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TRANSFORMER RESISTANCE CHECKS TRANSFORMED

24 October 2012

With six-winding capability and the ability to use the same lead set for turns ratio testing, users can perform transformer testing in less time and with increased safety, explains Charles Williams, product marketing manager at Megger

Accurate DC resistance checks on transformer windings are an invaluable diagnostic tool that can help with the positive identification of many types of transformer fault, including, for example, internal connection problems, shorted and open windings, and problems with tap changers.

Because of the very high inductance of power transformer windings, ordinary low-resistance ohmmeters are not well suited for carrying out this type of test. For this reason, instrument manufacturers have developed test sets that are specifically designed for transformer winding resistance measurements.

The first winding resistance test sets released to the market had two-winding capability.

So for a traditional sixwinding transformer, the user would climb the ladder, connect their leads to two windings, climb back down and then run the test. They would repeat this process another two times to test all six windings.

Changing a few connections sounds easy, but often it isn’t, as the transformer bushings where the test connections have to be made are, in many cases, relatively inaccessible. As a result, the apparently simple task of changing the connections during the test sequence can be surprisingly inconvenient and time consuming.

To address this issue, a new generation of automated trans - former resistance measurement test sets has been developed with the capability to connect and test all six windings. These new instruments are, of course, also capable of performing three-winding and even single-winding tests, should these be needed.

TIME SAVINGS

With these instruments, the connections to the windings are made only once, before the test sequence is started, and then the instrument automatically measures and stores the resistances of all six windings without further operator intervention. This means big time savings and in particular, the user spends less time up on the ladder and on the transformer, which reduces the chance of a safety event.


The new testers also offer other refinements to aid speedy and convenient testing, including simultaneous winding magnetisation, where both the LV and HV windings are energised at the same time during testing. This technique gives fast and reliable measurements, even in traditionally challenging applications such as those involving large transformers with deltaconnected low-voltage windings.

In addition, facilities are provided for demagnetising the transformer core when testing has been completed, which eliminates the risk of an abnormally high inrush current when the transformer is returned to service.

These demagnetisation facilities can also be used independently of resistance testing, for example to prepare a transformer for SFRA (sweep frequency response analysis) and other transformer testing.

Finally, the most capable of these new transformer resistance measurement test sets offer facilities for internal storage of test results and, in some cases, onboard analysis of the results. These features once again lead to big time savings. In the case of Megger’s new MTO3xx test sets, there is a further advantage – they use the same lead set as the Megger TTR3xx turns ratio test (TTR) sets.

This saves even more time, as the user doesn’t have to go up the ladder to disconnect the winding resistance leads and connect the turns ratio leads.

A NEW GENERATION OF AUTOMATED TRANSFORMER RESISTANCE MEASUREMENT TEST SETS HAS BEEN DEVELOPED


Megger MTO300 series transformer ohmmeters measure all six windings of a three-phase transformer automatically without changing connections. With test current capability up to 10 A, they provide simultaneous winding magnetization to ensure fast, accurate results, and autodemagnetization after test. And, by sharing the same lead set as Megger TTR turns ratio testers, they make transformer testing even more convenient.

Key Points

  • Instrument manufacturers have developed test sets that are specifically designed for transformer winding resistance measurements
  • Some systems offer facilities for internal storage of test results and on-board analysis

 
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