Assessing Remnant Life of Transformer based on Furan ...

1 Assessing Remnant Life of Transformer based on Furan Content inTransformer Oil and Degree of polymerization of Solid , , Jaspal. S. Wadhwa,The Tata Power Company Limited, Mumbaiemail: - Power Transformers are the most vital components in a sub-station / Receiving station. Failure of a Transformer leads to loss of Revenue besides affecting reliability of power supply to consumers. It can lead to non-availability of the Transformer for long durations. In order to ensure that Power Transformers provide long and trouble-free service, several diagnostic tests are carried out and remedial actions initiated throughout their operational lifetime. For the oil-filled Transformers, more particularly which are in service for more than 15 years, it is advisable that we should also estimate the residual life of the Transformers. It can be done by Assessing the extent of degradation of the cellulosic paper insulation through Furan content in Transformer oil and degree of polymerization of paper insulation.

2 This can help the utilities in making optimum use of the Transformers and also taking timely decisions regarding refurbishment / replacement of Transformers, The paper presents the real life case study of Assessing the health of cellulosic paper insulation of a MVA Power Transformer , which is in service for the last 31 years. I. Introduction Power Transformers are the most vital and costliest components of Electrical Power system. In order to ensure reliable and economic power supply, it is essential that we should utilize the installed transformers optimally. At Tata Power, we have detailed Condition Monitoring plan for Power Transformers, in order to assess the condition of windings, insulation, core, Transformer oil. The condition Monitoring Tests includes IR, Tan-delta, Mag-Balance, Winding resistance, Turns ratio, Bushing Tan-delta, DGA, Oil BDV, etc.

3 About 4 years back we have also started carrying out SFRA (Sweep Frequency Response test). We have also installed COPS (Constant Oil Pressure system) on most of the Transformers, which separates the Transformer oil from the atmosphere, preventing gas / moisture contamination of oil. Though the life of a Transformer is generally accepted to be 25 to 30 years, at Tata Power about 40 % of our Transformers are in service for more than 25 years because of the elaborate condition Monitoring process. Since a large population of the Transformers is in service beyond 25 years, it was felt that we should also assess the residual life of the Transformers, so that appropriate and timely action can be initiated for their refurbishment / replacement. This would also help us in avoiding unforeseen tripping of any Transformer , which could result in interruption of power supply.

4 As a first step, the Furan content measurement for all the Power Transformers was carried out in 2006. The Total Furan content of a MVA Transformer at our Saki s/s was the highest 1226 ppb, indicating that the paper insulation has deteriorated. This Transformer has been in service since 1977. The Total Furan content of the Transformer was monitored since then. It increased to 1404 ppb within the next 4 months. In Feb 2008, when the Test was carried, the Furan content had increased to 8530 ppb. The Furan content in the Transformer oil had reached 12739 ppb by April 2008. This set the alarm bell ringing. Reference [1] indicates that in case the total Furan content is more than 2500 ppb, the winding needs to be replaced / rewound. We also carried out FDS (Frequency Domain Spectroscopy) Test, in order to ascertain the moisture content in the solid insulation.

5 The moisture content in the solid insulation was found to be %, which was on the higher side. As the Furan content in oil had increased at a rapid rate, the moisture content in solid insulation was high and the Transformer was in service for 31 years, we decided to carry out the DP test in order to ascertain the extent of degradation of solid insulation. II Transformer Details and History The MVA, 110 KV/ KV Transformer Sr. No: 23976, manufactured in 1976, has been is in service at our Saki s/s sine Dec 1977. The Transformer is provided with Off load Tap-changer. The Transformer was up rated from 60 MVA to MVA in April 1985 with the installation of oil pumps and additional radiators. In Nov 1993, Transformer oil was replaced. In March 1997, B phase Tap changer assembly was replaced and A and C phase Tap changer moving contacts were replaced as they were damaged.

6 In March 2004, the conservator was replaced with COPS. During 1983-1995, the above Transformer was 90 % loaded for 4 hours daily on week days and the Max. winding Temperature reached 90 deg C. During 1995-2006, the Transformer was loaded upto 50-60 % of its rated capacity during peak hours. The peak load progressively increased thereafter and in the Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008508summer month of May 2007, the peak load was 80-85 %of its rated capacity for 4 hours. However the minimumload on the Transformer dropped to 20 MW during thenight. The maximum and minimum winding temperatureswere recorded as 90 deg C and 55 deg C Analysis and DiscussionThe Diagnostic Tests were done regularly for the aboveTransformer and no abnormality was DGA of Transformer oil was done regularly and noabnormality was indicated till Oct 2007. The values of thedissolved gases are tabulated in Table I Dissolved gases in Transformer Oil (ppm)13/09/0622/10/0708/04 + % v/vin DGA result of April 2008 indicated generaloverheating involving cellulose Furan Content in the Transformer oil was measuredfor the first time in 2006 and it was monitored since then,as the values were the highest amongst all the PowerTransformers in Tata Furan Content details are tabulated in Table II Furan Content in Transformer Oil (ppb)

