Copper shield and steel armor of high voltage power cable generally need to be grounded. What is the difference between grounding at both ends and grounding at one end?
Can steel armoring and copper shield be welded together when making cable end ends?
Can steel armoring and copper shield be welded together when making cable middle end?
More than 35 kv high voltage cable for single core cable, single core cable in the power, can form induced voltage in the shield, if on both ends of the shielding and grounding, shield and loop is formed between the earth, will produce induced current, cable shielding layer will be fever, loss of power, influence the normal operation of the line, in order to avoid the occurrence of this phenomenon, with the method of grounded at one end, usually when the line is very long also can be used in a neutral earthing and cross linking, etc.
When making the cable end, the steel armor and copper shield are welded and grounded separately to facilitate the inspection of the cable inner shield. During the inspection of the cable inner shield, a voltage is applied between the steel armor and copper shield. If it can withstand a certain voltage, the inner shield is proved to be intact.
If there is no such requirement, the steel armor and copper shield can be grounded together without testing the inner sheath of the cable (we advocate grounding after separate leads).
Why do HV single core XLPE insulated power cables need special grounding method?
Power safety regulations: 35kV and below the voltage level of cables are grounded at both ends, this is because most of these cables are three-core cables, in normal operation, through the total current through the three cores
And is zero, there is basically no flux outside the aluminum clad or metal shield, so there is basically no induced voltage at both ends of the aluminum clad or metal shield, so there will be no induced current flowing through the aluminum clad or metal shield after both ends are grounded.
But when the voltage exceeds 35kV, most of the single-core cable, the relationship between the wire core of the single-core cable and the metal shield, can be regarded as a primary winding of a transformer.
When the single-core cable core passes through the current, there will be magnetic field lines cross linked aluminum envelope or metal shield layer, so that the two ends of it will appear induced voltage.
The length of the cable line with the size of the induced voltage is proportional to the current through the conductor and, when the cable is very long, the induction voltage on the sheath add up to the degree of endangering the personal safety, fault occurred in the line and operating over-voltage or lightning hit, shielding will form a high induction voltage, and may even breakdown sheath insulation.
At this point, if the will still aluminum package or three connected to each other on both ends of the metal shielding layer, the aluminum bag or metal shielding layer will appear a lot of circulation, the core value of line current of 50% - 95%, loss of form, package or aluminium metal shielding layer of hot, this not only waste a lot of energy, and reduced the carrying capacity of the cable, and accelerate the cable insulation aging, so the single core cable should not be grounded at both ends.
[In a few cases (such as short cable or light carrier), two ends of the aluminum cladding or metal shield may be connected to each other three times. However, when one end of the aluminum cladding or metal shield is not grounded, the following problems follow:
When lightning current or over voltage wave along the core flow, cable aluminum package or metal shield grounding terminal will be the impact of the high voltage; the system short circuit, short circuit current flows through the wire core, cable aluminum package or metal shield insulated end also can appear higher induced voltage, power frequency on the cable outer sheath insulation can't bear the role of the overvoltage and damaged, will lead to a multipoint earthing, form a circulation. Therefore, when using grounding connected at one end, measures must be taken to limit the overvoltage on the sheath, when installation should be according to the different situation of line,
According to the principle of economy and rationality in the aluminum bag or a certain position of metal shielding layer adopts special connection and grounding method, and installing sheath protector at the same time, in order to prevent the cable sheath insulation breakdown. Therefore, high voltage cable line installation, should according to GB50217-1994 codes for design of electric power cable, single core cable metal sheath is only a little ground, a little metal sheath of induced voltage should not exceed 50-100 - v (not take not any contact with the metal sheath security measures less than 50 v, such as taking effective measures,
Shall not be greater than 100 v), and deal with insulation. If the voltage is greater than the rules, the metal sheath after graded insulation or insulation should be taken to connect into crossover interconnection wiring. In order to reduce the single core cable line on nearby the induced voltage of auxiliary cable and communication cable, should try to adopt cross Internet connection. The cable length is not long, can use single point grounding way. To protect the cable sheath insulation, without grounding should be mounted at the end of the sheath protector.