Connection and grounding of single core cable metal sheathed
HV and EHV cables due to its special structure, is not easy to manufacture low-voltage power cable structure, often adopts the structure of single core cable, and cables need to be buried, all need to have a metal armour layer, metal armour layer material usually adopts high permeability steel tape.
 
1. single core cable metal sheath over voltage and circulation
 
When the single core power cable conductor pass through the AC current, the magnetic field generated around is crosslink with the metal sheath, and the electric potential is generated on the metal jacket. The magnitude of the induced electromotive force is related to the size of the current in the conductor, the arrangement of the cable and the length of the cable. The three-phase cable arrangement of the equilateral triangle, if the two ends of the metal sheath grounding, it will form a circulation of the metal sheath, and the size of the corresponding circulation cable length, the current size of the conductor. For economic security considerations, in some of the cable is not long, the conductor of the current is not large, the circulation is very small, the impact on the cable load flow is small, can be directly grounded at both ends of the metal sheath.
If only the metal sheath of the cable is directly grounded at one end, the other end of the cable's metal sheath is not more than 50 V (or less than 100 V), otherwise the appropriate unit shall be set up to set up the insulation joint. In the fault, big electric current in the conductor, may produce a very high voltage in the metal sheath, endangering sheath insulation in cable line single-phase grounding, at cable ungrounded end, shall install overvoltage protector grounding.
 
2. In order to solve the problem of the existence of the circulation at the same time, and at the other end of the cable, the cable metal sheath should take the different ground form for the cable length and the current in the conductor.
 
Cable metal sheath should be directly grounded at one end, the other end of the cable is connected to the ground, the longer the cable is, the higher the induction voltage is. The non direct ground terminal sensing voltage should be limited to 50v, and the matching coefficient is not less than 1.4. Therefore, one end of the direct grounding cable can not be too long.
 
Intermediate of cable metal sheath directly ground, grounding ends through voltage protection device, one end extension directly grounded, the maximum length of end grounding cable connection and double end principle and direct grounding as.
Cable line is very long, even if the use of metal sheath in the middle ground, there will be a high induction voltage. At this time, can be used to cross the metal sheath. If the three-phase current is symmetric, then the cable end of the metal sheath is zero, can be directly to the ground, and not in the metal sheath in the circulation. Where the highest induction voltage is located in the insulation joint, it should be installed in the same place, the same as in the case of short circuit, metal sheath insulation of the impact of voltage and over-voltage protection under the impact of the residual voltage with the coefficient is not less than 1.4. If such a cross is connected to the ground, as a unit, because the metal sheath is directly grounded at both ends, any length of the cable can be divided into several units, which is theoretically applicable to various lengths of cable.
 
The above two methods all need to be installed in voltage protector, so it will increase the operation and maintenance work. If the cable line is short, the transmission capacity of a large margin, the induction voltage on the metal jacket is very small, can be used directly on the ends of the metal sheath. The circulation in the metal sheath is small, the loss is not significant, the impact of the cable load flow is little, and the operation and maintenance is less.
 

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