Epoxy‐based dielectrics are extensively used in grid‐connected energy systems and modern microelectronics as electrical insulation, adhesive, and packaging components. However, space charge accumulation in epoxy‐based dielectrics is a foremost factor threatening device stability and lifespan, especially under conditions of high voltage direct current and high temperatures during long‐term operation, and thus are investigated systematically. This article reviews the state‐of‐the‐art progress in understanding and regulating the space charge behavior of epoxy‐based dielectric materials. The development of space charge measurement techniques is first introduced. Then, experimental observations of macroscopic space charge characteristics in epoxy dielectrics under different conditions, along with the advancement of nanofiller‐doping strategies for improving space charge suppression capabilities, are discussed and emphasized. Afterward, simulation progress based on the bipolar charge transport models and quantum chemistry calculations are summarized to provide some insight from a microscopic perspective. It is concluded by providing a brief summary and highlighting future research opportunities in inhibiting space charge accumulation of epoxy‐based dielectrics for practical applications in power equipment and electronic devices.

Space Charge Behavior in Epoxy‐Based Dielectrics: Progress and Perspective

This paper presents a detailed review of the streaming electrification phenomena of different insulating fluids for power transformers. The comparison of different techniques used to assess the charging tendency of fluids is discussed depending on the flow type (planar or centrifugal), volume of oil, and interface material. The charge separation between the insulating fluid and metallic/pressboard interfaces is explained in terms of the electrical double layer formation involving a fixed layer and diffuse layer. Based on the experimental results, the streaming electrification is observed to be a function of various factors such as speed, temperature, electric field, and surface roughness. Depending on the molecular structure of insulating liquids that come into contact with solid insulation at the interface, the streaming current can increase; hence, a suitable additive (benzotriazole, fullerene, Irgamet 39) is selected based on the type of fluid and charge polarity. The degradation of the insulating liquid upon ageing, which increases the streaming current and reclamation of such aged fluids using adsorbents (Fuller’s earth, activated carbon, bentonite, and alumina), is a possible method to suppress the static current through improving its dielectric properties. The nanofluids show a higher streaming current compared to base fluid with no change observed even after the reclamation process. The energization process using alternating current (AC) and direct current (DC) impacts the streaming phenomenon depending on its magnitude and polarity. The diffusion of sulfur compounds in the insulating liquid is another major hazard to transformers because the sulfide ions affect the physio-chemical reaction at the interface material, which is responsible for the formation of streaming current.

/pdf/streaming-electrification-of-different-insulating-fluids-in-9m0394ck.pdf

Streaming Electrification of Different Insulating Fluids in Power Transformers

Silicone rubber (SR) insulators have been coated with different concentrations of NaCl to study the contamination level adopting laser-induced breakdown spectroscopy (LIBS) analysis. An elemental study of LIBS spectral data has identified the sodium peaks, thereby indicating the presence of salt deposits. A regression coefficient is used for a better understanding of the direct correlation between the salt deposit density (SDD) and the normalized intensity ratio, at various standoff distances. A marginal decrement in the normalized intensity ratio of the sodium peaks has been noticed in the case of readings taken at morning and midday, compared to those taken at night at different SDD values and different energies of the laser pulse. With increase in SDD, the intensity distribution characteristics have a right shift on the intensity scale. The polluted SR specimens are successfully classified by employing artificial neural network technique. Overall, the LIBS method is successful in identifying the variations in the salt deposition on the surface of the insulator, even at far distances of 15 m and at any time of the day.

