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  • 1.
    Abid, Fahim
    et al.
    KTH.
    Ghorbani, Hossein
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Edin, Hans Ezz
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Differences in morphology and polarization properties of heat-treated XLPE and LDPE insulation2016In: 2016 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (IEEE CEIDP), IEEE conference proceedings, 2016, p. 113-116Conference paper (Refereed)
    Abstract [en]

    Cross-linked polyethylene (XLPE) is the most commonly used insulating material for extruded high voltage cable applications. Degassing is a heat-treatment process that is performed to remove methane from XLPE insulation which is formed during the crosslinking reactions as a by-product. Apart from removing methane, heat-treatment influences the electrical properties through changing the morphology due to annealing and also removal of polar crosslinking by-products. Scanning electron microscopy (SEM) is generally used to observe the changes in crystalline structure of the polymer. Frequency domain spectroscopy (FDS) is widely used to study polarization properties of dielectric materials. In this study these two methods are used for a comparative analysis of XLPE and LDPE subjected to different heat-treatment time, with or without a diffusion barrier. Electrical measurements are performed at room temperature. From the SEM imaging conducted after permanganate acid etching, formation of spherulites due to heat-treatment is not obvious in neither LDPE nor in XLPE. However, distinctions between LDPE and XLPE in SEM micrographs are evident. From studies with dielectric polarization spectroscopy, it is found that the LDPE samples are less sensitive to heat-treatment in comparison to the XLPE samples while dissipation factor of XLPE samples are influenced by the choice of pressing film used during sample preparation.

  • 2.
    Ghorbani, Hossein
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering. ABB AB, High Voltage Cables, Karlskrona, Sweden.
    Characterization of Conduction and Polarization Properties of HVDC Cable XLPE Insulation Materials2016Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Since its first introduction in 1998, extruded direct current (DC) cable technology has been growing rapidly leading to many cable system installations with operation voltages up to 320 kV. Cable manufacturers invest heavily on technology development in this field and today extruded DC cable systems for operation voltages as high as 525 kV are commercially available.

    The electrical field distribution in electrical insulation under DC voltage is mainly determined by the conduction physics, therefore a good understanding of the DC conduction is necessary. In case of Cross-linked Polyethylene (XLPE) insulation, the presence of the peroxide decomposition products (PDP) is believed to influence its electrical properties. The PDP are volatile and therefore they may diffuse out of the samples during sample preparation and testing. Besides, the morphology of the XLPE is known to evolve over time even at moderate temperatures. Since the material may change during preparation, storage and even measurement, the procedure during all stages of the study should be chosen carefully.

    In this work, the physics of the dielectric response and conduction in XLPE is briefly discussed. The existing measurement techniques relevant to characterization of DC conduction in XLPE insulation materials are reviewed. The procedure for high field DC conductivity measurement is evaluated and recommendations for obtaining reproducible results are listed. Two types of samples are studied, i.e. thick press molded samples and thick plaque samples obtained from the insulation of in-factory extruded cables. For press molded samples, the influence of the press film used during press molding and the effect of heat-treatment on the electrical properties of XLPE and LDPE are studied. High field DC conductivity of XLPE plaque samples is measured with a dynamic electrode temperature to simulate the standard thermal cycles.

    Investigations show that using PET film during press molding leads to higher apparent DC conductivity and dielectric losses when compared to using aluminum foil. The influence of heat-treatment is different depending on the press film. High field DC conductivity measurements and chemical composition measurement of samples obtained from the cable insulation are in good agreement with the results obtained from the full scale measurements. Finally a non-monotonic dependence of apparent DC conductivity to temperature of some samples pressed with PET film is discovered which to the author’s best of knowledge has not been previously reported in the literature.

  • 3.
    Ghorbani, Hossein
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Characterization of Conduction and Polarization Properties of HVDC Cable XLPE Insulation Materials2018Doctoral 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.

  • 4.
    Ghorbani, Hossein
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Influence of press films on conduction in polyethylene plaque samples2016In: Dielectrics (ICD), 2016 IEEE International Conference on, France: Institute of Electrical and Electronics Engineers (IEEE), 2016, article id 16250905Conference 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.

  • 5.
    Ghorbani, Hossein
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Long-term conductivity decrease of polyethylene and polypropylene insulation materials2017In: IEEE Transactions on Dielectrics and Electrical Insulation, ISSN 1070-9878, Vol. 24, no 3, p. 1485-1493Article in journal (Refereed)
    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.

  • 6.
    Ghorbani, Hossein
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Observation of non-monotonic dependence of leakage current with temperature during thermal cycling2016In: IEEE Electrical Insulation Conference (EIC), 2016, Montreal: Institute of Electrical and Electronics Engineers (IEEE), 2016, article id 16263557Conference 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.

  • 7.
    Ghorbani, Hossein
    et al.
    ABB High Voltage Cables, Karlskrona, Sweden.
    Abbasi, Amirhossein
    Jeroense, Marc
    Gustafsson, Anders
    Saltzer, Markus
    Electrical characterization of extruded DC cable insulation — The challenge of scaling2017In: IEEE Transactions on Dielectrics and Electrical Insulation, ISSN 1070-9878, Vol. 24, no 3, p. 1465-1475, article id 16993052Article in journal (Refereed)
    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.

  • 8.
    Ghorbani, Hossein
    et al.
    ABB AB, High Voltage Cables, Karlskrona, Sweden.
    Farkas, Andreas
    ABB AB, High Voltage Cables, Karlskrona, Sweden.
    Hulden, Pierre
    ABB Power Products, Sweden.
    Conductivity measurement of plaque samples obtained from the insulation of high voltage extruded cables2015In: Nordic Inslu. Symp., 2015Conference paper (Refereed)
  • 9.
    Ghorbani, Hossein
    et al.
    ABB AB, High Voltage Cables, Karlskrona, Sweden.
    Hoq, Tanbhir
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of heat treatment on morphology and dielectric properties of PE cable insulation material2015In: Nordic Inslu. Symp., 2015Conference paper (Refereed)
  • 10.
    Ghorbani, Hossein
    et al.
    ABB AB, High Voltage Cables, Karlskrona, Sweden.
    Olsson, Carl-Olof
    ABB AB, Corporate Research, Västerås, Sweden.
    Englund, Villgot
    Borealis AB, Stenungsund, Sweden.
    Robust characterization of the DC-conductivity of HVDC insulation materials at high electric fields2015In: 9th International Conference on Insulated Power Cables, 2015Conference paper (Refereed)
  • 11.
    Ghorbani, Hossein
    et al.
    DONG Energy Wind Power Copenhagen, Denmark.
    Olsson, Carl-Olof
    Jeroense, Marc
    DC conductivity of polyethylene and crosslinked polyethylene measured with a dynamic temperature program2017Conference 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.

  • 12.
    Ghorbani, Hossein
    et al.
    ABB AB, High Voltage Cables, Karlskrona, Sweden.
    Saltzer, Markus
    ABB AB, High Voltage Cables, Karlskrona, Sweden.
    Abid, Fahim
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Heat-treatment and physical properties of high voltage DC XLPE cable insulation materialManuscript (preprint) (Other academic)
1 - 12 of 12
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