High-voltage cable head production technology and related knowledge
1. Pencil problem When making cable heads (terminations and joints), why cut the main insulation layer into a pencil head shape at the end of the cable? What's the harm in not cutting it? When making the terminal head, it is not necessary to sharpen the pencil end. However, if the sealing tape needs to be wrapped between the cable insulation end and the wiring fittings, in order to ensure the sealing effect, the insulation end is usually cut into a cone to ensure that the wrapped sealing tape and the insulation can be well bonded. When making intermediate joints, if the installed joints are prefabricated structures (including prefabricated joints and cold shrink joints), the insulating end should not be cut into a cone, because this type of joint has a shielding tube in the middle part of the joint , the length of the shielding tube is only slightly longer than the copper or aluminum connecting tube, if the cable insulation is cut into a cone, the root of the cone will leave the shielding tube, and the gap in the connecting tube will not be shielded, thus affecting the performance of the joint , causing the joint to break down in the middle. If the installed joint is heat-shrinkable or wrapped, the insulating end must be cut into a cone, that is, a reaction cone, and the tapered surface must be polished with an abrasive belt, because the length of the tapered surface is much longer than that of the insulating end. Therefore, the tangential field strength along the tapered surface is much smaller than that of the insulating right-angled side, and the possibility of breakdown along the tapered surface is greatly reduced, thereby improving the performance of the joint.
2. Stress tube and stress evacuation glue The stress tube and stress evacuation glue in the cable accessories are mainly used to ease and disperse the electrical stress. Can you introduce the material composition of the stress tube and stress evacuation glue, whether the stress tube and stress evacuation glue contain Semiconductor composition? The material composition of the stress tube and the stress evacuation adhesive is composed of a variety of polymer materials blended or copolymerized. Generally, the base material is a polar polymer, and then fillers with high dielectric constants are added. Whether the stress tube and stress evacuation glue contains semiconductor components depends on the material formula of the manufacturer, which may or may not be present.
3. Cable grounding issues The copper shielding and steel armor of high-voltage power cables generally need to be grounded. What is the difference between grounding at both ends and grounding at one end? Can the steel armor and the copper shielding layer be welded together when making the cable terminal? Can the steel armor and the copper shielding layer be welded together when making the middle head of the cable? Most of the 35KV high-voltage cables are single-core cables. When the single-core cable is powered on, an induced voltage will be formed on 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, and an induced current will be generated. The layer will generate heat, consume a lot of power, and affect the normal operation of the line. In order to avoid this phenomenon, one end is usually grounded. When the line is very long, midpoint grounding and cross-connection can also be used. When making the cable head, the steel armor and the copper shield are welded and grounded separately to facilitate the inspection of the inner sheath of the cable. When testing the cable sheath, a voltage is applied between the steel armor and the copper shield. The voltage proves that the inner sheath is intact. If there is no such requirement, there is no need to detect the inner sheath of the cable, and the steel armor and the copper shielding layer can also be grounded together (we advocate grounding after being separated). Why do high-voltage single-core XLPE insulated power cables adopt a special grounding method? Electric safety regulations stipulate that cables with a voltage level of 35kV and below are grounded at both ends. This is because most of these cables are three-core cables. In normal operation, the sum of the currents flowing through the three cores is zero. There is basically no flux linkage outside the package or the metal shielding layer, so that there is basically no induced voltage at both ends of the aluminum package or the metal shielding layer, so there will be no induced current flowing through the aluminum package or the metal shielding layer after the two ends are grounded. But when the voltage exceeds 35kV, most single-core cables are used. The relationship between the core of the single-core cable and the metal shield can be regarded as the primary winding of a transformer. When the single-core cable core passes current, there will be magnetic force lines interlinked with aluminum cladding or metal shielding layer, causing induced voltage to 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 superimposed to a level that endangers personal safety. When lightning strikes, a high induced voltage will be formed on the shield, which may even break down the insulation of the sheath. At this time, if the two ends of the aluminum clad or the metal shielding layer are still connected to each other, a large circulating current will appear in the aluminum clad or the metal shielding layer, and its value can reach 5095 of the core current, forming a loss, making the aluminum clad Or the metal shielding layer generates heat, which not only wastes a lot of electric energy, but also reduces the current carrying capacity of the cable and accelerates the aging of the cable insulation, so the single-core cable should not be grounded at both ends. In individual cases (such as short cables or light-load operation), the two ends of the aluminum clad or the metal shielding layer can be connected to each other. However, when one end of the aluminum cladding or the metal shielding layer is not grounded, the following problems are brought about: When the lightning current or overvoltage wave flows along the core, the ungrounded end of the aluminum cladding or the metal shielding layer of the cable will have a high impact Voltage; when a short circuit occurs in the system, when the short circuit current flows through the core, a high power frequency induced voltage will also appear at the ungrounded end of the cable aluminum cladding or metal shielding layer, and the insulation of the cable outer sheath cannot withstand this overvoltage effect When it is damaged, it will lead to multi-point grounding and form a circulating current. Therefore, when using one-end interconnection and grounding, measures must be taken to limit the overvoltage on the sheath. During installation, special connection and grounding should be adopted at a certain position of the aluminum clad or metal shielding layer according to the different conditions of the line and in accordance with the principles of economy and rationality. way, and at the same time install the sheath protector to prevent the breakdown of the cable sheath insulation. According to this, when installing high-voltage cable lines, it should follow the requirements of GB50217-1994 "Cable Design Regulations for Electric Power Engineering". When the metal sheath of a single-core cable line is only grounded at one point, the induced voltage at any point of the metal sheath should not exceed 50- 100V shall not be greater than 50V when no safety measures are taken to prevent arbitrary contact with the metal sheath; if effective measures are taken, it shall not be greater than 100V, and ground insulation shall be used. If the voltage is greater than the specified voltage, the metal sheath should be insulated in sections or insulated and then connected into cross-connected wiring. In order to reduce the induced voltage of single-core cable lines to adjacent auxiliary cables and communication cables, cross interconnection wiring should be used as much as possible. For the case where the cable length is not long, a single-point grounding method can be used. In order to protect the insulation of the cable sheath, a sheath protector should be installed at the end that is not grounded.
