Traveling Waves - Reliability in Fault Location

Authors: Marco Antonio Fernandes Ramos, FURNAS Centrais Eletricas, Joaquim Américo Pinto Moutinho, Eletronorte, Carlos Alberto Dutra and Sergio Luiz Zimath, Reason Tecnologia, Brazil

Considering this, fault location on long lines, sometimes with series compensation, is becoming more important to restore the transmission availability to the system.
The Takagi fault location algorithm for one line end is the most common used today to inform where a fault is in a line, but relies on some mathematical simplifications that are not valid in all situations like non-transposed lines, weak sources, high fault resistance, parallel lines, series compensated lines and measurement errors. Present communication availability allows for use of data from both ends of the transmission line and the calculation of fault location is fundamentally similar to single-ended methods except that now a means exists to determine and minimize or eliminate the effect of fault resistance, non-transposed lines and weak sources.

However, measurement errors, line impedance errors and compensated lines still influence in the fault position accuracy obtained by this technique.
Field experience shows that accuracy of impedance based methods are in the range of few percent of the line length, and vary according to fault conditions, meaning that large errors are sometimes expected. This makes the repair crews search for several tower structures around the tower indicated by the impedance algorithm, thus raising the time needed to repair the line.
Fault location using travelling waves, although not new, is getting now more attention from utilities because  of its high accuracy. But once utilities use it, another characteristic becomes more important:

  • Travelling waves achieve high accuracy in all faults

Repair crew members noticed that fault locations coming from travelling waves methods are always in the informed location, and this raised their confidence to go only to the tower indicated, significantly reducing the time needed to repair the line.

Travelling Wave Fault Locator
The system is composed of two devices, one at each line end and connected to the voltage or current transformers. They are both equipped with a GPS receiver to time tag the exact moment the traveling wave reaches each end of the line.
This principle uses only the information of:

  • The cable length of the line (l
  • The propagation speed of the traveling wave (k
  • The time difference between the times of arrival of the traveling waves of each side (ta - tb)

The equation that is used to calculate the position of the fault is therefore:

                     Simplicity! There is no need for simplification in the equation!      (1)

When comparing to impedance methods, the first difference that can be seen is the absence of information of line parameters related to the nominal frequency of 60Hz or 50Hz.
In fact, the first thing that is done in fault locators based on the traveling wave method, is to filter the nominal frequency content by applying a high pass filter in order to completely eliminate it.

It is important to state the difference between line length and cable length, because usually the information about length of the line is composed by the geographical distance between all the line towers, which is good for impedance methods as they inform the distance proportional to the impedance of the line. But as the principle of traveling waves actually measures the cable length as considered in the equation, we have to take account of the cable length that is due line sagging, and also the cable from the potential transformer until the fault locator.

Figure 1 shows the line length that is normally considered by impedance methods, composed by the sum of the distances between all the towers of a transmission line.
Usually not even the connection between the last structure and the bus bar is considered as this makes not a big difference in the overall line length and causes no problem for distance protective relays that are designed to work by zones and not by the exact distance to the fault.

On the other hand, traveling wave fault locators are expected to present results with errors in the range of a few hundred meters or less.
In this case, we do have to consider all the cable length that is connected between the two fault locators, including the cable from the potential transformer to the terminals of the fault locator. In some cases there are distances of cable of 500 meters or more from the PT to the fault locator device installed in the panel. If we are talking of a device with an accuracy of a few hundred meters, this for sure would make a huge difference in the final result.

So, the correct distance provided by the traveling wave fault locating device is the sum of the cable length and the cables inside the substation as shown in Figure 2.
If we return to the equation (1), other important information that has to be provided to the algorithm is the propagation speed of the traveling wave. For overhead transmission lines this value is usually around 98% of the speed of light, and for underground cables it is more likely to be around 50% of the speed of light. This value can be calculated based on the line parameters, or measured using the fault locator itself. Although calculated values are close to the real value, a small difference can have a great impact on the accuracy, thus it is recommended to measure the propagation speed. The measurement of the “real” propagation speed can be done by generating a traveling wave with the opening of a breaker, or switching of a shunt reactor of the line for example.

Once the correct cable length of the transmission line is used in the equation, and as the exact position of the breaker or shunt reactor relative to the cable length should be known, is just a matter of adjusting the propagation speed in order to obtain the correct position of the device.
As for the time difference for the equation (1), this is obtained from the traveling waves generated by discontinuity in power like faults or switching of breaker, shunt reactors or the protection of a series capacitor.

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