Avoiding unexpected failures - ageing, assessment and management of high voltage oil filled transformers
•Download as PPTX, PDF•
0 likes•92 views
This presentation outlines the ageing process of the liquid and solid insulation in transformers. It also discusses how understanding the extent of ageing in a transformer can be achieved by diagnostic monitoring, including assessing ageing markers in the oil. A number of examples of assets with problems will be presented. asset managers to extend asset life will conclude the presentation.
Recommended
More Related Content
More from NSW Environment and Planning
More from NSW Environment and Planning (20)
Recently uploaded
Recently uploaded (20)
Avoiding unexpected failures - ageing, assessment and management of high voltage oil filled transformers
- 1. Avoiding Unexpected Failures - Ageing, Assessment & Management of HV Oil Filled Transformers Philip Reilly
- 2. PRESENTATIONDETAILS OVERVIEW Ageing process of paper insulation Ageing assessment – Laboratory analysis of oil samples Case studies x 2 Maintaining control of condition / Life extension activities 1 2 3 4
- 3. AGEING PROCESS OF PAPER INSULATION CELLULOSEINSULATIONINTRANSFORMERS • Most economic solid insulation for oil cooled transformers • Polymer consisting of anhydroglucose joined together by glycosidic bonds. • Degree of polymerization (DP) is the average number (n) of glycosidic rings • Thermally upgraded papers have better resistance to loss of physical strength during operation.
- 4. AGEING PROCESS OF PAPER INSULATION CELLULOSEINSULATION–CRITICALCOMPONENTS Paper (conductor wrapping, soft paper bushing) Radial spacers (in the winding) Cylinders and strips (between windings) Corrugated board and cylinder wraps Angle rings, caps, snouts, lead exit insulation Washer ring plates and spacing blocks Clamping rings and plates Crepe paper (lead insulation, screen rings, RIP-bushings)
- 5. AGEING PROCESS OF PAPER INSULATION CELLULOSEAGEINGPROCESS • The ageing of paper insulation limits transformer life expectancy • Temperature, water and oxygen accelerate rate of ageing: • Doubles for every 6-8 o C increase • Free access to air / oxygen reduces expected life by a factor of 2 • 2% water content in paper reduces expected life by a factor of 10 • Some low molecular weight acids accelerate the degradation of cellulose by moisture
- 6. AGEING PROCESS OF PAPER INSULATION TRANSFORMERRELIABILITY • Failure rate < 1% across all voltage classes • Short circuits & lightening strikes more likely cause of unreliability than paper & oil ageing • Substation Tx Failures: • Windings > OLTC > Bushings (80%) • PD, Tracking & Flashover (38%) • GSUT Tx Failures: • Overheating, local hotspot (32%) *CIGRE WG A2.37 Reliability Survey (2017)
- 7. AGEING PROCESS OF PAPER INSULATION PAPERAGEINGANDMECHANICALSTRENGTH • DP of new paper (post dry-out) is approx. 1000 • Paper ageing results in decreasing DP & mechanical strength • DP 200 is considered as end of life • Mechanical strength at DP 200 = 30% of new paper • Reduced ability of transformer to withstand short circuit stress / frequent large inrush currents
- 8. AGEING PROCESS OF PAPER INSULATION OILAGEINGANDPAPERDIELECTRICSTRENGTH • It is also very important to properly maintain the quality of oil throughout the life of transformers. Measurements of paper conductivity in the laboratory show that there is no difference in conductivity between paper with a DP of 1132 and one with a DP of 188. By contrast, the conductivity of paper impregnated with highly oxidized oil has been observed to increase dramatically by a factor of 500,with dielectric failure of paper becoming much more likely than its mechanical failure due to its low DP. • Advances in DGA interpretation. CIGRE JWG D1/A2.47. 2019.
