Research On Ultrasonic Detection And Localization In Partial Discharge Based On Fiber Fabry-Perot Interferometer

Partial discharge in power system will cause insulation damage and long-term partial discharge has great safety risks.In order to ensure the safe and stable operation of the power grid,it is necessary to monitor and locate the partial discharge online.Traditional partial discharge detection methods presented by Pulse Current method and Ultra-high Frequency detection method are usually complicated in system setup and unfriendly in cost.Even more,some of them need to remain power off to maintain the equipment,which undoubtedly brings unnecessary trouble and economic loss.Fiber-optic sensors have attracted more and more attention due to their advantages such as high sensitivity,compact size,low cost and great ability to resist electromagnetic interference.This paper will combine the advantages of fiber-optic sensors to design an ultrasonic sensor based on External Fabry-Perot Interferometer（EFPI）structure in which the gold-diaphragm is used as sensing element.The work will focus on detection of ultrasonic signal,detection and localization of partial discharge which are as follows:（1）Theoretical analysis of ultrasonic generated in partial discharge is made in both electrical and optical aspects.The ultrasonic in partial discharge is analyzed in generation principle and characteristics.The principle of FPI is deduced,and the reflection spectrum of low-fitness FPI is analyzed in detail.Also,two kinds of forced vibration models,“plate vibration model”and“film vibration model”,are analyzed in detail.The acoustic response sensitivity of the diaphragm is simulated and how the sensitivity influenced by the material,thickness and size are investigated.（2）The paper proposed an EFPI ultrasonic sensing structure based on gold diaphragm.The preparation of diaphragm and the fabrication of sensor probe are introduced.An ultrasonic sensing system is setup for testing.The phase sensitivity and minimum detectable pressure（MDP）of the sensor can be figured out.In the experiment,ultrasonic sensing is performed in both air and transformer oil to simulate the environment of gas insulated switchgear（GIS）system and power transformer.The phase sensitivities of the sensor to 30k Hz and 80k Hz ultrasound signals are 1.31rad/Pa（-117.68d B re 1rad/μPa）and 1.05rad/Pa（-119.55d B re1rad/μPa）.The minimum detectable sound pressures are 8.98μPa/Hz^1/2@30k Hz and 6.42μPa/Hz^1/2@80k Hz,respectively.The results obtained is much better compared with our previous work.The frequency response can be considered flatness in the frequency band detected and there is only response peaks in very small ranges near the resonant frequencies,which is consistence with theoretical analysis.At the same time,the sensing scheme is easy to fabricated,friendly in cost,convenient to implement,and has the advantages of high sensitivity and good noise performance.（3）A needle-plate partial discharge simulated system is setup to test the performance of the proposed ultrasonic sensor in partial discharge detection and localization.The ultrasonic generated in three stages of partial discharge,corona discharge,spark discharge and arc discharge,are analyzed in detail.The main analysis objects are its pulse repetition frequency,ultrasonic signal frequency,signal amplitude in time-domain and component amplitude in frequency-domain,etc.How the characteristics changed by the voltage applied between two electrodes is studied.The results show that the proposed ultrasonic sensor can analyze the strength of partial discharge qualitatively.In addition,the directional insensitivity of the sensor can be applied in the localization of partial discharge.After the simulation of traditional TDOA（Time of Difference Arrival）localization method,a new localization method based on amplitude ratio is proposed and analyzed in both simulation and experiment which shows that four sensors in array can obtain the position of the partial discharge.