Transformer test - from conditioning to test

Matz Öhlén - Transformer test marketing manager

With an ageing power component population today’s utilities face tough challenges, as equipment failures, with their consequent repair costs and revenue losses, invariably have a major impact on profitability. In particular, the power transformer has become one of the most mission critical components in the electricity distribution network.

There is, therefore, a pressing need for reliable diagnostic methods for use in transformer applications, and this need is driving the world’s leading experts to evaluate new diagnostic technologies that will ultimately improve the reliability and optimise the operation of the distribution network.

One key requirement is to be able to identify “good” units in an ageing population of transformers. Adding just a few years to the expected end-of-life for a transformer means substantial cost savings for the owner. In fact, experience shows unequivocally that replacing transformers on a time-based plan is not a cost-effective option, as the replacement cost for these vital assets can be enormous.

Testing and diagnostic analysis of power transformers are, therefore, of the utmost importance. This is particularly true as it is virtually impossible to make a visual inspection inside a transformer but, nevertheless, internal problems left undetected and unaddressed are often final.

It is worth noting that, for many transformers, the last and only time they were tested was during the Factory Acceptance Tests (FAT), just before they were despatched to site. For a transformer that has been in service for some time, however, it is essential to have accurate information about its condition today, not just data that, in many cases, is decades old.

There are however, numerous test methods available for transformers, so selecting the best combination of tests can be something of a challenge. Detailed guidance on this important issue will be given in future articles, together with an explanation of the benefits of a structured approach to transformer testing and information about specific tests that have wide application. This article provides an introductory overview.

In general, the major standards organisations such as ANSI, IEC and the IEEE divide test methods into three groups: routine tests, type / design tests, and special tests. Many of the tests can be applied to a variety of different equipment, such as switchgear, disconnectors, reactors, cables, motors and so on, but in this article we will deal only with power transformers.

The three test groups just identified can be subdivided into factory/acceptance tests and field tests. The field tests can be further divided into subgroups according to the purpose of the test. The most important of these field test subgroups are commissioning, fingerprinting (benchmarking)/ characterisation, condition assessment and fault investigation.

Factory / acceptance tests are performed to verify that the transformer has been built as ordered, and that it does not have any manufacturing faults. Commissioning tests, usually carried out immediately before the transformer is put into service, verify that it has not been damaged in transit, and that it has been mounted correctly.

The preceding groups of tests are of course, carried out at the start of the transformer’s working life. When it has been in service for several years, fingerprinting/characterisation tests should be performed. These will not only provide information on the status of the transformer but also, and usually more important at this stage, they will provide updated baseline data for comparison with future test results.

Condition assessment should be carried out regularly on all transformers, but regular testing is especially important for those that have already been in service for a significant proportion of their design lifetime.

The results of these tests will enable decisions to be made about whether the transformer can continue in service in its present location in the distribution network, whether it would be preferable to transfer it to a less demanding application, or whether it is in imminent need of replacement. The results can also indicate developing problems that, if promptly and correctly addressed, can extend the useful life of the transformer.

The final group of field tests are those relating to the investigation of faults. The two essential objectives of the tests are to decide whether the transformer is damaged, and whether it can safely be returned to service. If the transformer is found to be damaged, further testing and investigation

Eliminate unnecessary capital expenditure by making full use of modern transformer test technologies will be required to determine if it should be repaired or scrapped, whether repairs are economically justified and, if repairs are to be made, whether they should be carried out on site or in a workshop.

It is interesting to conclude this short introduction to transformer testing by looking at two diagrams (Figures 1 and 2) that clearly show the potential benefits of routine condition monitoring. Curves in each diagram show the transformer’s capacity to withstand operational stresses and the actual stresses to which the transformer is subjected while in service. It will be seen that the withstand capacity, for the most part, declines relatively smoothly throughout the transformer’s life. If it is subject to adverse events such as overloads and short-circuits however, these produce a downward step change in withstand capacity.

In figure 1, the third of these adverse events results in transformer failure, even though the transformer was able to tolerate events of the same magnitude earlier in its life. This failure means that the transformer has to be replaced before reaching the end of its design life – potentially, several years of service have been lost.

