Measures to solve the thermal stability problem of OPGW optical cable
1. Increase the section of the lightning conductor
If the current exceeds not much, the steel strand can be increased by one size. If it exceeds a lot, it is recommended to use good conductor lightning protection wire (such as aluminum-clad steel stranded wire). Generally, it is not necessary to change the whole line, only the line section entering and exiting the power station can be changed, and the length is determined by calculation.
2. Isolation and insulation of OPGW optical cable lightning protection line for incoming and outgoing line stalls
The maximum current in the lightning protection line is at the incoming and outgoing line. If a string of insulators is added to the lightning protection line at this level, the current will not be able to enter the substation. At this time, the maximum current occurs in the second gear. Although the total short-circuit current changes very little, the grounding resistance increases a lot, so the lightning protection line current decreases more. Two issues should be paid attention to when taking this measure. One is the selection of the pressure resistance of the insulator string, and the other is the appropriate matching of the ground resistance of each tower to minimize the current in the lightning protection line.
3. Use a shunt line to reduce the current of the OPGW optical cable
The cost of OPGW optical cable is relatively high, and it is uneconomical to simply increase the cross-section of OPGW optical cable to bear the short-circuit current. If the other lightning protection line uses a good conductor with very low impedance, it can play a good shunt role and reduce the current of the OPGW optical cable. The selection of the shunt line should meet the following conditions: the impedance is low enough to reduce the current value of the OPGW optical cable below the allowable value; the shunt line itself needs to have a large enough allowable current; the shunt line should meet the lightning protection requirements. Have sufficient strength safety factor. Although the resistance of the shunt line can be reduced very low, its inductive reactance decreases slowly, so the role of the shunt line has a certain limit. The shunt line can be selected in sections according to the short-circuit current conditions at various parts of the line, but special attention should be paid to the fact that when the shunt line changes model, because the shunt line becomes thinner, more current is distributed to the OPGW optical cable, so The current of the OPGW optical cable will suddenly increase a lot, so the selection of the shunt line requires repeated calculations.
4. Select two specifications of OPGW optical cables
Because the short-circuit current of the incoming and outgoing lines of the substation is the largest, OPGW optical cables with large cross-sections are used here, while OPGW optical cables with small cross-sections are used for the incoming and outgoing lines far away from the substation. This measure only applies to longer lines and should be compared economically. When selecting two types of OPGW optical cables, two shunt lines should be considered at the same time. At the intersection of the two lines, attention should be paid to sudden changes in the current of the OPGW optical cable and lightning protection line.
5. Underground distribution line
If several grounding bodies are used to connect the grounding device of the terminal tower to the grounding grid of the substation, a considerable part of the short-circuit current will enter the substation from the ground, reducing the current of the incoming OPGW optical cable and lightning conductor. When using this measure, consult with the operating department.
6. Parallel connection of multi-circuit lightning protection lines
If the grounding devices of several terminal towers are connected, the short-circuit current can flow into the substation along the multi-circuit lightning conductor, so that the single-circuit current is much smaller. If there is still a problem with the thermal stability of the lightning protection wire of the second gear, the grounding device of the second base tower can be connected, and so on. However, it should be noted that when there are many connected towers, the problem of relay zero-sequence protection needs to be studied.
7. The incoming and outgoing line stalls use ADSS optical cables
When the OPGW optical cable is cancelled, and the ADSS (all-dielectric self-supporting) optical cable is used, the maximum short-circuit current in the OPGW optical cable can be considered as the current flowing to the substation when the second base tower fails, and this current is higher than that of the first base tower. The short circuit current is small. Therefore, when the ADSS optical cable is used for the entrance and exit line block, the maximum short-circuit current can be calculated according to the short-circuit current at the fault time of the second base tower during the thermal analysis of the OPGW optical cable, so that the thermal stability requirements for the OPGW optical cable are greatly reduced.
The thermal stability of the optical fiber overhead composite ground wire (OPGW) should be fully considered in the design and selection process, and different measures should be taken according to the specific structure and actual path of the OPGW optical cable to avoid damage caused by the single-phase grounding short-circuit current to the OPGW optical cable. harm, and improve the operational reliability of the OPGW optical cable.