In the actual parallel operation of cables, single core cables are more in parallel. Due to the influence of laying mode, the actual carrying capacity of single core cables may not meet the needs of actual loads, and overload may occur in actual use. In fact, when six cables are laid in the air in parallel without gap, the actual re flow can only reach about 60% of the theoretical current carrying capacity. If the cable load is added, the theoretical selection is not made according to the actual laying situation. It is likely to cause the cable to be in full load operation state in the actual power on process, resulting in the cable heating phenomenon during the power on operation. Therefore, in the process of parallel laying of cables, the actual current carrying capacity is not simply a “1 + 1 = 2” relationship. It is very likely that “1 + 1 = 1.5” or even “1 + 1 = 1” phenomenon may occur, resulting in serious heating phenomenon in the actual operation of the cable. Now let’s take a simple example. For example, for the load of three-phase asynchronous motor with capacity of 570kw and rated current of about 1140A, two yjv-0.6/1kv-1 * 300 cables are used for power supply in parallel. According to the theoretical design and calculation given value, the theoretical current carrying capacity of yjv-0.6/1kv-1 * 300 single cable laid in air is about 750A, and the theoretical parallel current carrying capacity of two cables can reach about 1500A, It can fully meet the actual needs of the equipment. We now assume that 32 cables are all concentrated in one cable tray and stacked side by side on the bridge, and the two yjv-0.6/1kv-1 * 300 with parallel power supply are also located in it. Referring to relevant materials, it is found that the actual current carrying capacity of the cable will drop to 60% of the theoretical calculation value when six cables are stacked in the air without gap. Then the actual current carrying capacity of the original cable is 1500 × 60% = 900A, and the actual current carrying capacity allocated to each cable is about 450A, which is nearly 300A different from the theoretical calculation current carrying capacity of 750A. In this way, serious overload heating phenomenon exists in the actual use process of the cable.
Moreover, the actual number of cables laid is far more than 6, so the actual cable re flow may be smaller than 900A. How to solve this problem, some people proposed to connect another yjv-0.6/1kv-1 * 120 cable in parallel to reduce the current distribution of the other two cables. Now let’s theoretically assume that after the three cables are in parallel, the actual distribution of load current is assumed to be 1 km in length and the laying temperature is all calculated at 20 ℃. Moreover, it is assumed that the conductor resistance of two yjv-0.6/1kv-1 * 300 cables 1 km in parallel are identical. In fact, due to manufacturing process problems, it is impossible to achieve complete consistency, and there is still a small difference in conductor resistance. In the actual calculation process, we ignore the above effects. The maximum DC resistance of copper conductor at 20 ℃ is 0.0601 Ω / km for 300mm2 and 0.153 Ω / km for 120mm2. The actual distribution of current at 1140A is calculated as (0.0601 * 0.0601 / 0.153 * 0.0601 + 0.153 * 0.0601 + 0.0601 * 0.0601) = 187a, the current distributed on the remaining 300 mm2 section is 953a, and the actual load current flowing through each 300 mm2 cable is about 477a, In this case, the actual power cable is still overloaded. However, the actual disaster discharge of cable 120 in this case is 435 * 60% = 261a, which still has a large margin, but the current distribution law will not distribute the current to the cable with section 120. In fact, the original problem has not been solved. Moreover, our assumption that there are only 6 cables does not meet our established requirements. Imagine adding another 300 The distribution rule of actual current carrying capacity of cables with cross-section of mm2 is 1140 * 1 / 3 = 380A. Therefore, in the actual process of parallel cables, the cross-section of cables must be calculated strictly before parallel use. Otherwise, adding cables in time may not solve the problem. The best case is to use cables with the same specifications and ensure the same length, so as to ensure the current The distribution is basically uniform. In fact, it is very difficult to re install and rework the cable after the field installation is completed. Therefore, the formal design and installation of cables in the early stage is very important, and the way adopted in the later stage is often only a remedial measure, which is difficult to fundamentally solve the problem.
Moreover, there are some problems in the parallel operation of multi-core cables. The main core A, B and C of each cable should be staggered in parallel for parallel use, and all wires of armored multi-core cable cannot be connected to one phase. If single core cable is used, eddy current effect will be generated in the armored steel belt of cable, which will cause heating and heat generation of the cable Breakdown fault. Although this is a very simple electrical principle, but in the process of the author’s many visits to users, sometimes some users put forward similar problems and practices. In the unbalanced lighting load of three-phase four wire system, the wiring and distribution mode of our load should ensure the uniform distribution of the load as far as possible, and ensure the three-phase current balance as far as possible. Otherwise, the serious imbalance of the three-phase current may cause alternating induction current in the armored steel strip, resulting in the heating of the cable.
In parallel operation of cables, attention should also be paid to the tightness of the connection nose at the end of each line, because the load capacity of the parallel cable is generally relatively large, and the conductor resistance per kilometer is below 0. If there is looseness and poor contact at any end of the line, the conductor resistance of the line will be doubled, resulting in uneven current distribution Bypass phenomenon, which will cause heating phenomenon of individual parallel cables and cause fault.
At the same time, it is possible that the conductor resistance of the actual line of the cable may not be completely consistent, so the current distribution of the same type and specification of cable can not be absolutely average distribution, and there may be some differences in the actual current distribution process.
Therefore, in the actual use of multiple single core cables in parallel, correction should be made according to the actual laying conditions, otherwise, heating phenomenon may occur during the parallel operation of cables, which will affect the normal use of cables.