Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Characterization of Conduction and Polarization Properties of HVDC Cable XLPE Insulation Materials
KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.ORCID iD: 0000-0003-1393-9511
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 [en]
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: urn:nbn:se:kth:diva-228220ISBN: 978-91-7729-817-5 (print)OAI: oai:DiVA.org:kth-228220DiVA, id: diva2:1208725
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
List of papers
1. Robust characterization of the DC-conductivity of HVDC insulation materials at high electric fields
Open this publication in new window or tab >>Robust characterization of the DC-conductivity of HVDC insulation materials at high electric fields
2015 (English)In: 9th International Conference on Insulated Power Cables, 2015Conference paper, Published paper (Refereed)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-180833 (URN)
Conference
JiCable 15, Versaile, France, 2015
Note

QC 20160125

Available from: 2016-01-25 Created: 2016-01-25 Last updated: 2018-05-22Bibliographically approved
2. Effect of heat treatment on morphology and dielectric properties of PE cable insulation material
Open this publication in new window or tab >>Effect of heat treatment on morphology and dielectric properties of PE cable insulation material
2015 (English)In: Nordic Inslu. Symp., 2015Conference paper, Published paper (Refereed)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-180834 (URN)
Conference
Nord-IS 25, Copenhagen, Denmark, 2015
Note

NV 20160125

Available from: 2016-01-25 Created: 2016-01-25 Last updated: 2018-11-20Bibliographically approved
3. Conductivity measurement of plaque samples obtained from the insulation of high voltage extruded cables
Open this publication in new window or tab >>Conductivity measurement of plaque samples obtained from the insulation of high voltage extruded cables
2015 (English)In: Nordic Inslu. Symp., 2015Conference paper, Published paper (Refereed)
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-180838 (URN)
Conference
Nord-IS 25, Copenhagen, Denmark, 2015
Note

NV 20160125

Available from: 2016-01-25 Created: 2016-01-25 Last updated: 2018-05-22Bibliographically approved
4. Effect of heat-treatment and sample preparation on physical properties of XLPE DC cable insulation material
Open this publication in new window or tab >>Effect of heat-treatment and sample preparation on physical properties of XLPE DC cable insulation material
2016 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, no 5, p. 2508-2516, article id 7736807Article in journal (Refereed) Published
Abstract [en]

Power cables with extruded crosslinked polyethylene (XLPE) insulation are used in HVAC and HVDC applications. These cables usually go through a degassing process to remove the methane formed in XLPE during the cross-linking reactions. In case of HVDC cables, the general belief is that the polar peroxide decomposition products (PDP) content significantly influences the conduction and space charge behavior of XLPE. This belief is mainly based on the results of experiments made on thin XLPE samples with different heat-treatments; but since heat-treatment also influences the morphology of the polymer, it is necessary to consider this effect as well. It is common to use polyethylene terephthalate (PET) film as a protective layer during sample press molding. Studies on the influence of the pressing film on the electrical properties of the sample are rare. In this work, the results of a series of experiments performed on 0.5 mm thick XLPE plaque samples in reference to additive free LDPE samples with different heat-treatment times are presented. Beside the PDP content, the morphology, DC conductivity and polarization properties are studied and analyzed. It was found that the pressing film used during sample preparation has a significant effect on the results and if not corrected, it may lead to wrong conclusions about the influence of the PDP content. Eliminating the effect of the pressing film, no clear correlation between the DC conductivity, dielectric loss and the PDP content was observed. The relation between the PDP content and DC conductivity is not found to be obvious, hence this correlation may need to be further evaluated.

Place, publisher, year, edition, pages
IEEE Press, 2016
Keywords
conductivity, cross linked polyethylene insulation, dielectric losses, HVDC insulation, power cables insulation, sample preparation, space charge, Cables, Crosslinking, DC power transmission, Dielectric devices, Electric conductivity, Electric space charge, Heat treatment, HVDC power transmission, Insulating materials, Insulation, Plastic bottles, Polyethylenes, Telecommunication cables, Crosslinked polyethylene, Effect of heat treatments, Peroxide decomposition, Polarization properties, Polyethylene terephthalates (PET), Power cables, Film preparation
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-201790 (URN)10.1109/TDEI.2016.7736807 (DOI)000389215000003 ()2-s2.0-84997282951 (Scopus ID)
Note

QC 20170217

Available from: 2017-02-17 Created: 2017-02-17 Last updated: 2018-05-22Bibliographically approved
5. Long-term conductivity decrease of polyethylene and polypropylene insulation materials
Open this publication in new window or tab >>Long-term conductivity decrease of polyethylene and polypropylene insulation materials
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.

