Research On Partial Discharge Localization Method Of Transformer Based On Pressure Balance EFPI Sensor

Power transformer faults can be divided into three categories:mechanical faults,conductor faults and insulation faults,of which insulation fault accounts for the largest proportion.The IEC has established corresponding standards for transformer insulation testing,and partial discharge is considered to be one of the most effective detection methods.Partial discharge(PD)is a discharge phenomenon caused by breakdown of localized areas in an insulating medium of electrical equipment.PD testing of transformers can accurately reflect their health and aging,reduce the probability of safety accidents,reduce losses and avoid excessive maintenance costs.For high-voltage transformers with complex internal structures,the degree of insulation degradation is also closely related to the PD localization.Therefore,it is necessary to detect and locate the position of PDs inside the transformer.In addition,for transformer manufacturers and power grid operators,quickly and accurately locate PDs during the fault detection of the transformer,which is beneficial to shorten the inspection cycle and improve work efficiency.The PD localization method mainly has electric method,UHF method and ultrasonic method.The traditional ultrasonic method is based on the piezoelectric sensors.The sensors are attached to the transformer case to detect the ultrasonic signal generated by PDs.Due to the compact internal structure of the transformer,the ultrasonic signal will undergo multiple times of reflection during the propagation process,and will also be reflected in the transformer case.There is attenuation,especially when the PD occurs inside the winding,the detection accuracy of the piezoelectric sensor is poor,and the localization success rate is low.In order to overcome the disadvantages of traditional piezoelectric sensors,this paper designs and fabricates a new fiber-optic EFPI ultrasonic sensor with pressure-balance structure to explore the propagation law of ultrasonic signals inside the transformer,and locates the PDs in oil based on EFPI ultrasonic sensor array.The EFPI sensor is applied to the PDs location of the transformer winding.The main research work and conclusions of the thesis are as follows:(1)A new welded fiber EFPI ultrasonic sensor with a new pressure balance structure is proposed.The new structure of the EFPI ultrasonic sensor immerses the transformer oil into the F-P cavity,and the sensitivity of the sensor is improved by coating the inner surface of the diaphragm,which is suitable for harsh conditions such as internal pressure and temperature changes of the transformer.The sensor diaphragm has a thickness of 50?m and a working radius of 1.3mm.The center frequency of the transformer oil is about 30kHz,the temperature coefficient is 0.03nm/?,and the response is-185dB?-210dB in the frequency range of 20kHz?60kHz(Re.1V/?Pa).(2)The ultrasonic signal of the transformer is simulated according to the FEM,which reveals the propagation law of the ultrasonic signal inside the transformer.The multilayer coil structure of the winding causes multiple reflections of the ultrasonic signal,which is not conducive to PD localization.The EFPI sensor was used to measure the wave velocity in the oil.The amplitude of the ultrasonic signal decreased after passing through the oil-impregnated insulating paper,but the wave velocity changed little.After passing through the multilayer composite medium,the ultrasonic amplitude was seriously attenuated and the waveform was distorted.Through simulation and experimental verification,when the source has a direct path to the observation point,the ultrasonic direct wave signal can be detected.(3)The PDs localization in oil based on SRP algorithm and EFPI sensor array is proposed.The position of PDs in oil is realized by four-element array,it is found through experiments that the positioning of ultrasonic array for PD is heavily dependent on direct wave signal.The acoustic performance of the sensor array is analyzed.The acoustic performance of the four-element array of plane cross array,plane diamond array and regular tetrahedral array is compared.The complex cepstrum de-reverberation algorithm is proposed to improve the array.positioning accuracy.(4)It is proposed for the first time that eight EFPI ultrasonic sensors are layered evenly in the oil passage between the high and low voltage windings of the transformer,and the arrival time difference algorithm is used for positioning.The ultrasonic signals in the high-voltage winding turn,the low-voltage winding turn,and the oil passage are simulated and tested respectively,the laboratory licalization error is less than 5cm,which provides a new method for detecting and locating the PDs signal inside the transformer winding.