Space Charge Measurements Research Articles

Enhanced charge injection in rough HVDC extruded cable interfaces

The insulation system of an HVDC extruded cable includes a variety of different material interfaces, especially within its cable accessories. Depending on interfacial manufacturing method, different degrees of surface roughness can be found and are here assessed. Based on detailed knowledge of insulation surface roughness, this paper proposes a new approach for calculations of electrical field distributions and injection current densities at the insulation surface. It is shown that local fields at rough material interfaces can be greatly enhanced, resulting also in field reduction in other areas along the surface. These calculations also reveal a distinct field threshold for space charge injection, likely leading to formation of homocharge layer adjacent to the interface. Existence of this homocharge layer at a rough surface (density of approximately 1 C/m3) is confirmed by means of space charge measurements.

The Industrial Applicability of PEA Space Charge Measurements, for Performance Optimization of HVDC Power Cables

Cable manufacturing industries are constantly trying to improve the electrical performance of power cables. During the years, it was found that one of the most relevant degradation factors influencing the cable lifetime is the presence of space charge in the insulation layer. To detect the accumulated charge, the pulsed electro-acoustic (PEA) method is the most used technique. Despite the wide use of the PEA cell, several issues are still present. In particular, the PEA output signal is strongly disturbed by the acoustic waves reflections within the PEA cell. This causes the distortion of the output signal and therefore the misinterpretation of the charge profiles. This, in turn, may result in an incorrect cable characterization from the space charge phenomenon point of view. In 2017, due to the proved degradation effect of the space charge accumulation phenomenon, the IEEE Std 1732 was developed. This standard describes the steps to be followed for the space charge measurement in cables specimens during pre-qualification or type tests. Therefore, cable manufacturing industries started to take a particular interest in these measures. In the light of this, the aim of the present work is to highlight that the enacted standard is not easily applicable since various problems are still present in the PEA method for cables. In particular, in this work, the effect of multiple reflected signals due to the different interfaces involved, but also the effect of the signal attenuation due to cable dielectric thickness, as well as the effect of the PEA cell ground electrode thickness in the output charge profile, are reported. These issues have been demonstrated by means of an experimental test carried out on a full-size cable in the Prysmian Group High Voltage laboratory. To better understand the PEA cell output signal formation, a PEA cell model was developed in a previous work and it has been experimentally validated here. In particular, simulations have been useful to highlight the effect of the reflection phenomena due to the PEA cell ground electrode thickness on the basis of the specimen under test features. Moreover, by analyzing the simulation results, it was possible to separate the main signal from the reflected waves and, in turn, to calculate the suitable ground electrode thickness for the cable specimen under test.

An improved pulsed electroacoustic system for space charge measurements under high temperatures and arbitrary waveform voltages

This paper presents the design and calibration method of an improved pulsed electroacoustic (PEA) system for space charge measurements, including a novel nanosecond pulse generator and an improved electrode system, which can be used under various temperatures and arbitrary waveform voltages. The measurement principle of the PEA system is first analyzed, and the calibration method for the applied voltage is proposed to solve the problem of waveform distortion caused by voltages with different frequency components. For the pulse generator, the main parameters of output pulses are an amplitude of ∼700 V, a full width of 4.6 ns, a jitter of 0.24 ns, and a repetition rate of 10 kHz on the 50 Ω load. For the electrode system, the flexible upper electrode design is first introduced to achieve consistently close contact between the sample and the electrode. Besides, the hybrid heating method and the replaceable coupling capacitor and high voltage electrode head are adopted. Space charge measurements are conducted at different conditions using this PEA system, which includes different temperatures and different voltage waveforms. The experimental results verify the effectiveness of the calibration method and the feasibility of the improved PEA system.

