Degradation of Cellulose Insulationin Liquid-Filled Power Transformers presented by: Thomas A. Prevost EHV-Weidmann Industries, Inc. W-ACTI 2005 Fourth Annual Technical Conference New Diagnostic Concepts for Better Asset Management November 15, 2005 A Member of the Group Title: Degradation of Cellulose Insulation in Liquid-Filled Power Transformers Thomas A. Prevost EHV Weidmann Industries, Inc. Abstract: The life of a transformer is limited to the life of its solid insulation. Many diagnostic techniques are used to assess the condition of the solid insulation. This presentation will give a review of cellulose insulation, both paper and pressboard, used in liquid filled power transformers. The manufacture of paper and pressboard will be reviewed with an emphasis on those critical properties that determine functional life. The degradation process of paper and pressboard will be reviewed including those byproducts of aging that are used in diagnostic analysis. Techniques to prolong the life of the solid insulation will be presented as well. A Member of the Group The life of a transformer is limited to the life of the solid insulation. Much of the diagnostics performed on power transformers is an attempt to determine the health of the insulation system. In order to understand the proper diagnostics to perform and interpret the results of these tests a fundamental understanding of the solid insulation materials is essential. Cellulose paper and pressboard is the most commonly used solid insulation in oil-filled power transformers. A Member of the Group short circuit stress. A Member of the Group .91-1995 “Guide for Loading Mineral-OilImmersed Transformers” Definitions: 3. or mechanical movement.5 transformer insulation life: For a given temperature of the transformer insulation. the total time between the initial state for which the insulation is considered new and the final state for which dielectric stress. and would cause an electrical failure.What is the Life of an Transformer? IEEE C57. which could occur in normal service. 91-1981 criterion) 180 000 20.92-1981 criterion) 65 000 7. oxygen-free 65 °C average winding temperature rise insulation system at the reference temperature of 110 °C Basis Normal insulation life Hours Years 50% retained tensile strength of insulation (former IEEE Std C57.12 Interpretation of distribution Transformer functional life test data (former IEEE Std C57.91-1995 “Guide for Loading Mineral-OilImmersed Transformers” Table 2—Normal insulation life of a well-dried.41 200 retained degree of polymerization in insulation 150 000 17.What is the Life of an Transformer? IEEE C57.42 25% retained tensile strength of insulation 135 000 15.55 A Member of the Group . Materials Critical to Functional Life of a Transformer •Conductor Insulation •Thermally Upgraded Paper •Duct Spacers •High Density Pressboard •Lead Insulation •Crepe Paper A Member of the Group . Critical Properties of Paper and Pressboard that Determine Functional Life •Chemical Purity •Mechanical Strength •Dielectric Strength •Thermal Stability A Member of the Group . A Member of the Group . Cellulose Basics: Part I) Fiber Source Boreal Forest •White Spruce •Black Spruce •Balsam Fir •Hemlock A Member of the Group . A Member of the Group . Chemistry of Wood Wood contains four major substances: ¾ Cellulose ¾ Hemicellulose ¾ Lignin ¾ Extractives For making paper and paper products. A Member of the Group . it is desirable to retain as much of the cellulose and hemicellulose as possible. Most extractives are removed during pulping. Lignin is the “chemical glue” that holds the fiber together. A Member of the Group . Kraft Pulp • Cellulose materials used for electrical papers and pressboard are usually manufactured from coniferous trees pulped by the Kraft process. pressure. • Kraft Process • “Cook” the wood chips using heat. and chemicals (pulping liquors) • Wash the pulp to remove the pulping liquor A Member of the Group . Kappa Number x 0. Softwood is generally cooked to a kappa number of 32 which corresponds to a lignin content of 4.Kappa Number The Kappa number measures the amount of lignin present in a pulp.8% A Member of the Group .15 = % lignin in pulp •Conventional kraft cooking removes 92-96% of the lignin from softwoods. A Member of the Group . A Member of the Group . Handbook for Pulp and Paper Technologists Figure 13-8. A Member of the Group . Photomicrographs of kraft softwood pulp before and after refining (Courtesy of Institute of Paper Science and Technology). BM2 Wet End A Member of the Group View from “Wet End” of 1.4 metre machine, BM2 This is a cylinder machine affording a multi-ply construction of the paper. The machine also features a CLUPAK ® facility, twin head MEASUREX computer control, float drying, size press, and on-line calendering. A Member of the Group Board Machine A Member of the Group Pulp .) Group . 