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XLPE MATERIAL


  The trend of cross-linked polyethylene insulated cables into power cables field is quite prevalent in the world as the substitutes of paper insulated cables, which had played the leading role in electric power transmission stage. Thus, nowadays after making its impact on its use in the medium voltage cables category, the stage seems to be turning in favour of XLPE cables in the high voltage and extra high voltage range. Especially in the higher voltage area, the monopoly of paper insulated cables, such as oil filled cable or gas filled or compressed type of cables, is gradually collapsing into the co-existence with XLPE cables, which have great advantages and economical reasons such as higher permissible temperature level, ease of jointing and maintenance techniques and omission of equipment inherent to the pressurized cable

 
  Electrical performance of XLPE is excellent. Di-electric breakdown strength and volume resistivity are high and both Di-electric loss (tan d) and dielectric constant (e) are low. Thermal resistivity is low. XLPE insulated cables can operate continuously at a temperature of 90°C, because they have excellent heat aging characteristics. This leads to large power transmission capacity. XLPE cables are lighter in weight, they are easy to handle, easy to install and jointing and termination is easy. They are of dry type because no oil impregnation is involved.

 
  The insulation material for types of XLPE cables is low-density (LD) polyethylene. PE has for a long time had a wide use as cable insulating and sheath materials owing to its excellent electrical and mechanical properties, its lightness, low temperature flexibility and good resistance to moisture, chemicals, ozone etc. as well as its comparatively low price.

 
  Low Density polyethylene, however, has properties which limit its use as cable insulation. Being a thermoplastic, its softening temperature is 105 - II5°C. Another disadvantage is its tendency to stress cracking when in contact with certain surface-active agents.

 
  By means of a process reminiscent of the vulcanization of rubber, PE molecules can be cross-linked, thus greatly improving the thermal and mechanical properties of the material, while its electrical properties are retained largely unchanged. This product, cross-linked polyethylene (XLPE), is therefore no longer a thermoplastic. It assumes elastic, rubber-like consistency, a property that it retains during a further rise of temperature. The tendency to stress cracking entirely disappears and the material also acquires very good resistance to aging in hot air.

 
 
CROSS-LINKED POLYETHYLENE

PE is made up of long molecular chains. By cross-linked these chains a network of strong bonds is created and PE is converted into cross-linked polyethylene, XLPE.

 
 
  THERMAL PROPERTIES

Owing to the cross-linking, XLPE is a very heat-resistant material. It cannot melt like polyethylene but decomposes and carbonizes if exposed for long periods to temperatures above 300°C.

 
  The permissible conductor temperature during short-circuit for 1 second has therefore been put at 250°C and under continuous load, conductors with XLPE insulation may have a temperature of 90°C. These temperatures are specified by International standards. In emergency conditions and for limited period, XLPE can withstand l30°C.

 
  XLPE, like PE, retains its low temperature flexibility down to - 40°C, which implies great advantages during laying of cables.

 
  ELECTRICAL PROPERTIES

The good electrical properties of PE remain largely unchanged during cross-linking process. XLPE therefore, like PE, has a very small and insignificant temperature dependant loss factor (tan d) and dielectric constant (e). As a result the dielectric loss of XLPE cables are small in comparison with those of PVC and paper insulated cables. XLPE cables are specially adapted for long cable routes and high voltages, in both cases where dielectric losses are of great significance.

 
  MECHANICAL PROPERTIES

Polyethylene has good mechanical properties. It is interesting that at normal temperature PE can resist local stresses better than PVC. In this respect XLPE has the same advantageous properties as PE and certain grades, such as filled XLPE insulation also resists abrasion much better than polyethylene.

 
  CHEMICAL PROPERTIES

Owing to the cross-linking of molecules XLPE has better resistance than PE to most chemicals such as ordinary acids, bases and oil.

 
  ENVIRONMENTAL POLLUTION AND CABLES

From the environmental aspect, both PVC and oil-impregnated paper-insulated cables have distinct disadvantages. When PVC cables bum they give off corrosive gases, and a leaking oil- filled cable may cause severe damage to environment.

 
  XLPE admittedly bums, but the products of combustion carbon dioxide and water do not cause damage. Filled XLPE used for low voltage cables can also be made resistant to flame propagation and the compound does not produce halogen.

 
  Cross-linking Process

Cross-linking is done by agent di-cumyl peroxide (DCP). Cross-linking takes place in the CCV Tube under heated and pressurized Nitrogen where DCP decomposes into two radicals, which react with Polyethylene thereby causing cross-linking.

