The University of Manchester will be building on three successful phases of the transformer research consortium, which has delivered world class research and results. Phases 1 and 2 focussed on the use of synthetic and natural esters in transformers at transmission voltage levels, this work led to sufficient understanding of the technology for National Grid to be able to deliver an inner London substation design incorporating synthetic ester filled transformers. Some of the successes delivered within Phase 3 were:
- progress was made in the interpretation of methanol and ethanol as potential early indicators of transformer insulation ageing. The rates of production by oil and paper and the partition between the two is now better understood. More data and case studies are required to evaluate the usefulness further
- a methanol measurement technique was developed, benchmarked within IEC and transferred to the oil analysis service provider for routine testing.
- study of oil result database for the participating utilities led to conclusions about the relative merits of different tests to show when oil is degraded. Different diagnostic techniques were compared and recommendations for possible improvements in IEC guidance were made.
- extensive laboratory testing has developed our understanding of dissolved gas production under different fault conditions. The same experiments have improved our knowledge on the response of on line gas monitors
- very useful insights have been gained into partial discharge inception and propagation in ester fluids that might lead to a revision of transformer test requirements
- Computational Fluid Dynamic (CFD) studies have shown up significant issues with existing techniques used by manufacturers offering the promise of better designs. Experimental studies have been used to successfully validate the CFD models.
Phase 4 will continue to investigate all of these areas in more depth.Under the original TOPICS project, the University of Southampton investigated corrosive sulphur issues in transformers, delivering improvements in oil reclamation operating procedures to prevent corrosion of silver, especially tap-changer contacts and a greater understanding of the failure mechanism involved with copper sulphide deposition. Research is required to investigate further aspects of oil reclamation and mitigation of corrosive sulphur in transformer oil.
Benefits
The outputs from the research being undertaken will provide the industry with improved knowledge and understanding of the optimised asset life and asset management of Transformers and Transformer Oil.
Learnings
Outcomes
TRC4
Bubbling behaviour was found to be affected by the degree of polymerisation (DP) of the solid insulation, the BIT (Bubble Inception Temperature) formula has been modified to include DP of the paper. As bubbling inception is affected by the rate of change of temperature as well as the absolute temperature, it was found that under some scenarios where electrification increases could increase the risk of bubbling in future in some transformers.
The risk of formation of bubbles is low in oil only, regardless of fluid type, even in the presence of increasing levels of particulate matter, water or dissolved gas. The solid insulation, paper, is the key source of bubbles and wetter insulation releases bubbles at lower temperatures. Thermally upgraded Kraft paper (TUK) was found to resist bubble formation.
Partitioning of 2-FAL between oil and paper has been shown to be affected by oil and paper types. At room temperature when impregnated with mineral oil, thermally upgraded paper absorbs less 2-FAL than non-thermally upgraded paper; and with the same type of paper, synthetic ester holds more 2-FAL than mineral oil.
Some transformers with obvious hydrogen and/or carbon monoxide increases during their service life were confirmed to be related to the introduction of metal passivators. Gassing arising from passivator addition was found to be generally short-lived. After four years, twenty-two of 150 were found to still show signs of stray gassing related to passivator.
It was found that ML techniques may be applied to single and multiple fault gases from an active transformer to highlight where trends are changing. The method developed could provide a more objective method of tracking trends and showing where faults or ageing may be accelerating. The method also enables the identification of anomalies in the data.
The radiator model simulations indicate that neglection of the steel walls in the model, commonly observed in the literature, would result in 5-10% reduction of the calculated total heat dissipation. Complete cooling loop (CCL) simulations using CFD have been shown to effective in demonstrating how oil temperatures and oil flow rates are influenced by loading and ambient temperatures.
