A MEDIUM-VOLTAGE CABLE FAILURES AND FIELD EXPERIENCE
A.5 Summary of Lessons Learned
This section summarizes the lessons learned from these failures.
A.5.1 Dry Cable Compression-Related Event
Although it is not clear exactly what caused the failure in this case, the event indicates that care must be taken to prevent abusive handling during the installation of cables. No loads should be rested on cables when they are laid out for installation, and no carts or vehicles should traverse them. Care should be taken to ensure that sheaves and pulleys used during pulling provide a smooth arc for the cable to follow and that cables do not run against edges of ducts or trays that could grab or indent their surface.
A.5.2 Overheated Butyl Rubber Failure
Some butyl rubber medium-voltage insulations will soften when subjected to long-term elevated temperatures from operation or environment. This problem is related to butyl rubbers having sulfur curing processes. Butyl rubber installation systems that remain in service should be checked to determine whether ohmic and ambient heating are well within the ampacity limits of the cable. This is especially important for cables known to be sulfur cured. However,
determination of whether a butyl rubber cable was sulfur cured might be difficult at this late date.
A.5.3 Failure of Continuity of a Zinc Tape Shield
When helically wrapped metal tape shields are used in cable, zinc tapes should be avoided.
These can corrode to the point of separation and result in an arcing and tracking path that can lead to insulation failure. The 2005 Nuclear Energy Institute survey results reported in NEI 06-05 indicate that the use of zinc tape shields is rare in the industry [11].
Medium-Voltage Cable Failures and Field Experience
A.5.4 Water-Related Failure of a Butyl Rubber Cable
Although the failure was related to a butyl rubber cable, most of the following lessons learned apply to all cable types.
The fault did not clear properly and continued to pop (seek ground) over an extended period.
The failure could have burned into a phase-to-phase fault. It is recommended that a circuit that has a suspected fault, as indicated by loud popping, be removed from service as soon as possible. Had the fault become a phase-to-phase fault, the source transformer could have been damaged. This fault eventually caused failure of the adjacent phase’s jacket and arced through that phase’s shield.
This concern also applies to cables that have phase-to-ground alarms that do not cause a circuit trip. When such alarms are received, it is recommended that the circuit be removed from service as soon as possible to preclude damage to the adjacent phases and the potential for the condition to become a phase-to-phase fault with high fault currents.
Having spare cable and necessary splice and termination kits available is critical to a rapid return to service. Spare cable alone might not be enough.
Had it been necessary to replace the entire circuit, including the dry sections, a long period would have been required to remove and replace fire stops and to maneuver the large, heavy cable through tightly configured tray systems.. Replacement of the section containing the fault might be the only viable option for return to service in a reasonable time.
Sealing of ducts on the interior of building walls can cause water to back up into the duct, covering cable in areas that previously were well drained.
Current splicing crews are familiar with modern cable construction in which shield semiconducting materials are easy to distinguish from insulation. Extra training will be necessary when such crews are working with old constructions in which semiconducting materials and the insulation are black.
Significant plant modifications might be required to allow replacement of complex below-grade circuits. Pulling cables at low temperatures is difficult and could lead to damage of the cable.
Tan δ testing was practical for this cable; however, due to the large attenuation of the corroded shield and butyl rubber, PD testing was not practical. On-line assessment had been performed on this cable. It is likely that the attenuation would have made identification of this degradation difficult using on-line techniques. The configuration of the circuit allowed on-line assessment at only one end, the one most distant from the degradation.
Medium-Voltage Cable Failures and Field Experience
A.5.5 Failure of an Ethylene-Propylene Rubber Insulated Cable from Long-Term Wetting
This event provided insights that insulation resistance measurement acceptance criteria that have been considered adequate in the past are unacceptable and should not be used. In this case, two failures occurred in a short period. At the time of the first event, the phases of the cable were found to have low insulation resistance values. The cables were cut in the manhole, the section with the low insulation resistance was replaced, and the cables with 10–18 MΩ insulation resistances, which were thought to be acceptable insulation resistances, were reused. However, 6 hours after reenergization, the reused section B and C phase failed, indicating that the 5 MΩ criterion was unsatisfactory.
Tan δ testing of cables with several hundred megohm insulation resistances in the case of the butyl rubber cable event (see Sections A.3.1 and A.5.4) indicated that even these cables were marginal. Acceptable tan δ results occurred only for cables have gigohm insulation resistances, indicating that insulation resistance testing is not a useful indication of insulation condition for medium-voltage cable. Insulation resistance remains a useful tool for troubleshooting, but it should be used with caution for determining serviceability for medium-voltage cables.
A.5.6 Failure of Cross-Linked Polyethylene Insulation from Water Treeing This failure indicates that water treeing of XLPE cable is not rampant, even in cables
manufactured in the early 1970s, and that the failures will likely occur at locations of minor and relatively infrequent manufacturing flaws. Even with these flaws, the water tree growth can take a significant time to lead to failure. In the case of this cable, it was 35 years.
A.5.7 Failure from Use of an Oversized Molded Termination
The lesson learned from this failure is that the Elastimold and other types of molded terminations (as well as cold-shrink and heat-shrink systems) must be sized properly to mate with the cable insulation and shielding system. Similar terminations throughout the plant were assessed to confirm that the correct size stress relief adapter had been used. If this type of problem is suspected, handheld PD detection devices can be passed near the terminations to determine whether PD is present.