Cable VLF testing - How does it withstand

[electricallearn]

Hi all,

I a trying to understand how it is that the HV cable can withstand the VLF hipot testing.

I thought that when a cable insulation is rated for a voltage, say 6.35/11kV, then the cable cannot be subject to voltages above 6.35kV ph-E and subjecting it to higher voltages would result in insulation damage.

However, I understand that VLF testing is performed by placing the cable up to 3 x Ph-E voltage for a period of time, say 15-30 minutes to check the integrity of the cable insulation.

How does this test not damage the insulation? Is it because it is done at a very low frequency (0.01 Hz)? Even if it is low frequency, I calculate that the peak voltage of 3 x Ph-E voltage will be applied to the cable for as many as 180 times over a 30 minute period. I thought the frequency was lowered purely so that the test kit could actually supply such a high voltage without pumping too much current.

Am I missing a fundamental point here??

 

[electricallearn]

Could anyone please shed some light on this?

 

[davidbeach]

All insulation is expected to withstand significant short term overvoltages without failure. The rated voltage level is a continuous rating with plenty of margin. The faster the rise time of the overvoltage the more likely the insulation will fail, thus a very slow rise allows a voltage well beyond rated with no risk to a properly constructed system.

 

[oldfieldguy]

It's been a while since I paid attention to it, but I seem to remember that the factory DC test voltage for a 15-kV cable was 80 kV. We used to proof test a 15 kV cable to 65 kVDC for acceptance tests on new installations.

Of course was was in the days when dinosaurs roamed the earth.

In a demonstration for electrical students I put over 50 kVDC on a length of THHN 600 volt cable before insulation failure.

old field guy

 

[electricallearn]

Great advice davidbeach and oldfieldguy - much appreciated

 

[VLFit]

Everything has a safety factor built in. In this case, this allows for safe operation during over voltage conditions that are very common in a distribution system, due to switching surges, reflected waves, etc. Also, insulation material will degrade over time, becoming weaker in terms of kV/mm of voltage stress it can withstand. For example, if we figure 1/2 of something is compromised over time, then we better design it with twice as much needed at the start. Anyway, solid dielectric insulation is factory tested at maybe 5-6 times its normal line to ground operating voltage. In the case of VLF, an Acceptance test to verify the installation of the system is sound, is performed at approximately 24 kV peak line to ground voltage on an 11 kV cable. That is a test of 2.7 Uo, meaning 2.7 times normal l-g voltage. A 15 kV cable has an IEEE 400.2-2012 designated test voltage of 30 kV peak Uo, or line to ground voltage.

In a nut shell, if I am going to energize an 11 kV cable with a line to ground voltage of 8.9 kV peak stress forever, it had better be able to hold off 2 - 3 times that for a 30 minute test.

Hope this helps

 

[electricallearn]

Excellent response VLFit - things are much clear for me now and appreciate the support...