Characterization Of High Voltage Dielectric Spectrum Of Oil-paper Insulation Degradation

OIP(Oil-impregnated paper)bushings are a common type in power systems and are widely used in transformers of 110kV and above.In practice,with the increase of bushing operation,its insulation will deteriorate to different degrees and is very prone to operational failure.The current non-destructive testing method for bushing insulation is frequency domain dielectric spectroscopy.However,due to the limitations of the dielectric spectroscopy device,which is mostly performed at a test voltage of 200V,the test is easily disturbed in the field,resulting in measurement errors.At the same time,insulation degradation will show nonlinear characteristics at high voltage.Using this characteristic high voltage dielectric spectroscopy has been successfully applied in the diagnosis of water tree defects in cross-linked polyethylene cables,however,it has not been widely used in the deterioration of oil-paper insulation.Based on the above phenomenon,three main aspects of research work are carried out in this paper.Firstly,the 10kV high voltage frequency domain dielectric spectrum measurement device and complete set of software are successfully developed to realize the accurate testing of dielectric spectrum curve at 10kV.The hardware is integrated with signal generator,high voltage amplifier,microcurrent sensor,homemade control board and signal acquisition card;the software includes three functional modules:data acquisition and processing,control of test loop and display of dielectric parameters.Through performance testing,the output voltage waveform of the device can be flexibly adjusted in the range of 0～10kV and 0.001 Hz～10Hz,and the maximum standard deviation of the dielectric value is 7.88×10-4,and the relative error of measurement is within 5%.In order to ensure the feasibility of field application,the high-voltage dielectric spectrum test was conducted for the actual 110kV casing,and the results showed that the maximum standard deviation of the dielectric loss value was 2.79×10-3,the data fluctuation was small,the maximum relative error of measurement was 6.68%,and the maximum absolute error was 0.001 98.Second,a model of oil-paper insulated bushings with different insulation degradation degrees was prepared,and the high-voltage dielectric spectrum characteristics of the bushing model were studied by combining the test temperature and test voltage,and a high-voltage diagnostic database of oil-paper insulated bushings covering different insulation states was established.The results show that 1)the 10 kV dielectric spectrum curve of casing increases with the deepening of insulation degradation in the frequency range of 0.001 Hz to 10 Hz;2)the overlapping standard curve can be obtained after temperature normalization for damp casing,but not for dry casing,so the simple frequencytemperature leveling method is not universal for oil-paper insulated samples;3)the FDS curve of casing without insulation defects does not change significantly with increasing voltage 4)As the insulation deterioration increases,the frequency point of voltage correlation of the curve moves to high frequency:the dry casing without aging shows only a weak voltage correlation at 0.01 Hz,while the aging casing with 4.121%moisture content for 28 days starts to show voltage correlation at 10Hz.5)No significant difference in the low frequency dielectric loss of dry casing under the excitation of cyclic voltage 0.2kV-10kV-0.2kV,but the rate of change of dielectric loss at 0.001 Hz increases with the increase of moisture content:the rate of change of dielectric loss of casing with 1.344%moisture content is negative,while the rate of change of dielectric loss of casing with 3.93%moisture content is 63.77%,so there is a voltage dependence of the damp bushing.Finally,based on the 10kV dielectric spectrum curve of the database,three diagnostic feature quantities characterizing the degree of moisture and aging state are extracted:the extended Debye model feature quantity Dx,the integrated value Tx of the full frequency band of the dielectric loss curve,the integrated ratio Rx of the low frequency to the full frequency band of the complex capacitance real part of the curve and a validation feature quantity:the ratio Cx of the low frequency complex capacitance real part to the high frequency.25 insulating intervals are divided according to the degree of casing insulation deterioration The relationship between the characteristic quantity and insulation state in each interval is established,and a quantitative diagnostic evaluation method for oilpaper insulated bushings at 10 kV is proposed.The error of insulation diagnosis results is less than 10%by the method verification database.The maximum relative error of the insulation diagnosis results of the laboratory casing model samples is 2.6%,which verifies the effectiveness of this insulation diagnosis method.