As part of my road test review of the Megger insulation tester, I have carried out some comparison of this instrument to the Keysight insulation multimeter and have commented that I have had issues with the on occasion. During the last test of a generator rotor winding I recorded some data videos on the and this blog is an explanation of the problems I have had.
Now before I get accused of assasinating the I do accept that I carry out an unusual role within the electrical industry. The machinery I work on is of the larger variety and has onerous and exacting test requirements that can take the functionality of some test instruments to their limits. I have had issues with a number of instruments over the years, from simple cheap voltage sticks and their proving units, up to specialised secondary injection test sets worth tens of thousands of pounds. Sometimes unforseen issues occur during tests and it is accepted as being just part of the job.
I stress that the has worked fine carrying out insulation tests on general electrical apparatus and does offer improved accuracy and reporting functions over a lot of other insulation testers on the market for general purpose work.
Just to prove that statement below is a video of the Keysight carrying out an insulation test on a 90kW three phase induction motor. The video lasts just over one minute.
Keysight had some reasonable reports that I could produce by either linking the to a laptop using the USB adapter, or via a wireless link to an iPad. Both produced similar reports as seen below on the left for the PC Logger Software and on the right for an iPad Insulation Test Report. Ideally I would have liked to have been able to export the data out to Excel to allow comparison to be made back to a baseline insulation resistance plot but I never got round to figuring out if I could do this.
I purchased the Keysight over 18 months ago now to use as an alternative to the Fluke 1555C insulation tester when testing out generator rotors. I also had the intention of using it for RSO winding resistance and impedance measurements where I need two multimeters to record the current and voltage at the same time. The longer term intention was to purchase the logging module to simultaneously record the voltage and current to improve accuracy during the test and the quality of the report to issue to the site. I never got this far due to the issue I encountered
This was all part of my drive to make testing the generator rotors easier, safer and more efficient.
I have tested three generator rotors so far with the and all have produced a similar result
- Rotor 1 - Static excited 412MVA hydrogen cooled generator
- Rotor 2 - Static excited 210MVA air cooled generator
- Rotor 3 - Static excited 161MVA air cooled generator
In all cases the produces a very erratic insulation resistance reading switching between MOhm and GOhm values rapidly. This was only ever noticed when the rotor insulation was in good condition and the current supplied by the instrument would have been quite low. When a rotor with a bad insulation reading was tested the would produce an insulation resistance value comparable to the reading from other instruments.
Rotor 3 Tests
When testing the last rotor with the Megger a 1 minute insulation value of 1.8 GOhms was obtained with DAR factor at 1.6 and PI factor at 3. It was therefore known that the rotor insulation was in good condition. The Megger is also a comparable meter to the Keysight offering the same insulation test ranges with a similar sized package and construction.
Time to see the Keysight in action. The video shows the problem with the meter and lasts just under three minutes.
In the video you can clearly see the insulation value on the Keysight bouncing around, where as when using the Megger unit the reading shows an increasing trend over the duration of the test as would be expected for a rotor with good insulation condition. In the end the fails the test before the one minute duration and displays an error message, no DAR value is calculated and the impression is that the rotor winding has an insulation fault on it. This has proven not to be the case for this rotor and the other two that were tested.
The conclusion I therefore came to was that some element of the large rotor winding was affecting the operation of the Keysight
To try and gather some more information on a potential cause of this error, I hooked up one channel of an oscilloscope via a high voltage probe to measure the voltage applied to the rotor and then a differential probe to the second channel on the oscilloscope via a shirt resistor in series with the rotor winding to measure the current applied. The video of these tests is below in two parts, first part is the voltage and current, lasting three minutes and then the second part is the voltage only and lasts two minutes.
I had issues with recording the current, for some reason the polarity would always go negative despite reversing connections through the shunt, which didn't change the polarity and then on the differential probe, which had no effect either. The second issue is that the insulation value recorded on the instruments was steady at around 2MOhms, so I assume that the input impedance of the differential probe was affecting the overall test circuit.
Despite these issues the only difference obvious at the time was the faster application of the test voltage when using the Keysight However both the voltage and the current from both instruments appeared steady on the oscilloscope screen through out the tests.
I therefore currently have no conclusion as to the cause of this and how it could be rectified. So at the moment, the Keysight is marked as unusable for rotor winding insulation tests, which is a huge disappointment both in terms of the money spent and the loss of the potential improvements to the testing methodology and the reporting facility.
Unfortunately the Megger whilst it can conduct the tests without a problem, it does not offer me the downloading and reporting facilities I desired. I do have my eye on another insulation tester that I need to do a little more research on to try and ascertain its suitability. In the mean time I have the option to use the Megger and manually record the results or use the oversized Fluke 1555C to obtain an element of the reporting option but live with lugging around a large case.