PROGRESS IN PAPER DEGRADATION ASSESSMENT IN SERVICE-AGED TRANSFORMERS

Transcription

1 PROGRESS IN PAPER DEGRADATION ASSESSMENT IN SERVICE-AGED TRANSFORMERS H. Ding, R. Heywood, J. Lapworth, S. Ryder and A. Wilson Presenter: Dr Hongzhi Ding Doble PowerTest Ltd UK INSUCON - 17 th May

2 Stating the obvious Why did this happen? And How can we detect it? 2

3 Outline of the presentation Introduction to Transformer Life and Paper Degradation Typical Ageing Factors Limiting Power Transformer s Lifetime Progress in Paper Degradation Assessment Using furans (2-furfural or 2FAL) Using methanol (MeOH) Using dissolved gas analysis (DGA) Using forensic investigation. Conclusions 3

4 Introduction to Transformer Life and Paper Degradation Depends on: Mechanical design Thermal design Dielectric design Electromagnetic circuit Tank and Oils Tap changers Loading Operation 4

5 Introduction to Transformer Life and Paper Degradation Oxygen Insulating Oil Acids are formed by oxidation Heat Autocatalytic ageing circle! Acids Oil Reclaiming Moisture Paper Water is formed by paper ageing Water & Acids accelerates paper ageing Traditional view is that life of power transformer is limited by paper ageing. This is frequently the case, but NOT always. 5

6 Introduction to Transformer Life and Paper Degradation 6 Authority Stage of Life Associated DP IEC New Good Average Aged <350 IEC End-of-life 200 Large utility UK End-of-life 200 Uncertainty in the minimum acceptable value for DP of paper remains!!

7 Typical Ageing Factors Limiting Power Transformer s Lifetime Winding hotspots and damage to winding conductor insulation Partial discharges in windings Creeping discharges on inter-winding insulation Deteriorated transposition joints and localised overheating in windings Corrosive sulphur in oils and copper sulphide contamination Moisture ingress from deteriorated gasket and metal corrosion Loose connections in supporting structures Loose core clamping and partial discharges Loose winding clamping and damage to winding conductor insulation Foreign objects and winding contamination

8 Using furans (2-furfural or 2FAL) Comparsion of 2FAL Trend in Three Single Phase GSU Transformers Degree of Polymerisation DP 2-furaldehyde Concentration (mg/g per g of paper) 2FAL (ppm) A Phase B Phase C Phase Time (Hours) /08/ /10/ /01/ /04/ /07/ /09/ /12/ /03/ /06/ /09/ /11/2016 Sampling Date Emsley, A., Xiao, X., Heywood, R. and Ali, M., Degradation of cellulosic insulation in power transformers. Part 2: Formation of furan products in insulating oil, IEE Proc Sci. Meas. Technol. Vol. 147(3): ,

9 Using furans (2-furfural or 2FAL) Log 10 2FAL A B* DP According to Xue Chendong, A=1.51, B= , which predicts a [2FAL] concentration of 6.5ppm corresponding to a DP of 200. Xue Chendong, Monitoring paper insulation aging by measuring furfural contents in oil, 7th International Symposium on High Voltage Engineering, Dresden, , August

10 Using furans (2-furfural or 2FAL) Example of High 2FAL with Higher Than Indicated DP A 240MVA 400/132kV transformer decommissioned after 49 years in-service, suspected to be significantly in risk of failure from severe paper ageing since for some years the indicated DP being 207 using highest 2FAL 6.12ppm. During scrapping investigation, however, the measured worst DP of 350 from selected winding samples are much better than furan results indicated DP 207 using highest 2FAL 6.12ppm. 10

11 Using furans (2-furfural or 2FAL) Example of Low 2FAL with Lower Than Indicated DP A 1000MVA 400/275kV transformer proactively decommissioned and scrapped after only 19 years in service, believed to be significantly in risk of failure from localised thermal ageing due to suspect manufacturing error causing oil flow restriction, albeit little amount of 2FAL (less than 0.5ppm) had been measured. Marker Lab 1 Lab 2 2FAL (ppm) MeOH (ppm) EtOH (ppm) CO (ppm) CO2 (ppm) During scrapping the measured worst DP was only 161 and much lower than indicated DP being 524 using measured highest 2FAL 0.473ppm and Chendong equation. 11

