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Polypropylene (PP)-based recyclable materials have attracted tremendous interest for HVDC cable insulation application due to their superior electrical properties, e.g., high thermal stability and superior recyclability. Compared with crosslinked polyethylene (XLPE), PP-based materials exhibit the advantages of not only higher working temperature but also facile and efficient cable manufacturing with reduced cost, which are highly desired in HVDC cable manufacturing. Considering their promising advantages, PP-based materials have received significant attention from both academia and industry in the field of HVDC cable insulation. In order to adopt PP as cable insulation material, the mechanical flexibility of PP should be improved. However, the regulation of the mechanical properties inevitably influences the electrical properties of PP. So extensive researches have been conducted on the regulations of the mechanical and electrical properties of PP. This review summarizes the research progress in recyclable polypropylene-based materials for HVDC cable insulation application. Particular attention is placed on the electrical property regulations and material structure-property relationship. The challenges that remain to be addressed and the opportunities for future studies in PP-based recyclable HVDC cable insulation materials are also presented.

Polymeric high-voltage direct-current (HVDC) cables are the core equipment in energy internet, which enable the long-distance large-capacity electric power transmission, large-scale utilization of renewable electric power and flexible interconnection of large power grids. Performance of the HVDC cables are determined by the properties of their insulation material. However, the traditional polymeric HVDC cable insulation material, crosslinking polyethylene (XLPE) is limited to relatively low working temperature which restricts the power capacity of HVDC cables. XLPE with thermosetting characteristics also presents a major barrier to material recycling and environmental pollution reduction. Here we report the ternary nanocomposites of polypropylene (PP), thermoplastic polyolefin (TPO) and MgO nanoparticles as an efficient way to recyclable HVDC cable insulation material that simultaneously possess excellent thermal, mechanical and electrical properties, especially at high temperatures. At an optimal composition, the ternary nanocomposites integrate the complementary properties of the multi-components to raise the mechanical flexibility at room temperature and greatly improve the electrical properties at high temperatures (including suppressed space charge accumulation and increased breakdown strength and volume resistivity) while retaining high melting temperature comparable to PP of about 160 °C. This work may pave a way for synergistic optimization of the overall properties of insulation materials and enabling the successful development of recyclable insulation material for large-capacity HVDC cable application.

Low-frequency resonance may seriously threaten the stable operation of a long-distance ultra-high-voltage-direct-current (UHVDC) system. In this paper, the impedance model of long-distance UHVDC systems was analyzed for various disturbances. For different disturbances, the positions of voltage sources in the simulated DC loop are different. Taking the first ±1100-kV DC power transmission project with a length of 2688 km as an example, the frequency characteristics of the DC-loop impedance were studied using the PSCAD/EMTDC simulation platform. The effects of various operating modes and the firing angle of the converters on impedance–frequency characteristics of the DC loop were discussed. The effect of the length of DC transmission lines on the DC-loop resonant frequency was studied. The distance is the most important factor determining the system low-frequency resonance. Faults in an AC grid, such as phase-to-phase short-circuit and single-phase faults, are shown to produce 50-Hz positive sequence harmonics on the AC side and 100-Hz disturbances in the DC loop. Two methods of mitigating low-frequency resonance were proposed and the effects simulated. Installation of the blocking filter in the neutral bus of the converter station can reduce the amplitude of the resonance overvoltage.

A spacer is a challenging part of high voltage direct current (HVDC) gas-insulated substations and gas-insulated transmission lines. This paper proposes a novel design of HVDC spacers by self-adaptively controlling the surface charge. The paper briefly reviews dielectric modification methods that aim to limit surface charge accumulation. A novel design of HVDC spacer is introduced that is based on the concept of adaptively controlling surface charges using nonlinear materials, and its surface charge control and regulation abilities are investigated. The surface charge accumulation and decay properties of the novel spacer are compared to a downsized spacer. The results show that the proposed model spacer can restrict surface charge accumulation in the charge adaptive control region of the spacer. Doping the nonlinear material can facilitate surface charge decay in the charge adaptive control region when the electric field exceeds a threshold. These positive results represent a new idea of designing a suitable spacer to avoid surface flashover in HVDC SF 6 apparatus.