Space Charge Measurement Using Pulse Electroacoustic Method with a Spin-coated Poly(vinylidene fluoride-trifluoroethylene) Thin Film

Space Charge Measurement Using Pulse Electroacoustic Method with a Spin-coated Poly(vinylidene fluoride-trifluoroethylene) Thin Film

Review of the PEA Method for Space Charge Measurements on HVDC Cables and Mini-Cables

This review takes into account articles and standards published in recent years concerning the application of the Pulsed Electro Acoustic (PEA) method for space charge measurement on High Voltage Direct Current (HVDC) cables and mini-cables. Since the 80s, the PEA method has been implemented for space charge measurements on flat specimens in order to investigate space charge phenomena and to evaluate the ageing of dielectrics. In recent years, this technique has been adapted to cylindrical geometry. Several studies and experiments have been carried out on the use of the PEA method for full size cables and HVDC cable models. The experiments have been conducted using different arrangements of the measurement setup and focusing attention on different aspects of space charge phenomena. In this work, the importance of space charge measurement is highlighted and the state-of-the-art PEA method application to full size cables and mini-cables is described. The main aim of this paper is to offer a complete and current review of this technique. In addition, limits on the use of PEA method are examined and main possible directions of research are proposed in order to improve the applicability, reliability, and replicability of this method.

Physical investigation of the Electro-Acoustic-Reflectometry method for space charge measurements

The Electro-Acoustic-Reflectometry method was recently proposed for the purpose of probing space charge with a high spatial resolution. In this article, the underlying physical principle of the method is studied in order to explain the observed signals. The effect of boundary conditions is exploited to experimentally validate the theoretical calculations.

Empirical Conductivity Equation for the Simulation of the Stationary Space Charge Distribution in Polymeric HVDC Cable Insulations

Many processes are involved in the accumulation of space charges within the insulation materials of high voltage direct current (HVDC) cables, e.g., the local electric field, a conductivity gradient inside the insulation, and the injection of charges at both electrodes. An accurate description of the time dependent charge distribution needs to include these effects. Furthermore, using an explicit Euler method for the time integration of a suitably formulated transient model, low time steps are used to resolve fast charge dynamics and to satisfy the Courant–Friedrichs–Lewy (CFL) stability condition. The long lifetime of power cables makes the use of a final stationary charge distribution necessary to assess the reliability of the cable insulations. For an accurate description of the stationary space charge and electric field distribution, an empirical conductivity equation is developed. The bulk conductivity, found in literature, is extended with two sigmoid functions to represent a conductivity gradient near the electrodes. With this extended conductivity equation, accumulated bulk space charges and hetero charges are simulated. New introduced constants to specify the sigmoid functions are determined by space charge measurements, taken from the literature. The measurements indicate accumulated hetero charges in about one quarter of the insulation thickness in the vicinity of both electrodes. The simulation results conform well to published measurements and show an improvement to previously published models, i.e., the developed model shows a good approximation to simulate the stationary bulk and hetero charge distribution.

The effect of filler loading ratios and moisture on DC conductivity and space charge behaviour of SiO2 and hBN filled epoxy nanocomposites

Nanocomposites that exhibit good insulation properties have already attracted a variety of research and their electrical properties are believed to be related to charge dynamics in bulk of materials. However, it is still unclear on how nanofiller loading ratios, surface treatment and resultant changes in morphology will influence the charge dynamics of nanocomposites. In this paper, we have clearly mentioned the influence of adding nanoparticles into epoxy resins and the characteristics of the movement of charges in the materials based on combining analysis on morphology, DC conductivity and space charge measurements. The presence of spherical nanoparticles (SiO2) introduced additional traps in bulk, which impaired the charge injection and reduced the mobility of charge carriers in samples of low filler loading ratios (e.g. 0.5 wt%). However, in silica-based samples of higher filler loadings, more nanoparticles further caused a higher density of traps, which resulted in lower average distance between arbitrary traps/inter-particle surface distances and thus charge carriers required less energy when moving from one to another by hopping or the quantum tunnelling mechanism. The surface treatment of SiO2 particles introduced deep traps which helped the separation of particles or related traps, and to some extent restricted the transport of charge carriers. In addition, hBN particles seem to act as barriers to charge injection and movement due to the layered structures and large numbers of resultant shallow traps in bulk. In terms of the moisture effect, the presence of water led to an obvious increase in charge injection and mobility, and resulted in the higher mobility of charge carriers in both base materials and within traps/particles of nanocomposites. The existence of water shells around spherical particles could contribute to a higher probability of the quantum tunnelling process and the formation of conductive percolation channels.

