Abstract:The healthy state of insulation in oil-impregnated bushings is traditionally evaluated by tanδ and capacitance at power frequency and mostly at 10 kV in the test standard. However, there has frequently been insulation accidents induced by moisture ingress (MI) for bushings that have passed the standard. The mechanism and new diagnostic features for MI into bushings were not distinct enough and an accurate test method is urgently needed research. To address this technical gap, a bushing model with a transparent sheath was designed and an ultrasonic humidifier device was adopted to simulate the environment of MI in bushings and recorded by digital camera. The parameters of dielectric dissipation factor, capacitance, partial discharge (PD), frequency domain response, and moisture content in oil were measured at room temperature with time. The results presented that both the increment dissipation factor at low frequency of 0.001 Hz and the increment dissipation factor of 1.2 U m could be used for detecting the earlier insulation defect of oil-impregnated paper (OIP) bushings. The phase resolved partial discharge (PRPD) can serve as the diagnostic basis of the severe state (S 3 ) of insulation deterioration caused by MI into bushings around the phases of 0-117 • , 151-303 • , and 325-360 • . The research findings would provide a useful reference for the condition diagnosis and maintenance of OIP bushings. Especially, the increment detection of Frequency Domain Spectroscopy (FDS) at the frequency of 1 mHz and 10 kHz was recommended firstly for the operative bushings in real sites.

Oil impregnated paper bushing is the key equipment connecting a transformer and a power grid. Insulation deterioration may cause partial discharge, which poses a great threat to the safe operation of power systems. In order to realize the online diagnosis for partial discharge and automatic identification of insulation defects of transformer bushings, an ultra-micro-convolutional neural network with only more than 3000 parameters is designed, which adaptively extracts partial discharge characteristics based on small samples, so as to judge the defect category and the reasons. The accuracy rate can reach 97.1%, the computational complexity is lower, the real-time performance is stronger, and it can be easily deployed on various embedded platforms.

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Frequency domain spectroscopy (FDS) has been widely used to evaluate the moisture fault of oil-impregnated paper (OIP) bushings. However, there is no reliable method for the location detection of moisture faults in bushings, which frequently initiate its insulation failure. In this study, we developed a distributed FDS measurement method for OIP bushings with two taps. Three times FDS will be measured to acquire the FDS parameters from 0.001 Hz to 10 kHz between the rod (a) and end foil (c), rod and second end foil (b), and second end foil and end foil at room temperature over time. The results show that the C1mHz /C10kHz ratio of Cab is more sensitive for detection of the inner damp layer. The rate of C1mHz /C10kHz of Cbc is more sensitive to the outer layer damp bushings. The layer ratios of complex capacitance (V12/V1 and V2/V1) are key features for the detection of non-uniformly distributed moisture for a capacitance bushing.