The copper shield and steel armor of high voltage power cable generally need to be grounded. What is the difference between two ends grounding and one end grounding? Can the steel shield and copper shield be welded together when making the cable terminal? Can the steel shield and copper shield be welded together when making the cable middle end?
35kV high-voltage cables are mostly single core cables. When single core cables are powered on, induced voltage will be formed in the shielding layer. If the shielding at both ends is grounded at the same time, a loop will be formed between the shielding layer and the earth, which will generate induced current. In this way, the shielding layer of the cable will heat up and lose a lot of power In order to avoid this phenomenon, one end grounding method is usually used. When the line is long, neutral point grounding and cross interconnection can also be used.
When making the cable head, the steel armor and the copper shielding layer are welded and grounded separately to facilitate the detection of the cable inner sheath. When detecting the cable sheath, a voltage is applied between the steel armor and the copper shield. If it can withstand a certain voltage, it proves that the inner sheath is intact. If there is no requirement in this respect, it is not necessary to detect the inner sheath of the cable, and the steel armor and copper shielding layer can also be connected together for grounding (we advocate separate lead out and grounding).
Why do high voltage single core XLPE insulated power cables need special grounding methods?
Electric power safety regulations stipulate that both ends of cables of 35kV and below voltage level are grounded, because most of these cables are three core cables. In normal operation, the total current flowing through three core cables is very small
If the sum is zero, there is basically no flux linkage outside the aluminum cladding or metal shielding layer, so there is basically no induced voltage at both ends of the aluminum cladding or metal shielding layer, so there will be no induced current flowing through the aluminum cladding or metal shielding layer after both ends are grounded. However, when the voltage exceeds 35kV, most single core cables are used. The relationship between the core of single core cable and metal shield can be regarded as the primary winding of a transformer. When the core of a single core cable passes through the current, there will be a magnetic flux cross-linking aluminum package or metal shielding layer, so that the induced voltage will appear at both ends of it. The magnitude of the induced voltage is proportional to the length of the cable line and the current flowing through the conductor. When the cable is very long, the induced voltage on the sheath can be added up to endanger personal safety. When the line is short-circuit fault, switching overvoltage or lightning impulse, a high induced voltage will be formed on the shield, and even the sheath insulation may be broken down. At this time, if the two ends of the aluminum package or metal shield are still interconnected and grounded, there will be a large circulating current in the aluminum package or metal shield, whose value can reach 50% - 95% of the core current, resulting in loss and heating of the aluminum package or metal shield, which not only wastes a lot of electric energy, but also reduces the current carrying capacity of the cable, and accelerates the insulation aging of the cable. Therefore, single core cable should not be double insulated The terminal is grounded. [in individual cases (such as short cable or light load operation), the two ends of aluminum clad or metal shielding layer can be interconnected and grounded. However, when one end of the aluminum clad or metal shield is ungrounded, the following problems will arise: when lightning current or overvoltage wave flows along the core, the ungrounded end of the aluminum clad or metal shield of the cable will have high impulse voltage; When the short circuit occurs in the system, when the short circuit current flows through the core, high power frequency induced voltage will appear at the ungrounded end of the cable aluminum package or metal shielding layer. When the cable outer sheath insulation can not bear the effect of this overvoltage and is damaged, it will lead to multi-point grounding and form loop current. Therefore, when one end interconnection grounding is adopted, measures must be taken to limit the Overvoltage on the sheath. During installation, special connection and grounding methods should be adopted at a certain position of the aluminum clad or metal shielding layer according to different conditions of the line and the principle of economic rationality, and protective layer protectors should be installed at the same time to prevent the cable sheath insulation from being broken down. Therefore, the installation of high-voltage cable lines should be in accordance with the requirements of code for design of cables in Electric Power Engineering (GB 50217-1994). When the metal sheath of single core cable lines is grounded at only one point, the induced voltage at any point of the metal sheath should not exceed 50-100v (no more than 50V if no safety measures are taken to prevent arbitrary contact with the metal sheath; no more than 100V if effective measures are taken), It shall be insulated from ground. If the voltage is higher than the specified voltage, the metal sheath shall be insulated in sections or connected to form cross interconnection wiring after insulation. In order to reduce the induced voltage of single core cable line to adjacent auxiliary cable and communication cable, cross interconnection wiring should be adopted as far as possible. When the cable length is not long, single point grounding can be used. In order to protect the cable sheath insulation, a sheath protector should be installed at the ungrounded end.