7 Aug06 Jan08 Feb08 Apr085- Hydroxymethyl-2 Furfural432814002-Furfuryl Alcohol1326002-Furfural11271308839012739 2-AcetylFuran 14110 05-Methyl 2-Furfural293100 Total Furan content12261404853012739 The Furan content above 2500 ppb indicates that the solidinsulation has the Transformer was in service for more than 31 years, it wasnecessary that we should evaluate the Remnant life of the solidinsulation and plan further suitable action for its replacement /refurbishment. We decided to carry out the DP test to ascertainthe extent of degradation of the paper DP test involved availing the outage of Transformer for 10days. The peak load of our Saki s/s was in the range of 173 MVA, whereas the firm capacity was only 145 theFuran content in oil had reached such alarming levels andmoisture content in solid insulation was also high, we could notafford to wait any longer. The failure of Transformer wouldhave resulted in load shedding of about 33 MVA load duringpeak hours.

8 Hence we decided to carry out the DP test in the lastweek of April 2008. We had to transfer 25 MVA load of RELand 8 MVA load of TPC-D for availing the outage of TheTransformer for the DP Transformer oil was drained, bell tank lifted and papersamples were taken from 25 different locations for the DP paper samples from top 2-3 layers of the winding leads andthe pressboard samples were taken for the measurement of DPfrom various locations. The winding leads were retaped with thefresh paper insulation, from wherever the paper samples werecut. The sample locations and test results are given in Table IIIT able III DP Test sample taken fromDP value1 Rphasewinding,Toplead(Outer layer)7672R phase winding, Center lead(Outer layer)5153R phase winding, Center(Innermost layer)8214 Barrier board between R & Yphase (Top)7045 Barrier board between R & Yphase (Center)5566 Barrier board between R & Yphase (Bottom)5397Y phase winding, Top lead(Outer layer)7828Y phase winding, Bottom lead(Outer layer)3439Y phase winding, Bottom lead(1stinner layer)86810Y phase winding, Bottom lead(Innermost layer)89211 Old barrier board between Y &B phase (Top)124412 Old barrier board between Y &B phase (Center)603 Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December sample taken fromDP value13 Old barrier board between Y &B phase (Bottom)71914 New barrier board between Y&Bphase(Top)131815 New barrier board between Y& B phase (Center)119316 New barrier board between Y& B phase (Bottom)

9 101717B phase winding, Top lead(Outer layer)45618B phase winding, Center lead(Outer layer)46919B phase winding, Center lead(Innermost layer)89020B phase winding, Bottom lead(Outer layer)47321B phase winding, Bottom lead(Innermost layer)764 Note: DP values are based on the viscometric average DP value of paper insulation is 645,indicating normal ageing of the paper insulation. Theaverage DP of pressboard is 877, indicating good conditionwith minimum life according to the Table IVTable IV Furan , DP and % of Remaining life2 FAL(ppb)DPValueEstimatedpercentage ofremaining lifeSuggestedInterpretation5880010013070 09029260079 NormalAgeing Rate65450066146440050172038046202136042 AcceleratedAgeing Rate237434038278932033327730029 ExcessiveAgeingDanger Zone385128024452426019 High Risk ofFailure5315240136245220773372000 Endofexpected lifeofpaperInsulationandTransformerTable III is extracted from An Introduction to the Hal-CenturyTransformerby theTransformerMaintenanceInstitute, Co.

10 IV is converted into graph DP vs Remnant life for findingout the intermediate values by interpolation. The graphs is givenbelow,The average DP value of thin insulation (paper) was 645,corresponds to remaining life of nearly 82 %, as per theabove DP value of pressboard was 877, indicatinggood condition of the pressboard with minimum lifeconsumption. This has given us an assurance that the solidinsulation has not deteriorated much and the Transformer cancontinue to remain in the solid insulation had not deteriorated, the high Furancontent in oil could be traced to a damaged pressboard, whichwas replaced along with the Tap changer contacts in 1998. Themoisture content in the Transformer oil was ppm, whichmight have also contributed to the high Furan have filtered the oil and filled the Transformer oil undervacuum, which has reduced the moisture content in , the moisture content in the solid insulation ofTransformer was %.