Sheath loss
The cylindrical metal sheath of the cable is in the alternating magnetic field of the core and generates an induced potential. Since the induced potential in the metal jacket of the cable is proportional to the product of the length of the cable and the current of the core, the induced potential of the single-core cable may reach a large value when the length and cross-sectional area of ​​the core are large. Especially when a short-circuit accident occurs in the system, the induced potential may reach the level of endangering personal safety. Mine cables may even cause cable operation accidents. Therefore, in most cases, two or more points of the cable sheath are firmly grounded to avoid the hazard caused by the sheath's induced potential, and the metal sheath can also serve as a grounding current path. However, the sheath is grounded to form a loop, and the induced potential causes an electric current to generate a loss, which is called a circulation loss. In order to limit the magnitude of the induced potential, it does not cause a large loss of the circulating current, and it is often necessary to take measures for the metal sheath of a large cross-section single-core cable. A single point of grounding can be used when the cable length is short. For long cable runs, special connection splice boxes can be used. To reduce grounding, transposition connections can be used. Single-core cable consisting of three-phase power transmission system can also use transposition and pressure drop method.
When the outside diameter of the sheath is comparable to the distance between the center of the cable and the center of the cable, the induced potential of the points on the same cross section of the sheath will be different. In the smaller distance, there will be less magnetic flux than in the larger distance, so the induced current is in the two sheaths. The density near the side is large, which is called the proximity effect. The increased losses of the sheath due to the proximity effect are called eddy current losses, also known as proximity effect losses. For single-core cables that are placed very close to each other, the proximity loss of the sheath in a single-phase circuit accounts for about 20% of the total loss; in a three-phase circuit, the proximity loss of an equilaterally laid cable sheath accounts for about 25 percent of the total loss. %, the proximity effect loss of the equidistant parallel-laying side-phase cable sheath is about 15% of the total loss, and the central cable is about 60%.
The calculation of the loss of the cable sheath is complicated. For the sake of simple calculation, the loss of the cable sheath can be considered! s is proportional to the square of the current in the core, so the ratio between it and the core loss is a constant. It should be pointed out that after the cable sheathing of mine communication cables is connected, the induced current will affect the effective resistance and inductance of the core. Other electrical parameters change their values.
Armored loss
The mine rubber cable has (strengthening layer), will change the induction current of the sheath to varying degrees, thus changing the sheath loss. At the same time, when the equipment is connected to the road, damage will also occur in the equipment.
Power cable heating and current carrying capacity
For the split core structure equal to the single point of the sheath multiplied by F;
For the equidistant parallel parallel laying of a regular transposing cable, the average loss per unit length of each phase of the three small segments can be calculated using the parameters of the equilateral triangle and the calculation formula in the table. For the cross-connected cable, if the lengths of the three segments are not equal, they are Equivalent to the appropriate laying position, multiplying the grounding multiplied by the cable at each end of the large section of the non-metastatic cable, if the mining control cable does not know the length of each section of the cable, then the cable buried in the cement tank pipe is 0.03 for the buried cable.
Reinforcement calculation with resistance: For a reinforcing band (transverse reinforcing band) with a large wrapping pitch, the resistance can be calculated as a cylinder with the same inner diameter and weight as the reinforcing band.
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