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In the field of high-voltage transmission systems, different degradation phenomena affect the reliability of the employed components. In particular, under dc stress, the space charge accumulation phenomenon is believed to be the most responsible of the dielectrics lifetime reduction. To measure the accumulated space charges in flat specimens, the pulsed electro-acoustic (PEA) method is one of the most used techniques. The working principle of the PEA cell is based on the acoustic waves propagation and detection. As is well known, the acoustic waves propagating in different means are partially transmitted and partially reflected. Therefore, the piezoelectric sensor of the PEA cell is subjected to both direct and reflected waves. The latter, which are also present in the PEA cell output signal, if are not taken into account, could be easily confused as a charge accumulation. For this reason, in this paper, the acoustic waves’ behavior within the PEA cell has been simulated in order to avoid incorrect interpretation of the PEA cell output charge profiles. In this paper, compared to other works present in the literature in which only the surface charges were considered, the space charges accumulated in the insulation bulk are also considered for a complete analysis of the waves behavior during the PEA measures. The results found in this paper are very useful for cable manufacturing industries in which the choice of the dielectric materials is strongly influenced by the space charge accumulation phenomenon. [ABSTRACT FROM AUTHOR]

A new perception of interface charge accumulation in HVDC cable joints was inspired by novel measurement methods of PEA and Q(t) and quantum chemical calculations. [ABSTRACT FROM AUTHOR]

This paper deals with the reliability of the Pulsed Electro-Acoustic (PEA) technique in the case of a specimen containing an air layer. The first approach to this study has been proposed by the authors in previously published works. In these papers, the mathematical description, the PEA cell simulation model, and some experimental tests have been reported. In this work, a more accurate description of the acoustic wave behavior within the PEA cell and specimen with and without an air layer is given. Moreover, the comparison between simulation and experimental tests for both cases (specimen with and without air layer) allowed the validation of the previously developed PEA cell simulation model. The latter was previously validated only for a single layer specimen, here the good performances of the model have also been confirmed in the case of a multilayer specimen, also with an air layer. Experimental and simulation results show that the air layer acts as a barrier for the acoustic signal, due to the different acoustic impedance between the air and the solid dielectric material which constitute the specimen. Therefore, the aim of the present work is to demonstrate that in the case of a three-layers specimen, composed as dielectric-air-dielectric, the PEA cell is not able to provide the complete profile of the entire specimen under test.

Abstract: Electron beam irradiated fluorinated materials, which are used for an optical solar reflector of spacecraft, have been investigated using a space charge measurement system under dc high stress. It was found that a large amount of space charge accumulated in the electron beam irradiated ethylene‐tetrafluoroethylene and fluorinated ethylene‐propylene copolymer under dc high stress, while it was not observed in nonirradiated them. In this report, a dependence of relaxation time after the irradiation on the space charge accumulation characteristics was also studied. [ABSTRACT FROM AUTHOR]

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.

Cross-linked polyethylene (XLPE) is used in electric power cables as insulation. The maximum thickness of an XLPE layer is 27 mm in a 500 kV class electric power cable. The pulsed electroacoustic (PEA) method is widely used to measure the space charge distribution in dielectric materials. When measuring the space charge distribution in dielectric materials using the normal PEA method, the acoustic wave (PEA signal) from which the space charge profile is obtained is detected by a piezoelectric device in the system. Usually, the sample thickness is less than 10 mm in a normal PEA system. When a thicker sample is used, the space charge measurement is very difficult to obtain with a normal PEA system, because the amplitude of the acoustic wave from the upper electrode decreases with thickness compared with that of the acoustic wave from the lower electrode. Using a redesigned PEA system, space charge distributions have been measured in the 27-mm-thick XLPE sample in a 1 kV/mm DC electric field. The experimental results show that the redesigned PEA system can detect a PEA signal from the lower and upper electrodes in the 27-mm-thick XLPE sample. The deconvolution technique applied to the dispersion in the thick XLPE sample has also been improved. [ABSTRACT FROM AUTHOR]

Investigation of space charge accumulation processes in a chemically cross-linked polyethylene for ac usage (XLPE) and a modified XLPE for dc usage (DC-XLPE) at voltage polarity reversal was carried out using pulsed electro-acoustic (PEA) measurement system. We found that a huge amount of so called packet-like charge, some of which partially increased the electric field in the material to more than twice of the average applied stress, generated in XLPE at the polarity reversal of DC stress. It enhanced the electric stress locally in the sample by almost twice of the applied average stress. On the other hand, we also found that the packet-like charge was not observed in DC-XLPE under the same experimental condition. [ABSTRACT FROM PUBLISHER]

Due to its excellent performance, oil-paper insulation has been widely used in many power apparatuses including converter transformer of high voltage direct current (HVDC) system. Space charge accumulation phenomena may significantly distort local electric field inside transformer and cause oil-paper insulation overstressing. As a result of long term operation, oil-paper gradually degrades. In most severe cases, the breakdown of insulation system may occur. In this work, the effect of space charge on breakdown characteristics of aged oil-paper insulation is investigated. Oil-paper samples with different aging states are obtained by thermally accelerated aging for various periods of time, categorized by the degree of polymerization (DP) of 1107 (new), 725, 504, 376 and 241, respectively. Experimental results show that the breakdown voltage of oil-paper samples without pre-stressing remains almost constant with thermal degradation. After the homo-polarity pre-stress, when the DP value is higher than ∼300 (from 1107 to 376), the breakdown voltage has an increasing tendency with an increase in thermal degradation. On the contrary, the breakdown voltage presents a decreasing tendency after the hetero-polarity pre-stress. When the DP is lower than ∼300 (DP241), the changes of breakdown voltage does not remain the same trend as previous samples after pre-stressing. Meanwhile, space charge distributions in different aged samples are measured by the pulsed electro-acoustic (PEA) method, and the trap parameters are correspondingly evaluated. It has been experimentally demonstrated that special attention should be paid to space charge accumulation and aging state of oil-paper insulation, which may lead to severe insulation accidents. [ABSTRACT FROM AUTHOR]