23 (Machine diagram for production of Transformerboard precompressed.Transformerboard Flow Sulfate Pulp Water Mixing Chests Storage Chests Stock Chests Refiners Machine Chest Deflakers Forming Roll Cutter Dryer Sheet Forming Cutting Table White Water Hot Press A Member of the Fig. A Member of the Group . DETC. Bus Bar etc.Transformerboard Mechanical Role • Support Windings During Short Circuits • Maintain Dielectric Clearances • Support High Voltage Leads • Support Auxiliary Equipment - LTC. A Member of the Group . A Member of the Group . Transformer’s Forces Core Outer Winding Inner Winding Radial Forces Axial Forces A Member of the Group . F Clamping Pressure = f(moisture.temperature.age) F transformer winding coil pressboard presspaper copper F rigid clamping distance A Member of the Group . A Member of the Group .>(. >3 Schematic of 550 kV BIL core and coil layout. '0$11 6. Types of Transformerboard * Difference is due to type of final drying • Calendered .Low Density Formable - • Dried Unrestrained Precompressed .High Density - Dried Under Pressure and Restrained A Member of the Group . Characteristics of Transformerboard Physical and Mechanical 25 Hi-Val T-IV 20 15 % 10 5 0 Oil Absorption Compression A Member of the Group . 57 4 .3 1 4 W ith S c re e n P a tte rn 3 2 . Compacting 3000 PSI A Member of the Group .Compression of Radial Spacers Effect of Screen Pattern Material = .059 Inch Thick T-IV 6 5 5.0 5 2 1 1 W ith o u t S c re e n P a tte rn 0 C o m p r e s s io n C o m p r e s s io n S e t Note: Tested in accordance with ASTM D-3394 Bedding Pressure 150 PSI. A Member of the Group .Aging of Pressboard Under Compression Spacer Stack Height (mm) 102 100 98 96 135 Deg. C 94 150 Deg. C 92 90 88 0 50 100 150 200 250 300 Aging Tim e (Days) Effect of aging on the thickness of a stack of Transformerboard. 0% Degree of Polymerization after 250 Days of Aging Inititial Values Aged at 135 C Aged at 150 C 1190 164 152 •Large difference in shrinkage versus Aging Temp. the thickness of the spacer material continues to decline.Shrinkage versus DP Shrinkage after 250 Days of Aging Aged at 135 C Aged at 150 C 4.8% 11. A Member of the Group . •While DP appears to have leveled off at a DP value that would indicate end of life. •Slight difference in DP versus Aging Temp. America. In 1962 NEMA officially recognized TUK in standard TR-11962 by establishing another temperature rise limit of 65 °C for oil-immersed transformers using TUK. Today 65 °C rise transformers are the norm in N. A Member of the Group .Thermal Upgrading of Insulation In the late 1950’s transformer manufacturers developed Thermally Upgraded Papers (TUK). The thermal limit of transformer windings is the insulation on the conductor at the winding hot spot. A Member of the Group . The average winding rise is calculated as follows: Ambient Average Wndg Rise Hot Spot Differential Hot Spot Temperature 55 C Rise 30 55 10 95 65 C Rise 30 65 15 110 * * Only attainable with thermally upgraded insulation. A Member of the Group . dicyandiamide. melamine. Addition of chemicals to protect the cellulose from oxidation: this is primarily achieved with nitrous compounds such as urea.Two types of Thermal Upgrading processes: Modification of the cellulose chains specifically at OH groups by cyanoethylation and acetylation. and polyacrylamide. Cellulose Molecule A Member of the Group . Single Glucose Ring A Member of the Group . Cyanoethylation A Member of the Group Ref. General Electric Company . A Member of the Group . •(ref Lundgaard) •Suppresses the self-catalyzing character of aging process by chemical reaction. •During this process the stabilizing agent is consumed.Dicyandiamide •Chemical Additive to paper.Amine Addition . •Consumes water as it is produced. •Neutralizes acids as they are produced. Aging Curves Aging Curves Thermally upgraded paper Regular Kraft paper (Paper severely aged below this line) Source: Westinghouse/ABB Brochure on Insuldur® A Member of the Group . Nitrogen •All of the various thermal upgrading processes contain nitrogen. •Nitrogen is not found in cellulose Nitrogen quantity is used to determine the amount of thermal upgrading agent added to