Fig. 1. Ultrasonic detection based on interferometric effect of Rayleigh scattering light in optical fiber

Fig. 2. Experimental setup for discharging fault detection of cable joint with Φ-OTDR fiber-optic sensors

Fig. 3. σ_r-Z_c curve obtained by distributed location detection using Φ-OTDR fiber-optic sensing system for acoustic wave generated by partial discharging in power cable joint with defect. Test voltage is 30 kV, and electrically calibrated partial discharge quantity is 4929 pC. Inset is enlarged diagram near acoustic wave monitoring position

Fig. 4. Phase-resolved partial discharge diagram (PRPD) obtained by partial discharge measurement with power frequency alternating voltage on cable joint with defect. Test voltage is 30 kV. (a) PRPD obtained by acoustic measurement method based on Φ-OTDR fiber-optic sensing system; (b) PRPD obtained by electric measurement method based on partial discharge measurement system in compliance with GB/T 7354-2018

Fig. 5. Variation of variance of location parameter σ_r of acoustic wave monitoring positions on cable joint with test voltage. Acoustic monitoring time is 100 ms

Fig. 6. σ_r-Z_c curves obtained by location calibration experiments in which five sensing fiber loops designed in Table 1 are mounted on ultrasonic monitoring positions

Fig. 7. Normalized amplitude varying with propagation distance according to acoustic wave attenuation formula u=10^-αz. At room-temperature of 20 ℃, the attenuation coefficients for XLPE and LSR are 0.74 dB/cm and 0.35 dB/cm, respectively, for the single frequency acoustic wave with frequency of 200 kHz^[21]

Table 1. Winding and layout parameters of five sensing fiber loops