doble biit-02 teardown ob type g bushings chavez

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2012 Doble Engineering Company -78th Annual International Doble Client Conference All Rights Reserved

FORENSIC TEARDOWN INSPECTION OF THREE OHIO BRASS 180KV TYPE-G BUSHINGS

Santiago Chavez

AES Alamitos

Darin Mitton Doble Power Services

ABSTRACT A forensic teardown inspection of three Ohio Brass 180kV type-G bushings was performed at Doble Engineerings High Voltage Lab. One of the bushings had failed in service and the other two had been installed at the same location. The purpose of the inspection was to discover what caused the bushing to fail and determine if there was a common problem with other bushings in the system with the same construction. The inspection consisted of an analysis of internal dielectric stresses, visual inspection of all internal components, chemical analysis and electrical testing of material samples taken from the bushings. A review of oil sample DGAs and power factor test records were also performed. Evidence of partial discharge and moisture contamination was observed in all three bushings. Gasket deterioration and moisture ingress was a root problem common to all three bushings. INTRODUCTION After a bushing failed in service, the decision was made to perform a forensic teardown analysis of it and two identical bushings from the same bank of transformers. The bushings were transported to the Doble High Voltage Laboratory in Watertown, Massachusetts where they were disassembled and inspected. The combined findings from of a dielectric stress analysis of the design, visual inspection during disassembly, and chemical and electrical tests of samples taken from the bushings were used to determine the cause of failure for the bushing that failed and the condition of the other two bushings. BACKGROUND On July 16, 2010 at 10:56 p.m. Unit 2 at AES Alamitos experienced a failure of A-phase Main Transformer. An Allis Chalmers 180kV transformer bushing failed violently, causing some damage to nearby equipment and unscheduled down time. The top portion of the bushing exploded leaving only the center conductor rod and cap. The bottom portion of the bushing stayed together and the internal components of the transformer were not damaged. The bushing was installed in a single phase generator step up (GSU) transformer connected to the A-Phase, in a bank of three transformers. The explosion caused some superficial damage to the C-Phase, but the B-Phase was not damaged. Nameplate information for the bushing and transformer information are as follows: Bushing Transformer

Manufacturer Ohio Brass Manufacturer Allis Chalmers Type Class-G-53X Type Single Phase GSU Voltage Class 180kV Capacity 56/74.6 MVA BIL 825kV Cooling Class ONAN/ONAF Current Rating 800A Temperature Rise 55C C1 Power Factor 1.0% Voltage 18,000 127,017GRDY/220,000 Manufactured 1956 Manufactured 1956 It was decided to perform a tear down and forensic analysis of all three bushings. The serial number of the bushing that failed (A-Phase) was 49310-8. The serial number of the B-Phase bushing was 49310-9, and C-Phase was 49310-7.

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AES Alamitos, in the City of Long Beach, is a 2,000 MW natural gas-fueled power plant, one of the largest in Southern California. POWER FACTOR TEST RESULTS Power factor tests were performed in November 2007, which was approximately 2.5 years before the failure. The measured values were very similar for all three bushing and showed a 56% increase over the factory nameplate values. The measured results are listed here in Table 1.

Table 1 Power Factor Test Results November 2007

Connected Phase

Serial Number

C1 Power Factor [%] Nameplate Tested W/Oil

A (failed) 41390-8 1.0 1.59 B 41390-9 1.0 1.53 C 41390-7 1.0 1.56

A query of historical test records using Doble DTA Web for the C1 measurements of Ohio Brass Type G bushings rated from 115 to 230 kV yielded 405 test results. Based on these test results, the acceptable range for the C1 power factor, corrected to 20 C, is between 0.12 0.64 percent. Figure 1 displays the historical power factor distribution.

C1 Power Factor Distribution History

Figure 1

The nameplate power factor values for the bushings under investigation were 1%. The query results showed few measurement results in this range. Interestingly however, there were noticeably more results close to the 1.6% measured in the latest tests. Given that all three bushing showed similar test values, the change was likely attributed to normal aging. In retrospect, it appears that the increased power factor values were likely due to a deteriorating condition inside the bushings, given the findings from the inspection.

0

10

20

30

40

50

60

0.12 to0.377

0.377 to0.634

0.634 to0.891

0.891 to1.148

1.148 to1.405

1.405 to1.662

1.662 to1.919

1.919 to2.176

2.176 to2.433

2.433 to2.69

%PF Corrected

115 - 230 KV OB Type G C1 Bushing Test

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DIELECTRIC STRESS ANALYSIS An Internal Dielectric Stress Analysis was performed to discover any inherent characteristics that can help to determine the failure mode and potential that a similar failure was likely in other bushings of the same design and vintage. Unlike modern capacitance graded bushing designs, which use layers of foil or conductive ink sandwiched between layers of insulating paper, these bushings used porcelain sleeves and spirally wrapped layers of resin impregnated cloth tape for grading the voltage between a fired-on ground shield and the bushing stud, as shown in Figure 2.

