Introduction

High voltage (HV) and extra-high voltage (EHV) underground cables commonly are plastic insulated and crosslinked either in catenary CV-line (CCV) or in vertical CV-line (VCV). There are some 400 catenary CV-lines and about 50 vertical CV-lines, or MDCV-lines. Practically all new CV-lines are radiant curing lines employing water or gas cooling.

The basic process has not changed very much since the introduction of triple cross-head. One of the few advances has been technology to produce heavy wall cores in catenary CV-line. Development in CV-line instrumentation has been more active. An X-ray system for layer adjustment and gravimetric control of extruder feeding are examples of measuring units which improve the operation of CV-line. Today, CV-line automation is based on a distribution system having programmable logic controllers (PLCs), operator panels, and process interface connected via field bus. A curing calculation programme is a vital part of this automation.

CCV-line versus VCV-line

Roundness and concentricity of the insulated core are essential for MV, HV, and EHV power cables with XLPE insulation. Specifications for roundness and concentricity have generally become tighter. The manufacture of reliably round and concentric layers can save significant amounts of insulation and semicon material, and also make further processing phases easier - including jointing and terminating during installation. Drooping insulation has limited the production of HV and EHV cores in inert gas catenary CV-lines. Generally, VCV-line gives better roundness compared to inert gas CCV-line, but the related building costs are high. Drooping in an inert gas CCV-line can be alleviated by reducing insulation viscosity and/or rotating the core. One solution is to use rotating caterpillars both before the cross-head and after the end seal. Conductors for HV and EHV cables are often taped with large sizes of the Milliken type, and are thus already sensitive to torsional forces. Round and concentric HV and EHV cores can also be produced with moderate rotating.

In recent years a significant number of CCV-lines have been equipped with the entry heat treatment (EHT) system, and these are now producing heavy-wall cores and giving excellent results. The system is based on reducing the overall viscosity of the insulation at the beginning of the CCV-line where drooping normally takes place. EHT is based on minimising that part of the insulation that is well over the melting point but not yet crosslinked (Figure 1). After, the cross-head conductor cools down the inner semicon and inner parts of the insulation and increases the supporting diameter. Large copper conductors, which are normally used with HV and EHV cores, have high heat capacity per unit length and thus will effectively cool the insulation.

Additionally, the surface of the core is cooled down by means of circulating nitrogen after the cross-head and before the heating zones (Figure 2). Circulation is controlled so that insulation temperature is at the level of the melting point on entering the first heating zone.

Figure 1: Melting and crosslinking inside insulation

Figure 2: EHT-system. Cooling circulation is applied after the cross-head

This short cooling section is followed by high heating, limited only by the maximum allowable surface temperature of the core. This minimizes the time during which the insulation is melted but not crosslinked. Table I shows some roundness values for cores produced in CCV-line equipped with EHT.

Table I: Some typical roundness values in Maillefer CCV-line with EHT-system

Maximising line speed

In many cases, however, VCV-line are preferred. Since building costs for CV-tower are remarkably high, it becomes vitally important to maximise line speed. Extruders, if correctly selected, can normally fulfill VCV-line output needs. It is a question of curing and cooling capacity. To fully utilise layout possibilities, a pressurised turn pulley is commonly used in VCV-line to extend cooling length. Chillers for cooling water can be used, but there is not much more that can be done to improve cooling capacity.

The situation is different for curing capacity. Inductive conductor heating can be utilized more effectively by means of post-heating (“post-heater”). Today the typical preheating temperature in VCV-line is in the range 60 to 100°C. There are several reasons why preheating temperature is limited. These include copper oxidation, conductor tape deformation, moisture-block material deterioration, etc. The post-heater is located after the cross-head in the pressurised tube, where the conductor can be heated up without these limitations. Post-heat temperature up to 180 - 200°C can be used. This allows a significant increase in line speed of 20-40%, depending on core and CV-line layout. Figure 3 shows a comparison for VCV-line with and without post-heater. It should also be observed that much shorter heating length is needed with post-heater. This length can be used for cooling.

