MV Cables | Electric Field & Stress Control

Published 10 Jan 2020

LV MV HV Cables

Electric field and stress control

All cables for MV and higher voltage ratings, whether paper or polymeric insulated, have a metallic screen which is connected to earth. In addition, polymeric cables have a conductive polymeric screen as described in earlier sections. This semicon screen ensures that the electric field generated by the energised phase conductor is wholly contained within the primary insulation.

At the position where a cable is to be jointed or terminated, outer layers of the cable including the conductive screen over the insulation must be cut back according to dimensions given in the installation instruction.

Figure 54 – Electrical equipotential lines indicating strong electric field at the screen edge

Figure 54 – Electrical equipotential lines indicating strong electric field at the screen edge

Figure 54 shows this situation, with the electric field represented by ‘equipotential lines’ joining points of equal voltage or ‘potential’. Where the equipotential lines are close together the electric field is strong.

This strong electric field at the screen edge (indicated by the red circle) would cause insulation damage and eventual electrical breakdown if nothing were done to reduce the electric stress at this position. This is why all MV joints and terminations have ‘stress control’ components whose function is to reduce the electric stress at the screen edge (and other positions of high stress) to acceptable levels.

Figure 55 shows the stress control method used from the earliest days of cable accessory development. This is a ‘stress cone’ which controls stress by positioning a conductive cone shaped component at the screen edge and thereby forcing the equipotential lines to separate exit the cable insulation more gradually. This is called a ‘geometric’ method of stress control. Stress cones for polymeric cables are made of flexible rubber and are commonly incorporated in cold-shrink or push-on accessories.

Figure 55 – Control of electric stress by a ‘stress cone’

Figure 55 – Control of electric stress by a ‘stress cone’

The common alternative to a geometric stress cone is a layer of material with special electrical impedance characteristics. The stress control function will depend on the resistivity and/or high relative permittivity of the material but the result is similar to that of a stress cone, in that the electric field is graded along the length of the layer and the field strength at the screen edge is reduced. The material may have ‘non-linear’ resistivity properties similar to those of surge arresters.

Figure 56 illustrates this type of electrical stress control. The materials may be in the form of heat-shrink or cold-shrink tubings, mastics or hot-melt compounds.

Figure 56 – Control of stress by a material layer with special electrical impedance characteristics

Figure 56 – Control of stress by a material layer with special electrical impedance characteristics

Partial discharges and their effects

Partial discharges are localised electrical breakdowns, typically occurring in small voids within insulation. When partial discharges happen in free air, for example on the surface of insulators, they are commonly called ‘corona’.

Partial discharges within the polymeric insulation of joints and terminations are likely to result in full breakdown of the accessory at some unpredictable time during service, depending on the size and number of the discharges and the progressive damage done by them.

Effective stress control components and their correct installation will reduce the likelihood of partial discharges occurring at working voltage or test voltages. The essential part to be played by the installer is to ensure that the accessory insulation is as void-free as possible, especially in high stress areas such as near the screen edge. Best practice will involve close attention to the following.

  • Ensuring that interfaces, such as those between layers of heat-shrink material, are clean and free from any contamination.
  • Correctly positioning cold-shrink and push-on components that make contact with the screen edge.
  • Applying adequate and uniform heating to heat-shrink components.
  • Applying insulation or stress control tapes exactly as required in the installation instruction (tension, positioning etc).

Further Reading

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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.

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