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MIT420/2 Insulation Tester Review Blog 10 - Retest of 120MW Generator Rotor Winding

three-phase

During my review of the Megger  MIT420/2MIT420/2 insulation tester in Blog 9, I tested a 120MW Generator rotor winding that actually failed the insulation resistance test. Following this issue, the bedplate heaters for the generator were reconnected and placed back into service. Around 4 weeks later, I returned to the generator to carry out a further insulation test with the expectation that the heat would have driven out the moisture from the winding and a healthy insulation reading would be obtained.

 

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With the heaters being disconnected during the last test, the winding temperature was taken as being the same as the ambient temperature. With the heaters in service, a higher winding temperature was expected and this was verified by measuring the resistance of the embedded winding temperature sensors. These are PT100 devices and tables are easily obtainable to cross reference the resistance obtained to a temperature value.

 

The resistance measured cross referenced to 21 degrees Centigrade which is circa 10 degrees above the ambient conditions. To maintain the winding in preservation, the recommendations are to maintain the winding temperature 5 degrees above the ambient and the humidity below 30% RH.

 

Initially the resistance was measured with the resistance function on the Megger  MIT420/2MIT420/2 However it was found that the resistance value in excess of 100 ohms was set to a scale without any decimal places This introduces a slight inaccuracy when using the insulation tester for this test For the purposes of an insulation test this isn't an issue but when measuring the winding resistance the corrected value can be affected due to the inaccurate temperature correction as detailed in the video below(lasts 1m 45s

 

 

A comparison of the rotor winding resistances obtained can also be seen in the table below.

 

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For this reason the winding temperature sensor resistance was measured with the Keysight  U1461AU1461A to obtain a more accurate value In fairness to the Megger I don't think it was ever designed to replace the resistance function on a multi-meter Even the Keysight  U1461AU1461A only displayed one decimal place for the scale so a good multi-meter would generally offer an extra digit over this as well

 

The 1 minute insulation resistance test was conducted at 250V and then 365V using the variable voltage function of the Megger  MIT420/2MIT420/2 This setting was used to match the working voltage rating of the rotor winding A final test was then recorded at 500V

 

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When carrying out multiple insulation tests at different voltages, the expectation would be to see a drop in insulation resistance as the voltage is increased. The results obtained didn't display this trait and this can sometime happen with large inductive apparatus when carrying out a succession of tests, as stored charges can sometimes remain and affect the next test result. This is resolved by allowing for longer discharge times between tests.

 

However, the values obtained during this set of tests were much better than the previous tests as shown in the comparison table below.

 

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The DAR values were found to be above the minimum value of 1.2 for both the 365V and 500V tests, where as during the previous test they were around 1.00, which would be considered too low. The video of the instrument set-up on the variable voltage and the full DAR test is below and lasts circa 3 minutes.

 

 

As such a good 1 minute insulation test value was obtained, the polarisation index test was conducted at 365V and then again at 500V to see if an increase in voltage would have an adverse effect on the insulation reading obtained.

 

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A good polarisation index value was obtained for both test voltages, 3.05 for the 365V test and 3.03 for the 500V test, well above the minimum value of 2.00. The insulation resistance plot seen above shows the classic response for a polarisation index test. This time the 500V test produces a slightly lower reading than the 365V test as expected. The drop however, is not significant enough to cause concerns.

 

The video below is the polarisation index test at 365V. I have sped up the test to reduce it down to around 4 minutes.

 

 

Overall the  MIT420/2MIT420/2 continues to perform very well and I am happy to continue using it It has done around 300 tests to date of various types and the battery voltage drops down to 50 during the heavier current tests so appears to have a reasonable life

  • in reply to jw0752

    Hello John,

     

    Thanks for the comments. Insulation testers are very easy to get hold of, I have often thought that I would like to get hold of a cheap unit to tear down for comparison to the likes of the Megger and Fluke professional units that I use regularly. Something like this unit from China seems widely available in both the states and the UK.

     

    VC60B+ Digital insulation tester

     

    You will also probably find a healthy second hand market for the testers as well, from old vintage hand cranked units all the way up to Megger and Fluke units from electricians upgrading or retiring.

     

    You don't need all the functionality of the professional units. Before these units offered all their functionality, all our timed tests were carried out with stop watches or a normal watch with a seconds hand and timing the pressing of the test button at 59 seconds. The only thing I would recommend is a start button that locks to save keeping it pressed down if you are carrying timed tests.

     

    Or you will just have to nudge to provide another road test for an insulation tester.......

     

    If you don't have motors available, for just for playing around with, I quite often have a resistor and coil circuit with a capacitor tee'd off for carrying out timed tests.

     

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    The coil is really just there for the resistance range on the meter, the resistor and capacitors from the charging circuit for the DAR and PI tests, you can change the values around to vary the response. Obviously just make sure the components are rated for the test voltage.

     

    I use circuits like this as they produce a consistent result for DAR and PI ratios, so if anyone ever queried a test result, I can use one of these to quickly verify the meter. Because of were I work and the safety factors involved mine are all built into cases.

     

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    This one I built resembles a 3 phase motor, so I can change the winding configuration between star and delta to see the different effects. You can spend as much time as you like on it.

     

    Also remember that what ever you test, it could retain a charge afterwards which won't have the same current limitation as the insulation tester if you touch it. The meters these days generally discharge, old ones may not, but if ever have any doubts I will apply a short across the tested item and leave it in place for twice the duration of the test. This becomes more important as the test voltage and duration is increased.

     

    Kind regards

  • Hi Donald,

    Thanks for the nice report. Due to posts like yours I have gotten an interest in experimenting with a megger. A more or less inexpensive one is on my 2018 items to buy list.

    John

  • in reply to three-phase

    Hi Donald,

     

    Thanks for the information! They look really good. I'm definitely going to keep that in mind, I have some items that would be nice to store properly.

  • in reply to shabaz

    Hello Shabaz,

     

    Thanks for your comments, glad you liked the videos!

     

    Yes I did cut out a piece of tool foam and glued it into the lid to house the accessories. The tool foam is available from a variety of places but for this one I used a company called shadowfoam. They were fine to deal with, but I did wait until I needed a few items so I could get a free fitting kit.

     

    They do have a video on their site of how to cut out the shapes in the foam and for the Megger accessories I did follow their methodology. In the past, for flat  items, such as my HV Probe, I have cut the shape out all the way through the foam and removed it, then thinned it down to the required depth and glued it back in with the outline, that way I get a smoother finish at the bottom of the cut out in the same colour as the rest of the foam, or you could use a different coloured foam for some contrast if you wanted to.

     

    Kind regards

     

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  • Hi Donald,

     

    Nice update  and awesome videos, they were worth the upload : )

     

    I notice you've got some shaped foam for the accessories, did you make that yourself? If so, how? It looks really neat.