Dielectric Graded Bushing Arrangement

Figure 2 The two inner concentrically arranged porcelain sleeves were separated by wedges cemented at the ends between the sleeves. The ground shield porcelain was attached to the mounting flange by wedges in a similar manner. It is well recognized that the radial electrical stresses are highest in the space between the bushing stud and the ground shield. The axial stresses are dependent upon the strike distance between the live electrodes at the top and bottom ends of the bushing and the fired-on ground shield. It is evident that this was a graded insulation bushing design where a series of porcelain barriers were used to control radial stresses in the space between the bushing stud and the ground shield. Since the oil gaps between these barriers were the weakest dielectric medium, the radial stresses at the outer diameters of each solid dielectric were calculated to check the integrity of the dielectric system. Table 3 shows dimensions and dielectric constants used to calculate the dielectric stresses.

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Table 2 Dielectric Constants and Dimensions

Component Material Dielectric Constant

Inside Radius

[in]

Outside Radius

[in]

Distance to Stud

[in]

Effective Distance

[in]

Effective Radius

[in]

Gradient At OR [kV/in]

Bushing Stud Copper - - 1.25 79.90

Tape Layer I

Cambric Cloth 2.32 1.25 1.55 0.30 0.30 1.55 64.44

Oil Duct I Oil 2.32 1.55 1.75 0.50 0.50 1.75 57.07 Inner

Porcelain Porcelain 6 1.75 1.88 1.63 1.03 2.28 43.74

Tape Layer II

Cambric Cloth 2.32 2.88 3.00 1.75 1.14 2.39 41.77

Oil Duct II Oil 2.32 3.00 3.40 2.15 1.06 2.33 42.80 Inner

Porcelain II Porcelain 6 3.40 4.50 3.25 2.04 3.29 30.35

Oil Duct III Oil 2.32 4.50 5.19 3.94 2.62 3.87 25.81 Shield

Porcelain Porcelain 6 5.19 6.45 5.2 3.19 4.44 22.49

Ground Shield

Conductive Coating - 6.45 - 5.2 - - -

The calculated radial stresses are tabulated in Figure 3.

Calculated Radial Stresses

Figure 3

The analysis of radial stresses indicates that all the radial stresses were within the limits of reliable bushing design. From the physical measurement information recorded during the teardown, the internal strike distance at the top end, from the bushing stud to the ground shield was estimated to be 47.68 inches. Corresponding to the 825 kV BIL, this gives an approximate average axial stress level of 17.3 kV per inch. Similarly, at the lower end, the internal strike distance from the bottom plate to ground shield was estimated to be 22.92 inches and corresponding to the rated BIL

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of the bushing, this gives an average axial stress level of 36 kV per inch. These stress levels are well below the recommended design limits for reliable bushings. VISUAL INSPECTION All of the porcelain above the ground sleeve of bushing that failed was gone, leaving only the center stud and top cap of the bushing. Photo 1 shows the condition of the bushing when it arrived at the lab. All of the porcelain above the flange was blown away in the failure.

Top End of Failed Bushing

Photo 1

Photos 2 and 3 show burn marks on the porcelain.

Burn Marks on Ground Shield Porcelain Burn Marks on Inner Porcelain Sleeve

Photo 2 Photo 3 Photos 4 and 5 show burn marks on the center stud and outer flange respectively. The shape of the crater and the burn marks indicate that this was an external flashover between the bushing stud and the bushing flange. The draw lead and its insulation had no involvement in this failure. It appears that an internal flashover created high pressure inside the bushing, causing the porcelain to rupture. This was likely followed by subsequent arcing Due to the extent of the damage, the precise cause and initiation point of the failure is not evident.

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Burn Marks on Center Electrode Burn Marks on Inner Surface of Flange

Photo 4 Photo 5 The impregnated paper layers taped over the bushing stud were examined and significant evidence of partial discharge (PD) activity was observed. These marks were not located near the areas where the flashover damage was observed. This indicates that a general problem existed throughout the bushing, such as contamination in the oil. Similar marks were found in the other two bushings, as they were disassembled. Photos 12 - 14 show black marks and burn holes in the insulation that are characteristic of partial discharge activity.

