How to Test Underground LV and HV Cables

If you are in the USA, check the National Electrical Code.

Some information would be helpful.

no i am in UAE Abu Dhabi

Did you lay the LV cables in the same trench as the HV cables.

Can't help you.

In the power utility I worked for, we hipot tested the buried cable for five minutes and logged the leakage current at 15 second intervals. This test record became the 'as built' documantation and reference point for any subsequent testing of faults.

We also kept at least a half meter separation of low voltage wiring such as street lighting above the lower placed high voltage cables. These HV were concentric neutral armored cables designed for an operating voltage of 27,600 volts RMS.

Our test voltage for hipot was 70,000 volts.

Occasionally we had to commission underground cables that had been buried for long periods of time; several months or up to a year without being energized. These cables always gave trouble. We had to 'cook' the cables to drive out the moisture that had migrated into the insulation. The initial leakage current was quite high as a result. It worked beter to start with a lower voltage such as the normal workign volts then wait until the leakage current stabilized for a period of several minutes.

Once that was accomplished we performed the normal hipot test.

Hipot the cable is the way to go, depends on the cable size you apply the hipot voltage accordingly could 50,000V - 70,000V etc. If it is DC voltage you will be using, make sure you discharge the test items from that capacitance that build up in them during the hipot test before attempt to put in service.

Hi, How do you mean " cook the cables to drive out moisture ". I am in a similar situation and would like the procedure on how to do this. Thanks

Start heating from the inside and move towards the end.

Sorry it was a slang jargon term. As you know Hi voltage will cause slight leakage current through the moisture absorbing insulation. This leakage is in the range of microamps. However it is measurable with the right test equipment. As the voltage rises, it tends to drive out the moisture. No one ever bothered to explain the physics behind what I observed, but after a while the leakage current dropped to normal values. I likened it to the 'skin effect' you also see on high voltage transmission lines in the 44,000V and above range.

A buried line will not absorb moisture because the high voltage potential seem to drive the water molecules out to the outer circumference. This is an effect quite apart from any internal heating effect associated with current flowing through the cable. We sometimes commissioned industrial sub divisions long before any building took place. As soon as we had tested the cable we energized it. It might be a year or more before the first customer connected and pulled load current.

Energizing the cable prevented moisture degradation.

This sounds like an old problem which one of my college professors used to test very high voltage cables before they were approved for use. They had to simulate actual use, which meant running current thru the cables, and creating the high voltage to simulate the possible leakage which might result.

What they did was to create a circulating current by making one or more of the phases a secondary part of a transformer and closing the circuit by interconnecting two or more of the phases. This runs a current down the center conductor and the high voltage was set up by just attaching the high voltage to the transformer secondary but with nowhere to go other than to ground.

At any rate, you can do this on an existing cable and the result is that there is heat generated all thru the heart of the buried cables due to resistance, and that will dry out the insulation without having to access anything but the two ends of the cable.

I have XLPE and paper Insulated lead clad cables in my system.

Since the PILC is no longer used in the industry , the XLPE is the cable of choice for the 132 , 69 and 33 KV cables in our system.

From research , I have realized that the industry is divided on whether to DC hipot XPLE cables .

Some do and some don't .

The ones that don't use the justification that DC hipotting results in voltage treeing of the cable and premature failure.

As a result , I use a Megger and test at 5000 V , after which I energise the cable with the working voltage and let it soak for a couple days.

I also hippotted aged cables before I started to use this technique and had failures .

I am not sure if the failures were as a result of the age of the cable or the hipotting.

All the new XLPE cables which I have installed using this 'no hipotting' technicque are still in service after 5 years.

As with anything else, experts will often differ in their opinion. It is my impression that formulation of the plastic insulation has changed over the years. This is no doubt a result of continuing research into better materials.

I recall hearing the power utility engineers discussing the pros and cons of annual validation tests using Hipot versus letting it run until a fault actually developed. No doubt about it a hipot test can be destructive depending on the voltage setting used. Our power utility opted to not do annual validation tests.

After the splicing crew was finished we also tested it but at a reduced voltage compared to what we used for new. Result logging was the same proceduer. Engineering then compared the 'new' test results with the re-test after repairs.

We took over a section that had been built by someone else and the cable left in the ground for over a year before we took over. The cable had not been energized. We had a lot of faults develop in that area. To me this was a clear indication that the buried cable being left not energized was detrimental to long term durability.

Hipot tests typically stipulate 2X working voltage plus a set amount over. A megger test of 5000 may not show minor weakness but it probably does eliminate all serious flaws in the dielectric. Your experienc sem to support the practice.