Better Options than Meggering for 4160V Alternator? 2

[HenryOhm]

All,

We have a 4160V, ~3MW diesel genset that went through some bad juju with arcing in the entrance cubicle of its attached switchgear cubicle. Previous loss of PT fuses on shutdown led the voltage regulator to take it very high on ramp up from idle to rated (perhaps as high as 150% and for a second or two prior to fault?). Excitation was cut immediately by protection relaying on fault but the decay may have taken 2sec. Surge arresters in the entrance cubicle were blown out so not sure what voltage spikes the alternator may have seen during this event.

Most of our installed base is in low voltage territory and we have not had problems with our newer 4160 equipment. Initially, the electricians meggered it at 1000VDC (looked good including polarization index). The OEM manual for it says to megger at only 500VDC?! I thought that a bit odd for a 4160V machine. Does a megger at 500VDC tell one much about insulation on a 4160V machine? Given the above scenario, does megger testing seem sufficient?

Also, trying to get up to speed on what testing, if any, might be suggested beyond this. Ramped vs Stepped DC Voltage Testing? This sounds preferable to AC or DC Hi-potting, even if either of these two are at some acceptable range less than what new test standards would say (but at least equal or higher than the machine rated voltage)?

I did find these two posts helpful and had already planned to investigate IEEE 95:

thread237-147223
thread237-301242

Any additional guidance or reference to links or other testing standards would be highly appreciated. We do have a spare alternator and can handle the associated downtime if necessary to replace it now. It would be much more painful for us to find out 6 months down the road that something was wrong and then have to replace it.

Thanks for any and all help!

 

[petronila]

Hello HenryOhm,

No problem if you have megged with 1000 Volts, like a guide value, EASA AR100-2010 referenced in IEEE-43 suggest that you can use 100-2500 Vdc for testing windings with voltages rated from 2501 up to 5000 Volts. You can take a look of IEEE-43.2000.

Regards

Carlos

 

[HenryOhm]

Thanks, petrinola!

Also, still trying to get up to speed, would partial discharge (PD) testing make sense in this situation or be preferable to ramped/stepped DC hipot?

 

[squeeky]

Becareful with the Hipot as it does stress the insulation and you could pop it. I would start low and work up. I am not in the office at the moment but If you have the PI test results and they look good are you comparing with previous results? 500v DC is like 1000v AC as the electrical stresses are based on RMS and the DC is a stronger force.

 

[petronila]

Hi HenryOhm,

Before HIPOT, Try first an ohmic resistance test and a surge test of stator and rotor windings in order to know if the problem could be overcome from shorted windings.

Regards

Carlos

 

[HenryOhm]

petrinola and squeaky, thanks for your feedback. We are going to go forward with additional testing, current-limited ramped DC hipot as well as surge comparison, etc.