Jean-Pierre Crine

Electrical aging of extruded dielectric cables: review of existing theories 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.

A molecular model to evaluate the impact of aging on space charges in polymer dielectrics

The understanding of space charge effects depends largely on how concurrent phenomena, such as thermal and electrical aging are understood. A model describing thermal and environmental aging of dielectric polymers is discussed. The model essentially is based on the rate theory associated with the name of Eyring. One basic feature of the model is that the proper energy term is a free energy of activation /spl Delta/G, which implies that the entropy change of the process cannot be neglected. Several examples are given to show that /spl Delta/G is related directly to some physical or chemical parameter involved in thermal or environmental aging. A model describing the mechanical aging of polymers is also discussed. It is shown that submicrocavities can be generated by mechanical stresses above a given critical level, the value of which depends on the energy of cohesion of the polymer. The size of these defects are in good agreement with the Griffith criterion. Finally, the same model adapted to electrical stresses is shown to describe the electrical aging of polyethylene. It predicts that the most deleterious influence of aging is above a given critical field (similar to the critical stress in mechanical aging), where submicrocavities are induced by the electromechanical forces associated with the ac field. Electrons can then move without scattering within the submicrocavities and this may lead to further degradation. The relations between thermal, mechanical and electrical aging and space charges in polymers are addressed briefly and domains requiring more work are mentioned.

Electrical aging of extruded dielectric cables. A physical model

A model is proposed to describe all experimental results on electrical aging of cables reported in Part 1. It is based on simple thermodynamics concepts in the Eyring theory, includes the concept of submicrocavity formation proposed by Zhurkov, and supposes that the first step in electrical aging is essentially a molecular process, as in Crine and Vijhs model. Our model of electrical aging under ac fields supposes that molecular-chain deformation is essentially a fatigue process and, therefore, that high frequencies generate more defects and thus reduce cable life, as indeed demonstrated by others. An original feature of the model is the submicrocavity formation above a critical field F/sub c/, whose value can be approximately predicted knowing the energy of cohesion of the polymer. This leads to a simple lifetime equation depending on just two physical parameters /spl Delta/G/sub 0/ (energy of activation of the chain deformation process) and /spl lambda//sub max/ (the maximum size of submicrocavities) with no adjustable unknowns. Above F/sub c/, there is an exponential relation between time and field, whereas below F/sub c/, the breakdown strength of the insulation varies very little with time; in other words, there is very limited (if any) aging. The slope of the exponential regime gives the value of /spl lambda//sub max/ directly whereas the intercept gives the value of /spl Delta/G/sub 0/. The predictions made by the model are discussed in correlation with existing experimental data. In addition to these basic assumptions, the model confirms that there is a relation between cable endurance and insulation morphology. Actually, the size of submicrocavities is ultimately limited by the amorphous-phase thickness. The max values deduced from the slopes of the exponential regime between F and log t for polyethylene (PE) (Part 1), XLPE and EPR insulation are in excellent agreement with the size of the amorphous phase of these samples, as measured by X-ray spectroscopy. It is also shown that the presence of water results in a lower /spl Delta/G/sub 0/ value, i.e. a shorter life. The precise relation between /spl Delta/G/sub 0/ and the nature and concentration of the impurity (including water) needs more work. The impact of these conclusions on the experimental limits of a reliable accelerated aging test and on the final breakdown process are discussed in a subsequent paper.

On the interpretation of some electrical aging and relaxation phenomena in solid dielectrics

Electrical aging and relaxations of solid dielectrics are phenomena studied for decades but still largely poorly understood. In this paper, we show that some of the theories often cited in the literature are wrong or inappropriate. The relations between aging, space charges and polarization of polyethylene (PE) under the influence of high electrical fields (above 20 kV/mm) are discussed in light of our electrical aging model. Below a so-called critical field, the activation volume of the aging process is dependent on the field-induced strain. It is our contention that strong charge injection occurs only after nanocavity formation, i.e. above the critical field. The amorphous phase is then significantly deformed and weak van der Waals bonds are broken, leading to another faster aging regime. The possible relation between the nanocavity formation at moderate fields and bonds breaking at higher fields proposed in our aging model and various polarization measurements is discussed. One objective of this paper is to encourage the development of more complex and complete theories specific to dielectrics. Some experimental work needed to achieve this goal is pointed out.

