First, the characteristics
cable line refers to the use of the cable line conveying power cable line, which mainly consists of a cable body, an intermediate cable connector, composed of a top end of the electrical line, further comprising a respective civil facilities, such as cable channel, Rows of pipes, shafts, tunnels, etc. Generally located underground, there are also overhead or underwater laying.
Compared with overhead lines, cable lines have the following main advantages: 1 not subject to 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 urban landscaping; 4 Does not occupy the ground corridor, the same underground pasغير مجاز مي باشدe can accommodate multiple lines; 5 is conducive to prevent electric shock and safe use of electricity; 6 maintenance غير مجاز مي باشدts are small. However, there are also the following disadvantages; 7 the same conductor cross-sectional area, the transmission current is smaller than the overhead line: 8 investment and construction غير مجاز مي باشدts exponentially increase, and increase with the increase of voltage; 9 fault repair time is also longer.
At present, the medium-voltage power distribution lines should adopt wire lines under the following conditions: 1 According to the city's planning, bustling areas, important areas, major roads, high-rise buildings and areas with special requirements for city appearance and environment; 2 overhead line corridors are difficult to solve; 3 High-reliability power supply or important load users; 4 Key scenic tourist areas; 5 Important power supply areas in major coastal cities that are vulnerable to tropical storms; 6 Power grid structure or operational safety needs; 7 Downtown areas with high load densities.
Second, the test and acceptance of the power cable
(1) Testing of Power Cables In addition to conducting the handover tests and preventive tests, the power cables should also be subjected to insulation tests during the construction process to identify the quality of the cables and the quality of the process during the inspection. Before laying, test on the cable tray to identify cable cups. After laying and before laying, conduct tests to identify whether there is any damage to the cable during laying. Conduct test after cable head construction to identify the quality of the cable head; before and after cable inspection. Tests to identify maintenance quality. The main contents of the inspection are as follows:
(1) The cables shall be arranged neatly. The fixing and bending radius of the cables shall be in accordance with the design drawings and relevant regulations. The electrical system shall be free of mechanical damage and the signs shall be fully installed, correct and clear. The terminals and intermediate joints of oil-impregnated paper-insulated cables and oil-filled cables shall be free of oil leakage; (2) There shall be no debris in the cable trenches and tunnels; the cover of the cable trench shall be complete; the lighting, ventilation, and drainage in the tunnel shall be complete. The facilities shall meet the design requirements; (3) The marked piles of buried cables shall conform to the actual path and the spacing shall meet the requirements. The signs shall be clear, firm and durable; (4) The signs and night lighting devices on the cross-strait, forbidden anchor areas of underwater cable lines shall meet the design requirements.
Third, the cable line operation considerations
(1) Do not run for too long or overheat. Therefore, do not ignore the monitoring of cable load current and external temperature and connector temperature; (2) Cable line feeder protection should not be put into reclosing. The faults of the cable line are mostly permanent faults. If the reclosing action is performed, it will inevitably increase the accident and threaten the stable operation of the power grid. (3) After the feeder line of the cable line is tripped, do not neglect the inspection of the cable. Focus on checking whether the cable path is excavated and whether the wire is damaged or not. If necessary, further tests shall be performed to judge; (4) Pay attention to the operation of the buried cable: The ground near the cable path cannot be easily dug; the ground near the cable path must not be scaled. Heavy objects, corrosive materials, temporary buildings; cable path signs piles and protection facilities cannot be easily moved or dismantled; (5) When the cables are retired after being decommissioned, they must be retested before they can be put into use. For cables that have been out of power for more than one week but less than one month, the insulation resistance shall be measured before being put back into operation. The insulation resistance shall not be reduced by 30% compared with the previous test record. Otherwise, the withstand voltage test shall be performed; the power failure exceeds one month but is less than one year old. , you must do surface pressure test, the test voltage can be half of the preventive test voltage; power outage time exceeds the test period, must be done preventive tests.
