Understanding of Pulse Reflection Waveforms

Author: Noyafa–CCTV Tester

Question: Understanding of the pulse reflection waveform Answer: The pulse reflection waveform reflects the fault and structure of the cable. Observing the low-voltage pulse reflection waveform of the cable can not only find the location of the fault point and determine the nature of the fault, but also help to understand the structure of the complex cable. Question: Understanding of pulse reflection waveform Answer: The pulse reflection waveform reflects the fault and structure of the cable. Observing the low-voltage pulse reflection waveform of the cable can not only find the location of the fault point and determine the nature of the fault, but also help to understand the structure of the complex cable. This advantage It is incomparable to the bridge method. However, the actual measurement waveforms often change a lot, which requires the operator to have at least test training and certain measurement experience and skills. Whether the reflected waveform can be correctly understood is the key to accurately measure the fault distance and understand the cable structure. Figure 3.2.a shows a cable with a low resistance fault with joint J in the middle, and Figure 3.2.b shows the pulse reflection waveform measured by injecting a low voltage pulse into the cable.

Similar to X-ray films for fluoroscopy, the cable structure (state) under test is visually displayed on the screen of the instrument in the form of waveforms. The pulse reflection waveform of the cable with a connector in the middle is multiplied by the time coordinate of any point on the waveform by one-half of the wave speed (ie, formula 3.1), and converted into a distance, then all the impedance mismatch points on the cable, such as the middle connector J, fault Point F and open-circuit terminal B of the cable appear on the waveform in the form of pulses according to the actual distance. The nature and severity of the impedance mismatch (eg low resistance or open circuit) can be determined based on the polarity and magnitude of the pulse.

In general, open and short fault reflections are stronger, while intermediate joint reflections are weaker. The smaller the resistance value of the low-resistance fault, the stronger the reflection. The pulse reflection waveforms of several typical faults are described below.

(1) The open-circuit fault pulse produces total reflection at the open-circuit point, and the reflected pulse has the same polarity as the transmitted pulse. Open Fault Pulse Reflected Waveform The open fault pulse reflected waveform is shown. After the reflected pulse of the first fault point on the waveform, there are several reflected pulses whose distance is still the fault distance, which is the result of the pulses being reflected back and forth between the measurement end and the fault point for many times.

Due to the loss of pulse transmission in the cable, the pulse amplitude gradually decreases, and the wave head rises more and more slowly. What actually works is the first reflected pulse, be careful not to mistake subsequent reflected pulses for reflected pulses from other fault points. (2) Short-circuit fault The pulse generates total reflection at the short-circuit point, and the reflected pulse is opposite in polarity to the transmitted pulse.

The reflected waveform of cable short-circuit fault pulse is given. The pulse polarity after the reflected pulse at the first fault point on the waveform shows a positive and a negative alternate change, which is because the reflection coefficient of the pulse at the fault point is -1, while the reflection at the measurement end is positive. Short-circuit fault pulse reflection waveform (3) When a low-resistance fault occurs in the low-resistance fault pulse reflection waveform cable, the voltage reflection coefficient and transmission coefficient at the fault point are given by the formulaρu=-1/(1+2K) andγ=2K/(1+2K) is given, where K=Rf/Z0 is the ratio of fault resistance to cable wave impedance.

Figure 3.5 shows the relationship between the voltage reflection coefficient and the transmission coefficient with the value of K. It can be seen that K>10, that is, when the fault resistance is greater than 10 times the wave impedance value, the amplitude of the pulse reflection coefficient is less than 5%, and the reflected pulse at the fault point is difficult to identify, so the low-voltage pulse method is not suitable for measuring the distance of such faults. Low-resistance fault point voltage reflection coefficient and transmission coefficient variation law 6.c gives the low-resistance fault pulse reflection waveform before the fault point in the cable midpoint. It can be seen from the pulse propagation grid diagram (Fig. 3.6.b) that the injected pulse generates a reflected pulse at the fault point, and returns to the measurement terminal at t1. The pulse returns from the measurement terminal, and is reflected again at the fault point, and again at t2. Once back to the measuring end.

The distance between the reflected pulse of the second fault point and the reflected pulse of the first fault point on the waveform is the fault distance. In practical applications, care should be taken not to mistake it for a new fault point or joint reflection. The transmitted pulse passing through the fault point is reflected at the end of the cable and returns to the measurement end at time t3. The pulse reflection waveform of the low-resistance fault after the midpoint of the cable is shown in Figure 3.7, and will not be described in detail here.

Low-resistance fault pulse reflection waveform after the midpoint of the cable 2. How to demarcate the starting point of the reflected pulse The general low-voltage pulse reflection instrument relies on the operator to move a ruler or an electronic cursor to measure the fault distance. In actual testing, people are often not sure where the cursor should be positioned to demarcate the starting point of the reflected pulse. According to Section 2.7, the farther the fault point is, the smoother the reflected pulse rises and the more difficult the calibration.

In the actual test, the inflection point caused by the reflected pulse on the waveform should be selected as the starting point of the reflected pulse, as marked by the dotted line in Figure 3.8.a, or a tangent line from the front of the reflected pulse, which intersects with the horizontal line of the waveform, can be used as a reflection The pulse start point, as shown in Figure 3.8.b.