Abstracts:This article presents a novel method for the direct observation of electric field profiles in solid dielectrics based on the step electroacoustic method. In the proposed approach, long pressure waves generated by a step voltage excitation are detected using a piezoelectric sensor that is significantly thicker than the test specimen. The transducer acts as a high-pass filter, allowing a step-function-like pressure signal to be observed for each space charge. As a result, the oscillogram directly shows the electric field profile in the specimen. Using an 80- $\mu $ m-thick PVDF sensor and a 50- $\mu $ m-thick PET film as the test specimen, we experimentally demonstrate that the spatial resolution of the profile after deconvolution can be less than $10~\mu $ m.

Abstracts:The inverter-driven motors are increasingly used in industrial and mobility applications, driving the demand for greater performance and reliability. Recent advances in power electronics have raised inverter output frequencies and slew rates, increasing the risk of discharge and insulation failure. A better understanding of discharge phenomena is thus essential. The authors are developing numerical simulations of dielectric barrier discharge (DBD) in twisted pairs of enameled wire. This study investigates the estimation and applicability of the secondary electron emission (SEE) coefficient ( $\gamma $ ) to the DBD simulations, addressing the lack of empirical data. As a result, fitting was found effective for estimating $\gamma $ from discharge voltage measurements. The estimated values were $4.7\times 10^{\text {-3}}$ for polyimide (PI) and $7.5\times 10^{\text {-3}}$ for polyethylene (PE). Applying these values in DBD simulations suggests the potential to estimate discharge voltages under various pressure conditions. These findings imply that DBD simulations can enhance the accuracy of predictions of discharge phenomena in twisted pairs of enameled wire.

Abstracts:As one of the most important equipment in the power system, it is of great significance to conduct fault diagnosis research on transformers. Aiming at the problem of difficult selection of parameters for support vector machine (SVM) in transformer fault diagnosis, a fault diagnosis model based on the improved pelican optimization algorithm (IPOA) optimized SVM is proposed. First, the standard pelican optimization algorithm (POA) is enhanced by introducing the Tent chaotic mapping, Levy flight strategy, and adaptive weighting strategy, and the superiority of the IPOA algorithm is verified by comparing its performance with other intelligent algorithms across four test functions. Second, feature dimensionality reduction of the data samples is performed using kernel principal component analysis (KPCA), and the IPOA algorithm is used to optimize the SVM parameters and then establish the transformer fault diagnosis model based on IPOA-SVM. Finally, the POA-SVM, northern goshawk optimization (NGO)-SVM, GWO-SVM, whale optimization algorithm (WOA)-SVM, and particle swarm optimization (PSO)-SVM models are used to conduct comparative experiments with the proposed method. The results show that the diagnostic accuracy of the IPOA-SVM model reaches 95%, which is 3.75%, 5.0%, 6.25%, 7.5%, and 8.75% higher than that of the POA-SVM, NGO-SVM, GWO-SVM, WOA-SVM, and PSO-SVM diagnostic models, respectively; and the proposed model exhibits better stability and greater adaptability.

Abstracts:The long-term synergistic effects of high-voltage direct current (HVDC) and temperature gradients inevitably stimulate charge accumulation in gas-insulated transmission line (GIL) insulators, which significantly increases the risk of insulation failure. Due to the limitations in charge measurement techniques, it is crucial to develop effective methods for accurately simulating and evaluating the dynamic charge behaviors of GIL insulators under complex operating conditions. In this study, we present a 2-D bipolar charge transport and interaction (2-D BCTI) model that accounts for both space charge and surface charge dynamics. Such a model is employed to investigate the charge behaviors of HVDC GIL insulators under varying electric fields, polarization time, and voltage polarities. Our findings indicate that higher voltages and extended polarization time lead to increased charge migration and localized accumulation within the insulator. Further analysis reveals that the surface potential at the high-voltage terminal interface is predominantly influenced by the tangential current ( ${J}_{\text {s}}$ ) along the insulator’s surface and bulky current ( ${J}_{\text {v}}$ ) passing through the insulator. In other regions of the basin insulator, such as the shielding ring and grounding terminal, ${J}_{\text {s}}$ is the dominant factor. This study provides valuable theoretical insights into the interaction between surface and space charge, offering a foundation for optimizing the insulation design of HVDC GIL systems.

Abstracts:The dissolved gas analysis (DGA) technology is a key method for monitoring large oil-immersed transformers. However, the large size, complex structure, and extended gas diffusion paths of high-capacity transformers make the diffusion characteristics of fault-generated gases within the transformer unclear, which limits the detection accuracy of DGA technology. This article investigates the diffusion characteristics of dissolved gases in a ±400 kV converter transformer under various fault types and analyzes the temporal changes in gas concentration at different monitoring points during the diffusion process. The results show that oil flow velocity is the primary factor influencing the diffusion of dissolved gases in the transformer. Although the concentration of fault gases does not affect the diffusion path of dissolved gases in the transformer, it does influence the diffusion rate. Moreover, the oil flow circulation at the turret on the ac side of the converter transformer is slow and directional, which impedes the diffusion of dissolved gases in the transformer oil, thereby hindering timely fault detection. The results of this study offer significant insights for the installation of dissolved gas monitoring devices in field converter transformers and the determination of the optimal monitoring cycle interval.

