Estimation of life expectancy of Transformers

Estimation of life expectancy of Transformers :
The evaluation of the life expectancy of a transformer is a key reason for having follow-up and 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 .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. It is implicit that 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 indepth 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.

Figure 4 represents the transformer size profile of the survey. It indicates that most transformers in use by small to medium sized industries are in the 500- 2500 kVA range.

From Figure 5 the variations which we observe are probably due to cycles in the economy. Characteristically, these small to medium sized industries are more prone to these economical cycles.

The decrease in the number of transformers more than fifty years old is probably due to the closing of small and medium sized industries. If the decrease was cause by a mechanism failure, the curve would have been less abrupt. Instead, the decrease would be spread over two decades The author is warning you not to use the curves from that reference for estimating the probable life span of your transformers.
In many countries power transformer kept off from service after completion of 20 or 25 years although with satisfactory test results. It is safe practice.