HV cable pressure testing

[wowski]

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?

 

[jbartos]

Please, provide more info.
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.

 

[wowski]

When we have done work on HV/MV underground cables (e.g. new joints) we apply a DC voltage at a slightly higher rating than what the cable is used for to test the insulation to ground and phase to phase. DC is used in preference to AC due to the capacitance on long cables. It is the standard test procedure to use the negative pole of the DC test set and not the positive, but nobody has really been able to tell me why.

 

[jbartos]

Suggestion: Visit
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

 

[wowski]

Thanks, the links have proved useful. As suspected the negative pole is used because it needs a higher negative potential to cause harmful discharge than the positive pole would. This means that he 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.

 

[jbartos]

Please, notice that some AC cable testing is also useful. E.g. one may be required to know AC leakage or AC capacitive current for a high-resistance grounded neutral system protection.

 

[kraigb]

This has been a very interesting thread. I have actually wondered about the use of the negative pole on the DC test set as well.
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

 

[wowski]

The cables which we test with DC sets are only used with 11 or 33kV EPR cables(we call it HV here, but a lot of companies would only see that as MV) . We only have a single 90kV XLPE cable which we would not use DC testing on for the reasons stated above. As for VLF ac testing, it's not something we've tried (or needed to yet).

 

[electricpete]

IEEE 400-1991 is a "Standard for HVDC testing of Power Cable Systems in the Field."

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.

 

[electricpete]

The Doble reference book on Cables states that moisture in insulation is attracted to the negative terminal due to electroendosmosis. They go on to state this may cause a difference in insulation resistance reading depending on which "electrode" (smaller center conductor or large outer shield) is negative. Also that if cable conductor is negative and moisture is entering at outside of the insulation, then current will increase over time during the dc test due to moisture migration nearer the smaller inner electrode.

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?

 

[jburn]

Very good discussion. I have always felt that DC Hipotting our XLPE cable (15KV) was destructive, and I have refused to do it. We do use DC Hipot on new XLPE cable to proof test it the first time. Because most of our XLPE cable is approaching 20 years old, I am think about Doble ( Power Factor) testing on a 3 to 5 year schedule. Is there another name for VLF (Very Low Frequency) AC hipotting? How does it compare to Doble or power factor testing. What are others doing with cable testing? Testing method, frequency of testing, on what types of cable, age etc. What has been your experence in finding problems with a cable. Is MV cable testing really worth the cost of doing it?

Thanks

 

[wowski]

In my previous post I did not mean to suggest that DC was more destructive than AC( although electricpete did make some interesting points on that subject) just that DC had a higher insulation breakdown voltage than AC. Maybe my reply could have been better phrased.
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.

 

[electricpete]

I think I remember reading some discussion of the polarity of DC megger test voltage in SDMyer's book on transformer maintenance. I know we're not talking about transformers but it may shed some light.

Unfortunately I don't have that book handy (does anyone else?) and the details don't reside in my head anymore.