Development Of High Voltage Frequency Domain Dielectric Spectroscopy Measuring Device

Oil-impregnated paper?OIP?bushings are the most commonly used type of bushings in power systems and are widely used in transformers with voltage ratings of 110 kV and above.Moisture defect is the main cause of OIP bushing failure.At present,the commonly used diagnostic method for moisture defect is the frequency domain dielectric response method.But the traditional dielectric spectroscopy measurement technology mostly apply a test voltage below 200 V,which leads to two shortcomings:First,the response current is quite small and is easy to be affected by environmental noise during the field test,resulting in large measurement error.Second,the dielectric loss will change with the voltage when the insulation contains moisture defect,while at low voltages,the change of dielectric loss is too small and can be easily covered by measurement error.Thus,this change may not be detected,especially for slight moisture defects.In order to improve the anti-interference ability of the device and the sensitivity of moisture detection,a high-voltage frequency domain dielectric spectroscopy measuring device for moisture diagnosis of OIP bushings is developed in this thesis.The main technical difficulties of high-voltage dielectric spectroscopy measuring devices include measurement circuit design,device architecture design,high-precision measurement and moisture diagnostic criteria,etc.Firstly,the dielectric response of different signal injection points and measurement points is analyzed through simulation.Then it is determined that the testing frequency range is 10 mHz?1 Hz,the highest test voltage and the maximum measuring power is 10 kV and 20 W,respectively.For the device architecture,the high-voltage filter circuits and the sensor protection circuits are designed for the problem of high power supply noise and potential breakdown risk in high-voltage dielectric spectroscopy testing.In order to improve the measurement accuracy,the real-time calibration method is adopted for the calibration of micro-current sensor to eliminate the influence of temperature change.The data processing algorithm,signal acquisition method and high-voltage dielectric spectroscopy test flow are optimized through comparative analysis and actual test.The development of dielectric response analysis software based on least squares fitting algorithm for waveform initial phase analysis was completed.Finally,through moisture injection and high-voltage dielectric spectroscopy test of the 40.5 kV and 15 kV OIP bushing models,the corresponding relationship between bushing insulation moisture content and high-voltage dielectric spectroscopy data is obtained.The same 40.5 kV bushing was tested with the IDAX 300 dielectric response analyzer and the high voltage dielectric spectroscopy measuring device developed in this paper.Based on the data aquired by ID AX 300,the absolute measurement error of the dielectric loss angle measured by the high voltage dielectric spectroscopy measuring device is less than 1.15×10^-3 rad,and the relative measurement error of which is less than 9.22%.The maximum absolute measurement errors of the real and imaginary parts of the complex capacitor are 16.77 pF and 0.585 pF,respectively,and the maximum relative errors are 8.59%and 13.08%,respectively.The result of moisture diagnose test conducted on the 40.5 kV bushing model indicate that when this type of bushing contains traces of moisture in part of the capacitor core,the commercial instruments may obtain erroneous diagnosis conclusions,while this defects can be detected using high-voltage dielectric spectroscopy technique.The result of moisture diagnose test conducted on the 15 kV bushing models show that when the bushing is well dried,the dielectric spectroscopy data remains almost unchanged with the increase of voltage;For the bushing with incomplete drying,the low-frequency dielectric loss rises increases linearly with voltage,but the magnitude of the increase is small;For the undried bushing,the low-frequency dielectric loss also increases linearly with the voltage increase,and the magnitude of the increase is quite large.Linearly fitting the curve of dielectric loss with voltage,the curve of ?tan?/?U with paper water content is obtained.It is found that ?tan?/?U is positively linear with water content.The test results prove that the high-voltage dielectric spectroscopy technology can be used for the moisture diagnosis of OIP bushings.