Space Charge Evolution Research Articles

Influence of Polarity Reversal Period and Temperature Gradient on Space Charge Evolution and Electric Field Distribution in HVDC Extruded Cable

During the operation of HVDC extruded cables, voltage polarity reversal (VPR) is considered one of the most severe conditions for cable insulation. In this paper, a bipolar charge transport model developed for cylindrical geometry is improved by introducing ionic carriers from impurity dissociation for the simulation of space charge and electric field in an HVDC extruded cable with thick polymeric insulation under VPR, and the construction of the geometric model is based on a practical 160 kV DC polymeric cable. The influence of polarity reversal period (PRP) and temperature gradient (TG), formed by the load current flowing through the conductor, on the space charge evolution and the electric field distribution are investigated. The mechanisms of the charge dynamics and the field distortion affected by the PRP and the TG are also discussed. The results show that under TG, the maximum transient field appears near the interface between the conductor shield and insulation in the early stage of complete VPR. In addition, the longer the PRP, the more serious the maximum transient field distortion. Moreover, an increase in TG intensifies the maximum field distortion under steady and transient states due to the enhancement of heterocharge accumulation.

Space charge evolution and its effects on electrical tree degradation in silicone rubber at varied temperature under polarity reversal voltage

Electrical tree is one of the main reasons for cable failure. In this paper, the space charge evolution and electrical tree characteristics under polarity reversal voltage with varied temperature in silicone rubber (SIR) are carried out. Results show that as temperature rises from 30 °C to 120 °C, electrical tree initiation voltage declines by 17.8%. Meanwhile, the trees become denser, and the tree length is larger correspondingly. In order to ascertain the charge transportation and its influence on tree initiation process under polarity reversal voltage, we build a bipolar carrier transportation model with needle-plate electrode. Space charge transportation characteristics and corresponding electric field are simulated. Moreover, the evolution model of electrical tree under polarity reversal voltage is discussed. The presence of homo space charge leads electric field to become larger during the voltage reversal process. The segment relaxation of SIR and more, deeper space charge injection make tree initiate more easily at higher temperature.

Characterization of ion beam induced polarization in scCVD diamond detectors using a microbeam probe

Diamond detectors are increasingly being used in many multidisciplinary areas due to their good spectroscopy properties. However, in certain cases of diamond employment as a nuclear detector, a significant deterioration of the signal properties due to the trapping of free charge carriers by defects can be observed. This phenomenon is known as polarization. Experimentally, a good understanding of the free carrier transport mechanisms can be obtained from direct measurements of the Charge Collection Efficiency (CCE) using the ion beam induced charge technique (IBIC). In this work IBIC was performed by focused proton and carbon ion beams of different energies in the MeV range, to probe the polarization induced changes of the CCE. The detectors being used in this study include a thin scCVD membrane detector (thickness 6 μm) and a scCVD crystal of 65 μm thickness. Furthermore, the detectors have been damaged with transmitted protons to study the influence of defects on polarization. The results that are presented show that the ion microprobe technique IBIC can be successfully used to fully characterize the polarization development in terms of time evolution in scCVD diamonds. More specifically, we show for the thin membrane detectors at high count rates and longer time irradiation a clear dependence of the pulse height signal decrease, induced by the polarization effect, with levels of radiation damage. A significant deterioration of the signal was observed in regions damaged with fluences between 1012 and 1014 ions/cm2 for both types of charge carriers. The results have shown that the energies used for the probing ion beams allow us to explore the electronic features from shallow to deeply penetration range and that has an effective impact on the temporal evolution of the build-up space charge. The experiments carried out have also shown that light can be used to suppress or amplify the polarization phenomena in damaged regions depending on the dominant type of charge carriers.

