Electric Cable Life Assessment and Condition Monitoring

John Clauss, 505-844-5449, jmclaus@sandia.gov
Curt Nelson, 505-845-9253, cnelson@sandia.gov

Sandia has an extensive program to study electric cables, primarily in the context of nuclear power plant safety. Cables are everywhere in power plants, transmitting power and information, and can be the origin of common mode failures, e.g. when many cables fail simultaneously during an accident. Generally, cables perform well in U.S. nuclear plants, but life extension and relicensing are leading to increasing needs for techniques to guarantee cable functionality under both normal and accident conditions.

Cable Life Assessment

– Cable Material Aging/Degradation Modeling – accelerated aging tests introduce new questions which must be taken into account, such as whether short exposure to high temperature is equivalent to long times at moderate temperature, and how to treat the combined effects of time, temperature and radiation dose rate. Ironically, low temperature can inhibit self-healing in polymers. A new promising technique involves measuring the oxygen consumption of the insulating material, which could permit very accurate aging measurements in a short time.

– Lab vs Natural Aging – The Sandia Cable Repository of Aged Polymer Samples (SCRAPS) – Sandia maintains a library of thousands of samples of common cable materials that have been aged in the lab, representing over 2000 separate experimental aging environments.

Condition Monitoring Research

– Electric NDE – a new Electric Cable Evaluation Facility (ECEF) is just going into operation, providing cable tray and conduit systems typical of power plants. The cables have fully characterized mechanical defects, hidden for NDE systems to locate as a “blind” test. To keep things interesting, the size, type, distance down the cable, etc. are varied from one cable to the next. Some cables have no defects. Special access ports permit visual observation of the defects following a “blind” NDE test.

ECEF will be used to test claims made by various developers for new cable NDE techniques, which historically haven’t faired well. It will also serve as a test bed for the development of new methods. Sandia has a proposal in to DOE and NRC for an evaluation of new Russian method called DIACS.

Sandia developed a new high-voltage fast pulse technique for electrical NDE (100kV, 1 nsec risetime, 10-500 nsec pulse width — low total energy insertion). It combines features of time-domain reflectometry (TDR) and partial discharge techniques, with electrical and acoustic indications of breakdown location. Sandia has done limited proof-of-principle testing that shows potential for use in a highly-constrained geometry.

(** Maybe a new approach for in-situ testing of underground distribution cable?? ** )

– Physical Monitoring – Density Technique
Preliminary research has shown a correlation between density and tensile elongation-at-break in many cable materials. Since density measurements can be made on very small samples of material, this may be a new NDE condition monitoring technique. For some materials, the density change occurs at a fairly constant rate. In others, the rate of density increase is slow, then reach a point at which it increases dramatically. The opposite can also be observed, i.e. fast at first, and then slow. Early results indicate that density measurements could represent a very useful condition monitoring technique.

“Density Measurements as a Condition Monitoring Approach .. ”
draft SAND report 1/21/98, KT Gillen, et. al.