HV cable pressure testing
HV cable pressure testing
(OP)
When DC pressure testing an AC MV/HV cable it is usual to test the cable using the negative pole. What is the reason for using the negative and not the positive pole considering that if a cable is likely to fail due to insulation breakdown, it is more likely to fail on the positive phase of an AC waveform?






RE: HV cable pressure testing
1. Are you following any standard(s)?
2. Are you also performing AC testing?
3. Clarify "DC pressure testing" since it is not listed in IEEE Std 100 "Dictionary..."
It appears that the cable might potentially have the positive charge against ground. This way, the reverse polarity of testing may remove/discharge the plus charge.
RE: HV cable pressure testing
RE: HV cable pressure testing
1.
http://standards.ieee.org/reading/ieee/std_public/description/inscond/400-1991_desc.html
2.
http://www.avointl.com/products/power/cablevan/cablevan.html
3.
http://www.usda.gov/rus/electric/engineering/1999/cabletesting.htm
4.
http://www.omnicontrols.com/lists/biddle1pm.html
5.
http://www.omnicontrols.com/lists/biddle3pm.html
6.
http://www.neetrac.gatech.edu/accessories.htm
7.
http://www.okonite.com/engineering/index.html
for more info
RE: HV cable pressure testing
RE: HV cable pressure testing
RE: HV cable pressure testing
We still use DC testing for new cables, but avoid it on field-aged cables.
We have been heading towards VLF (Very Low Frequency) AC hipotting for testing our field-aged solid dielectric cables. By using VLF hipots, you take advantage of the frequency-dependent characteristic of the cable's capacitance.
(Recall the impedance model of a capacitor Z = 1/(j*2*pi*f*C) )
A lot of the research floating around these days suggests that XLPE insulation does not perform well after a DC voltage has been applied. It has a sensitivity to DC stress that can be described as a 'memory' that shows up after the cable has been exposed to DC and then de-energized. The XLPE (and TRXLPE) molecules are polar and take on a set when exposed to a unidirectional electric field. The tendancy of the XLPE molecules to rotate back to the orientation that the DC field placed them in creates space charges in the insulation. (Return-voltage diagnostic methods rely on the development of the space charges that result from DC exposure).
So DC has it's uses, but when the cable is tested with DC and then re-energized too early with AC, the space charges (which may not dissipate for many hours) cause stress enhancements in the bulk of the insulation. Those areas of intensified voltage stress are likely to become failure sites, either at some point in the future or almost immediately upon re-energization.
OK. I've probably strayed outside the scope of this discussion, but I have one last question for those who have posted to this thread...
Is VLF testing the same direction others are taking?
Thanks,
Kraigb
RE: HV cable pressure testing
RE: HV cable pressure testing
I did see where in the test equipment section 4.2.1.1 they discuss the fact that negative voltage will be applied. But I didn't see any other discussion or explanation of it in this standard. I remember hearing before that insulation may act differently depending upon the polarity of applied voltage for oil-soaked paper insulation used in transformers.... seems reasonable the same applies for some types of cables.
Wowski - you mentioned "cable can be tested at a higher voltage without causing destructive damage to the insulation. This is also why DC is used in preference to AC."
I'm not sure I have heard that DC is less destructive than ac for cable testing. I agree with your assessment that DC is preferred due to simpler test equipment that doesn't have to charge the cable capacitance continuously. But there is at least 1 effects which may make the dc potentially more destructive:
1 - DC can be more destructive than ac for wet cables due to the treeing effect which occurs over time while dc applied... doesn't have enough time to occur during half-cycle of ac.
Also it may be possible to set the test set trip lower for dc since the expected continuous curent is lower for dc... possibly providing less destruction if test set does trip. But also note that cable will charge completely during dc but not ac... I think this creates the possibility for a more harmful magnification of a reflected wave if an insulation breakover does occur during a dc test.
Interesting subject. I'd like to hear more.
RE: HV cable pressure testing
I think this is the same phenomenon associated with "treeing" during DC testing of wet cables. Interesting to see that it is applicable to XLPE, but apparently not in EPR cables per jbartos' link to Okonite.
I also saw in that link where DC was claimed to be less destructive than ac testing, as wowski said. A new question for wowski - why do you say that cable is more likely to fail during the positive half-cycle of ac voltage?
RE: HV cable pressure testing
Thanks
RE: HV cable pressure testing
As for the statement that a cable is more likely to fail on the positive half-cycle of an AC waveform, this was information I'd been told by test engineers. I also remember one reference to this in ' High Voltage Engineering:Fundamentals ' which confirms this is true for corona discharge. I don't know whether this is true for other failure modes.
RE: HV cable pressure testing
Unfortunately I don't have that book handy (does anyone else?) and the details don't reside in my head anymore.