12 Using methanol (MeOH) Cigre JWG A2.D1.46 (Field Experience with Transformer Solid Insulating Ageing Makers) Measured methanol with a notable increasing trend with transformer in-service age in about 100 service-aged UK transmission transformers which are free breathing, have normal Kraft paper operating at 65K average rise and have been in service from 5 years to nearly 60 years. Wang, Z.D., Liu, Q., Tee, S.J., Matharage, S.Y., Jarman, P., Wilson, G., Hooton, R., Dyer, P., Walker, D., Krause, Ch., Smith, P., Mavrommatis, P., Noakhes, J., Gyore, A and Martin, R., Ageing Assessment of Transformers through Oil Database Analyses and Alternative Diagnostic Techniques, CIGRE SC A2 Colloquium, Shanghai, China, Paper FP0663, 20 th 25 th September,

13 Using methanol (MeOH) Correlating 2FAL with MeOH Marker GSU1 GSU2 Phase A B C A B C 2FAL MeOH EtOH

14 Using dissolved gas analysis (DGA) Thermal Fault Involving Paper Paper covered and bare metal faults can be differentiated by ethylene/ethane ratio, and the ratio <3 indicates winding fault whereas the ratio>3 indicates core fault. The main disadvantage of DGA method for the assessment of paper degradation in service-aged transformer is that it is impossible to establish the location and phase of thefaultifitoccurs. 14

15 Using dissolved gas analysis (DGA) 400/275kV 750MVA transformer increasing ethylene (up to 320ppm) Example of Indicated High Resistance Fault by DGA DGA signature is indicatives a high temperature overheating fault of paperwarped conductor. Levels of hydrogen together with CO as well as CO 2 /CO ratio indicate the fault not within the winding. 15

16 Using forensic investigation DP Data from Scrapped UK Transmission Transformers Worst DP Data of Scrapped UK Transmission Transformers Linear (Data of Scrapped UK Transmission Transformers) Years in Service The variation of paper degradation within these transformers is strongly design dependent! 16

17 Using forensic investigation DP Data from Scrapped UK Generator Transformers Lowest Degree of Polymerisation (DP) Ageing Investigation on UK's Generator Transformers Design 1 Design 2 Design 3 Design 4 Design 5 Design Years in Service Again, the variation of paper degradation within these transformers is strongly design dependent! 17

18 Using forensic investigation Worst Log (2FAL) Correlating 2FAL and DP for Scrapped UK Transformers Data of Scrapped UK Transmission Transformers Xue Chendong Equation (1991) Linear (Data of Scrapped UK Transmission Transformers) Worst DP The Chendong correlation is not unreasonable for the UK data, but tends to significantly underestimate the actual paper degradation for a particular 2FAL concentration. The UK experience so far shows a much lower 2FAL threshold value (~1.5ppm) than suggested by Chendong (~6ppm). 18

19 Using forensic investigation Additional Advantage of Forensic Method Mechanical damage to the paper of a transformer LV winding and discharges between loosened LV winding turns 19

20 Conclusions Measurement and analysis of the oil in in-service transformers for the concentration of paper degradation by-products such as furans, methanol and dissolved gases are good methods for determining the degree of paper degradation, but all these methods is suggestive and should be used with care. The trend in 2FAL, methanol and dissolved gases should be observed, as well as the absolute levels. Results should be compared with as many others as possible, preferably including scrapped transformers where the DP is known. The rates of paper degradation in service-aged transformers are more dependent upon the OEM/design groups than years in service.

21 Conclusions Forensic investigations on sister transformers of the same design are the best method for determining paper degradation and hence residual life. Knowledge from the forensic investigation of failed or scrapped transformers for any reason not only enable the results of oil testing for 2FAL, methanol and DGA as well as electrical condition assessment measurements to be understood correctly, but also provide unique insight into likely end-of-life scenarios for the increasing population of ageing power transformers and therefore offer significant financial and reliability benefits.

22 Thank You for Listening 22