Conductor shielding layer (also called inner shielding layer, inner semi-conductive layer)
The conductor shielding layer is a non-metallic layer extruded on the cable conductor, which is equipotential with the conductor and has a volume resistivity of 100~1000Ω•m. Equipotential with the conductor.
Generally, low-voltage cables of 3kV and below do not have a conductor shielding layer, and medium and high-voltage cables of 6kV and above must have a conductor shielding layer.
The main functions of the conductor shielding layer: eliminate the unevenness of the conductor surface; eliminate the tip effect of the conductor surface; eliminate the pores between the conductor and the insulation; make the conductor and the insulation in close contact; improve the electric field distribution around the conductor; for cross-linked cable conductor shielding layer, it also has the function of inhibiting the growth of electric trees and heat shielding.
Insulation layer (also called main insulation)
The main insulation of the cable has the specific function of withstanding the system voltage. During the service life of the cable, it must withstand the rated voltage and overvoltage during system failures for a long time, lightning impulse voltage, to ensure that no relative or phase-to-phase breakdown short circuit occurs under the working heating state. Therefore, the main insulation material is the key to the quality of the cable.
Cross-linked polyethylene is a good insulating material, which is now widely used. Its color is bluish-white and translucent. Its characteristics are: high insulation resistance; able to withstand high power frequency and pulse electric field breakdown strength; low dielectric loss tangent; stable chemical properties; good heat resistance, long-term allowable operating temperature of 90°C; good mechanical properties, easy processing and process treatment.
Insulation shielding layer (also called outer shielding layer, outer semi-conductive layer)
The insulation shielding layer is a non-metallic layer extruded on the main insulation of the cable. Its material is also a cross-linked material with semi-conductive properties and a volume resistivity of 500~1000Ω•m. It is equipotential with the grounding protection.
Generally, low-voltage cables of 3kV and below do not have an insulation shielding layer, and medium and high-voltage cables of 6kV and above must have an insulation shielding layer.
The role of the insulation shielding layer: the transition between the main insulation of the cable and the grounding metal shielding, so that they have close contact, eliminate the gap between the insulation and the grounding conductor; eliminate the tip effect on the surface of the grounding copper tape; improve the electric field distribution around the insulation surface.
Insulation shielding is divided into strippable and non-strippable types according to the process. For medium voltage cables, strippable type is used for 35kV and below. Good strippable insulation shielding has good adhesion, and no semi-conductive particles remain after stripping. Non-strippable type is used for 110kV and above. The non-strippable shielding layer is more tightly combined with the main insulation, and the construction process requirements are higher.
Metal shielding layer
The metal shielding layer is wrapped outside the insulation shielding layer. The metal shielding layer generally uses copper tape or copper wire. It is a key structure that limits the electric field inside the cable and protects personal safety. It is also a grounding shielding layer that protects the cable from external electrical interference.
When a grounding or short-circuit fault occurs in the system, the metal shielding layer is the channel for the short-circuit grounding current. Its cross-sectional area should be calculated and determined according to the system short-circuit capacity and neutral point grounding method. Generally, the cross-sectional area of the shielding layer calculated for a 10kV system is recommended to be no less than 25 square millimeters.
In cable lines of 110kV and above, the metal shielding layer is composed of a metal sheath, which has both electric field shielding and waterproof sealing functions, and also has mechanical protection functions.
The material and structure of the metal sheath generally adopt corrugated aluminum sheath; corrugated copper sheath; corrugated stainless steel sheath; lead sheath, etc. In addition, there is a composite sheath, which is a structure in which aluminum foil is attached to the PVC and PE sheaths, which is widely used in European and American products.
Armor layer
A metal armor layer is wrapped around the inner lining layer, generally using double-layer galvanized steel belt armor. Its function is to protect the inside of the cable and prevent mechanical external forces from damaging the cable during construction and operation. It also has the function of grounding protection.
The armor layer has a variety of structures, such as steel wire armor, stainless steel armor, non-metal armor, etc., which are used for special cable structures.
Post time: Jun-28-2024
