Discussion on some problems in parallel operation of multiple single-core cable

- May 06, 2018-

Cable in the actual parallel use of a single core cable in parallel, a single core cable in the actual parallel use of the process may be due to the impact of the laying mode, the actual load flow can not meet the actual load needs, the actual use may appear overload phenomenon. In fact, when the 6 cables without clearance of parallel stacking in the air after the laying of the actual flow can only reach about 60% of theoretical load flow, if coupled with the load of the cable according to the theoretical choice, did not follow the actual laying of the correction. It is likely to cause the cable in the actual power process at full load running state, resulting in cable power operation generated fever phenomenon. Therefore, in the parallel laying of cable, its actual load flow is not a simple "1+1=2" relationship, it is likely to appear "1+1=1.5" or even "1+1=1" phenomenon, resulting in the actual operation of the cable during the serious fever phenomenon. Now we give a simple example, such as capacity of 570KW, rated current of about 1140A of three-phase asynchronous motor load, using two yjv-0.6/1kv-1*300 cable in parallel to power supply, according to the theoretical design calculation of the given value, yjv-0.6/1kv-1* 300 single cable laying in the air theoretical calculation of the load flow of about 750A, two cable theory of parallel load flow up to about 1500A, can fully meet the actual use of equipment needs. We now assume that there are 32 cables all concentrated on one side of the bridge stacked at random stacking, and the above parallel power supply of two yjv-0.6/1kv-1*300 is also located. Check the related materials found that when the cable in the air 6 without gap between the stacking of the actual load of the cable will drop to the theoretical calculation of the given value of 60%.

     Then the actual load flow of the original cable is 1500x60%=900a, each cable is assigned to the actual load flow of 450A or so, and theoretical calculation of the load flow 750A is nearly 300A, so the cable in the actual use of the process there is a serious overload heating phenomenon. and the actual number of cables to be deployed far more than 6 root, then the actual cable may be less than 900A of the flow. How to solve this problem, some people put forward to parallel a yjv-0.6/1kv-1*120 cable to reduce the remaining two cable distribution of the current, we now theoretically assume that, after three cables in parallel, the actual distribution of load current, assuming that 3 parallel cables used for the length of 1 kilometers, The laying temperature is calculated by 20℃. It is assumed that the 1 km two yjv-0.6/1kv-1*300 cable conductor resistance in parallel is identical. In fact, due to the problems in the manufacturing process can not achieve complete uniformity, the conductor resistance is still a small difference. In the actual calculation process we ignore the above effects.

20℃ copper Conductor maximum DC resistance copper core 300mm2 for 0.0601ω/km,120 mm2 as 0.153ω/km, the actual distribution calculation of 1140A current The distribution current of the mm2 section is (0.0601*0.0601/0.153*0.0601+0.153*0.0601+0.0601*0.0601) =187a, the current on the residual mm2 section is 953A, and each root 300 The actual load current on the mm2 cable is about 477A, in which case the actual power of the cable is still overloaded. and the actual disaster flow of cable 120 in this case the load flow is 435*60%=261a, there is still a lot of margin but the distribution of current law does not assign current to the 120 section of the cable up, in fact, the original problem remains unresolved. And our assumption is that only the cable is 6 of the case, also does not meet our established requirements. Imagine adding a cable mm2 section, the actual load flow distribution law for 1140*1/3=380a, so in the actual parallel cable process to the cable section must be calculated after the serious, can be used in parallel, otherwise the timely addition of the cable may not solve the problem, The best case is to use the same size cable, and ensure the same length, so that the distribution of the current is basically uniform. In fact, the installation and rework of a live cable after the on-site installation is complete is difficult to achieve in general.

      Therefore, the early formal design and installation of cable is very important, the latter approach is often a remedial measure, it is difficult to fundamentally solve the problem. Moreover, there are some problems in the parallel operation of Multi-Core cable, armored cable in parallel to the main line of each cable a,b,c three-phase staggered corresponding to the use of parallel, can not be armored multi-core cable all the lines of new and connected to a single core cable use, if so, will be in the cable armor-shaped steel belt in the vortex effect, Causing the heat of the cable, resulting in thermal breakdown failure. Although this is a very simple electrical principle, but in the author of many times to visit the user's process, sometimes there are users to ask similar questions and practices.

     In the three-phase four-line unbalanced lighting load, our load wiring and distribution should be as far as possible to ensure that the distribution of load evenly, as far as possible to ensure that the three-phase current balance, or may be due to the serious imbalance of three-phase current caused in the armored steel belt alternating current, resulting in The parallel use of cables for each line end wiring nose tightness degree also should be noticed, because the use of parallel cable load capacity is generally larger, each of its conductor resistance is below 0, if at any end of the line once the line nose loose and contact undesirable phenomenon, will multiply the line of conductor resistance,

resulting in uneven distribution of current and even bypass phenomenon, which will cause a parallel to the individual cable generated fever phenomenon, causing trouble.

At the same time, it is possible that the actual conductor resistance of the cable can not be 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 have some differences. Therefore, in the actual parallel use of multiple single core cable in accordance with the actual installation of the correction, it may cause the cable parallel use of the process of heating phenomenon, affecting the normal use of cables.