Water Resistance and Power Cables

Use a megger to determine insulation quality.

1. The only way a VLF (very low frequency) signal is going to eliminate the moisture is thru heating. I would NOT recommend this (for obvious reasons). Do you mean VLF voltage injection (like the Tan Delta test) to test for cable faults?

2. Always ensure proper sealing and venting of cables and cable junction boxes to prevent water and moisture ingress and humidity and temperature variation moisture build up. use of special water-proof joints or heat shrink over the joint and cable interface helps too.

3. High voltage testing (such as AC Hi-Pot), partial discharge testing, Tan Delta testing, visual inspection. Depends on the power cabling, its location, its length, its construction, etc.

What type of cables are you talking about?

TQ for input...

1. Yes, Very Low Frequency (VLF) testing incorporates the application of an AC voltage at a low frequency in the range of 0.01 to 1.00 Hz by injecting 1.6Uo (our spec) to test cable fault.

2. Our design for XLPE 630mm2 1C 33kV Cable is included with swealable tape n aluminium foil to prevent water. For special water-proof joints, any suggestion?

3. Partial Discharge testing can only detect Electrical Tree, not water tree inside cable, correct me if i'm wrong... And the Partial Discharge Testing cannot be detect IF cable have water ingress.

The suggestion using VLF at XLPE cable (630mm2, 500mm2, 240mm2)

If you know you have water ingress into the insulation, then I do not recommend testing with anything higher voltage - power then a megger 500V or 1kV.

You want to test and record the readings,

Then hook up a DC welding machine to begin circulating current thru the phase leads in any type loop configuration. This is how generator-motor windings are dryed out. The Important part is start immediately and keep at it until the megger readings start going up, then keep with the program until you have the required readings for the voltage being applied to the conductors.

Please keep us informed as the dry out progresses with megger readings. Select a welding machine current that creates a temperature rise in the conductors. Guidelines may be a rise of 10-20C initially and when the megger readings start to increase then in daily steps increase the weld machine current to the conductor rated temperature rise.

I was on a job site where they didn't start immediately and a month later after heating the windings with a welding machine they were dried out and the readings came back up on the 4160v generator's. Two horizontal generators underwater up to the centerline of the shaft.

If you apply too high a voltage during testing there will be "treeing" within the insulation and failure, if during winter you can see where the snow is melted. If the moisture is trapped in the insulation then it may flash to steam and blow-out inside the insulating jacket.

An alternative is the gel-like insulating goo that is forced into conductor insulation, I'll look for the info and post later.

We had that problem when we assumed responsibility for a privately constructed sub division and the buried able had been left in ground for a year without being energized.

We used our normal cable testing hipot gear but kept the voltage low for many hours. Gradually the voltage expelled moisture from the insulation as noted by the decreasing leakage current. That in turn meant we could increase the voltage to once more bring up the leakage current to the maximum permissible level. We envetually reachd a point where the cable leakage current limit was acceptable and also the hipot voltage limit.

At that point we felt it safe to energize the new cable by connection to the utility power. Once enrgized, the cable remained dry and the insulation intact with low leakage current.

Another question, can we inject DC for long duration (let say few hour) at XLPE cable for drying processes? Injection DC Voltage for long duration can degraded insulation of cable (polyethylene), rite?

Hello,

No testing methodology available in the globe can pump out the water ingress in the cable espeicaly in MV and EHV cables.The trapped water inside the cable cannot be removed completly.Heating the conductor may release the water which is at the ends but not the trapped ones.The life of the cable will become half due to water ingress.

I thought the OP was talking about moisture ingress directly into the insulation. A common occurence if you bury cable in ground but do not energize. Especially if water table is high such as after spring thaw. But of course if you bury cable and do not seal splices and connectors, that's another story. I agree if water is ingress by capilliary action via open ends and unsealed splices then its a write off. But moisture ingress through the dielectric insulation, that's another matter. That is fixable. We had to do that several times. Which is why we made it a priority for me to go and hipot a cable as soon as it was terminated and buried. As soon as I passed the cable for Hipot test we then enrgized it even if no use was planned for many months afterwards.

yes, my problem about the water ingress into the insulation..

Hello,

water ingress into the insulation is the worst situation.The DC hipot and VLF cannot reveal anything about water ingress.These are either go or no go tests which will say whether the cable is good or bad.The amount of deterioation due to water ingress cannot be confirmed.