Support for the MWA300 and MWA330A three-phase ratio and winding resistance analysers
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Verify that the INT PC/EXT PC switch located on the side panel of the MWA is set to EXT PC. Power down the MWA and your computer. With your PC connected to the MWA, power on your PC and the MWA.
Some antivirus software can also block communication to your PC. If possible, temporarily disable your antivirus. This is a quick method but not the preferred, or may not be allowed by your IT department. Within your PC antivirus there are ways to allow exemptions for certain programs and IP addresses. Check to see if “PowerDB.exe”, “MTOTestXP.exe”, and “MTOSetup.exe” are approved programs; if not, add them to the exempt list. Additionally, allow an IP Address range of 169.254.1.1 through 169.254.1.10 to be exempt.
If controlling the MWA with your PC, close Power DB and open it again, open your form and start testing. If it happens again, a full power down of the MWA and your PC is recommended. If the problem persists, then we recommend that you update your software to the latest version of Power DB.
If the onboard computer of the MWA freezes, power down the MWA and restart. If this problem persists, then you will need to update the software on the onboard computer. Download the “Transformer Test Instrument Software Update” for the MWA330A from the update section. Make sure to read the instrument update instructions prior to installing.
There are files within Power DB that are used to control the MWA; occasionally, these may be bypassed during installation. If you are not running the latest version of Power DB, then you can update Power DB and install this additional software as part of the update. When installing Power DB, make sure to select “MWA USB Drivers” under the Additional Software prompt. If you are updated to the latest version of Power DB, or do not wish to update, then you can install the specific software needed.
- Click “No” on the prompt
- Close Power DB
- Browse to the following folder on your PC: C:\Program Files (x86)\PowerDB Inc\PDB Optional Files\MTOSetup
- Double click on MTO_PDB_Update_Release_XXXX.exe (NOT MTO_PDB_INSTALL…)
- Click “Next” and proceed with the install
- After the install, click finish and resume testing
Verify that the INT PC/EXT PC switch is set to INT PC. Power off the MWA and then power it back up. If still not able to communicate then the unit will need to be returned to Megger or an authorised repair centre.
Note: You can also connect a PC that has Power DB installed and operate the unit with your PC if you need a quick fix in the field. Just make sure the switch is set to EXT PC and power the MWA off and then on with your PC connected.
Once testing begins, several errors can appear that result from improper connections, improper instrument configuration, instrument malfunction, or a faulty asset under test. If an error occurs, repeat the measurement before performing any corrective action to verify it is not a glitch.
Verify that the form's nameplate data matches the transformer's nameplate. Pay particular attention to the transformer's vector diagram, voltage, and power rating.
Check all connections to make sure the clamps are on the correct bushing terminals and verify proper connection to the MWA. Make sure there are no unwanted grounds or shorts connected to the terminals.
You can perform functional tests with the MWA to verify that the unit is operating correctly. In some cases, it may be easier to check connections and swap leads or connection points on the transformer; other times, it might be more convenient to perform quick functional tests. Follow steps one and two below to verify that the MWA is working correctly.
- Megger provides a test jig with the MWA to test its TTR functionality. Use a form set for a YNyn0 transformer and connect the test leads to the metal portion of the test jig next to the markings. Perform the test; the results should be a ratio of approximately 1:1 with a phase deviation of +/- 6 minutes and an excitation current of 0 to 0.5 mA. Note: The ratio will be between 0.9980 and 1.0020; this is due to the internal measuring circuits of the MWA and is expected since you are essentially connecting the leads together and not connecting to a transformer. Also note that this test is only for checking the instrument's general function; you should not use the test jig to calibrate your MWA.
- To test the winding resistance functionality of the MWA, you will need some conductive metal, e.g., a wrench or pipe. Something approximately eight inches or more in length will be helpful. Use a winding resistance form set to YNyn0 and connect the H1 clamp to one side of the object and the H0 clamp to the other. You should obtain a resistance value. Continue to measure the resistance using the other leads, i.e., between H2 and H0, then H3 and H0. If connected at the same spot, they should yield the same resistance value with a little deviation due to the connection points. Repeat the test with the low side leads, i.e., X(1,2,3) leads to X0.
If the functional tests do not pass, there is an issue with the MWA or the test leads. You must return the MWA to Megger or an authorised repair centre for evaluation. If the functional tests pass, the MWA is working. In this case, the error is either due to your connections or an issue with the transformer. Proceed as follows:
- Verify that the clamps have a solid connection. The clamps are Kelvin type, so you need to connect both sides of the clamp to the bushing terminals. Oxidation can build up on the terminals, so make sure they are clean such that the clamps can make a solid electrical connection. Connect to a different part of the terminal, if need be, to check for a better connection.
