NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
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NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
Author: Noyafa–CCTV Tester
The above analysis of typical direct flash waveform is carried out under ideal conditions, which is different from the actual direct flash pulse current waveform (as shown in Figure 4.7). The actual waveform has the following characteristics: (1) The current in the cable As time increases, it gradually approaches 0. The above analysis of the typical direct flash waveform is carried out under ideal conditions, which is different from the actual direct flash pulse current waveform (as shown in Figure 4.7). The waveform has the following characteristics: (1) The current in the cable gradually approaches 0 with the increase of time, which is because the energy stored in the cable and the capacitor is completely consumed after the fault breaks through. (2) Due to the propagation loss of the current wave in the cable, the current waveform and the output of the linear current coupler become smoother and smoother with the increase of time, and the amplitude becomes smaller and smaller. (3) Since the capacitor C cannot be regarded as an absolute short circuit, after the current traveling wave arrives, the capacitor C is gradually charged and the current gradually decreases.
Therefore, we should observe a sawtooth-like wave head (Fig. 4.7.a), not a right-angled square wave as in Fig. 4.6.a. (4) As shown in Figure 4.7.b, the reflected pulse at the fault point has a small positive pulse appearing. This is the effect of the stray inductance Ls of the capacitor itself and the test leads.
Although Ls is generally only a few microhenries, its influence cannot be ignored for high-frequency traveling wave signals. Figure 4.8.a is the equivalent circuit of the measurement terminal, C is the capacitor capacitance, for high-frequency traveling waves, C can be considered as a short circuit. The current traveling wave from the fault point can be considered as a right-angle wave of negative polarity. Referring to Section 2.4, the reflection caused by the inductance Ls is shown in Figure 4.8.b.
At the beginning, the current on the inductor cannot change abruptly, which is equivalent to an open circuit. The reflection coefficient of the current traveling wave is -1, negative reflection occurs, and the waveform changes in the positive direction; as time increases, the current on the inductor enters a steady state, the inductor is equivalent to a short circuit, and the current traveling wave The reflection coefficient is +1, there is a positive reflection, and the waveform changes to the negative direction, so a small positive pulse appears on the waveform.
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