investigation of failures of 230kv copper conductor bushings

7/30/2019 Investigation of Failures of 230KV Copper Conductor Bushings

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2010 Doble Engineering Company -77th Annual International Doble Client Conference

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INVESTIGATION OF FAILURES OF 230-KV OIP COPPER CONDUCTOR BUSHINGS

Arturo Del Rio Keith Ellis

Trench Ltd, Canada

ABSTRACT

The fact that copper ions from conductor surfaces are taken into solution in some mineral transformer oils and then

deposited onto paper insulation in bushings and transformers has been well documented. These deposits have beenimplicated in a number of instances of equipment failure. Corrosive sulfur in transformer oil has been suggested as

the cause for both the migration and the deposition of copper on the paper insulation in these instances even though

the oil has consistently tested as non-corrosive. The migration and deposition of copper in transformer oils has been

demonstrated to be independent of sulfur, corrosive or otherwise. The process of taking copper into solution andthen depositing the copper as a fixed, stable compound on paper insulation is dependent upon the formation of

copper-organic polar compounds in solution in the oil, attraction of these compounds to the surface of Kraft paper,

and the formation of stable copper compounds in a pattern that decreases the dielectric strength of the paper

insulation [1]. This paper presents one case of a copper migration induced failure in three separate transformer

bushings manufactured by Trench France and oil filled with Shell Diala D mineral oil.

INTRODUCTION

In May and June of 2006, two Trench COTA 750-F012-27-AG3-01-ADP bushings failed on two separate

transformers on the Southern Companys system. Inspection of the failed bushings revealed very similar details:

The failures were below the flange and the air-side porcelain insulator remained intact.

The epoxy inboard end insulator was shattered.

Arc marks were evident from the same point on the ground sleeve to the copper conductor 5-8 inches

above the bottom terminal (breaker plate).

Arc marks were also evident to the breaker plate and/or the shield.

Preliminary Investigation revealed further similarities between the failed bushings:

The bushings were identical 750kV BIL copper conductor bushings.

The bushings were manufactured in the Trench France facility.

The bushings were of similar age (5-6 years service).

The bushings were applied on 230kV transformers.

The transformers were produced by the same transformer manufacturer.

Both transformers were relatively lightly loaded and the transformer service data indicated they had

not seen excessive temperatures.

Although the bushing nameplate rating is 1200 Amps, these bushings were physically built as 2000

Amps units and only saw service duties of maximum of 800 Amps.

Shell Diala D was the mineral oil used.

There was no identifiable cause of failure from the site investigation.

For reference, the first failed bushing, which failed May 2006 at Georgia Power, was identified as A. The sisterbushings from this bank were identified as B and C. The second failed bushing, which failed June 2006 at

Alabama Power, was identified as D and the sister bushings from the bank were identified as E and F.


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All Rights Reserved2

Bushings A, B, and D were dismantled and inspected at the Georgia Power Forest Park repair facility. No oil

remained in the failed bushings (A and D). Unfortunately, no oil was collected from bushing B. The paper

and foil in the vicinity of each failure was badly damaged and presented no clear identifiable cause of failure.

Georgia Power Bushing A S/N 99A6424 Failed May 2006

Bushing B S/N 99A6427 No failure - field tear down

Bushing C S/N 99A6429 No failure Trench Canada tear down

Alabama Power

Bushing D S/N 01A3930 Failed June 2006 Bushing E S/N 01A3931 No failure returned to Trench Canada

Bushing F S/N 01A3933 No failure returned to Trench Canada

Review of historical data revealed that the bushings from the Georgia Power bank, bushings A, B and C, had

been tested in January 2006 and were found to have elevated power factors. Subsequent to the failures and afterdielectric tests had been performed on bushing C, Alabama Power bushings E and F were field tested and also

found to have increased power factors leading to their removal from service [2].

