Depolarization Currents Research Articles

Application of L1 Trend Filtering Technology on the Current Time Domain Spectroscopy of Dielectrics

The current time domain spectroscopy of dielectrics provides important information for the analysis of dielectric properties and mechanisms. However, there is always interference during the testing process, which seriously affects the analysis of the test results. Therefore, the effective filtering of current time domain spectroscopy is particularly necessary. L1 trend filtering can estimate the trend items exactly in a set of time series. It has been widely used in the fields of economics and sociology. Therefore, this paper attempts to apply L1 trend filtering to the current time domain spectroscopy. Firstly, polarization and depolarization currents are measured in the laboratory. Then the test results are filtered by L1 trend filtering and the filtering effects are compared with several common filtering algorithms, such as a sliding mean filter and Savitzky–Golay smoothing filter. Finally, the robustness and time complexity of L1 trend filtering are analyzed. The filtering results show that because the polarization currents vary in a wide range of the time domain (about 2–3 orders of magnitude), smooth and undistorted curves in the whole test time range can hardly be obtained through common filtering algorithms, while they can be obtained by L1 trend filtering. The results of robustness analysis and time complexity analysis show that L1 trend filtering can extract the trend items accurately in the time series under given different noise levels, and the execution time is also lower than 176.67 s when the number of tested points is no more than 20,000. Those results show that L1 trend filtering can be applied to the time domain current spectroscopy of dielectrics.

Phase behavior and slow molecular dynamics in the glassy state and in the glass transformation of a nematic liquid crystal: 4CFPB

ABSTRACTThe molecular mobility in 4-cyano-3-fluorophenyl 4-butylbenzoate (4CFPB), a nematic liquid crystal, was studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). By TSDC a complex behavior was observed in which the main relaxation, with a wide distribution of activation energies, coalesces at high temperature with a poorly distributed mobility; this observation is in line with what was found by dielectric spectroscopy where a ‘double relaxation’ was observed. The step signal of the positional glass transition in DSC is a single and clean signal, which shows no evidence of the complexity revealed by the dielectric techniques; this signal seems to correspond to the usual main relaxation. The dynamic fragility values obtained by DSC and by TSDC indicate that 4CFPB is a moderately fragile glass former, in reasonable agreement with the values obtained by dielectric spectroscopy.Among the secondary relaxations in 4CFPB there are modes sensitive to physical aging, which makes them candidates for Johari-Goldstein’s relaxation status. It is emphasized, however, that this sensitivity to aging may not be a sufficient criterion for the attribution of this quality.

Investigation on thermal ageing impact on dielectric properties of natural ester oil

Ester oils are evolving as next-generation transformer insulants, and it is essential to understand their characteristic variation upon thermal ageing. Corona inception voltage reduces with thermally aged ester oil under AC and DC voltages. The UHF signal generated due to corona discharge activity has energy content in the range 0.4–4 GHz. The magnitude and energy content of the UHF signal formed due to corona activity increases with thermally aged oil. Ionic mobility studies are carried out to understand the impact of thermal ageing on polarization and depolarization current, ion concentration, and ionic radius formed in the oil. Thermal ageing of ester oil leads to an increase in the ionic mobility of oil samples along with an increment in the ionic concentration. The conductivity of oil is observed to increase with ageing, accompanied by a reduction in ionic radii. The effect of voltage stress shows enhanced conductivity for thermally aged oil samples. Dielectric response spectroscopy exhibits higher loss and permittivity values for thermally aged oils. The physicochemical analysis shows a gain in viscosity, moisture content and turbidity, and a marginal loss in thermal conductivity due to the formation of new molecular species. UV analysis exhibits a significant redshift in the oil sample aged at a higher temperature.

Structural, magnetic and electric-polarization properties of geometrically frustrated YBaCo4O7and DyBaCo4O7cobaltites

In the RBaCo4O7 (R = Ca, Y and rare earth) cobaltite family, only CaBaCo4O7 shows 3D long-range ferrimagnetic and spin-driven electric-polarization properties. In the present study, we have investigated the structural, magnetic and electric polarization properties in YBaCo4O7 and DyBaCo4O7 members of this family and the obtained results are compared with the CaBaCo4O7. The compound YBaCo4O7 showed a series of magnetic transitions in agreement with the reported literature, additionally, a cluster glass behavior below 5 K is observed. Powder neutron diffraction studies on DyBaCo4O7 cobaltite showed an orthorhombic Pbn21 symmetry and signature of short-range 120° magnetic correlations of kagome layer and a spin glass behavior below 65 K. Dielectric measurements on both the samples showed strong frequency dispersion at high-temperature and frequency-independent behavior at low temperature without dielectric anomaly. Pyroelectric current measurement has showed a broad peak around 50 K in both the samples; however, a careful analysis relates the peak to thermally stimulated depolarization current. This study signifies that the giant structural distortions and cobalt charge ordering at the kagome layer are the key factors to drive both long-range magnetic ordering and spin-driven electric polarization in this cobaltite family.

