Analysis and diagnosis_of_typical_transformer_dc_resistance

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Analysis and Diagnosis of Typical Transformer DC

Resistance

TuMingtao, Yang Qinghua

HIMALAYAL - SHANGHAI - CHINA

Abstract: To detect the defects of transformer winding and confirm the causes,the data of two transformers of Beijing Electrical Power Company are analyzed.The cause is that there is quality problem in coil welding. Measures are putforward. The method of determining the defect is summarized.

Key words: Transformer, DC resistance, defect, welding, detection, advice

Introduction

According to the Code forCommissioning Test and PreventiveTest of Electrical Equipment, thecommissioning test should be carriedout prior to the running of powertransformer. The power transformeris put into operation followed bypreventive test. The DC resistance ofpower transformer is one ofimportant aspects, which reflectphysical property of transformerwinding. The abnormality of DCresistance always indicates that thecoils are broken or poor partialcontact exist. The DC resistance testsof one set of 250000kVA /220kVtransformer and one set of50000kVA/110kV transformer areanalyzed in this paper.

1. Analysis of Defect in one set of250000kVA/220kV PowerTransformer

1.1 250000kVA/220kV PowerTransformer Test

An preventive test is conducted on

Beijing Electric company’s #3transformer. It is found that the linedifference of low voltage coil exceedsthe limit and other electricalindicators are normal. After one yearthe DC resistance test is conductedagain and the line difference furtherincreases. The detailed data is listed inTab.1.

Tab.1 DC resistance of250000kVA/220kV transformer lowvoltage line

Rab&Rbc&Rca represents the DCresistance of low voltage ab, bc and ca.

Ra, Rb and Rc represents the DCresistance of low voltage phase-a, lowvoltage phase-b and low voltagephase-c.

From Tab.1, we can conclude that the



difference of low voltage DCresistance in phase increases,exceeding the standard (＜2%). RabandRca increase obviously compared withthe commissioning value, whichindicates that low voltage phase-a DCresistance increase obviously. Basedon the formula (1)-(3), DC resistanceof low voltage phase can be calculatedby using the data shown in Tab.1. Thecalculated data is shown in Tab.2.

Tab.2 DC resistance of #3 transformerlow voltage phase

Ra= (Rca - Rp )-RabRbc/(Rca - Rp ) (1)

Rb=(Rab - Rp )-RbcRca/(Rab - Rp ) (2)

Rc=(Rbc - Rp )-RabRca/(Rbc - Rp ) (3)

In the formula, Rp= （Rab + Rbc + Rca）

/2.

The data of three tests are comparedin the Tab.2. It is assumed that thephase-c DC resistance changes underthe influence of temperature, DCresistances of both phase-a and b ofMay 30, 2000 rise, up by 0.15m Ω forphase-a and 0.08mΩ for phase-b.From the test data of September 10,2001, we can find that the DCresistance of phase-a further goes upby 0.28mΩ while that of phase-b doesnot change basically. The comparisonindicates that the DC resistancecondition of phase-a furtherdeteriorates.

In order to figure out the problem oflow voltage DC resistance, the Beijing

Electrical Institute conducted the testfor confirming the causes andposition.

1.2 Transformer Defect Locating

After the removal of transformer oil,all screw joints of low voltage coils arefastened followed by the test. The testdata is shown in Tab.3.

Tab.3 DC resistance of joint lowvoltage line after fastening

The comparison between Tab.1 andTab.3 indicates that there is noobvious improvement in maximumline difference pre-and-post fastening.However, the difference between DCresistance ab and bc decreases, whichshows that the DC resistance isimproved to a certain extent afterfastening all screws. The values ofeach phase are shown in Tab.4.

Tab.4 DC resistance of joint lowvoltage phase after fastening

The DC resistance of phase-b goesdown after the treatment while thereis no obvious change in that ofphase-a (not considering the effect oftemperature on the DC resistance).The DC resistances of phase a and bincrease but the causes are different.The joint screws loosen, causing DCresistance of phase-b to go up whilethe cause of phase a may be the defectin the cold joints. To further figure outthe causes of phase-a defect, the low



voltage Δ connection is open. The jointat the position represented by theletter y is unfastened shown in Fig.1and the DC resistances of phase a andb are measured. Ra = 4.129mΩ, Rb=3.799mΩ.

Fig.1 Diagram of low voltage coilstructure

The DC resistance of phase-a (Fig.11-6 ) is tested. The six joints arewaggled in order. To facilitate thecomparison, the similar test is alsocarried out for phase-b and its testresults are shown in Tab.5.