In conducting the CFD simulations as part of TRC4 a number of observations have been made about winding types. Comparisons of flow and temperature distributions between traditional disc-type windings and SMIT disc-type/layer-type windings show that SMIT disc-type/layer-type windings can have uniform liquid flow distributions in the axial cooling ducts regardless of the total liquid flow rate, while traditional disc-type windings have increasingly uneven flow distribution in the radial cooling ducts with increasing pass inlet Reynolds number. On the other hand, traditional disc-type windings can have multiple passes, leading to lower average winding and hotspot temperatures than SMIT disc-type/layer-type windings at the expense of much higher pressure drop over the winding. In terms of manufacturing deviation, the thermal effect of geometric deviations (paper bulging, local changes of oil duct sizes, winding disc crossovers, washer leak, etc.) were found in OD conditions not to be pronounced if these deviations are not so extreme as to cause complete blockage of certain oil ducts. These findings are informative for transformer specification and design review during the procurement process.
Observations were made of the shockwaves under LI testing in mineral oil, synthetic ester and gas-to-liquid. The oil and ester shockwaves were found to be similar with mach-cone shapes being observed under positive LI, while only spherical shock-waves observed under negative LI, with ester shockwaves being more obvious than those in oil but still weak. The gas-to-liquid mach-cone shockwaves were dissipated more quickly and transformed into spherical shockwaves. The propagation velocities and shockwave pressures were found to be lower in gas-to-liquid than in both mineral oil and synthetic ester.
TOPICS 2
Thermal ageing between 110 and 150 °C can be used to prepare Kraft paper samples in the laboratory spanning its full lifecycle in a transformer, from new to end of life. UVC (254 nm) radiation ages only the surface layers of the paper and hence, is a very energy efficient and time saving methodology for producing aged paper samples for flow testing. Significant roughening of the paper surface was observed following exposure to oil flow, particularly in aged samples under flow rates commensurate with reclamation. Despite this, no significant erosion was detected, even in samples which had undergone long-term temperature/flow cycling in an oil containing a significant particulate content. This conclusion, which was verified in a geometry approximating part of a transformer winding, shows that the paper is unlikely to be damaged during normal reclamation activities, even in a highly aged asset.
Two analytical techniques to detect trace levels of S8 in insulating mineral oil have been developed, the analytical methods were optimised and laboratory results that demonstrate the potential of the approaches adopted are presented in the deliverables. An effective laboratory reclamation strategy was developed, with respect to reducing silver corrosion in power transformers, that operates in practical conditions to be used in the field by the respective industry. This strategy demonstrated the possibility of transforming a corrosive sample into a non-corrosive one. This opens the possibility of reusing samples that presented a corrosive risk to the power transformer, without the environmental and economic concerns regarding the disposal of used mineral insulating oil
Thermal Endurance
The degree of polymerisation of paper in both ester types was found to reduce exponentially in the ageing experiments. With synthetic esters, the water content in both the solid and liquid insulation was noted to increase in the early stages of the ageing periods but then decrease to values similar to those found at the start. In natural esters the moisture content in paper also reduced but levelled rather than decreased appreciably in the fluid. Acidity was found to increase continuously with ageing but is believed not to cause paper degradation in esters in the same way as occurs in mineral oil.
End-of-life criteria for insulation systems based on synthetic and natural esters with non-thermally upgraded Kraft paper were obtained by ageing the reference insulation system for time and temperatures obtained from the standard life curve. Using the assumed gradient (from mineral oil curves) the calculated thermal classes (IEEE C57.154) for esters are the same or lower than for mineral oil (thermal class 120), a conclusion that is not credible based on the experimental results. Calculating new gradient values gives significantly higher thermal classes for both esters – at least 140. At this stage the life curves and thermal classes are preliminary results and will be improved by additional temperature points.
Recommendations for further work
Dynamic and larger scale tests investigating bubble inception temperatures would validate the findings from the work in this area. This would support the development of a load-temperature-moisture dynamic model. Such a model would enable the risk of bubble formation to be understood more accurately.
The effect of particulates on flow related erosion of Kraft paper was not studied explicitly (i.e. by adding this material to the oil) due to serious risks of clogging the existing pump/flow meter arrangement. To avoid this, the existing equipment could be modified with a top mounted impeller. This would then circulate the oil over the paper and stir the tank without risk of clogging, and hence permit such studies to be safely undertaken.