- 9. AGEING PROCESS OF PAPER AND OIL INSULATION HYDROLYSISOFPAPERINSULATION • The presence of water will increase the rate of cellulose degradation • What we want to see: • Oil low water content • Cellulose Insulation < 0.5% • Cellulose insulation water content can increase to 5% during operation • Rate of cellulose ageing x 20 • Hydrolysis creates organic acids that further degrade the cellulose insulation
- 10. AGEING PROCESS OF PAPER AND OIL INSULATION THERMALDEGRADATIONOFPAPERINSULATION • Chemical Ageing Markers • Carbon Dioxide • Water • Furanic Compounds * Furanic compounds may also be produced by hydrolytic degradation
- 11. Ageing assessment – Laboratory analysis of oil samples CELLULOSEAGEING–LABORATORYASSESSMENT • Regular oil testing includes Water Content, Furan Analysis, Acidity, DGA, Particle Count • Pyrolysis, hydrolysis and oxidation of paper insulation produces water • Paper hydrolysis produces 2FAL and 5HMF (Furans) • 5HMF degradation produces acids • Paper oxidation produces acids • Hydrophilic nature of these acids promotes reaction with paper • Carbon oxides and carbonic acid • Cellulose fibres (kV & particle count)
- 12. Ageing assessment – Laboratory analysis of oil samples TransformerCaseStudy-10MVA/66–11kV/YOM1986
- 13. Ageing assessment – Laboratory analysis of oil samples TransformerCaseStudy-45MVA/132–11kV/YOM2008
- 14. Maintaining Control of Condition / Life extension activities OILMAINTENANCE • Limiting the ageing process can extend service life • Reclaiming – acid & sludge • Reconditioning – water & particles • Degassing • Replacement • Oxidation Inhibitor (acid, sludge)
- 15. Maintaining Control of Condition / Life extension activities OILRECLAMATION–LIFEEXTENSION • Remove / reduce soluble and insoluble polar contaminants • Acid removal via Fullers Earth • Oil, windings and cooling system can be cleared of acid & sludge • Oil is returned to ‘as new’ specifications: • Rate of degradation of paper insulation reduced by removal of acid and water
- 16. Maintaining Control of Condition / Life extension activities LIFEMANAGEMENTOFPAPERINSULATION • Inhibited oil • Operating temperature • Moisture / Oxygen ingress • Oil Maintenance • Cellulose Maintenance • CIGRE WG D1.53 – Ageing of Liquid Impregnated Cellulose for Power Transformers (2018)
- 17. QUESTIONS? ANY OIL ANALYSIS REPORTS INDICATE A PROBLEM? PHILIP.REILLY@AMPCONTROLGROUP.COM
- 18. CONNECT WITH US
Editor's Notes
- The areas Iʼd like to address are We don’t have time today to discuss electrical testing and visual inspections. Both are important when assessing aging in transformers. Here I was recently conducting visual inspections on a fleet of transformers manufactured in the late 70’s. Problems we saw included extensive leaks, cracked bushing on a 75MVA GSUT, corrosion of cooling fins, OTI / WTI that had seen better days.
- There’s lots of paper in a transformer. What does it do? • Its an electrical insulator. • Itʼs a physical structure within the transformer. It separates the copper windings, and this is a very important role. Kraft paper is still typically used in most transformers. Cellulose is a repeated chain of glucose molecules. Itʼs a polymer. Degree of Polymerisation (DP) is a measure of the length of these chains. I will talk more about this throughout the presentation. An alternative to Kraft paper is the more expensive thermally upgraded paper. Thermally upgraded paper ages less because it is less sensitive to higher temperature, water and acid in the oil.
- Cellulose plays a very important role in the internal structure of a transformer. Photo on the right is of a core of an Ampcontrol transformer during the manufacturing process. We see the paper covered windings... Clamping structure. On the left Iʼve listed some structural components of items that are included in the winding structure.
- We just seen a photo of windings were the paper insulation is brand new. The photo on the right is of a winding weʼre the paper insulation is severely degraded. Possibly at ‘end of lifeʼ When the paper is at end of life the transformer needs to be replaced or the core rewound. Accelerated ageing of paper will reduce the life of the asset. The cellulose ageing process is well documented. Iʼve listed some interesting statistics about the effects of temperature, water content and oxygen content of the oil upon the rate of degradation of the paper insulation. The effect of moisture is striking. Sources of moisture in transformer oil includes ingress from the atmosphere, oxidation of oil, acid catalysed hydrolysis of paper insulation, and thermal degradation of paper insulation.
