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The development of high‐precision, non‐destructive, and three‐dimensional (3D) in situ imaging of micro‐scale damage inside polymers is extremely challenging. Recent reports suggest that 3D imaging technology based on micro‐CT technology causes irreversible damage to materials and is ineffective for many elastomeric materials. In this study, it is discovered that electrical trees inside silicone gel induced by an applied electric field can induce a self‐excited fluorescence effect. Based on this, high‐precision, non‐destructive, and 3D in situ fluorescence imaging of polymer damages is successfully achieved. Compared with the current methods, the fluorescence microscopic imaging method enables slicing of the sample in vivo with high‐precision operation, realizing the precise positioning of the damaged area. This pioneering discovery paves the way for high‐precision, non‐destructive, and 3D in situ imaging of polymer internal damage, which can solve the problem of internal damage imaging in insulating materials and precision instruments. [ABSTRACT FROM AUTHOR]

Epoxy-based composites with high insulation and thermal conductivity are desirable materials for electronic and electrical applications. However, resolving the tradeoff between insulation and thermal conductivity remains challenging. Based on the functional requirement, we designed and fabricated a cactus-like double-oriented epoxy composite by combining magnetic orientation and ice-templated methods. Semiconducting SiC endowed the composite with field-grading characteristics, thus relieving local electric field stress along the horizontal direction, while BN with high thermal conductivity promoted heat dissipation along the vertical direction. The composite exhibited its highest performance with 15 vol% filler, improving the breakdown voltage and thermal conductivity by 43.7% and 1312% compared with pure epoxy, respectively, outperforming recently developed packaging materials. It is believed that this work offers an efficient strategy for the fabrication of the filler structure and provides insights on the simultaneous enhancement in insulation and thermal conductivity of polymer composites. [ABSTRACT FROM AUTHOR]

The real-time monitoring and scientific prewarning of local overheating for electrical equipment and electronic devices are of great significance but remain challenging. Therefore, in this work, a temperature-responsive microcapsule is prepared to endow insulating materials with the ability to thermally sense external temperature fields. Specifically, in the microcapsule/epoxy composites, the epoxy resin served as the polymer matrix was doped with reversible thermochromic microcapsules, acting as a temperature indicator whose color transition temperature is centralized between 45 °C and 55 °C. The test results indicate that the color of the prepared composites can sensitively vary in response to the fluctuations in external ambient temperature, which was illustrated by the behavior of isothermal melting. Further analysis reveals that composites are able to precisely locate multiple tiny overheated areas and present remarkable thermal durability undergoing 480 h of aging at 70 °C. The thermochromic materials designed in this article provide a new technological route for large-scale thermal sensing applications of electrical equipment. [ABSTRACT FROM AUTHOR]

This article examines and calculates the probability of failure probability of a rod-plane air gap (RPAG) under a damped alternating overvoltage (DAOV) based on a large amount of experimental data and the probability density distribution of measured overvoltages in a power system. By changing the length of the RPAG, the authors propose a prediction model for the breakdown characteristics of any length of RPAG, with the variable of DAOV parameters (including the damping factor and frequency). The results show that the model can adequately predict the breakdown characteristics of any RPAG under the action of the DAOV. Based on color channel representation, we show the results of 4-D prediction in the form of a 2-D graph. The authors combine this with a system economic model to develop a model for dynamic insulation coordination (DIC). With changes in the length of the RPAG, this model can dynamically characterize the safety factor of the system to efficiently and accurately determine whether the insulation for the air gap meets the relevant safety requirements. The proposed DIC model helps improve the accuracy of insulation design and can be used in industrial applications as a statistical method for coordinating insulations in power systems. [ABSTRACT FROM AUTHOR]

Fiber optic current transformers (FOCTs) have advantages over conventional current transformers. However, the application of FOCTs is significantly limited by the birefringence effect caused by temperature and vibration. Thus, this study proposed a sensing scheme of coupling a fiber polarization rotator to improve the robustness of FOCTs. First, the mathematical model of the proposed FOCT was derived using the Jones matrix, and the error caused by the birefringence effect was computed. Then, the FOCT was modeled by applying the finite element method to verify the effectiveness of the proposed FOCT in improving the temperature and vibration robustness. Finally, experiments were conducted, including temperature, vibration, and frequency response characteristics. The proposed scheme was compared with the basic polarimetric detection (PD) scheme and several state-of-the-art schemes. Results show that the maximum error of the proposed scheme is 1.7% in the temperature range from $- 20 ^{\circ }\text{C}$ to $80 ^{\circ }\text{C}$ (that of the basic PD scheme ranges from −7.05% to 9.75%). In the vibration test, the maximum output error of the proposed scheme is 0.72%, while that of the basic PD scheme reaches 4.89%. Meanwhile, the proposed FOCT meets the requirements of the measurement of high-amplitude and high-frequency transient currents, such as lightning impulse current, in power systems. [ABSTRACT FROM AUTHOR]

Multi-chamber arc-extinguishing has been widely researched and used to protect transmission and distribution lines from lightning. Its principle is to divide a long-gap arc into multiple short-gap arcs using multiple semi-enclosed arc-extinguishing chambers. To study the dynamic evolution of arc plasma in an arc-extinguishing chamber, we establish a three-dimensional simulation model of arc plasma in a semi-enclosed arc-extinguishing chamber by solving the Navier–Stokes and Maxwell equations. We analyze the dynamic evolution of an arc from the inside of the chamber to the outside. An impact current test platform is built and the arc motion is photographed by a high-speed camera, revealing the acceleration of the arc in the semi-enclosed chamber. Finally, the influence of chamber dimensions on key arc parameters is analyzed. In the simulation, the time when the arc completely moves out of the chamber and is extinguished is consistent with the experimental images, which verifies the accuracy of the simulation model. The arc temperature, chamber pressure, and airflow velocity first increase in an oscillating manner and then slowly decrease with time. The change of the opening diameter will lead to the obvious change of the arc temperature maximum. The opening diameter is the main chamber dimension that affects the arc temperature. The electrode spacings have the most obvious effect on the maximum pressure in the chamber. The minimum pressure in different chamber dimensions is lower than the atmospheric pressure. The influence of opening diameter, distance from central axis to closed side, and opening wall length on the maximum airflow velocity is the same, and the maximum airflow velocity decreases with the increase in these dimensions. The influence of electrode spacing on the maximum airflow velocity is opposite to them. [ABSTRACT FROM AUTHOR]