Another concern is posed by lapped paper samples where, due to the tensile stresses imposed on the paper by the winding process, subsequent ageing (or drying during reclamation) could cause the paper to fracture and become detached from the windings. Whilst not directly related to flow erosion, and hence outside the scope of the current project, a study of the ageing behaviour of lapped bar samples in air and under oil could be highly beneficial.
Although chemical pathways were identified for the production of ethylene in transformer oil, the link to reclamation was not made and required further investigation to understand the mechanism that causes selective ethylene production which makes health assessments difficult based on DGA.
The calculation of the thermal class for insulation systems based on esters and non-thermally upgraded Kraft paper requires additional data to understand the gradient on the life curves. This project produced three points on the curve and a fourth was planned. Additional points beyond this would be necessary to obtain more accurate gradients.
Lessons Learnt
TRC4
It has been shown that not only the temperature but also the rate of temperature increase is important for moisture bubble formation, which should be considered for transformer loading capability calculations in future. An experimental set-up was established for investigation of bubble formation, moisture in solid insulation is the key source of bubbles and inception temperature reduces with more moisture.
In the experimental set up used for investigating bubble formation, it was found initially that they most frequently occurred at the point where the paper was fixed in place by copper wire. Further investigation showed that this was the hottest point in the cell. With a redesign the bubbles continued to form at the point predicted to be hottest when the fixing was moved further away. In these cases, bubbles formed between paper layers and left the paper at any point, not just where the joins between layers occurred.
In the scenario where mineral insulating oil-filled transformers retro-filled with ester liquids, the total liquid flow rate could be reduced to about 50-60% due to the high viscosity of the ester liquids, which will result in high temperatures within the transformers. Mitigating this may be possible but would require further investigation and may be required on a case by case basis.
Paper samples stored in plastic bags after preparation for ageing studies and at the end of ageing studies were found to contain higher moisture than expected. A separate investigation found that moisture increased the longer they were kept in the bags so glass bottles containing dry oil were used instead as they were shown not to allow moisture in the paper to increase.
Machine Learning (ML) techniques were found to be able to scan large datasets quickly while performing multi-variate analysis. The accuracy of DGA results could be established while trace abnormalities in gassing patterns could be detected. While the application of ML was able to rank transformers based on their DGA results this was based on differences not intrinsically good or bad results. Neither was the technique able to identify fault types. Further work on segmentation and linear regression of individual gases and their relationships with other gases shows promise for enhancing the ability of ML to identify fault types.
Experiments verified the possibility of using CFD to calculate hair – the heat transfer coefficient for air convection, which is a representation of the air domain. The hair equation was obtained and found to be applicable to CFD and CCL CFD simulations.
An optimal radiator CFD modelling strategy has been demonstrated by a parametric study based on radiator CFD models including air. The hair equation compared with average surface temperature can be obtained this has been verified by experiments and compared with empirical equations in the literature.
As the electric field becomes more uniform, a clear transition from discharge propagation dominated breakdown to discharge initiation dominated breakdown is observed and evidenced. Observations of streamers created in LI tests were made using high speed photography. It was found that these images were clearer and better measurements could be made using the Laser-Schlieren technique to produce shadowgraphs
TOPICS2
A detailed examination of the chemical pathways for ethylene production in transformers has shown that there are a number of different radical processes which contribute to the formation of ethylene. This work is enabling an ongoing investigation into the links between the oil reclamation process and ethylene production.
Whilst noticeable morphological changes (roughening) of the paper surface could occur to aged paper under reclamation conditions, no measurements showed any significant erosion, even after exposure to longer-term temperature/flow cycles mimicking reclamation. This shows that normal reclamation activities are unlikely to result in flow related damage to the paper insulation, even in an aged asset.
Two new analytical methods have been developed for the evaluation of sulphur content in mineral oils, with quantification limits as low as 30 ppb, despite a very low measurement time.
