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
Buried cable fault finding method With the development of electric power industry, various cables are more and more used in various fields of production and life, and most of them are buried underground. When the cable fails, finding the fault point of the cable is a problem that has troubled us for a long time. With the development of electric power industry, various cables are more and more used in various fields of production and life, and most of them are buried underground.
When the cable fails, finding the fault point of the cable is a problem that has troubled us for a long time. How to quickly and accurately find the fault point of the cable is our goal of Xinyijie Electric. Generally, we mainly use the following steps to find the fault point of the buried cable: 1. Judging the nature of the cable fault: mainly low-resistance faults and high-resistance faults Send a pulse voltage signal to the cable. When the cable is faulty, the pulse reflection is generated due to the mismatch between the impedance of the fault point and the characteristic impedance of the cable. The cable fault distance finder displays the characteristics of the tested cable on the screen in the form of a waveform. .Disconnection fault waveform b. Low-resistance and short-circuit fault waveforms are set from the time the instrument transmits the pulse until the time when the reflected pulse from the fault point is received is Δt, and Δt is the time for the pulse to go back and forth between the test point and the fault point.
Suppose the fault distance is S and the propagation speed of the pulse in the cable is V, then: S=VΔT/2Δt is automatically timed by the instrument, and the fault distance S is obtained in combination with the set wave speed V. In fact, during the propagation of the cable, the pulse encounters all impedance mismatch points, such as joints, remote ends, etc., and will produce reflections. The user can determine the distance of the fault point by identifying the starting position, shape and amplitude of the reflected pulse. The fault point is located at the starting point of the reflected pulse, that is, the position of the dotted cursor in Figure 5.1.
It can be seen from Figure 5.1 that the transmitted pulse is of positive polarity. If the reflected pulse is of positive polarity, it means a disconnection fault; if the reflected pulse is of negative polarity, it means that it is a low-resistance or short-circuit fault waveform. Pulse current method: When the insulation resistance of the fault point of the cable is large (greater than 10 times the characteristic impedance of the cable, Rf>10Zc≈200Ω), the reflection coefficient of the fault point is very small, causing the reflected pulse to be indistinguishable, so the low-voltage pulse method cannot measure the distance. Use a high-voltage generator to apply high voltage to the faulty cable, so that the fault taps through and discharges, and the discharge pulse is reflected back and forth between the fault point and the test end. Use the cable fault distance finder to sample and record this signal and measure the time difference, and the distance to the fault point will be obtained.
There are two ways to collect the discharge pulse signal: voltage sampling and current sampling. Using current sampling is the pulse current method: the current coupler collects the current flowing back to the high-voltage energy storage capacitor from the test ground (cable metal sheath) and is completely isolated from the high-voltage part. , safe and reliable, the waveform is easier to identify. Cable fault distance meter high voltage signal generator multiple pulse method: The multiple pulse test method in the cable fault tester is suitable for cable high resistance faults and cable flashover faults, and needs to be coupled with high voltage signal generators and multiple pulse signals Compared with the pulse current method, the test waveform of the multiple pulse test method is simple and easy to identify. The basic principle of the secondary pulse test method: applying high voltage to the cable causes the cable fault to break through and generate an arc. The AC resistance of the arc is very low, which can be considered as a low-resistance short-circuit fault.
Using certain technical means, the low-voltage pulse signal can be sent to the cable within the arc duration, and the arc at the fault point of the cable will reflect the pulse back to the measurement end, and its waveform is similar to the low-resistance fault waveform of the low-voltage pulse method. 3. Cable path search Mainly use the pipeline detector to find the accurate cable path and prepare for the precise point determination. Pipeline Detector 4. The precise location of cable faults mainly uses digital cable fault location meters, and adopts the most advanced acousto-magnetic synchronous fixed-point method to achieve accurate finding of fault points.
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