Keeping transformers online

Many of the transformers currently in use in the UK distribution network have long exceeded the service period for which they were originally designed. Though most continue to give reliable service, there is no doubt that the risk of unforeseen failures is increasing. Regular testing can help to reduce and manage this risk.

Transformers are undoubtedly among the most reliable and efficient pieces of equipment in any electrical distribution network, but after many years of service, the likelihood of failure inevitably increases. And, when failures do occur, the consequences can be expensive, with power utilities facing the costs not only of having to source and fit a replacement transformer on a very short timescale, but also the possibility of fines for leaving customers without supply.

Even though a transformer may have been well manufactured using high quality materials, as it ages there is no way of accurately predicting when a failure will occur, unless up-to-date information is available about its condition. Hence it is important, particularly with older transformers, to implement a regular programme of electrical performance testing.

Since all of the transformer’s component parts are susceptible to failure, a comprehensive battery of tests is needed to gather all of the necessary condition information. But how often should regular inspection and testing be carried out on transformers?

It is difficult to give a definitive answer, as much depends on the age of the transformer and its importance in the network. As a starting point, a testing interval of between one and five years is generally recommended. However, the testing techniques described later are ideal for plotting condition trends in transformer condition, and any adverse developments revealed in this way can be used to trigger more frequent testing.

Let’s now turn to the types of tests that can readily be performed on transformers.

One of the most useful is TTR (transformer turns ratio) testing. The turns ratio can be accurately determined by measuring the ratio of input and output voltages under no-load conditions. A greatly reduced primary voltage is used for this test for reasons of safety and convenience.

This form of test is designed to identify problems with shorted turns, one of the most frequent causes of transformer failure. It can also reveal other problems, such as faulty tap-changer or incorrectly set tappings.

Turns-ratio testing can be carried out on both voltage and current transformers, although care must be taken to avoid core saturation in current transformers that have a low knee point voltage. Problems can also arise when measuring CTs because their turns ratio is normally very high. It is, therefore, important to ensure that the test equipment used has appropriate measuring capabilities.

Oil is commonly used in transformers, both as an insulator and as a coolant. Routine testing of the oil provides an excellent indication of the overall condition of the transformer and facilitates failure prediction and fault diagnosis. There are many different tests that can be performed on the oil, including dielectric strength, power factor, resistivity and specialised tests for water and gas content.

The dielectric strength test is a good indicator of the level of contamination, such as water or conductive particles, in the oil, and is easily performed with readily available equipment. The breakdown voltage of the insulating oil depends on the conditions where the test is carried out and the contaminants present. Various standards, including IEC 156, ASTM D1816, BS 5874 and VDE 0370, which detail the applied test voltage, type, size and spacing of electrodes, have been created to ensure repeatability of testing.

Another method of evaluating the condition of transformer oil is Tan Delta testing. This is a highly accurate, if somewhat under-used alternative to dielectric strength testing. The test is carried out at a much lower voltage, as it does not attempt to break down the oil sample; instead it measures the ratio of resistive to capacitive current flows.

Since the test is carried out at power frequency and since it also measures the resistive current, it is possible to determine any change in insulation characteristics with a greater degree of accuracy than with the dielectric strength method.

Long established as a reliable method of insulation evaluation, Tan Delta testing can also be used to periodically check the winding insulation and insulator/bushing insulation of transformers. As Tan Delta testing is carried out at power frequency, it provides a useful indication of how the insulating
materials will react under load conditions.

Another useful test is to measure the DC resistance of transformer windings with a transformer ohmmeter. If tests of this type are carried out regularly under defined conditions, they are very good at detecting changes that may indicate an incipient fault. As well as providing this basic functionality, the best transformer resistance testers offer additional features, such as dual measurement and facilities for tap-changer testing.

Dual measurement allows the resistance of the primary and secondary windings of a single or three-phase transformer to be checked simultaneously. This can be a big time saver, especially when testing delta-delta connected windings on three-phase transformers, where the test currents in the primary help to balance those in the secondary, allowing stable readings to be obtained much more quickly.

Transformer ohmmeters that feature automatic surge detection and shutdown systems are also extremely useful for checking the windings and contact resistance on tap-changers with makebefore- break contacts.

The shutdown circuit will be triggered by a voltage kickback of a few microseconds if the tap-changer contacts are opened when the tap-changer circuit is operated through all of the tap positions. This action will check for pitted or misaligned contacts, as the instrument will shut down if either condition occurs.

Transformers are so reliable that it’s tempting to ignore them until something goes wrong. Given the often far-reaching consequences of transformer failures, however, this practice cannot be recommended, particularly when transformer testing can be carried out easily and inexpensively.

In short, the best way to keep transformers on line is to make sure that they’re regularly put to the test!