A process of selective and total removal of elemental sulphur from transformer mineral oil proved successful in the laboratory. A real transformer oil sample was successfully reclaimed, i.e., the sample that presented corrosive properties towards silver was transformed into a non-corrosive one. The laboratory reclamation process was conducted in a manner that a scale up to be used in the field should be feasible and economically viable.
Thermal Endurance
It has been noted that that waiting time for sampling is an important factor which contributes to the variations of chemical marker concentrations measured in the ageing experiments and ‘when to sample’ for the laboratory ageing experiments needs to be addressed in future IEC/IEEE standard revision.
During the thermal ageing experiments with esters, the acidity was observed to increase with ageing. This has been seen in other studies and in installed transformers. The acidity is not expected to cause premature ageing of paper insulation as is seen with mineral oil. The method for measuring acidity or the interpretation of the results may need to be reviewed for in-service transformers if the test is to have any meaning.
Determination of life-curves (1/Temperature vs Lifetime) enables a thermal class for an insulation system to be estimated according to IEEE C57.154. The equation for determining lifetime (See Equation 1 in the supporting file) requires the gradient of the life curve (B). For mineral oil the gradient is 15,000 and it was initially assumed the same gradient would be found for both natural and synthetic esters. However, the results suggest this may not be the case.
Dissemination
The research progress/findings were/will be disseminated in the following events, workshops and conferences:
- IEEE International Conference on the Properties and Applications of Dielectric Materials (ICPADM), Xi’an, China, 20-24 May 2018
- IEEE International Conference on Dielectrics (ICD), Budapest, Hungary, Jul 1-5 2018
- IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Cancun, Mexico, Oct 21-24 2018
- Cigre UK A2-D1 Liaison meeting, Manchester, 12-13 Nov 2018
- Euro TechCon, Cardiff, Wales, Dec 4-6 2018
- IEEE Electrical Insulation Conference (EIC), Calgary, Canada, 16-20 Jun 2019
- IEEE International Conference on Dielectric Liquids (ICDL), Rome, Italy, 23-27 Jun 2019
- International Symposium on High Voltage Engineering (ISH), Budapest, Hungary, 26-30 Aug 2019
- EuroDoble, Portugal, 20-23 Oct 2019
- Cigre SCA2-B2-D1 Colloquium, New Delhi, India, 18-23 Nov 2019
- Cigre UK A2-D1 Liaison meeting, Manchester, 9-10 Dec 2019
- IEEE Electrical Insulation Conference (EIC), Knoxville, USA, 7-11 Jun 2020
- IEEE International Conference on Dielectrics (ICD), Spain, 5-9 Jul 2020
- IEEE International Conference on High Voltage Engineering and Application (ICHVE), China, 6-10 Sep 2020
- International Conference on Condition Monitoring and Diagnosis (CMD), Virtual Conference, 25-28 Oct 2020
- EuroDoble virtual conference, 20-22 Oct 2020
- IEEE Electrical Insulation Conference (EIC), Virtual Conference, 7–21 Jun 2021
- International Symposium on High Voltage Engineering (ISH), Xi’an, China, 25-29 Sep 2021
- IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Vancouver, Canada, 17-20 Oct 2021
Publications:
- ‘Tracking the progression of sulfur corrosion in transformer oil using thin film sacrificial copper strip’, M.S.Ahmad Khiar, R.C.D.Brown and P.L.Lewin, ICD, Hungary, 2018