- Transformer reliability surveys are published on a regular basis. Here are some ‘headlines’ from this CIGRE survey published in 2017. Survey represent 167k operating years across 21 countries between 1996-2010. Their rate of failure is low, < 1% across all voltage classes. That’s very low but there are thousands of transformers out there. Most are filled with combustible liquids. So short circuits…. [read] But paper insulation at end of life is less able to withstand the physical forces associated with short circuits. The photo on the right is a 25 MVA transformer that was struck by lightening in 2019. The winding was damaged and we did a full rewind at our factory. Read the remaining points.
- When paper insulation ages, the consequence is loss of mechanical strength. When a new transformer leaves the factory, after its paper insulation has been through a dry out process, the DP of its paper should be > 1000. At DP 200, the paper insulation is thought to be at or approaching end of life. Mechanical strength of the paper is thought to be approx. only 30% of new paper. But the paper DP wont be uniformly the same - warmer parts. When the paper is at ‘end of lifeʼ the transformer may continue to operate for years. However, the risk of failure increases because the reduced mechanical strength of the paper insulation is less able to withstand short circuit stress / large inrush currents.
- The electrical insulating properties of the paper are unchanged by the ageing process providing the oil insulation remains in a good condition. The paper insulation is an electrical insulator. But conductivity can increase x 500 were the oil is heavily oxidised and acid levels are elevated. And the risk of a dielectric failure is elevated to equal / higher than failure due to loss of mechanical strength. Important to maintain the oil in good condition.
- Paper hydrolysis is the breakdown of the cellulose by moisture. A transformer should have low oil moisture levels (<10ppm, mineral oil) and water content of the paper should be less than 0.5% by dry weight. If, during operation, this increases to 5%, the rate of ageing of the paper insulation can increase by a factor of 20. Hydrolysis of paper creates acids that become dissolved in the oil and which further degrade the paper insulation. On the right are the results of a study of the effect of organic acids upon the rate of degradation of paper insulation. Note the strong effect of Formic acid upon the rate of degradation.
- Another mechanism for the degradation of paper insulation in a transformer is by elevated temperature - Pyrolysis. A widely quoted statistic in that Rate of ageing of the paper insulation is thought to double every 6-8oC rise in operating temperature. When we test oil samples and interpret data a sign of pyrolysis is when CO2 levels are elevated. The ratio of carbon monoxide to carbon dioxide is important. We also look for increases in water content and furanic compounds that are dissolved in the oil.
- From an oil sample our lab routinely performs these tests to evaluate the condition of the paper insulation. We look for discrete differences in test data to provide clues as to what may be happening to the paper insulation. Pyrolysis, Oxidation and hydrolysis of paper insulation produces water and so we may see increases in water content Side note - Itʼs important to use a 50ml glass syringe to avoid contamination of the sample with atmospheric moisture. Also a good idea to thoroughly flush the drain valve prior to taking the sample.
- Letʼs move on to Life Extension. The paper insulation of both the transformers I’ve just reviewed is in late and early stages of degradation respectively. Limiting the ageing process is a priority for both. This will help extend service life. On the left are ‘Oil maintenanceʼ options that can be considered for this and other transformers, where there are problems with the oil quality. Each of these either maintains or returns the oil in / to a state that doesn’t degrade the paper insulation. This will prevent occurrence of paper degradation and so, theoretically, will extend service life of the transformer. The paper insulation canʼt be returned to an ‘as newʼ condition but the oil insulation can.
- One of the life extension activities I mentioned in the previous slide was oil reclamation. Run through the bullet points. And the fullers earth can be regenerated up to 300 times by heating at 600 degC. Typical composition of fullers earth: SiO2 - 66.1% Al2O3- 12.6% MgO - 9.8% Other metal oxides - 10%
- Over the life of your transformer apply the following to minimise the ageing of the paper insulation to extend the life of the asset. Maintain sufficient levels of oxidation inhibitor in the oil. Monitor and control operating temperature. Ensure Correct operation of OTI and WTI. Ingress of moisture and oxygen can be managed by repairing oil leaks, replacement of gaskets, regular inspection of silica gel breathers, use of a bladder in the conservator. Filter the oil to remove acid and water. Oil can be de gassed to control oxidation of the paper insulation. In regards to the previous transformer, at what point should we intervene to stop any further paper degradation? f you keep the oil insulation in pristine condition the paper insulation degradation seen on the right can be avoided.