INVESTIGATION PROCESS BUSHING C

Bushing C S/N 99A6429 became the main focus for the investigation. This bushing was shipped to the Trenchfacilities in Ajax, Canada, where routine dielectric test including impulse test at 85% of rating were performed. The

bushing passed the partial discharge, hi-pot and impulse tests. The increased C1 power factor readings were

confirmed. There was no change in C1 or C2 capacitance when compared to the original factory tests and nosignificant change in C2 power factor was observed. Table 1 summarizes the C1 increased power factor readings

for bushing C.

Table 1

Bushing C Power Factor Results

C1 Nameplate 1999 0.26%

C1 Commissioning 0.30%C1 Jan 2006 0.37%

C1 When removed from service 0.52%

10kV: 0.516%

50kV: 0.465%

C1 After Impulse and Hipot Test

September 2006

102kV: 0.385%


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Oil Analysis

Oil samples were analyzed before and after the dielectric test and the results are included in Table 2, below:

Table 2

Bushing C Oil Test Results

Although some discrepancy was evident in the test results, there were no significant gases that could indicateelectrical discharges within the bushing and the moisture in the oil was relatively normal at 9 to 12 ppm. However

the oil power factor showed high values at 25C and 100C. This was determined to be significant to this

investigation, since an increase in oil power factor is an indicator of contamination.

Additional oil samples were taken from bushing C in order to verify the questionable oil power factor results;these results are shown in Table 3. Pilot clay treatment done by Doble on one of the samples was successful in

removing some degradation byproducts and/or contamination of the oil resulting in improved interfacial tension(IFT) and 25C power factor; this is indication that polar material was removed from the oil by the clay as this test isgreatly affected by such compounds. Because of the type of containers used to collect the samples, this test was

deemed inconclusive since the contamination could have been introduced from the materials of the sealing system of

the jars.

Table 3

Oil Test Results Verification

Source of Test Pf @ 250C (%) Pf @ 1000C (%)

Trench France n/a 2.741

Trench BU 2.150 18.800

Trench IT 1.683 14.290Doble* 0.664 7.55

Morgan Schaffer 1.499 9.830

*Worst case of multiple samples taken during bushing oil draining.

Bushing ID Bushing C, as received @ Trench Bushing C, after all voltage tests New

Tested by/date: Trench MS Trench MS bushing oil

(ppm) 2006.09.18 2006.09.19 2006.09.20 2006.09.21

Hydrogen (H2) 4 25 4 30 10

Oxygen + Argon (O2 + Ar) 4800 4720 4473 13500 -

Nitrogen (N2) 17900 63600 17900 68200 -

Carbon Monoxide (CO) 62 160 79 160 -

Methane (CH4) 2 5 2 5 5

Carbon Dioxide (CO2) 88 1330 91 1300 -

Ethylene (C2H4) < 0.04 ND < 2 ND


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The bushing was dismantled while collecting samples for suspect materials including the o-rings and flat gaskets,

cast epoxy parts, Belleville compression washers. Collected samples were sent for analysis failing to pinpoint the

source of contamination.

Analysis of the Paper around a Puncture Hole

The active part was unwound turn by turn with measurements taken of foil layer placement. There were noabnormalities found such as paper wrinkling or foil displacement. One significant finding was a puncture next to the

bottom of the first foil layer. It was thought that this probably occurred during the impulse test but it was

undetermined why. This finding and additional evidence collected from bushings E and F led to the finding of afine faint dark line at the edge of the aluminum foil where the puncture had occurred. The puncture area and the

dark treeing line are shown in Figure 1.

Puncture Mark and Dark Treeing Line

Figure 1

Analysis of the paper around the puncture hole showed Kraft paper in very good condition with a degree ofpolymerization (DP) of 938 and moisture content of 1.2% [3]. Under a stereo microscope the paper appeared to be

in good condition. The darkened material adjacent to the hole exhibited a smooth compressed area of fibers. Thechemical composition demonstrated a strong concentration of aluminum (~82.1 %), with minor amounts of copper

(~12%), calcium (~3.4%) and sulfur (~2.5%), along with the background of carbon and oxygen. See Figure 2,

below.


