The transmission capacity and transmission distance of transmission lines are all related to voltage. The higher the line voltage, the longer the transmission distance. The voltage of the line and system needs to be determined according to the distance and capacity of the transmission.
1. Overhead Transmission Lines Overhead transmission lines are composed of poles, conductors, and insulators and are erected on the ground.
Overhead transmission line conductors are made of a well-conducting metal with a sufficiently thick section (to maintain proper flow density) and a large radius of curvature (to reduce corona discharge). EHV transmission uses split wires. Overhead ground wires (also called lightning conductors) are placed above the transmission conductors to protect the wiring from lightning strikes. Important transmission lines usually use two overhead ground wires. Insulator strings are made of single suspension (or rod) insulators in series, and need to meet the requirements of dielectric strength and mechanical strength. The number of insulators per string is determined by the voltage level of the transmission. The towers are mostly made of steel or reinforced concrete and are the main supporting structure for overhead transmission lines. Overhead line erection and maintenance are more convenient and the cost is lower. Overhead transmission lines are designed to take into account the effects of various natural conditions such as temperature changes, severe storms, lightning flashes, rain, ice, floods, and wet fog. There must be sufficient ground width and clearance corridors for the overhead transmission lines.
The maximum transmission power determined after comprehensive consideration of technical, economic and other factors of the transmission line is called the transmission capacity of the line. The transmission capacity is roughly proportional to the square of the transmission voltage. Therefore, increasing the transmission voltage is the main technical means for realizing large-capacity or long-distance transmission, and it is also the main symbol of the development level of transmission technology. At present, domestic and foreign (including developed countries in Europe and America) generally use overhead lines as the most important way to transport electrical energy.
2. Power Cable Lines Power cables generally consist of single conductors, double conductors and triple conductors consisting of conductors, insulation layers and protective layers.
Underground cable lines are often used in areas where overhead lines are difficult to set up, such as transmissions in cities or in special crossings. At present, the use of cable power transmission, mainly from the perspective of urban landscape and line security. However, fault finding time and maintenance time of the cable line are very long, which has a serious impact on the reliability of the grid operation and the normal power consumption of the user. Therefore, in the construction of power grids, the replacement of overhead lines with cable lines cannot be achieved.
Cable line features:
(1) Power supply is reliable.
(2) does not occupy the ground and space.
(3) Do not use poles to save wood, steel, and cement.
(4) Simple operation and maintenance, saving line maintenance costs.
(5) The cable is expensive, the branch is difficult, the construction process of the cable joint is more complicated, the fault point is more difficult to find, and it is inconvenient to handle the accident in time.
3. Power frequency electric and magnetic fields of overhead lines and power cable lines 3.1 Power frequency electric fields generate electric fields, and the magnitude of the power frequency electric field strength of overhead lines is related to the voltage of the line and the distance to residents. According to HJ_T_24-1998 "Technical Code for Environmental Impact Assessment of Electromagnetic Radiation of 500 kV EHV Transmission and Distribution Projects", the standard of 4kV/m is implemented for the intensity of electric field in residential areas. Only when the grid construction project passes the environmental impact assessment, can it be constructed in full compliance with the standards.
The cable lines are buried deep within the 7-10 meter underground tunnel. In addition to the reinforced concrete of the tunnel and the earth's surface, the power frequency electric field on the ground surface is already very small and tends to the environmental background value. Therefore, the power frequency electric field strength of this cable line is basically negligible.
3.2 Power frequency Magnetic field currents produce magnetic fields. The frequency of magnetic induction of overhead lines is related to the size of the current of the line and the distance to the residents. According to HJ_T_24-1998 "Technical Specifications for Environmental Impact Assessment of Electromagnetic Radiation from 500 kV EHV Transmission and Distribution Projects", the frequency-frequency magnetic induction of residential areas is implemented at a standard of 0.1 mT.
Due to the shielding of the reinforced steel structure of the tunnel and the large distance from the ground surface, the power line frequency magnetic induction of the cable surface generally does not exceed 0.003mT. With the increase of the horizontal distance from the cable tunnel, the frequency-frequency magnetic induction intensity rapidly decays and tends to the environmental background value.
A Hose Clamp or Hose Clip is a device used to attach and seal a hose onto a fitting such as a barb or nipple. Many types are available, including American hose clamp, German hose clip, British hose clamp , Screw/band (Worm Gear) clamps, t-bolt clamp, Spring clamps, Wire clamps, Ear Clamp and so on.
Hose clamps are typically limited to moderate pressures, such as those found in automotive and home applications. At high pressures, especially with large hose sizes, the clamp would have to be unwieldy to be able to withstand the forces expanding it without allowing the hose to slide off the barb or a leak to form. For these high pressure applications, compression fittings, thick crimp fittings, or other designs are normally used.
Hose clamp Uses and applications
Hose clamps are frequently used for things other than their intended use, and are often used as a more permanent version of duct tape wherever a tightening band around something would be useful. The screw band type in particular is very strong, and is used for non-plumbing purposes far more than the other types. These clamps can be found doing everything from mounting signs to holding together emergency (or otherwise) home repairs.
Hose clamp Sealing and mechanical strength
One of the fundamental goals of most hose clamps is to ensure a tight seal between the hose and the barb, preventing the working fluid from escaping. To this goal, they are designed to provide even pressure on all sides, with no gaps. An example of this would be wire clamps. An obvious design would seem to be simply having a wire around the hose, one end attached to a nut, and the other end to the screw, and when tightened, pulling the ends of the wire towards each other. However, this will leave a gap where no pressure is applied (underneath the screw), and cause a leak. To combat this, the more complicated and weaker design of having the ends overlap and then be pushed apart from each other is used, as this ensures pressure around the entire circumference of the hose.
To ensure a good seal, the barb, hose nipple, or beaded tube must be smooth and free of nicks, scratches, or contamination. Stuck hoses should never be removed by slitting them, as this can leave a scratch on the fitting which will cause a leak.Hose Clamp
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