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The operational stability of a power transformer exerts an extremely important impact on the power symmetry, balance, and security of power systems. When the grid load fluctuates greatly, if the load factor of the transformer cannot be maintained within a reasonable range, it leads to increased instability in grid operation. Adjusting the transformer capacity based on load changes is of great significance. The existing control methods for on-load capacity-regulating (OLCR) transformers have low timeliness, and the daily switching frequency of the capacity-regulating switch is not controlled. To ensure the safe and stable operation of transformers, this paper proposes a control method for OLCR transformers based on load prediction and fuzzy control. Firstly, the operating principle of OLCR transformers is analyzed, and a multi-strategy enhanced dung beetle optimizer (MSDBO) combined with a CNN−LSTM model is proposed for load forecasting. On this basis, the daily switching frequency of the capacity-regulating transformer is introduced, and hesitant fuzzy control is used to select the optimal capacity-regulating strategy relying on three factors: loss, economy, and switching frequency. Finally, simulation models are constructed using the MATLAB/SIMULINK platform and simulation analysis is conducted to verify the effectiveness and superiority of the proposed control method. For the three scenarios in this paper, the method reduces daily power loss by 28.5% to 56.3% and daily operating costs by 25.4% to 50.8%. The method used in this paper can sacrifice 3.5% to 9.2% of the loss reduction capability in exchange for reducing the number of switch operations by 28.6% to 57.1%, significantly extending the lifespan of the switches and thereby increasing the operational lifespan of the transformer. [ABSTRACT FROM AUTHOR]

As a core component of the smart grid, capacitive equipment plays a critical role in modern power systems. When defects occur, they pose a significant threat to the safety of both other equipment and personnel. Hence, it is of great significance to predict whether defects occur in capacitive equipment in advance. To achieve this goal, we propose a novel method that integrates the weight of evidence (WOE) feature encoding with machine learning (ML). Five models, including support vector machine (SVM), random forest (RF), extreme gradient boosting (XGBoost), multi-layer perceptron (MLP), and linear classification, are employed with WOE features for defect prediction. Furthermore, based on the prediction of equipment with defects, an additional prediction is conducted to determine the potential defect level of the equipment. Experimental results demonstrate that the performance of each algorithm significantly improves with WOE encoding features. Particularly, the RF model with WOE encoding features exhibits optimal performance. In conclusion, the proposed method offers a promising solution for predicting the occurrence of defects and the corresponding defect levels of capacitive equipment. It enables relevant personnel to focus on and inspect equipment predicted to be at risk of defects, thereby preventing major malfunctions. [ABSTRACT FROM AUTHOR]

Changes in soil electrical parameters can affect the distribution of earth potential in high-voltage direct-current (HVDC) ground electrodes when climatic conditions changes. This paper proposes a model to describe the relationship between surface-layer soil moisture and surface-layer soil resistivity under short-term climatic influence, by using image recognition technology. Based on the relatively stable resistivity of lower soil layers, a soil model more reflective of the actual operating conditions is established for soils near the ground electrode, and a finite element method is adopted to calculate the earth-surface potential (ESP). The experimental results indicate the following: (1) Compared with other measurement methods, image recognition of surface soil resistivity is a low-cost, real-time, online, and accurate method; and (2) changes in surface-layer soil moisture affect both ESP and step voltage. These effects are large in the case of high resistivity for the soil layer where the ground electrode is buried. This large fluctuation in step voltage particularly results in a potential safety hazards during ground electrode operation. Therefore, in order to ensure personal safety and obtain more accurate electrical parameters, it is necessary to consider the effect of natural climate on the soil surface resistivity. Finally, the value of the step voltage can be observed using image recognition, this also provides a new method for the safety monitoring of the DC ground electrode.