Charge accumulation on the surface of high voltage direct current (HVDC) spacers decreases their surface flashover voltage and affects the reliability of the insulation systems. In part I of this study, a novel HVDC spacer in which the surface charges can be adaptively controlled is proposed, and the aim of this paper is to experimentally verify the design. The experimental results of three kinds of model spacers, including a regular spacer, a spacer with a novel shape, and a spacer with charge adaptively controlled are provided. AC and DC surface flashover experiments and a polarity reversal test were performed. The effect of surface charges on triggering the surface flashover phenomenon are discussed. The results show that the surface flashover voltage increases both for the novel shaped spacer and the charge-adaptivelycontrolled spacer under application of AC bias, owing to a modification of the spacer shape that increases the creep distance. However, the surface breakdown voltage is the lowest for the novel shaped spacer under DC application and polarity reversal test. The charge adaptive control spacer displayed more advantages than the other samples under application of DC bias and in polarity reversal tests owing to its superior surface charge dissipation properties. It is hoped that this work could offer a new perspective for the design of a suitable spacer to avoid surface flashover when implementing HVDC energy transmission systems.

Surface charges have the effect of changing the electric field over spacers in compressed gases, which may affect the stable operation of gas insulated equipment and could possibly trigger a catastrophic surface flashover without warning. In this paper, we review the literature on surface charge accumulation and related surface flashover phenomenon on spacers in compressed gases. A summary of experimental research of surface charge accumulation is presented and, the surface charge accumulation mechanism is also summarized, with some newly identified factors in mind that may affect surface charge transport and measurements thereof. Subsequently, the correlation between surface charge accumulation and the surface flashover phenomenon is thoroughly discussed, and the effect of surface charges on surface breakdown under applied voltage pulses of various waveforms is analyzed. Finally, several important research topics in this area are proposed. In addition, some ideas on potential surface charge control methods are presented to aid the design of a better spacer free from surface charges in future studies.

The design of spacers for operation under DC stress is a challenging part for gas-insulated substations (GIS) and gas-insulated transmission lines (GIL). This series of papers has proposed a novel way to develop a spacer for operation under high-voltage direct-current (HVDC) conditions. In this part, a model spacer based on adaptively controlling surface charges is briefly introduced and is verified using routine tests, including electric field strength simulations, X-ray imaging, mechanical simulation and mechanical testing. In addition, the thermal expansion coefficient and thermal conductivity of the spacer are also measured to investigate its operational stability under various thermodynamic conditions. The results show that the geometrical improvements and SiC doping are available to reduce both the normal electric field strength around the central electrode and the tangential electric field strength around the enclosure. The mechanical properties of the concave side of the spacer are slightly lower than those of the regular spacer, while the mechanical properties of the convex side demonstrate the major improvements during both simulations and water pressure-based destructive testing. It is hoped that this work will provide a new approach for the design of spacers for use in HVDC GIS or GIL.

In this paper, a 3-D iterative approach is presented to calculate the nonlinear ion-flow field around two cross-over HVdc transmission lines. The electric-field resulting from space charges is calculated by an integral method. An analytical formula to calculate the space charges along the electric-field lines determined by the electric field is derived based on a 3-D method of characteristics. The electric-field distribution and the space-charge distribution are calculated iteratively until a convergence is reached. Since both the electric field and the space charges are mainly calculated by analytical approaches, the approach greatly reduces the computing scale and can be used for 3-D calculation. The approach was validated by the experiment on a reduced-scale model. The ion-flow field at the crossing of two ±800-kV transmission lines was calculated with the method and discussed.

