(1) Function: It can electrically isolate the core from the ground and the cores of different phases from each other, thereby ensuring that no relative ground or phase-to-phase breakdown short circuit occurs when transmitting electric energy. Therefore, the insulation layer is also an indispensable component of the cable structure.
(2) Material requirements.
1) High voltage resistance. Since the phase-to-phase distance and the distance to the ground of the conductive part of the cable are relatively small, the insulation layer is subjected to a very high electric field strength, generally between 1~5kV/mm, 8~10kV/mm in 110kV cables, and as high as 14~16.5kV/mm in 500kV cables. The higher the voltage level of the cable, the higher the voltage resistance requirement for the insulation material.
2) Low dielectric loss tangent. Insulating media operating in an AC electric field, due to the presence of polar molecules, leakage current will pass through the insulation layer, causing the insulation layer (medium) to heat up. This part of the loss is called dielectric loss. The higher the cable voltage level, the greater the dielectric loss. This high loss results in increased heat generation, which accelerates insulation aging. Therefore, the insulation material must have a low dielectric loss tangent.
3) Good corona resistance. Air bubbles in the insulation layer or protrusions on the inner and outer surfaces are easily ionized under high electric fields, causing discharges. The ozone generated during discharges is destructive to the insulation layer. Corona resistance varies among materials, so materials with good corona resistance are required.
4) Chemical stability. Chemically unstable materials are susceptible to changes in properties due to external factors, which can affect their insulation level. These changes typically result in a deterioration in insulation performance, which has a direct impact on the cable's service life. Therefore, materials with stable chemical properties should be selected.
5) Low-temperature resistance. Generally, non-metallic materials have high strength but a high brittle point. Cable line installation (especially in northern regions) often requires installation in very low temperatures. Once brittle, they are easily damaged and cannot be installed. Therefore, low-temperature resistance is required. In northern winters, when the average temperature is below 0°C, high-voltage cables must be preheated before installation.
6) Good heat resistance. The maximum allowable operating temperature of a cable depends on the heat resistance of the insulation material. Specifically, the higher the maximum allowable temperature at which the physical and chemical properties of the insulation material remain unchanged, the better. This increases the current carrying capacity of the cable line. Therefore, the higher the heat resistance of the insulation material, the better.
7) Good machinability. Insulation materials must possess a certain degree of flexibility and mechanical strength to facilitate manufacturing, construction, and installation. These are essential properties of insulation materials.
8) Long service life. Insulation materials will age over a certain period of time, leading to performance degradation or even failure. Due to the high cost and construction expenses of cable lines and the difficulty of laying them, the service life of cables is highly demanding, requiring them to be durable. Currently, the service life of cables is generally no less than 30 years.
