Electrical Tester online
May 2009
Power transformers – are you covered?

Power transformers – are you covered?

01 May 2009

Damon Mount – UK power sales manager

It’s easy to assume that the substation on your site belongs to the power utility, but are you absolutely sure? If you get it wrong, and you’re unlucky enough to suffer a transformer fault, you could find yourself landed with a bill for tens or even hundreds of thousands of pounds.

In the substation, the power transformers are probably the most expensive items. And that’s not the worst of it. The delivery time for a replacement transformer is typically months, or even years for the largest types. The direct and indirect costs associated with a transformer failure can therefore be enormous.

But there’s surely no need for concern. All of the power transformers on your site are the responsibility of your energy supplier, aren’t they? It may be a very good idea to check again. In a surprisingly large percentage of installations, the power transformers belong to the owner of the premises, and not to the power utility.

Of course there’s still no reason to worry, because transformer failures will certainly be covered by insurance, won’t they? The answer is possibly not. Because of the huge costs involved, insurers are understandably cautious about making pay-outs relating to transformer faults and failures. If there is a claim, they will certainly ask for evidence to show that the transformer has been regularly tested and maintained.

Since many companies are not even aware that they are responsible for the power transformers on their sites, it’s not too much of a surprise that there are a lot of transformers that most certainly don’t get the regular attention they need.

This is a special concern with the many transformers currently in use that have long exceeded their design lives. Although they may apparently still be working well, it is inevitable that some of the materials used in their construction – in particular the insulating materials – will have started to deteriorate.

If an unmaintained transformer fails, whether it is old or new, it’s perfectly possible that the insurers will contest the claim or refuse to pay. Let’s take a look at what needs to be done to avoid this potentially devastating situation.

The first and most obvious step is for maintenance departments to check which of the transformers on their site are their responsibility. The next step is to implement a regular testing programme for these transformers.

But what form should the testing take? There are of course many types of conventional tests that can be applied to power transformers to check, for instance, the performance of the tap changers or the windings.

This means that to build up a reasonably complete picture of the transformer’s condition, a whole battery of tests is needed, which will take a considerable time to perform. During this time, the transformer will necessarily be out of service, which can be very inconvenient.

There are, however, two tests that between them can provide a wealth of information, not only about the presence of faults but also in many cases their type and location. These tests are sweep frequency response analysis (SFRA) and frequency domain spectroscopy (FDS).

Electrically, a transformer is made up of multiple capacitances, inductances and resistances. It is in effect a very complex circuit that produces a unique “fingerprint” when test signals are injected over a range of frequencies and the results plotted as a curve. In particular the distance between conductors affects the capacitances in the transformer.

Movement of the windings, which can be caused by electrical overloads, mechanical shocks or simply by ageing will therefore alter the capacitances and change the shape of the frequency response curve.

The SFRA test technique for transformers is based on comparisons between measured curves which allow variations to be detected. An SFRA test involves multiple sweeps and reveals whether the mechanical or electrical integrity of the transformer has been compromised.

SFRA tests are used to capture a “fingerprint” reference curve for each winding when the transformer is new or when it is known to be in good condition. These curves are subsequently used as the basis for comparisons during maintenance or when problems are suspected.

The best way to use SFRA testing is to take regular measurements on the same transformer over a period time, and to compare the results. However it is also possible to use type-based comparisons between transformers with the same design. Finally, a construction-based comparison can be used in some circumstances, when comparing measurements between windings in the same transformer.

A single SFRA test can detect winding problems that would otherwise require multiple tests with various kinds of test equipment, as well as problems that cannot be detected at all by tests of other kinds.

As a general guide, magnetisation and other problems relating to the core alter the shape of the SFRA curve at the lowest frequencies, up to around 10 kHz. Medium frequencies, from 10 kHz to 100 kHz represent axial or radial movements in the windings, and high frequencies above 100 kHz correspond to problems involving the cables from the windings to bushings and tap changers. In modern SFRA test sets, built-in analytical tools simplify comparisons between curves.

While SFRA tests provide a lot of information about the condition of a transformer, they do not give an accurate

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