Figure 3: VCV-line with and without postheater. Curves show conductor temperature and surface temperature. Core is Cu 1,000mm2 132kV

Figure 4 shows the inductive coil used for post-heating. It forms part of a 0.5m long CV-tube. Due to the insulation and semiconductive layers, coil diameter is large in relation to the conductor diameter. This of course means rather low heating efficiency, but becomes negligible in importance when compared, for instance. to the price of materials.

Figure 4: Inductive coil used with post-heating

Optimisation within process constraints

Even today, core temperature or crosslinking cannot be measured on-line. Numerical simulation and optimisation of the process, based on thorough process know-how and connected seamlessly to CV-line automation, are of vital importance. Simulation opens a window into CV-line process, showing both core temperature and crosslinking (Figure 5).

Figure 5: Numerically calculated core temperature inside CV-tube. Figure shows temperature from cross-head to end seal. Left side is conductor temperature; right side, surface temperature

Curing calculation consists of both simulation and optimisation. The optimisation part uses simulation to find and state the maximised line speed within given process constraints. Typically, curing calculation is used as recipe generator for CV-line automation system.

CV-line automation

An automation system for CV-line consists of PLCs with distributed process interface, PC-based process supervising unit as operator interface, and curing calculation as recipe generator. In a modern CV-line there can be several independent PLCs for such separate units or functions as line equipment and tube heating, implemented with Profibus, Interbus, or similar technology. The process supervising unit (Figure 6) includes process displays, trends, logging, alarms, and recipe system. Trends and data logging are of great importance, since abnormal situations during production can be traced afterwards.

Figure 6: User interface (PSU) of modern CV-line

Achieving savings in materials

Material savings during start-up and production has been the focus over recent years as a result of reduced profit margins in the cable business. Accordingly, in CV-line start-up it is important to reach acceptable process conditions and core quality as fast as possible. For core dimensions, this means the use of an X-ray system for centering. In the production phase it is possible to utilise either an X-ray system or a gravimetric method, or both. An X-ray system is included in practically all new CV-lines, and these units have been installed in many existing lines as well.

Figure 7: X-ray system installed after cross-head

Gravimetric control has also been shown to give significant material savings. This system is installed on the hopper of the extruder, for measuring granule flow gravimetrically and controlling screw speed. X-ray is irreplaceable for centring and start-up. But for controlling longitudinal variations also, gravimetric control should be considered.

The Control of contaminants

Dielectric strength of XLPE insulated cables depends primarily on the smoothness of the insulation-semicon interface as well as the purity and integrity of insulation. These in turn depend on material cleanliness, handling, and extrusion. Cleanliness of extruded insulation material can also be controlled in-line with an optical cleanliness scanning system. This is installed between the main extruder and the cross-head and inspects 100% of the insulation material. Polymer is flowing between two glass windows. Since molten LDPE is transparent, possible foreign particles can be seen and recorded. For large contaminants, size and shape are reported; for small ones, only the number of particles per category.

Figure 8: CSS installed in CV-line after insulation extruder

Conclusion

Radiant curing CV-line has not changed very much over the last two decades. Single-screw extruders, radiant heating, and water or nitrogen cooling are still used. Even triple cross-head has been in use for quite some time. Even so, some clear changes have taken place. HV and EHV cores are now insulated and crosslinked in catenary CV-lines. Extended conductor heating is used to improve CV-line efficiency. Curing calculation has been further developed to better utilise CV-line capacity. Instrumentation, notably X-ray and gravimetric systems, is utilised to save material and reduce scrap. And an X-ray system is already a practical standard.

Maillefer Extrusion Oy
Ansatie 6a B
FIN-01740 Vantaa
Finland
Fax: Int’l +358 9 88 66 57 71