Electrical treeing and breakdown of mechanically strained XLPE cable insulation

Ildstad, Erling; Hagen, Svein Thore

doi:10.1109/elinsl.1992.247035

Cited by 22 publications

(10 citation statements)

References 9 publications

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“…This may affect the electrical performance of power cables in service. Consequently, there are considerable studies in last few decades that explore the influence of mechanical stress on the dielectric breakdown strength [1][2], on the propagation of electrical tree [3] of crosslinked polyethylene (XLPE) power cable insulation. Ilstead et al [1] showed that a significant reduction in ac breakdown strength occurred when XLPE cable samples were subjected to 21% strain.…”

Section: Introductionmentioning

confidence: 99%

“…Consequently, there are considerable studies in last few decades that explore the influence of mechanical stress on the dielectric breakdown strength [1][2], on the propagation of electrical tree [3] of crosslinked polyethylene (XLPE) power cable insulation. Ilstead et al [1] showed that a significant reduction in ac breakdown strength occurred when XLPE cable samples were subjected to 21% strain. Our early research [3] indicated that electrical tree growing characteristics have been significantly changed when mechanical stress is introduced.…”

Section: Introductionmentioning

confidence: 99%

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Impact of mechanical deformation on space charge in XLPE

Chen

Kamaruzzaman

2007

2007 IEEE International Conference on Solid Dielectrics

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Abstract:In this paper we report the effect of mechanical deformation on space charge dynamics in crosslinked polyethylene. Thin films were peeled from a 66 kV commercial XLPE cable. Space charge measurements under dc electric fields have been monitored using the pulsed electroacoustic (PEA) technique. It has been found that charge dynamics in deformed XLPE are different from that from undeformed XLPE at lower voltages. At low applied electric field, space charge is dominated by heterocharge in the deformed XLPE while homocharge governs charge formation in undeformed XLPE. However, at high electric field, there is no significant difference between them. Space charge is dominated by homocharge in both deformed and undeformed XLPE. The results indicated that through deformation, molecular chains rearrange so small molecules such as crosslinking by-products, initially being trapped, can be released. Under the influence of the applied electric field they can be ionised to form heterocharge in the sample.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Impact of mechanical deformation on space charge in XLPE

Chen

Kamaruzzaman

2007

2007 IEEE International Conference on Solid Dielectrics

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“…The effect of internal stresses on electrical tree growth, determined by photo-elastic techniques, was also reported by Champion, et al [3] and David, et al [4] in 1992 and 1994, respectively. Whereas the above were concerned with laboratory specimens, similar conclusions have also been drawn from more practical applications, such as in cables, in numerous publications from Densley in 1979 [5] through to Ildstad and Hagen in 1992 [6].…”

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confidence: 74%

Mechanical prestressing improves electrical strength

Varlow

2002

IEEE Electr. Insul. Mag.

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T here is nothing original in the proposition that mechanical stress has a controlling influence on the propagation of electrical trees in insulating materials. Over a quarter of a century ago, Billing and Groves [1] and later Arbab and Auckland [2] were discussing the effect of externally applied mechanical stresses-for example by bending-on tree growth. In a simple bending experiment, it is immediately obvious that tree growth is accelerated in regions of tensile stress and retarded where compressive stress is present. The effect of internal stresses on electrical tree growth, determined by photo-elastic techniques, was also reported by Champion, et al. [3] and David, et al. [4] in 1992 and 1994, respectively. Whereas the above were concerned with laboratory specimens, similar conclusions have also been drawn from more practical applications, such as in cables, in numerous publications from Densley in 1979 [5] through to Ildstad and Hagen in 1992 [6].The theoretical analysis of the mechanical aspects of tree growth began with Zeller, et al. [7], [8], who attributed the growth of filamentary cracks to electrostatic forces, a process they named electrofracture. This theoretical approach has been developed subsequently by Hikata, et al. [9] and Fothergill [10].On the basis of this approach to an understanding of the physical processes, there is a strong temptation to interpret electrical tree growth in terms of fracture mechanics, but the analogy must not be pushed too far. Certainly, it is possible to draw a direct correlation between treeing characteristics and the mechanical properties of insulating materials. A preliminary study of tree inception and growth in materials of greatly differing mechanical properties [11] revealed that the mechanical characteristics of the material were paramount. It was proposed that the residual mechanical stress developed under alternating voltage application produced fatigue cracking in the inception phase. Since crack formation is determined by, among other things, tensile strength, it followed that this parameter would influence the tree initiation time. This view was supported by measurements of the tree inception times of polyester resin and an elastomer (PL-3). The tree inception time in the elastomer was less than 30% that of the polyester resin, which had a tensile strength five times greater than that of the elastomer.The buildup of internal tensile stress due to the alternating electrostatic forces, as well as leading to crack formation and tree initiation, will also increase the material's propensity for tree growth. It follows that tree growth in elastic materials may be less rapid than in inelastic materials, since the former will not suffer from long-term mechanical stress buildup. Thus, a material's modulus of elasticity could provide a measure of its resistance to tree growth. This was confirmed by a comparison between PL-3 and polyester resin, which demonstrated an inverse dependence of the time for a tree to cross the interelectrode spacing on the modulus of...

