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
The purity of the XLPE compound that is used for the insulation of subsea and EHV cables plays a critical role. The purer the compound (Picture 1), the lower is the risk for a breakdown. (Picture 2)
Picture 1:High quality insulation compound
Impurities of 50μm may already cause damage to the end product with high follow up costs. 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.
Picture 2:Cross section of an EHV cable with breakdown
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.
SUBSEA AND EHV CABLES REQUIRE A CHALLENGING PURITY DEGREE OF XLPE MATERIAL
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.
Picturec 3:Measurement of the melt temperature in a CCV line
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.
An alternative to conventional melt temperature methods (such as thermocouple sensors or infrared pyrometers) 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.
Measurement of the cleanliness of the melt
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.
Picture 4:Measurement of the cleanliness of the melt in a CCV line
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.
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.
Picture 5:X-ray and optical inspection
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 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.