At present, there are 247 kilometers of cable lines in the 10 kV distribution network in Rizhao city, and the cabling rate has reached 53.3 percent.
In 2003, a total of 9 non-external cable accidents occurred in rizhao urban cable lines, among which 7 accidents occurred in cable intermediate joints.
In 2004, a total of 8 non-external cable accidents occurred in rizhao urban cable lines, among which 5 accidents occurred in cable intermediate joints.
The cable intermediate joints in these 12 accidents are far from reaching the service life of 20 years. According to the analysis, the main reason is that the production technology of the cable intermediate joints is not up to standard.
The operation technology of the staff is very important in the production of the cable intermediate joint. However, according to the investigation, some staff lack of understanding of the cable structure and the function of each component, which should arouse our attention.
In this paper, the structure of 10 kV crosslinked cable and the fabrication process of intermediate joint are briefly described.
I. Cable structure
Any power cable from its structure analysis, can be roughly divided into three parts, namely the conductor, insulation shielding layer and protective layer.
Conductor is the cable core, the use of a number of round aluminum or copper wire tightly twisted.
Its surface is smooth, avoiding the concentration of electric field, preventing the extruded semi-conductive shielding layer of semi-conductive material from entering the conductor, greatly preventing the possibility of water entering the conductor along the longitudinal.
Insulation shielding layer includes: inner and outer shielding layer, copper shielding layer and main insulation.
In the manufacturing process, it is impossible to make the surface of the conductor and insulator smooth enough to even the electric field intensity on the surface of the conductor and insulator. Therefore, there is a semi-conductive shielding layer on the surface of the conductor and insulator to achieve this purpose, which is the reason for the existence of the inner and outer shielding layer.
The existence of semi-conductive shielding layer reduces the possibility of partial discharge, and also can effectively restrain the growth of hydropower branches.
The thermal resistance of the semi-conductor shielding layer can prevent the high temperature on the wire core from directly impacting the insulating layer.
In addition, the outer shielding layer and the metal sheath are equipotential to avoid partial discharge between the insulation layer and sheath.
The main insulation material is crosslinked polyethylene, and the cable insulation mainly depends on this layer.
The existence of copper shielding layer is because there is no metal sheath extruded insulated cable, in addition to the semi-conductor shielding layer, it is necessary to add a metal shielding layer wrapped with copper tape or copper wire.
The copper shielding belt is grounded at both ends during installation, so that the outer semi-conductor shielding layer of the cable is always at zero potential, thus ensuring the radial uniform distribution of the electric field.
During normal operation, the grounding capacitance current of the copper shield conducting cable is used as a channel for short circuit or ground current in the system, and also plays a role of shielding electric field to prevent the axial surface discharge of the cable.
The protective layer includes: lining layer, steel armor and outer sheath.
The lining layer and outer sheath are generally made of polyvinyl chloride (PVC), which, combined with steel armor, can prevent the insulation layer from being damaged by external forces and the invasion of water.
Two, cable intermediate joint production
There are many types of cable intermediate connector. The following is a simple description of the thermal shrinkable cable intermediate connector.
The intermediate joints of power cables and cables together constitute the power transmission network.
The process of making intermediate joint is actually the process of restoring each structure layer of cable.
Cable intermediate joint is mainly based on the characteristics of cable structure, which can not only restore the performance of cable, but also ensure the extension of cable length.
The purpose of making cable couplings is to restore the basic performance of the cable by restoring the structural layers of the cable.
When making the middle end of the cable, connect the two end core of the cable with conductor connection metal to make the cable core conduct;
The semi-conducting tape is wound on the surface of the connecting tube and overlapped with the inner semi-conducting shielding layer at both ends to ensure the conductivity of the inner semi-conducting shielding layer. The semi-conducting tape layer is filled with filling glue with a thickness of no less than 3mm to restore the insulation characteristics of the main insulation.
The composite pipe is fixed between two stress pipes by heating from the middle to both ends, and then wrapped around the waterproof adhesive at the step of both ends of the composite pipe. The semi-conductive tape is then wrapped around the waterproof adhesive, and the two ends are overlapped with the copper shielding layer and the semi-conducting layer of the composite pipe respectively to restore the outer semi-conducting shielding layer.
Wire with connector on both ends of the copper shield binding welded copper shield connected on both ends, and ground wire tied to rotate with a conductor, tied on the armoring weld on both ends, and on the shielding layer on both sides of the weld to restore cable grounding (if required the cable shielding grounding copper strip is separated from the strip, the ground on the shielding layer on both sides of the reoccupy after weld up inside the casing will be resumed within the cable sheath, then jumper with steel belt connect sheathed on both ends, and then install heat shrinkable outer protection of casing or metal casing).
After the metal sheath and sealing sleeve are installed, the cable's protective layer is restored.
Matters needing attention in making cable intermediate joint
Cable insulation is a typical example of composite insulation, which is composed of a variety of different media. It is easy to leave fine air gap at the junction of different media and layer to layer, and at the junction of different media and band to band, etc., making it the source of partial discharge.
The cable insulation structure is complex, so it is necessary to pay attention not only to the coordination of the tangential field strength component perpendicular to the dielectric layer, but also to the coordination of the tangential field strength component parallel to the dielectric layer.
The surface discharge at the edge of the electrode is more likely to occur than the dielectric breakdown.
Therefore, the following problems should be paid attention to in the process of making cable intermediate head.