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Darkened Material Adjacent to the Hole

Figure 2

The dark treeing line on this sample exhibited copper (~61.1%), sulfur (~20.2%) and calcium (~13.2%), with minoramounts of silicon (~4.4%) and aluminum (~1%).

OTHER SIGNIFICANT FINDINGS

Bushings E and F and another suspect bushing named G removed from service were also tested, dismantled

and samples of paper and oil obtained for analysis. Dielectric tests revealed increased power factor at 10 KV andtip-up test performed up to line-to-ground voltage showed a gradual increase in power factor at lower voltage levels

followed by a gradual decrease as it approached the 100 KV level resembling a typical Garton effect curve.

Results are shown in Figure 3. For comparison purposes, a new 138 KV bushing COTA 650 was tested in similar

way and the results were plotted in the same graph. It was found that the measured power factor for the newbushing was very consistent independently of the applied voltage with a slight drop in power factor as the test

voltage was increased. No changes in capacitance readings were observed.

Upon active part tear down, faint treeing lines were found at the edge of the lower end of the aluminum foils similar

to those found in bushing C (Figure 4). The extent and intensity of the treeing lines were somehow related to the

increase in power factor. It would be expected that these treeing lines would be comprised mostly of aluminum andcopper and oxygen. Then carbon would be formed from the degradation of the paper due to heating along that

aluminum/paper interface or heating due to partial discharge along that interface. If this treeing was due to partial

discharge at the edge of the foil/paper interface a lot of aluminum would be found embedded in the treeing line.

This was not the case in the samples examined. More copper and sulfur were found at these locations than

aluminum. Aluminum was found in very small concentrations or not present at all. Similar to the findings inbushing C, DGA analysis from oil samples did not indicate any type of incipient fault condition and DP tests did

not show significant aging of the paper.

Paper fibers

Burn Hole


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Tip-up Test Results for Bushing E

Figure 3

Treeing Lines from Bushing E

Figure 4

The Shell Diala D bushing oil was tested for corrosive sulfur according to several industry standards with consistentnon-corrosive results therefore failing to identify corrosive sulfur as the root cause of the failures. Corrosive

results were obtained only after prolonged aging test time at higher test temperatures [4, 5]. A summary of test

results for in-service aged Shell Diala D oil from bushing G in included in APPENDIX A for reference.

The dissipation factor of the oil showed a very strange behavior with time and temperature. A rapid increase of the

dissipation factor with temperature was observed. The further heating at the same temperature for 2 hours led to a

decrease in the power factor. A second heating procedure led to a much lower power factor. This strange kind ofbehavior may be an indication of volatile polar compounds dissolved in the oil and leading to an increase in the

power factor. Figure 5 shows the behavior of the oil conductivity versus temperature [6].

Moreover, the power factor of the oil impregnated paper from the bushing was 25% at 90C. After a vacuum

treatment and a further heating at 90C the value was 1.6%. This is a further indication, that some polar compoundsare adsorbed in paper and may undergo further reactions.


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Behavior of the Oil Conductivity versus Temperature

Figure 5

Analysis of Copper Core Sections

Two sections of copper core from bushing D were analyzed. Pictures of the blackened copper core are shown in

Figure 6.

Copper Core Samples from Bushing D

Figure 6

The concentrations for the SEM/EDX analysis clearly show that most of the content is copper and sulfur with some

carbon and oxygen. Therefore, the blackened area on the copper core of the bushing is in the initial stages of copper

sulfide formation or the sulfur content would be slightly higher. Figure 7 shows some flakes of copper sulfide at1000 times magnification which have formed on the surface of the copper conductor. The very high copper to sulfur

ratio and significant proportions of carbon and oxygen suggest the deposit is not only copper sulfide but mostly

organic compounds such as polymerized oil.


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Flakes of Copper Sulfide at 1000X

Figure 7

Analysis of the Treeing Lines

Figure 8 is a region along a random treeing line on the paper from bushing F. Four out of the five major tracksshown in the picture follow single paper fibers for most of the discharge. The fact that the discharges follow the

fibers indicates that the properties of the fiber/oil interface dominate the process. The discharge path tends to go

slowly through the thickness of the paper rather than following the surface on the aluminum foil side of the paper.

