Power transformers: to test or not to test?
Alex Rojas - Product marketing engineer
They’re efficient, dependable and they need little in the way of maintenance; as a result, power transformers often receive little attention or thought. There are even those who would suggest that, as so few power transformers fail in service, there is little point in routinely testing them, especially when staff levels are low and budgets are tight.
Unfortunately, just like people, transformers deteriorate with age, and they can be quickly damaged by excessive stress. Unlike people, however, they don’t complain. In fact, they’re likely to show few if any outward signs of ageing and stress damage. The result is a transformer that appears satisfactory in every way yet may, at anytime, fail suddenly and completely.
And when a power transformer fails, the consequences are costly and disruptive. If the defective transformer is part of a distribution system, consumers will be left without power; if it’s part of an industrial installation, production will be lost; and if it’s part of a rail network, travellers and freight will be stranded.
The cost of replacing the transformer itself is high, but the consequential costs can be enormous, easily reaching hundreds of thousands, if not millions of dollars.
So why do transformers fail? We’ve already mentioned ageing and stress, but it’s worth expanding a little on these issues. All insulating material slowly degrades as it ages, even when the transformer is operating under normal conditions. This means that transformers have a finite lifespan, typically of around 50 years.
ABB conducted a survey and found many transformers currently in service are close to reaching their 50th birthday. They also found that as long ago as 1998, the mean age of power transformers across 39 utilities was over 29 years. It’s most unlikely that this figure has fallen over the last decade.
Even for transformers that are not especially old, it’s important to realize that over-stressing reduces the ‘natural’ lifespan significantly, and results in a high risk of sudden premature failure.
Most transformer related stresses fall into three categories – electrical, thermal and mechanical
Electrical stress develops in the insulating material that separates parts of the transformer at different potentials. This stress ages the dielectric, even at rated voltage. If the ratings are exceeded, even momentarily, the ageing rate can increase dramatically – an over-stress condition lasting just a few milliseconds can reduce the overall life of a transformer by years and, possibly even more important, make it much more susceptible to immediate failure if it is subjected to another overstress event.
Thermal stresses also result in gradual ageing of the insulating material, even at rated temperature. Exceeding the rated temperature once again accelerates ageing, as do repeated temperature changes, even if the transformer always stays within its rated operating temperature range.
Damaging mechanical stresses are most likely to occur during shipping or installation, when a highshock event can deform windings or displace internal components resulting in, for example, reduced creepage and clearance distances.
So, can anything be done to prevent power transformer failures? In truth, it is simply not possible to eliminate all failures, but the number of sudden and unexpected failures can be reduced dramatically. The key is detecting deterioration and damage before it progresses to a full-scale fault. And, since we’ve already seen that failing transformers are unlikely to show any external signs of problems, the only way to detect incipient problems is by testing.
Many types of tests are applicable to power transformers, and all have their applications in particular circumstances. The vast majority of incipient problems can, however, be detected by just three types of test – dielectric power factor (sometimes known as tan delta), winding turns ratio, and resistance. Megger has state-of-the-art instruments that allow these and many other transformer-related tests to be carried out conveniently, quickly and inexpensively.
There is, however, something else to consider. While tests carried out in isolation may produce evidence of a developing problem, such problems are much more likely to be revealed by regular testing and trending of results over time. A sudden change in the trend associated with a given quantity, even if that quantity itself remains well within permissible limits, should always be cause for concern and reason for further investigation.
Unfortunately, routine testing creates challenges of its own. Specifically, these are cost, availability of skilled labour to carry out the tests, and the analysis and storage of results.
The solution is provided by modern measurement technology. Automated instruments cut testing time and therefore costs. They can also be pre-programmed with test sequences, which means that the knowhow required to actually perform the test is much less than that needed with traditional manually operated test sets.
Instruments have, of course, had internal result storage for some time, but the latest models from Megger take this concept much further. With the aid of the versatile PowerDB software package, which on some models is actually integrated with the instrument itself, test results can be loaded directly into reports, along with information such as site and equipment ID numbers.
No matter how tight the budget, or how low the staffing levels, dispensing with routine transformer testing is never a satisfactory option. With modern instruments, testing costs are modest, whereas the cost of not testing can be colossal.
