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Long-term conductivity decrease of polyethylene and polypropylene insulation materials
KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.ORCID iD: 0000-0003-1393-9511
2017 (English)In: IEEE Transactions on Dielectrics and Electrical Insulation, ISSN 1070-9878, Vol. 24, no 3, p. 1485-1493Article in journal, Editorial material (Refereed) Published
Abstract [en]

This work summarizes the results of a number of DC conductivity measurement studies on polypropylene (PP), low density polyethylene (LDPE), and cross linked polyethylene (XLPE). The main observation is that under apparently time-constant external conditions (voltage, temperature, etc.) no steady-state direct current (DC) was established even after very long measurement times. Nevertheless, this behavior seems to exhibit some common systematic features, and since the experiments were performed with different equipment at different R&D labs in different years by different teams, simple measurement artefacts can be excluded. One observation is that there are two electric field regimes with slightly different behavior, separated by crossover field of about 10-15 kV/mm. In this work we focus on the high-field region, where the main observation is that the conductance slowly decays sub-linearly with time, I~ t-n, with 0.3 <; n <; 1, n mostly around 0.5. We provide experimental indications that this behavior is rather independent of the presence of 1) the voltage and 2) the peroxide decomposition products (in PE). The observations are in favor of an underlying thermally driven relaxation process related to structural changes (morphology, free volume) of the polymer. A main implication of the results is that the use of steady-state conductivity values for the characterization of certain polymer insulation is not appropriate and instead the decaying behavior of the conduction current must be considered.

Place, publisher, year, edition, pages
2017. Vol. 24, no 3, p. 1485-1493
Keywords [en]
HVDC insulation, Conduction, Polyethylene, Polypropylene, Charge injection, Morphology
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-228361DOI: 10.1109/TDEI.2017.006218ISI: 000405000300021Scopus ID: 2-s2.0-85022335154OAI: oai:DiVA.org:kth-228361DiVA, id: diva2:1209373
Note

QC 20180523

Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2018-05-23Bibliographically approved
In thesis
1. Characterization of Conduction and Polarization Properties of HVDC Cable XLPE Insulation Materials
Open this publication in new window or tab >>Characterization of Conduction and Polarization Properties of HVDC Cable XLPE Insulation Materials
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High voltage direct current (HVDC) cables with extruded insulation system were introduced in 1998. Since then this technology has been growing rapidly with many installations that are today operational at voltages up to 320 kV. With fast developments during the last few years, extruded DC cable systems for operation voltages as high as 640 kV are now commercially available.

Due to the importance of DC conductivity in the distribution of electrical field across the cable insulation, a good understanding of the DC conduction physics is of key importance to the design of robust HVDC cables. Currently, crosslinked polyethylene (XLPE) is the dominating material used in the insulation system of extruded DC cables. Since XLPE includes peroxide decomposition products (PDP), understanding their role on conduction behavior of the insulation system has been of great interest. The PDP and other chemical species can move in the system through diffusion which makes the characterization of cable insulation more challenging. Besides, like other semi-crystalline polymers, morphology of the XLPE evolves with temperature and over time. These changes in the material pose challenges to their electrical characterization by requiring stringent control of parameters during preparation, storage and measurement.

In this work, different electrical characterization techniques relevant to polymeric insulation materials are discussed highlighting their applications and limitations. A set of considerations during preparation, storage and measurement of polymeric samples is listed and implemented in the studies. The design and performance of a high voltage DC conductivity measurement setup is evaluated.

Thick low density polyethylene (LDPE) and XLPE press molded plaque samples together with plaque samples extracted from cable insulation are prepared and studied using the high voltage DC conductivity measurement setup. Conductivity measurements are done both under static and dynamic temperature programs. Different preparations and test programs are used and the results are analyzed and compared.

