The MDCV Process By Peter Stewart-Hay
  The MDCV (Mitsubishi Dainichi Continuous Vulcanization) process is a commercial, continuous dry curing process co-developed by Mitsubishi Cable Industries Ltd. and Mitsubishi Petrochemical Co. Ltd. (Formerly the Dainichi Chemical Company of Japan.) The most fundamental part of the MDCV process is the cross-linking of the insulation and semi-conducting layers of insulation while still under slightly decreasing extrusion pressure in the long land die (LLD), an external extension of the extruder triple crosshead. These machines are designed to manufacture XLPE high-voltage and extra-high-voltage cables rated from say 138 kV to 550 kV. The MDCV extrusion lines are significantly different from vertical continuous dry cure vulcanizing lines (VCV) and catenary continuous dry cure vulcanizing lines (CCV) by virtue of the fact that MDCV extrusion lines are completely horizontal and thus more economical to build. High building bays are still required however so that the pay-off and take up reels can be accommodated and rewinding can be facilitated. Cable manufacturers are required under the licensing agreement to not disclose any essential details of the technology. An MDCV line uses a super clean, high molecular weight, low-melt index, cross-linkable polyethylene compound with a lower organo-peroxide concentration than that used in the other described dry curing methods (VCV and CCV). Because of this, MDCV lines produce fewer by-products such as acetophenone, methane, methyl styrene, cumyl alcohol, other trace organic compounds and water vapor at about 300 ppm. (Note: The cross-linking process is activated thermally by the chemical decomposition of the organo-peroxide.) Analysis after curing of the standard super clean insulations in VCV and CCV extrusion lines has shown that the more volatile by-products result in the creation of micro voids in the insulation in an order of about 100 per cubic millimetre. Moreover, the diameters of these micro voids range typically from one to ten microns. Today, this is considered adequate for high- and extra-high voltage cable manufacture. The MDCV process however, with its lower organo-peroxide level in the insulation and the process of cross-linking of the insulation while still under extrusion pressure, results in the formation of less gaseous decomposition by-products and thus fewer condensation micro voids in the insulation. One can conclude that the fewer the micro voids in the insulation, the better the quality of the insulation. This was learned from the evolution of very poor quality, steam cure XLPE insulation to high quality, dry cure XLPE insulation.
Below is the original, published design of the long land die system but secrecy prevents us from showing the second generation design of an MDCV line. The cross-section drawing however is adequate to allow the readers to have a fairly good feel for the long land die machinery and process.
To combat conductor sag by gravity in the insulation in the horizontal long land die, there are three items to consider: 1. The long land die is a confined space filled with the viscous plastic insulation. For the conductor to sink, it must displace this high viscosity insulation below the conductor and require it to flow around the conductor to a place either above or beside the conductor. This is not an easy task. Added to this is the increasing cross-linking of any cable segment as it moves along the long land die, toward the take-up. This significantly increases the viscosity of the insulation and thus the resistance to conductor sag. 2. The rheological properties of the insulation were tailored specifically to provide a higher viscosity, thereby reinforcing resistance to flow as per item one. 3. The cable inside the long land die is rotated continuously to ensure that the conductor is actually always falling back into centre. Many cable companies have declined to purchase a license to manufacture MDCV XLPE cables because of a perceived slower line speed and the high cost of the long land die segments and turn buckle components. The long land die is certainly an expensive tool but there are ways to reduce this cost. It has also been determined that at 230 kV and above the line speed of a MDCV line is comparable to that achieved by a properly designed VCV line. The basic components of an MDCV production line can be purchased from many wire and cable machine manufacturers but some components of the second generation lines are exclusively manufactured by Mitsubishi Cable Industries Ltd. Moreover, it would be impossible to build and electronically integrate a MDCV line without the instruction manual, which contains all of the theory of operation and the drawings provided to all licensees. The most recent manual, for example, is about 250 pages long. In April 2002, the EXSYM Corporation was formed in Japan through the integration of Showa Electric Wire & Cable Co. Ltd. (SWCC Showa Holdings Co. Ltd., Japan) and Mitsubishi Cable Industries Ltd. At this time, the MDCV licensing group in Tokyo was disbanded and the technical employees retired or reassigned. There will be no new MDCV licenses. Today there are only a handful of MDCV lines around the world and the majority of them are in Western Europe, with some in Japan and India. It is interesting to note that existing MDCV licensees with the older design MDCV lines, after careful legal review, decided to build sister MDCV lines for their licensed plant. The writer is aware of this has already happened at one location. The Indian MDCV line was relocated from Mumbai to Nashik a few years ago and at that time it was rebuilt and upgraded. It is owned and operated by the Cable Corporation of India. Independent machinery suppliers from Europe have advised the writer that this second generation line is now technically equivalent to any of the machines presently installed in Europe. The writer played a significant role in the repair and upgrade of this Indian machine as well as managing the construction of the final (newest) and most advanced MDCV line in the World in late 1991,1992 and early 1993. As with all HV and EHV CCV and VCV extrusion lines, a MDCV line requires clean room handling and conveying of the insulation and the use of super-smooth, super-clean semi-conducting polymers. This is of course extremely important when manufacturing undersea HV and EHV cables. Because there will be no new EXSYM licensees producing EHV cable on MDCV lines, this is where the story ends excepting that the writer is aware of a heavily used and retired MDCV line in Europe.
W - The Conductor R - The Triple Layer XLPE Insulation 1 - The Triple Crosshead 2 - The Inner Tip or Nipple 3 - The Obsolete Long Land Die Segments 3 - The Cooling Section
4 - The Lubricant Injector 6 - Supplementary Clamshell Die Heaters 7 - Crosshead Die Adaption Retainer 31 - Die Adaptation Crosshead to Segments 41 - Obsolete Porous Metal Lubricant Filter 42 - Lubricant Reservoir
43 - Lubricant Supply Tube 44 - Lubricant Injector Adaptor Passage 3 - Obsolete Cooling Water Die Segments 3 - Obsolete Gradient Cooling Segments Missing - Suction Device Plus Water Seals Missing - Gradient Cooling Water Seals