Non-intrusive load monitoring addresses the challenging task of decomposing the aggregate signal of a household's electricity consumption into appliance-level data without installing dedicated meters. By detecting load malfunction and recommending energy reduction programs, cost-effective non-intrusive load monitoring provides intelligent demand-side management for utilities and end users. In this paper, we boost the accuracy of energy disaggregation with a novel neural network structure named scale- and context-aware network, which exploits multi-scale features and contextual information. Specifically, we develop a multi-branch architecture with multiple receptive field sizes and branch-wise gates that connect the branches in the sub-networks. We build a self-attention module to facilitate the integration of global context, and we incorporate an adversarial loss and on-state augmentation to further improve the model's performance. Extensive simulation results tested on open datasets corroborate the merits of the proposed approach, which significantly outperforms state-of-the-art methods., Accepted by IEEE Transactions on Power Systems

Dielectric capacitor has drawn much attention recently both in academia and in industry because of its high power density and excellent cycle performances. In order to meet the demand for high energy density, high-e (dielectric permittivity) nanoparticles are dispersed into polymer matrices to form nanocomposites with both high e and high Eb (breakdown strength). Polymer-nanoparticle interface has great influence on energy storage properties. In order to find out whether we can prepare well-dispersed nanocomposites by forming a thin coating layer (less than 2 nm) on the surface of nanoparticles with strengthened interfacial interaction, such as hydrogen-bonding interaction, a specific coating molecule, i.e., 3,4,5-trihydrobenzoic acid (or Gallic Acid, GA), was selected for surface modification of BaTiO3 (BT) in this research. A coating layer of only 1.6 nm in thickness formed by the reaction between phenolic hydroxyl groups in GA and surface hydroxyl groups on BT to produce modified BT@GA nanoparticles. Hydrogen-bonding interaction did form between GA and the hydroxyl groups in PVA matrix. This ensured fine dispersion, higher e, higher Eb and higher energy storage density for PVA + BT@GA nanocomposites. For PVA + 40%BT@GA, e reached 51 at 1 kHz, while Eb was 144 MV/m, which is twice as that of PVA + 40%BT. The discharging energy density of PVA + 40%BT@GA reaches 0.554 J/cm3 at 50 MV/m, about fourth times as that of pristine PVA.

Surface charge accumulation on cone-type spacers restricts the development of dc gas-insulated substations (GISs) and gas-insulated transmission lines (GILs). In previous studies, many surface charge tailoring methods were investigated. However, indirect tailoring of surface charge by temperature field regulation has not been performed. In this paper, the relation between the temperature field distribution and the surface charge accumulation is studied by experiment. And a temperature control method by doping K2O·6TiO2 whiskers in the composite of epoxy resin is introduced and verified by experiment. The results show that the doping of K2O·6TiO2 is able to weaken the surface charge accumulation in both air and SF6.

The capacitor bushing is the most commonly used bushing in power systems. However, the use of the capacitor bushing is limited by the complexity of the insulation and grading electric field structure. At the same time, as the voltage grade of the power system increases, the bushing needs to be made larger and its manufacture is more complicated, limiting the use of the capacitor bushing for such power systems. In light of present high-voltage bushing problems, the present paper proposes a new type of high-voltage bushing structure that adopts a three-layer structure with nonlinear composites for internal insulation to replace the original bushing condenser in a capacitor bushing. From the inside to outside, the three layers are a uniform-voltage layer, leakage-current-limiting layer, and electrode extended layer. The uniform-voltage layer and electrode extended layer made from materials having nonlinear conductivity are used to make the electric stress uniform in the main insulation and at the edge of the flange, avoiding breakdown of the main insulation and flashover of the flange. The design of the new bushing is applicable to a variety of voltage levels of DC power systems. At the same time, the new bushing is smaller, the production of the bushing is simpler, and the risk of a bushing fault is reduced, marking a technical innovation for high-voltage bushings in power systems.

