Until the 1980s, most of the MV cables installed by utilities and industry were paper-insulated type in which the insulation over the conductor(s) is built up by applying multiple layers of special cellulose paper. This paper insulation has to be impregnated with insulating oil or compound to exclude air and impart good electrical properties. A common feature of all paper cables is a metallic sheath, normally extruded lead or aluminium, enclosing the paper-insulated core(s). The main purpose of the metallic sheath is to give the paper insulated cores complete protection from the environment outside the cable.
Figures 1, 2 and 3 show examples of 3-core cables with lead sheaths and steel tape or wire armour. The three common constructions are:
These three cable types each require their own accessory designs, so it is important to identify the cable construction details in order to select the right joint or termination.
Figure 1 – 12kV ‘belted’ paper-insulated cable with overall lead sheath and steel tape armour
Figure 2 – ‘H’ type or ‘screened’ 36kV paper-insulated cable with overall lead sheath and steel wire armour
Figure 3 – ‘HSL’ 24kV paper-insulated cable with individual lead sheaths and overall steel tape armour
In most European countries, paper insulated cables have been superseded by cables with extruded polymeric (plastic) insulation in place of paper. The insulation material in these modern cables is either crosslinked polyethylene (XLPE) or ethylene propylene rubber (EPR).
Figures 4 to 9 show typical single-core polymeric cables, though there are other design variations of cables for special applications. Similar design variety applies to 3-core cables, of which Figure 10 is one example. Figure 11 is an example of a 3-core bundle comprising 3 single-core cables supplied twisted together.
All these cables have an extruded polymeric outer sheath, usually PVC or polyethylene, and some form of metallic screen, typically copper wires or tapes, or longitudinally-applied aluminium or copper foil laminate. In addition, or sometimes in place of these screens, there may be an armour layer comprising steel tape or wire, or aluminium wire.
For protection against moisture entry, some cables are made with an extruded lead or aluminium sheath, and there may also be water-swellable tapes (Figure 9) or powder to prevent water passing along the cable under the sheath.
What is important for the installer is that he is aware of the function of each of the cable components and knows, with the help of the installation instruction, what to do with each of them as part of the procedure to prepare the cable ends.
At the heart of all modern MV polymeric cables is a three-layer extrusion comprising:
The outer conductive layer is often referred to as the ‘semicon screen’. An alternative insulation screen, typical of earlier designs of polymeric cables, is a conductive (graphite) paint layer covered by a conductive fabric tape (Figures 7 and 8).
The different types of insulation screen; bonded, strippable and paint & tape, each require different methods and different jointing tools for their correct removal as part of the accessory installation procedure.
Figure 4 – Single-core cable with bonded or strippable insulation screen and copper wire earth screen
Figure 5 – Single-core cable with bonded or strippable insulation screen and copper tape earth screen
Figure 6 – Single-core cable with aluminium or copper tape laminate earth screen bonded to the outer sheath
Figure 8 – Single-core cable with conductive paint & tape insulation screen and copper tape earth screen
Figure 9 – Single-core 36 kV cable with XLPE insulation, bonded insulation screen, copper wire earth screen, waterswellable tapes and PVC outer sheath
Figure 10 – Typical 3-core 12 kV cable with copper tape earth screens and steel wire armour
Figure 11 – Single-core cables supplied twisted together to form a 3-core bundle
Although it is many years since paper cables were installed as standard, there is still a demand for ‘transition joints’ to connect new polymeric cables into existing paper cable networks owned by electricity utilities. The requirements for correct handling and preparation of paper cables and polymeric cables are quite different, as will be seen in later sections.
LV, MV & HV Jointing, Earthing, Substation & Electrical Eqpt
Thorne & Derrick International are specialist distributors of LV, MV & HV Cable Installation, Jointing, Duct Sealing, Substation & Electrical Equipment – servicing UK and global businesses involved in cable installations, cable jointing, substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.
THORNE & DERRICK Product Categories: Duct Seals | Cable Cleats | Cable Glands | Electrical Safety | Arc Flash Protection | Cable Jointing Tools | Cable Pulling | Earthing | Feeder Pillars | Cable Joints LV | Joints & Terminations MV HV
Pictured : Type Testing Mosdorfer CCL HV heat shrink cable joints in KIT Germany.
Dual wall heat shrink tubes are used to rebuild the cable insulation with built-in stress control.
Image : Ahmad Haidar (Sales Business Development Consultant at Mosdorfer CCL Systems.
T&D, the leading supplier of Cable Jointing, Cable Pulling & Laying Equipment welcome all UK and international sales enquiries.
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Includes access to our popular JOINTERS BLOG with contributions from utility professionals, linesmen and cable jointers working on MV HV EHV cables and overhead lines typically at 11kV, 33kV, 66kV and up to 132kV.
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Image Courtesy of : Pete Thomasson – HV Cable Jointer / Linesman @ Ventia.
Pictured: 3M QTIII Cold Shrink Cable Terminations (33kV/6.6kV Step-Down – XLPE Single).
T&D stock an extensive range of 11kV-33kV 3M Cold Shrink Joints & Terminations for the jointing, terminating and abandonment of medium voltage power cables.
T&D are 3M UK’s largest stockist of their range of Scotchcast and Cold Shrink cable joints, terminations and abandonment kits for industrial and hazardous area installations.
3M Electrical
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Pictured : EHV Cable Jointer Installing Pfisterer Connex Size 5s Inner Cone Plug Cable Termination (132kV).
Image Courtesy : Dean Wilson (Owner Director at D.C. Cable Jointing Ltd)
T&D are Main Distributors & Stockists for the Pfisterer range of Medium Voltage Power Products including Connex Plugs, Cable Jointing Tools, Sicon Connectors, Surge Arresters and Electrical Safety Equipment.
The Pfisterer MV Connex range is ideal for use in high load ring main units, circuit breaker switchgear, high voltage motors, transformers, capacitors, transducers and cable joint boxes. Pfisterer connectors on the equipment side are designed to meet EN 50180 (bushings for liquid filled transformers up to 52kV), EN 50181 (plug-in type bushings up to 36kV for equipment other than filled transformers) and DIN 47637 (separable accessories for power cable for bushing with inside cone).
Pfisterer Connex Inner Cone Plug Terminations
Video: Pfisterer Tools – Pfisterer Connex Plugs (33kV Separable Connectors Size 3) – careful and professional MV cable preparation using approved cable jointing tools is essential to acheive a reliable installation.
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Image: Darren Street Cable Jointer at D+R Jointing Services Ltd.
Pictured: Nexans Euromold Cable Connectors Terminating 11kV Cables Into Siemens NXPLUS Gas Insulated Switchgear (GIS).
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