Subsea and EHV Cables Require a Challenging Purity Degree of XLPE-Material
This paper outlines the reason for the need of a high purity degree of XLPE material to be used for the insulation of subsea and EHV cables. Furthermore, there will be technological solutions introduced for purity assurance of XLPE pellets that are integrated at specific production stages.
Necessity of clean XLPE compound for subsea and EHV cables
Impurities of 50μm may already cause damage to the end product with high follow up costs. The repair of a defective subsea cable for example, which has been damaged by contamination, can lead to weeks of downtime. Furthermore, impure XLPE compound respectively defective cables and consequential crashes already affect the industry during the manufacturing process. As part of the production of EHV cables, they are tested in plant with a test voltage 2.5 times of the nominal voltage. Approximately, five to six breakdowns (Picture 2) a year are detected this way. Those breakdowns cause costs of 30,000 € minimum per crash even before the cable can be delivered to its dedicated position. In addition, valuable time is lost, making permitted delivery dates not accomplishable. Often, not agreed joints have to be used, damaging the quality image of the manufacturer consequently and may lead to contractual penalties. It is for these reasons that the Chinese Standard for high voltage cables, for example, demands the exclusion of contamination from 75μm in the processed materials. Moreover, there are guidelines from the AEIC (Association of Edison Illuminating Companies), which state that cables have to be designed in such a way that they are usable for at least 40 years. Accordingly, it is necessary to inspect the material for purity to 100% before it enters the end product. Sample tests are not sufficient to exclude all contamination reliably.
Today cable manufacturers use screens to catch impurities in the XLPE melt before they get into the cable. The screens are positioned directly in the melt flow after the extruder, before the crosshead. However, these screens can get clogged by scorches, or excessive amount of contaminants after certain run time. Then the melt pressure in the extruder may increase significantly. Finally, the production has to be stopped in order to change the screens, which in turn means that later a joint is required at that position. Joints, were the cables are welded together are manually made and always critical, in particular with regard to subsea cables for offshore-applications. That is why cable manufacturers aim at delivering large cable lengths with only a minimum number of joints as they contain a potential risk for breakdowns. One of the aspects to achieve long lengths is using highly pure raw material. As the integration of screens reduces the productivity of the line, an approach is to omit the screens. This requires, however, reliable methods to detect and sort out contamination in the XLPE material at an early stage.
Production of subsea and EHV cables
A subsea respectively an EHV cable is manufactured in a CCV or VCV extrusion line. It mainly consists of a conductor, an inner semicon, the insulation and the outer semicon. Inner and outer semicon as well as the insulation is often made of XLPE material as XLPE has excellent dielectric properties, making it useful for extra high voltage cables up to 500 kV AC-voltage, and 750 kV DC. In order to assure the highest purity of the XLPE material it is necessary to continuously measure and inspect certain material characteristics at specific stages before and during the production.
XLPE purity assurance between extruder and crosshead
A homogeneous and pure XLPE melt is decisive for the final cable quality. There are technologies used during the extrusion process that assure these material characteristics. They are installed between the extruder and the crosshead.
Measurement of the melt temperature
The temperature of the polyethylene material, which is used for the insulation of subsea and EHV cables, is a significant criterion for an optimum melt and in consequence for a maximum extruder output. The correct temperature assures a homogeneous polymer melt, eliminates early cross linking of the material, and thus assures that there are no scorches in the insulation material. Moreover, a melt temperature measurement system should be capable to detect inhomogeneities in the melt.
There are conventional methods used to measure the melt temperature such as thermocouple sensors. More often even simple hand-held meters are utilized to measure the melt temperature after the cross-head before starting-up the extrusion line. These techniques do not offer reliable measuring results as they are contact measurements with a relative slow response time. Moreover, they can influence the melt flow properties, which may result in cross-linking. By using an infrared pyrometer a contact-free temperature measurement with an improved response time is possible. Though, mainly the surface temperature of the XLPE melt can be measured, because the penetration depth of infrared radiation in LDPE (low density polyethylene) amounts to several millimeters only. Furthermore the existence of specific fillers in the PE melt can drastically decrease this penetration depth.
An alternative to these conventional melt temperature methods is a non-contact melt temperature measurement system based on non-invasive ultrasonic technology (Picture 3). It precisely measures the melt temperature during production and does not influence the melt flow properties. Moreover, it measures the average temperature of the melt and not the temperature in the center of the melt flow. The adapter of the system including the ultrasonic sensors is positioned in the flow channel between extruder and cross-head. In contrast to the methods described before, the ultrasonic sensors do not influence the polyethylene melt flow, because they are outside of the flow channel. In consequence, the extrusion process is not affected by the sensors, even if they have to be exchanged. The extremely high measuring rate allows a fast response time as well as the registration of small temperature variations. Only with the use of the ultrasonic system melt shear heating errors are eliminated. It ensures homogeneous melt viscosity for the extrusion process and helps to avoid premature cross-linking after screens, which may lead to ambers and scorches in the polyethylene material.