High-sensitivity and miniaturized measurement of electric fields is important for applications in power grid such as lightning waveform capturing and electrical equipment monitoring. In addition, the demand of wide-area information measurement in smart grid also necessitates the development of miniaturized and noncontact sensors. So far, there have already been a variety of electric-field sensing devices. However, these approaches meet the challenges of large size and low sensitivity. To solve the problem, this paper proposed a new electric-field sensor based on piezoelectric effect. The piezoelectric effect enables the conversion of electric fields into piezoelectric material deformations and then the air-gap capacitances in the device. The results indicate that the electric-field micro sensor can reach a high sensitivity of 30 V/m. The side length of the sensor chip can be as small as a few millimeters. Owing to the feasibility in mass production of the proposed sensor, it can potentially be extended to the industrial applications in power grid.

It is important for us to study what have changed on electrical property of polymer after electrical tree aging occurs when we explore the self-healing of electrical degradation of polymers. Leakage current characterization has been demonstrated in this paper and is useful to give these advices. Electrical trees of different shapes and different electrode structures are formed and the high field leakage current are measured. The mechanism of leakage current variation of electrical tree samples is explored and confirmed by experiment designed. The polarization index and absorption ratio are also calculated and seem to behave no obvious pattern during the extending of tree channels. In conclusion, the high field leakage current is an effective non-destructive characterization of the electrical tree defects.

Prefabricated rubber accessories affect high-voltage cable operation because they determine the electric field distribution within a cable joint. For a DC power cable, the electric field distribution depends on the electrical conductivity of the insulation material, which is strongly affected by temperature. In this paper, instead of traditional rubber cable joints with a semiconductive rubber material as the stress cone and inner deflector shield, a field grading material with a strongly nonlinear conductivity is used to achieve better grading electric field performance. To study the electric field distribution of a DC cable joint under various temperature conditions, a finite element numerical simulation is conducted using COMSOL Multiphysics. A comparison of the electric field distribution of various models shows that polymer composites with nonlinear conductivity effectively improved the electric field distribution in the cable joint, especially for high temperature gradients. It is shown that a field grading material has great potential for high-voltage DC cable joint design, facilitating the development of higher-voltage DC cables.

Microvaristor ceramics blended with insulation polymers can function as nonlinear dielectrics, which possess self-adaptive electrical parameters varying with the applied field. Broadening the switching (electric) field of the nonlinear dielectrics for their applications in various apparatus and voltage class is an important issue. This paper prepared ZnO microvaristor ceramics with different grain size through controlling the sintering temperature, as well as the microvaristor ceramic/polymer composites (MCPC) with different filler concentration, filler diameter, and filler grain size. The nonlinear V-I characteristics of both the single ZnO microvaristor filler and the MCPC were measured. The switching field of the MCPC is found to be adjusted in a wider range by controlling the filler grain size. The relationship between the nonlinear conducting behavior of single filler and the composite are analyzed. Suggestions were proposed for the preparation of ZnO microvaristor fillers to meet specific field grading requirement for MCPCs.

In this paper, we present the learning-based data analytics moving towards transparent power grids and provide some possible extensions including machine learning, big data analytics, and knowledge transferring. The closed loops of data and knowledge are illustrated and the challenges for establishing the closed loops are discussed. General ideas and recent developments in supervised learning, unsupervised learning, and reinforcement learning are presented together with extensions for power system applications. Furthermore, much emphasis is placed on privacy-preserving data analysis, transfer of knowledge, machine learning for causal inference, scalability and flexibility of data analytics, and efficiency and reliability of computation. Existing integrated solutions in the industry featuring the Industrial Internet and the digital grid are also introduced.

With the development of smart grid, it is of increasing significance to identify and cope with various types of overvoltages, faults and power quality disturbances effectively and automatically. In this paper, a framework for overvoltage identification and classification based on sparse autoencoder is proposed. By using single-layer and stacked sparse autoencoders, dimensionality reduction and automatic feature extraction of ferroresonance overvoltage waveforms in power distribution systems are achieved as an example, which does not require feature engineering to produce a series of features. Classification of different ferroresonance modes is then implemented with a softmax classifier, and favorable classification results are obtained after parameters of feature extraction and classifier models are determined. Application of the proposed framework in smart grids is discussed. The proposed framework provides a brand new idea for establishing a smart identification and classification system for overvoltages, which can be generalized to classification of faults and power quality disturbances.

