing can cause temperatures of over 3000 oC to be developed

The gas concentrations in transformer oil depend upon:◦The volume of oil involved (dilution

factors)◦The age of the transformer (new or

old)◦The type of transformer

Generator or Transmission Sealed or free breathing Construction of Tap changer

Interpretation

Gas Normal Abnormal

Interpretation

1.

Hydrogen <150 >1000 Arcing corona

2.

Methane <25 >80 Sparking

3.

Ethane <10 >35 Local Overheating

4.

Ethylene <20 >100 Severe Overheating

5.

Acetylene <15 >70 Arcing

6.

Carbon Monoxide <500 >1000 Severe Overloading

7.

Carbon Dioxide <10000 >15000 Severe Overloading

Recommended Levels (max. ppm)

Gas IEEE Recommended Conc.

Age Compensated

1. Hydrogen 100 20n+50

2. Methane 120 20n+50

3. Ethane 65 20n+50

4. Ethylene 50 20n+50

5. Acetylene 35 5n+10

6. Carbon Monoxide 350 25n+500

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