Transformer Life Extension and Adaptation to New Uses

1. Transformer Life Extension and Adaptation to New Uses Simon RYDER Doble Engineering 2013 Power Forum Istanbul 09-10 October 2013

2. Transformer Life Extension Some features of generator transformers: • Large and heavy • Expensive • Long lead times • Usually durable and reliable But: • Difficult to repair in the field • Faults can be difficult to diagnose • Failure can be catastrophic

3. Transformer Life Extension Failure can be catastrophic!

4. Transformer Life Extension One view of ageing processes affecting transformers (from CIGRE brochure 445)

5. Transformer Life Extension Many ageing process are affected by load, e.g. oil and solid insulation ageing. May be promoted by increased load as a result of plant upgrades. May also be promoted by increased load as a result of lower load power factor. Change from base load to daily starts or peaking should in principal result in reduced oil and solid insulation ageing. Cooler controls may need to be adjusted to respond more rapidly.

6. Transformer Life Extension Oil ageing is reasonably straightforward to manage. Oil ageing can easily be assessed by sample testing. Oil can be reinhibited, regenerated or changed if necessary. Solid insulation is more difficult to manage. Taking samples of solid insulation for testing is difficult. Can estimate solid insulation ageing from oil or electrical tests. Oil tests in particular are easy, but interpretation is difficult. Cannot renew solid insulation in the field.

7. Transformer Life Extension Severe solid insulation ageing, leading to end-of-life failure

8. Transformer Life Extension Some ageing processes are simply a result of exposure to the environment, e.g. external corrosion. Many power stations are at very unfavourable locations for external corrosion, so this may become a serious problem on older transformers. It can be prevented and sometimes also reversed by correct maintenance. Many generator transformers are fitted with compact heat exchangers. These can also suffer from corrosion and fouling, which reduced their effectiveness leading to increased oil and solid insulation ageing.

9. Transformer Life Extension Transformer failure owing to severe external corrosion leading to water leak and short-circuit inside cable box

10. Transformer Life Extension Transformers are fitted with a wide range of accessories, some of which are also subject to ageing processes, e.g. indicators going out of calibration, tapchanger contact wear. On older transformers, consumables may be difficult to obtain. On older transformers, site staff may lack or loose the necessary skills to perform specialised maintenance. Rare and obsolete models of tapchanger are a major maintenance challenge in some markets. Errors in maintenance and repair can result in failure.

11. Transformer Life Extension On-load tapchanger failure caused by error in maintenance

12. Transformer Life Extension Some practical tips: • Check oil and solid insulation ageing regularly through oil and electrical tests • Check correct cooler and breather function, adjust cooler settings if necessary • Maintain exterior in good condition • Ensure adequate supply of spares and consumables are available • Ensure all maintenance is made correctly, using specialist contractors if required