## 3 Traveling Waves (TW)  
In the event of dc fault on transmission line, transients of high frequency voltage and current waves are generated which tend to circulate between TL ends and fault point. These transients are known as traveling waves. [PERSON] and [PERSON] presented the phenomenon of traveling waves in 1950 for fault location estimation [49; 50]. Practically, TWs based techniques has replaced traditional reactance and impedance-based methods for fault location estimation because of its accuracy and high efficiency. However, TW based methods could not be applied on large scale due to shortcomings such as difficulty in detection of first TW wavefront (containing maximum information), measurement error in speed of propagated TW wavefront, inaccuracy in detection if dc faults appear near VSCs/terminals, or amplitude of dc fault TW is close to zero crossings. Traveling waves are mainly caused by the events of switching, faults, and lightning. Traveling wave technique is not effective for ac transmission networks due to small amplitude of the TWs [51]. TWs generated by the fault in a dc transmission line are different from the TWs in ac transmission line due to the dispersion and the attenuation occurring in AC TLs. Attenuation at rising edge results in reduced amplitude, while dispersion at falling edge makes TW difficult to be detected [52]. Fault location estimation with respect to voltage phase angle is not acceptable because of small fault angle [33; 53; 54]. Traveling wave-based protection system was first introduced in 1978, based on discrimination between forward and reverse waves [55]. Pick-up sequences of discriminant functions are used to find the direction of fault [56]. Correlation is used to determine the similarity between traveling waves generated from healthy and faulty locations within dc grid, helps in fault discrimination and identification [57]. Digital signal processing approaches are used to design a protection based on direction of propagation of traveling waves [58]. Surges due to faults are distinguished from others by correlation function-based traveling wave protection algorithm [59]. Precise detection and identification of wavefront arrival time at relays is the key in the assessment of fault location. High frequency components of terminal voltages are extracted by band-pass filter or by evaluation of current through the surge capacitors [34; 60; 61]. These high frequency components are employed to detect the arrival time of TWs of transients and faults. Required accuracy of results for detection of arrival time of wavefront is obtained by a high signal sampling rate, achieved through methods of digital signal processing and this is limitation of traveling wave method. However, this issue can be addressed through the latest transducer technology [53; 62]. Conventional time and frequency domain methods are not useful for detection of arrival time of wavefronts. However, wavelet transform is highly applicable for the analysis of wavefronts because of simultaneous localization capabilities of time and frequency, particularly for multi-terminal dc systems [63; 64; 65]. Discrete wavelet transform (DWT) is mostly applied to solve the research problems of dc line fault location Because of its speed, resolution, accuracy, and selectivity, [65; 66]. However, it is noted that continuous wavelet transform (CWT) is more efficient in generating detailed and continuous analysis for transients. In CWT, wavelet which is required to be analyzed is shifted precisely over the input signal domain, whereas in DWT, wavelet which is required to be analyzed is shifted in the dyadic grid (grid consisting of tiles of different width and length depending on actual time and frequency resolution of each partial DWT spectra component in the time scale plane [66; 67; 68]. Different dc fault location estimation methods in MT-HVdc systems are discussed in the subsequent sections, for different configurations, based on the parameters of traveling waves.