Acoustic Sensing Of Partial Discharge For 10kV XLPE Cable Terminals Based On Fiber-optic Mach-Zehnder Interferometer

Cross-linked polyethylene（XLPE）cables are widely used in power systems.PD testing is an effective means to assess the insulation status of XLPE cable terminals（reported to be generally at the n C level）.To address the above problems,this paper analyzes the main factors affecting the sensitivity of PD acoustic fiber detection,develops an external PD acoustic detec-tion Mach-Zehnder interferometer（MZI）fiber sensor,a 10 kV XLPE cable terminal with a built-in PD acoustic detection fiber MZI sensor,and passes the PD The paper presents the char-acteristic quantity and threshold value of PD acoustic wave detection by fiber optic MZI,studies the pulse and energy of PD acoustic wave detection by fiber optic MZI,and compares the pulse repetition rate and energy of PD current detected by electric measurement methods such as high-frequency current transformer（HFCT）and pulse current method.The theoretical analysis of the principle and response characteristics of PD acoustic wave detection by optical fiber MZI is presented.Based on the PD acoustic wave band distribution,the acousto-optical conversion process of mandrel type fiber optic MZI sensor under the acous-tic field static pressure and pressure gradient model is analyzed.The axial attenuation of the XLPE cable and the detection characteristics of the fiber optic MZI are studied experimentally.Theoretical and experimental analyses show that the axial sensitivity of the mandrel-type fiber optic MZI sensor to acoustic pressure is higher than the radial sensitivity,and the fiber optic MZI sensitivity can be adjusted by adjusting the size,position,and fiber length of the mandrel-type fiber optic MZI sensor.The paper produced XLPE silicone rubber flat plate air gap specimens to simulate the internal PD of cable terminals developed external fiber optic MZI sensors,built a fiber optic MZI sensing system,and tested the PD acoustic waves of flat plate air gap specimens.The phase distribution of the fiber optic MZI sensing signal and the phase resolved partial discharge（PRPD）of the PD detected by HFCT have a good correspondence.The acoustic waves of the air gap PD of the flat plate specimen are mainly in the frequency band of 2 k Hz to 200 k Hz.The paper combines the characteristics of the mandrel-type fiber optic sensor and the struc-ture of the 10 kV XLPE cable terminal to develop a 10 kV cable terminal with a built-in fiber optic MZI sensor to detect the PD acoustic wave.The feasibility of the fiber optic sensor im-plantation was investigated by simulation and experiment.The simulation shows that the elec-tric field distribution and the maximum value of the field strength of the 10 kV XLPE cable terminal do not change significantly before and after the fiber optic implantation.The 10 kV XLPE cable termination with the fiber optic MZI sensor embedded at 1.73 U₀ passed the PD test according to GB-7354 test requirements.Using 2 k Hz to 200 k Hz as the characteristic fre-quency band of PD acoustic wave,the test detects PD of defects such as bubbles and knife marks inside the 10 kV XLPE cable terminal and compares the study with HFCT to detect PD.The sparse and dense demarcation line（SDDL）of the normalized phase diagram of the fiber optic MZI sensing signal is proposed as the PD acoustic characteristic quantity of 10 kV XLPE terminal with a diagnostic threshold of 0.5.The smaller the SDDL,the more intense the PD is characterized.It was found that fiber optic MZI detects PD of less than 50 p C at 10 kV XLPE cable termination,which is better than the current domestic and international acoustic fiber optic Sagnac interferometer（SI）and optical time domain reflectometry（OTDR）cable.The sensitivity of the terminal PD detection n C level and the detection performance of the starting discharge voltage is higher than that of HFCT.The paper investigates the relationship between PD acoustic fiber MZI detection charac-teristics and electrical detection of discharge energy,pulse repetition rate,oscillating wave,ul-tra-low frequency cosine square wave voltage PD detection in the field environment.The test found that PD unit energy corresponding to the fiber MZI acoustic energy increases as the dis-charge power decreases,proving that fiber MZI detection of tiny PD is more sensitive.Fiber optic MZI can detect PD under an oscillating wave and ultra-low frequency cosine square wave voltage in the field,and its acoustic pulse number and pulse duration are positively correlated with the number of PD current pulses detected by the pulse current method and HFCT.PD acoustic wave detection by fiber optic MZI can be inferred from the sample current pulse rep-etition rate.The performance of fiber optic MZI detecting PD below 100 p C in field tests is better than that of the pulsed current method.