paper. ASTM D-982/ TAPPI T-418 “ Organic Nitrogen in Paper and Paperboard” A Member of the Group . Different thermal upgrade processes will have different nitrogen content levels to assure sufficient upgrading. •Presently no acceptance test will indicate if thermally upgraded paper is not used. •Currently being considered for IEEE C57.Verification of 65 °C Rise Insulation •Presently there is no clause in the standards which state that the transformer manufacturer must verify that Thermally Upgraded Paper is used.00 •The transformer purchaser needs to specify! A Member of the Group .12. Degradation of Cellulose Insulation Causes: •Moisture •Oxygen •Temperature Effects: •Breakdown of the Cellulose Polymer •Reduced Mechanical Strength •Shrinkage (Under compression) Byproducts: •Moisture •Gas •Carbon Monoxide/ Carbon Dioxide •Acids •Furans A Member of the Group . High Moisture Content in Insulation Can Cause: • Accelerated Aging of the Cellulose • Significant Reduction in Dielectric Strength • Bubble Formation and Dielectric Failure • Partial discharges in the Insulation Dry = Cellulose < 0.5% by weight & Oil < 10 ppm H 2O A Member of the Group . 000 20.000 30.1 80.5 A Member of the Group .9 185.6 93. S.3 81.1 240.2 115 115 132 154 230 Weight of Paper (kg) 453.15 0.000 10.000 16.605.16 0.11 0.1 kg/KVA 0.1 130.8 91.13 0.12 0.4 244.6 1.Paper and Water in Transformers KVA Rating 3.6 132.9 181.D.7 1837 2612.7 23.7 3637.000 Ref.12 5% Initial Moisture Kilograms Liters 22.000 40.8 4808. Meyers KV 13. Moisture Accelerates Ageing Process Ageing acceleration factor 25 20 15 10 5 0 0 2 4 6 8 10 12 Moisture content in paper (% W/W) A Member of the Group . Effect of moisture on Dielectric strength of Insulation 60 Voltage U(kV) 50 x= x= x= x= x= 40 30 20 1% 4% 6% 8% 10% 10 0 30 40 50 60 70 80 90 100 Temperature (°C ) Power factor tan (%) 30 25 x= x= x= x= x= 20 15 10 1% 4% 6% 8% 10% High-voltage insulation systems of Transformerboard must be properly dried and impregnated with oil. The insulation has to be dried because moisture increases the dielectric power factor and increases the risk of thermal breakdown. 5 0 30 40 50 60 70 80 90 Temperature (°C ) 100 110 A Member of the Group . Moisture Promotes Bubble Evolution • Residual moisture in winding insulation can lead to generation of gas bubbles at high temperature • This is the dominant concern in the selection of a limiting hot spot temperature for safe operation • Determinant factors for bubble generation have been identified : – Moisture content in insulation – Hydrostatic pressure – Duration of the high temperature A Member of the Group . V.Generation of gas bubbles at high temperature T. Atlanta. 2001 A Member of the Group . Oommen et al. Brian. Sparling. 2005 A Member of the Group . Tutorial Transformer Insulation Condition Monitoring RVP-AI Mexico.Critical temperature for bubble evolution 190 Kobayashi rapid heating Kobayashi slow heating 170 Temperature 150 Davydov 130 Oommen gas free 110 Oommen gas saturated 90 70 50 0 2 4 6 8 10 WCP % w/w Ref. GE Energy. Diagnostics techniques for assessing the condition of insulation •Moisture of Oil •Dissolved Gas Analysis (DGA) •Degree of Polymerization (DP) •Furans •Power Factor •Polarization Index •Return Voltage A Member of the Group . Norris A Member of the Group .Equilibrium Conditions Water in Oil & Paper 20°C 30°C 50°C 40°C 7 60°C Water in Paper (%) 6 70°C 5 4 80°C 3 90°C 2 100°C 1 0 0 10 20 30 40 50 60 70 80 90 100 Water in Oil (ppm) Ref. (theoretical) Von Guggenberg (theoretical) 60 Sokolov et al. Brian. 2005 A Member of the Group . Sparling. (pressboard) Temperature (°C) 120 Griffin (insulated conductor) Sokolov andVanin (full size transformer) 100 Oommen (distribution transformer) 80 Du et al. (theoretical) 40 FARADAY™ Model approximation 20 0 1 10 100 1000 10000 Diffusion time constant (hours) Ref. Tutorial Transformer Insulation Condition Monitoring RVP-AI Mexico.Diffusion Time Constant on Insulation Material 140 Davydov et al. (winding model) Davydov et al. GE Energy. Arcing When the insulation system is thermally overstressed. Hydrogen from the Oil CO and CO2 from the insulation A Member of the Group . gasses are produced and they will dissolve in the oil.Dissolved Gas Analysis The causes of fault gasses are classified into three categories: 1. Partial discharge 2. Thermal Heating 3. •DP of Insulation Components prior to processing ~1200 •DP of Insulation Components following processing ~1000 •DP level considered as “over-processed” ~800 •DP level considered end of life ~200 A Member of the Group .Degree of Polymerization Measurement of intrinsic viscosity after dissolving the cellulose in a specific solvent. Gives an average measurement of the number of glucose units per molecular chain. failure .shortening of cellulose chains – DP lowered . A Member of the Group .loss of electrical and mechanical strength.by-products contaminate the oil IEEE Transformer Committee Panel Session – October 25. Inc. 2005 Source ABB Power Technologies. and acidic .darkening of color .Effects of aging: .paper becomes wetter. trans. Aging process : Cellulose depolymerization CH2OH O OH OH O OH CH2OH OH OH OH O H O O OH O CH2OH O OH O OH OH CH2OH O OH CH2OH CH2OH O OH OH A Member of the Group . Cellulose Degradation CH2OH H O O H OH H O H H OH Glucose Unit A Member of the Group . Degradation of Cellulose CO HOH CH2OH CARBON MONOXIDE O H O O H H C OH H WATER CHO O HH H H OH FURAN HOH HOH WATER WATER A Member of the Group . 3. 5. 4. A Member of the Group .Furans Most labs determine the concentration of five furanic compounds: 1. 2-furaldehyde 5-methyl-2-furaldehyde 5-hydroxylmethyl-2-furaldehyde 2-acetyl furan 2-furfuryl alcohol (2FAL) (5M2F) (5H2F) (2ACF) (2FOL) Note: 2FAL is stable for years while all other furanic compounds are less stable. 2. They tend to form and then degrade to 2FAL over a time period of months. . A.926 vol.D...3 A Member of the Group .S. 1-5 June 2003 Page(s):921 . Myers. D.Furans Causes of Specific Furan Compounds: Compound Cause 2-furaldehyde (2FAL) 5-methyl-2-furaldehyde (5M2F) 5-hydroxylmethyl-2-furaldehyde (5H2F) 2-acetyl furan (2ACF) 2-furfuryl alcohol (2FOL) General overheating. R. 2003.B. Normal ageing High temperatures Oxidation Rare. Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials. Volume 3. Shkolnik. “Furanic Compounds in Dielectric Liquid Samples: Review and Update of Diagnostic Interpretation and Estimation of Insulation Ageing”. Causes not fully defined High Moisture Ref: Stebbins. Myers on 13 units [4] A Member of the Group .Relationship between 2FAL concentration and DP Source:1999 data from S.D. 2.Furfural vs. DP Correlation Plots A Member of the Group . GE Energy RVP-AI 2005 A Member of the Group . microg/L) 0% 25% 50% 75% 100% Residual Life 10000 VIT ST2 PAL T3 ALK 1-2B ALK 7-8A 1000 ALK 5-6B ALK 3-4B KLY 2RX2 PAL T2 ASH T-1 KLY SP5RX RYL SPT1 100 RLY SPT3 10 MCA TX 200 300 400 500 600 700 800 900 1000 1100 1200 DEGREE OF POLYMERISATION Ref.CORRELATION BETWEEN 2-FAL and DPV 2-FURALDEHYDE (ppb. particles. acids. gasses •Resets the Furan levels •Dry the transformer •Removes moisture from solid insulation •Reduces the clamping pressure on windings A Member of the Group .Techniques to Mitigate the Ageing Process •It is not possible (today) to reverse the ageing of the cellulose insulation •Control (slow down) the ageing process •Remove the catalysts •Moisture •Acids •Oxygen •Process the oil •Removes moisture. Techniques to Mitigate the Ageing Process •Control (slow down) the ageing process •Reduce oxygen •Maintain/Upgrade the Oil preservation system •Membrane in oil conservator •Reduce the temperature •Increase cooling •Control load A Member of the Group . •Measured by Kappa number= low lignin content •High mechanical strength •High Density Pressboard Spacers with Surfaces Milled •Improved compression characteristics= Short Circuit Withstand •Thermally Upgraded Paper •Determined by level of Nitrogen. A Member of the Group .Summary and Conclusion Degradation of Cellulose Insulation in Liquid-Filled Power Transformers •Selection of proper raw materials will prolong insulation life •Pure/Clean cellulose processed with the Kraft process. •Removal of these by-products will slow down the ageing process •Measurement of these by products can also be used to assess insulation life. •The byproducts of insulation ageing are: •Moisture •Gas •Carbon Monoxide/ Carbon Dioxide •Acids •Furans •These by-products are also catalysts for the ageing process. oxygen and temperature.Summary and Conclusion Degradation of Cellulose Insulation in Liquid-Filled Power Transformers • The rate of Insulation degradation is related to the presence of moisture. A Member of the Group . •Include TUK vs Non-TUK A Member of the Group . •Diffusion •Equilibrium •Continue to verify Furan vs DP •Need to measure retired/failed insulation.Summary and Conclusion Degradation of Cellulose Insulation in Liquid-Filled Power Transformers •Future Work •Further development of moisture models. Thank you for your attention Questions?? A Member of the Group .