By Ben Lanz, Director, Applications Engineering, IMCORP. Member, EPRA
(This article was written for Practitioner Perspectives: A Journal of the Electric Power Reliability Alliance and will feature in the upcoming issue, which is free for EPRA members.)
Power cables are one of the most commonly overlooked power system assets and are, by default, typically addressed reactively after failure. They are also one of the most misunderstood. This article provides insight into how new scientific findings on cable care address these misunderstandings. In this article I will provide insight into some of these findings through case studies, and I’ll explain how a holistic, condition intelligence-based strategy can easily double the expected life of most assets—including cables—and maximize reliability with optimal lifecycle cost.
Most medium- and high-voltage cable systems consist of plastic and rubber (solid dielectric) insulation. A significant percentage of assets containing those materials are reaching—or have passed—their assumed 30- to 40-year end-of-life. Asset managers are now facing a significant challenge in balancing reliable power delivery with budget priorities. As new assets are installed, reliability professionals are asking, “how can we make cable systems last longer with higher reliability?”
This broad reliability question comprises several smaller and more practical questions, which have been answered here with input from cable owners from all over the world. With their help, we have assembled the largest condition assessment database of its kind—and the findings are good news for cable owners.
We discovered that to extend the life of medium- and high-voltage cables, there are two key things to consider when developing a reliability plan, supported by four best-practice recommendations:
A. Practice “cable care” techniques.
With the proper care and maintenance, aged cable systems can outperform new cable systems.
B. Eliminate physical stressors and extreme duty stressors.
With these additional stressors removed, it is possible to extend the reliable life of cable to 100 years—and beyond.
C. Follow best practices:
Install and maintain over voltage protection.
Perform IR inspection of connectors.
Perform offline 50/60Hz PD testing to ID insulation defects.
Eliminate or minimize over voltage tests such as fault location ‘thumping’ and withstand/hipot testing.
Medium and high voltage solid dielectric cable system insulation fails due to an erosion process associated with phenomena called partial discharge (PD). PD is an electrical discharge (or ‘micro arcing’) that does not completely bridge the insulation. PD can arise from an extreme focus of electric stress, a lack of the appropriate solid insulation, or a combination of both.
A focus of electric stress, or stress enhancement, can be caused by issues such as accessory interface contamination, a foreign object, a protrusion of a semiconducting layer, or area of extreme moisture concentration. A lack of appropriate solid insulation filled by a gas, or a void, can be caused by such issues as:
• a damaged semiconducting layer,
• overheating of the cable or accessory insulation,
• an insulation cut,
• a lack of accessory/cable interface void filler,
• or an incorrect accessory/cable interface dimension.
PD, and its associated erosion process at a defect site, is rarely active at steady-state operating voltage unless the failure is imminent. PD is initiated when localized electric stress overcomes the local dielectric strength.
Voltage transients—fast, short duration electrical transients—are the primary driver to turn on PD and propagate insulation failure. The sources of transients include:
• circuit switching,
• restoration activities, such as breaker operations and fuse reclosures,
• fault location and withstand tests,
• momentary flashovers and grounds (momentary contacts with air insulated components),
• complete faults elsewhere in the system,
• capacitor banks switching,
• transformer tap changes,
• and, especially, lightning.
Transients reflect and resonate within the power system and can increase in magnitude exponentially. Voltage transients typically occur in the microsecond to millisecond timeframe. This is more than enough time for PD, which occurs in the nanosecond range, to turn on, erode the insulation, and turn off.
Successive transients can cause intermittent growth of a carbonized fault channel sometimes described as an electrical tree. As the electrical tree grows, the turn-on voltage drops, and eventually the PD is active at the operating voltage. The erosion process fails this insulation.
Industry Questions Answered with Science
Many questions have been asked about proper cable care over the last few decades. These questions have often been answered with reasonable theories, which have helped the industry make reasonably accurate asset decisions. But now, with the aid of scientific research, a more precise understanding is driving more optimal solutions.
Answers to some of those frequently asked questions now follow, and each is informed by this research:
D. Does cable only last 30 to 40 years?
Research indicates for most applications, provided there is no extreme loading or voltage events or discrete physical defects in the cable system, there are no known significant long-term aging mechanisms to cause cables to fail short of 100 or more years.
