Hossein Ghorbani

It is known that the heat treatment process in theproduction of extruded cables, affects the electricalproperties of polymeric insulation materials; this effectis via two main phenomena; first, it leads to diffusionand removal of polar substances from the system;second, it affects the polymer morphology by increasingthe polymer chain relaxation and creation of crystallites,hence increasing the crystallinity of the material.In this study, thick pressed samples with additive freelow density polyethylene (LDPE) insulation materialwere heat treated at different conditions for differentperiods of times. Differential scanning calorimetry(DSC) measurements confirmed that heat treatmentleads to higher crystallinity and higher lamella thicknessin LDPE. Dielectric properties of the material wasstudied by conduction current measurement anddielectric spectroscopy.Conduction current of samples with different heattreatment times was measured at high electric fieldswhich shows that the conduction current of LDPE isaffected by heat treatment. Generally, the apparentconductivity decreases by heat treatment.Dielectric spectroscopy measurements show that heattreatment does not have a significant effect on real andimaginary permittivity of the polymer. Interestingly, anincrease in the sample thickness via heat treatment, wasobserved both by sample thickness related to thememory effects in the polymer which needs to be takeninto consideration for accurate capacitancemeasurements.

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.

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.

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.

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.

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.

A new 525 kV DC cable system with a power rating range up to powers above 2 GW has been developed for both subsea and underground applications. The 525 kV extruded DC cable system can transmit at least 50% more power over extreme distances than previous solutions (i.e. the 320 kV extruded DC system). The technology enables the lowest cable weight per installed megawatt (MW) of transmission capacity and the higher voltages provide reliable transmission and low energy losses. This system utilizes a new cross-linked polyethylene (XLPE) DC insulation material, an oil- and porcelain-free termination based on wall bushing technology as well as a land joint and a flexible sea joint. The paper describes the product development of the cable as well as the accessories, the extensive testing procedures and the capability of the new cable system.

A new 525 kV DC cable system with a power rating range up to powers above 2 GW has been developed for both subsea and underground applications. The extruded HVDC cable system technology is appropriate when power needs to be efficiently delivered through populated or environmentally sensitive areas, or in coastal and open-sea applications. HVDC cable links are essential components of sustainable energy systems to connect energy markets and transmit vast amounts of electricity over very long distances, across or between continents. The successfully tested extruded 525 kV HVDC cable system is the result of long-term focused innovative R&D work across business units and corporate research centers and collaboration with a material manufacturer. This system utilizes a new cross-linked polyethylene (XLPE) DC insulation material, an oil- and porcelain-free termination based on wall bushing technology as well as a land joint and a flexible sea joint. This paper describes the product development including challenges and solutions, for the cable as well as the accessories, the extensive testing procedures and what the cable system is able to do.

The conductivity of XLPE insulation plates has been measured with different levels of water concentration. Tapes have been used to humidify the insulation plates in a heat treatment step. Three different levels of water concentration was provided by the tapes. The apparent conductivity was measured for 24 h at 70°C and 30 kV/mm. It is found that the conductivity is increasing with increasing water concentration. It is also found that other polar substances emanating from one of the tapes had a dominating influence on the conductivity. In combination with high water concentration, the conductivity is increased to more than 100 times the conductivity of the reference plate.

HVDC cable systems are growing in number. This is a result of globally defined targets leading to an increase of investments in renewables. The HVDC cable systems enable us to integrate these new energy sources in a manner that reduces losses and stabilizes operations. In this paper, the focus is on extruded HVDC cables. Starting from a general description of the forces that drive us toward more cables, this paper quantifies the push for higher voltages. The extruded cable system on a product level is described followed by design aspects. Particularly, the effect of thermal runaway is highlighted. The accessories, joints, and terminations are described. The advantages of the concept nonlinear field grading are described. This paper ends with some words on realized commercial projects and future trends.