11 KV XLPE cable hi pot test

[byjuvc]

Friends,
During my recent discussions with my seniors, I was told that hi pot test on XLPE cables is damaging to the cables . Only megger should be used to check the Insulation Resistance. If it is a PILCA cable then hi pot test is ok. What are your commencts on this?

Byju

"Principle without practice is useless and practice without principle is dangerous"

[Hurricane Harry]

Our company has been using dc hypot test for years. As of late they are starting have us us lower voltages for proof tests. A 13 kv test is now at 26kv for five minutes. The theory is, that resudual dc voltage remains in the cable, even after it has been grounded, go figure. Anyway when the cable is put back into service, the ac volatage creates a spike, causing treeing, and damage to the xlp.

[byjuvc]

Our company has been using dc hypot test for years. As of late they are starting have us us lower voltages for proof tests. A 13 kv test is now at 26kv for five minutes. The theory is, that resudual dc voltage remains in the cable, even after it has been grounded, go figure. Anyway when the cable is put back into service, the ac volatage creates a spike, causing treeing, and damage to the xlp.

I don't understand Sir. You said "they are starting have us us lower voltages for proof tests" and "A 13 kv test is now at 26kv for five minutes". Isn't that contradictory ? I think u must be using a lower voltage for a longer duration of time..isn't it ? Is there any literature or external link which I can make use of to get more info on this phenomenon?

Thanks in anticipation

Byju

[Hurricane Harry]

At one time we tested 13kv cables at 45kv. This is mostly derived from IEEE standards. http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=69557

[Trampbag]

The site you quote says:

“The inadequacy of DC testing in determining the cables' AC breakdown strength, the danger of programming the cables with faults when DC and impulse testing at unnecessarily high voltages, …”

You said:

“At one time we tested 13kv cables at 45kv.”

I wouldn’t test XLP at this voltage unless directed by an engineer or manager and only then if I had the orders in writing signed by that engineer or manager.

[Hurricane Harry]

That is exactly what we are doing, testing at the voltages requested by engineering. It is in writing in our specs. We have records of all of our network feeders going back 20+ years. This establishes a base llne that can be used as a comparison for future tests. Each time we take a feeder out of service and add a transformer or splice, it is tested, and compared with previous tests. If the leakage is high, 200ua or more it is suspect and may or not be put back into service.

[Trampbag]

Ah, OK Harry. Running a set of tests based on a history proven over a period of time under supervision of an engineer is entirely different than testing used cable having an uncertain condition with a high pot at voltages higher than the rating of the cable especially when the owner of the system is looking to claim liability against anyone.

I haven’t experienced any problems with testing cable using a high pot when test voltage dose not exceed the cable rated voltage or the operating voltage, whichever is less.

Keep in mind I am talking cable only. Any equipment still connected has to be considered when selecting the test voltage.

[byjuvc]

From the valuable inputs received from you friends my conclusion is that if I have to do a hi pot test on the 11 KV UG cable during breakdowns I will apply only 6.5 KV line to earth . it is a fact that 99 % of our system is on XLPE . So I wont take a risk.

With warm regards

Byju

[DaveR]

DC Testing is bad for Medium to aged xlpe cables. Thats why one tests with Very Low Frequency Test Sets (VLF) .1Hz. Testing values have been set by IEEE and other orgs. Ask your company to buy them. You should be Testing. If it fails under the the correct test voltage it is FAULTY. Testing and rooting out faults results in a better performing , safer to operate network. It saves money long term.

One can measure the Tangent Delta when your are testing with some VLF test sets , which will give you a very good idea what condition your cable is in. i.e. how much treeing is taking place.

Look at HVDIAGNOSTICS INC. or Baur. It will be well explained and you can see what equipment I am talking about.

Regards

Dave

[CHICAGO HAND.]

Over Here We Call The "vlf", The Atm Machine.
We Love That Thing. It Has Made Me More Money These Last Few Years Than Anything Else.

[Hurricane Harry]

We bought one but never used it. Some new studies show it may not be as good for the cables as first thought.