Flakes of Copper Sulfide at 1000X

Figure 8

For illustration purposes, the single trace of the treeing line shown in the red circle is analyzed here. This trace isshown in Figure 9. In Figure 9, the plane of focus for the picture is set near the aluminum foil side of the paper. The

branches near the opposite side of the paper are out of focus midway through the paper. The drawing in Figure 10

helps to illustrate that. In the illustration the aluminum foil would be on the left side (dotted line).

The path followed by the traces often reaches the opposite side of the paper and into the adjacent paper layers.

There is an indication that the pattern begins as an electrostatic deposition of particles on the surface and along theedge of fibers adjacent to the aluminum foil. The particles that have collected at that location are semi-conductors at

best. They included sludge particles, copper oxides, copper sulfides, and other particles. This deposition modifies

Aluminum Foil Location


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the electrostatic field and collects more particles at greater distance from the aluminum foil forming fractal

agglomerates and chains along the edges of single paper fibers and resulting in very low level energy discharges.

The discharge patterns follow the edges of paper fibers in the same pattern seen for the deposited particles. Finally, a

discharge path is created along the particles on the surface of the paper fibers not affecting the fiber itself.

Trace from a Treeing Line

Figure 9

Discharge Path

Figure 10

Copper and sulfur are associated with the discharge paths but they form following the discharge and are not thecause of the discharge. Copper sulfide forms on the surface of the discharge as very small filamentous fragments.

That indicates the mobility of both copper and sulfur ions in the system. Figure 11 is a photomicrograph taken with

a scanning electron microscope of one part of one of the traces. The individual paper fibers can be clearly seen as

smooth-walled structures. Some of the fibers show a dark grainy deposit on their surface. This is the trace line,

which is made up of carbonaceous debris primarily from the oil. On top of this debris are a number of white thread-like deposits. These white thread-like deposits are the copper sulfide deposits. They form after the trace has

formed. The area between these threads is deficient in copper and sulfur. The trace is not the result of copper

sulfide but seems to provide a sight where copper and sulfur are brought together.

AluminumFoilEdge


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Figure 11

Photomicrograph of One Part of One of the Traces

DISCUSSION

Since materials at the point of failure were destroyed and dispersed it is not possible to attempt to reconstruct initial

condition from the failed materials. It is also not possible to know the state of the system at the moment the failure

was initiated. Consequently, the search for the cause of failure relies more significantly on inferences that can bedrawn from the investigation of comparable circumstances.

It is apparent that at the moment of failure the bushing insulation could not withstand the condition or conditions towhich it was subjected. Transient and harmonic conditions may have been present and should not be eliminated as

potential contributing factors.

The work conducted during this investigation helped to clarify the involvement of some of the potential factors that

could have contributed to the failures. Specifically:

1. Heating of the oil or the paper has not been identified as a contributing factor from dissolved gas analysisperformed.

2. Arcing has not been identified as an active incipient fault process from dissolved gas analysis performed.

3. Oxygen levels in tested oils were found to be consistent with vacuum processed oils. The one directly

measurable oxidation product, total acid number, was at low levels in the tested oils.

4. Moisture levels were acceptable for both paper and oil samples collected during the dismantling and

inspection of bushing C. The moisture in the paper in the vicinity of the puncture hole was anacceptable 1.2%. The moisture in oil for oil sampled from bushing E was comparable to values obtained

for bushing C oil.

5. Acid levels were found to be low in tested oils.

6. Broad surface contamination with sludges, waxes, films, or sediments was not observed.

7. Fluid particle counts were not unacceptable in the fluids tested, although some of the ASTM D1816dielectric breakdown voltage measurements of the fluid samples were unacceptable. This particulardielectric breakdown voltage method is sensitive to particles and moisture and typically shows more

variation than other methods.

