Moisture Equilibrium and Moisture Migration Within Transformer Insulation System

Transcription

1 Moisture Equilibrium and Moisture Migration Within Transformer Insulation System Tutorial of CIGRÉ WG A2.3 Conveners: Victor Sokolov, Ukraine Maik Koch, Germany 1

2 Outline Introduction: background and definitions Main sources of water contamination Water in oil and in cellulose materials Moisture distribution, equilibrium and migration Mechanism and criteria for bubble evolution Methods for moisture determination 2

3 History Initiator and convener: Victor Sokolov 21 Presentation on the CIGRE A2 website 25 Attempt to restart work 26 Maik Koch took over organization of WG 27 Final version finished Victor Sokolov, convenor, passed away suddenly 28-6 Brochure 349 available at 3

4 Moisture - Frequently Discussed Large population operates under aged conditions Hazardous effects: 1. Decreases the dielectric withstand strength 2. Accelerates cellulose decomposition 3. Causes the emission of bubbles at high temperatures Uncertainty of traditional measurement methods New methods available: Capacitive sensors Dielectric response methods Life expectance / a Dry 1% 2% 3% 4%, Temperature / C 4

5 Many Measures of Water Water content (absolute water content) W Water saturation RS Relative humidity RH Water activity a W Water vapor pressure p Dew point 5

6 Measures of Moisture Water content (absolute water content) W Water mass related to dry mass or mass as sampled Measure: (,5 5) % for cellulose 6 or (1-~8) ppm (mg/kg) for oil 5 Water saturation RS (relative humidity RH) Water vapour pressure p relative to saturation RS 1 % Measure - 1 % p S Information about water availability / activity Important for damaging effects, migration, equilibrium, drying potential Water activity = RH under equilibrium / 1% Moisture in Kraft paper / % Moisture isotherms 21 C 4 C 6 C 8 C Moisture saturation / % 6

7 Outline Introduction: background and definitions Main sources of water contamination Water in oil and in cellulose materials Moisture distribution in transformers Moisture equilibrium and migration Mechanism and criteria for bubble evolution Methods for moisture determination 7

8 Sources of Water Leaky seals Installation, repair Breathing Water from aging Residual moisture 8

9 Sources of Water: Atmosphere Main mechanism through poor seals Typical leaks: top seal of draw-lead bushings, the seals in explosion vents, and leaks in forced-oil circulation systems between the main tank and the coolers Rainwater can be sucked in when there is a rapid drop of pressure Free breathing: significant, but limited up to.2 % per year Leaky seals Membrane-sealed: about.3 to RH.6 % water breather =1% Installation, repair RH air =5% Water from aging RS oil =RS cellulose = 3% 9

10 Ingress due to Breathing 1. Grid transformer GT 2. Grid w. heat recovery HE 3. GSU Rated power (MVA) Oil 2C 11 m³ 45 m³ 1 m³ No. of conservators and breathers 2 / 2 1 / 1 2 / 2 Moisture sensors 1 per conservator and 1 per breather 1 per bre. RH 1

11 Air Volume Oil volume [m³] Oil volume Air volume 2,4 1,8 1,2,6 Penetrating air volume [m³] Change of oil volume and penetrating air volume for GT GT: typ,5 m³/h, max,8 m³/h HE: typ:,15 m³/h, max. 4,2 m³/h GSU:,5 m³/h, max.,1 m³/h Air volume [m³] Time [day.month] Volume of air entering the transformers 2 weeks in February and in May most air enters HE, volume only 45% of that of other transformers GT GT HE Feb HE May GSU GSU MOISTURE Feb EQUILIBRIUM May AND MOISTURE MIGRATION Feb WITHIN May TRANSFORMER INSULATION SYSTEMS Tutorial of CIGRÉ WG A2.3 11

12 Moisture Contamination Water mass [g] Moisture ingress in a period of two weeks Water mass [kg/a] GT Feb GT May HE Feb 16 HE May GSU Feb Water mass [kg/1a] GSU May 4 4 1,5 15,1 2 2 MOISTURE EQUILIBRIUM GT AND MOISTURE HE GSU MIGRATION WITHIN TRANSFORMER GT HEINSULATION GSU SYSTEMS Tutorial of CIGRÉ WG A Extrapolated for 1 resp. 1 years 12

13 Exposure to the Atmosphere Moisture adsorption of 3 mm high-density pressboard at 23 and 4 C at various humidity s of the ambient air Water mainly in the outer insulation components, depth mm Weight increase (%) C, 97 % rh 23 C, 75 % rh 23 C, 5 % rh 4 C, 25 % rh Time (days) Leaky seals Installation, repair RH breather =1% RH air =5% Water from aging RS oil =RS cellulose = 3% 13

