Threshold For Space Charge Accumulation Research Articles

Design Method of Changing Medium Voltage Alternating-Current Cable to Direct-Current Operation

Changing the existing medium voltage alternating current cable to transmit direct current is a feasible technical transformation scheme, which has obvious advantages in cable capacity increase and operation reliability improvement. At present, there is only one project case introduced in the alternating current to direct current conversion of medium voltage alternating current cables. Most of the studies are carried out from the perspective of theoretical analysis and design calculation. The research focuses on the constraints of alternating current to direct current conversion of different cables and the design of ampacity and voltage. This paper studies the design method of converting the existing alternating current cable to direct current, analyses some problems in the current design and the technical details that need to be further improved, and discusses the constraints of converting the medium voltage alternating current cable to direct current from the aspects of current carrying capacity design, system design and system design, insulation temperature difference and electric field inversion, voltage design, space charge accumulation threshold calculation, electrical type test, etc. On this basis, this paper puts forward a design idea of alternating current to direct current transformation of medium voltage alternating current cable based on comprehensive constraints and suitable for practical engineering transformation and makes an example calculation. Finally, it gives implementation suggestions for the electrical type test of alternating current to direct current transformation of medium voltage alternating current cable.

Numerical analysis of thermo-electric field for AC XLPE cables with different service times in DC operation based on conduction current measurement

This paper presents an estimate for the influence of service time on the conduction current characteristics of a 10 kV AC XLPE. The experimental conduction current results show that the space charge accumulation threshold increases with service time. Fourier transform infrared (FT-IR) and differential scanning calorimetry (DSC) experiments show increased thermo-oxidative aging and reduced crystallinity as the service time increases. This results in deeper traps in the XLPE material which can be verified from isothermal surface potential decay experiments. Based on the experimental results, the numerical analysis of thermo-electric fields is carried out for 10 kV AC XLPE cables with different service times under bipolar DC operation mode. It shows that the DC power for 10 kV AC XLPE cable in bipolar DC operation is higher than the AC power, whereas the DC operating voltage and DC power increases with the service time. In order to ensure a safe and long service life of the cables in DC operation, a conductor temperature of 60 °C and DC operating voltage of 11 kV are recommended for 10 kV AC XLPE cables in bipolar DC operation.

Investigation on space charge and charge trap characteristics of gamma‐irradiated epoxy micro–nano composites

Epoxy nano–micro composite specimen prepared with micro silica and ion trapping nanoparticle, by shear mixing process, was exposed to gamma radiation and its performance for space charge and charge trap characteristics were analysed. The threshold for space charge accumulation of epoxy nanocomposites reduces and rate of space charge accumulation increases with an increase in dosage of gamma irradiation. The average growth of space charge density during poling and charge decay rate during depoling are relatively higher for gamma-irradiated specimens than the virgin specimen. The initial surface potential has a marginal reduction with increase in the dosage of gamma radiation, but the surface potential decay rate has increased significantly. Trap distribution characteristics indicate more number of shallow traps and increase in charge mobility after irradiation. The relative permittivity and loss tangent of the specimens have high impact due to gamma irradiation. The activation energy calculated from DC conductivity by Arrhenius law reduces with increment in radiation dose. Laser-induced breakdown spectroscopy reflected no change in elemental composition with gamma-irradiated specimen. The variation in plasma temperature and ion line to atomic line intensity ratio with dosage of gamma radiation have direct correlation to the Vickers hardness number of the specimens.

Short-term and long-term breakdown analysis of electroactive polymer with and without nanofillers

Electroactive polymers have drawn widespread interest in the electric engineering community primarily due to the large shape deformations that can be obtained under the application of an electric field. The energy storage for a linear dielectric elastomer is determined by its relative permittivity and the applied electric field (magnitude squared), and is maximized by operating the electroactive polymer at high electric fields and large stretches. In a previous study, it was shown that the relative permittivity of a silicone elastomer can be increased by adding titania nanoparticles without sacrificing the electrical characteristics such as the electrical conductivity and space charge dynamics. It was demonstrated that the threshold for space charge accumulation is about 20 kV/mm and the absence of deep traps in the titania-filler modified elastomer leads to fast space charge depletion. In the present work, the short-term and long-term breakdown characteristics of neat and nano-titania silicone elastomer films are discussed.