8. Corrosive sulfur was not identified in any of the tested fluids by any existing standard or method. A

determined effort was made to produce a positive CCD corrosive sulfur test result for the tested oil. To

produce a positive result, it was required that the sample be subjected to a temperature of 140C for 15

days. It should be noted that under these conditions other serious issues would arise within the equipment.


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9. Copper and sulfur have been identified on paper surfaces, but copper sulfide formations have not been

specifically identified on any of the paper samples.

10. Polar compounds have observed and directly related to elevated fluid power factor measurements for the

fluid from bushing E and another service aged Diala D. High fluid power factor measurements have been

observed for all of the bushing fluids tested. Observation of elevated power factor and the Garton Effect

in power factor testing on bushing E also demonstrated the presence of polar compounds in the fluid.

Elevated power factors have been observed for all of the bushings.11. Ionic species transported by ligands are possibly indicated by some of the SEM measurements

12. Discharge like treeing patterns were observed in bushings C, E, F and G. Also, discharge like

lines were found in one inspected bushing from the Georgia Power Dorchester substation failure [7].

CONCLUSIONS

It could not been determined if the tree pattern seen on the paper of the examined sister or failed bushings are related

to the cause of the actual failure in the Trench bushings A and D; however, the puncture hole found at the edgeof the foil in bushing C, which coincides with the treeing line, is a good indication that the area had become a

weak point in the bushings insulation. The following conclusions have been drawn from this investigation.

The preponderance of information indicates the failures are due to insulation failure.

The failures are related to insulation compromising phenomena linked with the oil.

Shell Diala D is the identified oil that has developed the features allowing this type of failure to occur.

Not all Shell Diala D develops these features.

The specific features are a high power factor that displays the Garton Effect and elevated levels of oil soluble

copper.

Some copper conductor bushings with Shell Diala D will be at risk for this type of failure.

Most copper conductor bushings with Shell Diala D will not be at risk for this type of failure

The copper conductor bushings without Shell Diala D are not at risk for this type of failure.

The aluminum conductor bushings with Shell Diala D are not at risk for this type of failure.

Compromised bushings can be identified by routine field tests.

Recommendations:

Identify those Trench France copper conductor bushings that contain Shell Diala D, manufactured between

1998 and 2003.

Identify which of these bushings has an increasing power factor.

When the C1 power factor increase is 1.5 to 2 x nameplate value and when the power factor indicates a tip up,

typically:

Test KV PF %

2 0.39

4 0.40

6 0.42

8 0.4310 0.55

replace the bushing.


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ACKNOWLEDGEMENTS

The authors would like to express gratitude to Russ Crutcher, Microlab Northwest; Dave Hanson, TJH2b; Danny

Bates, Alabama Power; Joseph Benefield, Georgia Power, for their contributions during this investigation.

REFERENCES

[1] E. R. Crutcher, E. R. and Warner, Ken, Copper Mobility and Failure in Electrical Equipment: It Is Not

Corrosive Sulfur, Proceedings of EuroTechCon, 2009.

[2] Bates, D., Back to Back Bushings Failures - An Ongoing Investigation, Proceedings of TechCon USA 2008.

[3] Doble Laboratory Test Analysis Report 71090, November 30, 2006.

[4] Doble Materials Report 72486, March 2007.

[5] Doble Materials Report 75585, August 2007.

[6] Siemens Test Report MCSL 068/07, April 2007.

[7] Del Rio, Arturo and Hanson, Dave, Copper Migration in Bushings: Update to Southern Co.Trench Bushing

Failure Investigation, Siemens Transforming Know-how into Solutions Conference, 2008.

BIOGRAPHY

Arturo Del Rio started his professional career as a Field Engineer in the oil fields ofColombia and as an Electrical Engineer for Hatch Associates consulting firm in Toronto,

Canada, were he was involved in several projects and studies related to electric arc furnaces

and the metallurgical industry.

He joined Trench Canada in 1991 where he has held various design and engineering

positions in the fields of instrument transformers, power electronics and air-core reactors. He

is currently Engineering Manager at the Trench Transformer Bushing Division in Ajax, ON,

Canada.