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“…It is not an inert material. It is denatured by water, which produces moisture and initiates teeing [11,12]; the polymerized chains which have been produced during the curing sequence can be broken or naturally split by time and temperature. This is a natural aging.…”

Section: The Design Of High Voltage Insulation Systems and The Main Rmentioning

confidence: 99%

The Insulation for Machines Having a High Lifespan Expectancy, Design, Tests and Acceptance Criteria Issues

Barré¹,

Napame²

2017

Machines

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Abstract:The windings insulation of electrical machines will remain a topic that is updated frequently. The criteria severity requested by the electrical machine applications increases continuously. Manufacturers and designers are always confronted with new requirements or new criteria with enhanced performances. The most problematic requirements that will be investigated here are the extremely long lifespan coupled to critical operating conditions (overload, supply grid instabilities, and critical operating environments). Increasing lifespan does not have a considerable benefit because the purchasing price of usual machines has to be compared to the purchasing price and maintenance price of long lifespan machines. A machine having a 40-year lifespan will cost more than twice the usual price of a 20-year lifetime machine. Systems which need a long lifetime are systems which are crucial for a country, and those for which outage costs are exorbitant. Nuclear power stations are such systems. It is certain that the used technologies have evolved since the first nuclear power plant, but they cannot evolve as quickly as in other sectors of activities. No-one wants to use an immature technology in such power plants. Even if the electrical machines have exceeded 100 years of age, their improvements are linked to a patient and continuous work. Nowadays, the windings insulation systems have a well-established structure, especially high voltage windings. Unfortunately, a high life span is not only linked to this result. Several manufacturers' improvements induced by many years of experiment have led to the writing of standards that help the customers and the manufacturers to regularly enhance the insulation specifications or qualifications. Hence, in this publication, the authors will give a step by step exhaustive review of one insulation layout and will take time to give a detailed report on the standards that are linked to insulation systems. No standard can provide insurance about lifespan, nor do any insulation tests incorporate all of the operating conditions: thermal, mechanical, moisture and chemical. Even if one manufacturer uses the standards compliance to demonstrate the quality of its realization; in the end, the successful use in operation remains an objective test. Thereafter, both customer and manufacturers will use the standards while knowing that such documents cannot fully satisfy their wishes. In one 20-year historical review, the authors will highlight the duration in insulation improvements and small breakthroughs in standards writing. High lifespan machines are not the main interest of standards. A large part of this publication is dedicated to the improvements of the insulation wall to achieve the lifespan. Even if the choice of raw materials is fundamental, the understanding of ageing phenomena also leads to improvements.

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Electrical treeing and breakdown of mechanically strained XLPE cable insulation

Cited by 22 publications

References 9 publications

Impact of mechanical deformation on space charge in XLPE

Impact of mechanical deformation on space charge in XLPE

Mechanical prestressing improves electrical strength

The Insulation for Machines Having a High Lifespan Expectancy, Design, Tests and Acceptance Criteria Issues

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