Black Hole in Insulation Dark Spots on Insulation

Photo 6 Photo 7 They are representative of what was found in all three bushings. Many of the spots showed a tree like pattern typical of partial discharge damage when examined under a microscope, while others did not. These spots were determined to be inclusions from the manufacturing process, The innermost layer of the insulation had a dark greenish appearance where it came in contact with the copper bar running through the center of the bushing, as can be seen in Photo 8. This appears to be a copper oxide deposits resulting from moisture on the copper. This coating was very uniform, indicating that they were caused by a reaction that had been taking place over a long period of time.

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Green Color on Insulation on Center Conductor

Photo 8 The top seal gaskets were found to be badly deteriorated, as shown in Photo 9. Signs of moisture ingress and corrosion were found in all three bushings. Photos 10 and 11 are representative examples of green deposits, which appear to be copper oxide, found in many locations throughout the bushings. Photo 12 shows dark stain marks on the center copper rod that appear to be caused by moisture in the oil.

Deteriorated Gasket Green Deposits on Gasket Surface

Photo 9 Photo 10

Green Deposits on Spring Assembly Dark Stains on Center Copper Rod

Photo 11 Photo 12

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LABORATORY ANALYSIS Samples of materials were extracted from each bushing during the teardown and labeled appropriately. These samples were analyzed at the Doble Material Laboratory by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis. Scanning Electron Microscopy (SEM) Energy Dispersive X-ray (EDX) analysis are two separate tests conducted at the same time. EDX is a technique in which an electron beam of the scanning electron microscope enters the bulk of a sample producing an x-ray emittance. The X-ray peak positions, along the energy scale, identify the elements present in the sample and can provide the percent concentrations of each of these elements thus providing an elemental breakdown of the material or particles. SEM is an analysis in which a beam of electrons, a few hundred angstroms in diameter, systematically sweeps over the specimen. The intensity of secondary electrons generated at the point of impact on the specimen surface is measured, and the resulting signal fed into a cathode-ray-tube display which is scanned in synchronism with the scanning of the specimen to produce a picture. The cellulose insulation primarily used in these three bushings is cambric cloth. Cambric cloth is open weave cloth impregnated with varnish and was used quite extensively in the electric industry in the first half of the twentieth century. The material shown in Photo 39 was chosen for testing as there seems to be a residue accumulating at the edges of the cloth layer overlap. A sample of cambric cloth that had a blackened edge was analyzed. An SEM micrograph of the edge of the cloth is shown Photo 13.

SEM Micrograph of Cambric Cloth Edge

Photo 13 Composition from the EDX spectra of the blackened edge is provided in Table 3.

Table 3 Composition of Cambric Cloth Edge

Element Composition, wt. % Carbon 57.3 Oxygen 40.6

Aluminum 0.9 Silicon 1.2

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Most of what was found was just carbon and oxygen so it is most likely thermal degradation of the oil, paper and varnish that collected in area. Edges of paper which still have individual fiber ends exposed tend to collect carbon more readily than smooth, non-porous surfaces. Because of the green discoloration observed on the center copper rod, a wipe sample was taken for analysis. The EDX composition of the two predominant particles from the wipe sample is provided in the Table 4.

Table 4 Composition of Particles from Wipe Sample

Element Particle 1, % Particle 2, %

Carbon 61.8 21.9 Oxygen 26.4 2.9 Chlorine 1.6 0.4

Potassium 0.8 -- Calcium 0.5 --

Iron 0.9 -- Copper 5.0 74.1

Zinc 3.0 -- Silicon -- 1.1

The copper is most likely from the conductor. The presence of potassium, calcium and chlorine would suggest salts from seawater although they are in a very low percentage. The wipe sample was taken because of the green hue which usually indicates corrosion of the copper surface. It appears that some moisture from the external environment did enter the bushing containing some salt water and did react with the copper to a small degree. A sample of the innermost layer of insulation next to the conductor with green deposits was analyzed. SEM Micrograph of insulation sample is shown in photo 14.

SEM Micrograph of Insulation with Green Deposits

Photo 14 The EDX composition of the deposits from the sample of insulation is provided in Table 5.

Table 5

Composition of Green Deposits Element Composition, wt. % Carbon 67.7 Oxygen 22.1 Copper 10.3

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A large amount of the copper was present in the cambric cloth and most likely combined with carbon and oxygen to form copper carbonate (CuCO3) on the cambric cloth surface giving it the green coloration. The formation of copper carbonate is a result of the slow chemical alteration of copper in which a humid (water) environment would have to exist so that the reaction could take place. A razor blade was used to remove some black material from one of the porcelain sleeves. Photo 15 shows the SEM micrograph of the black material.

SEM Micrographs of Black Material

Photo 15 The elemental composition is provided in Table 6.

