Transformer Oil Analysis and Health Index

Oil analysis for transformers offers an effective condition assessment tool to enable effective and economic asset management of the equipment. On going oil analysis can identify performance problems, maintenance, refurbishment and end of life requirements.

Working with many owners and operators of high voltage equipment, EA Technology has developed a process for deriving and populating health indices for a wide range of HV assets. The intention is to combine relevant information in order to provide a means of ranking equipment by proximity to end of life. The final number for each piece of equipment is normalised onto a scale of 0 to 10, 0 representing the best condition and 10 the worst condition. The detailed formulation of a health index for each population of equipment is specific to that population, based on the available condition information and the background history/operation of the units in the population. The formulation is designed such that increasing values represent increasing levels of degradation and probability of failure. However, this is not a linear scale. By design the relationship between the health index and the probability of failure is exponential. Individual items of equipment with a health index of 7 indicate ‘poor’ condition with a significantly increased probability of failure that will continue increase relatively fast in the short term.

Asset in ‘good’ condition (HI less than 4) would be expected to continue to operate satisfactorily for the foreseeable future (to have a long remnant life) and do not require any significant change to existing operation and maintenance.

Assets in ‘moderate’ condition (HI 4-7) are not at immediate risk but may become increasingly unreliable in the medium term (5-10years). Assets in this condition are potentially candidates for life extension measures, enhanced maintenance, refurbishment etc.

Assets in ‘poor’ condition (HI greater than 7) are at risk in the short term, this risk will increase relatively quickly. Significant investment (replacement) is required to prevent unacceptable probability of failure. For electricity network assets this is often effective End of Life (EOL).

Having derived the initial health index, the health index in future years can also be estimated based on the understanding of the degradation processes and the rates at which these proceed. Definition of end of life for assets will vary depending on the application. However, the approach is based on identifying units where the probability of failure is significantly raised, i.e. those with a health index of greater than 7. The actual end of life, and therefore the remnant life, of an existing unit will depend on the level of risk acceptable to the owner. In the present case any transformer with a health index of greater than 7 would be deemed to be approaching end of life.

Benefits

• Maximises the use of available information, including oil analysis results, duty, environmental factors, fault history, age etc
• Identifies transformers that would benefit from life extension measures or replacement
• Accurate indicator of transformer condition identifying units at risk of failure
• Enables informed asset management decisions to be made for transformer populations
• Determines the most appropriate method of managing the transformer population both at present and in future years
• The health index approach offers the initial steps to adopting a condition based risk management (CBRM) strategy.