Some tips for choosing copper bars
32
Bus bar, also called bus bar or current-carrying bar, is a kind of conductor that carries current.In switchgear and control equipment, it is mainly used to collect, distribute and transmit electric energy, and connect to primary equipment.According to relevant statistics, short-circuit accidents mostly occur directly or indirectly at the busbar. Busbar faults are one of the most serious faults in electrical equipment..Therefore, the correct selection and application of the bus is extremely critical.Below on medium pressure 3.6kV—40.Preliminary analysis, calculation and summary of several aspects related to the bus in 5kV switchgear and control equipment.In indoor switchgear and control equipment, busbars are divided into rectangular, circular, D-shaped, U-shaped, etc. according to their cross-sections. Among them, rectangular busbars with the same cross-section have lower resistance and larger heat dissipation surface than round, D-shaped, U-shaped, and other busbars. , High current-carrying capacity and other reasons, the rectangular bus is at 40.The most widely used in voltage levels of 5kV and below.Round and D-shaped busbars have good skin effect and anti-corona effect, and they are also used, but the connection is more complicated..U-shaped busbars are generally used in equipment with large current and high force effect requirements, such as generator outlet switch cabinets.Its rated current is large, generally above 5000A, and the rated peak withstand current (IP) is large, generally above 50kA.According to the material, it can be divided into copper bus bar, aluminum bus bar and iron bus bar. Among them, copper bus bar is the most widely used due to the large current carrying capacity, corrosion resistance and force effect..Aluminum busbars are also used in low current, non-coastal and non-petrochemical systems.The use of iron busbars is mainly from economic considerations, mainly used in PT connection lines.According to the natural state, it can be divided into hard bus and soft bus.Soft busbars are mainly used in places where the connection is inconvenient and can absorb some force effects, such as inside the circuit breaker.The current carrying capacity of the busbar refers to the effective value of the current that the busbar can carry under specified conditions.The main indicator of the specified conditions is temperature. For indoor switchgear and control equipment, the upper limit of the ambient temperature is 40°C and the lower limit is -25°C..The current-carrying capacity of the busbar when placed in a vertical position is higher than when it is placed horizontally. Generally, when the busbar is placed horizontally and the width is less than 60mm, the current-carrying capacity of the busbar is 0 when placed in a vertical position.95 times, when the width is greater than 60 mm, the current carrying capacity is 0 when placed in a vertical position.92 times, this is because the heat dissipation performance is better when placed vertically than when placed horizontally..According to the material of the busbar, its current carrying capacity is also different at the same temperature.Switchgear and control equipment mainly use rectangular copper busbars as the main rectangular aluminum busbar as auxiliary.The current-carrying capacity value can generally be found according to various design manuals.Based on many years of experience, the author summarized a simple calculation formula for bus current carrying capacity (communication) during vertical release through the statistics of bus current carrying capacity in various manuals..The current-carrying capacity of single-layer rectangular copper busbar at 40℃ Id(A): Id =k(b+8.5) In the h formula, b is the thickness of the busbar (mm); h is the width of the busbar (mm); k is the coefficient (A/mm).The relational formula I2=(1.56～1.58) In the Id formula, I2 is the current-carrying capacity of the double-layer copper busbar (A); Id is the current-carrying capacity of the single-layer copper busbar (A).1.56～1.58 is the coefficient, generally choose 1 for 100×10 bus.58; 80×10, 80×8 choose 1.57,60×6 choose 1.56.The relational formula of the current-carrying capacity of the 3-layer copper busbar and the single-layer copper busbar at 40℃ is I3=2Id where I3 is the current-carrying capacity of the four-layer copper busbar (A).The relational formula of the current-carrying capacity of 4-layer copper busbar and single-layer copper busbar at 40℃ is (this kind of choice is not recommended, it is better to use special-shaped busbar instead) I4=2.In the 45Id formula, I4 is the current carrying capacity of the four-layer copper bus (A).Note: 2-layer and above copper busbars means that there is an air gap equal to the thickness of the busbars between the two busbars.The conversion relationship of the busbar when the ambient temperature is 40℃ and when the ambient temperature is 25℃ is: I40=0.In the 85I25 formula, I40 is the current-carrying capacity of the bus at 40°C (A); I25 is the current-carrying capacity of the bus at 25°C (A). The conversion relationship of the download flow rate of the copper bus bar and aluminum bus bar of the same specification under the same ambient temperature condition is: IAl=ICu/1.3 In the formula, ICu is the current-carrying capacity of the copper bus; IAl is the current-carrying capacity of the copper bus. For example, according to the above formula, the current-carrying capacity of TMY100×10 bus is calculated as: Single layer: 100×18.5 = 1850A The manual is 1860A. This data is based on the “Regulations for High-Voltage Power Distribution Equipment” of the Ministry of Water and Electricity published by China Industry Press. Double layer: 2 (TMY100×10) carrying capacity: 1850×1.58=2923(A); [Check the manual for 2942A]; Three layers: 3 (TMY100×10) current carrying capacity: 1850×2=3700A [Check the manual for 3780A] All the above calculations are quite close to the manual data, you can Carry out the calculation of bus current carrying capacity according to the above formula.Support and fixation of the busbar (1) The support of the busbar should fully consider whether its bending strength meets the force effect requirements of the configured system.(2) The bus bar is generally fixed by hardware, and bolts are not used to directly drill holes on the bus bar..(3) For insulators used for fixed busbars, when they are subjected to large electric power (such as when the system’s rated peak withstand current is above 40kA), it is necessary to verify whether the bending strength of the insulator meets the requirements of structural design.The force effect of the insulator can be checked according to the following formula: Fjs≤0.6FphFjs=1.76KLich2×10-2/a Where: Fjs-stress acting on the insulator, kg; Fph-bending strength of the insulator, can be checked for related samples; L-span between insulators (bus bar support spacing) (cm); a —Phase distance (cm); ich——Rated peak withstand current (KA) (4) The busbar splint and fixing hardware should also be able to withstand the force caused by the rated peak withstand current.(5) The fixed hardware or other supporting hardware of the bus bar should not form a closed magnetic circuit.(6) When the bus bar is flat, the upper pressing plate of the bus bar supporting splint should be kept 1-1 with the bus bar.With a clearance of 5mm, when the busbar is placed upright, the upper pressing plate should be kept at the same distance as the busbar.5-2mm gap.(7) Between multiple rectangular bus bars, a gap not less than the thickness of the bus bar should be maintained.(8) The metal splint (except fasteners) at the end of the bushing for busbars 600A and above should be made of non-magnetic materials. There should be a metal connection between it and the busbar, and the contact should be firm. The thickness of the metal splint should not be less than 3mm. When there are two or more bus bars, the bus bars themselves should be fixed.(9) The fixing between the busbar fixing hardware and the pillar insulator should be flat and firm, and the busbar supported by it should not be subject to additional stress.Several issues related to busbars in medium voltage switchgear and control equipment, the relationship between universality and particularity should be fully considered in the structural design and manufacturing, so as to meet the requirements of relevant standards and take into account economy and practicality. The nature and development of the busbar system will directly affect the operation of the power supply system. Enough attention should be paid to the three aspects of use, design and manufacturing..