Testing Advice

Sure, you can test an 11kV cable with only 500V from a megger. You just should be careful to write up your report carefully to indicate that you tested this cable with a voltage 22 times less than the operating voltage. So assuming that non-linear breakdowns won't occur at higher voltages, you can then extrapolate how much leakage one should expect at operating voltages. But I would be sure to point out that insulation breakdown is well known to be a non-linear event.

Yes you can. However, the information you get is almost worthless.

What will be the normal minimum voltage to test this cable?

Does it make sense to use a test voltage higher then the 11 kV ?

Yes, it makes sense to test to a higher voltage. This grants you a safety margin.

These cables as per IEC standard should be pulsed for test with 33KV pulse minimum and up to 55KV

Rudy quote "Does it make sense to use a test voltage higher then the 11 kV ?"

Yes. A typical test might be done at 50 KV.

DEAR ALL THIS IS FOR YOUR INFORMATION AND ATTENTION

Putting Hipot Out to Pasture

Oct 1, 2003 12:00 PM, By Wally Vahlstrom, Electro-Test, Inc.

As DC high-potential testing starts to show its age, test methods recommended by IEEE are ready to fill the void

For many years, high-voltage DC testing has been the traditionally accepted method to judge the serviceability of medium-voltage cables. DC high-potential (hipot) tests worked well as a withstand and condition assessment test for paper- and oil-insulated, lead-covered (PILC) cable. Even when plastic insulated cables were first introduced in the '60s, it was still the preferred method.

Alhough applicable industry standards began to change in 1994, manufacturers, testing firms, and standards groups still agreed that DC hipot test equipment could be used to perform factory tests, acceptance tests, and maintenance tests. As far as the installer or maintenance electrician was concerned, cables that withstood the DC hipot test were good enough to be placed—or placed back—into service. But recent research and field data is calling that practice into question and raising serious doubts about whether DC hipot testing might damage or cause extruded cables—especially field aged cross-linked polyethylene (XLPE) insulated cable—to fail prematurely.

After receiving reports in the early '90s that DC hipot testing could be to blame for latent damage experienced by extruded medium-voltage cable insulation, the Electric Power Research Institute (EPRI) funded two studies relating to XLPE and ethylene propylene rubber (EPR) cables. These studies, EPRI Reports TR-101245 and EL-6902, yielded the following conclusions regarding XLPE cable:

· DC hipot testing of field-aged cable reduces its life.

· DC hipot testing of field-aged cable generally increases water tree growth.

· DC hipot testing before energizing new medium voltage cable doesn't cause any reduction in cable life.

As noted in IEEE Standard 400-2001, Guide for Field Testing and Evaluation of Shielded Power Cable Systems, "testing of cables that have been service aged in a wet environment (specifically, XLPE) with DC at the currently recommended DC voltage levels may cause the cables to fail after they are returned to service. The failures would not have occurred at that point in time if the cables had remained in service and had not been tested with DC." This standard also indicates other testing has shown that "even massive insulation defects in extruded dielectric insulation cannot be detected with DC at the recommended voltage levels."

The shift in opinion of hipot testing that began in 1994 when applicable industry standards began to change is only just now gaining industry-wide acceptance. Current versions of some of these standards no longer provide for DC hipot testing of extruded cables as a maintenance test. Of those that still do, all have reduced the recommended test duration from 15 min to only 5 min. None endorses DC hipot testing as a factory test for extruded cables, but all continue to provide for DC hipot testing as an acceptance test on newly installed extruded cable. These industry standards also no longer endorse DC hipot testing as a maintenance test for extruded cables that have been in service for more than five years.

Vahlstrom is director, technical services for Electro-Test, Inc. in San Ramon, Calif