Keywords
HVDC insulation, Conduction, Polyethylene, Polypropylene, Charge injection, Morphology
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-228361 (URN)10.1109/TDEI.2017.006218 (DOI)000405000300021 ()2-s2.0-85022335154 (Scopus ID)
Note

QC 20180523

Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2018-05-23Bibliographically approved
6. Observation of non-monotonic dependence of leakage current with temperature during thermal cycling
Open this publication in new window or tab >>Observation of non-monotonic dependence of leakage current with temperature during thermal cycling
2016 (English)In: IEEE Electrical Insulation Conference (EIC), 2016, Montreal: Institute of Electrical and Electronics Engineers (IEEE), 2016, article id 16263557Conference paper, Published paper (Refereed)
Abstract [en]

Conductivity under high voltage DC, is one of the most important characteristics of insulating materials used as insulation in HVDC applications. The volume conductivity of the insulation can be measured on press molded plaque samples by applying DC voltage and measuring the leakage current passing through the insulation. Such measurements are often performed at constant temperatures and constant voltage levels. In this work, the leakage current is measured on 1 mm thick press molded low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) samples under DC voltage of 30 kV while the different temperature conditions including thermal cycling with temperatures between room temperature and 90 °C is applied. While the obtained results with constant temperature are in good agreement with the previously reported measurements, during thermal transients an interesting phenomenon was observed. Based on the theories and previously reported measurements, the leakage current is expected to have an Arrhenius dependence to temperature. But in the new experiments during thermal transients, the leakage current shows a non-monotonic temperature dependence and during heating and cooling, peaks are observed in the measured leakage current. It was discovered that the behavior of leakage current is influenced by the type of protective film used during sample preparation and using PET film leads to a higher apparent conductivity than using aluminum foil. A non-monotonic temperature dependence is observed in samples pressed using PET film while samples pressed with aluminum foil exhibit an Arrhenius temperature dependence.

Place, publisher, year, edition, pages
Montreal: Institute of Electrical and Electronics Engineers (IEEE), 2016
Keywords
HVDC insulation, power cable insulation, cross-linked polyethylene insulation, conductivity
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-228363 (URN)10.1109/EIC.2016.7548644 (DOI)2-s2.0-84988380478 (Scopus ID)978-1-4673-8706-4 (ISBN)
Conference
IEEE Electrical Insulation Conference (EIC), 2016
Note

QC 20180523

Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2018-05-23Bibliographically approved
7. Role of thermal and electrical relaxations for the long-term conduction current in polyethylene
Open this publication in new window or tab >>Role of thermal and electrical relaxations for the long-term conduction current in polyethylene
2016 (English)In: Proceedings of the 2016 IEEE International Conference on Dielectrics, ICD 2016, Institute of Electrical and Electronics Engineers (IEEE), 2016, p. 1106-1109Conference paper, Published paper (Refereed)
Abstract [en]

The characterization of the electrical conduction behavior of polymeric insulating material is a challenging task. Typical reasons are the underlying complexity of the physical processes, the smallness of the current, and the long relaxation times. Moreover, even if the external conditions like applied voltage and temperature are time-constant, often a steady-state direct current (DC) is not established during the measurement time, which may last up from hours to months. This work reports on DC current measurements on 1 mm thick press molded LDPE and XLPE samples. The results show that the conductance is slowly decaying sub-linearly with time, I∼tp, with 0 &lt;-p &lt; 1, p often around-0.5. In certain cases an intermediate current maximum is observed. There is strong indication that the decay of the effective conductivity is due a thermal process rather than to an electrical process.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2016
Keywords
Conductivity, cross-linked polyethylene insulation, HVDC insulation, power cable insulation, Electric conductivity, Insulating materials, Insulation, Cross linked polyethylene insulation, DC current measurement, Effective conductivity, Electrical conduction, Electrical relaxations, External conditions, Polymeric insulating materials, Polyethylenes
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-194952 (URN)10.1109/ICD.2016.7547812 (DOI)000389639800256 ()2-s2.0-84988443740 (Scopus ID)9781509028023 (ISBN)
Conference
1st IEEE International Conference on Dielectrics, ICD 2016, 3 July 2016 through 7 July 2016
Note