The Simultaneous Evolution of Space Charge and Conduction Current in LDPE

The development of a system to measure simultaneously the current and space charge in a dielectric is described. The results obtained for low density polyethylene samples show that the relationship between space charge accumulation and the time-dependent current is not straightforward. In accord with previous work on space charge limited currents, the transient currents initially decreased, but after a while an increment or peak appeared. Additionally, the generation and extraction of space charge packets caused an impulse-like current peak such as previously observed. By comparing the electric field and current at the anode, it was found that the increment and the peak of the anode current are determined by injected negative carriers reaching the anode rather than the increase of electric field at the anode. It is shown that the dynamical evolution of the current can be divided into three stages similar to transient space charge limited current theory.

Effects of Trapping Characteristics on Space Charge and Electric Field Distributions in HVDC Cable under Electrothermal Stress

Space charge behavior has a strong impact on the long-term operation reliability of high voltage–direct current (HVDC) cables. This study intended to reveal the effect of trap density and depth on the space charge and electric field evolution behavior in HVDC cable insulation under different load currents and voltages by combined numerical bipolar charge transport (BCT) and thermal field simulation. The results show that when the load current is 1800 A (normal value), the temperature difference between the inside and the outside of the insulation is 20 °C, space charge accumulation and electric field distortion become more serious with the increase in the trap depth (Et) from 0.80 to 1.20 eV for the trap densities (Nt) of 10 × 1019 and 80 × 1019 m−3, and become more serious with the increase in Nt from 10 × 1019 to 1000 × 1019 m−3 for Et = 0.94 eV. Simultaneously decreasing trap depth and trap density (such as Et = 0.80 eV, Nt = 10 × 1019 m−3) or increasing trap depth and trap density (such as Et = 1.20 eV, Nt = 1000 × 1019 m−3), space charge accumulation can be effectively suppressed along with capacitive electric field distribution for different load currents (1800 A, 2100 A and 2600 A) and voltages (320 kV and 592 kV). Furthermore, we can draw the conclusion that increasing bulk conduction current by simultaneously decreasing the trap depth and density or decreasing injection current from conductor by regulating the interface electric field via simultaneously increasing the trap depth and density can both effectively suppress space charge accumulations in HVDC cables. Thus, space charge and electric field can be readily regulated by the trap characteristics.

Time-resolved measurement of space-charge evolution in dielectric films or slabs by means of repeatable laser-induced pressure pulses.

A new variant of the Laser-Induced Pressure-Pulse (LIPP) method for repeatable, time-resolved space-charge profile measurements is proposed and demonstrated. Automated deposition of a fresh laser-target film before each illumination leads to good repeatability of the LIPP and thus allows for the detection of time-resolved changes in the space-charge distribution over many hours. We describe and discuss the experimental setup and its features, compare the repeatability of the LIPP measurements on the same sample without and with re-preparation of the test cell, and present the time-resolved evolution of the space-charge profile in a two-layer arrangement of a silicone-grease and a silicone-elastomer film as an example. Finally, the temperature dependence of the space-charge evolution during polarization under high voltage and during depolarization in short circuit is shown. Possible uses and future developments of the new LIPP approach are also discussed.

Analytical model for homocharge accumulation in LDPE — role of conduction, injection and diffusion

Homocharge, particularly, positive charge near anode has been observed inside low-density polyethylene (LDPE) by many researchers. However, the phenomenon is yet to be understood theoretically and mathematical analysis is pending, although, simulations have been reported. Simulation of evolution of space charge in LDPE, with time and space, using bipolar charge transport model considered only drift component of charge carriers inside the dielectric. The experimental results suggest significant concentration gradient in space charge distribution. The authors derived a comprehensive drift and diffusion based analytical models, for the time/space evolution of space charge and electric field in dielectrics using Maxwell's equations, current continuity equation, conduction and injection mechanisms. The proposed analytic models of electric field and space charge distribution entwines charge injection at electrodes and charge transport inside the medium, which seem to have not been reported until now. Remarkable, believed to be unprecedented, agreement between the analytical and experimental results has been observed. Using the model, the role of conduction, injection, diffusion on homocharge accumulation has been investigated. Pulsed electroacoustic method is used for the measurements of the space charge dynamics.

