Summary of power cable knowledge (two)

Conductor shielding layer (also called inner shielding layer, inner semi-conductive layer)

①Conductor shielding layer is a non-metallic layer squeezed on the cable conductor, and is equipotential with the conductor, and the volume resistivity is 100~1000Ω•m. Equipotential with conductor.

② Generally, low-voltage cables of 3kV and below have no conductor shielding layer, and medium- and high-voltage cables of 6kV and above must have conductor shielding layer.

③The main function of the conductor shielding layer: eliminate potholes on the surface of the conductor; eliminate the tip effect on the surface of the conductor; eliminate the pores between the conductor and the insulation; make close contact between the conductor and the insulation; improve the electric field distribution around the conductor; The cable conductor shielding layer also has the function of inhibiting the growth of electrical trees and heat shielding.

Insulation layer (also called main insulation)

①The main insulation of the aaac conductor  has the specific function of withstanding the system voltage. During the service life of the cable, it has to withstand the rated voltage and the overvoltage of the system failure, lightning impulse voltage for a long time, and ensure that no phase-to-earth or phase-to-phase interaction occurs under the working heat state. Breakdown short circuit. Therefore, the main insulation material is the key to the quality of the cable.

②Cross-linked polyethylene is a good insulating material and is now widely used. Its color is bluish white and translucent. Its characteristics are: higher insulation resistance; able to withstand higher power frequency and pulse electric field breakdown strength; lower dielectric loss tangent value; stable chemical performance; good heat resistance, long-term allowable operating temperature of 90℃; Good mechanical properties, easy to process and process.

Insulation shielding layer (also called outer shielding layer, outer semi-conductive layer)

① The insulating shielding layer is a non-metallic layer extruded on the main insulation of the cable. Its material is also a cross-linked material, which is semi-conductive and has a volume resistivity of 500-1000Ω•m. Equipotential with ground protection.

② Generally, low-voltage cables of 3kV and below have no insulation shielding layer, and medium- and high-voltage cables of 6kV and above must have insulation shielding layer.

③The role of the insulating shielding layer: the transition between the main insulation of the cable and the grounded metal shield, so that there is close contact, eliminates the pores between the insulation and the acsr conductor; eliminates the tip effect on the surface of the grounded copper strip; improves the insulation around the surface Electric field distribution.

④Insulation shielding is divided into peelable type and non-peelable type according to the process. Generally, medium voltage cables use peelable type for 35kV and below. A good peelable insulating shield has good adhesion and no semi-conductive particles remain after peeling. 110kV and above adopt non-peelable type. The non-peelable shielding layer is more closely integrated with the main insulation, and the construction process requirements are higher.


Metal shield

①The metal shielding layer is wrapped outside the insulating shielding layer. The metal shielding layer is generally made of copper tape or copper wire. It is a key structure to limit the electric field inside the cable and protect personal safety. It is also a grounding shield that protects the cable from external electrical interference.

②In the event of a grounding or short-circuit fault in the system, the metal shielding layer is the channel for short-circuit grounding current. Its cross-sectional area should be calculated and determined according to the short-circuit capacity of the system and the neutral 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 meters. Mm.

③The metal shielding layer in the cable line of 110kV and above is composed of a metal sheath, which has the function of electric field shielding and waterproof sealing, as well as mechanical protection.

④The 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 affixed to the PVC and PE sheaths, and is used more in European and American products.

Armor layer

① A metal armor layer is wound around the inner lining layer, generally double-layer galvanized steel tape armor. Its function is to protect the inside of the cable and prevent damage to the cable by mechanical external forces during construction and operation. It also has the function of grounding protection.

②The 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.

Outer sheath

①This is the outermost protection of the acsr conductor chart. Generally, polyvinyl chloride (PVC) polyethylene (PE) is used, which are all insulating materials and are formed by extrusion. According to technical requirements, flame-retardant polyvinyl chloride (PVC) is generally used. Adapt to the requirements of cold winter and hot summer without cracking or softening.

②The main function of the outer sheath is to seal to prevent moisture intrusion, protect the armor layer from corrosion, and prevent the expansion of fire caused by cable failure.

③The characteristic information of the cable is also printed on the outer sheath, such as specification, model, production year, manufacturer, continuous meter length, etc.