Abstracts:The tremendous demand for electricity requires higher operating voltages, which in turn impose great stress on the transformer insulation system. Condition-based maintenance and liquid decontamination are activities for assessing the risks and extending the service life of such systems. It is therefore important to explore the reclamation of the new biodegradable liquids. It is known that adsorbent-based reclamation is a prominent approach to treating insulating liquids. The present article provides the results of an experimental study on the reclamation of two low-pour-point synthetic ester fluids and a typical synthetic ester liquid using magnesium silicate-based adsorbents. The experimental results also include mineral insulating oil for a baseline reference. All four of these liquids were subjected to accelerated aging under open beaker conditions in the presence of cellulose. The aged liquids were then regenerated by pressure and gravity percolations with two magnesium silicate-based adsorbents. Physicochemical and electrical characterizations were conducted on both the feed and the filtrate. It is inferred that magnesium silicate-based adsorbents have some potential for the reclamation of synthetic esters by removing the polar compounds that evolved with the liquid’s service life.

Abstracts:In this study, aging impact analysis on dielectric behavior of mineral oil (MO), natural and synthetic ester (SE) is performed and their aging status is estimated using the hump phenomenon obtained from dielectric modulus spectrum. For this purpose, equivalent aging of 5, 10, 15, and 20 years of each oil is emulated by mixing formic, acetic, and levulinic acid with mineral and ester oils. Thereafter, Fourier transform infrared spectroscopy and frequency-domain spectroscopy (FDS) are conducted on the prepared aged samples to assess the impact of aging on physicochemical alteration and low-frequency interfacial polarization characteristics at different temperatures, respectively. Based on the FDS results, dielectric modulus spectrum is obtained, from which hump phenomenon is identified. Besides, another important factor, i.e., conduction dominance factor (CDF), is introduced to quantitatively investigate the dominance of conduction mechanism over relaxation polarization. Based on the experimental results, a correlation with the aging status of liquid insulation with the three aging sensitive parameters (hump frequency, hump peak, and CDF) is established, which can reliably be used for estimation of their aging status.

Abstracts:Hydrophobicity is one of the vital properties of outdoor polymeric insulators which prevents the water accumulation on the insulator’s surface. Due to extreme environmental conditions, polymeric insulators tend to lose its hydrophobicity property. Accurate hydrophobicity classification is essential to understand the condition of the insulators in operation within the power system network. In this study, a hybrid swin transformer (HST) is adopted to enhance the hydrophobicity classification accuracy. Traditional image classification methods struggle with variation in droplet shapes, sizes, and surface patterns which can complicate the classification process, however, the model adopted in this study integrates hybrid shifted windows with advanced vision transformer techniques to capture both short-range and long-range dependencies in images, providing a more robust understanding of complex visual patterns associated with different hydrophobicity levels. This Hybrid approach is evaluated with laboratory generated dataset (according to International Electrotechnical Commission (IEC) Standard 62073) and an online available dataset. Extensive experimental and analytical results demonstrate that the hybrid model outperforms existing state-of-the-art techniques. Additionally, an Android application for image classification is developed with a simple graphical user interface (GUI) to enhance the maintenance of the insulators.

Abstracts:A residual charge method is useful for measuring water tree in 22-kV crosslinked polyethylene (XLPE) cable systems. However, the conventional residual charge method cannot locate a water-tree degraded cable section because the time-resolved signal response from a water-tree degradation section cannot be obtained. In this study, we inject a sine wave pulse with a frequency of 1 MHz into measuring object cables when ac voltage is applied to the cables to release trapped charges due to the water-tree. We examine to locate a water-tree degraded cable section in 22-kV XLPE cable systems, including water-tree degraded cables and a Y-branch cable joint. The time-resolved residual charges induced by the voltage pulse induced by an injection coil (IJ coil) can locate a water-tree degraded cable section.

Abstracts:Power electronic converter (PEC)-fed transformers and motor insulation face a considerable challenge from the nonsinusoidal repetitive steep high-voltage pulses leading to premature breakdown. The increased local electric stresses in the winding insulation cause partial discharge (PD), leading to the degradation and aging of the windings. This study investigates functional capacitive field-grading composites (FCFGCs) to prevent the incidence of PD. PD tests are conducted on simplified test objects of motor windings, such as twisted pair samples, subjected to steeply rising square voltage waves. The FCFGC materials are fabricated by dispersing ferroelectric barium titanate (BTO) powder into polydimethylsiloxane (PDMS) matrix. The FCFGC demonstrates higher relative permittivity compared with pure PDMS, and this characteristic also shows dependence on the electric field. The current-voltage (I–V) characterization reveals that the electric field dependence of the FCFGC increases as the BTO filler contents are raised. PD experiments are carried out using unipolar square waveforms with slew rate as high as 0.22 kV/ns on enameled twisted pair wires. This involves applying a coating of pure PDMS and five distinct FCFGC samples with BTO loadings of 10%, 20%, 30%, 40%, and 50%. Experimental findings indicate that FCFGCs can reduce PD magnitudes and enhance PD inception voltage (PDIV) compared with neat PDMS insulation. Increasing the BTO filler loading reduces PD magnitudes and increases the PDIV by 2.68 times (i.e., 168%). In addition, numerical models are used to conduct electric field analyses of the twisted pair experiments. These models incorporate the field-dependent permittivity (FDP) of the FCFGC characterized experimentally, providing a more comprehensive understanding of the experimental outcomes.