The simulation of electrical discharges has been attracting a great deal of attention. In such simulations, the electric field computation dominates the computational time. In this paper, we propose a fast tree algorithm that helps to reduce the time complexity from $O(N^{2})$ (from using direct summation) to $O(N\log N)$ . The implementation details are discussed and the time complexity is analyzed. A rigorous error estimation shows the error of the tree algorithm decays exponentially with the number of truncation terms and can be controlled adaptively. Numerical examples are presented to validate the accuracy and efficiency of the algorithm.

Grounding faults are common faults on dc transmission lines. Characteristics of the short-circuit current, such as amplitude and frequency, and its following distribution are very important for analyzing the influence of the fault. Based on the ±500 kV Yongfu dc transmission project in China, several artificial grounding short-circuit tests at the terminal or middle of the dc transmission line were carried out. This paper reports the measurement and analysis of the short-circuit current and its distribution in a dc system for the first time. The short-circuit current distribution includes currents in the ground wires of the fault tower and currents in ground wires, which are connected to the converter station grounding grid, such as the optical ground wire (OPGW) currents of ac outgoing lines, neutral point currents of different transformers, and the grounding electrode bus arrester current. In addition, the frequency components of the fault currents are analyzed. Based on the PSCAD/EMTDC software, the current distribution is simulated and discussed. The simulation results are in good agreement with the test results.

This paper proposes the Powell (PPSO) method. The particle swarm optimization (PSO) algorithm is combined with the Powell algorithm to identify the parameters of lightning channel-base current function. The method can overcome the problem of premature convergence for PSO algorithm, and improve the analysis accuracy. The Heidler function is used to represent the lightning channel-base current. We have compared the PPSO method and genetic algorithm (GA) on reaching the desired peak value of the current $I_{m}$ and the maximum time rate of change of current ( di / dt )max. The results show that the parameters of Heidler function evaluated by the PPSO method can achieve more accurate values of $I_{m}$ and ( di / dt )max than those evaluated by GA. Also, we have compared the PPSO method, the Nelder–Mead particle swarm optimization (NMPSO) method, and PSO method using the measured channel-base current. The results show that the PPSO method is better than the NMPSO and PSO method to determine the channel-base current function parameters. The PPSO method is an efficient method to identify the channel-base function parameters, which can improve the digitalization of lightning monitoring equipment. This approach is also useful in research related to lightning characteristics and lightning protection.

Localized overstress due to electric field concentration is a threat to the long-term performance of the insulation in almost all high voltage apparatus. Resistive field grading using field grading materials (FGMs) may be a solution to this problem. Insulation matrixes such as ethylene propylene diene monomer, epoxy, or silicone rubbers, blended with fillers such as ZnO microvaristors, are reported to possess field dependent conductivity with greater and more stable nonlinearity than traditional FGMs [1]. Their application in high voltage apparatus such as bushings [1], cable accessories [2]–[4], insulators [5], [6], and stator coils [7] has been studied recently. It is well known [8] that the most important parameters for FGMs are the nonlinear coefficient α and switching field Eb; the challenge is to tailor these two parameters to a specific application. FGMs can function effectively in most cases when α >10 [9], but the adjustment of Eb is more complex.

We studied the effects of indium doping on the microstructure and electrical properties of ZnO varistors co-doped with Al 2 O 3 and Ga 2 O 3 . Scanning electron microscopy, current–voltage testing, capacitance–voltage testing, and X-ray diffraction were performed. The results showed that the leakage current of sintered varistors decreased as the indium dopant concentration was increased. The average grain size decreased slightly with increasing indium content, which improved the voltage gradient. A certain amount of the doped In 3+ dissolved into the grains causing the grain resistance to decrease, which resulted in a low level residual voltage. The sintered samples with 0.024 mol% indium, 0.2 mol% aluminum, and 0.42 mol% gallium showed excellent overall performance, exhibiting a nonlinear coefficient of 58.8, a leakage current of 1.82 µA/cm 2 , a 1-mA residual voltage of 433.9 V/mm, and a residual voltage ratio of 1.53. The obtained co-doped ZnO varistors will provide better protection and stability in applications as metal oxide arresters for power systems.