E. Does moisture fail cable?
No. Random moisture only creates a more ‘leaky’ or lossy insulation. Aside from losing a tiny amount of power to operate the cable, this is not problem.
However, in the extremely rare event that moisture concentrates due to the higher stress of an original manufacturing or installation defect, extreme voltage transient can cause local stresses to exceed the insulation strength, start carbonizing insulation, and creating a fault channel (electrical treeing).
F. Does cable fail rapidly once a carbon track is formed?
In the vast majority of cases, No. Most carbon tracks or ‘electrical trees’ are not active at the operating voltage and thus only grow during short voltage transients. They can take years—or even decades—to grow to failure.
G. Do installation defects fail quickly?
Installation defects often take years or decades to fail. Since the defect erosion process is only turned on intermittently and the fault channel path is driven by the highest stress path which often is not the shortest path, the growth rate can be surprisingly slow. The first failure is often associated with an installation or manufacturing defect. However, once a cable is a couple of decades old, we need to be very careful as the voltage transients associated with the fault location process often damages the cable causing new defects.
H. Can DC or VLF tests at least detect most of the gross defects?
DC and VLF (very low frequency or 0.1Hz) present such different stress distributions in cable insulation compared to in-service and factory test conditions, most defects are missed. By adding dielectric loss or tangent delta measurements—or even PD measurements—a few more percent of defects can be detected.
That said, these approaches are generally less than a ten percent solution and unfortunately can cause damage without warning. Best practice recommends, these tests be kept to less than the line to ground voltage for less than a minute just to check for existing shorts.
I. Are most cable failures due to overheating?
Overheating is generally a connector problem, not a cable issue. Most failures are actually due to insulation defects, not overheating problems. However, in high load applications, termination and joint connector installation problems on aluminum conductors are notorious for overheating and damaging cable insulation systems.
J. Is helical copper tape a robust shield design?
Helical copper tape functions just fine in paper insulated cable systems, since it is in oil and moisture is kept out by a lead sheath. Solid dielectric cable only has a polymer jacket to protect it and does not stop moisture from causing corrosion, initializing a process of arcing and pitting from the outside inwards which is commonly observed on aged cable.
Generally concentric neutral and longitudinally applied copper tapes have been shown to have a much better long-term performance.
K. Are there any standards we can use test solid dielectric cable systems?
Quality tests by cable and accessory manufacturers are performed on all new system components at the manufacturing plant prior to shipping and installation. All factory-built products must meet standards such as ICEA (Insulated Cable Engineers Association) or IEEE (Institute of Electrical and Electronics Engineers).
The manufacturers’ quality control tests require 50/60Hz partial discharge (PD) diagnostics at an elevated voltage, with generally better than 5 or 10pC sensitivity [Table 1]. These standards can be used to judge the performance of cable system in the field.
L. What can be done to enhance longevity?
The recipe for cable longevity is to push quality to the earliest point in the life of the cable system, and then minimize any extreme duty cycle events. As early in life as possible a cable system should be baselined with an offline 50/60Hz PD test to check for insulation issues and an IR test under high load conditions to check for connector issues.
Once any necessary repairs are complete, confirm sufficient overvoltage protection is installed at all significant impedance change points (primary cable end points). Finally, monitor the cable system for any extreme overvoltage or overloading events (including, reclosing on faults, thumping, withstand or Hipot testing). If such events occur, a new baseline for the insulation and connector will need to be established.
Ben Lanz has spent 20 years in the power cable industry. He currently holds the position of Director of Applications Engineering at IMCORP and has technical oversight of power cable life cycle consulting. He is a Senior Member of the IEEE Power & Energy Society, a voting member of the IEEE Standards Society, and a member of the IEEE Dielectrics and Industrial Applications Societies. He has served as Chairman of the Insulated Conductors Committee (ICC) technical committees responsible for cable testing, cable reliability and surge arresters, Chairman of the American Wind Energy Association (AWEA) O&M Balance of Plant technical subcommittee, a UL technical study committee member for MV and HV DC cables, and is a reviewer and voting pool member for InterNational Electrical Testing Association (NETA) standards. Mr. Lanz received his electrical engineering degree from the University of Connecticut (UCONN) under mentorship of Director of the Institute of Material Science Electrical Insulation Research Center (EIRC), Dr. Matthew Mashikian. He has published dozens of papers on power system reliability, asset management, and diagnostics and regularly presents on the topics.