| Distributed feedback fiber laser(DFB-FL)has the advantages of narrow line width,high detection sensitivity,and small size when it is used as a partial discharge(PD)acoustic emission(AE)sensor in power equipment.Compared with the interferometric fiber optic sensor,its sensing unit is independent of the demodulation system,which is beneficial to the long-distance monitoring of power equipment.Compared with the EFPI fiber optic sensor,its fully insulated structure and broadband detection characteristics are conducive to capturing more complete PD information in a high-voltage field.However,the research on DFB-FL mainly focuses on the field of low-frequency underwater AE detection.Compared with underwater AE detection,the AE caused by PD in power equipment is higher and its energy is lower.The performance of DFB-FL is greatly restricted by external interference,the sensor’s intensity noise in the PD detection frequency band,and the external optical feedback(EOF)noise in long-distance applications.In addition,the widely used intensity demodulation(ID)method also greatly limits the reliability of DFB-FL detection.To solve the above problems,this thesis is guided by the PD detection application of DFBFL,which conducts an exploratory study on the noise mechanism and system structure of DFB-FL in different application scenarios.Firstly,the feasibility of applying DFB-FL to PD detection of power equipment is judged by the theory of limit noise and the acoustic pressure model of DFB-FL.A DFB-FL is used as the sensing unit,which has a 3 cm equivalent cavity length and106.8 μPa minimum theoretically measurable acoustic pressure.By optimizing the Michelson fiber interferometer,a demodulation system with a non-equilibrium length of 50 m is obtained.The basic structure of the DFB-FL fiber AE sensing system which can be used for high-frequency AE detection is proposed.According to the external interference and intensity noise suffered by DFB-FL during PD detection,a 60-300 k Hz bandpass filter is used to remove the external interference.The intensity noise model of DFB-FL is established,and the relationship between pump power and intensity noise is defined.The Erbium-ytterbium co-doped DFB-FL,which has easily regulated intensity noise,is selected as the sensing unit,and the intensity noise of DFB-FL is suppressed by the frequency division suppression method.DFB-FL fiber AE sensing system with high stability is designed and obtained.Experimental results show that the average detection sensitivity of the high-stability DFB-FL fiber AE sensing system is about 37.15 times that of the traditional PZT sensor in the detection frequency band of 60-300 k Hz.To solve the problem that the performance of DFB-FL is reduced in long-distance applications due to the characteristics of the intrinsic light source,the EOF theory is applied to clarify the relationship between the EOF coefficient and the emission line width of DFB-FL.An optical feedback suppression method is proposed to stabilize the emission line width of DFB-FL by placing an optical isolator at a distance of 0-200.6m from DFB-FL.In addition,an optical feedback introduction method is proposed to reduce the emission line width of DFB-FL by constructing an EOF modulation structure using a mirror and a 1×2 coupler.According to the above two methods,an EOF suppression(S-EOF)system and an EOF introduction(I-EOF)system are constructed respectively.Experiments show that the S-EOF system has a more stable detection performance under the condition of large discharge capacity,and its average response SNR is 3.08 d B higher than that of the I-EOF system and 5.36 d B higher than that of the PZT.The I-EOF system has a higher detection sensitivity,and its PD detection onset voltage is 21.8% lower than that of the PZT,and 45.6% lower than that of the S-EOF system.Finally,to further improve the sensitivity and stability of the DFB-FL fiber AE sensing system.Considering the many factors affecting the output of the coupler,the demodulation system is improved by using the structure of an asymmetric 3×2 coupler with a large-value low-frequency phase modulator,and an improved method suitable for PD AE signal demodulation is proposed based on the NPS method.It is verified by experiments that the improved method can achieve effective demodulation for both continuous and instantaneous signals of specific frequencies.Under the same AE signal excitation,the average SNR of the improved method is 6.11 d B higher than that of the traditional ID method.The improved method can successfully solve the widely used ID methods’ problem of working point drift caused by changes in external factors such as temperature. |
