Cable manufacturing process in Demirer Kablo includes wire drawing, stranding, insulating, degassing, screening, lead sheathing if required, outer sheathing and testing.
Wire drawing is the thinning process of copper and aluminium wires. A certain number of wires with a certain diameter are required to form the conductor and in order to obtain these wires, standard conductor rods are drawn.
First, the conductor rod is fitted at the entry of the die unit. Then, by means of eleven disks turning with a constant angular velocity, dies placed in front of every disk and propulsion from a single engine within the machine, the wire is thinned down to the required diameter. The disks wind the wire by drawing it, force it through the neighboring die and move it to the next disk. Drawing process ends with the last disk, but the wire coming out is not suitable for direct use as it has become harder after stretching and thinning during the process.
The annealing unit on the line is used to soften the wire again, making it suitable for cable production process. This unit consists of two conductive pulleys having an electrical potential. The wire passing through them forms a short circuit. Joule effect increases the wire temperature up to approximately 400 °C. The temperature is affected by rotational speed of pulleys, ambient temperature and applied voltage. The voltage is controlled by adjusting the line speed in order not to anneal the wire excessively. After it is cooled down, annealed wire is wound on a spool at take-up.
Stranding is the process of forming the conductor, current carrying component of the cable. A certain number of thinned wires are loaded to the stranding machine. Number of wires is dependant to the conductor cross section and currently used numbers are shown in below table. Over 1000 mm2, Milliken structure is used.
The loading pattern must conform to the geometric structure of conductor. The carriages are loaded according to the number of wires and the wires, in the form of bundles, are passed through the dies placed in front of the carriages. Diameter of these bundles is lower than the geometric diameter of the wires. The conductor bundle is drawn by means of a dual capstan placed on front section of the machine. During the standard process, while the capstan is drawing the conductor, every carriage turns in opposite directions. In this way the rigidity of the conductor is provided by means of reverse pitch from each level. The drawn wires are compacted while passing through the dies with lower diameters and thus a better contact with each other is provided.
The cross section obtained herein is not a geometric cross section, but an electrical one. The electrical resistance must not exceed the value defined in the standards according to the type of conductor.
If required, longitudinal water tightness can be provided in conductor by applying swelling powder and yarn. If segmental conductor is used, triangle segment slots are created instead of circle ones in stranding machine, and in another machine called the drum twister, these slots are put together to create a round conductor.
Completely dry curing and cooling (CDCC) is the production process of insulation, another main component of cable.
Cross-linked polyethylene (XLPE) is the ideal insulation material for high voltage cables due to its excellent electrical and physical properties. XLPE is obtained through reaction of high molecular polyethylene with organic peroxide cross-linking additives under specific temperature and pressure conditions. In cross-linking, processing technique and raw material’s purity degree are of vital importance.
In the design and manufacturing of underground XLPE medium and high voltage cables, perfection of the interfaces between the insulation and inner and outer semi-conductive layers is one of the most critical issues, increasing in importance at higher voltage levels. Insulation and semi-conductive layers are processed together by a three layer extrusion system.
In Demirer Kablo factory, there are three catenary type CDCC lines. Two caterpillars and two capstans placed on the line work synchronized, in order to ensure the continuity of the whole process from the transmitter to take-up. In the process, first, the conductor is loaded to the pay-off and by means of entrance capstan, it is moved to the main machine compartment at the height between 20 to 100 meters. The simultaneous processing of inner semi-conductive layer, insulation and outer semi-conductive layer is undertaken by specially developed high performance cross heads settled in the CDCC line, using super smooth, extra clean raw materials with immediate curing. Three extruders placed in the machine compatment are connected to the triple cross head. During the passage of conductor through this head, inner semi-conductor, insulation and outer semi-conductor are simultaneously extruded by the three extruders in form of three seperate layers. Transfer of XLPE granules from clean room to the extruder is handled with utmost care in order to prevent contact with the atmosphere and potential contaminants.
As soon as triple extrusion process is completed, the cable is inserted in a 100-200 m downsloping tube containing nitrogen at 10 bars pressure. Through first 40-60 meters, the tube is heated by current transformers in order to enable cross linking of the insulation material. Pressure is required to prevent insulation’s deformation by high temperature. This hot zone is called the curing zone and the remaining part is used as a cooling zone. Cable passed the curing zone is cooled in the cooling zone with cold nitrogen in order to prevent deformation due to softness after exiting the tube. In order to prevent the cable getting damaged by contacting the tube, devices detecting the position of the cable and keeping it in the center are used in the curing zone. Furthermore, wall thickness and eccentricity of each layer is controlled with x-ray devices. Following the cooling process, cable is taken out to the atmosphere and coiled to the drum on the coiler. CDCC process is thus completed.