The influence of AC and DC voltages on electrical treeing in low density polyethylene

Electrical tree growth is a well-documented process leading to failure of high voltage polymeric insulation under AC stresses. However, tree growth in HVDC insulation failure is not well understood. This work considers electrical tree degradation in polymeric insulation subjected to combined AC and DC voltages. Tests in LDPE samples of needle-plane geometry yield three types of electrical trees, which grow depending on the magnitude of the AC components, irrespective of the DC voltages. In tests with an AC component of 10 kV (peak amplitude), 12 kV and 15 kV trees are distinguished by both the conductivity of tree channels and the tree shape, and are referred to as either conducting trees, non-conducting branch trees or non-conducting bush trees respectively. With 10 kV AC, tree initiation was significantly accelerated by superimposing −20 kV DC. With +20 kV DC, the incepted trees had more bifurcations, but there was no major change to tree initiation time. Space charge measurements on thin LDPE films provide a basis for understanding the difference between tree initiation with +DC and −DC voltages. The subsequent propagation of conducting trees were not influenced by biasing the 10 kV AC. With 12 kV AC and 15 kV AC, the growths of non-conducting branch and bush trees were both accelerated by positive biasing. A retardation was observed in bush tree growth with −15 kV DC. The effects of DC voltage polarity on tree morphology are different for branch and bush trees. Different relationships between PD magnitudes and tree growth were also found between branch and bush trees. The tree length determined PD magnitudes in branch trees and the evolution of PD in bush trees suggest that the additional DC stress has no impact to PD magnitudes. Nevertheless, DC bias effects on the symmetry between positive and negative discharges have been evidenced through PD analysis for bush trees. Moreover, the modifications on the shape of bush trees are believed to be associated with the changes in PD asymmetry. This work has illustrated the importance of AC ripples in the failure mechanisms in HVDC insulation.

Relationship between space charge behavior and trap energy density distribution: A simultaneous measurement

This paper gives a detailed introduction of a simultaneous measurement system that can obtain the space charge distribution and the thermally stimulated current (TSC) of the same sample over the temperature range 213–373 K. This measurement system adopts a 4-electrode structure and a time sharing strategy to avoid the mutual interference between the space charge measurement and the current measurement. In addition, a space charge recovery method is proposed to obtain the real space charge distribution from the original data under varying temperature situation. Furthermore, a simultaneous measurement is performed on a cross-linked polyethylene (XLPE) sample. The experimental results show that the positive charges accumulate and dominate in the bulk during polarization and the cooling stages. During the thermal stimulation stage, the accumulated charges decay after ∼330 K, and the thermally stimulated current increases significantly after ∼330 K. The analysis results imply that the release of trapped charges may be the main cause of the thermally stimulated current, and most of the charges are trapped in traps more than 1 eV.

Polyethylene blends with/without graphene for potential recyclable HVDC cable insulation

This paper presents investigations on a usage of graphene as a voltage stabilizer for polyethylene blends in the potential recyclable high voltage direct current (HVDC) cable insulation. Thermal and electrical properties of the blends with/without graphene were evaluated by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), electrical treeing tests, DC conductivity and space charge measurements. Trap energy distributions in the blends were estimated using the measured space charge decay data. The obtained results show that additive-like amounts of graphene can improve the thermal stability of the polyethylene blends. The influence of the graphene on the tree inception voltage (TIV) of the polyethylene blends is not significant, whereas the low density polyethylene/high density polyethylene (LDPE/HDPE) with/without graphene demonstrates the highest TIV comparing to the LDPE, LDPE/polypropylene, and their corresponding graphene added materials. It is found that the DC conductivity of the polyethylene blends with/without graphene increases consistently with the increase of the temperature. The conductivity of the LDPE/HDPE decreases apparently with the inclusion of graphene. It has been shown that space charge accumulations can be suppressed by the addition of graphene in the polyethylene blends. It is postulated that the high trap level and trap energy density of the polyethylene blends with graphene are closely connected to the suppression of the space charge accumulation in the blends.

