Transformer resistance measurement

DC resistance measurement is an important weapon in the armoury of every engineer involved with the testing of power transformers but, to achieve reliable results quickly and safely, it is essential to use an ohmmeter that has been specifically designed for this type of work. What, however, are the key features to look for when buying or specifying an instrument of this type?

Appropriate functionality is, of course, essential but safety must always be the first priority. Let’s start, therefore, with safety considerations. A transformer ohmmeter will most often be used for measuring the resistance of the transformer windings and injecting a DC test current into a highly inductive load is always a potentially dangerous procedure. During the test, the transformer stores a large amount of energy and, if that energy is not discharged in a properly controlled way, arcing is likely to occur with consequent risk of injury to the instrument user as well as damage to the instrument and the equipment under test.

For these reasons, it is essential for a transformer ohmmeter to offer automatic discharge not only at the end of each test but also in the event of a failure of the power supply to the instrument during a test, if the current test leads are accidentally disconnected, or if the test is terminated prematurely. In many applications it is also highly desirable for the instrument to provide a clear warning, such as a bright flashing strobe light, while a test is in progress.

Having examined the major safety issues, let’s now turn to operational requirements. Power transformers, in most cases, have a vital role in the transmission network and taking them out of service is usually costly and inconvenient. This makes it imperative to keep testing time to a minimum and one of the best ways of speeding up winding resistance measurements is to use an ohmmeter capable of delivering high test currents.

The latest instruments can, for example, deliver 50 A DC into windings at a compliance voltage of 50 V DC, which includes the majority of secondary windings in power transformers. This high test current allows tests to be completed up to ten times faster than is possible with traditional 10 A test sets. Another desirable timesaving feature is provision for measuring the resistance of two windings simultaneously which, as might be expected, means that testing time can be reduced by 50% compared with conventional single-winding measurements.

An issue that is sometimes overlooked when carrying out DC tests on transformer windings is that, unless appropriate measures are taken, the transformer core is likely to remain magnetised after the test. This can lead to a number of problems.

If, for example, AC tests are carried out on the transformer after the DC test, remnant magnetism in the core will almost certainly lead to erroneous results. Also, if a power transformer with a strongly magnetised core is returned to service, the inrush current may be excessive and cause an expensive system trip event. In the case of current transformers, magnetised cores can result in improper operation of protection relays. To eliminate these potential problems, good transformer ohmmeters provide integrated facilities for safely demagnetising the core when testing is complete.

While transformer ohmmeters are most frequently used for measuring winding resistance, the best instruments also have another very useful function: verifying the continuity and correct operation of load tap changers (LTCs). These are the only moving parts of a transformer and, like all mechanical devices, they are susceptible to wear and damage. In fact, LTCs result in more failures and outages than any other power system component, so frequent testing to ensure reliable and safe operation is essential.

A transformer ohmmeter can, of course, be readily used to check the winding plus contact resistance for LTCs, but good instruments can do more. With these, if the LTC is operated while the resistance test is being performed, correct make-before-break operation can be verified and any unexpected resistance fluctuations detected. These fluctuations may, for example, be indicative of contact pitting, weak springs or contact misalignment in the LTC, all of which are issues that require attention if future problems are to be avoided.

A good transformer ohmmeter is a versatile tool. It can be used to perform field and factory testing not only of power transformers and LTCs but also of current transformers, voltage transformers and of motor and generator windings. Instruments capable of delivering high currents continuously also provide a useful and convenient way of carrying out heat runs.

Clearly a good transformer ohmmeter is an invaluable addition to any power engineer’s portfolio of test equipment and we have already looked at many of the factors that need to be considered when specifying or buying such an instrument. One important factor remains, however, and that is convenience. Any instrument that’s inconvenient to work with quickly becomes an encumbrance rather than an aid.

Portability is an important factor and modern high current test sets should weigh no more than about 15 kg, especially if they are primarily intended for use on site. A wide operating temperature range is also often important for on-site applications. Other key factors are an intuitive operator interface, a display that’s easy to read in all lighting conditions, and facilities for internal storage of test results for later recall to the instrument’s own display and for downloading to an external computer.

Finally, the instrument should be complemented by easy-to-use test leads. Leads fitted with universal Kelvin clips are particularly convenient since they allow safe and secure connections to be made easily, and they mean that the separate current and voltage leads traditionally used are replaced by a single lead.

At first sight, a transformer ohmmeter may appear to be a specialist tool of rather limited usefulness. As we have seen, however, modern instruments like those featured in the Megger MTO family have much to offer and are an indispensible aid to reducing downtime in power distribution networks, industrial installations and transport systems that employ electric traction.