I. The Difference Between Low-Voltage and High-Voltage Cables
High-voltage cables, from the inside out, consist of conductor - inner semiconducting layer - insulation layer - outer semiconducting layer - metal armor - sheath.
Low-voltage cables, from the inside out, consist of conductor - insulation layer - steel tape - sheath (many low-voltage cables do not even have steel tape).
The main difference between high-voltage and low-voltage cables is that high-voltage cables have additional semiconducting layers and shielding layers. Therefore, the insulation layer of high-voltage cables is much thicker than that of low-voltage cables, and their structure is more complex, requiring higher manufacturing processes.
(I) Semiconductor Layer
The inner semiconducting layer primarily serves to mitigate the electric field effect. Because there is a gap between the conductor and insulation layer of high-voltage cables, localized discharge (PD) can easily damage the insulation layer. To mitigate this, a shielding layer of semiconducting material is added between the metal conductor and insulation layer to act as a transition. The outer semiconducting layer serves the same purpose as the inner semiconducting layer: it prevents localized discharge between the insulation layer and the metal sheath.
(2) Shielding Layer
The metal shielding layer of a high-voltage cable has three main functions:
1. Shielding the electric field
As the name suggests, it provides a shielding effect. The voltage within a high-voltage cable is extremely high, and the electric field it generates is extremely strong. The shielding layer effectively prevents the high-voltage cable from being disturbed by the outside world.
2. Capacitive current during operation
The inner layer of a high-voltage cable is a conductor, with an insulating layer in the middle and a metal sheath on the outside. The two conductors (the inner copper wire + the metal sheath) are separated by an insulating medium (the insulating layer), which can be thought of as a capacitor. The basic principle of a capacitor is two metal plates separated by an insulating medium. Alternating current charges and discharges the capacitor, so the shielding layer provides a circuit for this.
In addition, both the metal shielding layer and the metal armor of a high-voltage cable need to be grounded at one end.
3. It can serve as a short-circuit current path.
If the insulation of a high-voltage cable is damaged, without a shielding layer, the cable will leak to the ground, posing a safety hazard. If a shielding layer is present, the leakage current will flow through the shielding layer to the ground.
2. How to distinguish low-voltage cables from high-voltage cables
1. Look at the cable structure. High-voltage cables have many layers. When you peel off the outer layer, there are armor, shielding layer, insulation layer, conductor, etc. inside. Low-voltage cables generally peel off the outer layer, and the inner layer is the insulation layer or conductor.
2. Check the cross-section. The cable contains a conductive core (copper core or aluminum core), and the outer layers are: insulation layer, semi-conductive layer, shielding layer, filling layer, steel protective layer, rubber protective layer (water-blocking).
3. Check the thickness of the insulation layer: the insulation layer of high-voltage cables is relatively thick, and the insulation layer of low-voltage cables is relatively thin. The insulation layer of low-voltage cables is generally within 3 mm, and the insulation layer of high-voltage cables is generally above 5 mm.
(1) Low voltage (below 1kv) is 1~3mm thick, 10kv cable is 5~8mm, and 35kv cable is about 10mm; low-voltage or weak-current cables generally have only one layer of insulation layer and protective layer.
(2) After removing the outer sheath, high-voltage cables have an insulation layer, which is a white plastic main insulation layer wrapped around the cable core. Low-voltage cables do not have this main insulation layer, but only a rubber protective layer.
4. Check the cable outer nameplate. The outer layer of the cable is generally printed with relevant cable parameters, including cable model, cross-sectional area, rated voltage, length and other parameters.
5. Check the voltage registration. The drawings generally have voltage levels such as YJV-1KV-4*150 or YJV-10KV-4*150.
Cables are usually divided into the following categories according to voltage levels: 1) Weak-current cables: 450/750V and below; 2) Low-voltage cables: 0.6/1kv; 3) Medium-voltage cables: 3-35kv; 4) High-voltage cables: 35-110kv; 5) Ultra-high-voltage cables: 110-750kv.
