Tower Top Voltage

A transient voltage at a tower top, to which lightning strikes, is easily calculated by applying a traveling wave theory. An example has been already explained in Section 1.7. See Problem 1.19. 

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Influence of a tower model on a tower top voltage, (a) Measured results, (b) Frequency-dependent tower model with a resistive-footing impedance, (c) Distributed line tower model with various footing impedances. 

Thus, the tower-top voltage Vg t) is changed to the following value ... 

From Equations 2.28, 2.35, and 2.34, an anal54ical wave is drawn as shown in Figure 2.59. The waveform explains a numerical simulation result of the tower-top voltage when lighming strikes the tower top. This data has been published in many papers [14, 15, 16, 17, 18, 19, 20, 21-22]. 

Calculate the tower top voltage in Figure 1.66 for 0 < t<40 ns, taking 01= 10 = 1/ arid compare with a measured result shown in Figure 1.57b. 

Figure 2.46 shows the effect of tower footing impedance on the tower top voltage in comparison with a measured result on a 500 kV transmission tower [23,31]. The inductive footing impedance shows a reasonable agreement with the measured result, but the resistive/capacitive impedance shows a far more oscillatory waveshapes. In fact, many measured results of the grounding electrode impedance show the inductive characteristic [32]. It should be noted that the effect of the tower surge impedance, that is, the effect of the... 

Figure 1.58 shows another example of a transient voltage at the top of an 1100 kV transmission tower (height 140 m, average radius 6.3 m) [41]. (a) is the measured result of an injected current and the voltage at the tower top, and... 

Transients generated on the antenna side because of a lightning discharge are no less troublesome. The tower is a conductor, and any conductor is also an inductor. A typical 150-ft self-supporting tower may exhibit as much as 40 /xH inductance. During a fast-rise-time lightning strike, an instantaneous voltage drop of 360 kV between the top of the tower and the base is not unlikely. If the coax shield is bonded to... 

FIGURE 1.58 Measured transient response at the top of a 1100 kV transmission tower, (a) Injected current and voltage at the top of the tower, (b) Step response of the voltage at the top of the tower. 

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