Space Charge Accumulation Phenomenon Research Articles

Time Evolution of Partial Discharges in a Dielectric Subjected to the DC Periodic Voltage

Partial discharge (PD) detection can be considered one of the most useful tools for assessing the insulation conditions of the power apparatus in high-voltage systems. Under AC conditions, this analysis is widely employed in online and offline tests, such as type tests or commissioning, and can be carried out by applying the phase-resolved PD (PRPD) method, since the patterns can give information about the defect classification. Under DC voltages, the classic pattern recognition method cannot be performed, and the measurements show complexities related to the nature of the phenomena. For this reason, to date, a standard for PD measurements under DC does not exist. In previous papers, a new method for PD detection under DC stress voltages has been proposed by the authors. It is based on the application of a direct current periodic (DCP) waveform useful in obtaining PRPD patterns. The dependence of partial discharge inception voltage (PDIV) and PD repetition rate (PDRR) on the δ shape parameter of the DCP for different materials, as well as the capability to recognize different discharge phenomena, provided valid indications on the behavior of PD in the transition from AC to DC. The aim of this paper is to evaluate the time dependence of PD occurring in a dielectric by applying the DCP waveform. In our previous studies, the investigations were focused on the PD behavior under different values of the DC voltage periodic part. In another work, the DCP waveform with both positive and negative polarity was applied to several dielectric materials. In the proposed work, instead, the DCP waveform is applied for a long time in order to observe its effect on the PD behavior for 72 h. In this way, due to the space charge accumulation phenomenon, the aging effect, also due to the space charge accumulation phenomenon, is evaluated. The methodological approach was to acquire PRPD patterns over time and evaluate their trends in comparison with the sinusoidal case. The experimental results show that, with a DCP waveform having δ = 0.6, the aging effect similar to that provided by pure DC stress is observed, while the acquired PRPD patterns are easily interpretated, as in the AC case.

Partial Discharges in HVDC Cables - The Effect of the Temperature Gradient During Load Transients

The paper investigates the role of the space charge accumulation phenomena in the inception of Partial Discharges (PD) in a defect within the bulk of the dielectrics of a High Voltage Direct Current (HVDC) cable. It is widespread accepted that the number of PD occurring in an HVDC cable during its lifetime is largely less than that under AC voltage stress. This is essentially due to higher values of the voltage between the conductor and the screen needed to trigger PD under a steady DC voltage than the same value under AC stress. Nevertheless, due to the dependency of the electrical conductivity of the insulation on the electric field and temperature, PD in HVDC cables is strongly influenced by the temperature distribution and, consequently, on the load current. Thus, the main goal of this research is to evaluate the main parameters of the PD occurring in an HVDC cable during a load transient. Numerical simulations by using Comsol Multlphysics® are validated by the results obtained with an ad hoc experimental setup. A model DC cable, subjected to transient phenomena due to a current step and with an air void defect in the insulation material, is simulated and setup. On the other hand, an experimental setup is used to carry out PD measurements on the same case study. The results of this research show the strong dependency of the PD parameters on the temperature distribution varying over time during a load transient.

Space-charge accumulation and band bending at conductive P3HT/PDIF-CN2 interfaces investigated by scanning-Kelvin probe microscopy

Charge transfer processes and space charge accumulation phenomena are fundamental topics concerning the technological applications of organic heterointerfaces.

Simulation research on space charge characteristics of zinc oxide varistor

Zinc oxide (ZnO) lightning arresters have good protection performance and have been widely used in power systems. ZnO varistor is the core component of the arrester, and its electrical performance determines the overvoltage protection level of the arrester. ZnO varistor will produce space charge accumulation phenomenon under the action of electric field. In order to study the space charge characteristics of ZnO varistors more deeply, a bipolar carrier transport model considering the change of grain boundary structure was established, and the law of carrier migration in ZnO grain boundaries was simulated. From the three aspects of pressure time, ambient temperature, and depletion layer density, the space charge distribution inside the grain boundary, Schottky barrier height, and grain boundary leakage current are systematically studied.

The Effect of the Axial Heat Transfer on Space Charge Accumulation Phenomena in HVDC Cables

To date, it has been widespread accepted that the presence of space charge within the dielectric of high voltage direct current (HVDC) cables is one of the most relevant issues that limits the growing diffusion of this technology and its use at higher voltages. One of the reasons that leads to the establishment of space charge within the insulation of cables is the temperature dependence of its conductivity. Many researchers have demonstrated that high temperature drop over the insulation layer can lead to the reversal of the electric field profile. In certain conditions, this can over-stress the insulation during polarity reversal (PR) and transient over voltages (TOV) events accelerating the ageing of the dielectric material. However, the reference standards for the thermal rating of cables are mainly thought for alternating current (AC) cables and do not adequately take into account the effects related to high thermal drops over the insulation. In particular, the difference in temperature between the inner and the outer surfaces of the dielectric can be amplified during load transients or near sections with axially varying external thermal conditions. For the reasons above, this research aims to demonstrate how much the existence of “hot points” in terms of temperature drop can weaken the tightness of an HVDC transmission line. In order to investigate these phenomena, a two-dimensional numerical model has been implemented in time domain. The results obtained for some case studies demonstrate that the maximum electric field within the dielectric of an HVDC cable can be significantly increased in correspondence with variations along the axis of the external heat exchange conditions and/or during load transients. This study can be further developed in order to take into account the combined effect of the described phenomena with other sources of introduction, forming, and accumulation of space charge inside the dielectric layer of HVDC cables.

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.

The Role of Right Interpretation of Space Charge Distribution for Optimized Design of HVDC Cables

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.

Contributed Review: Review of thermal methods for space charge measurement.

The space charge accumulation phenomenon has garnered great interest over the last two decades because of the increased use of direct current in high voltage electrical systems. In this context, a significant relevance has been achieved by the thermal methods, used for solid dielectrics. This paper presents a review of this non-destructive measurement system used for the measurement of space charge. The thermal pulse method, the thermal step method, and the laser intensity modulation method are described. For each configuration, the principle of operation, the thicknesses analyzed, and the spatial resolution are described, reporting also the main related applications.

Effect of space charge on breakdown characteristics of aged oil-paper insulation under DC voltage

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.