Effect of temperature on space charge characteristics in epoxy resin

Epoxy resin has been widely used as the dry-type insulation in high voltage direct current bushing. The temperature of the central conductor can go up to 90-105°C when working at the rated power flow. Thus, the inner insulation made of epoxy resin sustains these high temperatures. In order to study the effect of temperature on the dielectric performance, the space charge characteristics of epoxy resin at different temperatures (21, 40, 60, 80 and 100°C) have been studied with an improved pulsed electro-acoustic set-up. Moreover, the temperature dependence of the volume resistivity has also been measured. Only homocharge close to the electrodes can be found in all tests. The higher the temperature, the more serious the electric field distortion is in the bulk of a sample. The threshold for space charge accumulation decreases remarkably with increasing temperature. Both the apparent trap-controlled charge mobility and the dc conductivity show similar positive temperature dependence, which increases with the elevation of temperature, resulting in different charge decay characteristics at different temperatures. The apparent trap depths corresponding to different temperatures have also been obtained.

Study on charge transport mechanism and space charge characteristics of polyimide films

As a core component of AC drive electric locomotive, performance of inverter-fed motor directly influences the stability and reliability of locomotive running. However, insulation early failures occur frequently. Presently, polyimide film, as a special type of engineering plastic film, widely applied in turn to turn insulation and turn to ground insulation of inverter-fed motors is a kind of basic insulating material. This paper investigates charge transport mechanism and space charge characteristics of polyimide films based on measuring and comparing space charge accumulation threshold field between corona-resistant and common polyimide films and analyzing the effect of temperature on space charge distribution. The results reveal that space charge accumulation threshold field of corona-resistant polyimide film is higher than that of common polyimide film; the adding of nano-particles effectively increases dielectric properties of corona-resistant film. Additionally, raising temperature promotes the electrode injecting charge and increases the charge energy and conductivity which gradually enhances space charge number and extends trapping position into the bulk, closely related to polyimide film insulation aging and breakdown which is an important characteristic.

Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface

From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps

Quantities extracted from space-charge measurements as markers for insulation aging

The relation of space charge-derived quantities with aging stresses and time is discussed in this paper. Polyethylene-based materials, aged for different times under electrical and/or thermal stress, are subjected to space charge measurements. Quantities are derived, that is, the threshold for space charge accumulation and the apparent, trap-controlled mobility, obtained by the threshold and the depolarisation characteristics, respectively, which show time variations depending on the level of applied stress and aging time, i.e. on the extent of degradation. This indicates that such quantities can be considered as aging markers useful for the diagnosis of bulk degradation in insulation systems aged under electrical field, dc or ac, and temperature.

Investigating dc polyethylene threshold by space charge. Current and electroluminescence measurements

It has been proposed recently that the lowest level of dc field, above which electrical degradation may occur, is associated with the formation of space charges in insulation. Therefore, inference of field levels above which space charge accumulates in a given material may provide fundamental information for material characterization and system design. Threshold field values were estimated through evidence of space charge accumulation derived from conduction current, space charge amplitude and electroluminescence (EL) counts vs, electrical field characteristics. Press-molded plates of polyethylene-based materials were used for this investigation. In order to be able to detect even by EL experiments the presence of the space charge accumulated at low fields, measurements were carried out during both polarization and short-circuit of specimens. It is shown that the threshold corresponding to the onset of EL upon grounding the specimen is close to the space charge accumulation threshold derived from space charge and conduction current measurements. This earlier value is considerably lower than the one previously detected from constant dc-voltage EL measurements, thus confirming that even at low fields and in the presence of voltage reversals or transients, space charges can trigger luminescence phenomena. The methodology developed in this paper enlarges the applicability of the EL technique to accommodate a wider electrical field range in view of insulation characterization.