In this letter, a new method is proposed to measure the magnetizing inductances of dual reversible models of single-phase two-winding transformers. Different from all available methods, a dc source is used to excite the transformer to produce deep-saturation through an air-core inductor. A small ac source is applied to the other winding to measure the two magnetizing inductances with only one test, significantly reducing the cost. The cost is further reduced because the necessary equipment is of smaller size than that for the available methods. The proposed method is validated experimentally and with finite elements simulations for two single-phase transformers. [ABSTRACT FROM AUTHOR]

• Environmentally friendly super-insulating material with 3D nanoporous structure. • High breakdown field strength of the material composited with atmospheric CO 2. • Novel insulation form and carbon sequestration capacity for the future GIL. • Theoretical calculation for a substantial improvement of breakdown field strength. Gas-insulated transmission lines (GILs) play a pivotal role in facilitating the future long-distance transmission of power generated by renewable energy. However, the extensive employment of gaseous SF 6 presents formidable obstacles to scaled application of GILs, due to its relatively low breakdown strength and substantial greenhouse effect. In this work, a fantastic phenomenon was discovered that the breakdown field strength of insulating gases can be significantly enhanced under the confinement of nanostructures, based on it, the starch/polyorganosiloxane biocomposite insulating dielectric (S/PBID) with three-dimensional nanoporous structure was designed and fabricated. Specifically, insulating gases (CO 2 , N 2 , SF 6) composite with S/PBID, the breakdown field strength is significantly improved by 744.44 %, 520.74 %, and 310.95 % compared to the original gases, respectively. The relative permittivity of S/PBID (<2) is considerably lower than that of most existing insulating materials. Theoretical analysis suggests that the nanopores within S/PBID restricts electron multiplication and transport processes at the lateral scale, leading to a substantial enhancement in gas breakdown field strength. The nanoporous material is expected to be composited with atmospheric CO 2 to overturn the conventional gas–solid composite insulation form of the GILs, thereby providing a novel insulation form of high-performance, environmental-friendly, and carbon sequestration. [ABSTRACT FROM AUTHOR]

Faults on the dc line threaten the stability and reliability of the modular-multilevel-converter-based HVDC (MMC-HVDC) systems. Fault current limiting is one of the crucial steps to handle the dc line fault. In this paper, a mutual-inductance-type fault current limiter (MFCL) using coupled reactors and few power electronic switches is proposed. Compared with the conventional dc reactors, it can automatically limit the fault current during the current rising stage with an increased equivalent inductance and accelerate the decline of the current by reducing the inductance of the discharge circuit. Consequently, the fault current stress and the total capacity of required arresters are decreased. Compared with the existing power electronic-type fault current limiters, the proposed MFCL provides an inexpensive and potential solution to the suppression of dc fault current using simple fault current limiting topology with fewer power electronic switches. The simulation case study and hardware experiments are carried out to verify the performance and practicality of the proposed MFCL. [ABSTRACT FROM AUTHOR]

Nano-dielectrics, which were discovered more than 20 years ago, were once considered as promising insulators. Adding or hybridizing inorganic oxides into polymer matrices inevitably results in poor compatibility between multiphase materials and thus results in limited breakdown strength improvement (generally lower than 20%). Therefore, we bypassed the traditional nano-doping method and proposed the direct construction of matrix materials on nanostructures, that is, a ceramic fiber-porous silica composite insulating paper. The breakdown strength of prepared dielectric was remarkably improved by 120.5%. To the best of our knowledge, such a high improvement in electrical performance has not been reported in previous studies. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Enclosure coating is a promising technique of metallic particle contamination deactivation in DC gas-insulated transmission lines (DC GIL). However, the conventional insulation film cannot inhibit partial discharge (PD) at triple junction around the contact between the particle and the film, which has adverse effects on metallic particle deactivation. In the present work, we therefore fabricated electrically nonlinear conductive film by doping ZnO micro-varistors in epoxy resin matrix. And we measured the conductivity and dielectric characteristics of film with various doping concentrations. We experimentally and theoretically investigated the effects of film properties and particle radius on suppression of PD and particle lift-off statistics. The developed nonlinear conductive film was proved to effectively suppress PD and better increase the particle lifting voltage than conventional insulation film, which is of significance in the long-term reliable operation of DC GIL. [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]

The lack of dc fault clearance capability is a major obstacle that restricts the development of high-voltage direct current (HVdc) systems. Employing an active mechanical dc circuit breaker (M-DCCB) is an effective method to handle the dc fault. However, the existing active M-DCCBs can interrupt the fault current once but have problems in making the second interruption instantly in case of reclosing a permanent dc fault. In this paper, a novel topology of the active M-DCCB is proposed by inserting a precharged capacitor into a bridge formed by four spark gap switches. The operating strategy for the proposed M-DCCB is discussed to handle the dc faults. Its parameters, including the oscillation inductor and the oscillation capacitor, are determined by inequations derived from the requirements of dc fault interruption. The selection standard for the spark gap switch is set based on its breakdown characteristics. Finally, simulations are conducted to investigate the performance of the proposed M-DCCB. Results show that the capacitor can accumulate a sufficiently large reverse charge voltage automatically charged by the fault current. The proposed active M-DCCB topology and the dc fault handling strategy can interrupt the dc fault in HVdc power systems consecutively. [ABSTRACT FROM AUTHOR]

