High Voltage最新文献

Recently, significant efforts have been exerted to replace mineral oil with environmentally friendly oils due to safety and environmental issues. However, there is a need to clarify the physical mechanisms behind the ageing impact of these oils. The authors use advanced optical spectroscopy techniques in correlation with dielectric measurements to understand the ageing processes in environmentally friendly oils as well as mineral oil. Firstly, different samples of environmentally friendly oils and mineral oil were utilised to investigate the ageing mechanism. The samples were subjected to different ageing periods using a thermal accelerated ageing process. Secondly, the severity of the produced byproducts due to the oil degradation is examined based on several measured properties representing macroscopic and microscopic categories. The macroscopic category was evaluated through dielectric properties, including breakdown voltage, dielectric permittivity, and dissipation factor. The microscopic category, on the other hand, was assessed using techniques such as ultraviolet–visible absorption spectroscopy and photoluminescence spectroscopy. These techniques enabled a deep understanding of the molecular-level changes occurring in the oil under ageing conditions, thereby getting new insights into oil ageing mechanisms. It is worth mentioning that natural ester oil demonstrated the most favourable performance across various properties under ageing conditions.

This paper proposes a method of impulse current generator modulated DC arc by combining the advantages of pulse and the RF to solve the low electron energy problem of direct current arc. Through experimental analyzing the electrical, spectral, and optical characteristics of the arc, the effect of impulse current generator (ICG) on improving electron energy is discussed. The results show that the ICG consumes more energy to enhance the strength of arc discharge, and therefore electron energy is increased in a microsecond scale. In addition, it is found that the electron energy of the arc discharge can be adjusted by varying inductance, capacitance, and discharge tube: increasing the inductance or capacitance can increase the electron energy firstly and then decrease it. In adjusting the three adjustable components, adjusting the inductor is the most effective method, followed by adjusting the capacitor, and adjusting the repetition frequency has the least effect. The reason is discussed, and it is believed that the results are related to leakage inductance and distributed capacitance.

In recent years, the powdering phenomenon often has been found in suspension composite insulators operating in outdoor environments, and there is currently a gap in research on the composition and formation process of powdered substance. A method for simulating powdering of silicone rubber (SiR) in a high humidity salt-fog environment is proposed, and the obtained powder is compared with the natural powder. Test results show that the powder obtained from artificial environments is similar to the natural powder in coastal areas, which proves that this method is reasonable to simulate the powdering process. Powdered substances are generally composed of two types of substances, Type I is an inorganic filler and its dehydration product and Type II is a small molecular siloxane. The average particle size of artificial powder is 8–10 μm, while that of natural powder is 3–5 μm. All the dielectric properties of powdered SiR decreased, and after 12 h of ageing, the dielectric properties of the artificial sample were close to those in coastal areas, and the element composition was also similar. Aluminium hydroxide (Al(OH)₃) crystals were detected in both powders. The change trend of the characteristic functional groups in the infrared spectrum of the artificial powder is consistent with that of the natural powder, but the degree of molecular chain bond destruction is lower.

Stress grading systems using non-linear resistive coatings are a key component to suppress surface corona in the end-windings of rotating machine. Compared to a sinusoidal-fed motor, the high slew rate of the voltage at the flanks of the repetitive square voltages from the inverter cause large capacitive currents to flow in the main wall insulation. These large currents, if not properly considered in the design phase, lead to severe electrothermal stress of the grading system. Experiments and simulations were conducted on a stress grading system whose structure arises from limitation posed by the motor structure. Measurements performed with different rise times show that the maximum potential along the conductive armour tape (CAT) increases non-linearly with increasing axial distance, and the potential at the edge of the CAT reached nearly twice the peak-to-peak voltage at 500 ns rise time, leading to corona inception. As metal plates are used in the machine to dampen vibrations in the end-winding, similar plates were also fastened to the stress grading system, worsening the already inadequate corona suppression performance. The stress grading system was therefore modified, avoiding the surface corona while, at the same time, reducing the temperature in the grading system to acceptable levels.

A strategy was proposed to predict accurately the dielectric permittivity and dielectric strength based on the Onsager local field theory. The molecular dynamic simulation was utilised to analyse the dipole moment fluctuation in polyimide (PI) to reflect the polarisation response of applied electric field. The simulation results revealed that the optical dielectric permittivity and static dielectric permittivity were 2.62 and 3.34, showing that the electronic displacement polarisation of the PI acted as a major contributor. The deviation between simulated and measured values of the frequency-dependent relative dielectric permittivity was no more than 5%, which exhibited high accuracy. As the polarisation response of the polar groups in the PI was at infrared frequencies, the conductive loss may be the dominant role in 10²–10⁶ Hz at room temperature. Moreover, the effect of Joule heat on the structure, dielectric permittivity and Young modulus was considered to accurately predict the dielectric strength of the PI (359 kV/mm), which was in agreement with experimental values in the literature. These results establish a clear correlation between structural characteristics and dielectric properties of the PI, which would be the theoretical insights into the design and synthesis of the PI with tailored dielectric properties.

