Hossein Ghorbani

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.

Inclined-Plane Tracking and Erosion Test according to the IEC 60587 Standard

The main goal of the paper is to summarise experiments using the tracking and erosion test according to the IEC 60587 standard. Various polymeric materials, e.g., high temperature vulcanised silicone rubber, liquid silicone rubber, epoxy resin and fibre reinforced materials were tested and differences in performance noted. A leakage current monitoring system has also been developed and the results of leakage current analysis are presented. Possible improvements of the present standard are also discussed.

Inclined-plane tracking and erosion test according to the IEC 60587 Standard

The present IEC 60587 IEC standard needs several improvements which would help manufacturers of power equipment and material suppliers to assess outdoor polymeric materials in a more reliable way.

Long-term conductivity decrease of polyethylene and polypropylene insulation materials

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.

Role of thermal and electrical relaxations for the long-term conduction current in polyethylene

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 <; -p <; 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.

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.

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.

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.

Electrical characterization of extruded DC cable insulation — The challenge of scaling

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.

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.