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Discussion On Some Problems In Parallel Use Of Multiple Single-core Cables
- May 18, 2017 -

The actual parallel use of cables with a single-core cable parallel more, single-core cable in the actual parallel use of the process may be due to the impact of laying mode, its actual load flow does not necessarily meet the actual load needs, in actual use may be overloaded phenomena. In fact, when the 6 cables with no clearance of the parallel laying in the air after the actual flow can only reach the theoretical load of 60%%, if coupled with the cable load according to the theoretical choice, did not follow the actual laying of the correction. It is likely that the cable in the actual power of the process of running a full load, resulting in the cable power running to produce fever. Therefore, in the course of the parallel laying of cables, the actual load flow is not a simple "1+1=2" relationship, it is likely that "1+1=1.5" even "1+1=1" phenomenon, resulting in the actual operation of the cable in the process of serious fever phenomenon. Now let's give a simple example, such as the capacity of 570KW, rated current is about 1140A of three-phase asynchronous motor load, using two YJV-0.6/1KV-1*300 cable in parallel to power supply, according to theoretical design to calculate the given value, YJV-0.6/1KV-1*300 single cable in the air laying the theoretical calculation of the load of about 750A, two cables of the theoretical parallel load flow can reach 1500A around, It can satisfy the actual use of equipment. We now assume that there are 32 cables all concentrated on one side of the bridge, stacked with random laying, and the two YJV-0.6/1KV-1*300 of the parallel power supply. Consult related materials found that when the cable in the air 6 of the actual load of the cables with no clearance after the stacking of the line will fall to the theoretical calculation of the given value of 60%. Then the actual load of the original cable is 1500x60%=900A, each cable allocated to the actual load of 450A, and the theoretical calculation of the load of 750A difference near 300A, so that the actual use of the cable in the process of serious overload fever phenomenon.

and the actual laying of cables is far more than 6 roots, then the actual cable can be more traffic than 900A is smaller. How to solve this problem, some people propose to shunt a YJV-0.6/1KV-1*120 cable to reduce the distribution of the remaining two cables, now we theoretically assume that the calculation of the three cables in parallel, the actual distribution of the load current, assuming that 3 of the cable in parallel to the use of the length of 1 kilometers, the laying temperature in all 20 ℃. And assuming that the parallel 1 km of two YJV-0.6/1KV-1*300 cable conductor resistors are identical. In fact, because the problem in manufacturing process can not achieve complete consistency, the conductor resistance is still slight difference. In the actual computation process we ignore the aforementioned effects. 20 ℃ copper conductor Max DC resistance copper core 300mm2 for 0.0601 ω/km,120 mm2 for 0.153 ω/km, the actual distribution of 1140A current calculation 120 mm2 cross-section distribution current for (0.0601*0.0601/0.153*0.0601+0.153*0.0601+0.0601*0.0601) =187A, the remaining 300 mm2 section of the allocated current is 953A, And each 300 mm2 cable actual flow of the load current is about 477A, in such cases, the actual power of the cable still overload phenomenon. and cable 120 of the actual disaster flow in this case the load flow is 435*60%=261A, still has a great margin but the current distribution law does not allocate the current to 120 cross-section of cables, the original problem is still not solved. And our assumption is that only the cable is 6 roots and does not conform to our established requirements. Imagine that another 300 mm2 cross-section of the cable, the actual load distribution of the law is 1140*1/3=380A, so in the actual parallel cable process to the cable section must be calculated after solemn, can be used in parallel, otherwise the cable may not be able to solve the problem, the best case is to use the same specifications of the cables, and ensure the same length, so that the distribution of the basic uniform current. In fact, the on-site installation is complete after the installation of a site cable reinstall and rework, in general, it is difficult to achieve. Therefore, the formal design and installation of cables is crucial, and the latter approach is often only a remedy, it is difficult to fundamentally solve the problem.

And there are some problems in the parallel use of multi-core cables, and the main core of each cable is staggered A,B,C three-phase in parallel connection with armored cables. Can not be armored multi-core cables all the line new and connected to a single-core cable used, if so, the cable will be in the armored steel strip in the vortex effect, causing the cable fever, resulting in thermal breakdown. Although this is a very simple electricity principle, but in the process of the author repeatedly visited users sometimes, there are similar problems and practices. In the three-phase four-line unbalanced illumination load, we load wiring and distribution to ensure that the distribution of the load evenly, as far as possible to ensure the three-phase current balance, otherwise may result in the three-phase current serious imbalance in the armored steel strip produced alternating currents, resulting in cable fever.

The connection of the cable's parallel connection to the end of the wiring of the nose to attract attention, because the use of parallel cable load capacity is generally larger, the conductor resistance of each kilometer is under 0, if the line on any end of the line once the nose is loose and contact bad phenomena, will multiply the conductor resistance of the line, resulting in uneven current distribution even bypass phenomenon, which will cause a parallel connection of individual cables to produce fever, causing failure.

At the same time possible the actual line of the conductor resistance of the cable is not completely consistent, so the same type of cable in the distribution of the current can not be an absolute average distribution, may be in the actual distribution of the current process may still exist some differences.

Therefore, in the actual parallel use of multiple single-core cables to be adjusted according to their actual laying situation, otherwise may cause the cable parallel use process to produce fever phenomenon, affect the normal use of cables.

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