Tab.5 Low voltage phase DCresistance of #3 transformer

The Tab.5 data indicates that the DCresistance of phase-a decreases whenwaggling the No.1 and 3 joints. Oneday later, the DC resistance restores tothe original value; there is no obviouschange in the DC resistance of phase-bdespite of waggling all joints. The

above-mentioned condition showsthat there is quality defect in phase-ajoints.

1.3 Treatment

It is confirmed that there is qualitydefect in the low voltage joints oftransformer. The press plier is used toweld the joints on site. The test dataafter welding is basically the same asFig.5. The defect in phase-a is notcorrected. Hence, the current joltshould be replaced by the new one.The test data after treatment is shownin Tab.6

Tab.6 DC resistance of #3 transformerlow voltage line

The comparison between the resultsof this test and that of commissioningtest indicates that the law of lowvoltage DC resistance is different. Forfurther analyzing the causes ofdifference, the line DC resistance isconverted into single-phase DCresistance. The data is specified inTab.7.

Tab.7 DC resistance of No.3transformer low voltage phase afterreplacing the joint

The DC resistance values of all phasesin the Tab.4 and 7 are compared. Theimpact of temperature on the DCresistance is excluded. It is found thatthere is no change in the DCresistances of phase b and c while the



phase-a DC resistance decreases.There is serious defect in the phase-acoil before installation.

2. Analysis of Defect in One Set of50000kVA/110kV Transformer

2.1 50000kVA/110kV TransformerTest

Based on the above Tab.8, it is foundthat DC resistance value of each tap is12-14mΩ more than that of phase-Band C, which shows that there issomething wrong with main part ofphase A.

Tab.8 DC resistance of #2 transformercoil

To figure out the causes, the No.9position of phase A are testedrepeatedly. The DC resistance value ofone time is 424.4mΩ and the earlierone is 428.7mΩ. It is speculated thatthe decline is caused by thetemperature change. But the changein the tests of phase B and C is notobvious, which indicates that the DCvalue of phase A is not stable. Theworkers knock the shield and outletwires outside the transformer coil onsite. As a result, the DC resistance ofphase A changes obviously when theoutlet wires are knocked. At last, thevalue stabilizes at 405.9mΩ. Nothingabnormal is detected at the weldingpoints. The cause lies at the end closeto the outlet wire within the coil.

2.2 50000kVA/110kV transformer

return-to-factory inspection

First of all, the test is performed onthe transformer coil and test resultsare shown in Tab.9. Through theanalysis, it is concluded that the DCresistance of phase A (RdA) increasesas the testing current decreases.However, the DC resistances of phaseB and C do not change when thetesting current varies. Hence, it isconfirmed that the cause is that thereis weld defect within the phase A coil.The phase-A coil is composed of upperhalf a packet and lower half a packetin parallel, which is made of twostrands in parallel. The upper andlower half a packet are unfolded. ItsDC resistance is measured and the testdata is shown in Tab.10.

No.9 DC resistance of #2 transformercoil

Item RdA/mΩ

RdB/mΩ

RdC/mΩ

Imbalancerate/%

Measuringcurrent/A

20 358.0 357.0

355.3 0.76

10 360.2 357.1

355.3 1.37

5 363.2 356.8

355.1 2.26

No.10 DC resistance of #2 transformerphase A coil

From the Tab.10, we can see that thereis obvious difference between the DC



resistance of lower half a packet #1coil and that of #2 coil. In conclusion,there is a problem with weldingquality of lower half a packet #1 coil.

After the problem is identified,phase-A coil is pulled out and the DCresistance of phase-A lower half apacket is measured. The DC resistanceof #1 coil is not stable, a woodhammer is used to knock the firstwelding point to measure the opencircuit. It is found that the weldingpoint snaps fully.

To further analyze the causes ofsnapped welds, factory test data andsemi-finished product test areanalyzed. The data is shown in Tab.11.

No.11 DC resistance of #2semi-finished transformer coil

In the semi-finished product test ofphase-A coil, the DC resistance valuesof phase-A upper and lower half apacket are abnormal but sufficientattention is not paid to; in the producttest, the parallel of upper and lowerhalf a packet decreases the differenceamong phase A, B and C, covering thedefect. Hence, the defect in thephase-A weld exists during themanufacturing process of coil.

3. Conclusions

a) The analysis of phase DC resistanceis more direct than line DC resistance.

b) After measuring value of DCresistance stabilizes, the problem partcan be knocked or waggled andmeasuring values are observed. If the

obvious change is found, there is adefect in that part.

c) When there is defect in the coilcontact, the DC resistance value canchange according to the test current.

d) The sufficient attention should bepaid to the small change of DCresistance in the future work.

Technology

Analysis and diagnosis_of_typical_transformer_dc_resistance