Influencing Factors And Improving Methods Of Dielectric Loss Measurement Stability At 0.1hz

The measurement of cable dielectric loss at 0.1 Hz ultra-low frequency voltage is more conducive to the detection of cable aging,damp and other defects,which has been applied in large-scale power grid.However,the current dielectric loss measurement technology under 0.1 Hz ultra-low frequency voltage is still monopolized by foreign enterprises,and the instruments and equipment rely on imports.The factors affecting the stability of dielectric loss measurement and the methods to improve it are not studied enough.One of the important reasons is that the key technologies of measurement stability control are not fully mastered.Firstly,in terms of hardware,the measuring loop of digital Xilin bridge is designed and the parameters are optimized.Then in the aspect of software,the full phase Fourier decomposition algorithm is used to effectively suppress the spectrum leakage and reduce the interference of fence effect,and the stable phase difference results are obtained.Finally,the effectiveness of the proposed method is verified by comparing the measurement with that of foreign equipment.The results show that:1)The ultra-low frequency power supply uses PWM to control the breaking of the thyristor to generate high frequency signals to form the ultra-low frequency signals.The use of resistance and capacitance voltage division measurement circuit will form the effect of RC filtering,resulting in the waveform collected on the resistance is a high frequency signal,hinders the measurement.The software algorithm of dielectric loss measurement under ultra-low frequency is not suitable for direct use of time domain comparison.Time domain signal comparison requires relatively dense data points,so it requires a high sampling rate,but the number of data points is not high,and the use of high sampling rate will lead to the waste of data points.Moreover,the time domain comparison method is prone to interference,so it is difficult to meet the stability requirements in the actual measurement process.2)Pure capacitance partial voltage is used to replace resistance and capacitance partial voltage,and the collected voltage signal is converted to calculate the phase of the voltage at both ends of the cable and the current flowing through the cable through the software algorithm,which can greatly reduce the difficulty of measurement.The software algorithm adopts all-phase Fourier decomposition,which has a good ability to suppress spectrum leakage,and reduces the interference of fence effect by adjusting the sampling rate.By calculating the signal phase in the frequency domain,relatively stable results can be obtained.3)The hardware influencing factors of stability measurement are mainly the channel resistance of the acquisition card and the dielectric loss value of the high-voltage capacitor C1.The phase difference obtained by direct measurement is not the required dielectric loss Angle,and the stable dielectric loss Angle can only be obtained by using the software algorithm to correct the error combined with the vector diagram.