Electrical aging of extruded dielectric cables: review of existingtheories and data

Despite the huge amount of data on so-called electrical aging of extruded HV cables, the fundamental phenomena responsible for it or evolving with aging time, are still far from well understood. It is therefore not surprising why it is so difficult to predict reliable cable lifetimes in service from accelerated aging experiments in the laboratory. The objective of this paper is to review critically the existing theories of electrical aging of solid dielectric materials. A relatively large number of models and theories exist but none of the most often used is known to yield reliable life predictions. One conclusion is that there is a need for a more comprehensive model of electrical aging of extruded dielectric cables. In order to develop this model, an extensive review of existing literature data was undertaken. This paper summarizes the data collected from more than 200 papers on aging of PE, XLPE and EPR cables. It appears that cable breakdown strength should not be plotted on log field vs. log time graphs to yield long-time (i.e. low-field) values, since results obtained over a long time period do not obey an inverse power law. In fact, high-field results are better described by an exponential relation between time and field. The models of Simoni, Montanari and Crine seem to give the best fit to experimental results obtained under a wide variety of experimental conditions. It is also shown that the lower field limit for the exponential regime with XLPE cable is in the 8 to 15 kV/mm range, which corresponds to the onset of strong charge injection. The influence of environment, insulation nature and morphology, and testing temperature are discussed     

Successful testing of 345-kV XLPE cables and premolded joints atIREQ

Extruded polymeric cables and accessories are an alternative to self-contained fluid filled (SCFF) cables for extra-high-voltage (EHV) systems. Crosslinked polyethylene (XLPE) insulated cables have many advantages over the traditional fluid-filled pressurized cable system with the elimination of the hydraulic system and the associated equipment and complications during the installation and operation/maintenance of such systems. Other advantages of the extruded cable system arise from the new accessory technologies and the use of splices and terminations that are prefabricated and pretested in the factory and require less time to install on site. Concerns over the long term reliability of high voltage cable systems, in particular the accessories and the lack of service experience above 300 kV led to the decision of Hydro-Quebec to carry out a prequalification (long-term) test program to assess the reliability of the cable materials and to verify the cable and accessory installation methods to be employed. The cables were installed in duct banks and manholes simulating actual installation conditions used in Hydro-Quebec underground cable network. This paper describes the program and results of the prequalification tests of 345-kV XLPE cables and accessories performed at Hydro-Quebec's Research Institute (IREQ) in partnership with three international cable manufacturers, Alcatel, Fujikura and Pirelli     

Mental addition versus subtraction in counterfactual reasoning: On assessing the impact of personal actions and life events.

Assessing the consequences of actions and events often requires comparing a mental simulation of the world in which the action is present to one in which the action is absent. We propose that people perceive more impact when asked to assess whether an action would increase the likelihood or degree of a potential outcome (mental addition) than when asked whether it would reduce the probability or extent of a potential consequence (mental subtraction). This judgmental asymmetry occurs because people (a) give more weight to features of the particular mental simulation (the action or its absence) serving as the subject of comparison and (b) give more weight to factors that produce as opposed to inhibit the relevant outcome. In 4 studies, Ss assessed the impact of personal actions (e.g., studying for an exam). Ss perceived more impact when the assessment was placed in an additive frame (e.g., "how many more questions will you get right if you study?") as opposed to a subtractive one (e.g., "how many fewer will you get right if you do not study?"). This effect was not influenced by the hedonic value of the event or by whether the S had actually experienced it. Discussion centers on the relevance of these results for the undoing of scenarios and causal attribution. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Classifying college students as depressed or nondepressed using the Beck Depression Inventory: An empirical analysis.

In Study 1, the classification of 150 college students as nondepressed or mildly or moderately depressed was compared using Beck Depression Inventory (BDI) scores obtained on 2 occasions, either on the same day or 1 wk later. Whereas the overall test–retest reliability coefficients were acceptable, particularly when both administrations occurred on the same day, the consistency of classification of Ss into the mild and moderate levels of depression was poor. In an attempt to determine the factors contributing to the poor classification consistency, a 2nd study with 207 Ss compared the effect of exposure to the BDI on subsequent BDI responses. Ss who completed the BDI on 2 occasions showed significantly lower BDI scores compared to Ss exposed to an alternate depression inventory (the Self-Rating Depression Scale [SRDS]) or no prior questionnaire. In addition, a large percentage of Ss classified as moderately depressed following the initial BDI administration failed to be classified within the moderate categories on retesting the same day. Ss tested using the SRDS maintained the same depression classification when retested on the BDI. Item analyses failed to reveal significantly different amounts of change among individual BDI items. (39 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of internal mechanical stress and strain on electricalperformance of polyethylene electrical treeing resistance

Crosslinked polyethylene (XLPE) and low-density polyethylene (LDPE) insulations used in HV cables are not only subjected to electrical and thermal stresses, but also exposed to mechanical stresses, whether residual internal stresses created during the cooling process of the fabrication, external forces when cables are bent during installation or thermomechanical stresses caused by differential thermal expansion between the conductor and the polymeric material. In order to investigate the possible influence of mechanical stresses on dielectric properties of polyethylene, measurements were conducted on pin-plane XLPE and LDPE samples with various magnitudes of residual mechanical stresses around the embedded electrode. The time to inception, the growing rate and the shape of the electrical trees under different voltages are reported in this paper. Specimens with the highest values of residual stresses were found to have the shortest inception times and the longest trees after one hour of aging under different voltages. When the mechanical stress was allowed to relax, the treeing resistance was measured to be significantly improved     

The mysteries of multifactor ageing

Ageing of solid insulating materials and systems, according to IEC and IEEE standards, is the occurrence of irreversible deleterious changes that critically affect performance and shorten useful life. What causes ageing is not an easy question to answer, nor is it a function of an isolated event or applied stress. Years of experience have shown that the degree and rate of ageing of insulation depend on the physical and chemical properties of the material, the nature and duration of applied/induced stress(es), and material processing and treatment during manufacturing and subsequent use in equipment. Stresses that affect the integrity, reliability, and service life of electrical equipment and components, such as switchgears, transformers, and cables, include electrical, thermal, mechanical, radiation and/or environmental ones. These stresses applied sequentially or simultaneously lead to ageing and deterioration, but the rates of ageing are different and are not easily explainable