A critical evaluation of analytical. techniques for the characterization of extruded dielectric cables (original) (raw)
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Electrical characterization of extruded DC cable insulation — The challenge of scaling
IEEE Transactions on Dielectrics and Electrical Insulation, 2017
HVDC cable technology with extruded insulation systems have been growing rapidly in the recent years. Different insulation concepts including crosslinked or thermoplastic polymers with or without particle fillers have been studied intensively. The DC conduction in the insulation systems is one of the most important mechanism in dielectric physics; therefore reliable and representative methods are needed to characterize it. In the development process of HVDC cables from small scale plaque sample experiments to full scale cable testing, high field DC conductivity measurement and space charge measurement with the pulsed electro-acoustic (PEA) method are common. These two methods provide two different views into the conduction physics in the insulation and provide different types of information. But both of these methods have their own drawbacks and limitations which are important to keep in mind when choosing measurement methods in different stages of the development process. Another important aspect is the inherent differences between the different types of test samples. Thin pressed plaque samples are easy to produce and require less complicated testing equipment than experimental cables but there are major differences which should not be ignored. In this paper, the principles, advantages and limitations of DC conductivity and PEA measurements on samples of different scales are described and their relevance to the evaluation process is discussed. Some of the less discussed challenges of these measurement methods on different objects are discussed in more details and recommendations are made for obtaining more useful results.
During the last decade, Very Low Frequency (VLF) testing for extruded distribution cables has gained interest among the North American utilities. The increasing interest is evidenced by recent research publications and discussions inside the expert community in which standards are being proposed and continuously discussed. While there is a general consensus as to the meaning of insulation dielectric properties, many open issues still remain for discussion in order to produce a more accurate evaluation. Consequently, this paper will discuss a number of the practical issues that arise when making these measurements at VLF on field aged and non-aged cables, particularly Tan δ measurements. The discussion is based on data from laboratory experiments and field testing.
Polymer Testing, 2015
Dielectric spectroscopy was evaluated as a condition monitoring technique for aged polyethylene electrical insulation in nuclear power plants. Bare core insulations of crosslinked polyethylene were aged at 55 and 85 C under exposure to 60 Co g-radiation at different dose rates (0.42, 0.76 and 1.06 kGy h À1) to different total doses. The samples were studied by dielectric spectroscopy and tensile testing, and the crystallinity, mass fraction of soluble component and density were determined. The oxidation profiles along the depth of the insulations were assessed by infrared microscopy. The aged samples showed an increase in both the real and imaginary parts of the dielectric permittivity over the whole frequency range studied, an increase in the mass fraction of soluble component and in the material density, and a decrease in the strain-at-break. The imaginary part of the dielectric permittivity at 100 kHz increased in a linear fashion with increasing material density, the latter being strictly related to the extent of oxidation of the material according to infrared spectroscopy and differential scanning calorimetry. The generic relationship between the imaginary part of the permittivity and the density included all the data obtained under different ageing conditions. The results suggest that dielectric spectroscopy can be used for in-situ measurements of the degree of oxidation of polyethylene cables, in order to obtain information about the condition of the cable insulation to enable the remaining lifetime to be predicted.
New very low frequency methods for testing extruded cables
IEEE International Symposium on Electrical Insulation, 1990
R. F e u r s t e i n and A. F i l z BAUR P r u f -und Messtechnik, Sulz, A u s t r i a 1. A b s t r a c t Without exception, b u r i e d p l a s t i c cables f o r m e r l y were t e s t e d w i t h d.c. voltage. Long l i f e space charges thus developed, which i n j e c t e d e l e c t r i c a l t r e e s a t t h e f o l l o w i n g a.c. voltage s t r e s s . Hence, a complete breakdown i s programmed. The examination o f b u r i e d cables w i t h mains frequency o f t e n i s impossible due t o t h e h i g h weight o f such a t e s t system and because o f t h e l a r g e c a p a c i t i v e power consumption. In combination w i t h a l o s s f a c t o r measurement equipment, t h e introduced mobile 0.1 Hz t e s t system w i t h s i n u s o i d a l a.c. voltage o f f e r s t h e p o s s i b i l i t y t o t e s t even l o n g cables w i t h l a r g e capacitance a t s i t e and nominal voltage U,. Thus, t h e r e i s t h e p o s s i b i l i t y t o r e c e i v e c l e a r statement on c o n d i t i o n o f cable i n s u l at i o n , w i t h o u t endagering t h e cable. P r a c t i c a l t e s t s on n a t u r a l l y aged cables show a strong r e l a t i o n s h i p between l o s s f a c t o r and breakdown voltage. 2. I n t r o d u c t i o n In t h e past, t h e i n s u l a t i o n o f b u r i e d hv cables was t e s t e d w i t h h i g h d.c. voltage. As here no r e a c t i v e power i s consumed, t h e t e s t i n g instruments can be small and l i g h t -w e i g h t . Though i n most o f t h e standards t h e d.c. h i g h v o l t a g e t e s t i n g o f b u r i e d PE/XLPE cables i s assigned, t h i s method can cause problems. So i t was observed several times t h a t breakdown occured i n p l as t i c cables when they were p u t i n t o o p e r a t i o n again s h o r t l y a f t e r they had been s u c c e s s f u l l y t e s t e d w i t h h i g h d.c. voltage. 3. Space charge phenomena A t needle-plate arrangements i n PE/XLPE negative space charges were observed i n t h e peak area which a r e decomposed even w i t h time constants o f hours. These space charges a r e caused by e l e c t r o n s which can be r e t a i n e d a t f a u l t y areas o f t h e cable. There a r e t r a p s i n t h e band model between t h e conduct i o n and valence band which can r e c e i v e e l e c t r o n s and thus b u i l d up negative space charges. It i s however time dependent t o f i l l these t r a p s w i t h e l e c t r o n s ( 1 ) . A t new i n s t a l l a t i o n s , where no water t r e e s a r e t o be expected, i t i s p o s s i b l e t o t e s t w i t h h i g h e r voltages. But t h e cables have t o be discharged v i a s u f f i c i e n t l y l a r g e r e s i s t o r s so t h a t t h e r e w i l l n o t occur any czc i l l a t i o n . I n a d d i t i o n , t h e cable has t o be shortened f o r approx. 24 hours, b e f o r e i t i s connected t o t h e mains voltage. A spark a t t h e cable terminal should be avoided i n any case since, due t o propagation o f waves o f h i g h p o t e n t i a l very f a s t p o l e changing stresses occur, which can damage t h e cable. For t h i s reason, on no account should p r o t e c t i v e gaps be f i x e d a t t h e c a b l e terminals. 5. a.c. voltage t e s t i n g With a.c. voltage s t r e s s you need n o t f e a r damage o f t h e i n s u l a t i o n by b u i d i n g l o n g l i f e space charges ( 6 ) .
