Frequency Domain Dielectric Response Research Articles

Extraction of Frequency Domain Dielectric Characteristic Parameter of Oil-paper Insulation for Transformer Condition Assessment

In modern power transmission and distribution systems, power transformers are critical components and their electrical properties together with service lives are determined by the insulation property of oil-paper insulation. To better apply the frequency domain spectroscopy technique to assess transformer insulation condition, a series of experiments were performed under controlled laboratory conditions. First, this article presents the frequency domain dielectric response measurement results of oil-paper insulation samples under different moisture contents and aging states, and then the extracted frequency dielectric characteristic parameters determined from the imaginary part of relative permittivity (ϵr′′) and dielectric dissipation factor (tanδ) curves are used to assess the moisture contents and aging states of oil-paper insulation. Finally, the quantitative relationships among dielectric characteristic parameters, moisture content, and degree of polymerization are also established. Results show that in the frequency range of 10–3–102 Hz, ϵr′′ and tanδ obviously increase as the moisture content of oil-paper insulation rises, while ϵr′′ and tanδ are very sensitive to aging only in 10–3–10–1 Hz. Consequently, ϵr′′ and tanδ in different frequency ranges can be used to distinguish the influences of moisture content and aging state on frequency domain spectroscopy measurement results.

Quantitative analysis of insulation condition of oil-paper insulation based on frequency domain spectroscopy

In order to distinguish the influences of moisture and aging on the frequency domain dielectric response of oil-paper insulation and better apply frequency domain spectroscopy (FDS) to assess the insulation condition of power transformers, the oil-paper insulation samples with different moisture contents and different aging states were prepared in the laboratory. The FDS of the samples were tested and a group of characteristic parameters were extracted from dissipation factor (tanδ) curves which could be used to assess the moisture content and aging states of oil-paper insulation respectively. The quantitative relationship among characteristic parameters, degree of polymerization (DP) and moisture contents (Km.c) was accurately established. The observations show that the proposed characteristic parameters are sensitive to the moisture in 10-3-102Hz, while the aging states influence the characteristic parameters in 10-3-10-1Hz. Meanwhile, an exponential relationship equation which could be used to assess the oil-paper insulation condition was established among the characteristic parameters, DP and the moisture content. Finally, the evaluation technique proposed in this paper was used to diagnose the insulation condition of several field transformers in this way, and its validity was preliminary and reasonably verified.

Distribution of relaxation times: An inverse problem

This article is a slightly amended and extended version of a short paper presented at ISE14 2011. The details of inverse problem solving method developed by the author are presented with examples consisting in the distribution of relaxation times in the physics of dielectrics. Frequency and time domain dielectric responses were used from synthetic and experimental data respectively. The method can be applied to other problems related to response theory in other research fields.

Stator end-winding currents in frequency-domain dielectric response measurements

This is a study of the components of capacitance, loss and harmonic currents that the nonlinear 'corona protection' coating used on stator end-windings adds to frequency-domain dielectric measurements. The work is based on measurements on simple laboratory models, compared to one-dimensional numerical models having discrete or continuous parameters and linear or nonlinear resistance. The necessary components and parameters of the numerical models are discussed by comparison with the measurements. Commonly used models of the conductivity of the SiC-based nonlinear material are compared to dc measurements on samples. The studied range of the applied voltage has amplitudes from low values up to the levels of operation and testing of stator windings, and frequency from line-frequency down to millihertz.

On the frequency domain dielectric response of oil-paper insulation at low temperatures

Results of Frequency Domain Spectroscopy measurements are known to be largely influenced by environmental conditions, such as the temperature. Because field measurements, last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care are required to interpret the results when performing tests at relatively low surrounding temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behavior of the insulation system in response to the ambient conditions. In this contribution, the dielectric behavior of a composite oil paper insulation system has been explained from the properties of Debye basic model. A series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. The equivalent circuit parameters of a laboratory made oil paper condenser bushing model were obtained using a non-linear optimization procedure. Since the dielectric parameters values are geometry dependent, poles, calculated from resistances and capacitances, were used as they are independent of the geometry. It was shown that the poles can be regarded as parameters able to be used for insulation condition assessment.

A new dielectric response model for water tree degraded XLPE insulation - part b: dielectric response interpretation

Dielectric response measurements, in both time and frequency domains, can generate valuable information about the condition of cables affected by water tree degradation. However, the interpretation of how these dielectric response measurements relate to water tree density and length is a difficult task. This difficulty in assessing in particular the longest water tree length from dielectric response measurements is a substantial limitation, as it is well known that the remaining electrical strength of a water tree degraded cable is related to the longest tree length. This paper will detail a study whose goal was to enhance the understanding of how water tree degradation (in particular long vented trees) influences the dielectric response, and therefore seeks to reduce this interpretation limitation. The paper examines the application of a finite element model for water tree degraded insulation. This application involves using the dynamic electrical behavior as predicted by the model to generate frequency domain dielectric responses. The predicted electrical behaviour of both bow-tie and vented trees and the dielectric response that such trees will produce is examined and compared to actual Frequency Domain Spectroscopy (FDS) measurements on water tree degraded cable samples. It is shown that the developed model can accurately reproduce such measurement behavior, including the non-linear response. Because of the success of the model in reproducing actual dielectric response measurements of water tree degraded cables, it has proven itself to be a useful interpretive tool for water tree detection and assessment.

Structure order-disorder and dielectric response in perovskite-related SrO-Fe2O3-Nb2O5 system: The oxygen-deficient composition Sr(Fe1−xNbx)O2.5+y (x=0.17, 0.25, 0.50)

Microstructures of materials with composition Sr(Fe1−xNbx)O2.5+y (x=0.17, 0.25, 0.50) have been investigated by powder x-ray diffraction (XRD) and transmission electron microscopy (TEM) examinations. XRD data for ceramic samples annealed under different P(O2) conditions could be indexed to a cubic perovskite. Superlattice diffraction spots have been observed by TEM investigation for the samples which were re-equilibrated by a subsequent annealing in a reducing atmosphere. A microdomain model is proposed to describe the observed phenomena. According to the model, the structure of reduced Sr(Fe1−xNbx)O2.5+y with x=0.17, 0.25 is considered to be a result of disordered intergrowth between brownmillerite-type Sr2Fe2O5 domains and perovskite-type Sr(Fe0.5Nb0.5)O3 domains. Brownmillerite-type domains are formed because of vacancy ordering. The order is extremely short ranged. Electron diffraction patterns constructed in terms of this microdomain model agree well with the observed results. The order-disorder transition can be altered by varying ambient oxygen partial pressures. For both ordered and disordered samples the frequency-domain dielectric response as a function of temperature is reported. Directly measured complex conductivity Y(ω) has been transformed by Kramers–Kronig relations in order to subtract the disturbances of G(0) and C(∞). The transformed temperature dependences have been further normalized and re-expressed in the form of master curves. For all the samples measured, the frequency-domain dielectric response deviates from the Debye model but follows the so-called universal power law behavior. The dielectric spectra for the samples in which brownmillerite-type ordering is identified show an anomalous low-frequency dispersion, while for the disordered samples, the relaxation behavior with a loss peak is observed. The variation of spectra shape with composition indicates that increasing Nb content tends to weaken the cooperative many-body interactions and causes the dielectric behavior to approach the Debye model.