Impact Of Liquid Insulation On Fiber Fabry-perot PD-induced Ultrasound Sensor Characteristic Parameters

Partial Discharge(PD)is the main factor that causes the insulation destruction of the equipment.Online monitoring of insulation status of power equipment can detect latent faults,accurately predict and judge the location and time of insulation faults and then take relevant measures to reduce the accident probability and economic loss.Partial discharge acoustic method has the characteristics of high sensitivity and small electromagnetic interference.The advantage of fiber sensor is that it has small size and high precision measurement and positioning,and has the advantages of anti electromagnetic interference,has become one of the main methods of on-line monitoring of electrical equipment insulation.Extrinsic fiber Fabry-Perot(F-P)sensors have been used to detect Partial Discharge of liquid-solid composite insulated in power equipment.In spite of this,the influence of liquid insulation dielectric on the characteristic parameters of the Partial Discharge testing of the Fabry-Perot sensor is not proved.In this paper,the working principle of the extrinsic optical fiber F-P sensor and two important parameters-natural frequency and static pressure sensitivity of diaphragm are introduced,and the relationship between the diaphragm material and size with the natural frequency and the static pressure sensitivity is also analyzed.It provides a theoretical basis for selecting suitable diaphragm.The vibration characteristics of F-P sensor diaphragm in air and insulating oil were theoretically analyzed and amplitude frequency responses were simulated calculation.The influence of the temperature of the transformer oil on the amplitude frequency response of the sensing diaphragm was analyzed.The dynamic viscosity of transformer oil was tested by using viscosity tester.The ultrasonic testing system of extrinsic fiber F-P sensor was established of partial discharge.The amplitude frequency characteristic curves of the F-P sensor in transformer oil at 20,50 and 80 degrees were tested respectively.The encapsulation structure of the sensor was designed and the amplitude-frequency response of same sensor in air at 20,50 and 80 degrees were tested.The two groups of experimental datas were compared and concluded.Experimental results show that the least thickness of the diaphragm has the highest static pressure sensitivity when the natural frequency of the quartz diaphragm is a certain value.The transformer oil medium has a great influence on the vibration of the sensor,which can reduce the natural frequency of the diaphragm vibration,and the reduction coefficient is around 0.58.In addition,it can make the bandwidth of the amplitude-frequency response curve larger and reduces the peak response at the natural frequency.The temperature rise of transformer oil increases the natural frequency of diaphragm vibration and makes the bandwidth of frequency response curve narrow,but the change is not big.The natural frequency and bandwidth of the amplitude-frequency response of the encapsulated sensor are basically unchanged with the increase of temperature,but the peak frequency response is increased with the increase of oil temperature.