Transformer turns ratio testers: who needs them?

Tony Wills - UK Application specialist

In principle at least, measuring the ratio of a transformer is a straightforward task – simply connect an AC supply to the transformer and measure the voltages on the primary and secondary. This can be done with no more equipment than a suitable AC source and a couple of multimeters.  So why would anyone spend money on a  dedicated transformer turns ratio (TTR) test set? Actually, there are some very good reasons, as we’ll shortly discover.

In reality, the deceptively simple description given above for transformer ratio testing applies only to single-phase transformers. When testing three-phase transformers, things start to become rather more complicated. The same basic technique can be used but, depending of the configuration of the transformer, calculations may be needed to convert the meter readings into useful results. Table 1 shows some of the factors that need to be applied in specific cases.

Further, if the testing is being performed with just two multimeters, it is only possible to test on a phase-by-phase basis, which means that a lot of connecting and reconnecting of the meters and the power source will be needed. Alternatively, if a three-phase supply is available, it might be possible to speed things up a little by using six multimeters, but few test engineers are likely to have so many multimeters readily to hand and, in any case, a test set up using six instruments is certain to be cumbersome and unwieldy. And, of course, testing with such basic equipment is even more complicated if the transformer has multiple taps.

It’s also important to think about the power source that will be used in a basic test set up, particularly when working with three-phase transformers. A three-phase source may be available, but how can it be safely connected to the transformer? And how will it be protected against overcurrent or short circuits that might occur if there’s a problem during testing? In many cases, there will be no easy answers to these questions.

Now let’s move on to the accuracy of the test results. When working with ordinary multimeters, an accuracy of around 0.5% is typically the best that’s likely to be achieved. This may be adequate in some instances, but higher accuracy makes the ratio test a more sensitive tool for uncovering potential transformer problems that may later lead to an in-service failure.

While measuring transformer ratios, it’s also very useful to measure the test current, which will give an indication of the transformer’s magnetising current, and to measure the phase difference between the HV and LV windings, as an anomalous result here may indicate core problems. When using only basic equipment, these supplementary tests are difficult or even impossible to perform.

We’ve seen that a very basic test set up for measuring transformer ratios, while attractive because it uses only standard test equipment, has many limitations in terms of safety, accuracy and convenience. The solution, of course, is to use a dedicated TTR test set, but what benefits can an instrument of this type be expected to offer?

One of the most important of these is safety. A modern TTR test set will apply no more than 80 V to the transformer under test – less for compact handheld single-phase testers. It will be arranged for easy connection to the transformer, and will minimise the need for disconnection and reconnection during testing. For the best three-phase testers, a remote tap controller will be available as an accessory, and this makes testing even faster and more convenient.

A modern TTR will provide high accuracy, 0.1% being typical, and once the transformer configuration has been set up, it will automatically apply the appropriate calculation factors to the measured test results, eliminating the need for tedious and error-prone manual calculations. As would be expected, the instrument will also include provision for measuring test currents and phase differences.

Another major benefit of using a modern TTR is that it will make it easy to handle the data produced by ratio testing. This may not immediately seem to be a significant issue, but it’s worth remembering that for every winding there will be a ratio, current and phase measurement and, for a three-phase transformer the number of results is multiplied not just by three, but also by the number of taps – and it’s possible for a transformer to have as many as 32 taps. Ratio testing does, therefore, produce a lot of data!

A good TTR test set of modern design will store and organise all of this data, and populate the appropriate forms while the test is being carried out, producing a result like that shown in Table 2. Not only does this automatic handling of data save time and eliminate the risk of transcription errors, it also means that the results can be quickly assessed before leaving the site to ensure that the necessary tests have been correctly completed and that there are no major problems needing urgent attention.

It’s easy to think that transformer ratio testing is so simple that there’s little point in investing in dedicated test equipment. However, as we have seen, the reality is very different, and a good TTR test set will quickly repay it’s modest cost in terms of time savings as well as the usefulness and accuracy of the results it produces.