unit 3lightning damages and risk assessment

1、Department of Communication EngineerLightning Damages and Risk Assessment23 Basic Mechanism of Lightning Damagev The type and amount of lightning damage that an object suffers depend on both the characteristics of the lightning discharge and the properties of the object. The physical characteristics

2、 of lightning of most interest are various properties of the current waveform and of the RF electromagnetic fields. Damage can also occur from electromagnetic radiation in other frequency bands and from the acoustic shock wave.v Four distinct properties of the lightning current waveform can be consi

3、dered important in producing damage: () the peak current, () the maximum rate of change of current, ()the integral of the current over time (i.e., the charge transferred), and ()the integral of the current squared over time, the “action integral”. We now briefly examine each of these properties and

4、the type of damage to which they are thought to be related.v 物体遭受雷电危害的类型和程度取决于雷电放电的特性和物体本身的性质。最感性趣的雷电物理特性是各种雷电流波形和射频磁场属性。雷电危害还可以由（其它频段）电磁辐射和声音振动波引起。v 重点考虑引起危害的四种不同雷电流参数是（1）电流峰值；（2）电流变化率的最大值；（3）电流的时间积分值（如传输电荷）；（4）电流平方的时间积分值即作用积分。现在我们简单地分析一下这些参数以及与之相关的危害类型。v () The peak current For objects or systems

5、that present an essentially resistive impedance such as , under certain conditions, a ground rod driven into the earth, a long power line, or a tree, the voltage V on the object or system with respect to remote ground will be proportional to the current I via Ohms law, V=RI, where R is the effective

6、 resistance at the strike point. For example, a 30 kA peak current injected into a power line phase conductor with a 500 characteristic impedance (the effective resistance is 250,since 500 is ”seen” in each direction) produces a line voltage of 7.5MV with respect to the earth. Such a large voltage c

7、an lead to an electric discharge from the stuck phase conductor to adjacent phase or neutral conductors or to ground, across insulating materials or through the air.v ()峰值电流 在一定条件下，呈现阻抗的物体和系统中，例如一个埋在地下的接地棒、一条长的供电电缆或者一棵树，根据欧姆定律V=RI ，物体和系统相对于遥远的大地的电压V与电流I成正比例，这里R是雷击点的有效阻抗。例如30VA的峰值电流流入特征阻抗为500的供电相线，将产

8、生7.5MV相对于大地的线电压。如此高的电压能导致雷击相线与相邻的相线、零线或地线之间放电，击穿绝缘材料和空气。v () The maximum rate of change of current For objects that present an essentially inductive impedance such as, under some circumstance, wires in an electronic system, the peak voltage will be proportional to the maximum rate of the lightning c

9、urrent (V=LdI/dt, where L is the inductance of the length of wire and V is the voltage difference between the two ends of the wire). For example, if a “ground” wire connecting two electronic systems (for example, in a communications tower and in an adjacent electronics building) has an inductance pe

10、r unit length of 10(-6)Hm(-1) and if 10 percent of the direct lightning current flows in the wire, producing dI/dt=10(10)As(-1), then 10 kV will be produced across each meter of the wire . It is easy to understand how even a very small fraction of the lightning current circulating in grounding and b

11、onding wires can cause damage to solid-state electronic circuits that have communication, power and other inputs “grounded” at different location. v ()The integral of the current over time The severity of heating or burn-through of metal sheets such as airplane wing surface and metal roofs is, to th

12、e first approximation, proportional to the lightning charge transferred, which is in turn proportional to the energy delivered to the surface. Generally, large charge transfers are due to long-duration (tens to hundreds of milliseconds) lightning currents, such as long continuing current, whose magn

13、itude is in the tens to hundreds of amperes range, rather than return strokes having large currents but relatively short duration and hence producing relatively small charge transfers. Additionally, even those impulse currents which do have relatively large charge transfers cause only relatively min

14、or surface damage on metal sheets, apparently because the current duration is too short to allow penetration of heat into the metal. v () Action integral The heating and melting of resistive materials, which may or may not be relatively good conductors, and the explosion of poorly conducting materia

15、ls are, to the first approximation, related to the value of the action integral, that is, the time integral of the Joule heating power I2R for the case that R=1. Thus the action integral is a measure of the ability of the lightning current to generate heat in a strike object characterized by resista

16、nce R. About five percent of negative first strokes in ground flashes have action integrals exceeding 5.510(5)A(2)s; About five percent of positive strokes have action integrals exceeding 10(7)A(2)s. v in the case of most materials that are poorly conducting, this heat vaporizes the internal materia

17、l and the resultant gas pressure causes an explosive fracture. In addition to heating effects, the action integral is also a measure of some mechanical effects such as the crushing of hollow metal tubes carrying lightning current. The effect is a function of both the instantaneous force, which is pr

18、oportional to the square of the current, and the duration of application of the force. In this case, the applied force must also exceed some threshold value.v Electromagnetic fields from lightning that impinge on any conducting objects induce currents and resultant voltages in those objects. Two pro

19、perties of the electromagnetic fields are sufficient to describe most of the important damaging effects, commonly the destruction of electronic components: () the peak values of the electric and magnetic fields and () the maximum rates of change of the fields. v For certain types of unintended anten

20、nas, such as elevated conductors that are capacitively coupled to ground, the peak induced voltage in the conductors with respect to ground is proportional to the peak electric field. For other unintended antennas, such as a loop of wire in an electronic circuit, some underground communication cable

21、s, and elevated conductors resistively coupled to ground, the peak voltage is proportional to the maximum rate of change of the electric or the magnetic field. The degree of coupling of fields through holes or apertures in the metal skins of aircraft and spacecraft is generally proportional to the r

22、ate of change of the electric and magnetic fields. v mechanism meknizm机制, 构造,机械装置,办法, 技巧, 途径v Characteristics特征，规格，参数v physical characteristics物理特征，物理性质v property 性质, 特性, 性能v acoustic ku:stik声音的, 听觉的v distinct distikt不同的,清楚的v integral intirl积分,整数,整体v with respect to关于，至于,相对于v phase conductor相线v neutral conductors零线，中线v circumstance s:kmstns 1.环境, 条件, 情况2.境遇, 经济状况3.事件；事实v bond 1.砌合；黏合；结合2.结合在一起，团结一致，团结在一起 (to， with) v bonding bndi结合,粘合,焊接v severity sverti:严重性，严重度，严重程度v burn-throughru:烧穿v approximation 近似；略计；逼近v to the first approximation大致上,相当近似地 v wi