Testing Solid Dielectric Cable

[CosmoVA]

Does anybody do routine maintenance testing of solid dielectric cable? If so, what testing do you perform?

Thanks

[SecondGen]

Does anybody do routine maintenance testing of solid dielectric cable? If so, what testing do you perform?

Thanks

What voltage is the cable rated for? Test voltages are dependent on cable rating, here is a basic rundown of routine maintenance of MV cable per NETA specs:

- Inspect exposed sections of cables for physical damage and evidence of overheating and corona.

- Inspect terminations and splices for physical damage, evidence of overheating, and corona.

- Inspect bolted electrical connections for high resistance by DLRO, infrared, or mechanical check.

- Insulation-resistance test individually on each conductor with all other conductors and shields grounded.

- Shield-continuity test on each power cable by ohmmeter method.

- Dielectric Withstand in accordance with ICEA, IEC, IEEE and other power cable consensus standards by means of direct current, power frequency alternating current, or very low frequency alternating current.

- Diagnostic Power factor/dissipation factor (tan delta) and partial discharge

In accordance with ICEA, IEC, IEEE and other power cable consensus standards, testing can be performed by means of direct current, power frequency alternating current, or very low frequency alternating current.

These sources may be used to perform insulation withstand tests, and diagnostic tests such as partial discharge analysis, and power factor or dissipation factor.

The selection can only be made after an evaluation of the available test methods and a review of the installed cable system.

Reference NETA-MTS Section 7.3.3 for more information

[CosmoVA]

What voltage is the cable rated for? Test voltages are dependent on cable rating, here is a basic rundown of routine maintenance of MV cable per NETA specs:

- Inspect exposed sections of cables for physical damage and evidence of overheating and corona.

- Inspect terminations and splices for physical damage, evidence of overheating, and corona.

- Inspect bolted electrical connections for high resistance by DLRO, infrared, or mechanical check.

- Insulation-resistance test individually on each conductor with all other conductors and shields grounded.

- Shield-continuity test on each power cable by ohmmeter method.

- Dielectric Withstand in accordance with ICEA, IEC, IEEE and other power cable consensus standards by means of direct current, power frequency alternating current, or very low frequency alternating current.

- Diagnostic Power factor/dissipation factor (tan delta) and partial discharge

In accordance with ICEA, IEC, IEEE and other power cable consensus standards, testing can be performed by means of direct current, power frequency alternating current, or very low frequency alternating current.

These sources may be used to perform insulation withstand tests, and diagnostic tests such as partial discharge analysis, and power factor or dissipation factor.

The selection can only be made after an evaluation of the available test methods and a review of the installed cable system.

Reference NETA-MTS Section 7.3.3 for more information

I understand what NETA recommends I am looking for what tests people perform and have they had success. Does everybody perform thermography, power factor test, and partial discharge on their cables? I would be surprised if companies do all of the NETA recommended maintenance testing and inspection on their cables routinely. I am hoping to find a good test to trend the health of the cable so we can plan replacement when they get near the end of life. Thanks

[SecondGen]

I understand what NETA recommends I am looking for what tests people perform and have they had success. Does everybody perform thermography, power factor test, and partial discharge on their cables? I would be surprised if companies do all of the NETA recommended maintenance testing and inspection on their cables routinely. I am hoping to find a good test to trend the health of the cable so we can plan replacement when they get near the end of life. Thanks

What do you typically do? At my shop, we will do a visual/mechanical inspection then megger and VLF as a basic cable maintenance test. If customer requests more diagnostics we may do a tan/delta, rarely PD. We will also do complete NETA testing when asked or specified. I've done IR on MV cable a few times as well as ultrasound but haven't come across many issues using those tools.

[CosmoVA]

What do you typically do? At my shop, we will do a visual/mechanical inspection then megger and VLF as a basic cable maintenance test. If customer requests more diagnostics we may do a tan/delta, rarely PD. We will also do complete NETA testing when asked or specified. I've done IR on MV cable a few times as well as ultrasound but haven't come across many issues using those tools.

As of now, we do not do anything. I was looking for some type of test (PD, power factor,??) that might reliably be able to trend the condition of the cable.

[roberts311]

As of now, we do not do anything. I was looking for some type of test (PD, power factor,??) that might reliably be able to trend the condition of the cable.

A power factor test will give you some good values for trend analysis as to the condition of the cable. It typically requires a separate inductor to offset the capacitance of the cable.

A DC Hipot or VLF tests (per the NETA MTS for in-service equipment) will let you know if the insulation is failing. This test is pretty much a go-no-go test.

If you can't take the equipment out of service, an IR scan could be an acceptable means of testing. IR scans would primarily be used to locate high-resistance connections or any cable load imbalances. Not really much information here about the quality of the insulation.

[Kalbi_Rob]

As of now, we do not do anything. I was looking for some type of test (PD, power factor,??) that might reliably be able to trend the condition of the cable.

If your talking about low voltage cable, I haven't seen any testing that will assist in trending other than resistance testing.
As for MV Cable, I've attached a slightly outdated pdf below on the different types of testing. Now here is my point of view on the main 3 types of testing that I have done:

DC Hi-Pot testing is still used today, but only recommended for acceptance testing on cables before energization. It is only used for acceptance testing before energization because this is the last test performed at the factory. That being said, I have found cables with critical splicing deficiencies, which still passed DC HiPot. We had one cable with the insulation cut to the cable, and passed because it was pointed toward open air vice ground. Also, DC HiPot has been found to be highly destructive to extruded oil cables (XLPE), and only recommended on paper based insulations.

VLF (very-low frequency) is preferred over DC HiPot due to the fact it does not have the destructive features DC HiPot has, but performs the same basic over-potential characteristics. IEEE 400.2 sets the standard for VLF testing and will tell you that this test has not been studied enough to provide value in trending cables, but is to be used as a pass/fail only. The note states that the test can be performed at 30 min if no change is noted for 15 min, but if any change is noted you should continue testing for atleast an hour to attempt to drive the cable to complete failure.

The other test I have performed is Tan-Delta, which is my recommendation for trending. The initial test sets the baseline, and should be fairly flat. I have had the best results in finding splicing errors with tan-delta testing compared to the other 2. It does cost more, and requires more training, but the benefits outweigh the costs in finding errors and electrical treeing.

My previous company performed online partial discharge, but still required tan-delta testing at specific intervals since olpd cannot detect the electrical treeing that can be detected with tan-delta testing.

Strategies for field testing medium voltage cables.pdf