Sensing the Polarization and Depolarization Current of Solid Dielectrics Used in High-Voltage Applications

This article describes a practical method to sense and measure the polarization and depolarization current of a solid dielectric under high voltage excitation. The proposed method has the possibilities of evaluating the capacitance, dielectric dissipation factor, and conduction current from the measurand with reasonable accuracy as compared to the other conventional methods. The present method has inherent noise immunity in acquiring the current in the picoampere range. The principle of the data acquisition technique of the present scheme and its implementation is discussed in detail in this article. A comparative study between the proposed scheme and the conventional method has also been discussed. The result shows a good agreement between the proposed method and conventional complex setup.

Effect of Semi-Conductive Layer Modified by Magnetic Particle SrFe12O19 on Charge Injection Characteristics of HVDC Cable.

For high voltage direct current (HVDC) cable, a semi-conductive layer lies between the conductor and the insulation layer; as the charge migrates the path from the conductor to the insulation material, it will affect space charge injection. In this work, the research idea of changing the injection path of moving charges within semi-conductive layer by magnetic particles was proposed. Semi-conductive composites with different SrFe12O19 contents of 1 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.% were prepared, and the amount of injected charges in the insulation sample was characterized by space charge distribution, polarization current, and thermally-stimulated depolarization current. The experimental results show that a small amount of SrFe12O19 can significantly reduce charge injection in the insulation sample, owing to the deflection of the charge migration path, and only part of the electrons can enter the insulation sample. When the content is 5 wt.%, the insulation sample has the smallest charge amount, 0.89 × 10−7 C, decreasing by 37%, and the steady-state current is 6.01 × 10−10 A, decreasing by 22%. When SrFe12O19 content exceeds 10 wt.%, the charge suppression effect is not obvious and even leads to the increase of charge amount in the insulation sample, owing to the secondary injection of charges. Most moving charges will deflect towards the horizontal direction and cannot direct access to the insulation sample, resulting in a large number of charges accumulation in the semi-conductive layer. These charges will seriously enhance the interface electric field near the insulation sample, leading to the secondary injection of charges, which are easier to inject into the insulation sample.

Use of Interfacial Charge for Diagnosis and Activation Energy Prediction of Oil-Paper Insulation Used in Power Transformer

Activation energy is popularly used for the estimation of remaining life of transformer insulation. It is defined as the average rate of all reactions that happen with cellulose. Existing literature shows that the activation energy of oil–paper insulation can be obtained from polarization depolarization current (PDC) and return voltage measurement (RVM) data that are measured at a specific temperature. It is practically difficult to ensure the same measurement temperature for both PDC and RVM data. On the other hand, PDC data and its analysis get influenced by de-trapping current. This de-trapping current is generated by ionic charge carriers that get freed from trap sites during PDC measurement process. Formation of these trap sites is related to physical, chemical reactions that happen at oil–paper interface. This paper proposes a methodology which uses de-trapped charge, dislodged from deep and shallow traps, to assess insulation condition and for the prediction of activation energy. Thus, eliminating the need of RVM data. The proposed method is tested using data collected from various real-life in-service transformers.

Relaxation, Thermal, and Interphase Effects in Polymer–Ferroelectric-Piezoelectric Ceramic Composites of Different Structures

The relaxation and thermal processes and interphase phenomena in composites based on ferroelectrics and a polymer matrix are studied. It is shown that the charge stabilized at the interface of the composite during its electrothermopolarization is mainly determined by the structure of the polymer matrix and the piezoelectric phase. The results obtained make it possible to reveal the main factors affecting the piezoelectric properties of the heterogeneous polymer–ferroelectric ceramic system. Polyolefins and fluorine-containing polar polymers are used as the organic phase, and ferroelectric ceramics of rhombohedral, tetragonal, and mixed structures serve as the inorganic phase. The relaxation processes and interphase phenomena are studied using a differential scanning calorimeter, and the charge state is analyzed by recording the thermally stimulated depolarization current. The charge-state stability is determined by the electret potential difference of the composites. The molecular relaxation is analyzed by the dielectric method. It is established that the composites in which the interphase interaction is more pronounced are characterized by high piezoelectric properties.