- ‘Effect of temperature changes on thin film sacrificial copper strips due to sulfur corrosion’ M.S.Ahmad Khiar, R.C.D.Brown and P.L.Lewin, CEIDP, Mexico, 2018
- ‘Determination of elemental sulphur in insulating mineral oil used in power transformers’, S.B.Garcia; R.C.D.Brown; G.J.Langley; P.Birkin; J.Pilgrim; P.L.Lewin; G.Wilson, Euro TechCon, Cardiff, December 2018
- ’Review of Experiments Investigating Transformer Insulation Condition on Bubble Inception Temperature’, J. Hill, Z.D. Wang, Q. Liu, Ch. Krause and D. Walker, ICPADM, Xi’an, 2018
- ‘Analysing the Power Transformer Temperature Limitation for Avoidance of Bubble Formation’, J. Hill, Z.D. Wang, Q. Liu, Ch. Krause and G. Wilson, IET Journal High Voltage, 2019 (open access)
- “Flow and Temperature Distributions in Disc Type Transformer Windings-Part II: Natural Cooling Modes", X. Zhang, M. Daghrah, Z.D. Wang, Q. Liu, P. Jarman, D. Walker, Applied Thermal Engineering, vol. 165, 2019. (open access)
- “Improvements to the Construction of Bubble Inception Formulae for use with Transformer Insulation,” J. Hill, Z.D. Wag, Q. Liu, S. Matharage, A. Hilker, D. Walker, IEEE Access, Volume 7, pp. 171673-171683, 2019. (open access)
- "Effect of Electric Field Uniformity on Positive Streamer and Breakdown Characteristics of Transformer Liquids", S. Shen, Q. Liu, and Z.D. Wang, IEEE Transactions on Dielectrics and Electrical Insulation, Volume 26, issue 06, 2019
- ‘Investigation of the Total Flow Rates in Oil Natural Transformer Retrofilling Scenarios’, X. Zhang, Z.D. Wang, Q. Liu, A. Gyore, K. Rapp, ICDL, Rome, Italy, 2019
- "Effect of Field Uniformity on Positive Streamer Characteristics of a Synthetic Ester Under Lightning Impulse", S. Shen, Q. Liu and Z.D. Wang, ICDL, Rome, Italy, 2019.
- “Investigation of Alternative Liquids and Ambient Temperature on Transformer Thermal Behaviour”, X. Zhang, M. Daghrah, Z.D. Wang, Q. Liu, P. Jarman, G. Wilson, R. Hooton, L. Dorpmanns, M. Wilkinson, CIGRE SC A2-B2-D1 Colloquium, New Delhi, India, 2019.
- “Effect of Waiting Time on Concentration of Chemical Markers measured on Laboratory Ageing Samples”, S. Y. Matharage, Q. Liu, Z.D. Wang, G. Wilson, Y.H. Kim and R. Martin, ICD, Spain, 2020
- ‘Rapid analytical method for elemental sulphur detection in power transformers insulation oil’, S.B.Garcia; R.C.D.Brown; G.J.Langley; P.Birkin; J.Pilgrim; P.L.Lewin; G.Wilson, EIC, Calgary, June 2019
- ‘Sacrificial copper strip sensors for sulfur corrosion detection in transformer oils’ M.S.Ahmad Khiar, R.C.D.Brown and P.L.Lewin, Measurement, Volume 148, pp 1-6, December 2019
- "On-Line Quantification of Corrosive Sulphur Content in Large Autotransformers", M.S.Ahmad Khiar, S.Garcia, P.L.Lewin, Richard.C.D.Brown, J.A.Pilgrim, G.J.Langley and G.Wilson, IEEE Transactions on Dielectrics and Electrical Insulation, Volume 26, issue 06, 2020
- “Effects of aging on the mechanical and dielectric properties of transformer grade kraft paper”, I. L. Hosier, P. L. Lewin, J. Pilgrim, G. Wilson, EIC, Knoxville, USA, 2020.
- “Observations of Abnormal Gassing Rise After Metal Passivation in Transformer”, T. Herath, Z. Wang, Q. Liu, G. Wilson, S. Tee, CMD, Phuket, Thailand (Virtual conference) 2020.
- “Effect of thermal conduction on transformer radiator CFD modelling”, S.C.Zhao, X.Zhang, Q.Liu, M.Wilkinson, M.Negro and M.Daghrah, CMD, Phuket, Thailand (Virtual conference) 2020
- “Development of a dual-temperature test cell for laboratory ageing experiment of transformer insulation systems”, B. Mebrahtom, S.Y. Matharage, Q. Liu, Ch. Krause, A. Gyore and L. Van der Zel, ICHVE, Beijing, China, 2020.