Highlights • The Preisach model denoted by I - Φ is determined by the centered cycles method. • The combination of the Preisach model and the three-component model is proposed. • The composite curve with hysteresis part and deep saturation part is proposed. • The inverse dynamic Preisach model is implemented in ATP-EMTP. • The transformer model can simulate the dynamic magnetization and deep saturation. Abstract The transformer model is very important for the transient simulations of power systems. Although a large number of researches have been carried out on the developments and applications of the transformer model, there are still some deficiencies in the study, such as hysteresis and frequency-dependence. This paper proposes an improved transformer model for the calculation of low-frequency transients in power systems. The magnetizing branches are simulated by the composite curves with hysteresis part and deep saturation part. The expression of the inverse static I - Φ Preisach model (ISPM) is derived from the classical Preisach model (CPM). The Preisach distribution function (PDF) is determined by the centered cycles method. Based on the dynamic performance of the grain-oriented electrical steel, the inverse dynamic Preisach model (IDPM) with three components is proposed. The deep saturation inductance can be obtained from the modified AC plus DC test. Then, the modified dual reversible model of the single-phase transformer is developed in ATP-EMTP. At last, the ISPM and IDPM are validated by the comparison of the measured and simulated loops. The steady-state and transient performances of the proposed transformer model are also validated by the comparison of the experiments with simulations including no-load test, DC bias test, and inrush current test. [ABSTRACT FROM AUTHOR]

The imaging results present the spatial development morphology of electrical trees inside the polymer. Nondestructive 3D Imaging of Microscale Damage inside Polymers - Based on the Discovery of Self-Excited Fluorescence Effect Induced by Electrical Field (Adv. Sci. 25/2023) B Nondestructive 3D Imaging of Microscale Damage b In article number 2302262, Potao Sun, and co-workers report for the self-excited fluorescence of a microscale-damaged microchannel inside a polymer under the influence of an electric field. [Extracted from the article]

Lightning usually contains multiple return strokes. Under multiple impulse currents, the main characteristics of soil media are not only those of breakdown but also those associated with recovery of electrical parameters. In this paper, the discharge recovery characteristics of soil under successive current pulses and the influences of various main factors are studied. The structural characteristics of soil subject to successive impulse discharges are analyzed from X-ray images taken during discharges. On the basis of experimental results, the modification method of soil resistivity under successive impulse currents is discussed. Depending on the conditions, these results show that the peak value of the impulse current and the water content of the soil have a significant effect on the rate and duration of recovery. However, the effect of soil salt content on recovery is not significant. Under successive impulse currents, the electrical characteristics of soil take about tens to hundreds of milliseconds for full recovery. [ABSTRACT FROM AUTHOR]

The impulse voltage wave invading a transformer is not the double exponential impulse wave (DEIW) specified by the IEC but a non‐standard waveform with oscillation attenuation characteristics, and this fact leads to doubts about the scientificity and optimality of insulation coordination. A key solution for this issue is the establishment of an equivalent relationship between the measured non‐standard impulse voltage wave and the DEIW. In this study, a test platform of multiparameter impulse voltage was set up. Then, the authors obtained the 50% breakdown voltage (U50%) of the oil‐impregnated paper (OIP) for transformer under the bipolar oscillatory attenuated impulse wave (BOAIW) and the DEIW with different waveform parameters. The effects of waveform parameters on the breakdown characteristics of the OIP were finally revealed. On the basis of experimental data and several mathematical methods, the authors established the U50% prediction mathematical models for the OIP under the BOAIW and DEIW. Based on the prediction models and the microfluctuation characteristics of measured overvoltage's waveform parameters, the authors proposed a method for evaluating the waveform equivalence. The characteristics of voltage amplitude and change rate were regarded as the two key factors for the waveform equivalence of this method. [ABSTRACT FROM AUTHOR]

The development of high‐voltage direct current circuit breaker (HVDCCB) has progressed rapidly. One of the major reasons for the failure of a circuit breaker is the metallic particle contamination. An enhanced mathematical model is proposed to simulate the motion dynamics of a spherical copper particle in the break of sulphur hexafluoride (SF6)[AQ ID=Q2] HVDCCB. By linking COMSOL and MATLAB, the interaction between particle motion and electric field distribution and plastic random movement in the collision process is considered. The effects of voltage magnitude and particle size on the particle motion are discussed. By means of statistical methods, the obtained findings show that, as voltage magnitude decreases or particle size increases, the particle–anode collision frequency decreases, and the particle–anode collision region increases in the x‐direction but decreases in the y‐direction. The accuracy of the proposed model is verified by experimental observations via high‐speed video camera. [ABSTRACT FROM AUTHOR]

Since the first application of oil-paper composite insulation in power transformers 100 years ago, researchers are now trying to improve the performance of transformer oil by suspending nanoparticles in the dielectric. However, the performance of paper impregnated with nano-modified oil remains unclear. We previously found nano-oil impregnated paper had a good capability to sustain repeated impulses. The amount of space charges accumulated in the nano-oil impregnated paper is much smaller than that in fresh oil impregnated paper. We considered this behavior to be the main reason for the better electrical performance of the nano-oil-impregnated paper. In this work, based on those achieved results, we focus on the modification mechanism of nano-oil impregnated paper. We analyzed the effects of TiO2 nanoparticles on the relative permittivity and their relationship with the charge injection process. We also discuss the influence of TiO2 nanoparticles on charge trap states and the space charge behavior. The dielectric characteristics and trap energy of both nano-oil-impregnated paper and fresh-oil-impregnated paper are determined in this work. [ABSTRACT FROM AUTHOR]