An Improved Calibration Method for the Measurement of Space Charge Inside Insulating Materials

Accurate measurement of the space charge inside insulating materials facilitates reliable assessment of insulation performance. In this paper, an improved calibration scheme based on the pulsed electro-acoustic (PEA) methodology is proposed for space charge measurement. First, according to the transmission-line model of the measurement system, the simulation results indicate that the acoustic impedances of the upper electrode and the sample may affect the recorded waveforms. Then, taking into account the actual situation that the acoustic impedances of the semiconducting electrode and the sample are different, an improved calibration method is presented by analyzing the propagation process of the acoustic waves. Comparison of the results further indicates that the material of the semiconducting electrodes may affect and lower the accuracy of the traditional PEA method. In contrast, the recovered results by the proposed method are more accurate and reliable. The improved method is thereby applied to calibrate the signals from polyimide (PI) samples. Combining with the simulation results, it is concluded that different semiconducting electrodes render sensible impact on the accumulation rate and process of the space charge. Furthermore, two groups of calibrated results under the same electric field are used to verify the effectiveness of the proposed method. Since the electrode material may be an influential factor in the measurement, it should be properly chosen according to the practical applied conditions of the insulation samples to achieve a better evaluation of the charge accumulation process.

Experimental and simulation study on space charge characteristics of epoxy resin filled with graphene oxide

Graphene has attracted much attention due to its advanced properties. In previous work, the relationship between temperature and the conductivity of epoxy-based nanocomposite filled with graphene oxide has been experimentally studied. To understand the charge transport behaviour, the space charge characteristics at different temperatures and field strengths are measured with an improved pulsed electroacoustic measurement system, which is designed for high-temperature space charge measurement. From the measurement results, epoxy resin (ER) filled with multi-layer graphene oxide (MGO/ER) shows suppressed charge migration while ER filled with single-layer graphene oxide (SGO/ER) shows a hetero charge accumulation, which is assumed to be caused by the partial reduction of graphene oxide to graphene. Compared to pure ER, the apparent mobility of the nanocomposites is smaller while the trap depth and number of trapped charges are larger, which is probably caused by the introduction of traps. In the simulation, the relationship between mobility, trap depth, trap density and charge distribution are firstly studied. Then the calculated values of mobility and trap depth are given to the parameters to compare the simulated charge profiles with experiment results. The simulation results agree well qualitatively with the experimental results, giving some support for the proposed explanations.

Erratum: Pressure wave propagation method for space charge measurement in coaxial geometry [Dec 18 2139-2146

Erratum: Pressure wave propagation method for space charge measurement in coaxial geometry [Dec 18 2139-2146

A new method for carrier mobility measurement in oil immersed cellulose paper insulation

This paper reports on a new method to measure the mobilities of both positive and negative charges in insulating materials based on space charge measurement in two-layer samples. To fulfill the condition of a unipolar carrier system, a blocking material is used as a barrier on one electrode thereby preventing charge injection to ensure the unipolar charges injected from the other electrode are transported through the bulk and accumulate at the barrier. Experimentally, the carrier mobility of oil immersed cellulose paper was obtained using the proposed method, and the result of mobilities of both positive and negative charges is presented. The temperature and field dependence of carrier mobility were also investigated, and the activation energies for negative and positive charges were calculated. The obtained carrier mobility of negative charges (majority electron carriers) is approximately 2 times of that of positive charges, and the charge migration can be interpreted by the thermally activated hopping conduction mechanism.