The weak interfacial strength and porous structure of the meso-aramid paper cause mechanical and insulating deficiencies. Enhancing density, regulating pore structure, and improving interfacial interactions of meso-aramid are crucial for promoting the performance of meso-aramid papers. A PMIA/polyethylene terephthalate (PET) composite paper was prepared using the wet method with meso-aramid short cut fibres (PMIA) and PET pulp as raw materials. The sandwich-structured PMIA/PET paper was achieved by covering PET microfiber non-wovens served as the surface layer while the PMIA/PET composite paper acted as the core layer. During the high-temperature hot pressing process, the PET pulp transformed into a viscous melt that coated on aramid fibres in between layers, forming a typical ‘reinforced concrete’ interface structure within the core layer of the PMIA/PET composite paper. The PET non-wovens on top and bottom surfaces were converted into a dense PET film that filled and covered the holes and defects in the PMIA/PET composite paper. This unique structure enabled the sandwich-structured PMIA/PET composite paper to exhibit excellent tensile strength (80.41 N/cm) and breakdown strength (56.35 kV/mm), surpassing most reported performances of meso-aramid papers in literature. This work not only provides novel insights for preparing high-performance meso-aramid papers, but also shows potential applications for other materials and structures.

The authors explore the limitations of silicone rubber (SR) in tropical environments due to its lack of antimicrobial activity. The use of a 2-methylisothiazol-3(2H)-one compound with 5-chloro-2-methylisothiazol-3(2H)-one is proposed as an efficient and environmentally friendly antimicrobial agent to enhance the fungistatic properties of SR. The fungicide added specimen was obtained by directly adding a quantitative amount of isothiazolinone to the polymer system. Through an in-depth analysis of the material properties of MCMSR, the form in which 2-methyl-4-isothiazolin-3-one/5-chloro-2-methyl-4-isothiazolin-3-one (MIT/CMIT) exists in the molecular structure of the SR crosslink system was carefully explored. The accelerated hydrolysis process of polysiloxane, which caused fungi secretion, was proven to be suppressed due to the effects of MIT/CMIT. The antimicrobial mechanism of isothiazolinone in preventing microbial growth was interpreted as the apoptosis and necrosis procedures of Aspergillus niger cells, which were verified by flow cytometry results. By choosing the appropriate concentration of isothiazolinone, the original properties of the SR were successfully maintained while effectively preventing the aggregation of isothiazolinone. This opens up a new research direction and application prospect for the antimicrobial modification of SR.

When designing high-voltage elements, knowing if the corona effect will be present during their life cycle is relevant. Therefore, designers consider several prediction criteria based on physical features related to breakdown discharge principles to predict the corona effect. The introduced practical set-up consists of a concave cone electrode with a hemispheric tip above a plate to evaluate selected corona prediction criteria. The hemisphere has a fixed diameter of 7 mm, and the electrode separation ranges from 2.5 to 39 cm. Information about the corona mode inception under different voltage sources was extracted using an intensified charge-coupled device camera and a partial discharge metre. The prediction criteria were connected to a specific corona mode depending on the main discharge structure behind its development. The average deviation between these criteria and experimental results was around eight percent. Underlying assumptions in criteria are also discussed in light of the experimental results.

With the increasing penetration of DC systems into the high voltage and medium voltage power industry, DC voltage is becoming common for distribution/transmission and to supply different typologies of loads. The allowable extent of AC ripple superimposed to DC, and its effect on insulation ageing, is a long-term discussed topic. The most harmful phenomenon causing extrinsic ageing acceleration and insulation system premature failure is partial discharges (PD); thus, the risk of incepting PD due to AC ripple could become a primary issue for electrical asset equipment reliability. In this work, the impact of AC sinusoidal ripple on insulation system life and reliability is dealt with, considering both intrinsic and extrinsic ageing but focusing on the latter, that is, the PD aspect. Experiments are performed to assess how the jump voltage (due to AC ripple) and the DC component impact on PD activity in terms of amplitude and repetition rate. For the first time, the correlation between the magnitude of jump voltage associated with ripple and PD inception is established, shedding a light on the allowable ripple extent which does not impact significantly on ageing and premature insulation failure. This approach can provide straightforward tools for design specification and ageing inference of insulation systems.

Polymeric insulation is a critical component of high voltage systems. However, exposure to high electric stress can cause partial discharges (PDs) to occur and may result in the deterioration of insulation and lead to dielectric failure. These PD events are accompanied by the production of acoustic pressure impulses in the polymer. Detection of these acoustic pressure impulses can reveal the presence of PDs and locate their source. However, analysing the detected acoustic emission (AE) signal is challenging. The acoustic pressure source's nature and the propagating medium's properties, such as density, viscosity, and elasticity, significantly affect the propagating AE signal. The effects of the hardness of the polyurethane (PU) on the propagating AE signal are reported by the authors based on results obtained from laboratory experiments. It was observed that the decay rate in the magnitude of the acoustic impulse was high in PU at all hardness levels following an exponential behaviour. The analysis of the frequency spectra indicates that the higher frequency components attenuate more strongly with distance. These laboratory results can be valuable for engineers and industries as they provide valuable insight into how the physical characteristics of a material affect the propagation characteristics of AE signals during the detection and location of PD source using the AE detection technique.