Air bubbles during PD of 11kv cable

We are manufacturing MV cables regularly on a Maillefer CCV line, but before and after Partial Discharge testing, we saw air bubbles coming out from the conductor after dipping core ends in the oil. What are the reasons for this? Will it affect the properties of the cable?

Hello ramawa,

Before I answer your question I need to know if you are performing a proper cable degassing procedure before performing the partial discharge test?

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641-3212

Dear sir,
we have installed new Maillefer CCV line this year. we set up all parameters required by their NCC receipe, we set minimum % of decomposed peroxide to 99%. as per them degassing is not required for 11kv cables. we saw this bubble phenomenon with 2-3 months old cable also.

Are these cable have waterblocking design. means you are using tapes or powder in between conductor layers

these are the normal cables as per IEC60502-2.

Hello again,

Although you are pretty well next to the lowest voltage of triple layer low voltage power cable, many higher voltage power cable companies work with a maximum internal methane pressure "p" of 0.2 kg/ cm*2 and graphs for mathematical degassing times are developed on the basis of that final internal methane pressure and the total thickness all three of the XLPE layers. This pressure limit was experimentally determined to give not more than 1 mm cable jacket swell and that was found to be adequate.

If you use woven type semiconducting tapes on top of the conductor, there should be no retarding of the methane gas coming out of solution so the degassing times should not change.

If you place air tight end caps on the cables that have not been properly degassed and and leave them outside, a methane internal/external pressure equilibrium will be formed and the cables will not be properly degassed.

If your problem is indeed methane coming out of the cables, collect the bubbles and have your laboratory determine if you have actually collected methane. Be careful of small explosions however. The chemists will know how to handle this properly.

Then you may wish to talk with Maillefer some more.

Regards
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com
519 641-3212