Natural ester is viewed as the most promising future alternative to mineral oil due to its high degradability, but its inferior breakdown characteristics prevent its application of large power equipment. The purpose of this research is to investigate the breakdown characteristics of natural ester. First, the natural ester's ionization potential and molecular number density are calculated. Then, an improved natural ester streamer discharge model is developed using the Poisson's equation and the carrier continuity equation, taking into account the effect of temperature on carrier mobility and incorporating the computed ionization potential and other factors. The results for different lightning impulse voltages were analyzed by COMSOL software. Comparing the mineral oil, the results indicate that the natural ester breakdown voltage is close to and slightly lower than that of the mineral oil but the acceleration voltage is very similar to the breakdown voltage. The lower ability of esters in prevention against the development of fast and energetic streamers is consistent with the description of the experimental results. In addition, the model takes into account the effect of temperature on the carrier mobility. As the applied voltage increases, the change of temperature in the streamer channel accelerates the carrier motion and promotes the propagation of the streamer. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Nanocomposite (NC) polymers are commonly employed because of their high breakdown strength, but the underlying mechanism of this phenomenon is not well understood. In the present study, specimens of polyethylene terephthalate (PET) films coated with epoxy resin (EP) and filled with nano-SiO 2 and Si 3 N 4 at concentrations of 0, 0.5, 1, 2, 5 and 10 wt % were subjected to DC and impulse voltages to breakdown. The effects of the types of nano-fillers and concentrations on the dielectric properties, crystallinity, and space charge trap characteristics were investigated on the breakdown strength. The experimental results show that improved DC breakdown strength of a sandwich structure of PET/nano-EP film is obtained even at 10 wt%, which is attributed to the effect of increased trap density for deep traps. In addition, to experimentally establish the dominant factors during impulse breakdown, a systematic theoretical model was used to account for the variations of impulse breakdown performance. It is concluded that the frequency-dependent dielectric characteristics plays an important role in determining the absorption of activation energy for trapped electrons, which affects the instantaneous insulating performance of NC polymers. [ABSTRACT FROM AUTHOR]

As the core component of composite insulation for dry-type high-voltage equipment, polytetrafluoroethylene (PTFE) films are subjected to high temperatures and complex electric fields. Therefore, this study explores the effects of electric field and temperature on the space charge characteristics of double-layered PTFE films based on the measurement of space charge distribution in double-layered PTFE films under different DC voltages and temperatures using the pulsed electro-acoustic measurement system. Results show that more charges accumulated in the PTFE sample subjected to a positive electric field during polarization and the space charges dissipated more rapidly during depolarization compared with those subjected to a negative electric field. The existence of a PTFE/PTFE interface increased the density of trap and deepened the trap. The positive charges that accumulated at the PTFE/PTFE interface during polarization gradually held a dominant position with the increase of temperature. Finally, the formation of charge in the PTFE/PTFE interface mainly depended on two factors. When the applied DC voltage was not large, the charge formation was mainly dominated by the Maxwell-Wagner polarization effect. When the voltage increased to a certain threshold, the charge formation was mainly caused by the charge injection and migration characteristics. [ABSTRACT FROM AUTHOR]

Polyimide film (PI) is widely used as dry-type smoothing reactor insulation for ultra-high voltage direct current (UHVDC) projects. In practice, the temperature of a central conductor can reach 80 °C to 100 °C at the rated power flow. Thus, the inner insulation composed of PI sustains these high temperatures. In this study, the space charge characteristics of PI at different temperatures (40 °C, 60 °C, and 80 °C) and thermal aging degrees (336, 500, 1000, and 1500 h) were examined with an improved pulsed electro-acoustic (PEA) measurement system to investigate the effects of temperature and thermal aging on dielectric performance. The apparent trap depths corresponding to various aging degrees were obtained according to the space charge decay dynamics at the interface. Results indicate that positive charges likely accumulate within the bulk of the material because of the thermal aging of homogeneous materials. The polarity of space charge at the interface also changes from positive to negative as aging degree increases. The space charge decay dynamics at the interface further shows that the trap energy increases as the thermal aging degree increases. The amount of residual space charge also increases remarkably as the electric strength and aging degree increase. The electric field distortion is closely related to temperature, thermal aging degree, and electric strength. [ABSTRACT FROM AUTHOR]

