Assessing the Reliability of Medium Voltage Cable

In my days as a young automation engineer putting in power, control, and safety systems on offshore oil and gas platforms, I had the “opportunity” to see improperly terminated motor leads burn up during a startup after power was supplied to them from a variable speed drive. So, it was with great interest that I listened to a presentation during the Emerson Exchange by Wally Vahlstrom and Cliff Kirby in Emerson’s Electrical Reliability Services organization. I could have used their expertise back then to prevent that sinking feeling I had when I smelled smoke.

Their presentation, Early Detection of MV Cable Problems Improves Overall System Reliability, described how failures can occur and steps to diagnose impending failures.

Cable terminations and splices are the area where most prone to deterioration and failure since these are typically assembled by hand. The junctions account for 80 percent of the failures. Typical problems include nicked insulation, incorrectly connected or no drain wire, physical abuse, and environmental contamination-all of which can produce partial discharge (PD). Also, voltage transients caused by lightning and other sources, and manufacturing defects can create reliability problems.

Cables themselves can also fail caused by many things including manufacturing defects, damage caused by installation or physical abuse, metallic shield corrosion, water migration, and even cable test methods like DC Hi-Pot methods which can damage older cables. Typically, the cable will pass the test, but fail after AC power is reapplied after some period. There are many suspected reasons for this but one may be that ‘space charges’ develop in the insulation during application of the DC test voltage.

Wally discussed a form of deterioration known as water trees found in extruded dielectric cables. These trees are water-filled micro channels that develop in the insulation of cables operating in a wet environment. The patterns that form resemble trees that have lost their leaves. Water trees can continue to grow under operating voltage until they bridge the insulation. This often leads to cable failure.

Cliff discussed some of the US standards and guides for testing cables in the field. IEEE 400 warns against testing the cable using DC Hi-Pot methods on older medium voltage cables, especially in wet environments because it accelerates failure. Other test methods described by the IEEE 400 standard include AC Hi-Pot, Partial Discharge, Very Low Frequency (VLF), Dissipation Factor (Tan delta) and Oscillating Wave (OSW).

The Electrical Reliability Services team uses on-line partial discharge detection methods to test the reliability of the cable system. It is the only test of the ones mentioned that can be performed while the cable is energized and in service. This testing method is a non-destructive, non-invasive predictive maintenance tool that assesses aging cables. This test is also used to test for workmanship in new cable installations, given the 80% failures occurring around the handmade terminations and splices. Ah yes, this is what triggered my memory of those smoking motor terminations!

A spectrum analyzer, RF analyzer, and U-shaped sensor are used to identify partial discharge. This testing can see about 500 feet each way down a cable.

Cliff showed some installations with corrosion in other areas outside of the cables including medium voltage switchgear. Typically, this is caused by non-operational space heaters in the switchgear. These space heaters prevent condensation that causes this corrosion to occur.

Cliff recommends a site assessment be done which can be performed over time. What to assess should be based on criticality, past failure rates, and environmental conditions to prioritize how and where the partial discharge testing is done.

Update: I’ve removed the picture and associated text for the picture I incorrectly attributed to IEEE.

Posted Monday, April 14th, 2008 under Asset Optimization, Electrical Reliability, Emerson Exchange.

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