Fig. 1. Φ-OTDR system. (a) Functional block diagram; (b) photo of terminal equipment

Fig. 2. Φ-OTDR test environment and signal in ultra-high voltage direct current control and protection system. (a) Converter with optical fiber cable link; (b) optical fiber cable interface on the control and protection system; (c) Φ-OTDR terminal equipment placed in cabinet; (d) measurement signal displayed by oscilloscope

Fig. 3. Threshold level and sampling period signal processing results. (a) Threshold level; (b) relationship between SNR and number of sampling cycles

Fig. 4. Flow chart of optical cable link SNR and fault location

Fig. 5. Results of addition short-time over double-level rate method. (a) Results of original signal subtraction processing by Eq. (8); (b) comparison of results by two algorithms

Fig. 6. Results of multiplication short-time over double-level rate method. (a) Results of original signal subtraction processing by Eq. (8); (b) fault signal multiplication short-time over two-level rate

Fig. 7. Comparison of processing results for time-domain signal by three algorithms

Fig. 8. Signal processing results of frequency band by RSMD method. (a) Signal frequency band standard deviation at different positions; (b) relationship between SNR and in-band segmentation number