STRESS MANAGEMENT STRETCHES CABLE LIFE
Electrical stresses in medium-voltage distribution cables must be carefully managed to get good cable performance.
- Published in Southwire's T&D Update newsletter in June 1997
- Reprint permission granted
Managing medium-voltage cable stress doesn't mean putting your cable buyer on tranquilizers.
"In cables operating in the medium-voltage range and above, reducing or distributing electrical stress is the key to good cable life," says Nick Ware, manager of Southwire's Cofer Technology Center.
Core Components Can Control Stress
Stress concerns start with the conductor. A larger diameter conductor puts less stress on the surrounding insulation. Why? A larger conductor radius distributes the stress over a larger surface.
In stranded conductors, the uneven surface of the twisted strands will create concentrations of electrical stress. Even with solid conductors, imperfections in the conductor surface form stress concentrations. That's why designers call for a layer of semiconducting material around the strands. The semiconductor distributes the stresses evenly into the insulation.
For high-quality, long-life cable, the smoothness of the semiconducting layer itself becomes important. Pits or projections at the interface with the insulation will also generate stress concentrations.
Cable designers must consider manufacturing realities. If the conductors are well-centered in the extruded insulation, electrical stresses will be distributed evenly. If not, stresses will be higher where the conductors are closer to the insulation surface. Voids and contaminants in the insulation are issues, too. They create localized electrical stresses that can lead to insulation problems. Closer manufacturing control allows designers to predict stress more accurately. That can reduce cable diameters and help control overall costs.
Shielding Distributes Internal Stresses
In most medium-voltage cable designs, the insulation layer is surrounded by a second semiconducting layer at ground potential. This distributes the electrostatic stress evenly around the cable. Without the insulation shield, the stress will be concentrated between the cable and the nearest ground potential.
Also, at voltages above 2,400V, potential problems include shock hazard to personnel, surface discharges, and tracking that can lead to cable failure. Southwire recommends using shielded cable designs for applications above 2,400V.
In shielded cable, leakage currents through the insulation to the outer semiconducting layer must be routed to the nearest ground by a metallic wire or tape shield. The metallic shield must carry the summation of the leakage currents with an acceptable voltage drop. In some applications, the metallic shield must also carry fault currents.
"In every cable component from the conductors out, the design goal is to reduce and distribute electrical stress," says Ware. "Good designs and high-quality manufacturing deliver better cable performance and longer life." |