This one step continous process ensures the following:
Degassing is the removal of waste gases emitted by chemical reactions during CDCC process. Although most of waste gases are removed before leaving the tube, some still remain within the cable.
Reel coiled with the core coming out from CDCC line is put into a degassing room. The room is isolated from external environment and air at 60 °C is ventilated therein. Accordingly room temperature and core temperature increase to 60 °C consecutively. Gases within the core are thus emitted to the air. The air in the room is replaced with fresh air at certain intervals. Depending on the insulation thickness, the core is kept in the room up to 1 month.
Sample taken from degassed core is then tested by the quality control department and upon receipt of positive results the reel is taken out and left to cooling.
Screening is the implementation of metallic wires in such a number and with such a diamater that is able to withstand the short circuit current. The short circuit current depends on the cable type.
In the process, the core is loaded to the pay-off. Primarily, a tape is helically wound to the core for water sealing and cushioning. Then, metallic wires coiled on small spools are also wound helically. A metallic tape and a swelling tape are then applied on the wires in order to prevent them from scattering. Finally, the core pulled by a caterpillar is wound to the coiler.
Taping, armouring, wire screening, assembling capability of straight or pre-spiralled 6 segmental conductor.
Taping, armouring, wire screening, assembling capability of straight or pre-spiralled 5 segmental conductor.
Lead sheathing is another type of screening. It is simply covering the core with lead. Lead ingot is melted in the melting pot and by means of an omega pipe it is transferred to the cylinder. The screw revolving in the cylinder pushes the lead towards the extruder head. The cylinder is meanwhile cooled down gradually in order to prevent any damage to the core. Extrusion process is carried out as the core passes through the extruder head placed at the end of cylinder.
As soon as this process is completed, the intermediate product is started to cool down and until it is brought to the open environment, its temperature is decreased down to the ambient temperature. Finally the intermediate product is wound to the coiler.
Aluminum sheathing is an alternative metallic screening method.
As the core passes through the line, Al tape, meeting the cross sectional area, is cut precisely in order to obtain exact diameter. Then, by means of forming unit it is shaped in circular form and finally TIG welding is performed. At this stage, aluminum is in a loose tube form. Depending on customer’s requirement, that loose tube is transformed into a corrugated or a compact shape by corrugator or compacting unit. At this point, if the aluminum taped cable is wound to the drum directly, a very large barrel diameter should be used in order to avoid wrinkles on the aluminum tape. This is the reason for having the tandem jacketing in the line.In the end of the process, in-line common jacketing process is performed.
Outer sheating is the last stage of cable manufacturing process. Cable, after its placement in the pay-off, is drawn through the cross head connected to the extruder by means of caterpillars placed at both ends of the line. Cable is sheathed with PE or PVC as it passes through the cross head and enters the cooling channel right after embossing, marking and metering. PE laminated aluminium tape can also be sheathed for radial water sealing during outer sheating by means of an apparatus placed near the cross head entry. Outer sheath thickness is obtained by adjustment of synchronised extruder cycle and line speed. Cooled cable is then wound to the reel on the take-up following the caterpillar and thus manufacturing process is completed.
Being aware of the fact that high and extra high voltage cable production requires capable quality control equipment and relevant experience besides highest possible precision at manufacturing stage, Demirer Kablo continously invests on quality control, follows new technologies and keeps highly trained personnel available as well as using high quality raw materials supplied from selected and periodically inspected reputable providers only. Some of the quality control equipment of Demirer Kablo are as follows:
Long term high voltage tests System type tests AC step voltage tests
2.400 kV – 120 kJ lightning impulse 1.540 + 1.050 switching impulse Lightning & switching impulse voltage tests
Partial Discharge Detector High voltage tests Partial discharge tests
1.400 kV – 4 kJ lightning impulse Lightning impulse voltage tests
Long term voltage tests Type tests
DC voltage tests Site tests
Tan measurement Capacitance measurement
Heating cycle voltage test Water penetration tests
Insulation and SC materials moisture content measurement
Spectral analysis of insulation and sheath materials
Insulation and sheath materials By product concentration OIT Carbon black measurements
Insulation, sheath and various materials Thermal analysis Melting point measurements
DC resistance measurement
Flame spread test
Smoke density test
Tensile strength and elongation measurements