The term “travelling wave based 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 phenomena used on transmission lines for the last two decade for accurate and consistent location of permanent and intermittent line faults typically to the nearest tower or span. Modern travelling wave fault recorders use a double ended (Type D) method for fault location that does not rely on operator intervention to determine 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 generates a travelling 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 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 Technique
The calculation of distance to fault is quite simple. If we know the speed of the travelling wave (its 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 travelling 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 travelling 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 travelling waves in power system joints. These can cause attenuation of the travelling waves. This means that there is a limit to the number of taps that can be accommodated in the line being monitored.
Why does travelling waves in power systems matter?
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 fault”. This is where a fault record taken from the relay or fault recorder was analysed. 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 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 ground therefore invalidating the impedance calculation and causing errors in the distance to 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 over all 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 down time and identifying trouble spots that can go on to cause further outages.
Step in the travelling 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.