## 21 Practical Realization of Traveling Wave (TW) Based Commercial Relays  
Traveling waves of current are being used to identify the reflections from the fault location, other end of TL or any discontinuity within TL [126; 127; 128; 129]. TWs of the neighboring stations can be eradicated with the help of reference data train of TWs generated from the external faults, line energization and auto reclosing and, switching of capacitor banks. This information is employed with TW generated during fault to find out propagation time and to estimate fault location on TL. The distance between sensing terminal and the fault point is calculated with the help of propagation time and velocity of TWs. Arrival time is determined by using differentiator smoother [127; 128]. In differential smoother, leading \begin{table}
\begin{tabular}{c c c} \hline \hline
**Serial Number** & **Fault Location Methods** & **Limitations** \\ \hline \multirow{3}{*}{1} & \multirow{3}{*}{Impedance Based Method} & Only Applicable for Permanent Faults \\  & & Error in fault estimation is about 10 percent \\  & & & of line length \\ \hline \multirow{3}{*}{2} & \multirow{3}{*}{Travelling Wave Method} & Not Good for Faults Near Terminals \\  & & & Not Good for Faults Having Waveform Very \\  & & & Close to Zero Crossings \\ \hline \multirow{3}{*}{3} & \multirow{3}{*}{Arrival Time Detection Method} & Detection of Wave-head is Difficult \\  & & & Velocity of Propagation Dependent \\ \hline \multirow{3}{*}{4} & \multirow{3}{*}{Frequency Based Detection Method} & Band-pass Filters are required for extraction \\  & & & of high frequency components \\ \hline \multirow{3}{*}{5} & \multirow{3}{*}{Transient Current Detection} & Surge Capacitors are required for \\  & & & generation of transient currents \\ \hline \multirow{3}{*}{6} & \multirow{3}{*}{Digital Signal Processing Based Method} & High Sampling Rate is Required \\  & & & \\ \hline \multirow{3}{*}{7} & \multirow{3}{*}{Pure Time and Frequency Based Methods} & Fault signal requires both high time and \\  & & frequency resolution \\  & & & Not suited for travelling or non-stationary \\  & & & waves \\ \hline \multirow{3}{*}{8} & \multirow{3}{*}{Discrete Wavelet Transform Based Method} & Less Computational Time (Feature) \\  & & Method \\ \hline \multirow{3}{*}{9} & \multirow{3}{*}{Continuous Wavelet Transform Based Method} & Full shifting of analyzing wavelet over the \\  & & full domain of input signal (Feature) \\ \cline{1-1}  & & Better Accuracy as Compared to DWT \\ \hline \multirow{3}{*}{10} & \multirow{3}{*}{Knowledge Based Methods Neural Networks Fuzzy Logic Bayesian Networks Analysis} \\ \cline{1-1}  & & & Failure in case of incomplete information of \\ \cline{1-1}  & & & fault \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Fault location methods and their limitations. edge is tracked by excluding signal distortion and allowing interpolation between samples. When incident TWs are passed through a junction point (tap) of TL, these TWs are reflected and transmitted in accordance with the characteristic impedance of TL segments. In a single end measurement-based method, time difference between first TW and successive reflected TWs are utilized for estimation of fault location. Sorting out of TWs is required when sensing terminals (VSCs) are closer compared to the distance between fault and sensing terminal of TL [127; 128]. Single end fault locator (SEFL) based on TWs is developed by [PERSON] et al. for HVdc system in 1984 [128]. SEFL separates the incident and reflected TWs for dc fault estimation. Practically, TWs based method of fault location is successfully implemented on Brasada-Harney transmission system, and Bonneville Power Administration's field [127], USA with least error. Attenuation and dispersion are the two phenomena associated with TWs generated in TLs and are caused by energy losses and variations in the inductances and capacitances of TL. It results in decaying of TWs magnitude which in turn, challenges the accuracy of fault location.