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
Direct flash pulse current waveform When the time t = 0, the fault point of the cable breaks down under the action of the applied voltage -E, forming a short-circuit arc, so that the voltage of the fault point jumps to zero. At this time, a positive sudden jump voltage E0 opposite to -E is generated at the fault point and the corresponding current direct flash pulse current waveform is set at time t=0, the cable fault point breaks down under the action of the applied voltage -E, forming a short circuit arc, so that the voltage at the fault point jumps to zero. At this time, a positive jump voltage E0 opposite to -E and the corresponding current i0=-E/Z0 are generated at the fault point (it is specified that the current flowing from the measurement point to the cable is positive, and the current generated by the jump voltage E is from The fault point flows to the measurement point, so it is negative, Z0 is the cable wave impedance) and is transmitted to both ends of the cable, as shown in Figure 4.5.
At time t=τ, the current wave i0 reaches the measurement terminal, and the capacitor is in a short-circuit state to the high-frequency traveling wave signal. According to the introduction in Section 2.4, the current is completely reflected back to the fault point at the measurement terminal; and at the fault point due to the arc short circuit It is completely reflected back again; it reaches the measurement point at the moment of t=3τ, resulting in a second reflection; this is back and forth until the entire transient process ends. The current at the measurement point is the sum of all current waves. Adding the current waves on the time axis of Figure 4.5 point by point, the current shown in Figure 4.6.a can be obtained. The initial value of the current is 2i0, that is, the incident current wave i0 reaches After the measurement point, the current doubling phenomenon occurs, and the output of the linear current coupler only reflects the abrupt component of the current, as shown in Figure 4.6.b. It can be seen from Figure 4.6.b that two negative pulses appear when t1=τ and t2=3τ respectively. The first negative pulse is caused by the discharge pulse at the fault point reaching the measurement point, which can be simply called the discharge pulse at the fault point; The negative pulse is caused by the reflection pulse at the fault point, which is called the reflection pulse at the fault point.
The distance between them corresponds to the time difference Δt = t2 - t1 = 2τ when the current pulse moves from the measuring end to the fault point and returns. The calculated fault distance is: Lx = V·Δt/2 where V is the propagation velocity of the traveling wave in the cable.
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