MWA300 and MWA330A three-phase ratio and winding resistance analysers

Dissolved gases in transformer oils act on the contact surfaces of RA switches and LTCs. Usually, you will note higher resistance measurements on taps that are not used or are used infrequently. You can rectify this apparent problem by operating the switch a few times, as the design of most LTC and RA switch contacts provides a wiping action that removes surface oxidation.

The DC resistance of a winding varies as its temperature changes. For copper windings, the variation is 0.93 % per ºC. Temperature is usually not a significant consideration when comparing phase results in a power transformer because the load on power transformers is generally well-balanced. Similar phase loads mean that the winding temperatures should be very similar. However, when comparing results to factory or previous field measurements, you should expect small, consistent changes. In addition to loading, temperature (and therefore resistance) variations can be due to cooling or warming of the transformer during the test, particularly on large transformers with an LTC where the time between the first and last measurement is often an hour or more. Note that the temperature of a transformer that has been serving load is likely to change significantly during the first few hours with no load. Another cause for temperature changes is the use of too high a test current. When measuring the DC resistance of smaller transformers, you should ensure that the test current does not cause heating of the windings. For this reason, the test current should not exceed 15 % of the winding rating.
 

There are many sources of error, including incorrect or poor connections, use of a defective test instrument or one requiring calibration, incorrect operation of the instrument, mistakes in recording results, and ambiguous or poorly defined test data. Note also that there is often more than one way of measuring the resistance of a transformer winding. Field measurements are taken via connections to external bushing terminals, whereas factory measurement connections are not limited to these terminals. Additionally, internal winding connections can be opened in the workshop or factory, making measurements possible that are not practical in the field. Unfortunately, details of factory test setups and connections are often omitted in test reports, which can lead to confusion when comparing test data.

Generally, yes, you should disconnect the bus from the bushing terminals. A physically isolated transformer eliminates the uncertainty of new electrical paths that you may unknowingly introduce with the bus left connected and increases safety.For example, a TTR test is an open circuit test. Any closed circuit created across the secondary winding during the test will result in current flow through the winding, pushing your ratio results outside the expected limits.A DC winding resistance test is a Kelvin type measurement meaning that you need to measure both current and voltage for accurate readings. Current is injected through the winding, and the voltage drop is measured across the winding section of interest. Using Ohm’s law, resistance is calculated. Suppose you create an alternative path for the current to flow, e.g., by grounding both sides of the transformer at the respective disconnect points of the attached bus. In that case, some of your test current flows through the ground circuit, and your measured resistance will be invalid.We recognise that in exceptional cases, disconnecting the bus from transformer terminals can represent upwards of an 8-hour task. Such an investment in time may not be feasible. Rather than forego testing, we recommend performing these tests with the bus attached while being safety vigilant to your expanded test perimeter. We invite you to review your results with us afterward, as they may be challenging to assess.
 

IEEE C57.152, “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors,” and C57.12.90, “IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers,” both reference the 2 % difference between windings. The guide and the standard are excellent references for transformer testing in general.

Two things can be at play here: core magnetisation or the test voltage. If test voltage hasn’t changed and the excitation current is different from a previous measurement, a difference in the transformer core’s magnetisation is likely to blame. The core can be magnetised in multiple ways. One of the most common is when a DC winding resistance test is performed, and the transformer core is not demagnetised afterward.Excitation current is dependent upon the test voltage used. The MWA uses a maximum of 80 V, whereas a power factor excitation current test is typically performed at 10 kV. Thus, you will not be able to compare values.

If you are performing a commissioning or acceptance test, you should test at all tap positions. In these instances, you would perform tests on each DETC tap while keeping the LTC at its nominal position (e.g., N). Then, you would continue with tests on all LTC taps while keeping the DETC at its nominal position (e.g., C). When investigating a suspected tap changer problem, you should again test all LTC taps while keeping the DETC at its as-found position. However, we generally do not advise performing tests on every DETC position if you have not moved the DETC for a long time. Doing so could damage the DETC, as they can become stuck in place when left in one position for an extended time. For routine wellness tests, you should check all the positions of the LTC in one direction and the first couple of tap positions in the opposite direction to verify a typical pattern.Whether performing a TTR or winding resistance test, if you detect an abnormality on one LTC tap, you should test all LTC taps. Specifically concerning winding resistance tests, if the results on the lower positions of a reactive LTC are not symmetrical to those on the higher positions, then you should test all the LTC taps.  
 

