LV and MV Cable Testing 5

[EEbyChoice]

I have quite a few on-site backup power generation facilities (located at customer sites, commercial and industrial)ranging in size from 16 MW down to 350 KW. Age ranges are from 3 years to 12 years or maybe a little older.

I am trying to establish an electrical preventative/predictive maintenance program for these facilities that includes cable diagnostic testing. I am interested in determining which types of testing should be used at the various facilities.
I have experience with DC Hi Pot testing and DC insulation resistance testing, but there are many other types of testing now available. We already conduct regular thermographic surveys of each site, but I have some concern about cable insulation system integrity at our older sites.

Most of these sites have never had any type of cable testing done so I don't have much history to work with.

I need to minimize risk to installed assets and also prevent the possibility of testing related customer outages or unavailability of equipment.

Any feedback would be appreicated.

 

[alehman]

It's good you have raised this point. There is a difference.
My understanding was they could provide both types of services. Perhaps I am mis-informed.

 

[mc5w]

Disclaimer:
This is NOT an official testing method but it should work for detecting water in contact with waterproof insulation or impregnating insulation.

One test that you could try is low voltage high frequency alternating potential to ground and phase to phase at 2,000 hertz or 4,000 hertz. I have had an instance where water raised the capacitance of waterproof (THWN) insulation in PVC conduit to the point where a variable frequency drive would not work. The mechanism is that the water acted as a giant capacitor plate that was in intimate contact with the insulation which created enough capacitance to short out the high frequency carrier of the drive. The wires tested fine using a DC megger or when energized with 60 Hertz line power but at the carrier frequency (usually 2 or 4 KHz) the wires were a poor excuse for a short circuit.

The carrier of a VFD has a very strong zero sequence component at zero speed and a variable amount of positive sequence component at different motor speeds. You will need to do some playing around. One possibility is to connect a 4,000 Hertz 660 volt primary transformer to one of the output leads and then use 2 AC/DC capacitors to connect the other end of the transformer primary to the DC bus. With a line voltage of 480 volts the DC bus voltage will be more like 660 volts - be careful.

Another possible high frequency source would be a motor, belt, automotive alternator, and zero to 15 volt DC power supply. You would throw away the diode stack and use the power supply to drive the field. At the full speed of the alternator (7,000 maybe 10,000 RPM) you can get somewhere between 80 to 240 volts AC out of it at a rather high frequency. See thread about getting mains power out of an automotive alternator.

Essentially, this would amount to capacitance testing the power cable as an audio cable which could shake down some kinds of defects. You should use a current limiting resistor in series with a high frequency ammeter for this test as well. You would also need to measure brand new insulation with this test to get a baseline as to what is normal.

Mike Cole, mc5w@earthlink.net