Influence of Standoff Distance and Sunlight on Detection of Pollution Deposits on Silicone Rubber Insulators Adopting Remote LIBS Analysis

The service of mineral insulating oils for power transformer insulation and cooling aspects cannot be disavowed. However, the continued use of mineral oils is questionable due to environmental unfriendliness and the divestment from fossil fuels. This has provoked the quest for green alternative insulating liquids for high-voltage insulation. Natural esters are among the remaining alternatives that are renewable and environmentally friendly. Regardless of their environmental and technical merits, natural esters have some limitations that are slowing down their total acceptance by transformer owners and utilities. Critical limitations and concerns include esters’ pour point, viscosity, oxidative stability, and ionization resistance. In this work, the state of the art of “natural esters for transformers” is explored with the aim of potential improvements. The sections of the article are geared towards technical viewpoints on improving the overall workability and serviceability of natural esters in high-voltage applications. A comprehensive review of the existing literature is achieved, based on performance improvements of the natural ester using “additives” and “chemical modification”. The authors hope that this report may be helpful to transformer owners as well as influence the progression of natural esters for power transformer applications.

/pdf/natural-esters-for-green-transformers-challenges-and-keys-2kfr7461.pdf

Natural Esters for Green Transformers: Challenges and Keys for Improved Serviceability

The globally observed trend toward electrically supplied devices is common in the transportation segment. Traditionally, the performance of electrical insulation has been assessed with respect to electric stresses and endurance. In all current-carrying devices, magnetic fields will also be superimposed on electric ones; therefore, the influence of a magnetic field’s exposure on partial discharge (PD) dynamics is an actual research topic. This problem is important in the transportation segment as well, where low- and medium-voltage levels are mostly used for supply and energy conversion (which are associated with high load currents). This article is focused on the influence of magnetic fields on the dynamics of PDs in the voids of insulation systems of transportation power devices and refers to both ac and dc cases. A rather weak magnetic field (80 mT) was established in order to detect and analyze early stages of PD behavior. The measurements of PDs that were carried out in a dedicated setup revealed their impact on PD intensity, which have been visualized in time-sequence diagrams and PD images. This detected effect is attributed to both the elongation of the charged particle trajectory and the enhancement of the electron energy due to acceleration. It has been shown that the impact of a magnetic field can be observed within a supply voltage frequency range of 20–400 Hz (which is typical for the transportation segment). The PD intensity was amplified in the above range at a magnetic field induction of 80 mT (even up to 50%). The effect of a magnetic field can be recognized as an additional modulation factor that influences the dynamics of PDs.

https://ieeexplore.ieee.org/ielx7/6687316/7098407/09804795.pdf

Effect of Magnetic Field on Partial Discharge Dynamics in Insulation Systems of Transportation Power Devices

This article reports an experimental investigation of the impact of magnetic field on corona discharge activity in mineral oil, using both ultrahigh-frequency (UHF) and fluorescence techniques under ac voltage. The results show that corona inception voltage (CIV) increased under ac voltage with varying harmonic frequencies without considering any phase shift and reduced marginally under magnetic field. In addition, the impact of the magnetic field shifted the dominating frequency of the UHF signal (0.9 GHz) obtained under ac voltage toward lower frequencies (0.3 and 0.6 GHz). Under high magnetic fields, the parameters such as rise time, fall time, and pulsewidth of the fluorescence signal increased in its magnitude compared to its effect without a magnetic field. Also, using both the UHF sensor and the fluorescence sensor, there was no clear difference in the phase-resolved partial discharge (PRPD) pattern caused by corona discharge (with and without magnetic field). Furthermore, the number of discharges is observed to be high under the impact of harmonic ac voltages with third- and seventh-order containing total harmonic distortions (THDs) of 4% and 40%.

Impact of Magnetic Field on Corona Discharge Behavior of Mineral Oil Under AC Voltage

This study reports experimental investigations of the effects of different surfactants (CTAB, Oleic acid and Span 80) on silica based synthetic ester nanofluids. The positive and negative potential observed for the ionic (CTAB) and non-ionic surfactant (Span 80) from zeta potential analysis indicates an improved stability. The optimization of nanofillers and surfactants is performed considering the corona inception voltage measured using ultra high frequency (UHF) technique and fluorescent fiber. Rheological analysis shows no significant variation of properties with shear rate, implying Newtonian behavior even with the addition of surfactant. In addition, the permittivity of the nanofluid is not much affected by adding surfactant but a marginal variation is noticed in the loss tangent with the effect of temperature. The fluorescence spectroscopy shows no change in the emission wavelength with the addition of silica nanofiller and surfactants. Flow electrification studies indicate an increase in the streaming current with the rotation speed and temperature, with a higher current magnitude observed in the case of nanofluids.

