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Causes and Countermeasures of the cable inlet

Information Source:Henan Huadong Cable Co., Ltd  Release date:2014-02-26

Harm

The cable inlet, under the action of electric field, water treeing phenomena occur, resulting in the cable breakdown. Water treeing is void diameter in 0.1M to several microns with water collection. Insulation in the presence of impurities, porosity and insulation and inner and outer semi conductive layer combined with local high electric parts surface uneven formation of premises is the starting point for occurrence of water tree. Water treeing development process more than 8 years in general, humidity, temperature, the higher the voltage, the ion containing more water, the faster development of water tree.

Reason

1 barrel of the cable into the custody when new, its two ends are sealed with plastic sealing sleeve, but to a section, and the rest is wrapped with plastic paper, outside with the rope, the sealing property is not good, after a long time, moisture will penetrate the cable.

2 when laying the cable when laying the cable, the cable head live wrapped with plastic paper sometimes soaked in water, the water entering into the cable; the traction and wear tube rupture, sometimes outer sheath phenomenon.

3 after laying laying, not timely cable head manufacturing, which has not been cable port sealing treatment of long-term exposure in the air, and even dipped in water, the water vapor into cable.

4 Production in the cable head cable head manufacturing time (including the terminal head and joints), due to the production staff, cable end cable wells sometimes slip into the water.

5 cable cable intermediate joint operation, occurrence breakdown fault cable wells, water will be along the gap into the cable; in the construction site, caused by external force cable breakage or breakdown, also have cable inlet.

Countermeasures

Cable water after the drying processing is very difficult (such as hot nitrogen pressure blow dry), in general there is no corresponding equipment. In actual operation, if the cable feed R6, we just cut in front of a few meters, such as the entire cable has water, we would not be desirable. Therefore, to prevent the cable inlet, should give priority to prevention, adopt the following measures:

1 cable head cable end sealing sawed off, both stacking or laying, shall be sealed by plastic (with special cable sealing sleeve), prevent moisture penetration.

2 Making timely after laying wire cable head.

3 of the purchase of cable, must choose the excellent quality of the manufacturers. Because the insulating impurities in pores, is the starting point for water treeing occurs, thus the quality of cable to prevent water treeing is essential.

4 strengthen the cable head manufacturing process management once the cable inlet, appeared breakdown phenomenon is often the cable head, so the wire head made good, whole life can be extended cable. Such as cable stripping in the semiconductor layer, we end up struck several road on the semiconductor layer, and then like sugarcane peeling off the semiconductor. But in the cut, if is too deep, will hurt the insulating layer, provide opportunities for water treeing generation. In addition, the solder, because there are no power, will directly use a blowtorch to melt the solder, at this time, the flame will damage the copper shielding layer and the insulating layer, thus to eliminate this phenomenon, the correct way to configure the UPS, because the soldering time normally required only 10min, power but 500W.

5 the cold shrink silicone rubber cable accessories shrinkage cable head of 3M company, the manufacture is simple and convenient, without a blowtorch, do not solder. And silicon rubber cable accessories and elastic, tightly on the cable, to overcome the disadvantages of heat shrinkable material (heat shrinkable materials not elastic, in the process of expansion and contraction of the cable, and the cable body will appear gap, between which for water treeing has facilitated the development of). At present, cold shrinkable cable accessories main cable intermediate joint all my bureau is used for 3M company.

6 long cable with several long cable branch box my bureau, each of length 3km, for such cable intermediate joint, in addition to the outside, we also use one to two cable branch box, once one of the cable inlet, will not spread to other segments, but also facilitate the segmentation look at the cable fault.

7 10kV system used in cable 8.7 / 10kV grade of the grade of cable insulation thickness of 4.5mm, and the thickness of insulation 6 / 10kV level cable for 3.4mm. Due to the increase in thickness of cable insulation, reduce the strength, can also aging, prevent water tree, due to the lokV low current neutral grounding system in single phase grounding, phase voltage cable should bear 1.73 times, and required to run the 2H, therefore, it is necessary to thicken the cable insulation.

8 using PVC plastic double wall corrugated pipe, the pipe corrosion, smooth inner wall, good strength and toughness, so the cable directly buried installation, can greatly reduce the cable outer sheath damage.

9 cable channel (pipe) and the design of cable well because of the limited conditions, we adopt the direct buried cable or cable trench, and buried for many, I belong to the coastal rainy area, cable or cable wells for water. Because the cable or cable and the depth of the well will exceed the sewer depth, drainage is difficult, so in the planning, it should be coordinated, facilitate cable ditch (well) drainage. If unable to do cable well not water, should be the joint cable wells up with bracket. In addition, our region is the heavy chemical industry zone, large area chemical companies, found in the inspection, cable trench chemical plant in the vicinity of the wire, some outer sheath has serious deformation, therefore, cable trench chemical plant near the must have drainage facilities. In addition, in the cable duct design, as far as possible to reduce the elbow straight, the cable laying, convenient; at the same time, the cable well production, we divided into large cable wells and small cable wells, cable wells can be used for traction cable, coil, do the middle joint, while in the middle of the road to do well the inconvenience of wire however, there must be a corner, we changed a cable wells, the cable well only in the laying of cables used for the placement of a steering pulley.

10 Test cable head cable production is completed. After doing a high-voltage DC leakage test, before the commissioning, we only for substation cable pre trial, other cable not test. Because, once the substation outlet cable fault, short circuit current will cause great impact on the substation equipment, thus found the wire has a problem, it is necessary to strengthen the operation and management of change in a timely manner. We think, the cable fault after treatment, and the cable test found fault cable, both are processed like trouble: to find the point of failure, or replace the cable. The disadvantages are: the impact of unplanned outage, the advantages of short circuit current is: do not test can prolong the service life of cables (some cable test do not ideal, but still can run for a long time, and DC test will increase after the breakdown of the cable may be), the fault point is obvious, easy to find. The advantages and disadvantages of the latter coincided with the opposite. Therefore, the cable users don't do experiments, we focus on the reliability of power supply, such as the 10kV switch on the power supply station, adopt dual power supply, power dispatching automation, once a line cable fault, immediately switch to another cable power supply. In fact, electric power equipment preventive new "test procedures", the cross-linked cable no longer specified time intervals for DC voltage test, only measuring insulation resistance, preventive test which can simplify the cable.

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