Arturo holds a bachelors degree in Electrical Engineering from the Universidad Industrial de Santander, Colombia,

and an M.A.Sc. degree in the field of power devices and systems from the University of Toronto, Canada. He has

been an IEEE member since 1988 and is a registered Professional Engineer in Ontario.

Keith P. Ellis is responsible for the development, promotion, sales and technical support forbushings for the Trench Bushing Group including HSP, serving as Bushing Product Manager

and OEM Sales Manager, Americas. Before joining Trench Keith was Sales Manager for the

ABB Power T & D Companys Components Division. This position was also held under the

company names of Westinghouse/ABB and ASEA Electric. Before assuming the Sales andMarketing responsibilities of transformer components, Keith was Senior Sales Representative

for RTE and RTE-ASEA in Upstate New York. Keith began his career in the power

transformer industry with RTE/ASEA as an Application Engineer.

Keith graduated from Mare Island Naval Shipyard with a journeyman certificate in Machine Technology. Attended

the University of California, where he majored in Mechanical Engineering. After serving with distinctions in the US

Navy during the Vietnam War he continued his education at the University of Wisconsin, Milwaukee. He is amember of IEEE, PES, Transformers Committee and Working Group Chairman for C57.19.00. He takes particular

interest in component applications to power transformers with special interest in high voltage bushings and on-load

tap changers.


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APPENDIX A

Summary of Results for Service Aged Diala D from Bushing G


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Doble Test Results

Service Aged Diala D from Bushing G

TABLE A1

DGA Analysis , ASTM D 3612, ppm v/vGAS Syr. 2759 Syr. 2013

Conc, Conc,Hydrogen 2.7 3.0

Oxygen 3,960 3,700

Nitrogen 50,500 50,600

Methane 3.5 3.6

Carbon Monoxide 100 102

Ethane 0.0 0.0

Carbon Dioxide 2,183 2,234

Ethylene 0.0 0.0

Acetylene 0.0 0.0

Total Combustible Gas 106 109

TABLE A2

Oil Quality Results After Clay TreatmentTest Method Before

Treatment

After

Treatment

Interfacial Tension, mN/m ASTM D 971, ISO 6295 46 48

Neut. No., mgKOH/g ASTM D 974


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TABLE A4

Additional Sulfur Test Results on the Oil As received, Doble CCD Test at 140 C4-day Testing, 18 gauge needle Method Result

CCD Test by Air Ingress - Paper Doble Method Light Deposit, dull in color

Dielectric Strength of Paper used in CCD test (Air Ingress) ASTM D 149*

CCD Test by Air Ingress - Copper Doble Method Non-corrosive

CCD Test by Air Ingress Copper Tarnish Level ASTM D 130 1bCCD Test, Sealed Doble Method Moderate Deposit, dull in color

Dielectric Strength of Paper used in CCD test (Sealed) ASTM D 149*

CCD Test, Sealed- Copper Doble Method Non-corrosive

CCD Test, Sealed- Copper Tarnish level ASTM D 130 2b

6-day Testing, 20 gauge needle Method Result

CCD Test by Air Ingress - Paper Doble Method Moderate Deposit, dull in color

Dielectric Strength of Paper used in CCD test (Air Ingress) ASTM D 149*

CCD Test by Air Ingress - Copper Doble Method Non-corrosive

CCD Test by Air Ingress Copper Tarnish Level ASTM D 130 2b

CCD Test, Sealed Doble Method Moderate Deposit, dull in color

Dielectric Strength of Paper used in CCD test (Sealed) ASTM D 149*

CCD Test, Sealed- Copper Doble Method Non-corrosive

CCD Test, Sealed- Copper Tarnish level ASTM D 130 2c

*Analysis was performed under the following conditions:

Sample Preparation: Paper dried for 16 hours at 80C and then oil impregnated

Electrodes: brass, 0.25 inch diameterVoltage Rise: 500 volts/second

Temperature: CMaterial Thickness: 6 mils (paper was in 2 layers to get an accurate result)

investigation of failures of 230kv copper conductor bushings

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