The MWA clamps have banana jacks on the side of the clamps, allowing a standard banana to alligator clamp wire to be used to fit into tighter spaces.
Note: you must make sure the clamp’s jaws are not touching and that you connect wires to both sides of the clamp since this is a Kelvin type measurement. 

You can perform a ratio and phase deviation test, as well as a winding resistance test, on a CT with the MWA. The MWA will not be able to give you a saturation curve since its maximum output voltage is 80 V. Since the MWA is designed to apply voltage on the high side and measure on the low side, you must swap the leads to perform CT testing, which is secondary injection. Although the MWA can be used to make a ratio measurement on a CT, it is generally easier and faster to use a dedicated CT test set, such as the MRCT. This quicker and more convenient functionality is particularly appreciated when testing multiple CTs.

It is often thought that higher test currents speed up the saturation of large transformers. However, in the majority of cases, this isn’t true. Voltage determines the saturation rate of the test, as given by: (flux) ɸ = Volts * t (seconds). Therefore, compliance voltages above 40 V DC are preferred when selecting a test instrument. Make sure, however, that the test current is greater than 1 % of the current rating of the winding under test. To improve saturation on transformer windings when charging time is slow, connecting the primary and secondary windings in series may help. This setup speeds up the test by providing more volt-turns of charge. It is essential, however, to ensure that the windings are connected so that the current through them produces a unidirectional flux in the core. With this setup, the test current will flow through the high voltage winding and the low voltage winding in series, while the DC voltage drop will only be measured across the low voltage winding, for example. Since current will not flow into the X1 bushing terminal, be aware that the X1 bushing connection integrity will not be assessed during this variation of the test.

With DC resistance measurements, the objective is always to try to saturate the transformer core, as this reduces the effective inductance of the winding and allows the test current to stabilise more quickly. Saturation typically occurs when the test current is around 1 % of the rated current for the winding. It is, however, usually advantageous to use a somewhat higher test current than this to minimise the effect of noise on the measurements. If the test current is too low, it will often be found that successive measurements give inconsistent results. Nevertheless, test currents of more than 15 % of the rated current should be avoided, as they are likely to lead to erroneous results because of the consequent heating of the winding. In most cases, the optimum test current is between 1 % and 15 % of the rated current. Power DB will automatically select an appropriate current to test the winding resistance when the transformer nameplate information, specifically rated voltage and power, is filled out correctly.

The low side of very large power transformers can have a current rating in the thousands of amps. If this is the case, a dual injection method where you inject current in both the high side and low side of the transformer, taking advantage of the transformer's physics, can increase the overall current and saturate the core. A dual injection option within the Power DB settings allows you to do this with your standard connections.  

Before reaching a conclusion, check your test connections. Loose connections can cause higher resistance values, so ensure the connection surface is clean, and you have good clamping pressure. Variations from one phase to another or inconsistent measurements can indicate many problems, including short-circuited turns, open turns, poor brazed or mechanical connections, defective ratio adjusting (RA) switches, or defective load tap changers (LTCs). If testing smaller distribution-type transformers that have winding resistance in the micro-ohm range, a larger difference may be due to the low resistance value and different connection points for the phases. 

The industry standard for factory tests permits a maximum difference of 0.5 % of any 'phase winding' resistance value from the average resistance of the three 'phase winding' results. Measurements made in the field may vary slightly more than this because there are more variables than in the factory. Thus, if the measurements are within 1 % of each other, they can be considered acceptable. Comparing absolute resistance values measured in the field with factory values can be difficult, principally because of the difficulty of accurately estimating the winding temperature. Values within 5 % of factory results are generally acceptable.

In general, you want to perform AC tests before DC tests. Since the TTR is an AC test and WR is a DC test, you should perform the TTR first. If you do perform the WR test first, you should demagnetise the transformer before performing TTR tests. A WR test should magnetise the core, which will affect the excitation current and possibly the ratio results, rendering subsequent data analysis challenging.Conversely, when you have confidence in a test instrument’s ability to demagnetise a core, some would argue in favour of always doing the WR test first. This way, after a successful demagnetisation, the transformer core is always in the same state of magnetisation - test after test - for the TTR test.