This paper proposes an improved centered cycle method for identifying the Preisach distribution function based on the non-uniform discretization. The principle and the limits of the centered cycle method are first introduced. Then, the non-uniform discretization method is proposed based on the curvature of the anhysteretic magnetization curve. The numbers of loops for identifying the distribution function are decided by the proportion of the integral of curvature. The method to determine the cell values of the non-uniformly discrete Preisach triangle is also introduced. The proposed method is validated by the laboratory test of a toroidal sample. The results show the higher precision of the proposed method for predicting the hysteresis loop than the centered cycle method by the same number of loops used for identifying. [ABSTRACT FROM AUTHOR]

Overvoltage data collected by the measurement systems in substations contain vital information about the power grid. With the development of smart grids, the mining and processing of this overvoltage data are of increasing significance. Unfortunately, there is currently no way of preventing error waveforms from being incorporated into the overvoltage database. In this paper, a cleansing framework for the measured overvoltage data is proposed. By automatically extracting and clustering features, error waveforms at the substation can be successfully isolated. In terms of implementation, a step-by-step scheme for pre-cleansing and full cleansing is established to improve the quality of the overvoltage data. Experimental results using measured overvoltage data demonstrate the effectiveness of the proposed solution. The proposed method and framework support the cleansing of overvoltage data, and are conducive to the analysis of the generation, propagation, and distribution characteristics of the overvoltages. [ABSTRACT FROM AUTHOR]

There are multiple discharges in one breakdown during a lightning incident. A lightning incident usually contains 3–5 return strokes. In this case, the lightning incident will not be well simulated by the first stroke. To date, a single impulse current waveform has been used to simulate the lightning impulse incident in research on the lightning impulse characteristics of soils, leading to some differences with observations of actual lightning discharges. In this paper, a time-delayed control technique and a controlled gaps-group isolation technique were used to produce the multiple high-impulse currents discharge to a soil sample, while nonintrusive X-ray imaging technology was used to obtain images of the discharge channel in the soil. The impulse characteristics of the soil under multiple impulse currents are studied. The differences between the discharge characteristics of the soil under multiple impulse currents and a single pulse are compared. It can be found that there has been a reduction in the effective soil resistivity under multiple impulse currents, and this reduction will be recovered over time. At the same time, a brief analysis of the discharge mechanism of the soil under multiple impulse currents is proposed, based on analysis of X-ray images of discharge channels. [ABSTRACT FROM AUTHOR]

Despite remarkable achievements in the field of Jiles–Atherton (JA) hysteresis theory, JA hysteresis modeling remains challenging given that most commercial software still lacks a practical dynamic JA hysteresis reactor model for electromagnetic transients. A novel voltage-driven dynamic flux linkage–current ( ψ–i ) JA hysteresis reactor in Electromagnetic Transients Program–Alternative Transients Program (EMTP–ATP) is developed in this study. The proposed model is based on flux linkage and current instead of magnetic flux density and magnetic field. Voltage-driven dynamic losses are incorporated into the static ψ–i JA hysteresis model by using Type-94 element in EMTP–ATP. The two proposed models in this paper are validated using current tests under 50 and 150 Hz. The performance of the proposed dynamic Model 1 matches better with experiments than the dynamic Model 2. Ferroresonance tests are carried out to validate the performance of the proposed reactor. The results show that the proposed reactor has a broad application prospect in electromagnetic transient studies. [ABSTRACT FROM AUTHOR]

Breakdown in dielectrics, the mechanism of which is still not yet fully revealed, exhibits very complicated test phenomena and characteristics. The breakdown voltage of a dielectric is believed to be related not only to the dielectric properties but also to the waveform of the applied voltage. For most dielectrics, the impulse breakdown voltage decreases with increasing wavefront time. However, we found for oil-impregnated paper (OIP) used in power transformers, increasing wavefront time in lightning impulses (at microsecond timescales) may lead to a significant increase in breakdown voltage. In contrast, the variation in the wavetail time produces very little effect on breakdown voltage. Combined with our previous test results on the space charge distributions in OIP, we tried to provide two explanations regarding this interesting phenomenon: First, the wavefront time of the applied impulse voltage affects the movement process of the positive particles in the oil-filled micro cavities in OIP, which is strongly related to the local electric field in the oil filled micro cavities and the breakdown voltage of the OIP. Moreover, the wavefront time is closely related to the trapping and detrapping process of the dielectrics. Therefore the effects of the traps in dielectric on the breakdown characteristics are different when the wavefront time varies. [ABSTRACT FROM AUTHOR]

Statistical results have shown that the waveforms of impulses invading power transformers differ from those of the standard impulse voltage recommended by the IEC Standard. However, the effect of such differences on the modification effect of nano-modified transformer oil has rarely been studied. The present work therefore experimentally investigated the effects of impulse waveform parameters on the breakdown characteristics of nano-TiO 2 modified transformer oil (NMTO). We determined the breakdown characteristics of NMTO with different nano-particle concentrations, and compared the breakdown characteristics under both standard impulse voltage and non-standard impulse voltage. Specifically, the influence of the wave front time and wave tail time of the applied voltage on the breakdown characteristics of NMTO were experimentally identified. Finally, mechanisms of observed phenomena were analyzed. Test results indicate that both the waveform parameters and polarity have important effects on the modification effect of NMTO. Such effects are attributed to the migration of positive particles and charged nano-particles. [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]

Under DC voltage, charge accumulates on the break insulation of mechanical DC circuit breakers, distorting the local electric field, which increases the risk of break insulation failure. Glass-fiber vacuum-impregnated epoxy resin can be used as an insulation material for DC circuit breakers owing to its excellent mechanical and electrical properties. We have created a system to measure the surface potential of an epoxy resin in different gases. The accumulation and dissipation of the surface charge in air and SF6 are determined, and the effects of air and SF6 are compared. The results show that the polarity of the surface charge is the same as that of the applied DC voltage, and that the main source of the charge is from Schottky charge injection. In SF6, the charge density is higher than that in air, and its decay rate is slower. The decay law of the surface potential in SF6 can be characterized by two exponential superposition formulas. In the early stage, surface charge is attenuated mainly by being transferred along the surface of the insulator and neutralized by gas ions. In the late stage, surface charge is attenuated mainly by transferring through the volume of the insulator. [ABSTRACT FROM AUTHOR]

