The voltage range of medium voltage cables is usually between 5kV and 35kV. They are mostly used for power transmission in industry, utilities, petrochemicals and construction. The MV cable design can be of different sizes and types as well as the construction is much more complex than that of LV cables. A metal shielding layer is an essential structure in medium voltage power cables and is typically composed of copper tape or copper wire wrapping. The so-called “shielding” on the cable structure is also a measure to improve the distribution of the electric field, which has the following effects:
- Protection against electromagnetic interference (EMI). Since the power cable through the current is relatively large, the current around the magnetic field will be generated, in order not to affect other components, so the shielding layer can be added to the cable energized electromagnetic field caused by shielding in the insulated core. So as to reduce the electromagnetic interference generated by the outside world.
- Safety and grounding. If the cable sustains any internal damage, the interference shield can also be considered as a grounding route, which allows the cable to be protected from damage and improves safety by diverting leakage currents to the ground.
- Lightning and power plant protection. The metal shield has good electrical conductivity and can provide effective lightning protection. In power stations or transmission lines, it quickly conducts away the overvoltage generated by lightning strikes and protects the cable from direct lightning damage.
- Pathway for capacitive and short-circuit currents. In normal operation, the metal shield can carry the small capacitive current generated by the capacitive effect of the cable. In the event of a short-circuit fault, the metal shield becomes a low-impedance path for the short-circuit current to allow the current to pass quickly.
- Homogenize the electric field to prevent axial discharge. Due to the semi-conductive layer has a certain resistance, when the metal shielding layer is poorly grounded, in the axial direction of the cable due to the uneven distribution of potential caused by the cable along the surface of the discharge.
- Moisture Resistance and Durability: Since many MV cables are used underground, shielding helps to resist moisture and protects the cable core from deterioration due to humidity or water ingress.
- Anti-static protection: The shielding also acts as an electrostatic shield. It prevents electric shocks or sparks caused by static buildup during laying and use of the cable which enhances the safety of the cable.
