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
Question: Pulse current method and linear current coupler Answer: High resistance and flashover faults of cables Due to the large resistance at the fault point (greater than 10 times the cable wave impedance), the low voltage pulse has no obvious reflection at the fault point (the reflected pulse amplitude less than 5%), so no problem: the pulse current method and the linear current coupler answer: the high resistance and flashover faults of the cable are due to the large resistance at the fault point (greater than 10 times the cable wave impedance), and the low voltage pulse does not occur at the fault point. Obvious reflection (the amplitude of the reflected pulse is less than 5%), so the low-voltage pulse reflection method cannot be used for ranging. The pulse current method is to break down the fault point of the cable with a high voltage, use the instrument to collect and record the current traveling wave signal generated by the breakdown point of the fault, and calculate the round trip time between the measurement end and the fault point by analyzing and judging the time of the current traveling wave signal. fault distance. The pulse current method uses a linear current coupler to collect the current traveling wave signal in the cable.
The application diagram of the linear current coupler is the wiring diagram of the impulse high-voltage flashover test. The linear current coupler L is placed next to the grounding lead of the energy storage capacitor C connected to the cable sheath. L is actually an air core coil that is in turn with the magnetic field generated by the current in the ground wire. Assuming that the currents at time t2 and t1 are i2 and i1 respectively, t1 is less than t2 but close to t2, and the output voltage of the coil is obtained according to the law of electromagnetic induction: V=K(i2-i1)/(t2-t1)=KΔi/Δt ( 4.1) The parameter K is a constant that depends on the number of turns of the coil, the shape and the relative position to the ground wire, the current variation: Δi=i2-i1, and the time variation: Δt=t2-t1.
Equation (4.1) shows that the output voltage of the linear current coupler is proportional to the rate of change of the ground wire current, not the current itself in the ground wire. a. The current in the ground wire b. The output of the linear current coupler gives the current in the ground wire and the output of the corresponding linear current coupler. It can be seen that the output of the linear current coupler starts to rise when the current in the ground wire starts to rise. is a spike, and after the current in the ground wire has leveled off, the output is zero. Therefore, after the current traveling wave generated by fault breakdown arrives, the linear current coupler outputs a pulse signal, and whether there is a pulse signal output from the linear current coupler can be used to determine whether there is a current traveling wave at the measurement point.
Different from the pulse voltage method using resistance and capacitor voltage dividers for voltage sampling, the linear current coupler used in the pulse current method is placed in parallel next to the low-voltage side ground wire, and has no direct electrical connection with the high-voltage circuit. It is very safe and convenient. There are two test methods for pulse current: DC high voltage flashover and impulse high voltage flashover, which will be introduced separately below.
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