Time Domain Reflectometry for Cable Fault Locating. Pulse Echo

Time-Domain Reflectometer RIF‑9

In the previous article we began introducing cable fault locating methods and discussed the surge wave generation, or thumping technique. Another time-proven cable fault locating method is time domain reflectometry (TDR), which has some variations, including the pulse echo, arc reflection, and impulse current. Here I am going to focus on the pulse echo method, its principle, benefits and associated issues.

The Basics of Pulse Echo

The pulse echo technique is performed with a time domain reflectometer (TDR), which combines a transmitter and a receiver. The transmitter sends out a low voltage high frequency pulse of a short duration onto a cable. This pulse of energy travels along the surface of the cable until it encounters some type of disruption manifested in a change within the characteristic impedance of the cable; these mismatches in the characteristic impedance of the cable may be caused by the start of the cable, splices in the cable, transformers, faults, etc. Depending upon the magnitude of the impedance change, either part or all of the transmitted energy reflects and travels back to the time domain reflectometer. In essence, this technique creates an electronic or a graphical roadmap of the underground cable showing different events along that path.


The elapsed time of a transmitted pulse traveling the entire length of a cable and the pulse reflections produced by deviations from the harmonious structure of the cable are shown on a screen. These reflections are then shown in a time sequence.

So, on the graphic display of a time domain reflectometer, the first event is the pulse or the start of the cable and then the first mismatch impedance, second mismatch impedance, etc. Everything is shown in a time domain format. Thus, time domain reflectometers make it possible to see into a cable, visualizing different cable landmarks such as splices, transformers, cable transition areas, the start and end of a cable, etc. In addition to this, TDR also provides the ability to take measurements of the cable length and relatively easily locate faults that are in series with the cable transmission path, such as a broken or open conductor, neutral or severe corrosion building up on a concentric neutral, separated splices or sealed off cable ends, etc. To a lesser extent the pulse echo technique can be used to locate a shunt cable fault or faults in the dielectric material in the cable, provided that the resistance of the insulation fault is of value less than 10 times the characteristic impedance of the cable being viewed.

Connecting the System to a cable under the test in «TDR» mode

Finding the Distance to the Cable Fault 

Measurements on a time domain reflectometer are made by measuring time. The TDR actually measures the amount of time it takes a pulse to launch out onto a cable, and then it starts to measure the elapsed time to when those reflections start to come back in. To find out the distance to where the pulse traveled, the TDR automatically takes the total amount of time that a pulse takes to leave and to return, divides that by half and then multiplies that by the speed at which the pulse is traveling.

A very important setting on the time domain reflectometer is velocity of propagation. Velocity of propagation is the speed at which high frequency pulses are traveling on a given cable and the speed at which these pulses travel will be influenced by the type of dielectric material and the thickness of the cross-sectional geometry of the cable.

Yet, even not knowing the details of the given cable, like its velocity, total length, and not knowing the particularities of the circuit, it is still possible to get very accurate results using a time domain reflectometer by simply taking advantage of the fact that the cable has two ends. To find a cable disruption in this case, it is necessary to measure the distance to the fault from both ends with the same velocity value; even if the velocity value is wrong, the fault will be pinpointed accurately.

To locate the fault, the distance should be first measured from end “a” and then marked on the ground. After that, the distance to the fault should be measured from end “b” and marked on the ground as well. In result, if the velocity was set too low, the fault will be undershot from both sides, with the true fault location being between the two markers. Naturally, if the velocity was set too high, the fault is going to be overshot from both sides, with the true fault location still being between the markers. Hence, even with very little information about the cable and no knowledge of the right velocity value, accurate results can be received by simply doing bidirectional measurements. When measurements are made on a longer cable, the total cable length can be divided into smaller sections.

Advantages and Disadvantages of Pulse Echo

The strong point of the pulse echo technique is that it enables relatively easy and fast faultfinding. All the results are shown on the TDR screen, which means there is no need to walk along the cable path tracking the fault. Another important advantage is that this approach uses safe low voltage pulses, preventing stress to the cable and making the process safe for technicians. The time domain reflectometer can be used on various cables all the way up through a low voltage, medium voltage and high voltage on electrical power cables. The obtained pulse echo signatures can be used for diagnostic values. For instance, to examine the splices within a three phase circuit, it is possible to capture all three phases, overlay them together and examine some of the issues that may be occurring on one phase as occurring on another phase.

One of the drawbacks of the pulse echo technique is that traces that result from complex circuits can be difficult to interpret. For example, in a circuit with several “T” or “Y” connections, when the reflections from them are overlaid, it can result in a very complicated wave form. This means that it may be very difficult to make meaning of all these reflections. The recommended practice in this situation is to use differential pulse echo: basically, a picture of a good cable phase is taken, and then a picture of the faulted cable phase is made. Everything that is common, all the areas where they share common “T” and “Y” connections, transitions, etc., all these images will be the same. So, the two common data points from these two waveforms are subtracted, which will display the points of difference or potential fault on the bad phase.

Another problem is that it is impossible to locate with pulse echo alone a dielectric fault or insulation fault where the resistance value is ten times or greater the characteristic impedance of the cable being viewed. In this case the recommended technique is arc reflection, and it will be discussed in the following article.

Alexei Tiatiushkin

Marketing manager

KharkovEnergoPribor Ltd.

marketing@keppowertesting.uk

http://www.kep.ua