In addition to melt temperature measurement, it is important to examine the XLPE-material for purity in the flow channel directly before the crosshead, because much of the contamination results from cleaning of the extruder screw or abrasion of the extruder. In order to detect the contamination in the insulating material a high-speed CCD camera system transilluminates the insulated material and informs about such contamination in the material, as well as amber and scorches (Picture 4). In this way, manufacturers have relevant information for the decision to switch from start-up to production.
XLPE purity assurance before material processing: Inspection and Sorting
The two technologies described before assure a homogeneous, pure XLPE melt and detect contaminants in the melt that are caused in the extruder. It is moreover important to inspect the XLPE material (pellets) for purity before it gets into the extrusion process.
Today, a pellet inspection is realized by systems used either in laboratories or for online monitoring during the production process. The majority of the systems are based on optical technology to detect contamination on the pellet. Contamination inside the pellets cannot be detected by these systems.
The inspection and sorting system described in the following allows for a 100% online quality assurance by using X-ray technology and an optical technique. Contamination that are detected are identified by an image processing software, characterized as contamination and automatically separated. The technology allows for the detection of impurities down to a size of 50μm.
The basic detection principle of the X-ray technology uses the different attenuation of the material. XLPE mainly comprises of Carbon. The carbon atom has 6 protons in its core. A typical contamination would be steel particles from the extruder or granulator, which is mainly Iron (FE). Iron has 26 protons in its core. These 26 protons have a much higher X-ray attenuation than the 6 protons from the carbon and results in a contrast between the two materials in the X-ray image.
Regarding the optical inspection, the illumination plays an essential role. By using a special light construction technique, smallest contamination is detected. In order to allow precise recordings of material flows at industrial speed, modern camera technologies are used. A powerful image processing software similar to the one used for the X-ray inspection is used to detect contamination with the optical system. Therefore, by setting a certain threshold, all contaminated pellets, which are above the threshold in the mathematical algorithm, are sorted out.
Typical contamination detected by X-ray and optical technology
The combination of both X-ray and optical technologies enables the detection of contamination in the pellet itself and on its surface (Picture 5). The X-ray system inspects transparent and colored (e.g. black) pellets as well as semi-conductive XLPE material for impurities. Typical impurities detected with X-ray are metallic as well as organic contamination and inhomogeneities (TiO2) inside the pellet. In addition, the optical system detects for example black specs on the pellet, foreign objects and foreign pellets as well as other organic or metallic contamination.
In order to avoid contamination from the feeding itself, the transport of the pellets is carried out via a vibrating ramp made of stainless steel. This avoids contamination that might occur by conveyor belts. The pellet transport system is hermetically sealed assuring that there is no risk that dust or other contaminants can get into the flow of the XLPE pellets. In addition, the transport system can be operated with an overpressure. The pellet inspection and sorting system is designed for a throughput of 500, 1,000 or 2,000 kg/h and can be integrated into new and existing feeding systems.
Integration of the system in the production line
The system is typically installed between the hopper that is fed from the XLPE supply (octabin, bag or silo) and the hopper of the extruder, whereas the compound is fed by gravity.
In summary, this paper outlined the reasons for the need of a high purity degree of XLPE material used for the insulation of subsea and EHV cables. Furthermore, the paper introduced systems for quality control of XLPE material that inspect the material at different stages of the cable production, before and during processing. They represent alternative respectively additional technologies to the integration of screen.
Technologies for melt temperature measurement and melt monitoring in the flow channel assure purity of the processed material between the extruder and the crosshead. In this way, contamination that has been caused in the extruder are avoided respectively detected.
In addition, there are pellet inspection systems used for purity assurance either as laboratory or online devices. The presented inspection and sorting system detects contaminated pellets and separates them before they get into the extrusion process. Accordingly, even if cable manufacturers keep on using screens, this technology assures that screens are not getting clogged with impurities from contaminated pellets and allows, therefore, a longer production run. By using X-ray and optical technology contaminants inside and on the pellet surface are detected, which guarantee 100% quality control.
Taken these advantages together, the use of different technologies for quality control of XLPE material at different production stages is essential to assure high-quality subsea and EHV cables. Simultaneously, by ensuring a high purity degree of XLPE material, the efficiency of the cable production line can be improved to a great extent. In addition, cost for re-manufacturing cables that failed the discharge tests can be saved.
(Source: SIKORA AG)