The term “traveling wave fault location” refers to the method of locating a fault or disturbance on an overhead or underground cable that is used to transmit power across an electrical network. Taking a closer look at traveling wave fault location, this phenomenon used on transmission lines for the last two decades for the accurate and consistent location of permanent and intermittent line faults typically to the nearest tower or span.
Modern traveling wave fault recorders use a double-ended (Type D) method for fault location that does not rely on operator intervention to determine the distance to fault. Results are automatically calculated and immediately available for use. The power arc at the fault site and the resulting step change in voltage generate a traveling wave that propagates along the line in both directions to the line ends. TWS fault locators positioned at the line ends accurately tag the arrival time of the waves using GPS as a reference. These time tags are sent to a central location where they are used to calculate the distance to fault using the line length and the velocity of propagation. Further details are given in Fig 1.
Figure 1 Type D Traveling Wave Fault Location Technique
The calculation of the distance to fault is quite simple. If we know the speed of the traveling wave (close to the speed of light) and the length of the line being monitored, then we can work out the distance to fault by using the time difference of the arrival times of the traveling waves at each end of the line.
Distance from end A = [Line length + (Time end A-Time end B).v] / 2
Similarly distance from end B = [Line length + (Time end B-Time end A).v] / 2
Where “v” = propagation velocity of the travelling wave.
Note, the application has generally been applied to traveling waves in transmission lines due to the simple two-ended nature of the single line diagram. However, it can also be applied to more complex sub-transmission lines where taps and multi-ended circuits are more common.
In these scenarios, it is necessary to take note of the reflection and refraction of the traveling waves in power system joints. These can cause attenuation of the traveling waves. This means that there is a limit to the number of taps that can be accommodated in the line being monitored.
Why is traveling wave fault location important?
Historically finding the location of a fault on a power line was done by protection relays and fault recorders using a technique called “Impedance based distance to a fault”. This is where a fault record taken from the relay or fault recorder was analyzed.
A fault would generally last a number of milliseconds before the protection system kicked in and cleared the fault. During this time the voltage would dip and the current would rise. By using the value of the voltage and current at the time of the fault it would be possible to calculate the impedance at this moment in time.
Assuming all of this impedance is caused by the cable and the cable manufacturer provides the impedance per unit length it is then possible to calculate how much cable is in play and therefore how far it is to the fault location.
However, this method is fraught with errors. If the fault is caused by a high impedance object such as a tree, this would create extra impedance to the ground, therefore, invalidating the impedance calculation and causing errors in the distance to the fault. Other factors also come into play such as incorrect line parameters, mutual coupling, unstable fault arc, etc. These cause inaccuracies that could put a distance to fault calculation out by 1-15% of the overall line length.
If we imagine a 100km line that is an error between 1k and 15km. Therefore, a more accurate fault location method is necessary to reduce manpower, reduce time searching, reduce downtime, and identifying trouble spots that can go on to cause further outages.
Step in the traveling wave fault location method. It is immune to high resistance faults, incorrect line parameters, unstable fault arcs, and mutual coupling. In addition, the accuracy is to within one tower span no matter what the line length.
So again thinking of a 100km line, that’s accuracy to +/-150m.