The space charge phenomenon in PE (polyethylene) particularly in LDPE (low density polyethylene) is being widely researched for more than two decades. Despite various theories proposed by researchers, the charge dynamics in general and charge packets in particular, inside the polymeric insulation are yet to be completely known. The authors have investigated the dependence of charge dynamics on electric field using PEA (pulsed electroacoustic) technique. The experiments were carried out at 25 °C on 100 μm LDPE film samples under applied electric fields of 100–150 kV/mm. External current measurements were also performed under similar conditions. The analysis of charge profiles and external currents shows that apart from other factors, charge formation and polarity of charge also depend on history of electric field application. Interesting observations were made on the consequence of such results. Experiments were performed repeatedly on virgin samples, taken from same lot, and the results were found to be almost same. It is believed that the effect of history of voltage application on charge formation, presented in the paper, is useful for understanding practical behavior of LDPE which, if used in power cables, is expected to serve as a dielectric for several years — accumulating a lot of history of voltage application. [ABSTRACT FROM AUTHOR]

As the temperature gradient always exists in the insulation of power equipment, the influence of temperature gradient on oil-impregnated paper insulation is studied by experiments and simulation. It's found that in the both single-layer and double-layer structures, the heterocharges always appear near the lower temperature electrode, and increase with the enhancement of temperature gradient and field stress, resulting in more considerable field distortion. However, there are more heterocharges in the single-layer samples while the field distortion is more severe in double-layer samples. Numerical simulations were carried out to investigate the mechanism of temperature gradient effect. It is indicated that the space charge accumulation is mainly due to the temperature dependence of the charge injection and conduction. The behavior of interface charges can be well described by assuming an energy barrier for charge transportation at the interface. [ABSTRACT FROM AUTHOR]

Abstract: Space charge is a key insulation problem of HVDC equipment. In this paper, the influence of voltage polarity and electrode material were studied by the pulse electroacoustic (PEA) method. It had been found that the type of space charge accumulating at the interface was the same with the voltage polarity. With different electrode materials, the space charge injection was consequently different, but not evident. The interface made the drift velocity slower in two-layer samples. It suggested that the interface had a significant impact on space charge distribution and the contact state was complex, so a modified Maxwell–Wagner model was proposed. [Copyright &y& Elsevier]

In our previous paper, we found an interesting packet-like charge behavior in low-density polyethylene (LDPE) under high dc electric field of more than 1 MV/cm. When such a high dc electric field was applied to LDPE film, a huge amount of positive packet-like charge of more than 100 C/m³ was injected into the bulk of the film and it moved towards the cathode side increasing its amount of charge. The movement of the packet-like charge gradually became slowly, and then it stopped in the bulk as it seems to be a stable state. Consequently, the electric field near the cathode was sometimes enhanced by up to more than twice the applied electric field. To analyze the feature of the positive packet-like charge behavior in LDPE, we carry out a simple numerical simulation based on the experimental results in the previous paper. The model is considered the principle of conservation of charge and the hypothesis that there are two different conductivity in front and rear of the packet-like charge in the sample. The hypothesis is based on the idea that the zone where is passed through by the packet-like charge has a higher conductivity than that in front of the packet-like charge because the charge traps are filled by passing of the packet-like charge. By considering such a simple model, we have a good agreement between the numerical simulation and the experimental results. [ABSTRACT FROM AUTHOR]

The pressure wave propagation (PWP) method was developed 25 years ago to study space charges in solid dielectrics. This method has been a powerful tool in this field of research but has also proved its efficiency in adjacent fields for the characterization of materials or processes. In this paper the history of this method and its principle are recalled. The pulsed electro-acoustic (PEA) method, which can be physically described in a similar way and for which data can be analyzed in an equivalent fashion, is also presented. Examples of applications and developments are given which indicate the direction which these methods might take in the future. [ABSTRACT FROM AUTHOR]

This paper deals with the reliability of the Pulsed Electro-Acoustic (PEA) technique in the case of a specimen containing an air layer. The first approach to this study has been proposed by the authors in previously published works. In these papers, the mathematical description, the PEA cell simulation model, and some experimental tests have been reported. In this work, a more accurate description of the acoustic wave behavior within the PEA cell and specimen with and without an air layer is given. Moreover, the comparison between simulation and experimental tests for both cases (specimen with and without air layer) allowed the validation of the previously developed PEA cell simulation model. The latter was previously validated only for a single layer specimen, here the good performances of the model have also been confirmed in the case of a multilayer specimen, also with an air layer. Experimental and simulation results show that the air layer acts as a barrier for the acoustic signal, due to the different acoustic impedance between the air and the solid dielectric material which constitute the specimen. Therefore, the aim of the present work is to demonstrate that in the case of a three-layers specimen, composed as dielectric-air-dielectric, the PEA cell is not able to provide the complete profile of the entire specimen under test. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]