We performed SEM, electric field-current density measurements, capacitance-voltage testing, and X-ray diffraction analysis to study the effects of indium doping on the microstructure and electrical properties of ZnO varistors co-doped with Ga2O3 and Y2O3. Our results showed that the residual voltage ratio of the sintered samples decreased with the introduction of indium at defined aluminum and yttrium content. The introduction of indium clearly improved the nonlinear coefficient. The ZnO varistor samples showed optimal performance at a composition of 0.025 mol% indium, 0.4 mol% gallium, and 0.9 mol% yttrium. This sample exhibited a residual voltage ratio of 1.45, a nonlinear coefficient of 76.2, a 1-mA residual voltage of 454 V/mm, and a leakage current of 1.44 µA/cm2.

Space charge accumulation is a critical issue for the deterioration of the elevated temperature insulating property of polymeric materials. We present the influence of surface-modified ZnO nanoparticles on the space charge distribution and direct current (DC) resistivity of polypropylene (PP)/elastomer blends under elevated temperature. Octyltrimethoxysilane, a silane coupling agent, was used for the surface modification of nanoparticles. Morphology characterization results indicated that the elastomer and coated ZnO were well dispersed in the PP matrix. It was observed that the coated ZnO can significantly improve the insulating properties, including a minimized electric field distortion (4.0%) and increased DC volume resistivity (1.41 × 1018 Ω m) under an electric field of 40 kV/mm and 70 °C. The DC resistivity of 2 phr PP/elastomer/ZnO ternary composites was improved by 13.4 times compared with that of pure PP/elastomer blends. The suppression of space charges may originate from deep traps existing in spherulite boundaries and interfacial zones between polypropylene and ZnO. This work provides an effective method to endow a recyclable insulating material with outstanding elevated temperature insulating performances.

The current monitoring of bundle conductors is essential for precisely locating operation failures in transmission lines, leading to a more stable and reliable power grid. This paper proposed a novel method for current reconstruction of bundle conductors, utilizing tunneling magnetoresistive (TMR) sensors. Both the normal and abnormal situations are discussed. The optimal and sub-optimal measurement points for sensors were obtained for concise current measurement with only one uniaxial TMR sensor. The current in each bundle conductor was calculated separately based on the comprehensive algorithm. Validation experiment indicated the error of the measurement and calculation system was no more than 10%, proving the robustness and feasibility of the measurement system and the algorithm. The reconstruction process takes full advantages of TMR sensors and provides a promising solution for transmission line monitoring in power grid.

High voltage cables now have been widely applied in electric power systems. A prefabricated rubber stress cone is the key part of high voltage cable terminal, and its property is the main factor in determining whether the high voltage cable operates safely. In this paper, the finite element numerical simulation is applied to analyze the distribution of electric field intensity. Instead of using materials with fixed conductivity, materials with nonlinear conductivity are used as the insulation material for rubber stress cones. The polymer composites with nonlinear conductivity that depends on applied electric fields can smartly smooth the nonuniform electric fields in insulation components of power systems through varied material parameters with the electric field. Combined with theoretical analysis, we can determine the parameters range of nonlinear conductivity materials. By comparing the electrical field distribution of different materials, it is found that nonlinear conductivity polymer composites can effectively improve the electric field at the rubber stress cone face and the junction of stress cone terminal, which is of significance in the development of higher voltage cable in the future.