The polyethylene terephthalate (PET) is widely used in dry-type insulation application because of its excellent insulation properties. In practice, thermal aging of PET is often encountered, but the related mechanisms and effect of thermal aging on the PET dielectric properties still remain unclear. In the present paper, we presented a systematic and comparative study on the effect of thermal aging on surface microscopic morphological characteristics, DC breakdown strength, trap depths and the apparent mobility. The results indicate that when aged at 130?C, the surface topography becomes coarser than that aged at 80?C. The polarity reversal at the PET/PET interface during the polarization indicates that the injection abilities of Al and Sc electrodes are gradually enhanced when aged at 80?C and 130?C, respectively. Moreover, the measured isothermal discharge currents during depolarization show that the trap depth corresponding to the maximum trap density increases significantly in the beginning of aging, and then it decreases gradually when aged at 80?C, but it increases evidently when aged at 130?C with increasing the aging degree. It also has been found that the apparent trap-controlled charge mobility is greatly enhanced when aged at 80?C with the increase in thermal aging degree. This enhancement is proved to be associated with the decrease in trap energy level at the PET/PET interface. Also, it can be preliminary concluded that the enhancement of DC breakdown strength (at 1500 h) is closely related to the increase in electron type trap energy and its density when aged at 130?C. [ABSTRACT FROM PUBLISHER]

Grounding resistance is an important basic index used to ensure the safety and reliability of grounding systems. In this paper, a method for grounding resistance measurement based on the fall of potential curve is proposed that can be performed using a 2–2.5D lead wire length. In this method, the potential distribution near the auxiliary current electrode is tested and is used as the main analysis and measurement parameter. Based on the relationship between the potential distribution generated by the grounding grid and the current electrode, the grounding resistance of the grounding grid can be deduced. First, the voltages between the grounding grid and the earth surface potential near the current electrode are measured; then, the ground potential rise (GPR) of the grounding grid is calculated through theoretical analysis and numerical calculation; and finally, the grounding resistance is calculated by dividing the potential by the test current. This method avoids the complex task of accurate compensation point location required for traditional measurement methods, and can be performed easily and quickly using a short lead wire. Measurement results from spot testing show that the proposed method can meet the accuracy requirements of engineering testing. [ABSTRACT FROM AUTHOR]

To understand the effect of thermal ageing mechanism on space charge formation and dissipation in double-layered polyester films (PET), the time dependence of space charge characteristics at physical interface and in the bulk of PET films have been investigated. The measurements of space charge characteristics in specimens of different thermal ageing conditions (80 and 130 °C) were conducted under different DC field strengths (polarization) and short circuit conditions (depolarization) using the Pulsed Electro Acoustic (PEA) measurement system. The influence of electric field distortion caused by the space charges on average AC breakdown strength of PET specimens was also investigated. Results indicate that the thermal ageing leads to polymer interface acts as positive charge traps at ageing temperature of 80 °C, but acts as electron traps at ageing temperature of 130 °C and provides deeper trapping level for positive charge carriers. Besides these, average breakdown strength of the composite decreases due to the electric field distortion caused by the space charges. A preliminary study of this phenomenon is given and further investigation is needed. [ABSTRACT FROM AUTHOR]

The MIE (Minimum Ignition Energy) characterizes the danger of a kind of combustible gas and its ignition behavior, and the antistatic test for the non-metallic materials is of great value to mining industry. This paper describes an antistatic level testing system for the non-metallic materials used in mines, and the methods to calculate the MIE of the ignition system, electromagnetic energy and scale the antistatic grade are also presented in details. According to the structure of the antistatic level testing system, the model to calculate the electromagnetic energy of the ignition system is set up firstly, then the MIE and electromagnetic energy before and after ignition are computed respectively. The ignition energy coupling coefficient is defined to indicate the ratio between the energy used for ignition and the total electromagnetic energy, based on which the quantity of triboelectric charge produced by the testing material can be obtained. The numerical relation between the ignition energy coupling coefficient and electrode gap is investigated, for the system described in the paper the threshold of ignition energy coupling coefficient increases from 0.42 to 0.77 when the gap distance increases from 1 mm to 5 mm. The system designed in the paper can be used to test the possibility of ignition and explosion for non-metallic devices used in mine, and the safety limit space for different material can be determined. In addition, it also offers reference for the scaling of triboelectric charge. [ABSTRACT FROM PUBLISHER]