Avalanche dynamics of ferroelectric phase transitions in BaTiO3 and 0.7Pb(Mg2∕3Nb1∕3)O3-0.3PbTiO3 single crystals

The motion of phase fronts during a ferroelectric phase transition is intermittent and follows avalanche dynamics. In the present study, we show that an intermittent propagation mode generates spikes of depolarization currents at an extremely slow heating rate of 0.05 K/min in BaTiO3 (BTO) and 0.7Pb(Mg2∕3Nb1∕3)O3-0.3PbTiO3 (PMN-PT) single crystals. Such “jerks” are indicative of avalanche dynamics, and their energy exhibits a power law distribution with exponents of ε = 1.3 ± 0.10 and ε = 1.5 ± 0.10 for BTO and PMN-PT, respectively. The rate of aftershocks after big events decays as an Omori-like power-law and interevent times are characterized by a universal double power-law distribution, indicating the critical temporal correlations between the avalanche events.

Electrophysical properties of a multicomponent PZT-type ceramics for actuator applications

A multicomponent lead zirconate titanate (PZT)-type ceramics doped with Mn4+, Sb3+, W6+, and Ni2+ with a perovskite structure was obtained. The designed material had a chemical composition of Pb(Zr0.49Ti0.51)0.94Mn0.014Sb0.02W0.014Ni0.02O3. Ceramic powders were synthesized by the calcination method, and they were densified by the free sintering method. We present the results of electrophysical measurements of the PZT-type ceramics, including scanning electron microscopy microstructure, dielectric, ferroelectric, pyroelectric, and thermally stimulated depolarization currents, piezoelectric properties and DC electrical conductivity. The results obtained for the multicomponent ceramics predispose it to micromechatronic applications as part of actuators.

Orientational glass, orientationally disordered crystal and crystalline polymorphism: A further study on the thermal behavior and molecular mobility in levoglucosan

Levoglucosan has very particular features in its phase diagram. On the one hand it has two crystalline phases, one orientationally ordered (ORC), stable at room temperature, which is transformed by heating in an orientationally disordered crystal (ODIC), a plastic crystal. On the other hand, it is difficult to form the structural glass by cooling down from the isotropic liquid, but it is easy to form the orientational glass (O-G) of levoglucosan by cooling from the ODIC phase. The stability of the O-G and ODIC phases allowed the study of the orientational glass transition, O-G → ODIC, by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC) with the determination of the respective dynamic fragility. In this context a critical analysis of the procedures using the effect of the DSC scanning rate on the temperature location of the DSC glass transition signal was performed.Two polymorphs of levoglucosan, not yet described in the literature, were discovered. These crystalline ORC forms are converted into an ODIC phase by heating, as happens with the most stable polymorph (as received). The three ORCs show ORC → ODIC transitions that differ greatly in temperature, enthalpy, and entropy of transition. We must therefore consider four levoglucosan polymorphs, three ORCs and one ODIC.TSDC was also used to study the secondary relaxations in the levoglucosan orientational glass, and it was possible to identify a β-type mobility (Johari-Goldstein) in addition to fast relaxations. Finally, the two dielectric techniques, dielectric relaxation spectroscopy (DRS) and TSDC, were compared in their ability to provide relevant information on the secondary mobility.

The Effect of Humidity on Dielectric Properties of PP-Based Nano-Dielectric.

Nano-dielectrics are sensitive to humidity and easily degraded in damp environment because of the high surface energy of nanoparticles. In order to study the effect of humidity on the dielectric properties of nano-dielectric, polypropylene (PP) was modified by polyolefin elastomer (POE) and nano-SiO2, and the samples with obvious filling concentration were pre-selected by breakdown strength for damp aging. The aging experiments were carried out in different relative humidity. The dielectric properties of new, hygroscopic saturation and samples after drying were measured and analyzed. It is found that the breakdown strength of hygroscopic saturation nano-dielectrics decreased obviously compared with new samples, and it was difficult to recover after drying. The damp degradation resulted in different changing trends of permittivity of PP and nano-dielectric, but there were relaxation loss peaks of water in both of them. The influence of damp degradation on the trap distribution was studied by thermally stimulated depolarization currents (TSDC), and it was found that the traps level introduced by water molecules was different in PP and nano-dielectrics. All experiment results showed that the performance of nano-dielectrics degraded obviously in humid environment, and it was difficult to recover even after complete drying because of the existence of bounded water molecules in nano-dielectrics.