Modified lifetime prediction for multilayer ceramic capacitors based on space charge evolution

Accurate lifetime prediction for multilayer ceramic capacitors (MLCCs) is essential to ensure the reliability of electronic systems. A prediction formula based on a physical model has been recently proposed as an alternative to the widely used empirical formula. In this study, we used thermally stimulated depolarization current measurements to demonstrate that the physical model should be modified for low electric fields (<10 V/µm). We propose a new formula for the conditions as well as a modified model that involves an overpotential factor. The new formula suggests that if the applied dc field is less than the threshold value, the breakdown of MLCCs will not occur.

Asymmetric injection and distribution of space charges in propylene carbonate under impulse voltage

Space charge can distort the electric field in high voltage stressed liquid dielectrics and lead to breakdown. Observing the evolution of space charge in real time and determining the influencing factors are of considerable significance. The spatio-temporal evolution of space charge in propylene carbonate, which is very complex under impulse voltage, was measured in this study through the time-continuous Kerr electro-optic field mapping measurement. We found that the injection charge from a brass electrode displayed an asymmetric effect; that is, the negative charge injection near the cathode lags behind the positive charge injection near the anode. Physical mechanisms, including charge generation and drift, are analyzed, and a voltage-dependent saturated drift rectification model was established to explain the interesting phenomena. Mutual validation of models and our measurement data indicated that a barrier layer, which is similar to metal-semiconductor contact, was formed in the contact interface between the electrode and propylene carbonate and played an important role in the space charge injection.

On the anomalous charging and discharging currents in LDPE under high electric fields

Dielectric characterization and conduction modeling of polymers are considered important owing to increasing use of polymeric insulation worldwide in power cables for electric power distribution. Electric conduction in polymers is a complex phenomenon and differs considerably at high electric fields where phenomena like space charge formation and charge injection from the electrodes come into the picture. This paper investigates the charge injection during polarization and subsequent discharging of low density polyethylene (LDPE) films at high electric fields (> 10 kV/mm) and under different temperatures. An attempt was made to study the short term Debye-type exponential behavior and long term power law behavior. Apart from the variation in the power law exponent for absorption current with increasing field, anomalies were noted in both types of relaxation currents at high electric fields and temperatures. Evidence of the polarization current anomaly was correlated with space charge behavior of samples. Interestingly, the space charge evolution under step voltage application was observed to be oscillatory in some cases, wherein, the charge trapping inside the dielectric bulk showed an oscillatory profile. The experimental factors which could contribute to anomalous discharging currents have also been thoroughly investigated and reported, which seem to have not been paid attention until now.

Influence of voltage reversal on space charge behavior in oil-paper insulation

The presence of space charge is a serious threat to the reliability of insulation material under voltage polarity reversal. The results of space charge evolution in oil-paper insulation during external voltage polarity inverse at room temperature were presented in this paper. A mirror image effect charge was observed in the steady state, but the formation of it was affected by the first polarizing, which could cause a change of electrical property. The results showed that the mirror image effect was independent of the involved nature, type and dynamics of charge. Experiments of different reversal polarities and periods indicated that the slow decay of heterocharge retained from previous homocharge injection was the cause of electric field enhancement. And the reversal period therefore had an effect on voltage polarity reversal, namely the shorter the reverse period, the higher the stress enhancement. The reliability of oil-paper insulation could be influenced by the polarity reversal because charge injection and distribution were unpredictable, and space charge modified and in return was affected by the field as well.

Influence of voltage reversal on space charge behavior in oil-paper insulation

The presence of space charge is a serious threat to the reliability of insulation material under voltage polarity reversal. The results of space charge evolution in oil-paper insulation during external voltage polarity inverse at room temperature were presented in this paper. A mirror image effect charge was observed in the steady state, but the formation of it was affected by the first polarizing, which could cause a change of electrical property. The results showed that the mirror image effect was independent of the involved nature, type and dynamics of charge. Experiments of different reversal polarities and periods indicated that the slow decay of heterocharge retained from previous homocharge injection was the cause of electric field enhancement. And the reversal period therefore had an effect on voltage polarity reversal, namely the shorter the reverse period, the higher the stress enhancement. The reliability of oil-paper insulation could be influenced by the polarity reversal because charge injection and distribution were unpredictable, and space charge modified and in return was affected by the field as well.