The development of voltage-source-converter (VSC)-based high-voltage direct current (HVDC) systems has increased significantly. However, the implementation of VSC-HVDC systems is limited for lacking fast, low-loss, and reliable circuit breakers. Hybrid dc circuit breakers (DCCBs) may help to improve this situation. In this paper, an efficient modular multilevel converter (MMC) HVDC system model based on the Zhangbei–Beijing \pm \text500\,\textkV bipolar MMC-HVDC transmission (overhead line) project is first established. Then, the system transient characteristics are investigated involving permanent line-to-ground faults (LGFs), permanent short-circuit faults (SCFs), temporary LGFs, and temporary SCFs with either ac circuit breakers (ACCBs) or DCCBs in service. Finally, the performances, including the fault clearance and recovery, comparison of DCCBs with two different breaking methods, and comparison of ACCBs and DCCBs, in the MMC-HVDC system are presented. This paper provides a deep understanding of LGF and SCF in the MMC-HVDC systems, operation of DCCB, and comparison of faults clearance and recovery under these faults conditions using ACCB and DCCB, respectively. Results also benefit the application of the DCCB and the operation strategies in high-voltage and high-power MMC-HVDC systems with overhead lines. [ABSTRACT FROM AUTHOR]

Amorphous alloy is increasingly widely used in the iron core of power transformer due to its excellent low loss performance. However, its potential harm to the power system is not fully studied during the electromagnetic transients of the transformer. This study develops a simulation model to analyze the effect of transformer iron core materials on ferroresonance. The model is based on the transformer π equivalent circuit. The flux linkage–current (ψ–i) Jiles–Atherton reactor is developed in an Electromagnetic Transients Program-Alternative Transients Program and is used to represent the magnetizing branches of the transformer model. Two ferroresonance cases are studied to compare the performance of grain-oriented Si-steel and amorphous alloy cores. The ferroresonance overvoltage and overcurrent are discussed under different system parameters. Results show that amorphous alloy transformer generates higher voltage and current than those of grain-oriented Si-steel transformer and significantly harms the power system safety. [ABSTRACT FROM AUTHOR]

Ferroresonance, excited by saturated electromagnetic voltage transformers, can severely damage power systems because it produces long duration large overvoltages. In this paper, a method is developed to eliminate potential transformer (PT) ferroresonance through a model-free method. First, a structural diagram of the ferroresonance eliminating device is established based on a practical ferroresonant circuit. The effect of the adjustable parameters of the device is investigated, including resistance, pulse frequency, and pulse duty cycle. Then, Iterative control strategies are developed to adjust the gate signal duty cycle of the controller for limiting the ferroresonance to a normal status and then switching off the elimination device. Finally, simulations are conducted demonstrating that the method proposed was effective. [ABSTRACT FROM AUTHOR]

Ferroresonance, which is a common phenomenon in power systems, have threatened the security of power systems for a long time. Although several ferroresonance suppression measures have been applied in power systems, ferroresonance still occurs occasionally. In this study, a flexible control strategy to control different ferroresonant modes is proposed. The behavior of a typical ferroresonant circuit acquired from a ferroresonance testing system to obtain the electromagnetic transient and control details of the control method is also investigated. The key parameters in the control module are determined, including the switching frequency and the coefficients of the proportional–integral (PI) controller. Then, six typical ferroresonant overvoltages are selected as control systems using the proposed method. Given the effect of a damping resistor and fully controllable power electronic switches with a gate signal controlled by a proportional–integral (PI) control system, these six typical ferroresonant overvoltages can all be suppressed in 0.12 s to the non-ferroresonant state after the controller is activated. The electromagnetic transients, including the essential system state quantity transients in the control process, are also investigated. At last, a laboratory prototype is implemented, and the experimental results show that the proposed method is useful for ferroresonant overvoltage control. [ABSTRACT FROM AUTHOR]

Impulse breakdown test results show the breakdown strengths of propylene carbonate modified by Al2O3 nanoparticles and Fe3O4 nanoparticles are improved by 13 % and 20 % than that of propylene carbonate without nano-modification, respectively. In order to explore the nano-modification mechanism, the electric field and space charge distributions in propylene carbonate with and without nano-modification were measured through a modified Kerr electro-optic measurement method. The measurement results show the space charge densities are decreased and the space charge transportations are inhibited in PC-based nanofluids compared with that of pure propylene carbonate due to the space charge adsorption of nanoparticle. Therefore, the streamer propagation in nanofluids is impeded, and the breakdown voltage is improved. The calculation of electric potential outside the nanoparticle indicates that Fe3O4 nanoparticle has stronger space charge adsorption capacity than Al2O3 nanoparticle. Accordingly, the Fe3O4-modified propylene carbonate shows greater improvement of breakdown performance than Al2O3-modified propylene carbonate. [ABSTRACT FROM PUBLISHER]

Advanced high-voltage and overvoltage measurement techniques are required for smart grid construction. The existing overvoltage measurement methods that are currently used for power system measurements are mostly based on the use of electromagnetic voltage transformers and capacitive voltage transformers, which have contradiction in measuring accuracy, measuring distance, antijamming, and system compatibility. A batteryless sensor for contactless measurement of the overvoltage on overhead transmission lines based on a combination of the electrooptic effect in LiNbO3 with stray capacitance in the air is designed in this work. On the basis of this design, a dual-crystal structure-based electrooptic conversion unit is presented that eliminates the natural birefringence and improves the operating stability of the sensor. Testing platforms were set up to measure the characteristics of the sensor in thermal stability. In combination with a data acquisition device, the newly designed sensor was applied to online monitoring of the overvoltage in the ac bus and the overhead transmission lines of a 500-kV transformer station and a ±500-kV convertor station in the China Southern Power Grid. [ABSTRACT FROM PUBLISHER]

