Markus Saltzer

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.

Observation of space charge dynamics in air under DC electric fields

Even under corona-free conditions insulating structures (materials) may be charged in DC fields. This is due to the fact that air has a finite ionic conductivity. The finite conductivity of air results from charges generated in the air due to background radioactivity and cosmic radiation. Typical ion concentrations on ground are 107 to 1010 1/m3 which leads to a conductivity ranging from 10-16 to 10-13 S/m. An experiment has been conducted to measure order of magnitude, time scales, and boundary influences of air ionic motion under DC electric fields. The experimental setup consisted of two concentric cylinders of metallic mesh. The radius of the inner cylinder was 0.5 m and that of the outer cylinder 2 m. The current was measured as a function of time after a potential was applied to the inner cylinder. The background ion concentration was monitored by an ion counter. The results were analyzed and compared with simulations using an ion-drift model considering small cluster ions of positive and negative polarity. The measurements have been found in good agreement with the model.

Effect of heat-treatment and sample preparation on physical properties of XLPE DC cable insulation material

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.

DC field distribution around an HVDC cable termination

DC field measurements of the electric field around a cable termination have been performed. The field variation along the surface as well as its time development after voltage application is studied. The agreement between measured and simulated results is relatively good, for short times after voltage application. After longer time, deviations from the simulated results occur. Possible reasons for this discrepancy are discussed.

DC field measurements around a cable end

Even under corona-free conditions, insulating structures (materials) may be charged in DC fields. This is due to the fact that air has a finite ionic conductivity. The finite conductivity of air results from charges generated by background radio activity and cosmic radiation. Typical ion concentrations on ground are 107 to 1010 ions/m3 which leads to conductivities ranging from 10-16 to 10-13 S/m. An experiment has been conducted where the end of a terminated high voltage XLPE cable was used as an insulating structure. The field distribution around the stripped cable end was measured with an applied DC voltage of -140 kV using a rotating field probe. The measurements are compared to simulations, which do not specifically take into account space charge effects. A big discrepancy is observed, showing the relevance of taking into account space charge effects when modeling HVDC insulation components subjected to air.

Measurements of high voltage DC fields in air

The global demand for electric power continues to increase in most countries but in particular in non-OECD (Organisation for Economic Cooperation and Development) countries [1]. Often the generation is located far from the consumers, and this requires transmission of electric power over long distances. For various reasons, high voltage direct current (HVDC) is often the preferred technology for these links [2]. To keep losses at a minimum, the voltage must be as high as possible, and this has stimulated the development of ultra-high voltage DC transmission. Over the last decade, 800 kV has been established as a new standard level for long transmission links [3].

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.

Charging insulating barrier by corona in air in large coaxial system

Results of the experimental study and computer simulations of corona charging of an insulating barrier in a large scale coaxial arrangement are reported. A computational model describing transport of charged species generated by corona and their dynamics on dielectric surfaces is presented. The measured and computed voltage-current characteristics of corona discharge are compared and discussed. It is shown that charges deposited on gas-solid dielectric interfaces reach a steady state after few cycles of the applied triangular ac voltage. The electric field induced by the surface charges modifies the field in the entire system and alter corona characteristics.

Extra high voltage DC extruded cable system qualification

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.

Measurements and simulations of corona currents due to triangular voltages in large scale coaxial geometry

Corona currents in air measured in a large scale coaxial geometry (douter = 1 m, dinner = 0.26 mm), under both DC and AC voltages of a triangular shape with frequencies in the range (1-50) Hz are analyzed. A computational model describing dynamics of positive and negative ionic charges is employed to examine the results of the measurements. The performed simulations show a good overall agreement with the experimental data.