Enhanced energy storage property in glass-added Ba(Zr0.2Ti0.8)O3-0.15(Ba0.7Ca0.3)TiO3 ceramics and the charge relaxation

Abstract Glass additive BaO-SrO-TiO2-Al2O3-SiO2-BaF2 is employed to enhance the microstructures and energy storage properties of the Ba(Zr0.2Ti0.8)O3-0.15(Ba0.7Ca0.3)TiO3 ceramics. To clarify the energy storage mechanism, the charge transportation and polarization process are investigated by thermally simulated depolarization current (TSDC). The dielectric breakdown strength increases from 4.3 kV/mm to 10.8 kV/mm for BZT-0.15BCT ceramics with 11 wt% glass additives, indicating that glasses could refine the grain size, uniform the structure, and decrease defects. Due to the micro-domain region, dielectric relaxation behavior is observed with a broadened and reduced dielectric constant peak at a large dielectric constant of about 3000 at room temperature. The largest charge energy density of 1.45 J/cm3 and discharge density of 0.17 J/cm3 are achieved for BZT-0.15BCT glass ceramics with 7 wt% glass additives. TSDC results demonstrate that dipole origin movement and charge transportation have an important effect on the dielectric properties and dielectric breakdown strength, respectively, which are largely influenced by the defects distribution state at the interfaces. Moderate domain walls could restrain the defects to inhibit the charge transportation and are harmful for the dielectric properties inversely. To achieve excellent energy storage performance, moderate domain walls are compromise of slightly degrading dielectric properties and greatly improving dielectric breakdown strength.

Effect of electrodes material on transient currents in dielectric liquid

In this study, results of polarisation and depolarisation currents measured on an insulating liquid are presented. The goal is to contribute to a better understanding of mechanisms occurring at the metal-liquid contact. Electrodes of different metallic nature are associated with the same mineral oil, the metal / liquid / metal structure is submitted to DC voltages and the transient current is recorded with a high sensitive electrometer. The current versus time feature shows that the conduction process is in good compliance with the metal work function rather than with its electric conductivity. The current versus field characteristics indicate a predominance of the ohmic behaviour for the low fields. The space charge limited conduction regime appears as soon as the electrical constraint exceeds ~1 MV/m, which corresponds to 10% of the breakdown voltage. Schottky injection is suspected to occur from the metal into the liquid since the applied fields are uniform and non-extremely intensified.

Effects of UV radiation on the charge trapping capability of PET

Poly(ethylene terephthalate) (PET) is able to retain space charge in traps, as is usual in dielectric materials. This allows the material to attain an almost permanent polarization when space charge is displaced by an external electric field or is injected from an electrode. We have studied the influence of UV irradiation on the charge trapping capability of PET in samples exposed for different periods of time, up to 10 weeks. The pulsed electro-acoustic technique (PEA) has been used to determine the charge profile. The injected charge that the material is able to retain on the irradiated surface increases with irradiation time. This indicates the formation of new traps. An extensive characterization of these localized states has been performed by the technique of thermally stimulated depolarization currents. Parameters of charge relaxation kinetics have been obtained by fitting spectra of the space charge relaxation current of a semicrystalline polymeric material peak (), related to injected charge, to the general-order kinetics model. A relaxation map analysis shows that relaxation times become more distributed and the activation energy decreases as irradiation time is increased. The activation energy decreases by approximately 10% after 10 weeks of exposure. These results show that UV irradiation creates additional traps on the treated surface, which agrees with PEA results, and that the energy depth of these traps is shallower and it is more distributed than in the case of pre-existing traps.

Polarization and depolarization characteristics of different structure oiled paper composite insulation under high applied voltage

Transformers are an important part of the power transmission and distribution process of power systems. Designing and manufacturing a stable transformer is essential for the stable operation of the entire power system. Accordingly it is critical for the stable operation of the entire power system to design and manufacture a stable transformer. In order to improve the composite insulation performance of the oil-paper in power transformer, studying the polarization characteristics of oil-paper composite insulation is essential. In this paper, the polarization characteristics of high external applied voltage and different oil-paper composite insulation structures are studied. In the three-electrode system, the cardboard has obvious polarization characteristics at the two poles, and the polarization characteristics are most obvious when the cardboard is at the high pressure pole. The initial value and the attenuation function of the polarization and depolarization current do not match the laws determined by the RC circuit. When the voltage is at a higher level, the difference between the initial values of the polarization and depolarization currents at different positions of the cardboard at the same voltage level is greater, and the polarity effects are more obvious. the above rules still exist when the distance between the electrodes is changed.