Investigations show that the type of protective press film used during press molding of samples can strongly influence the results and using polyethylene terephthalate (PET) film appears to lead to the higher measured conductivity levels. Studies with dynamic temperature program reveal a non-monotonic temperature dependence of apparent DC conductivity of samples with certain

preparation which to the author’s best of knowledge has not been previously

reported in the literature.

Studies on XLPE and LDPE plaque samples with different heat-treatments show that the behavior of conductivity during thermal dynamics depends on the type of the press film. Upon degassing, samples pressed with PET film show a decline of conductivity at all temperatures and a reduction of the non-monotonic behavior. Based on the results from the experiments, a set of hypotheses as potential explanation of this behavior is put forward and discussed.

Abstract [sv]

Extruderade likspännings (DC) kablar infördes 1998. Sedan dess ha likspännings (DC) kabeltekniken utvecklats snabbt och har lett till många existerande kabelsysteminstallationer med driftspänningar på upp till 320 kV och typprovade extruderade DC kabelsystem har nått driftspänningar på upp till 640 kV.

Den elektriska ledningsförmågan påverkan på den elektrisk fältfördelning i isolationsmaterial vid DC, kräver en god förståelse av hur den elektriska konduktiviteten påverkas av fältstyrka, temperatur och en mängd materialparametrar relaterade till polymerernas morfologi och kemi. Idag är tvärbunden polyeten (PEX) det dominerande material som används i isolationssystem för extruderade DC-kablar. PEX innehåller tvärbindningsrestgaser och det har lett till stort intresse för bättre förståelse av tvärbindningsrestgasers påverkan på materialets elektriska egenskaper. Restgaserna och andra kemikalier är flyktiga och kan röra sig i systemet genom diffusion som gör karakterisering av kabelisoleringen mer utmanande. Dessutom, liksom andra semi-kristallina polymerer, ändras PEX materialets morfologi med temperatur över tiden. Med tanke på att materialet kan ändras under provpreparering, lagring och även vid mätning, så måste samtliga steg ovan väljas mycket försiktigt.

I detta arbete, diskuteras olika karakteriserings tekniker för polymera isolationsmaterial och deras möjligheter och begränsningar. Viktiga aspekter för provpreparering, lagring och mätning är listade till vilka tagits hänsyn till under detta arbete. Design och prestanda av mätuppställningen för DC ledningsförmåga har utvärderats.

Tjocka plattprover av pressad LDPE och PEX samt plattor ursvarvade från kommersiellt tillverkade högspänningskablar preparerades och studier genomfördes på deras ledningsförmåga under högspänning DC. Mätning av ledningsförmåga genomfördes både med konstant och med varierande temperatur enligt specifika program. Olika provprepareringar och provprogrammen användes och resultaten analyserades och jämfördes.

Resultaten visar att typ av pressfilm som använts under provpreparering kan ha en stor betydelse för resultaten och användning av PET filmer leder till högsta ledningsförmåga. Studier med varierande temperatur program hos PEX och LDPE

plattor med visa prepareringar, visar ett icke monotont beroende av DC ledningsförmåga på temperatur som tidigare inte rapporterats i litteraturen.

Studier på PEX och LDPE plattor med olika värmebehandling visar att beteenden beror på typ av press film som används under provpreparering. Efter avgasning, uppvisar prover pressade med PET film en minskning i ledningsförmåga i samtliga temperaturer och en minskning av det icke monotona temperatur beteendet. Baserade på experimentresultaten har ett antal hypoteser som förklaring av observationer tagit fram och diskuterats.

Place, publisher, year, edition, pages
stockholm: KTH Royal Institute of Technology, 2018. p. 89
Series
TRITA-EECS-AVL ; 2018:45
Keywords
HVDC insulation, power cable insulation, cross-linked polyethylene insulation, conductivity, dielectric losses, space charge
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-228220 (URN)978-91-7729-817-5 (ISBN)
Public defence
2018-06-08, Sal Q2, Osquldas Väg 10, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20180521

Available from: 2018-05-22 Created: 2018-05-18 Last updated: 2018-05-23Bibliographically approved

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