- “Effect of paper type and water content in paper on the partitioning of 2-FAL between liquid and paper insulations”, S.Y. Matharage, Q. Liu, Z.D. Wang and D. Walker, ICHVE, Beijing, China, 2020.
- “Effect of electric field uniformity on streamer and breakdown characteristics in a gas-to-liquid oil under positive lightning impulse”, H.C. Yu, Q. Liu, Z.D. Wang, Ch. Krause and A. Hilker, ICHVE, Beijing, China, 2020.
- “Quantitative UHPUHPSFC-MS analysis of elemental sulfur in mineral oil via derivatization with triphenylphosphine: Application to corrosive sulfur-related power transformer failure”, S. B. Garcia, J. Herniman, P. Birkin, J. Pilgrim, P. Lewin, G. Wilson, G. J. Langley and R. C. D. Brown, Analyst, 2020, 145, 4782-4786, (open access)
- “Identification of elemental sulfur in mineral insulating oil - Standard corrosive test (DIN 51353) vs. Analytical approach”, S.B.Garcia, P.Birkin, J.Pilgrim, P.Lewin, G.Wilson, G.J.Langley and R.C.D.Brown, ICD, Valencia, Spain, 2020.
- "Shockwave characteristics of streamer propagation in insulating liquids under positive lightning impulse" S. Shen, Q. Liu and Z.D. Wang, IEEE Transactions on Dielectric and Electrical Insulation, vol. 28, issue 02, pp. 493-501, April 2021.
- “Oil flow damage in Kraft paper 1: Thermal aging”, I. L. Hosier, P. L. Lewin, G. Wilson, 2021 EIC 2021. Accepted.
- “Oil flow damage in Kraft paper 2: UV aging', I. L. Hosier, P. L. Lewin, G. Wilson, 2021 IEEE EIC 2021. Accepted.
- “An assessment of the potential for oil flow damage in Kraft paper under normal operation and reclamation in a high voltage transformer, Part 1: Thermal aged paper”, I. L. Hosier, P. L. Lewin, G. Wilson. In preparation.
- “An assessment of the potential for oil flow damage in Kraft paper under normal operation and reclamation in a high voltage transformer, Part 2: UV aged paper”, I. L. Hosier, P. L. Lewin, G. Wilson. In preparation.
- “Removal of elemental sulfur in mineral oil via a system based on trisubstituted phosphine-loaded polymers: Application to corrosive sulfur-related power transformer failure”; S. B. Garcia, J. Herniman, P. Birkin, Neil J. Wells, J. Pilgrim, P. Lewin, G. Wilson, G. J. Langley and R. C. D. Brown, 2021. In preparation.
- “Long term DGA trend evaluation of transmission power transformers”, T. Herath, Z. Wang, Q. Liu, G. Wilson, R. Hooton, S. Tee, ISH, Xi’an, China, 2021. In preparation
- ‘Transformer Thermal Behaviour Comparisons between Conventional Disc Type Windings and A Disc Type Winding with only Axial Cooling Ducts’, X.Zhang , Q.Liu , Z.D.Wang , M.Wilkinson, IET Generation, Transmission & Distribution. In review
- “Development of an Experimental Setup to Study Temperature Distribution of Liquid Naturally Cooled Power Transformers”, Ch. Pößniker, Z.D. Wang, D. Walker, Ch. Krause, A. Hilker, CEIDP, Vancouver, Canada, 2021. In preparation
- “Mach-cone Shockwave Characteristics of Gas-to-Liquid Transformer Oil under Positive Lightning Impulse”, H.C. Yu, Qiang Liu, Z.D. Wang, A. Hilker, A. Gyore, CEIDP, Vancouver, Canada, 2021. In preparation
- “Understanding the Temperature Profile of the IEC TS 62332-1 Dual-temperature Ageing Cell”, B. Mebrahtom, S.Y. Matharage, Q. Liu, Ch. Krause, A. Gyore and L. Van der Zel, CEIDP, Vancouver, Canada, 2021. In preparation
