1. Why is it necessary to install a return line parallel to the cable in the single-core cable jacket earthing mode?
In the cable line that is earthed at one end of metallic sheathing, to make sure the inductive voltage in protecting a jacket does not exceed permitted standard, must install the conductor that lays along cable line parallel, and the two ends of conductor ground, this kind of conductor calls backflow line. When single-phase ground fault occurs, the grounding short circuit current can flow back to the center point of the system through the backflow line. Because the magnetic flux generated by the grounding current through the backflow line cancels the magnetic flux generated by a part of the grounding current of the cable conductor, the induced voltage of the sheath during the short circuit fault can be reduced.
2. How many projects can the cable project be divided into?
- Site transportation: including loading and unloading of engineering materials from warehouse to construction site, transportation and return trip of empty vehicle.
- Local engineering: including road excavation, tunnel, trench construction, etc.
- Laying engineering: including laying, making the middle head, lifting the cover plate, burying pipe, adjusting the tide, making the drawing head, etc.
- Both ends of the project: including the production and installation of the support, suspension bridge and its foundation, the production of the end head, the installation of oil pressure and signal device, electrical performance testing, etc.
- Stop-off works: including the production of oil-filled cable stop-off, the installation of oil tank, automatic drainage and signal devices, etc.
- Grounding engineering: insulation joint, transposition box, protector, grounding box installation, etc.
3.What inspection should be conducted for the acceptance of cable lines?
- Cables shall be in accordance with the specifications, arranged neatly, without damage, with complete, correct and clear labels;
- The fixed bending radius and relevant distance of the cable and the wiring of the metal sheath of the single core cable shall meet the requirements;
- The cable terminal and intermediate head shall be installed firmly without oil leakage. The oil pressure and meter setting value of the oil-filled cable shall meet the requirements;
- Good grounding;
- The cable terminal has correct color, and the metal parts such as the bracket have complete paint;
- There shall be no sundries in the cable trench and tunnel and on the bridge frame, and the cover plate shall be complete.
4. What are the requirements during the measurement of positive sequence impedance of cable line?
The phasor sum of the ac resistance of the cable conductor and the three same inductive reactance of the cable is called positive sequence impedance.
Positive sequence impedance of cable line generally can directly measure on cable, generally use the lower voltage measurement, therefore, need to be done with a step-down transformer step-down, step-down transformer USES the star connection, generally for more than 10 kva capacity, has a wide range of voltage regulation, when measuring ac power should be stable, so as to ensure the measurement to achieve the requirements of the current, the actual meter reading values must be the cable voltage, test current best close to cable allows carrying capacity for a long time, read the meter measurement value, after close the current read at the same time, the value of three tables.