PROGRESS IN PAPER DEGRADATION ASSESSMENT IN SERVICE-AGED TRANSFORMERSH. Ding, R. Heywood, J. Lapworth, S. Ryder and A. Wilson

Presenter: Dr Hongzhi DingDoble PowerTest Ltd UK

INSUCON - 17th May 20171©2017 Doble Engineering Company. All Rights Reserved 

Stating the obvious

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Why did this happen?

And How can we detect it?

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

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Introduction to Transformer Life and Paper Degradation

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Depends on:• Mechanical design• Thermal design• Dielectric design• Electromagnetic circuit• Tank and Oils• Tap‐changers • Loading• Operation

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Insulating OilAcids are formed by

oxidation

PaperWater is formed by

paper ageing

Moisture

Oxygen

Acids

Water & Acidsaccelerates paper ageing

HeatOil

Reclaiming

Autocatalytic ageing circle !

Traditional view is that life of power transformer is limited by paper ageing. This is frequently the case, but NOT always.

Introduction to Transformer Life and Paper Degradation

6

Authority Stage of Life Associated DP

IEC 60450 New 1000-2000

Good 650-1000

Average 350-650

Aged

• 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

Typical Ageing Factors Limiting Power Transformer’s Lifetime

Progress in paper degradation assessment

Using furans (2-furfural or 2FAL)

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0 1000 2000 3000 4000 5000

Con

cent

ratio

n (m

g/g

per

g of

pap

er)

Deg

ree

of P

olym

eris

atio

n

Time (Hours)

DP2-furaldehyde

0.000

0.500

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01/08/2004 25/10/2005 18/01/2007 12/04/2008 06/07/2009 29/09/2010 23/12/2011 17/03/2013 10/06/2014 03/09/2015 26/11/20162F

AL (p

pm)

Sampling Date

Comparsion of 2FAL Trend in Three Single Phase GSU Transformers

A Phase

B Phase

C Phase

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): 110-114, 2000.

Progress in paper degradation assessment

Using furans (2-furfural or 2FAL)

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DP*BAFAL2Log10

According to Xue Chendong, A=1.51, B= 0.0035, 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 InternationalSymposium on High Voltage Engineering, Dresden, 139-142, August 1991.

Progress in paper degradation assessment

Using furans (2-furfural or 2FAL)

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Example of High 2FAL with Higher Than Indicated DPA 240MVA 400/132kV transformer decommissionedafter 49 years in-service, suspected to be significantlyin risk of failure from severe paper ageing since forsome years the indicated DP being 207 using highest2FAL 6.12ppm.

During scrapping investigation, however, themeasured worst DP of 350 from selected windingsamples are much better than furan results indicatedDP 207 using highest 2FAL 6.12ppm.

Progress in paper degradation assessment

Using furans (2-furfural or 2FAL)

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Example of Low 2FAL with Lower Than Indicated DP

Marker Lab 1 Lab 2 2FAL (ppm) 0.295 0.473 MeOH (ppm) 0.132 0.097 EtOH (ppm) 0.280 0.225 CO (ppm) 642 747 CO2 (ppm) 2490 2576

A 1000MVA 400/275kV transformer proactivelydecommissioned and scrapped after only 19 years inservice, believed to be significantly in risk of failure fromlocalised thermal ageing due to suspect manufacturingerror causing oil flow restriction, albeit little amount of2FAL (less than 0.5ppm) had been measured.

During scrapping the measured worst DP was only 161and much lower than indicated DP being 524 usingmeasured highest 2FAL 0.473ppm and Chendongequation.

Progress in paper degradation assessment

Using methanol (MeOH)

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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 andAlternative Diagnostic Techniques,” CIGRE SC A2 Colloquium, Shanghai, China, Paper FP0663, 20th – 25th September, 2015.

Progress in paper degradation assessment

Using methanol (MeOH)

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Correlating 2FAL with MeOH

Marker GSU1 GSU2

Phase A B C A B C 2FAL 1.522 1.503 1.453 2.467 2.416 2.588 MeOH  1.311 1.322 1.286 1.695 1.724 1.672EtOH  0.203 0.203 0.206 0.448 0.454 0.450

Progress in paper degradation assessment

Using dissolved gas analysis (DGA)

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Thermal Fault Involving Paper

Paper covered and bare metal faults can be differentiated by ethylene/ethane ratio, and the ratio 3 indicates core fault.

The main disadvantage of DGA method for the assessment of paper degradation inservice-aged transformer is that it is impossible to establish the location and phase ofthe fault if it occurs.

Progress in paper degradation assessment

Using dissolved gas analysis (DGA)

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Example of Indicated High Resistance Fault by DGA 400/275kV 750MVA transformer increasing ethylene (up to 320ppm)

DGA signature is indicatives a high temperature overheating fault of paper-warped conductor.

Levels of hydrogen together with CO as well as CO2/CO ratio indicate the fault not within the winding.

Progress in paper degradation assessment

Using forensic investigation

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DP Data from Scrapped UK Transmission Transformers

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0 10 20 30 40 50 60

Wor

st DP

Years in Service

Data of Scrapped UK Transmission TransformersLinear (Data of Scrapped UK Transmission Transformers)

The variation of paper degradation within these transformers is strongly design dependent!

Progress in paper degradation assessment

Using forensic investigation

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DP Data from Scrapped UK Generator Transformers

Again, the variation of paper degradation within these transformers is strongly design dependent!

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Lowe

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egre

e of

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P)

Years in Service

Ageing Investigation on UK's Generator Transformers

Design 1Design 2Design 3Design 4Design 5Design 6

Progress in paper degradation assessment

Using forensic investigation

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Correlating 2FAL and DP for Scrapped UK Transformers

‐1.5

‐1

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Wor

st Lo

g (2F

AL)

Worst DP

Data of Scrapped UK Transmission TransformersXue Chendong Equation (1991)Linear (Data of Scrapped UK Transmission Transformers) 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).

Progress in paper degradation assessment

Using forensic investigation

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Additional Advantage of Forensic Method

Mechanical damage to the paper of a transformer LV winding and discharges between loosened LV winding turns

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.

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.

Conclusions

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