Soil discharge occurs when it is subjected to high impulse current, and it is closely associated with the impulse performance of grounding systems. In this paper, a new method is put forward to visualize the inner impulse discharge region of simulate soil (fine sand) directly, using X-ray imaging technology. According to the three views of the discharge region, the boundary surface of the discharge region can be obtained based on image filtering and edge extraction by 3-D reconstruction. On this basis, the ionization parameters (i.e., the residual resistivity of soil discharge region \rhores and the critical breakdown electric-field strength E_c ) are calculated by a finite-element method. The results show that discharge region volume enlarges with water content increasing, and \rho_{0} . E_c$ increases from 100 to 150 kV/m, as soil water content decreases from 5% to 2%. [ABSTRACT FROM AUTHOR]

The ultrafast mechanical switch is one of the core technologies in hybrid high-voltage direct current breakers. However, the insulation between contacts may suffer surface insulation failure at moments of large current interruption. Studies show that surface charges play an important role in surface insulation failure. To date, there is still a lack of complete physical understanding of the rules governing the generation, migration, and accumulation of surface charges. In this paper, we present a nonequilibrium plasma model to investigate the development rules of stage transformation (corona discharge, streamer discharge, and spark discharge) of surface discharge. Surface charge distributions considering the effects of voltage amplitude and insulation materials are also discussed. The results show that the stages of surface discharge can be identified from the discharge current. The surface charge will increase under a continuous voltage, and most of the surface charge density is within 100~\mu \textC/\textm^2 . Increasing the voltage amplitude is not conducive to the accumulation of positive charges. However, the surface charge density and discharge current increase along with the voltage amplitude. The discharge current variations of different materials are basically consistent. The surface charge density may be positively associated with the relative dielectric constant, whereas it is difficult to judge the direct relationship between the surface charges and discharge current. [ABSTRACT FROM AUTHOR]

Ferroresonance is a common and complex phenomenon in power systems. Some types of ferroresonances are remarkably difficult to distinguish from others. This study adopts non‐linear analysis methods to directly extract non‐linear characteristic quantities from ferroresonance overvoltage time series (FOTS). Simulated and practical FOTS are investigated to validate the proposed method. Non‐linear characteristic quantities, i.e. average grey value of reconstructed attractor and improved largest Lyapunov exponent of ferroresonance overvoltage, are extracted based on phase space reconstruction of voltage time series to identify different types of ferroresonance. Practical FOTS acquired from a transformer substation is used to validate the proposed method. Non‐linear characteristic quantities obtained in this study can be applied to identify different types of ferroresonance. These characteristics allow further understanding of the non‐linear ferroresonance phenomenon from another point of view and benefit the study of ferroresonance identification and suppression. [ABSTRACT FROM AUTHOR]

Transformers are the most widely used apparatus in power systems to transfer electrical energy from one side to another. Despite remarkable achievements in transformer modelling over the past few years, three‐phase transformer modelling techniques have not progressed to that of their single‐phase counterparts, mainly because of topological correctness of core and coil structure, complicated interaction between fluxes of the three iron core legs, and non‐linear saturation of ferromagnetic material. An improved three‐phase transformer model with extended inverse Jiles–Atherton (JA) hysteresis theory is proposed in this study. The extended inverse JA model with anisotropic anhysteretic magnetisation function is addressed, and the classical eddy‐current‐loss term and anomalous‐loss term are directly incorporated into the original energy balance equation to represent the non‐linearity and iron core losses rather than representing them with a parallel combination of resistances and saturable inductances (R//L model) as most previous studies have done. The results of inrush current case indicate that the proposed transformer model can predict the residual flux, include eddy‐current‐loss and anomalous‐loss, and predict the inrush current peak values with relative error less than 6%. [ABSTRACT FROM AUTHOR]

For a long time, thermal effect, as a major factor for insulation aging, is a great challenge to the safe operation of power transformers. During the decades-long operation, cellulose fibers contained in the oil-paper insulation, break and deform, accompanied by the generation of new chemical products and variation of mechanical/electrical properties. Therefore, in this work, the influence of thermal aging on the electrical performance of the transformer oil paper insulation was investigated. 50% breakdown voltage and average AC breakdown voltage of oil impregnated paper were experimentally determined, and the water content measurement, surface morphology observation and frequency domain spectroscopy measurement were carried out on the oil impregnated paper samples with different aging degrees. The test results indicate that the electrical performances of aged oil impregnated paper improved during the aging process, which could be attributed to the forming of micro globules caused by the effect of thermal aging. [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]

During the decades-long operation, oil paper insulation in power transformers may suffer the effect of thousands of lightning impulses. Such interesting but rarely studied phenomenon, also known as accumulative effect, may cause irreversible damage to the oil paper insulation, or even premature failure. Therefore, in this work, the influence of repeated lightning impulses on the oil paper insulation, especially the mechanism behind this phenomenon, was experimentally investigated. We determined the accumulative breakdown characteristics of the oil paper insulation and observed the influence of the repeated lightning impulses on the dissolved gas in oil and the degree of polymerization. Specifically, the space charge distribution in the oil paper samples and the relationship between the interval time and accumulative breakdown characteristics were experimentally determined in the end. The test results indicate that the accumulative effect only exists in the cellulose paper rather than the transformer oil. Such effect was considered to be the consequence of electrical degradation and accumulation of space charge. [ABSTRACT FROM AUTHOR]

