1. S. Bhattacharya Manager - Substations Department POWER TRANSFORMER REFURBISHMENT

2. Power Transformer is one of the few vital and single most costliest equipment in any T&D network Hence keeping Power Transformer in good health and extending its service life have always remain challenging tasks to the professional engineers, specially in today's regulatory regime.

3. Refurbishment It is a major overhauling work to restore a transformer to healthy condition. Sometimes, it also involves some degree of upgrading / uprating. To maximise service life and to enjoy consequent financial benefits and reliability.

4. Major overhauling involving core & winding inspection, thorough washing and sludge removal Re-clamping of winding Drying out Gasket replacement Corrosion treatment and painting Oil reconditioning or replacement Terminal bushing replacement, if required OLTC replacement, if required Refurbishment work usually comprises different combinations of

5. Refurbishment drive Condition assessment Condition improvement

6. Condition assessment, through Oil condition monitoring test, once a year Insulation Resistance measurement, once in 3 years

7. Condition improvement, through On-line filtration Oil replacement followed by on-line filtration Drying out followed by filling with new oil On-site By external heating and vacuum pulling By hot air blowing and impedance heating At repairers’ works In vacuum drying oven / autoclave Replacement of terminal bushing, OLTC etc. as and when required.

8.  Removal of sludge, foreign contaminants, moisture extraction and degassing of oil  Moisture removal from solid insulation (most difficult task in the drive) Condition improvement of solid and liquid insulation effected by

9. Refurbishment work prompted Mostly by poor health Sometimes by a suspected problem Sometimes by age Sometimes by an intended relocation Sometimes along with repairing

10. Poor oil condition but with acidity < 0.5 and satisfactory / moderate IR values – on-line oil filtration Both oil condition and IR values are low – attempts are made for condition improvement through on-line oil filtration Poor IR values and oil condition, specially with acidity >= 0.5 – oil replacement followed by on-line oil filtration Refurbishment Strategy

11. IR very poor and IR improvement through oil replacement is marginal / not sustainable – on-site drying out and oil replacement During resiting, transformer with very low IR values or aged / last refurbished 20-25 years back – drying out at repairer’s works and oil replacement While carrying out any repair work – drying out and oil replacement

12. Achievement Power Transformer refurbishment is being carried out since 1962 and now-a-days, on an average, refurbishment of 5/6 transformers are carried out each year. Out of a current population of 214 nos. of power transformers, 40 transformers are serving over 50 years and 23 transformers are in the age group of 35 to 50 years as against common life expectancy of 35 years.

13. Life extension on Refurbishment Supports 0102030 40 5060 Years Borrowed LifeEstimated Life 35 Barrackpore D/S T2 Barrackpore D/S T3 P. St. D/S T2 Canal D/S T2 Sinthia D/S T1 Rep. & Drying Out Drying Out Oil Replacement Oil Filtration 23285459 532428 15183944 53 1322334144 20263343 45

14. Future Pathway Objective : Life extraction from the present day transformers to that extent as achieved in case of transformers of yesteryears. Through : 1. Using means for reducing oil deterioration, such as Air-bag fitted conservator, Thermosyphonic filter and insulating oil conforming to stringent specification.

15. Future Pathway Objective : Life extraction from the present day transformers to that extent as achieved in case of transformers of yesteryears. Through : 2. Carrying out periodical oil maintenance, guided by traditional means of oil and transformers’ IR monitoring.

16. Future Pathway Objective : Life extraction from the present day transformers to that extent as achieved in case of transformers of yesteryears. Through : 3. Taking care of any abnormalities in their nascent stage, practicing transformer monitoring through DGA.

17. Future Pathway Objective : Life extraction from the present day transformers to that extent as achieved in case of transformers of yesteryears. Through : 4. Attaching much more importance to insulation condition improvement drives, timely, such as oil filtration / oil replacement / drying out, most importantly based on health monitoring and residual life assessment adopting state-of-the-art tools / techniques.

18. Conclusion Refurbishment is an essential requirement to extend / maximise service life of transformer State-of-the-art techniques / tools for condition monitoring should be practiced for remaining life assessment study and to select right transformer for refurbishment at the right moment Cost-benefit analysis should be made to ascertain viability of the refurbishment work