Fig. 1

Figure 2 shows the same transformer but illustrates the effects of testing and informed equipment management. In this case, a routine test at the point marked “measure” on the curve has revealed that the remaining withstand capacity is borderline for the application in which the transformer is being used.

Fig. 2

Acting on the test results, actions – which might for example include oil regeneration, degassing or drying – were taken to boost the withstand capacity, and the transformer was also transferred to a less demanding duty. As the curve shows, the result is that the transformer not only continues to operate for the whole of its planned working life, but goes on to provide valuable extra years of service.

In today’s cash-strapped commercial environment, eliminating unnecessary capital expenditure is a business imperative. For utilities, making full use of transformer testing technologies is without doubt, one of the most effective ways in which this imperative can be addressed.

indication of the presence of contaminants – in particular water – in the transformer insulation. Standard tests, such as the widely used Karl Fischer test are of course available for accurately assessing the moisture content of transformer oil, but this is not the whole story.

In fact, it is usual for a much greater percentage of the moisture in a transformer to be held in solid insulation such as paper than is held in the oil. To further complicate matters, the moisture moves between the solid insulants and the oil in a way that is influenced by many factors including, in particular, temperature.

Measuring the moisture content of the oil may not therefore provide dependable information about the moisture content of the transformer’s solid insulation. This is a serious concern, as moisture in the insulation significantly accelerates the ageing process in transformers and it can cause bubbles between windings that lead to sudden catastrophic failures.

To establish the moisture content in the transformer, the second of the tests mentioned earlier – frequency domain spectroscopy (FDS) – could be used. Initially, this may sound a lot like SFRA, as it involves measuring transformer

characteristics over a range of frequencies. This time, however, it’s the dielectric properties of the insulation (capacitance, loss and power factor) that are measured over a range of frequencies, typically from one millihertz to one kilohertz.

These are, in essence, the same dielectric tests that are often carried out at power frequency, but testing at a single frequency provides far less information than is revealed by FDS testing. Unlike single-frequency testing, FDS can for example reliably distinguish between a transformer that is dry but has bad oil and one that is wet but has good oil. In the first case, the oil needs refurbishing or replacing; in the second the transformer only needs drying out.

FDS testing also has other benefits – it can be performed at any temperature, and the test can be completed quickly. Software can be used to calculate the water content in percentage terms, and modern FDS test sets typically provide accurate and detailed results in less than 20 minutes.

As we have seen, regular testing using the SFRA and FDS test techniques provides a reliable insight into the condition of power transformers, but how can this information best be used by the transformer owner?

A short-circuit fault on the transformer may cause unseen damage inside, and a damaged transformer put back into service could fail catastrophically. An SFRA test can be done before re-energising and compared to a reference trace taken while the transformer was in good working order. If the two traces match, nothing has changed and the transformer can be safely returned to service. Carrying out this test takes less than an hour, reducing outage time and saving money.

Ageing, mechanical damage and moisture content can be seen as a change in the frequency response of the transformer over time and may indicate that remedial action such as drying out the transformer is needed to guard against future failures. In other cases, it may show that the transformer is inevitably coming to the end of its useful life, but even then the information is invaluable.

In this situation, it may be possible to minimise the load on the transformer so that it can continue in service until a replacement is obtained. Even in the worst case, there is at least a warning that failure is imminent, which can allow time for contingency plans to be made and put into place.

There is another very valuable aspect of regular testing, which we touched on earlier. Insurance companies are more likely to honour a claim for failure of a power transformer that’s been regularly tested and properly maintained so as to remedy any issues identified by the tests. Such a transformer is, of course, less likely to fail, but if it does there is at least the consolation that the insurers will foot the bill!

Even for those who are aware of their responsibilities in looking after power transformers, regular testing may appear as something of a burden. However, tests with modern instruments can be performed quickly and easily, and they yield dependable informative results. And, if the test regime eliminates just one unforeseen transformer failure that would otherwise have occurred, the effort involved in testing and the cost of the instruments used will have been repaid many times over.