Transport and Two-Species Capture of Electrons and Holes in Ultrapure Germanium at MilliKelvin Temperature

This work describes the transport and capture processes of electrons and holes in ultrapure germanium \(\langle 100 \rangle \) at low temperature and low electric field. Dynamic space-charge effects are responsible for the most significant systematics of the Cryogenic Dark Matter Search. To understand the relationship of transport dynamics to space charge evolution, it is important that we understand charge carrier transport under these nonequilibrium operating conditions. Here, we present measured data and a consistent model of electron and hole dynamics and capture. A charged and a neutral capture probability for both electrons and holes appears to explain the multiple, field-dependent power laws observed in data. This offers a quantitative understanding why germanium crystals operating under these conditions should “neutralize” when grounded, but accrue space charge while under bias.

Investigation Into Surface Potential Decay of Polyimide by Unipolar Charge Transport Model

To predict spacecraft charging levels and mitigate electrostatic discharges, it is very important to understand the charge transport properties of highly insulating materials such as polyimide. The combination of surface potential decay (SPD) experiment and unipolar charge transport (UCT) model investigates the charge transport properties of polymide. In a simulated space environment chamber, the SPD experiments are performed on polymide. After irradiated by electron beams, the surface potential distributions of the sample are measured by a noncontact potential probe (Trek). The UCT model that consists of relaxation polarization, charge injection, charge migration, charge trapping, and detrapping processes simulates the SPD results. In the numerical UCT model, we solve the charge continuity equation by Runge-Kutta discontinuous Galerkin method and Poisson's equation by boundary element method. The least square error between the experimental and the simulated SPD results is searched by genetic algorithm (GA). From the GA calculation, we optimize the charge transport parameters of polyimide. Then, we calculate the space charge evolution properties during the SPD process of polyimide by the UCT model. These charge transport parameters, then, can be used to study the surface charging or deep dielectric charging of the insulator.

Thermal pulse measurements of space charge distributions under an applied electric field in thin films

The thermal pulse method is a powerful method to measure space charge and polarization distributions in thin dielectric films, but a complicated calibration procedure is necessary to obtain the real distribution. In addition, charge dynamic behaviour under an applied electric field cannot be observed by the classical thermal pulse method. In this work, an improved thermal pulse measuring system with a supplemental circuit for applying high voltage is proposed to realize the mapping of charge distribution in thin dielectric films under an applied field. The influence of the modified measuring system on the amplitude and phase of the thermal pulse response current are evaluated. Based on the new measuring system, an easy calibration approach is presented with some practical examples. The newly developed system can observe space charge evolution under an applied field, which would be very helpful in understanding space charge behaviour in thin films.

Effect of additives on the performance of cross-linked polyethylene subjected to long term single and periodically reversed polarity DC voltage

The performance of cross-linked polyethylene with and without tree-retarding additives, aged with single and reversed polarity DC voltage was investigated. Under single polarity field of a 50 kV/mm, cross-linked polyethylene without the tree retardant (XLPE) showed significantly longer time to breakdown than the material with a tree retardant (TR-XLPE). This is attributed to the difference in how the two materials accumulate and retain space charge. In the case of polarity reversals, TR-XLPE showed better performance than XLPE. This seeming contradiction was studied by measuring the dynamics of space charge evolution with time. The Thermal Step Method (TSM) of space charge measurements was used. It showed that after the polarity reversal the TSM current rapidly inversed in the TR-XLPE but not in the XLPE. This implies more rapid charge dissipation in TR-XLPE. The retained charges cause the electric field enhancement in the material when the polarity is reversed. Thus, after each polarity reversal XLPE was subjected to higher local electric stress for a longer time and thus broke down sooner than TR-XLPE. For both materials the space charge decreased faster when the polarity was changed from negative to positive than vice versa. This could be explained by more efficient electron than holes injection. Since the control of the power flow in DC networks requires reversals of voltage polarity, their detrimental effect on XLPE-insulated equipment, such as cables, cannot be avoided. The effect could be mitigated by the application of carefully designed additives. However, as the present study indicates, additives that improve insulation performance under polarity reversals can be harmful under single polarity voltage.