 
  Peroxide is already mixed at the material supplier's plant with proper balance of anti-oxidant and peroxide to ensure the required thermal stability and optimum curing level. Therefore, no mixing whatever is done during production. This will prevent problems which might occur due to the unbalanced mixing of material during production stage. Un-packing and handling of the material is also done in similar super clean environment at Riyadh Cables Group of Companies.

 
  The mixing ratio is about 1 to 1.5 PHR with very small amount of anti-oxidant. The cross-link residual is gas, which defuses out of the insulation gradually. Other residuals are Acetophone and Cumyl-alcohol which are in very low ratio, Research in this regard has proven that this material has very good effect on insulation such as:
  1. It improves the breaking strength when inclusion occurs in XLPE material.
  2. It slows water tree growth in the XLPE material under service.


 
  RCGC employs in-line dry curing for all its CCV lines alongwith State-Of-The-Art Triple Cross-Head extrusion where the Conductor Screen, Insulation and Insulation Screen are extruded simultaneously by means of a Triple Cross Head which has the following advantages:

 
 
  • Reduces micro voids and moisture content in insulation and ensures enhanced and stable breakdown strength and uniform insulation structure.
  • Ensures extremely accurate layer thickness.
  • Ensures high purity in the frontier limit between the semi-conductive layers and the insulation.
  • Provides optimal fusion of the individual layers without contamination.
  • Ensures a firm bond and smooth interface between each layer thus improving electrical properties.
  • Prevents unforeseen damage to the conductor or insulation screen during manufacturing process.
 
  The above are optimized by the use of an X-Ray unit located immediately after the cross-head which provides a transparent view of all three layers. This arrangement also facilitates recording the trend every 2 seconds. The unit continuously scans 360° geometry of the cable and displays the maximum, minimum and eccentricity of all three layers separately. Any deviation between the specified values and the measured values are recorded and adjusted automatically.

 
  Also incorporated in the CCV line is the "Twin-Rot" system, one of the latest in manufacturing technology which ensures superior control of eccentricity and eliminates the possibility of "Pear Drop" since the cable rotates during manufacture.

 
  QUALITY ASPECT

It is the policy of Riyadh Cables Group of Companies to supply customers with products meeting fully their stated needs. The products perform their required functions safely, consistently and reliably for their intended use. They fully meet the specifications which they are designed to meet whether Customer, Country or International.

 
  RCGC sources its raw materials from reputed suppliers from all over the world. The most important cable constituents such as XLPE material and conductor and insulation shielding material are procured from the world's leading suppliers of cable components. Right from the beginning, all incoming material and cable constituents are analyzed and tested to ensure their quality and compliance with specifications before being used. During manufacture again complete tests are performed on physical, mechanical and electrical properties of insulation and sheath material.

 
  The raw materials and products undergo rigorous and regular testing by local and overseas independent inspection agencies. Products have already been type tested at international agencies. Several of the products are also Type tested and certified from KEMA, Netherlands. Type tests are also performed in-house on a pre-defined regular basis to ensure and guarantee the quality of manufactured products.

 
  Testing facilities are equipped with up-to-date most modem and advanced laboratories. The laboratories are equipped with facilities for complete testing of its products both for incoming raw material as well as finished goods as per International Standards. Test fields include the following which are used mainly for research activities, performing trials and also for performing type tests.

 
 
  • Routine test field with large shielded enclosures for carrying out the routine high voltage tests, Partial discharge measurements upto 400 kV at a noise level of below 2 pC, measurement of target delta and capacitance.

  • Type test field consisting basically of 2400 kV impulse generator, 400 kV AC test system extendable upto 800 kV. Heating cycle equipment for conducting Special tests and Type tests on High Voltage Cables and Accessories, Qualification tests as per AEIC CS 8, AEIC CS 7, BS Standards, Long term tests and for determination of cable breakdown.


 
 
  Riyadh Cables has implemented in its system total quality management. Its commitment towards quality is reflected by achievement of ISO 9001 : 2000 (TUV certified) and has also obtained certification for its Quality Management System to BASEC (British Approvals Service for Cables).

 
  Research and Development:

The company has a realistic approach towards improvement and development of its products, therefore, it has established a Research and Development Department whose main objectives are:

 
 
  • Selection of the best raw material available for cables after long term and short term testing and after in-depth analysis and review.

  • Co-ordinate with other research bodies both in the Kingdom and World, to study cable phenomenon such as cable aging and insulation de-gradation and provide measures to minimize this effect.

  • Co-ordinate with local electric utilities in order to optimize the most economical cable construction taking into consideration their needs and local environment conditions.


 
 




XLPE INSULATED MEDIUM VOLTAGE CABLES
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