A thermodynamic model for the compensation law and its physical significance for polymers

Abstract This paper describes how the compensation law can be explained by a linear relation between the activation entropy and enthalpy of a given process in a polymer. These two variables are related by the thermal expansion coefficient and a constant approximately equal to the Rao acoustical parameter. A relation between the activation free energy and some thermodynamic parameters is presented. The activated volumes for the α and β relaxations of polyethylene are shown to vary with temperature and cry-stallinity. The activated volume has also been calculated for some other polymers and is of the order of 1 to 6 molar volumes at 295 K.

Electrical, chemical and mechanical processes in water treeing

Water treeing is a complex phenomenon involving several processes with many synergistic effects. Although a huge number of papers on the subject have been published over the last 25 years, there is no comprehensive theory able to describe the often contradictory experimental results. However, there are some tendencies that are always observed, whatever the experimental conditions. A critical review of some electrical, chemical and mechanical processes is made. In fact, water treeing is likely to be due to a mixture of all these processes. When existing data is examined carefully, it appears that very few theories can explain the results. It is concluded that voltage and frequency are major aging factors, and the role of oxidation may be less important than sometime was suggested. The energy of ion reduction is pointed out as a factor that may possibly influence water tree initiation and growth. The solubility parameter of additives and solutions is another factor that deserves some attention. Finally, the polymer morphology appears to affect water treeing greatly through modifications of its mechanical properties. Before a detailed model could be proposed, several studies remain to be done and thorough investigations devoted to the initiation phase and to synergistic effects are required. The development of a reliable and significant accelerated aging test simulating field conditions is also suggested.

Polymer oxidation and water treeing

In order to determine whether or not oxidation of polymer influences water treeing, more than 200 vented and bow-tie trees from 31 field-aged cables were investigated. Micro-IR spectroscopy analysis has not shown any consistent excess of carbonyl content in water trees as compared with the adjacent non-treed regions of the insulation. Although the levels of carbonyl content of the bulk of the polymer within the treed regions are similar to those in untreed regions of the polymer, vented trees are more susceptible to oxidation if subjected to high temperatures in the presence of oxygen. It was observed that vented trees initiate at similar rates in XLPE in either a nitrogen or air atmosphere. This indicates that tree initiation is rather independent of the presence of oxygen. However, the tree growth rate is slower in nitrogen than in air, the actual difference being affected by the type of ionic solution used. This further suggests that some, not yet known, chemical reactions between oxygen, XLPE and ions play an important role during water tree propagation. The IR absorption band at (1585 cm/sup -1/), typical of carboxylates, was detected in some water trees. It should be noted that the large absorption band of water at (1640 cm/sup -1/) often masks the smaller carboxylate band. Under laboratory conditions carboxylate groups were detected on oxidized nontreed XLPE surfaces. These results do not imply that carboxylates are responsible for tree propagation but confirm only that carboxylate groups are formed during XLPE oxidation. Thermal pre-oxidation of XLPE, to the levels measured in typical field-aged cables, has little or no effect on the initiation and growth of vented water trees. Very high levels of pre-oxidation (as determined by the carbonyl content), at least 80/spl times/ the average oxidation level measured in typical field-aged cable, retard the growth of vented water trees. However, these high levels of oxidation negatively affect the dielectric properties of the insulation.

A model of aging of dielectric extruded cables

The authors present a model of electrical aging of extruded cables based on actual data and realistic experimental conditions. It is shown that a simple model partially based on the rate theory associated with the name of Eyring describes very well the aging of polyethylene (PE), crosslinked PE, and ethylene-propylene rubber cables under a wide variety of conditions. The significance of the physical and thermodynamic parameters used in the model is discussed in relation to the polymer morphology. It is speculated that severe and irreversible electric aging of dielectrics is preceded by the formation of microcavities in the 50-100-AA range.<<ETX>>

Influence of internal mechanical stress and strain on electrical performance 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.

A new analysis of the results of thermally stimulated measurements in polymers

It is shown that the compensation law observed in thermally stimulated measurements (TSC and TSDC) between the activation energy E and the preexponential term τ0 in the Arrhenius equation for various polymers can be explained by a linear relation between the activation enthalpy ΔH‡ and entropy ΔS‡ of the process. When this relation is combined with the well‐known rate theory, TSC or TSDC results with polymers indicate that both techniques yield very similar results. It is also deduced that they are especially sensitive to entropy changes in the polymers studied. This points out the non‐negligible value of ΔS‡ in polymer relaxations, and this implies that the proper energy term describing a polymer relaxation is not E but a free‐energy term ΔG. The relation between the Vogel–Tammann–Fulcher and rate theory equations is also made. The correlations between the ΔG, ΔH‡, and ΔS‡ values, and the polymer thermodynamic and morphological properties are also briefly discussed.