Carl-Olof Olsson

HVDC Cable Systems—Highlighting Extruded Technology

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.

Influence of press films on conduction in polyethylene plaque samples

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.

Observation of non-monotonic dependence of leakage current with temperature during thermal cycling

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.

Conductivity of crosslinked polyethylene influenced by water

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.

Measurements of leakage current and field enhancement in extruded model cables at HVDC

In the characterization of insulation materials for extruded HVDC cables, measurements of leakage current and space charge distribution are often used. The apparent conductivity that can be obtained from leakage current should be in agreement with the conductivities that can be evaluated from the time evolution of space charge from voltage application until steady-state. In this paper, examples of leakage current data and space charge data for two cables having different levels of conductivity are presented. The similarities in the results are shown, and differences as well as future challenges are discussed.