Measurement and Modeling of Low-Frequency Current From Hybrid AC/DC Corona

This paper aims at investigating the mechanism of hybrid ac/dc corona which is usually accompanied with the hybrid HVAC/HVDC power transmission lines. A coaxial wire-cylinder corona cage is designed in this paper, and the combined ac/dc voltage is applied to the wire and cage. The low-frequency component of corona current from the hybrid ac/dc corona in this corona cage with five types of wires is measured. To obtain the detailed characteristic of the interaction between the space charge and ionized electric field, a 1-D method is developed to simulate the hybrid ac/dc corona in the symmetrical corona cage. This method is validated through the comparison with measurement results. Based on this simulation method, analysis of the evolution of space charge and ionized electric field in one time cycle is presented. The regular pattern of charge injection from the discharge electrode is summarized, where the charge injection is found not linearly dependent on the applied voltage. Finally, the corona loss of hybrid ac/dc corona is calculated, and the similar result with the quadratic law in ac corona is found from the calculation results. This research provides useful knowledge for the engineering computation of the effects of hybrid ac/dc corona.

Optically Induced Measurement of Electron and Hole Drift Velocities in a Germanium 〈100〉 CDMS Detector at 50 mK

We report on precise drift velocity measurements of electrons and holes in 50 mK, ultrapure (≈1010 net shallow impurities per cm3) germanium 〈100〉 CDMS dark matter detectors as a function of electric field up to 4 V/cm. A laser diode connected to an optical fiber extending from room-temperature to the detector creates electron-hole pairs on one surface of the crystal. High-speed electronics measure the drift current as the generated carriers travel to the opposite face of the crystal. CDMS detectors measure the ionization and phonon response of particle interactions within the crystal. Stable charge collection is necessary for successful background discrimination when looking for a possible dark matter signal. While biased, however, ionization performance degrades over time due to the build-up of space charge. Free electrons and holes created by particle interactions are subject to drift-diffusion dynamics occurring simultaneously with the trapping of carriers to localized surface and bulk states. The combination of these processes determine the evolution of space charge within the crystal, making it important that we understand carrier transport under our unique operating condition of low-temperature and low-field. We find good agreement between our measured drift velocities and our theoretical predictions, indicating carrier scattering is dominated by spontaneous phonon emission. In addition, we present preliminary measurements of effective longitudinal carrier trapping lengths for both n-type and p-type crystals at 50 mK.

Electric field and space charge measurements in thick power cable isulation

This paper describes the evolution of electric field and space charge in high voltage AC and DC cable polymeric insulation, followed using the thermal step method.

Space charge formation and its modified electric field under applied voltage reversal and temperature gradient in XLPE cable

The results of space charge evolution in cross-linked polyethylene power cables under dc electrical field at a uniform temperature and during external voltage polarity reversal are presented in the paper. A mirror image charge distribution was observed in the steady state, but the pre-existing field altered the way in which the steady state charge distribution was formed from that obtaining when the cable was first polarized. Polarity reversing charge was generated in the middle of the insulation and moved towards the appropriate electrodes under the influence of a field in excess of the maximum applied field. Our results show that the mirror effect is a steady state effect that is due to cross-interface currents that depend only on the interface field and not its polarity. Measurements on cable sections with an elevated mean temperature and temperature gradient show that the interface currents are temperature dependent, and that differences between the activation energies of the interface and bulk currents can eliminate, and possibly even invert the polarity of the space charge distribution.