Improve mechanical properties in XLPE

Hi,

For low voltage (0.6/1 kV) XLPE insulation over bunched copper conductor we preheat the conductor to improve mechanical properties (elongation and tensile) of insulation layer. The conductor size we use is 0.5 to 4.0 sq mm and the insulation thickness is approx 0.8mm.

However the same is not practical when we use taped conductor and we always get a lower elongation and tensile value on taped conductor.

I would like understand various methods which will help to improve the mechanical property of XLPE insulation other than preheating the conductor.

Hello brn,

OK well lets think what is going on for a moment.

As the conductor is insulated, the first portion of the plastic to freeze is that immediately in contact with the conductor which naturally is a heat sink. The second portion of the XLPE plastic to be frozen is that in contact with the atmosphere and then cooling water (If a moisture cure type insulation.) or that in contact with the atmosphere inside the CV tube and then in contact with the cooling water. (If a standard XLPE compound.) Because the inner and outer surfaces of the insulation are frozen before the central portion of the insulation, the result is inbred stress inside the insulation. This results in lower mechanical properties of the insulation (Elongation and tensile strength.) along with increased shrinkback of the insulation.

To improve the situation, conductor preheating is employed often with gradient cooling. This slows down the heat transfer from the insulation. A lower melt temperature may also be used but you must be very careful here as you might degrade the mechanical properties even further.

Because you cannot use conductor preheating with your taped conductor, the amount of inner insulation freezing is still far too much even though heat transfer is somewhat retarded by the taped layer over the conductor.

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

Dear Peter...
First, let me thank you for your time and effort in replying my query.

Precisely the same phenomenon you explained is what happening with XLPE insulation. Just to clarify, the insulation line is normal horizontal (not CV) with apx 150m cooling section (straight + multipass).

Conductor preheating and gradient cooling we do to improve mechanical properties with non-taped conductor.

For any insulation mechanical properties can be improved by tubing extrusion; however lots of insulation breaks happen with this solution esp with taped conductor.

Hello again,

Not sure how you get improved mechanical properties by tubing extrusion but I suspect you are not using a vacuum to pull the insulation down and thus have less positive contact with the conductor.

With helical taping , we would never recommend a tubing process because the melt would be pulling on the tape.

There is one thing however that you might be able to do if you are using an induction preheater. You could turn down the power level so that the heated conductor still doesn't damage the tape or the process. Experimentation with direct measurement is of course required.

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