|
发表于 2010-12-4 04:39:23
|
显示全部楼层
Short-Circuit Heating of Bars
Copper busbars are normally part of a larger generation or transmission system. The continuous rating of the main components such as generators, transformers, rectifiers, etc., therefore determine the nominal current carried by the busbars but in most power systems a one to four second short-circuit current has to be accommodated. The value of these currents is calculated from the inductive reactances of the power system components and gives rise to different maximum short-circuit currents in the various system sections.
These currents are very often ten to twenty times the continuous current rating and therefore the transitory heating effect must be taken into account. This effect can, in many cases, lead to dangerous overheating, particularly where small conductors are part of a large heavy current system, and must be considered when determining the conductor size. To calculate the temperature rise of the conductor during a short circuit it is assumed that all the heat generated is absorbed by the conductor with none lost by convection and radiation as for a continuous rated conductor. The temperature rise is dependent therefore only on the specific heat of the copper conductor material and its mass. The specific heat of copper varies with temperature, increasing as the temperature rises. At normal ambient temperatures it is about 385 J/kg K and at 300°C it is about 410 J/kg K.
Short-circuit heating characteristics are not easy to calculate accurately because of complex a.c. and d.c. current effects, but for most purposes the formulae below will normally give sufficiently accurate results:
where t = maximum short-circuit time, s
A = conductor cross-section area, mm2
I = conductor current, kA
 = conductor temperature rise, K
If  = 300°C, then
The value of t obtained from the above equation should always be greater than the required short circuit withstand time which is usually 1 to 4 seconds. |
评分
-
参与人数 1 | 贡献 +5 |
E币 +50 |
威望 +5 |
收起
理由
|
无限创意
| + 5 |
+ 50 |
+ 5 |
辛苦提供资料 必须加最高分 |
查看全部评分
|