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Operation and maintenance of power cables

Update:2020-12-04
Abstract:

The operation of power cable lines is one of the import […]

The operation of power cable lines is one of the important links in power production. The safety of cable lines is related to the safe production of enterprises and the safety of people's lives and property. Today, I will briefly introduce you to related issues in the operation and maintenance of power cables.

A. Characteristics of cable lines
A cable line refers to a line that uses cables to transmit electrical energy. It is mainly composed of the cable body, cable intermediate joints, and electrical line ends. It also includes corresponding civil construction facilities, such as cable trenches, pipes, shafts, and tunnels. It is generally installed underground, but also overhead or underwater. Compared with overhead lines, cable lines have the following main advantages:
1. Not be disturbed by natural weather conditions (such as lightning, wind and rain, smoke, pollution, etc.)
2. Not affected by the growth of trees along the line;
3. Conducive to the beautification of the urban environment;
4. It does not occupy ground corridors, and the same underground passage can accommodate multiple circuits;
5. Conducive to preventing electric shock and safe use of electricity;
6. Low maintenance cost. But there are also the following shortcomings:
7. With the same cross-sectional area of ​​the wire, the transmission current is smaller than that of the overhead line:
8. Investment and construction costs increase exponentially, and increase with the increase in voltage;
9. The fault repair time is also longer.
At present, medium-voltage distribution lines should use wire lines under the following conditions:
1. According to city planning, prosperous areas, important areas, main roads, high-rise building areas and those with special requirements for the city appearance and environment;
2. The overhead line corridor is difficult to solve;
3. Users with high power supply reliability or important load;
4. Key scenic tourist areas;
5. Important power supply areas in major cities that are vulnerable to tropical storms in coastal areas;
6. The need for grid structure or operational safety;
7. Downtown areas with high load density.

B. Test and acceptance of power cables
In addition to the handover test and preventive test for power cables, insulation tests should also be carried out during the construction process to identify and inspect the cable quality and process quality in each link of the construction. Perform a test on the cable reel before laying to identify a good cable cup; perform a test after and before laying to identify whether the cable is damaged during the laying; perform a test after the construction of the cable head to identify the quality of the cable head; before and after the cable repair Test to identify the quality of maintenance. The main contents of the inspection are as follows:
1. The cables should be arranged neatly, the fixing and bending radius of the cables should conform to the design drawings and relevant regulations, the electrical system should be free of mechanical damage, and the signs should be fully installed, correct and clear. The terminals and intermediate joints of oil-impregnated paper insulated cables and oil-filled cables should have no oil leakage;
2. There should be no debris in the cable trench and tunnel, the cover of the cable trench should be complete, and the lighting, ventilation, drainage and other facilities in the tunnel should meet the design requirements;
3. The marking piles of directly buried cables should be consistent with the actual path, and the spacing should meet the requirements. The signs should be clear, firm and durable; (4) The signs and night lighting devices on both sides of the underwater cable line, in the anchor-forbidden area, should meet the design requirements.

C. Matters needing attention in cable line operation
1. Don't overload or overheat for a long time. Therefore, do not ignore the monitoring of cable load current, external temperature, and joint temperature;
2. Cable line feeder protection should not be used for reclosing. Cable line faults are mostly permanent faults. If the reclosing action is taken, it will inevitably expand the accident and threaten the stable operation of the power grid;
3. After the feeder of the cable line trips, do not neglect the inspection of the cable. Focus on checking whether the cable path is excavated and whether the wire is damaged. If necessary, further check and judge through tests;
4. Special attention should be paid to the inspection of the operation of directly buried cables: the ground near the cable path cannot be digging; the ground near the cable path is not allowed to scale heavy objects, corrosive substances, and temporary buildings; the cable path marker piles and protective facilities cannot be moved or removed casually;
5. The cable line must be retested before it can be put into use when it resumes operation after being stopped. For cables that have been out of power for more than one week but less than one month, the insulation resistance should be measured before being put into operation again, and the insulation resistance should not be reduced by 30% compared with the previous test record, otherwise, a withstand voltage test should be performed; power outages for more than one month but less than one year , The surface pressure test must be done, and the test voltage can be half of the preventive test voltage; if the power outage exceeds the test period, the preventive test must be done.

D. Operation and maintenance of cable lines
The operation and maintenance of cable lines is important to do a good job in three aspects: load monitoring, cable metal sheath corrosion monitoring and insulation supervision, to keep the cable equipment in good condition and prevent sudden cable accidents. The main projects include: establishing technical data of cable lines, conducting patrol inspections of cable lines, cable preventive tests, preventing external damage to cables, analyzing the causes of cable faults, detecting cable faults, and repairing cable faults. Cable lines need to add special content, such as trapping and killing termites, well water sample analysis, water branch section inspection, live measurement and monitoring of insulation, etc.
1. Load monitoring. Generally, the cable line specifies the maximum current value according to the cross-sectional area of ​​the cable conductor and the insulation type. Various meters are used to measure the load current of the wire line or the temperature of the cable sheath as the main load monitoring measure to prevent the cable insulation from exceeding the maximum allowable temperature. Shorten cable life.
2. Temperature monitoring. The temperature of the cable should be measured in summer or when the wire is at the maximum load. When measuring the temperature of directly buried wires, the temperature of the soil in the same section without other heat sources should be measured. When the cable crosses or is close to the laying of the underground heating pipe, the soil temperature around the cable should not exceed the soil temperature of the same depth in other parts of the local section by more than 10°C under any circumstances. Check the temperature of the cables. Choose the place where the cables are arranged densest or where the heat dissipation is the worst, or where there is an external heat source.
3. Corrosion monitoring. Use a special instrument to measure the surrounding soil adjacent to the cable line. If it belongs to the anode area, corresponding measures should be taken to prevent electrolytic corrosion of the cable metal sheath. In the moist soil around the cable line or the soil filled with domestic garbage, the cable metal sheath often undergoes chemical corrosion and microbial corrosion. According to the measured voltage value of the anode area, select the appropriate cathodic protection measures or drainage devices.
4. Insulation supervision. For each cable line, prepare a preventive test plan according to its importance, find the weak links in the cable line in time, and eliminate the defects that may occur in cable accidents. For cable lines that require insulation from the metal sheath to the ground, the outer sheath generally needs to be subjected to a DC voltage test separately after the preventive test, so as to find and eliminate the defects of the outer sheath in time.

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