Fourth, the operation and maintenance of cable lines
The operation and maintenance of cable lines is important to do well in three aspects: load monitoring, corrosion monitoring of cable metal sheaths and insulation supervision, keeping cable equipment in good condition and preventing cable accidents. The main projects include the establishment of technical data for cable lines, inspection of cable lines, preventive testing of cables, prevention of damage to external cables, analysis of causes of cable faults, detection of cable faults, and troubleshooting of electrical wiring. The cable line needs to add special content, such as trapping termites, human well water sample analysis, water branch slicing and live measurement, and monitoring insulation.
1. Load monitoring. Generally, the maximum current value is determined according to the cross-sectional area and insulation type of the cable conductor, and various meters are used to measure the load current of the wire line or the jacket temperature of the cable. As the main load monitoring measure, the cable insulation is prevented from exceeding the maximum allowable temperature. Reduce cable life.
2. Temperature monitoring. The temperature of the cable should be measured during the summer or when the cable is under maximum load. When measuring the temperature of buried wires, measure the soil temperature without other heat sources in the same area. When the cable intersects with or is placed near the underground heat pipe, the soil temperature around the cable should not exceed 10°C above the soil temperature at the same depth in other parts of the local segment. When checking the temperature of the cable, select the place where the cable is arranged in the densest place or where the heat is dissipated most or where the heat source is outside.
3. Corrosion monitoring. Special measuring instruments are used to measure the soil surrounding the adjacent cable lines. If they belong to the anode area, corresponding measures shall be taken to prevent the electrolytic corrosion of the cable sleeves. The wetted soil around the cable line or the soil filled with domestic waste, the metal sheath of the cable often undergoes chemical corrosion and microbiological corrosion. According to the measured voltage value of the anode region, suitable cathodic protection measures or drainage devices are selected.
4. Insulation supervision. For each cable line, according to its importance, a preventive test plan was prepared, the weak links in the cable line were discovered in time, and the defects that may cause a cable accident were eliminated. Metal cable sets have insulation requirements for cable lines. Generally, after the preventive test, the outer jacket needs to be separately subjected to a DC voltage test to detect and eliminate defects in the outer jacket.
Fifth, the side fault of cable fault
After the cable fails, the general side-seeking steps are as follows:
(1) Determine the nature of the fault. According to the phenomenon occurring at the time of the fault and some simple tests, the nature of the fault is initially determined, and it is determined whether the fault resistance is a high resistance or a low resistance. It is a flashover or a closed fault, and is a short circuit to ground, a broken wire, or a mixture thereof. Phase, two-phase or three-phase fault. For example, if the cable in operation is faulty, only the grounding signal may be a single-phase ground fault; if the relay is over-current tripped, two-phase or three-phase short circuit may occur, or a short circuit and grounding may occur. Mixed failures. Through preliminary judgment, the nature of the fault cannot be completely determined yet, the insulation resistance must be measured and the continuity test must be conducted; (2) Burn-through of the fault point. That is, through burn-through, the high-impedance fault or flashover fault can be changed into a low-impedance fault for rough testing; (3) rough testing. Use the instrument to measure the fault distance on one side of the cable, and use the technical data of the cable circuit to calculate the location of the fault point; (4) Find the path. For drawing data is not complete or cable path is unknown, through the audio-induction detection method and pulse magnetic field method, find the fault cable laying path and buried depth, in order to conduct fixed-point fine measurement. Audio inductive detection method is to pass the audio signal current into the wire, determine the path according to the strength of the audio signal received by the receiver in the receiving coil; (5) the fine measurement point of the fault point. The exact position of the fault point was determined by methods such as impact discharge acoustic measurement, audio induction, and acoustic-magnetic synchronous detection. The sound measurement method is only suitable for low-resistance grounded cable faults and has a poor effect on metallic ground faults. The induction method is suitable for metallic ground faults and phase-to-phase short-circuit faults.
The above five steps are general search steps. When the actual side seeks, some of the steps may be omitted according to specific conditions. For example, if the cable laying path is accurate, it is not necessary to search the path. For high-impedance failures, the flashover method can be used directly without burn-through. For some flashover faults, no fixed point is required, and the distance data can be obtained according to the lateral search. Access to information can be directly checked and judged on the intermediate joints. For cable faults in cable trenches or tunnels, shock discharge can be performed and direct monitoring can be used to determine the fault point.
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