Table 6 Composition of Black Material

Element Composition, wt. % Carbon 43.4 Oxygen 25.2 Sodium 3.2

Magnesium 0.5 Aluminum 1.0

Silicon 5.0 Sulfur 8.4

Chlorine 1.1 Potassium 2.4 Calcium 4.0

Iron 1.9 Copper 4.0

Copper and a large amount of sulfur are present. There is no suspicion of corrosive sulfur being an issue as the copper was quite clean. There is a good percentage of copper away from the copper conductor itself. This may be a combination of sludge and other materials. Sludge usually contains carbon, oxygen, sulfur, copper and other elements to a lesser degree and is the byproduct of oxidation within the bushing. This would confirm that possible outside environmental contaminants such as air and water were diffusing into the system over time.

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HIGH VOLTAGE LABORATORY TESTS Dielectric Breakdown Measurements were performed using an Associated Research Breakdown Tester in accordance with ASTM D 149; 1 inch diameter electrodes in oil. Dielectric strength of cambric cloth samples as determined by high voltage breakdown test are listed below.

Sample Description [V/mil]

Clean with No Discoloration 1400 With Green Discoloration 1060 With Green Discoloration 1190 With Signs of Partial Discharge 970

The voltage breakdown tests show reasonably good insulation strength, giving no evidence that deteriorated cambric cloth contributed to the failure. DISSOLVED GAS IN OIL ANALYSIS (DGA) Oil was drained from the two bushings that did not fail, prior to shipping them to the Doble high voltage lab. DGA and moisture content measurements were taken on the two samples labels as B-Phase and C-Phase corresponding to transformer from which they were taken. The levels of dissolved combustible gasses present are given here: Dissolved Combustible Gasses [ppm] B-Phase C-Phase Hydrogen 66 69 Methane 35 40 Ethane 109 167 Ethylene 68 74 Acetylene 1 1 Carbon Monoxide 459 356 The elevated levels of ethylene and presence of acetylene indicate localized overheating was occurring within the bushings. Elevated levels of carbon monoxide indicate overheating of the cambric insulation material. Given the evidence of moisture air ingress observed, less soluble gasses have likely leaked out. Generation of hydrogen and carbon monoxide was likely higher than indicated by the DGA samples, which would support the evidence of partial discharge observed. The levels of dissolved non-combustible gasses present are given here along with the moisture content. Dissolved Non-Combustible Gasses and Moisture [ppm] B-Phase C-Phase Carbon Dioxide 20471 19556 Nitrogen 79821 79255 Oxygen 6495 9722

Moisture in Oil 20 17 Oxygen and water content were lower than anticipated, given the evidence of moisture air ingress observed. This suggests that they were likely consumed in the chemical reactions, and that the process had occurred steadily over a long period of time.

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CONCLUSION The radial and axial electrical stress analysis of these bushings indicates that the design was adequate and didnt expose any particular weakness that could have led to the failure of S/n: 41390-8. The teardown observations from all three bushings indicate that the failure of S/n: 41390-8 was most likely caused by moisture, oxygen, salt and other contaminates entering the bushing through deteriorated gasket seals that compromised the dielectric integrity of the internal bushing insulation. The elevated power factor readings from November 2007 were likely caused by this deteriorating condition inside the bushings. The results from dielectric breakdown strength tests on solid insulation samples taken from the bushings were quite high. This suggests that dielectric deterioration was occurring mainly in the bushing oil. It is possible that the evidence of partial discharge activity observed during the teardown was the result of decreased dielectric breakdown strength of the oil. The oil from bushings S/n: 41390-9 and S/n: 41390-7 was drained before they were shipped to Doble. Therefore it was not available for dielectric strength or partial discharge testing. BIOGRAPHY Mr. Darin D. Mitton. Mr. Mitton is a Transformer Engineer with Doble Engineerings Global Power Services. He has 13 years of experience in power transformer and shunt reactor design, followed by 8 years of consulting. He has been with Doble Engineering for 3-1/2 years, providing consulting services, including Design Reviews, Factory Inspections, Factory Test Witnessing, Forensic Analysis, and Condition Assessments. He holds a B.Sc. in Engineering from Idaho State University and is a registered Professional Engineer. Mr. Santiago E. Chavez. Mr. Chavez is the Team Leader of IC&E of AES Alamitos. He has 30 years of power generation experience. He has been with AES for 12 years; first as Project Support then 11 years as a Team Leader. He previously worked at Southern California Edison for 18 years as an Industrial Electrician, Maintenance Planner and Safety Environmental Specialist III (CMMS) (REP 5973). He worked 11 years at Magma Copper Company as an Industrial Electrician.

doble biit-02 teardown ob type g bushings chavez

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