Space Charge Measurement by Electroacoustic Method: Impact of Acoustic Properties of Materials on The Response for Different Geometries

Space Charge Measurement by Electroacoustic Method: Impact of Acoustic Properties of Materials on The Response for Different Geometries

Pressure wave propagation method for space charge measurement in coaxial geometry

The existence and accumulation of space charge in the insulation of polyethylene cable has a significant influence on the conductivity, aging and breakdown of the cable. Therefore, it is of great importance to study the space charge distribution in high voltage polyethylene cable. This paper proposes a simplified approximate analysis of pressure wave propagation measurement method in coaxial geometry. The experimental results show that, as in the case of planar sample, the peak of current in coaxial geometry obeys the simple model proposed.

Space charge characteristics in 160 kV DC XLPE cable under temperature gradient

A temperature gradient (TG) will be formed across a high voltage cable insulation resulting from Joule heat in the conductor. Therefore, it is important to measure space charge profiles in a full-scale cable under TG. In this study, an improved pulsed electro-acoustic system used for coaxial cables was established by injecting nanosecond pulses into the outer semi-conductive layer. An induced current heating and a water circulation system was used to regulate the temperatures of a cable conductor and the the outer semi-conductive layer. By this method, a TG between the cable conductor and the outer semi-conductor layer was controlled and set at various temperatures and the TG was formed across the radial insulation so as to avoid the influence of heat from the cable conductor. Space charge profiles in a 160 kV DC coaxial XLPE cable was measured under 10 kV/mm stress with polarity reversal. According to CIGRE TB496, the load cycle operation of the cable under 15 kV/mm with temperature difference of 40°C was performed for 48 and 72 h, including cable conductor cooling and heating. The results show that there is little space charge injection in the bulk at room temperature, but some remnant hetero-charges accumulates near the outer semi-conductive layer during voltage reversal. Under TG, hetero-charges appear near the outer semi-conductive layer on the low temperature side and hetero-charges increased with increasing TG. In addition, the hetero-charges increase during cable conductor heating and decrease during cooling.

Space charge and conductivity measurement of XLPE nanocomposites for HVDC insulation–permittivity as a nanofiller selection parameter

Cross-linked polyethylene (XLPE) insulation is successfully used for high-voltage AC transmission. However, it is still under development for high-voltage DC application due to space charge accumulation, which distorts the internal electrical field distribution and leads to its aging/failure. Therefore, the space charge should be measured and carefully analysed. On the other side, conductivity measurement helps to forecast the degradation probability of the insulation. Higher conductivity represents the severe degradation. Nanofiller addition, such as SiO2, TiO2, MgO and so on (<5 wt%), particularly surface-modified nanofiller due to its better dispersion significantly suppresses the space charge accumulation and conductivity. Nevertheless, the choice of suitable nanofiller has still remained a challenge. With this context, space charge and conductivity of XLPE-silica and XLPE-magnesium oxide (MgO) surface-modified nanocomposites are measured. This study proposes a parameter for nanofiller selection that will deliver optimal properties for the intended application. Results show that nanocomposites with higher nanofiller permittivity (i.e. MgO) have less space charge accumulation and low conductivity and are justified with the help of a band gap theory model.

Electrical characterization of epoxy-based molding compounds for next generation HV ICs in presence of moisture

The effects of moisture on the electrical properties of epoxy molding compounds containing high quantities of silica filler microparticles have been investigated by means of dielectric spectroscopy, steady-state conductivity and pulsed electro-acoustic space charge measurements. It has been shown that the presence of water at the filler/epoxy interfacial areas affects the low-frequency dielectric response of the materials and significantly increases their electrical conductivity, especially at higher temperatures. In addition, in presence of moisture, space charge measurements have shown the accumulation of heterocharges which significantly increase the electric field at the electrodes. These aspects are of primary importance, as the electrical properties of the encapsulation materials strongly influence the space charge distribution in high voltage devices.