147 / 2018-08-25 14:23:08

Time Domain Filtering Technology for Polarization and Depolarization Currents of Dielectrics

Polarization and depolarization currents,time domain filtering,dielectric,log-partitioning fitting

Polarization and depolarization currents of insulation dielectrics are the currents flowing through the insulating dielectrics during application of step voltage and short circuit. Polarization currents also include instantaneous displacement currents, polarization absorption currents and steady-state conductive currents. Depolarization currents include instantaneous displacement currents and absorption currents. The macroscopic parameters (conductivity, permittivity and loss factor) and microscopic parameters (susceptibility, space charge) can be obtained by measuring polarization and depolarization currents, and then the internal microstructure corresponding can be inferred, which is an effective ways to understand the physical nature of dielectrics. At present, most scholars apply the polarization and depolarization current to the diagnosis of aging of insulation. The insulation is equivalent to the extended Debye model, and then the relationship between the model parameters and aging is solved according to the current results to judge the aging state of insulation. However, in the process of measuring polarization and depolarization currents, there will always be some interference in both laboratory and on-site testing, such as spatial noise in the external space, the interference of testing instruments and the undulation of DC high voltage caused by high voltage power. Therefore, how to reduce the influence of interference and extract the accurate polarization and depolarization currents is particularly important for subsequent analysis of dielectric properties correctly.
Through reading lots of literatures, some of scholars analyzed the polarization and depolarization currents by the original measured data, and others scholars filtered the results of polarization and depolarization currents, but they have not pointed out the filtering algorithm and filtering effect in detail. The algorithm and effects of filtering will affect the follow-up analysis of experimental results directly. However, there was little research about the filtering technology of polarization and depolarization currents.
In this paper, time domain filtering technology for polarization and depolarization currents of dielectrics was studied. Firstly, the polarization and depolarization currents of common insulating materials are measured in laboratory. Then some common filtering algorithms were used to filter the test results in time domain, and the filtering effects of different filtering methods are compared and analyzed in detail. The filter results show that some common filtering algorithms ware not suitable for the polarization and depolarization currents. Considering that the range of the currents is about 2-3 orders of magnitude in the measurement time and the currents become relatively weak when they tend to steady state, a method of log-partitioning fitting filtering is proposed. Firstly, the polarization and depolarization currents were taken logarithm to narrow the range of the currents. And then the test results are divided into n time intervals, and each time interval increases with the testing time, to ensure that the currents values at each time interval are subject to the same change rule. Finally, the currents in each time interval were fitting by functions to eliminate the interference.
The filter results show that all the values of filtering are within the fluctuation range of the test results, and the curve of fitting was very smoothing. This means the filter method proposed in this paper is effective and accurate for polarization and depolarization currents, and can be used to filter polarization and depolarization currents in the future.