INVESTIGATION OF FAILURES OF 230-KV OIP COPPER CONDUCTOR BUSHINGS. Arturo Del Rio Keith Ellis Trench Ltd, Canada

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1 INVESTIGATION OF FAILURES OF 230-KV OIP COPPER CONDUCTOR BUSHINGS ABSTRACT Arturo Del Rio Keith Ellis Trench Ltd, Canada 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 been implicated 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 and then 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-F AG3-01-ADP bushings failed on two separate transformers on the Southern Company s 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 sister bushings 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.

2 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 after dielectric 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 Trench facilities 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 no significant 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 % C1 Commissioning 0.30% C1 Jan % C1 When removed from service 0.52% C1 After Impulse and Hipot Test September kV: 0.516% 50kV: 0.465% 102kV: 0.385% 2

3 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 Bushing ID Bushing C, as Trench Bushing C, after all voltage tests New Tested by/date: Trench MS Trench MS bushing oil (ppm) Hydrogen (H 2 ) Oxygen + Argon (O 2 + Ar) Nitrogen (N 2 ) Carbon Monoxide (CO) Methane (CH 4 ) Carbon Dioxide (CO 2 ) Ethylene (C 2 H 4 ) < 0.04 ND < 2 ND <0.04 ND < 2 ND 2 Ethane (C 2 H 6 ) < 0.04 ND 3 <0.04 ND < 2 ND 2 Acetylene (C 2 H 2 ) < 0.04 ND < 2 ND <0.04 ND < 2 ND 2 Total gas content% 2.28% 6.98% 2.25% 8.33% - Water content RT N/A N/A 4.38% N/A 0.05% 90C N/A N/A 25.4% N/A Although some discrepancy was evident in the test results, there were no significant gases that could indicate electrical 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 25 C and 100 C. 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 25 C power factor; this is indication that polar material was removed from the oil by the clay as this test is greatly 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 25 0 C (%) C (%) Trench France n/a Trench BU Trench IT Doble* Morgan Schaffer *Worst case of multiple samples taken during bushing oil draining. 3

4 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 no abnormalities 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 a fine 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 of polymerization (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. The chemical 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. 4

5 Paper fibers Burn Hole 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 minor amounts 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 and tip-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 new bushing 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 and copper 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 in bushing 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. 5

6 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 consistent non-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 of behavior 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 90 C. After a vacuum treatment and a further heating at 90 C the value was 1.6%. This is a further indication, that some polar compounds are adsorbed in paper and may undergo further reactions. 6

7 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 at 1000 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. 7

8 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 tracks shown 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. Aluminum Foil Location 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 is shown 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 the edge 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 8

9 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. Aluminum Foil Edge 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 the cause 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 threadlike 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. 9

10 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 be drawn 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 to which 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 analysis performed. 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 an acceptable 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 D1816 dielectric breakdown voltage measurements of the fluid samples were unacceptable. This particular dielectric 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 140ºC for 15 days. It should be noted that under these conditions other serious issues would arise within the equipment. 10

11 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 edge of 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 bushing s 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 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 % replace the bushing. 11

12 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, [2] Bates, D., Back to Back Bushings Failures - An Ongoing Investigation, Proceedings of TechCon USA [3] Doble Laboratory Test Analysis Report 71090, November 30, [4] Doble Materials Report 72486, March [5] Doble Materials Report 75585, August [6] Siemens Test Report MCSL 068/07, April [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, BIOGRAPHY Arturo Del Rio started his professional career as a Field Engineer in the oil fields of Colombia 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 bachelor s 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 for bushings 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 Company s Components Division. This position was also held under the company names of Westinghouse/ABB and ASEA Electric. Before assuming the Sales and Marketing 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 a member of IEEE, PES, Transformers Committee and Working Group Chairman for C He takes particular interest in component applications to power transformers with special interest in high voltage bushings and on-load tap changers. 12

13 APPENDIX A Summary of Results for Service Aged Diala D from Bushing G 13

14 Doble Test Results Service Aged Diala D from Bushing G TABLE A1 DGA Analysis, ASTM D 3612, ppm v/v GAS Syr Syr Conc, Conc, Hydrogen Oxygen 3,960 3,700 Nitrogen 50,500 50,600 Methane Carbon Monoxide Ethane Carbon Dioxide 2,183 2,234 Ethylene Acetylene Total Combustible Gas TABLE A2 Oil Quality Results After Clay Treatment Test Method Before Treatment After Treatment Interfacial Tension, mn/m ASTM D 971, ISO Neut. No., mgkoh/g ASTM D 974 <0.01 <0.01 Power Factor at 25 C, % ASTM D Power Factor at 100 C, % ASTM D TABLE A3 Sulfur Related Test Results on the Oil As Received Test Method Result Corrosive Sulfur ASTM D 1275B Non-corrosive Tarnish Level ASTM D 1275B/D 130 1b Corrosive Sulfur Extended Test (8 days) ASTM D 1275B - Extended Tarnish Level Extended Test (8 days) ASTM D 1275B/D Extended Total Sulfur, mg/kg ASTM D Elemental Sulfur, mg/kg UOP 286 < 1 Dibenzyl disulfide (DBDS), mg/kg Doble < 1 14

15 TABLE A4 Additional Sulfur Test Results on the Oil As received, Doble CCD Test at 140 C 4-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 1b 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 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 80 C and then oil impregnated Electrodes: brass, 0.25 inch diameter Voltage Rise: 500 volts/second Temperature: C Material Thickness: 6 mils (paper was in 2 layers to get an accurate result) 15