1. Why is it necessary to install a return line that runs parallel to the cable in the grounding method at one end of the single-core cable sheath?
Answer: In the cable line where one end of the metal sheath is grounded, in order to ensure that the induced voltage in the sheath does not exceed the allowable standard, a conductor laid in parallel along the cable line must be installed, and both ends of the conductor are grounded. This kind of conductor is called a return. Streamline. When a single-phase ground fault occurs, the ground short-circuit current can flow back to the center of the system through the return line. Since the magnetic flux generated by the ground current through the return line cancels a part of the magnetic flux generated by the ground current of the cable conductor, it can reduce The induced voltage of the sheath in the event of a short-circuit fault.
2. How many projects can the cable project be divided into?
answer:
(1) On-site transportation: including the loading and unloading, transportation and empty return journey of engineering materials from the warehouse to the construction site.
(2) Local engineering: including road excavation, tunnel, trench construction, etc.
(3) Laying engineering: including laying, intermediate head production, cover plate, buried pipe, school tide, traction head production, etc.
(4) Two-end engineering: including the production and installation of the support, suspension bridge and its foundation, the production of the terminal head, the installation of the oil pressure and the signal device, and the electrical performance test of various types.
(5) Stopping works: including the production of oil-filled cable stop heads, the installation of fuel supply tanks, automatic drainage and signal devices, etc.
(6) Grounding engineering: including the installation of insulation joints, transposition boxes, protectors, and grounding boxes.
3. What inspections should be carried out for the acceptance of cable lines?
Answer: (1) The cable specifications should meet the requirements, the arrangement should be neat, no damage, and the signs should be complete, correct and clear;
(2) The fixed bending radius of the cable, the related distance and the wiring of the metal sheath of the single-core power cable should meet the requirements;
(3) The cable terminal and the middle head should not leak oil, and the installation should be firm. The oil pressure of the oil-filled cable and the meter setting should meet the requirements;
(4) Good grounding;
(5) The color of the cable terminal is correct, and the metal parts such as the bracket are completely painted;
(6) There should be no debris in the cable trench, tunnel, and bridge frame, and the cover plate should be complete.
4. What are the requirements for the positive sequence impedance measurement of the cable line?
Answer: The phasor sum of the AC resistance of the cable conductor and the three-phase inductance of the cable is called the positive sequence impedance.
The positive sequence impedance of the cable line can generally be measured directly on the cable reel. Generally, a lower voltage is used for measurement. Therefore, a step-down transformer is required to step down the voltage. The step-down transformer adopts star connection, and the capacity is generally more than 10kVA. Wide voltage adjustment range, the AC power supply should be relatively stable during measurement to ensure that the current meets the specified requirements during measurement. The actual reading value of the voltmeter must be the voltage at the end of the cable. The test current should be close to the long-term allowable current carrying capacity of the cable. For the value of each meter, read the values of the three meters at the same time after turning on the current.
5. According to the "Electrical Industry Production Safety Regulations", what are the requirements for electrical workers?
Answer: The following conditions must be met:
(1) Appraised by a doctor to be healthy and free from illnesses that hinder work;
(2) Possess the necessary electrical knowledge, master the relevant regulations, professional techniques and safe operation techniques according to their duties and nature of work, and have passed the examination;
(3) Proficiency in first aid methods for electric shock.
6. What should be paid attention to when the cable is transported and unloaded?
Answer: (1) In the process of transportation, loading and unloading, the cables and cable reels should not be damaged. It is strictly forbidden to push the cable reels directly from the car. Generally, the cables should not be transported and stored flat.
(2) Before transporting or rolling the cable reel, ensure that the cable reel is firm, the cable is wound tightly, the oil pipe between the oil-filled cable and the pressure oil tank should be fixed without damage, the pressure oil tank should be firm, and the pressure indication should meet the requirements.
7. What are the measures for cable fire prevention?
Answer: (1) Use flame-retardant cables;
(2) Use fireproof cable tray;
(3) Use fire-resistant paint;
(4) Fire partition walls and fire baffles are installed at cable tunnels, mezzanine exits, etc.;
(5) Overhead cables should avoid oil pipelines and explosion-proof doors, otherwise local pipes or heat insulation and fire prevention measures should be taken.
8. Which aspects should be considered when choosing the cross section of a power cable?
Answer: The following aspects should be considered:
(1) Long-term allowable working current of the cable;
(2) Thermal stability once short-circuited;
(3) The voltage drop on the line cannot exceed the allowable working range.
9. What are the advantages of power cables and overhead lines?
Answer: (1) It is reliable in operation. Because it is installed in a hidden place such as underground, it is less damaged by external force, has less chance of failure, and the power supply is safe, and it will not cause harm to people;
(2) Maintenance workload is small and frequent inspections are not required;
(3) No need to erect towers;
(4) Help improve power factor.