14 Cellulose Decomposition Degree of polymerisation new: 12 end of life: 2 Aging causes molecular chain scission and formation of by-products 4 including water and 3,5 furanic compounds 4 scissions ~ 1 % W ater [%] 3 2,5 2 1,5 1,5 Moser Dahinden Fabre Pichon Fallou Chain scissions 14

15 Moisture Concentration over Age of 77 Transformers in DE, PL, SE, REDIATOOL research project Moisture content in cellulose/ % 5 4,5 4 3,5 3 2,5 2 1,5 1, Age / years Moisture 25 C / % 15

16 Preservation Systems Free breathing Self-drying breather Breathing via a desiccant Flexible diaphragm or rubber bag Nitrogen cushion either at atmospheric or high pressure Refrigerated drier Hermetical seal Thermo-electric modules Air Molecular sieve Oil Breather Dehydrating breather for free breathing system Drain tube Refrigerated drier Main oil pipe connection to tank Temporary connection to bottom of tank 16

17 Outline Introduction: background and definitions Main sources of water contamination Water in oil and in cellulose materials Moisture distribution in transformers Moisture equilibrium and migration Mechanism and criteria for bubble evolution Methods for moisture determination 17

18 Solubility of Water in Oil Nonpolar molecules Very low solubility Increases with aromatics, aging products (acids) Moisture Saturation [ppm] Oil 1 Oil 4 Silicone Oommen Aged W S B / T WOil e B oil specific constant Free water droplets if saturation level exceeded Ester liquids dissolve 2 to 4 times more water Temperature [ C] 18

19 Solubility due to Acids Moisture Solubility in Four Service Aged Oils Compared to a Non- Aged Laboratory Used Oil. Neutralisation value = value for all acids / value for water soluble acids [mgkoh/g] Saturation humidity [ppm weight] Non-acidic lab-used Nytro 1X, NV=.5 Service aged, NV=.11/.1 Service aged, NV=.22/.3 Service aged, NV=.34/.6 Service aged, NV=.42/ Temperature [ C] 19

20 Adsorption to Cellulose Polar and therefore hygroscopic Attraction to OH-groups Water solubility ~ 2-fold higher than oil H H H H H O O H H H H O O H H O O H H H H H H H O H O O O H O H O C H H H H O H O OH OH OH OH OH OH OH OH OH monolayer adsorption polylayer adsorption capillary condensation 2

21 Moisture Isotherms Water content (%) Strongly bound monolayer Adsorption Less strongly bound water layers and capillary adsorbed water Desorption Solvent and free water Increasing pressure and/or temperature General moisture isotherm for cellulose material Relative humidity (%) 21

22 Moisture Isotherms Water content [%] ,5 1 Piper Fessler In vacuum In air Water vapor pressure [mm Hg] Water-Paper Isotherms (23 C) Experimental and Based on the Formulas of Piper and the Measurements of Fessler 22

23 Outline Introduction: background and definitions Main sources of water contamination Water in oil and in cellulose materials Moisture distribution, equilibrium and migration Mechanism and criteria for bubble evolution Methods for moisture determination 23

24 Moisture Distribution in the Solid Insulation 1. Thick structures : supporting components, about 5 % of insulation mass, diminutive contribution to moisture migration due to a few years time constants 2. Thin cold structures pressboard barriers, end caps, etc 2 3 % of insulation mass, at bulk oil temperatures, large amount of the water 3. Thin hot structures paper wrapped on conductors close to conductor temperature, relatively dry Clamping Plate Angle Ring Cylinder Spacer Block Paper Wrap around Copper Wire 24

25 Mass and Surface Area of Insulation Structures Mass Thick 43 Thin Hot Thin Cold Thick Thin Hot Area Insulation Structures Classified by Mass and Area for a 4 MVA GSU Transformer 18/347 kv 25

26 Moisture Distribution 125/95 C 1,4/2,1% 27/42 Distribution example: 15 MVA, 7 t cellulose, 7 t Mineral oil, Temperature 4 C cellulose C w = 3 % 21 kg water T+ T 85/65 C 2,4/2,9% 441/115 Oil 16 ppm 1,1 kg H 2 O Temp. Moisture DP Important to know how wet the paper/pressboard is, not the oil! 26

27 Moisture Migration Diffusion time Oil free pressboard Impregnated pressboard Temperature 2 C 7 C 2 C 7 C Time constant 1,7 h,3 h 333 h 6 h Conditions: water content in pressboard,5 %, pressboard thickness 1 mm 3,5 3 2,5 W=2,% T=2 o C W,% 2 1,5 W=1,8% T=4 o C 1 W=1,2% T=2 o C Effect of Temperature on Moisture Distribution in Turn Insulation,5,1,2,3,4,5,6,7,8,9 1 x/d 27