As striking lightning impulse waveforms in power transformers are quite different from the standard lightning impulse waveform (SLIW) used in the withstand voltage tests, it is essential to identify the breakdown characteristics of oil-impregnated paper (OIP) under non-SLIW and to quantitatively compare them with those under SLIW. On that basis, the impulse insulation levels of power transformers can be rationally determined and then the insulation specifications can be evaluated to maintain high equipment reliability. In this paper, with damped alternating oscillation waveform as a representative basis, the breakdown characteristics of oil-impregnated paper were experimentally obtained by changing the frequency and damping rate of the applied voltage. All experiments were conducted using a column-column electrode model. In addition to the 50% breakdown voltages (U50), Minimum breakdown voltage Umin and V-t characteristics under different circumstances, the influence of various parameters on breakdown characteristics and the possible reasons were discussed. After comparing waveforms and corresponding results, the specific relationship between waveform parameters and U50 were expressed as a single equation. As a result, the breakdown voltage was 1.106 to 1.435 times higher than that under SLIW depending on frequency and damping rate. The steepness of the wave peak and the duration of high voltage around wave crests are considered to be two important factors affecting the breakdown voltage. An effective way was proposed to predict U50 of oil-impregnated paper as a reference for insulation specifications irrespective of wave shape. These results support the rationalization of insulation for actual lightning impulse waveforms in the power transformers. [ABSTRACT FROM AUTHOR]

This paper presents a finite-element method (FEM) based on Maxwell differential equations to analyze the impulse characteristics of the grounding system in a complex soil structure. The geometric coordinate transformation is leaded into the FEM and the soil parameters in the transformation domain are determined by comparing the original coordinates without drawing the geometric construction of the soil. The unit smoothing function with continuous partial derivative is adopted to solve the convergent difficulty caused by the resistivity jumping on the medium interface. The model is validated by comparing the results with the classic methods combined with field-circuit and simulation experiments. The simulation results show that the current dispersal proportion from the external conductors of the grounding grid, especially from the four corner points, is higher than that of the internal conductors. The resistivity of the soil that surrounds the grounding electrodes reduces under impulse current and the resistivity of the soil that surrounds the external conductors is lower than the resistivity of the soil that surrounds the internal conductors. These situations change more drastically under higher injected current. [ABSTRACT FROM PUBLISHER]

According to the IEC Standard, the insulation performances of power apparatus under lightning impulse voltages are evaluated by the withstand test of standard lightning impulse voltage, with the wave front/tail time of 1.2/50 ?s. However, the standard lightning impulse voltage originally obtained from the transmission lines or triggering lightning towers, is quite different from the invading lightning impulse voltage in the power transformers. Hence, in this work, an attempt has been made to obtain the waveform characteristics of invading lightning impulses in power transformers using an overvoltage online monitoring system. Energy Method was then proposed to convert the recorded oscillatory waveform into double-exponential waveform defined by IEC Standard. And the statistical waveform parameters of recorded invading lightning impulses, namely, the non-standard lightning impulses, were determined based on the Energy Method. Current insulation assessment methods, for example, the V-t characteristics, are far from satisfactory to meet the demand of insulation design for the power apparatus, whereas the impulse breakdown voltage-number of voltage applications characteristics (V-N characteristics) could well evaluate the long term performance of insulation system. Therefore, we experimentally identified the V-N characteristics of oil paper insulation under impulse voltage with different waveforms, including non-standard lightning impulse voltage, and then mathematically analyzed the obtained data. At last, the influence of wave front/tail time on the V-N characteristics of oil paper insulation was experimentally studied. The test results indicate that the insulation performances of oil paper insulation under repeated nonstandard lightning impulse voltages and under standard switching impulse voltages were better than that under repeated standard lightning impulse voltages. And the capacity to withstand repeated voltage became increasingly stronger with the increase in wave front time or the decrease in wave tail time. The breakdown voltage, however, was only related to the wave front time of applied impulse voltage. [ABSTRACT FROM AUTHOR]

A penetrating X-ray imaging technology is applied to study the soil ionization under surge current, by which grayscale images are obtained. Based on soil breakdown mechanism, there are series of discharge channels generating in the soil. By comparing the discharge time and imaging time, the filamentary paths, which can be directly observed in these grayscale images, are determined to be discharge air channels. The principle and process of imaging are introduced in detail in this paper. According to the Beer–Lambert law, the relationship between grayscale value and equivalent air thickness of any points in the discharge channel area has been proposed. Image processing technology is used to dispose the grayscale images to make them more distinct for analysis without losing the accurate information of the original images by maintaining their grayscale value percentage. Analyses of experimental imaging results are also presented, and effects of factors, such as configuration of grounding electrode and amplitude of surge current, are considered on the distribution of discharge channels as well. In addition, soil residual resistivity of discharge area was calculated by a finite-element method, according to experiment parameters and 3-D diagram of discharge area. [ABSTRACT FROM AUTHOR]

This study presents a method for the estimation of soil structure based on genetic algorithm (GA), which can reveal actual soil structures through the experimental curve of apparent resistivity that was measured near the ground grid. GA involves the use of the errors produced by the experimental and theoretical curve of soil apparent resistivity to achieve soil structure optimisation. In this study, new soil structures were generated by GA, and finite element method (FEM) was used to compute the theoretical curve of apparent resistivity under these soil structures. The relative position of ground grid and the measurement profiles was considered in FEM, which can reveal the true measurement site. Compared with present methods, the proposed method that combines FEM and GA can adapt to a solution domain of complex geometry shape while ground grid and soil exist simultaneously. Thus, the proposed method can handle the experimental curve which is influenced by the grounding system. In addition, the solution accuracy of the soil parameters was reasonably established and thus allowed the algorithm to exhibit convergence within 50 generations. [ABSTRACT FROM AUTHOR]

