Estimation of life expectancy [9].

The evaluation of the life expectancy of a transformer is a key reason for having diagnosis systems. This preoccupation is closely related to the need of the suppliers of electricity to predict the time of replacement in order to maximize the useful life of the equipment, as well as minimizing the risks of failure leading to power reliability problems.

The ultimate question to answer is how many years are left before the equipment has a failure?

The evaluation of the life expectancy is often subject to a number of erroneous interpretations [10, pages 92-98].. First, it is important to define what we agree upon as end of life.

The end of life is attained when the transformer is incapable of fulfilling its functions. Certain organizations distinguish between technical, planned and economical end of life. The tendency is to give too much importance to the technical end of life. It is rare that a transformer is replaced for only technical reasons; the main reasons to retire a transformer from service are related to costs. The operational expenses must be minimized. These reasons are of a planning nature (modification to load profile, voltage changes, etc.)

Second, we should distinguish between the life expectancy of the insulation and that of the transformer. It has often been the case where the transformer was kept in service several years after the insulation was classified obsolete. The life expectancy of the insulation is not that of the life expectancy of the transformer.

The technical life expectancy of a transformer is determined by several factors. It depends upon design, historical events, operating conditions, its actual state and future conditions.

Most of the present methods put too much emphasis on the condition of the insulating material. We could easily appreciate that not only temperature, load and water content have an effect on the capacities of a transformer to fulfill its functions but also the number of short-circuits, over-voltage, design weakness, repairs and moving, etc.. To be able to use a multi-factor evaluation, it is necessary to have an in depth understanding of the interrelations between the internal components. Once this is acquired, the historical information of the transformer will be needed. It is, therefore, important to gather the information as quickly as possible at the time in order to easily access it.

The eternal question is, "How long will my transformer last?". In order to answer this question we have extracted data from a survey of 251 transformers used by small and medium sized industries. From this survey, we have the transformer size and age profile with which we can estimate the life span of your transformer.

In terms of frequency of failure, the transformer is classed among the elements having a relatively high rate of failure, in the electrical power distribution network. The failure rate of an oil transformer is 0.0062 failure/unit year.

A failure rate of 0.0062 signifies that a transformer will have a failure within the next 160 years. Consequently, within a group of 10 transformers, 1 of these will have a problem within the next 16 years. So, the failure rate increases in proportion to the number of transformers. The reader should take into account the component location and how it will affect the downstream electrical distribution when comparing reliability data. More precisely, how will the plant be affected if a particular piece of equipment fails? Will it shut down a critical plant process?

When we take a failure rate and multiply it by the downtime per failure, we obtain the average downtime per year.

The transformers of an industrial class of 600 volts to 15 kV, have a better reliability rate. The category of transformer more than 10 MVA operating at a higher voltage than 15 kV, has a three times greater chance of failure.

Ą strong proportion of failures is related to insulation. The failure of insulation is often caused by the substantial reduction of the mechanical strength of the cellulose, with water being one of the principal agents of deterioration. In this condition, the insulation system does not have the necessary capacity to support the stress which is imposed upon it. This will eventually lead to an irreversible breakdown.

The second source of failure is related to damage created by external short-circuits. About 8 per cent of failures are related to a defect in the protection system. This could be prevented by carrying out a complete verification of the protection system every 5 to 10 years, depending on operating conditions and the immediate environment of the transformer protection network.

20 per cent of transformer failure removals are done manually. This indicates that the use of a prevention program for tracking equipment problems allows for equipment withdrawal from the circuit where the condition is judged doubtful - avoiding unscheduled downtime.

We have noted that the periods most susceptible to failure are those that are related to equipment manufacturing, transport and installation. These problems manifest themselves within the first years after installation. The failures relative to aging start to appear around the twenty-fifth year, according to the surveys.

Based on a failure curve developed from the preceding information, two periods of intense observation have to be implemented. These tracking periods follow premature failure and random failure, as well as when insulation begins aging.

The first tracking period follows problems related to premature failure experienced in the first year of utilization or following a major repair. The tracking starts when equipment is put into service, followed by consistent observation and a tight sampling campaign. This sampling rate is about two to three times the normal frequency since the probability of failure is higher during that period.

The second tracking period is related to random failures or insulation aging. A large percentage of breakdowns of transformers are related to a problem in the insulation system. These problems, generally detectable, are a consequence of a condition that evolved during a certain period. Since the principal element of failure is the insulation system, the latter should receive the biggest part of the maintenance effort. However, the other components should be inspected, but at less frequency intervals. If the transformer is equipped with an OLTC, then greater maintenance effort should take place on the tap changer.

Besides considering these statistics, we must keep abreast of any changes in the transformer market which could affect the reliability of the components. We will need to put in place an information system accessible to the industrial user, which will allow failure causes to be put into perspective.

This information system will be continually updated by the users. This update can be done from a database software specialized for transformers [5] or any other means judged efficient. From the database, new tendencies could be revealed allowing us to adjust our maintenance efforts or to modify future purchasing specifications.

LINKS

1. IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power System, IEEE Std 493-1990.

2. Annual survey by Doble Engineering Company.

3. Kelly J.J., A Guide to Transformer Maintenance Transformer Maintenance Institute, SD Myers, 1981.

4. Report of Transformer Reliability Survey - Industrial Plants and Commercial Buildings, James W. Aquilino, IPSD 80-7.

5. PERCEPTION 4.0, Software for tracking and diagnosis of oil insulated equipment, SEIDEL.

6. Diagnostic du transformateur. Recherche du dysfonctionnement, Mˇthodologie de lÕentretien, Expertise. Michel Bˇlanger, mars 1999.

7. Status and Trends in Transformer Monitoring. C. Bengtsson, ABB Transformer, Ludvika, Sweden, IEEE transaction on Power Delivery, Vol. 11, No. 3, July 1996.

8. R. Sahu, Using Transformer Failure Data to Set Spare Equipment Inventories, 1980.

9. Status and Trends in Transformer Monitoring. C. Bengtsson, ABB Transformer, Ludvika, Sweden, IEEE transaction on Power Delivery, Vol. 11, No. 3, July 1996.

10. IEEE Guide for Loading Mineral-Oil-Immersed Transformers, C57.91-1995.