28 Moisture Equilibrium Water potential the same everywhere in the system Moisture diffusion depends on differences in moisture saturation / water vapour pressure / water activity (same temperatures, same pressure assumed ) RS RS Pb p p S RT ln M 1 % RS p W p S Oil RH Air p Cellulose p Oil p Air RS Breather = 1 % a W, Cellulose aw, Oil aw, Air RH Air = 5% RS Oil = RS Cel = 3 % 28

29 Traditional Equilibrium Diagrams Used for determination of moisture in paper: Onsite oil sampling, transportation to laboratory Moisture content determination (ppm) Application of an equilibrium diagram 29

30 Errors Moisture by weight in cellulose [%] ,6 % 3,2 % 2,9 3 2,8 % 2,3 % 2 1,7 % 1 21 C New paper 4 C New paper 6 C New paper 8 C New paper 6 C Aged oil + PB 6 C New pressboard 6 C Aged oil + paper ppm ± 5 ppm Moisture by weight in oil [ppm] Diagrams not applicable! Unless adapted to the cellulose and oil Conditions: 1. Equilibrium exists 2. Temperature- and moisture distribution 3. Sampling, moisture measurement ± 13 % error 4. Results vary for different authors 5. Absorption capacity of oil 6. Absorption capacity of cellulose 1 % error 3

31 Based on Water Saturation Moisture in paper [%] C%P 4 C%P 6 C%P 8 C%P Moisture relative to saturation [%] Onsite and online application Conditions: 1. Equilibrium exists 2. Temperature- and moisture distribution 3. Sampling, moisture measurement ± 13 % error 4. Steep gradient in low moisture region 5. Absorption capacity of oil 6. Absorption capacity of cellulose 31

32 Application for Online Monitoring Moisture in oil is identical to moisture in cellulose if equilibrium exists Equilibrium through long time mean value Type and aging of oil have no influence Oil temperature RS in oil RS in cellulose Time, date Top oil temperature / C Relative saturation / % 32

33 Outline Introduction: background and definitions Main sources of water contamination Water in oil and in cellulose materials Moisture distribution, equilibrium and migration Mechanism and criteria for bubble evolution Methods for moisture determination 33

34 Bubble Evolution 1 layers Kraft paper 3,9% moisture content Inception at 12 C Temperature measured at conductor surface, below paper External player 34

35 Bubble Evolution 1 layers Kraft paper 3,9% moisture content Inception at 12 C Temperature measured at conductor surface, below paper 35

36 Bubble Evolution p in p external p Formation of vapour-filled cavities (bubbles) decrease of the dielectric strength, De-impregnation of the turn insulation Begins at gaseous residues in paper R = R >> r R = r R > r pe pe pe p e p e 2r R = pi p e pi pe p pi pe p ma x. pi p e p pi p e 36

37 Experimental Results Amount of evapourable water in paper Microstructure of paper, depends on aging Gaseous residues in paper Surface tension of oil, depends on polar impurities (aging) and temperature Temperature [ C] New KP new oil TUP new oil Aged KP new oil New KP aged oil Aged KP aged oil Oommen Moisture in paper [%] Moisture in paper [%] 37

38 Risk Estimation Hot Spot is rather dry, has less gaseous remnants Steep temperature rise supports formation Danger limited to areas with high field strength Inception temperature [ C] Temperature gradient [K/min] Large bubbles might be torn by the electric field. "As a conclusion, the risk of a dielectric breakdown due to bubble evolution is considered to be low." 38

39 Outline Introduction: background and definitions Main sources of water contamination Water in oil and in cellulose materials Moisture distribution, equilibrium and migration Mechanism and criteria for bubble evolution Practical experiences Methods for moisture determination 39

40 Dielectric Strength of Oil Breakdown voltage in oil depends on Relative humidity 1 Particles 8 Contaminations (soot, acids ) 6 Temperature 4 Consider sampling temperature! 2 Breakdown Voltage [kv] Particles 5g/t Particles 2g/t Relative Humidity in Oil [%] 4

41 Operation at Cold Temperatures Dissolved water (high relative saturation) Failures after energizing wet transformers in winter time Ingress of free water 4 MVA, 22 kv: breakdown of the oil space between the bushing and the tank Rainfall low pressure inside sucked in about 5 g of water through a broken seal in the draw-lead bushing Rapid cooling of a wet transformer after high loading may result in super-saturation of oil in the cooler and formation of free water 41