Proceedings of the Nordic Insulation Symposium, 2017
Good understanding of insulation conductivity is of great importance in development and design of high voltage DC cables. Conductivity of polymeric insulation materials is a property which is quite sensitive to many parameters such as temperature, electric field, chemical composition, thermal history, morphology, etc. Therefore, due to different process history, the results obtained from pressed plaques are not necessarily representative of the insulation behavior in an extruded cable. A method was developed to obtain samples from HV cable in form of plaques with a thicknesses up to a few millimeters. Plaque samples were extracted from two cables, their conductivity at high field was measured and compared to those obtained from fullscale tests; the results, confirm a very good agreement between the small scale and full-scale measurements; this hints to a promising prospect for such small scale characterization techniques.
Electrical aging of extruded dielectric cables: review of existing theories and data
IEEE Transactions on Dielectrics and Electrical Insulation, 1996
A model is proposed to describe all experimental results on electrical aging of ci bles 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 si ipposes that the first step in electrical aging is essentially a molecular prucess, as in Crine and Vijh's model. Our model of electrical aging under ac fields supposes that molecul ir-chain deformationis essentially a fatigue process and, therefore, that high frequencies g,enerate more defects and thus reduce cable life, as indeed demonstrated by others. An wiginal feature of the model is the submicrocavity formation above a critical field F,, whose Value can be approximately predicted knowing the energy of cohesion of the polylr er. This leads to a simple lifetime equation depending on just two physical parameteis AGO (energy of activation of the chain deformation process) and La, (the maximunl size of submicrocavities) with no adjustable unknowns. Above F,, there is an exponeni ial relation between time and field, whereas below F,, the breakdown strength of the inoulation varies very little with time; in other words, there is very limited (if any) aging. TI ie slope of the exponential regime gives the value of A , , , directly whereas the interce 3t gives the value of AGO. 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 insulationmorphology. 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 1 >g t for polyethylene (PE) (Part 11, XLPE and EPR insulation are in excellent agreement ivith 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 AGO value, i.e. a shorter l de. The precise relation between AGO and the nature and concentration of the impurity including water) needs more work. The impact of these conclusions on the experiment 11 limits of a reliable accelerated aging test and on the final breakdown process are discus sed in a subsequent paper.
Aging of Oil-Filled Cable Dielectrics
IEEE Transactions on Power Apparatus and Systems, 1970
A description of aging stability for self-contained oilfilled cable dielectrics is given. The power factor of oil-filled cable dielectrics over an extended period and the dielectric properties of low-pressure self-contained oil-filled cable withdrawn from service were studied in the laboratory. From the test results, the authors suggest that the increase in the power factor of oil-filled cable after long use is mostly attributable to the deterioration of the mineral insulating oil. This conclusion clearly differs from the results
Dielectric and Mechanical Behavior of Thermally Aged EPR/CPE Cable Materials
2018 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP)
Power and instrumentation cables play a crucial role in the safe operation of Nuclear Power Plants (NPPs). Thermal and other stressors present in the reactor environment cause the cable materials to degrade. In this work, dielectric and mechanical properties of cable insulation and jacket materials are studied as they are thermally aged, supporting development of non-destructive evaluation sensors for monitoring cable aging. Materials selected for this study are found in certain types of single-core unshielded power cables. These utilize ethylene propylene rubber (EPR)-based insulation material and chlorinated polyethylene (CPE)-based jacket material. Flat mats of these materials were obtained from the cable manufacturer and thermally aged at 140 °C in an air-circulating oven. Elongation-at-break was measured on tensile specimens stamped from the aged mats, and dielectric properties were measured from 100 Hz to 100 kHz using a parallel plate capacitor and precision LCR meter. In the case of aged EPR-based materials, rapid decrease in elongation at break indicating end of useful life was accompanied by a significant increase in dissipation factor, D, measured at 100 kHz. Capacitive measurement of D shows promise, therefore, as a non-destructive indicator of corresponding mechanical property changes in thermally-aged EPR-based insulation materials.
Recent experiences with after-laying tests of solid dielectric transmission class cables
2007
Over the past two years, a number of transmission and distribution class XLPE cable circuits located primarily the Middle East have been subjected to after-laying tests consisting of ac high potential over voltage tests and partial discharge tests as part of a commissioning test package. The hi-potential tests were performed in accordance with IEC 62067 [1] and IEC 60840 [2] and the partial discharge tests were performed using commercially available partial discharge monitoring technology. The results obtained and experience gained are presented in this paper. Słowa kluczowe: kable XLPE, próby pomontażowe, wyładowania niezupełne.