1. Yunnan cable manufacturers of high voltage cable why grounding?

1. Grounding of high-voltage three-core cables: grounding at both ends of cables with a voltage level of 35kV and below. This is because most of these cables are three-core cables. In normal operation, the total current flowing through the three cores is zero, there is substantially no magnetic flux linkage outside the aluminum clad layer or the metal shielding layer, and there is substantially no induced voltage at both ends of the aluminum clad layer and the metal shielding layer. Therefore, after both ends are grounded, the induced current will not flow through the aluminum clad layer or the metal shielding layer.

2. Grounding of high-voltage single-core cable: When the cable is running with load, induced voltage will be formed on the shielding layer. If the shields at both ends are grounded at the same time, a loop will be formed between the shielding layer and the ground, and an induced current will be generated. In this way, the cable shielding layer will generate heat, lose a lot of power, and affect the normal operation of the line. When the cable line is short-circuited, subjected to lightning impulse or operating overvoltage, a very high induced voltage will be formed on the shield. This will endanger personal safety and may even pierce the outer sheath of the cable. In order to avoid this phenomenon, one end grounding is usually used. When the line is long, midpoint grounding and cross interconnection can also be used.

When making the cable head of high-voltage single-core cable, the steel armor and copper shield must be welded and grounded separately. Its purpose is to facilitate the detection of the quality of the inner sheath of the cable. When inspecting the cable sheath, a voltage is applied between the steel armouring and the copper shield. If it can withstand a certain voltage, it proves that the inner sheath is intact. If it is not necessary to test the inner sheath of the cable, the steel armor and copper shield can be connected together to ground (it is recommended to ground after separate extraction).

2. Yunnan cable manufacturers single core cable grounding several methods:

1. Direct grounding at both ends: This grounding method can reduce the workload, but there is a circulating current on the metal sheath, and the applicable conditions are harsh. It requires short cable lines, low transmission power, and a large margin of transmission power. In this case, there is no need to install a cable sheath protector, which can reduce the operation and maintenance workload. However, there is a circulating current on the metal sheath, so this method is not recommended.

2. One end of the metal shielding layer is directly grounded, and the other end is grounded through the sheath protector: when the cable line is short (within 500m), the metal sheath is generally directly grounded at one end of the sheath, and the other end is grounded through the sheath protection. The grounding insulation does not form a loop, which can reduce and eliminate the circulation, which is beneficial to improve the transmission capacity of the cable and the safe operation of the cable. According to the requirements of the code, the induced voltage of the metal sheath part that is not directly grounded shall not exceed 50V. If connected to an overhead line, the direct grounding is generally installed at one end of the overhead line, and the protector is installed at the other end.

Cross interconnection grounding: When the cable line is long (generally more than 1km), the metal sheath of the cable can be cross-connected and installed. Cross interconnection is to divide the cable line into three equal length sections (deviation is not more than 5%), and install insulated joints between each section. The metal sheath is led out from the coaxial cable insulation joint, and after being cross-interconnected through the interconnection box, it is grounded through the cable sheath protector, and the metal sheaths at both ends of the cable are directly grounded to form an interconnection section. If the cabling is long, a multi-segment interconnect can be formed by connecting multiple interconnect segments.

After grounding through cross-interconnection, the potential difference between the two grounding points is zero, so it is impossible to generate circulating current on the sheath. At this time, the line has a very high sheath voltage, that is, the induced voltage determined according to the length of each small section can be limited to 50V. The cross interconnection method can reduce the induced voltage and circulation of the metal sheath, which is beneficial to improve the transmission capacity of the cable.