42 Classification Based on Relative Saturation CIGRÈ WG A2.3 CLASS I: good : RS below 5%@2 C, mc below 1. % CLASS II: fair : RS below 8%@2 C, mc below 2. % RS of oil < 5% at lowest T CLASS III: probably wet : RS of oil ~ 5% at lowest temperatures CLASS IV: wet : RS of oil up to 1% at lowest temperatures IEC 6422 Saturation / content Dry < 6 / < 2,2 Moderately wet 6-22 / 2,2-3,7 Wet 22-3 / 3,7 5 Extremely wet > 3 / > 5 Moisture content [%] Moisture contamination Dry Moderately wet 21 C 8 C Wet, > 3 % extremely wet Moisture saturation [%] 42

43 Outline Introduction: background and definitions Main sources of water contamination Water in oil and in cellulose materials Moisture distribution, equilibrium and migration Mechanism and criteria for bubble evolution Practical experiences Methods for moisture determination 43

44 Karl Fischer Titration Reference for other methods 2 H 2 O + SO 2 + I 2 H 2 SO HI Measures water content Water relative to weight [µg, %, ppm] Possible errors: Transportation to laboratory Sample preparation Titration system Electrolysis electrodes Measurement of bound water depends on heating temperature and time Sample injection Detection electrode Water Vapour Sample Heating Scattering results obtained by Round Robin Tests 44

45 Capacitive Probes Based on moisture equilibrium Moisture relative to saturation Moisture Saturation[ppm] diffusion Oil 1 Oil 4 Silicone Oommen NN,49 upper porous electrode Ions Water polymer film bottom electrode, glass substrate Hygroscopic polymer film Change of capacity Result: -1 % or -1 aw Possible errors: Diffusion of aging byproducts Corrosion of electrodes Calibration necessary Calculation of ppm (μg/g) by oil specific coefficients C w,s = 28 ppm 2 C w,s = 122 ppm Example: C w,rel = 1%, 4 C New Oil: C w = 12 ppm Aged oil: C w = 28 ppm Calibration to oil essential 45

46 On-Line Application Top oil temperature / C Oil temperature RS in oil RS in cellulose Time, date Long time average for mathmatical equilibrium RS in oil and RS in paper Water content in cellulose by moisture isotherms Relative saturation / % Moisture in aged Kraft paper / % ,2 Aged KP 21 C Aged KP 4 C Aged KP 6 C 4,1 Aged KP 8 C Moisture relative to saturation / % 46

47 Chilled Mirror Dew Point Instruments Dew point of water vapour in gases sample equilibrated within sealed chamber thermoelectric cooler chills the mirror until condensation detected by an optical reflectance sensor relative saturation calculated from sample temperature and dew point temperature Mirror Sample Optical Sensor Sample Gas 47

48 Dielectric Response Methods Dissipation factor at power frequency Polarisation index, tip up test At that time: No reliable method for onsite moisture diagnostics 1991 RVM for water determination Soon questioned by users (Kachler 1996) Polarisation Depolarisation Currents ETH Zurich, Switzerland Frequency Domain Spectroscopy KTH Stockholm 27 Combination of PDC and FDS Universities of Hannover and Stuttgart U U(t) U r,max t c t d t peak U, I U c (t) i pol (t) Dissipation factor T C i dep (t) Frequency t t 48

49 Measurement Voltage source ~ Current meter HV-winding LV-winding Guard Main insulation Tank Main insulation between HV and LV winding will be measured Voltage to shorted HV, current from shorted LV Guard to tank Measurement of: Time domain voltage after charging RV method Time domain current during charging and discharging PDC Frequency domain current and dissipation factor FDS Combination of TD and FD 49

50 Current (na) ( Dissipation factor Interpretation ,1 1,1,1 high low oil conductivity high low moisture of cellulose and aging insulation geometry insulation geometry low I dep I pol high oil conductivity moisture of cellulose and aging Time (s) moisture of cellulose, aging high high low low,1,1,1,1, Frequency (Hz) Time Domain Fast measurement No information for t<1s Frequency Domain Wide frequency range Long test duration Basically same information in TD and FD 5

51 Uncertainties Determination of "correct" insulation temperature Aging by-products increase conductivity, aged transformers may appear moister than they really are. Average moisture content of main insulation indicated Accuracy in most of the cases better than ±.5 % 51

52 Summary Motivation: risks of water contamination, new methods Sources are installation, repair, aging, breathing, leaks Most water stored in cellulose: oil/cellulose = 1/2 Moisture equilibrium is based on relative saturation Bubble evolution at 1 C possible Inaccuracy of ppm-equilibrium Moisture determination by capacitive probes and dielectric response methods Brochure 349 available at 52