[ناشناس]

بهترین نحوه مهار کابل های قدرت به لدر ها چیست؟
ایا مهار کردن کابلهای قدرت 33 کیلوولت با بست چنگالی یا با بست فلزی به بدنه اسراکچر یا لدر مورد قبول است؟….

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بهترین نحوه مهار کابل های قدرت به لدر ها چیست؟
ایا مهار کردن کابلهای قدرت 33 کیلئوولت با بست چنگالی یا با بست فلزی به بدنه اسراکچر یا لدر مورد قبول است؟
ایا این بست چنگالی یا بست فلزی تولید حرارت یا نقطه حرارتی بعلت بسته شدن مسیر جریان یا تشکیل لوپ، مشکل ساز و تولید نقطه حرارتی نمیکند؟

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[حمید]

آنچه در این ارتباط حائز اهمیت است کنترل تلفات حرارتی ناشی از شار مغناطیسی اطراف کابل است. این شار در مورد کابلهای تک رشته و یا کابلهای سه فاز درگیر نامتعادلی زیاد جریان و یا هارمونیکهای مضرب سه قابل توجه بوده و می باید در عملیات نصب مورد توجه واقع شوند. همانطور که می دانید صرف فلزی بودن بست کابل مشکل ساز نیست بلکه این بست فلزی نباید فرو مغناطیس باشد. برای مثال در بستهای آلومینیومی با وجود تشکیل جریان گردشی تلفات حرارتی قابل توجهی ایجاد نمی شود. The magnetic fields surrounding single-conductors can extend much farther than those surrounding multiconductor cables. Concerns have been expressed in some circles that magnetic fields may be linked with cancer. The cautious consultant or end-user may wish to exercise the option of minimizing magnetic field strengths until more definitive medical evidence is available. The nuisance effects of the magnetic fields can be both elusive and expensive to correct. Typical nuisance effects involve the actions of the magnetic fields on sensitive electronic equipment, such as computers. In extreme cases, shielding and filtering of power supplies may be the only way to remedy the situation. The magnetic field from each conductor is nullified by those of the neighbouring conductors in a three-phase system with a pure sine wave form. In a four-conductor cable, the magnetic fields neutralize almost totally within the cable. However, the fields of single-conductors can extend much farther, depending on the spacing between the conductors. Magnetic fields are amplified in circuits with high levels of third harmonic currents and multiples of the third harmonic. These currents are common today due to electronic devices which chop the wave form of the voltage. One of the unexpected results is that third harmonic magnetic fields in each of the three phases are additive, so the magnetic field surrounding a group of three conductors can be much greater than would be expected. This amplified magnetic field is cancelled only by the field from the neutral conductor. In large singleconductor feeders, the neutral conductor is often located a significant distance from some of the phase conductors, leading to propagation of third harmonic magnetic fields to greater distances. The use of multi-conductor cables will eliminate this concern, owing to their close proximity. Special precautions are necessary with single-conductor systems, in addition to those previously mentioned. Accessories which totally surround single-conductor cables, such as clamps and connectors, must be nonferrous to avoid magnetic hysteresis and eddy current losses, which could lead to major overheating. Third harmonic currents will greatly increase the magnetic losses in such components. The current sharing between parallel conductors of the same phase must be ensured by attention to phase configurations. Sometimes imbalances in the current sharing can be present due to inherent difficulties in complying with spacing requirements. It is virtually impossible to balance the third harmonic currents equally with any type of single-conductor phase configuration, although this is automatically achieved with 4-conductor cables.

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