Estimation of the degree of aging and the remaining lifetime of composite insulators is an important issue. In this paper, one shed of a 500 kV AC composite insulator that has operated for three years on a transmission line was studied. The insulator shed was cut longitudinally into four layers from the upper surface to the bottom surface and tested by the thermally stimulated current (TSC) method, and each layer was also tested by Fourier transform infrared (FTIR) spectroscopy. The results show that the first layer has fewer trap charges than the other layers in the TSC test, but it also has the highest trap level, and the trap depth of the first layer is deeper than that of the other layers. The trap levels and the quantities of shallow traps decrease successively from the second to the fourth layer. In the FTIR spectra, the absorption peaks of the first layer are lower than those of the other layers, and the other three layers have similar absorption peaks. The aging state of the first layer is more serious than that of the other layers. The study indicates that long-term contamination and ultraviolet (UV) radiation are the main reason for the aging differences among the different insulator shed layers. [ABSTRACT FROM AUTHOR]

The large differences in the load peak and valley of rural distribution networks always bring severe problems to system planners and operators. Given this issue, this paper deals with the hot-spot temperature (HST) of the transformer and its overload capability, and proposes a modeling method-based online monitoring of practical parameters. In the current work, a temperature-fluid coupling field of the 315 kVA vegetable oil distribution transformer is developed in both the two-dimensional and three-dimensional geometry, by which the convection and heat dissipation process can be studied. The grid of the model is divided into regions to increase the calculation speed and ensure the accuracy of the calculation. Secondly, tests related to the temperature rise of the transformer are carried out. The accuracy of the three-dimensional model is later discussed in terms of temperature and fluid velocity distribution. Finally, the temperature distribution laws of the amorphous alloy vegetable oil distribution transformers (AVDT) are compared and analyzed under different load conditions. Findings reveal that the AVDT has low no-load loss and strong overload capacity, which is capable of reducing the internal overheating accidents of the transformer. [ABSTRACT FROM AUTHOR]

Under operational conditions of high electric fields and elevated temperatures, the accumulation of space charges at multilayer insulation interfaces is often considered as an important factor affecting insulation performance. This study experimentally explored the influence of different thermal aging degrees (110 °C for 0, 720, 1600, 2100, and 2900 h) on physicochemical characteristics. The space charge dynamics in two-layered thermally aged PET-PET films were measured using the pulsed electro-acoustic (PEA) method and simulated on the basis of a one-dimensional modified bipolar charge transport model. The parameterization for key parameters involved in the model was analyzed through parameter sensitivity. Results indicated that the molecular structure, crystallinity, and dielectric spectra of the PET films are affected by thermal aging. The thermalization process also has noticeable effect on the surface state characteristics, which are characterized by deeper trap depth and larger trap density. Several experimental phenomena measured by the PEA method were observed on the basis of numerical simulation. [ABSTRACT FROM AUTHOR]

A detailed research on the streamer discharge local mechanism is necessary to enhance and develop the streamer theory. In the present paper, the characteristics of a streamer discharge in air at an atmospheric pressure are investigated using a hydrodynamic model based on plasma chemistry. Aside from continuity equations for electrons and ions, as well as Poisson?s equation for the electric field, the model includes the electron mean energy density equation and the neutral particle density continuity equation. The electric field strength and the velocities of streamer propagations obtained from proposed model are in good agreement with experimental and simulation results in literature. The mean electron energy is nearly constant in the streamer channel, except for a peak value at the streamer tip. Although the mole fraction of oxygen is only about 0.2 in air, oxygen ions are the dominant charged particles in the streamer discharge. At the electrode surface, the charged particle densities significantly decrease in the direction of the electrode because of surface reactions. [ABSTRACT FROM AUTHOR]