QC 20161123

Available from: 2016-11-23 Created: 2016-11-01 Last updated: 2018-05-22Bibliographically approved
8. Influence of press films on conduction in polyethylene plaque samples
Open this publication in new window or tab >>Influence of press films on conduction in polyethylene plaque samples
2016 (English)In: Dielectrics (ICD), 2016 IEEE International Conference on, France: Institute of Electrical and Electronics Engineers (IEEE), 2016, article id 16250905Conference paper, Published paper (Refereed)
Abstract [en]

Press molded plaque samples are commonly used for characterization of polymeric insulation materials. Such samples are prepared by pressing polymer granulates sandwiched between two layers of protective press films at high temperatures and pressures. Polyethylene terephthalate (PET) film is a common press film since it tolerates high temperatures, acts as a good diffusion barrier and can be separated from the molded polyethylene sample easily. In this work, studies are performed on the influence of the pressing film on the dielectric properties of press molded polyethylene insulation samples. Volume resistivity is measured under high voltage DC at different conditions on samples pressed using different press films and different preparations. Furthermore, PEA measurements under high voltage DC is performed on plaque samples press molded with different press films. It is found that press molding using PET film leads to a considerably higher apparent conductivity of the samples in comparison to using aluminum foil. Choice of press film, also influences the space charge measurement results. It is concluded that the influence of the press film used during sample preparation cannot be neglected and this effect should be evaluated carefully in material characterization research. According to the results in this work, aluminum foil seems to be a better option as a press film in comparison to PET press film.

Place, publisher, year, edition, pages
France: Institute of Electrical and Electronics Engineers (IEEE), 2016
Keywords
HVDC insulation, power cable, polyethylene, conductivity, polyethylene terephthalate, space charge
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-228364 (URN)10.1109/ICD.2016.7547828 (DOI)2-s2.0-84988410105 (Scopus ID)978-1-5090-2804-7 (ISBN)
Conference
Dielectrics (ICD), 2016 IEEE International Conference on
Note

QC 20180523

Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2018-05-23Bibliographically approved
9. DC conductivity of polyethylene and crosslinked polyethylene measured with a dynamic temperature program
Open this publication in new window or tab >>DC conductivity of polyethylene and crosslinked polyethylene measured with a dynamic temperature program
2017 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Electrical conductivity of HVDC cable insulation materials is important for its function. It is very practical to evaluate this parameter by DC conductivity measurements on press molded polymeric plates samples. While in real operation conditions, the insulation undergoes both static and dynamic thermal conditions, most of the published research in this area is still focused only on steady state thermal conditions.

In this work, the focus is instead on the behavior of electrical conductivity under dynamic thermal conditions. Press molded XLPE and LDPE plate samples with different preparations are tested under 25 kV/mm DC field with a dynamic temperature profile ranging from room temperature to 90 °C.

It was discovered that in many cases, the measured conductivity during dynamic measurements strongly deviates from the expected Arrhenius temperature dependence; instead the conductivity shows a non-monotonic temperature dependence manifested as conductivity peaks during heating and cooling. The behavior is found to be strongly related to the type of protective film used during press molding of the sample; further degassing leads to a reduction of the non-monotonic temperature dependence and with long degassing the behavior tends to the expected Arrhenius temperature dependence.

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-228365 (URN)
Conference
Nordic Insul. Symp. (Nord-IS 17)
Note

QC 20180523

Available from: 2018-05-22 Created: 2018-05-22 Last updated: 2018-05-23Bibliographically approved
10. Electrical characterization of extruded DC cable insulation — The challenge of scaling
Open this publication in new window or tab >>Electrical characterization of extruded DC cable insulation — The challenge of scaling
Show others...
2017 (English)In: IEEE Transactions on Dielectrics and Electrical Insulation, ISSN 1070-9878, Vol. 24, no 3, p. 1465-1475, article id 16993052Article in journal, Editorial material (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2017
Keywords
HVDC insulation, power cables, conductivity, space charge.
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-228367 (URN)10.1109/TDEI.2017.006124 (DOI)
Note

QC 20180523

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

Open Access in DiVA

fulltext(1825 kB)163 downloads
File information
File name FULLTEXT01.pdfFile size 1825 kBChecksum SHA-512
0e54d2a49fe551730ca9dfd02a7847ca2ce7e336b5c9b150ae143ce82cf3ae9178f97e36b3b8edf82c0051f93b10056f2a65b6de63991c6cc53fc2a15f08ca53
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Ghorbani, Hossein
By organisation
Electromagnetic Engineering
Other Electrical Engineering, Electronic Engineering, Information Engineering

Search outside of DiVA

GoogleGoogle Scholar
Total: 163 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 963 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf