The metal shielding of insulated cables is different from that of oil-paper (or oil-filled) cables. In oil-paper (or oil-filled) cables, the metal shielding is accomplished by lead sheath. Therefore, the actual shielding layer is cut off in accordance with the calculated value. In insulated cables, lead sheaths are not used (only 110 kV or higher grade cables are used), but copper strips are wrapped around the semi-conductive shielding layer in a certain way. This kind of manufacturing brings a lot of trouble to the calculation of shielding layer.
I. Shielding Layer Function
1. Make the direction of electric field the same as that of insulation radius.
For medium and low voltage cables, although three-phase cables are formed, they can still be regarded as three single-phase cables installation and test. There is no electrical connection between the phase and the phase.
2. Unbalanced current
As the center line, there is also capacitive current under normal conditions. When the cable fails, the copper strip acts as the short-circuit fault current loop.
3. Preventing Axial Surface Discharge
In a good grounding environment, due to the resistance of the semi-conductive shielding layer, the non-uniform potential distribution along the cable axis may result in the discharge along the cable surface. When there is a bad grounding in the middle of the cable axis, the distributed capacitive current forms a high potential zone at the two grounding points. The closer to the two ends, the greater the voltage drop (U), the greater the unit resistance R0 of the cable surface is, the same at the two ends, A and B. A high electric field is formed, which causes the phenomenon of discharge ignition. When the current is low, metal shielding can be used to eliminate it. However, for single-core cables, due to the high induction potential, even with metal shielding, it can not be eliminated. Cross-transposition method must be used to compensate, which will be discussed in detail in the future.
II. Structural Forms of Metal Shielding
One or two annealed steel strips are spirally lapped to form a cylindrical concentric conductor. Multiple thin steel wire wrapping and Waichuan copper belt spiral interrogation gap wrapping can increase the short-circuit capacity the most, and can offset the L dagger sense when wrapping in opposite direction.
III. CALCULATION OF SECTION OF SHIELDING LAYER
When three-phase short-circuit or two-phase short-circuit fault occurs in power system, no short-circuit current flows through the metal shield layer. Only when a single-phase short-circuit fault occurs in the cable or a two-phase grounding short-circuit fault occurs in the system, can the short-circuit current flow through the metal shield. From the operation experience, cable accidents, especially those of XLPE insulated cables, rarely occur inter-phase short-circuit. Most of them occur after the germination of grounding fault due to high temperature. The inter-phase short circuit occurs in cable accessories, such as insufficient inter-phase distance, improper installation, poor material performance and so on. Interphase short circuit usually occurs within seconds of single-phase fault. The current flowing through the shield layer may be very large in the initial stage, but after the short circuit between phases occurs, the current in the metal shield layer will decrease. Secondly, for example, in China's neutral point direct grounding system (for high voltage cables), in the metal shield layer, the short-circuit current flowing through the conductor core of the cable is the same as the short-circuit current flowing through the conductor core. In the neutral point non-direct grounding system (area of medium and low voltage distribution system), the short-circuit current will not be too large except in special cases (double grounding). For this reason, GB 12706-91 has stipulated that when the nominal cross-section of the metal shield layer is 16, 25, 35 and 50 mm, it can be selected according to the fault current capacity, but it should be ensured that the maximum temperature of the short-circuit current passing through the metal shield layer does not exceed 300 C.