Repeated impulses have long been considered to affect the insulation properties of electrical equipment. This interesting but rarely studied phenomenon may pose great risks to the reliable operation of power equipment. Therefore, in this work, we focus on the accumulative effect of repeated lightning impulses on the oil-impregnated paper (OIP) used in power transformers. Specifically, we study the effect of space charge on the accumulative characteristics of OIP. A repetitive impulse accumulating test platform was set up, and the tests were conducted to obtain the accumulative characteristics of OIP under repeated lightning impulses. The accumulative characteristics of OIP with various interval time were experimentally identified to study the influence of interval time on the accumulative effect. Then, space charge in OIP samples were investigated using the pulsed electro-acoustic technique. Charge behavior in OIP was analyzed under different conditions, i.e., changing the applied times, amplitude and interval time of the applied lightning impulses. Charge injection and transport occurred during lightning impulse accumulation. The increased application of lightning impulses, high applied voltage, and short interval time contributed to the accumulation and transport of space charges. At last, the possible mechanisms of the accumulative effect are discussed in this article based on the experimental results. [ABSTRACT FROM AUTHOR]

Artificial intelligence has performed exceptionally on power systems, motivating its application in electromagnetic transient (EMT) diagnosis on smart grids. However, owing to the insufficient number of measured EMTs, existing models for EMT diagnosis tend to be built using simulated EMT data which are not distributed like measured EMT data, leading to degrading generalization performance of the models in industrial applications. This paper proposes a novel DRN-LSTM model applicable to a small-sample measured EMT dataset and data-augmentation methods that boost the generalization of the model. The DRN-LSTM model includes a deep residual network, long short-term memory, and a fully connected layer, and it can make an effective diagnosis on measured EMT data. Data-augmentation methods, namely random cropping and data consolidation, effectively enlarge the data volume of the small-sample dataset. Experimental comparisons show that the proposed DRN-LSTM model has better generalization than classical and state-of-the-art artificial-intelligence-based models on measured EMT data, after random cropping and data consolidation of the small-sample set. The proposed DRN-LSTM model and data-augmentation methods provide a competitive solution for the diagnosis of EMTs and other transients for which it is difficult to collect sufficient data. • The proposed DRN-LSTM model and data-augmentation method could be applicable to the diagnosis of EMT and other transients that are difficult to sufficiently collect. • The input of the DRN-LSTM model is the time-frequency matrix of the EMT transformed by a short-time Fourier transform. The model uses the DRN to extract features from the timefrequency matrix and then passes these features to the LSTM layers for temporal modeling and the extraction of temporal sequential patterns relating to the measured EMTs. The model finally outputs these patterns to the fully connected layer, producing a feature representation that is more easily separable. • Data-augmentation methods, namely random cropping and data consolidation, are proposed to enlarge the data volume of the small-sample measured EMT dataset and thus improve the generalization performance of the DRN-LSTM model in industrial applications. • The random cropping is proposed to increase the data volume of the datasets by randomly cutting the time-frequency matrix of EMT data into many different time ranges. The proposed data consolidation augments the dataset by integrating measured and simulated EMT datasets, especially when the volume of EMT measurements is insufficient. Adopting the two dataaugmentation methods, the generalization performance of the DRN-LSTM model can be boosted, even if the volume of the measured EMT dataset is limited. [ABSTRACT FROM AUTHOR]

The working current of DC grounding electrodes is constantly increasing with the rapid development of high‐voltage direct current systems. Thus, the temperature rise of grounding electrode conductors is increasingly becoming an important safety factor. Therefore this study measured the soil temperature characteristics of water content and resistivity; established an electric heat coupling finite element model for DC grounding electrodes that is based on current field and temperature field theories to analyse the temperature changes in common DC grounding electrodes. In contrast to traditional models, the proposed model realises electric heat coupling field calculation by considering the mutual influence of temperature rise and soil parameters. The results show that the soil resistivity increases with increasing temperature and the soil resistivity change further influence the temperature changes of the grounding electrodes. The change in grounding electrode temperature rise is not steady and includes a sharp growth stage. [ABSTRACT FROM AUTHOR]

Among the ultra‐high‐voltage (UHV) grids in eastern China, the construction of multi‐circuit transmission lines with mixed voltages on a single tower is a novel transmission technology that can significantly reduce line corridor coverage, maximise the use of tower height and significantly improve power transmission capacity per area. However, the electromagnetic interaction between lines is complicated and interferes with the secondary arc current. This study proposes a hybrid tower model that incorporates double‐circuit transmission lines with alternating currents at 1000 and 500 kV. Its secondary arc current is investigated according to circuit analysis and EMTDC in consideration of the electromagnetic coupling effect, along with the influences of line length, compensation, phase sequence and transposition on the secondary arc current. Results show that a strong secondary arc current is generated for 1000 kV transmission lines in accordance with the UHV hybrid transmission project from Ximeng to Shanghai. This finding should be emphasised. Furthermore, the influence of quadruple‐circuit line length on the secondary arc current depends on line voltage rating. Finally, this study presents several methods that may reduce the secondary arc current, such as transposing lines evenly and installing suitable neutral reactors. [ABSTRACT FROM AUTHOR]

The mechanisms by which conductive and dielectric nanoparticles (NPs) trap electrons are explained by the potential well distribution caused by induced or polarized charges on NPs. Thus, the distributions of surface and saturation charges on conductive and dielectric NPs are determined. Given conductive Fe3O4, semiconductive TiO2, and dielectric Al2O3 NPs, insulation performance tests are conducted and ionization models of nanofluids (NFs) based on transformer oil are developed. These models are compared with those of NFs based on pure oil. The NP whose conductivity or permittivity does not match that of the dielectric liquid has a potential well and an increased amount of saturation charges on its interface. This NP influences streamer development strongly and enhances the breakdown of oilbased NF. [ABSTRACT FROM AUTHOR]