Temperature and electric field dependence of charge conduction and accumulation in XLPE based on polarization and depolarization current

Temperature and electric field are important factors affecting space charge accumulation in high voltage dc (HVDC) cable insulation. Charge conduction and accumulation in XLPE have been studied by correlating the polarization and depolarization processes. The experimental results indicate that the steady current of XLPE increases about 2 or 3 orders with the increasing temperature from 25 °C to 90 °C, and the stronger electric field, the less the increased amplitudes. The trap level in XLPE stressed by different electric fields and temperatures have a tiny change from 0.89 eV to 1.15 eV. Besides, there exists an inflection temperature of charge accumulation in XLPE, around 50 °C∼ 60 °C. At the room temperature, the trapped charges are difficult to release from the traps, and these homo-charges near the electrode can depress the further injection of charges. With the increase of temperature, part of trapped charges near the interface will gradually migrate towards the bulk of the material, and more charges are injected. When the temperature exceeds around 50 °C, the molecular movement is accelerated which can dramatically enhance the hopping probability of charges between the adjacent traps, and little accumulated charges are left.

Релаксационные, термические и межфазные эффекты в композитах полимер-сегнетопьезокерамика различной структуры

AbstractThe relaxation and thermal processes and interphase phenomena in composites based on ferroelectrics and a polymer matrix are studied. It is shown that the charge stabilized at the interface of the composite during its electrothermopolarization is mainly determined by the structure of the polymer matrix and the piezoelectric phase. The results obtained make it possible to reveal the main factors affecting the piezoelectric properties of the heterogeneous polymer–ferroelectric ceramic system. Polyolefins and fluorine-containing polar polymers are used as the organic phase, and ferroelectric ceramics of rhombohedral, tetragonal, and mixed structures serve as the inorganic phase. The relaxation processes and interphase phenomena are studied using a differential scanning calorimeter, and the charge state is analyzed by recording the thermally stimulated depolarization current. The charge-state stability is determined by the electret potential difference of the composites. The molecular relaxation is analyzed by the dielectric method. It is established that the composites in which the interphase interaction is more pronounced are characterized by high piezoelectric properties.

Dynamic mechanism of breakdown in polypropylene-based nano-dielectric

The dynamic mechanism of breakdown in polypropylene (PP) were represented based on the discharge-avalanche theory. PP is a typical semi-crystalline thermoplastic polymer with low loss and high thermal stability, which had broad application prospects in recyclable and environment-friendly insulation. Dielectric strength was one of the important properties for polymer insulation, hence research of breakdown mechanism was of great significance for insulation reliability increase and advanced materials development. In this paper, the dielectric strength of PP was enhanced by nano-silica filling, and mechanism of enhancement was discussed based on the change of activation energy measured by thermally stimulated depolarization currents (TSDC).

Structure evolution and ferroelectric properties in stoichiometric Bi0.5+xNa0.5−xTi1−0.5xO3

The influence of B-site deficiency on the stability of electrically induced long-range ferroelectric order of the stoichiometric Bi0.5+xNa0.5−xTi1−0.5x□0.5xO3 (BNT-xVTi) (“□” denotes vacancies) ceramics is studied. The depolarization and ferroelectric to relaxor transition are identified as separate and discrete processes in BNT-based materials. For BNT-0.02VTi, the resonance and anti-resonance peaks on dielectric permittivity-frequency curves indicate dominating ferroelectric phase at room temperature. The depolarization temperature, determined by thermally stimulated depolarization current, is ~ 65 °C. However, the ferroelectric to relaxor transition temperature is absent, as no distinct frequency-independent anomalies for the dielectric permittivity exist. This depolarization process can be ascribed to nanoscale ferroelectric domain at room temperature for BNT-0.02VTi, which is induced by chemical disorder and strong random field as VTi generated. Hence, the results imply that the B-site deficiency in BNT is a very effective route to tailor the stability of electrically induced long-range ferroelectric order.