Impact of Surfactants on the Electrical and Rheological Aspects of Silica Based Synthetic Ester Nanofluids

This work reports an experimental and theoretical analysis of the rheological properties of mineral oil-based SiO2 nanofluid for their potential applications in transformer insulation. The flow electrification mechanism on the nanofluids with different surfactants such as cetyl trimethyl ammonium bromide (CTAB), oleic acid, and Span 80 is studied using a spinning disk technique. The results show a higher streaming current for the nanofluids with CTAB as a surfactant compared to oleic acid and Span 80. The rheological behavior of nanofluids is explored with the double gap concentric cylinder geometry. The variation of shear stress with shear rate follows a power law relationship along with a yield stress observed for all the nanofluids. A transition is seen from storage modulus to dominant loss modulus for the nanofluids during the frequency sweep analysis, whereas no transition is observed in the case of mineral oil. In addition, regression analysis using artificial neural network (ANN) algorithms are performed on the experimentally measured viscosity of the nanofluids in order to estimate theoretical parameters and provide insights into the streaming current formation. The desirable rheological characteristics of nanofluids are identified for achieving enhanced insulation performance in transformers.

Experimental Study and ANN Analysis of Rheological Behavior of Mineral Oil-Based SiO2 Nanofluids

The present study deals with the characterization of electrical and rheological behaviour of the Synthetic ester fluid incorporated with various concentration of Aluminium nitride (AlN) nanoparticles. Zeta potential measurement has shown an excellent stability of the ester fluid mixed with the nanoparticle. Corona inception voltage (CIV) measured using ultra-high frequency (UHF) technique and the results confirmed higher discharge resistance for nanofluid dispersed with 0.005% of AlN. Phase resolved partial discharge (PRPD) analysis with higher harmonic voltages has shown increase in pulse magnitude and number of discharges. Dielectric constant and tan <inline-formula> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> measurements have revealed a slight marginal increase in its value with the inclusion of filler. Streaming current magnitude was found to increase with the disc rotational speed, concentration of nanoparticle and ambient temperature. Fluorescence spectroscopy analysis has shown increase in fluorophores intensity with filler dispersion in the ester fluid. Rheological properties of fluids mixed with nanoparticles are inspected with cone plane geometry configuration. Synthetic ester and its nanofluid exhibited Newtonian behavior of flow pattern and are independent of the applied shear rate. An exponential decay of viscosity of nanofluids was observed with temperature and it follows an Arrhenius law. The calculated activation energy has increased with the incorporation of filler concentrations. The complex modulus of AlN nanofluids was observed alongside exhibiting the viscous behavior where the loss modulus <inline-formula> <tex-math notation="LaTeX">$\text{G}''$ </tex-math></inline-formula> is much larger than the storage modulus <inline-formula> <tex-math notation="LaTeX">$\text{G}'$ </tex-math></inline-formula>, with the applied frequency.

https://ieeexplore.ieee.org/ielx7/6287639/6514899/09745099.pdf

Somasundaram Karthikeyan Amizhtan

Papers

Impact of Magnetic Field on Corona Discharge Behavior of Mineral Oil Under AC Voltage

Impact of Surfactants on the Electrical and Rheological Aspects of Silica Based Synthetic Ester Nanofluids

Experimental Study and ANN Analysis of Rheological Behavior of Mineral Oil-Based SiO2 Nanofluids

Investigation on the Electrical and Rheological Properties of AlN-Based Synthetic Ester Nanofluids

Study on water droplet initiated discharges and charge trap characteristics of laser textured silicone rubber nanocomposites