- Swap the leads with a different phase's leads to ensure that the error is not following the leads and remains on the phase you were initially testing.
If you have verified that you do not have a connection problem or setup mistake and have confirmed the MWA's integrity, then the error message may be due to a problem with the transformer. See the “Interpreting test results” section for interpretation and recommendations.
Note: If the excitation current is too high while performing TTR tests, you can lower the test voltage to 40 V or 8 V. If the error persists, you may have a short-circuited transformer winding, and further investigation is warranted.
This error means that the instrument cannot produce the required current. Verify that all leads are connected correctly to the MWA and the transformer under test. The clamps are Kelvin type, so both sides of the clamp need to connect to the asset. Next, verify the error on multiple phases. If connections are correct and sound, and the error occurs on multiple phases, it is most likely a problem with the MWA. You must return the instrument to Megger for repair.
Interpreting test results
The MWA is a combined transformer turns ratio (TTR) and winding resistance (WR) test set, so the data interpretation criteria differ for each test. Still, there are a few commonalities to the assessment approach you should adopt for each. First, the best evaluation method is comparing newly acquired measurements against previous test results to ensure nothing has changed. If no previous test results are available, then comparing the test results for each of the three phases to the others is also helpful. Finally, differences in measurements indicate winding problems, assuming that you have performed your tests like the previous ones, e.g., the same test settings and conditions (temperature). The MWA can detect winding problems that include short-circuited winding turns, open-circuited turns, or broken or corroded connections.
A TTR test is used to verify that the transformer vector diagram and ratio are correct per the nameplate. The measured ratio should be within 0.5 % of the nameplate ratio for all taps.
In addition to comparing against the nameplate, you should compare the measured ratio of one phase to the ratios measured for the other two phases. The industry standard (factory) permits a maximum difference of 0.5 % from the average of the three ‘phase winding’ ratios. Field readings may vary slightly more than this due to the many variables. If all ratios are within 1 % of each other, they are acceptable.
Changes in ratio values or deviations from nameplate are an indication of shorted turns or open windings, incorrect assembly or connections in tap changers, and high resistance connections.
Comparing absolute resistance values in the field with factory values may be difficult due to the problem of precisely estimating the winding temperature. A transformer OEM can control the winding temperature in a factory test setting, and direct access to the windings is often available. On the other hand, field testing requires winding resistance to be measured through the bushings, and the test is performed at the winding temperature available, which is not always precisely determined. For this reason, if comparing field measurements to factory values, measured values within 5 % are generally acceptable.
Since field measurements are generally made in different conditions than factory measurements, comparing each ‘phase winding’ resistance value to the others is better. IEEE allows a maximum resistance difference of 2 % between phases for liquid-filled transformers.
Changes in resistance values (after temperature compensation is taken into account) are indications of shorted turns, broken strands, faulty or corroded connections between winding and bushings, or within a tap changer.
If performing a winding resistance test on a transformer equipped with a load tap changer (LTC), it is advisable to continue injecting DC current into the respective winding while switching through the taps. By maintaining the current, you test the LTC’s ‘make before break’ functionality, verifying that it is operating correctly. If the current is interrupted during a tap transition, then this indicates that the LTC is not working correctly in that transition.
User guides and documents
Software and firmware updates
Transformer Test Instrument Software Updates for MWA330A and DELTA4310A
Please read these instructions before performing the update, you can download them here.
DELTA and MWA Updater
The following components have been updated:
PowerDB ________________ V11.2.10
MTOTestXP ______________ 2019.12.03.1
Delta Manual Control ______________ 188.8.131.52.0
Instrument Config ______ 1.0.20023.1919
Splash Screen __________ 1.0.21075.830
Factory Config _________ 1.0.21122.850
Megger Update Manager __ 1.0.21165.1032
- Megger recommends that you return your instrument annually for calibration verification.
- Any instrument returned for re-calibration will be updated with the latest firmware and software versions.
- Certified Factory Calibration is valid for one year.
Incorrect installation of updates or incomplete updates may cause the equipment to become unusable.
If damage occurs from improper updates, the customer may be responsible for repair costs.
Software updates for MWA330A and DELTA4310A
Download this zip file, extract, and run the executable.
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.
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.
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.
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.
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.
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.
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 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.
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.