The interpretation of the surface charge pattern of spacers in gas-insulated transmission lines (GIL) is a research topic with great importance, which requires a comprehensive understanding of charge behaviors at physical interfaces. Here, we report a surface charging phenomenon of spacers in the presence of metal particles inside the direct current (DC) GIL. The charging of spherical metal particles on the spacer surface appears as dotted discontinuously distributed charge spots, which are due to the charge injection from the induced charge of the metal particle and gas ionization over the metal particle surface. Concentrically distributed bipolar charge patterns are seen to build up, which are due to the surface trapping of charges from gas ionization at the end tip of the acicular metal particle. The lateral type of ionization obeys the law of dielectric barrier discharge of the needle-plate electrode in an intense DC electric field, which may bring significant influence on the safety of the spacer. The study in this letter provides direct evidence demonstrating the surface charge patterns of spacers in the presence of metal particles, which serves as an important basis for the design of DC GIL.

Electrostatic discharge (ESD) is a phenomenon that commonly exists on the insulation surface in electrical and electronic devices at different environmental conditions. In this letter, the surface charge pattern of PMMA subject to ESD at the different relative humidity (RH) is presented by Lichtenberg figures. The influence of RH on discharge processes is discussed. The results show that the voltage amplitude, polarity, and the shape of the electrode during ESD all have a significant influence on the surface discharge process. The surface discharge pattern at positive ESD is plum blossom-like while at negative ESD the surface discharge pattern presents a star-like pattern. The ESD process can be suppressed under the high RH condition. The study of this paper provides direct evidence on how ESD process is mitigated at different RH.

Polymer nanocomposites with high dielectric permittivity (e) are urgently demanded for power energy storage. In this research, in order to prepare nanocomposites with strong interfacial interactions and higher e, a novel surface modification molecule, 3,4,5-trihydroxybenzoic acid (or Gallic acid, GA), was developed. There are two main advantages for selecting GA as the coating molecule. Firstly, the polar groups, such as hydroxyl and carboxyl groups in GA, could form hydrogen-bonding interaction with poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE). Therefore, though thickness of coating layer is only 1.6 nm, nanocomposites with better dispersion were prepared by strengthening hydrogen-bonding interaction at polymer-nanoparticle interface. Secondly, functional groups with high bond dipole moment, such as carboxyl groups and phenolic groups, are beneficial to obtain high e in PVDF-CTFE + BT@GA. e is 25.6 at 1 kHz with 12.9% BT@GA. This is about 2.5 times higher than that of PVDF-CTFE matrix (10.4 at 1 kHz). The hypothesis in this research might offer a feasible approach to prepare capacitors with higher e and higher energy density by regulating bond dipole moment of functional groups in the coating molecules.

Polymeric materials filled with ZnO varistors microspheres can exhibit the intriguing nonlinear current-field characteristic that is promising for electrical field grading applications. However, the switching field of the composite material is hardly controllable by current approaches, and the nonlinear current-field characteristic of the composite material is attained at the expense of significantly compromised mechanical properties. In this study, we developed silicone rubber (SR)-based ternary composites containing ZnO microspherical varistors and carbon fibers (CF) for simultaneous control over the electrical and mechanical properties to satisfy the practical requirement in electrical field grading. The measured J-E characteristics showed that introducing CF into conventional ZnO microspherical varistors/SR binary composites could largely broaden the range of switching field and improve the nonlinear characteristic. The mechanical tensile tests of the ternary composite samples revealed remarkably enhanced mechanical properties compared with ZnO microspherical varistors/SR binary composites. This paper demonstrated the synergistic effect in tailoring the nonlinearity and mechanical performance of nonlinear polymeric composites for advanced field grading materials.

The zinc-oxide (ZnO) varistor is widely used to limit the overvoltage in the power systems, and its quality is closely related to the safety of the entire protected system. In this paper, accelerated aging experiments under different current levels were carried out on the ZnO varistors to study the harmonic characteristics of the leakage currents in the aging process. It is found that the proportions of higher order harmonics in leakage currents increase faster after several impulse tests and then a sudden decrease appears before the varistor fails, which is caused by the decrease of the varistor voltage. And based on this, a new criterion for the aging of the ZnO varistor is put forward.

Due to the effect of the space charges, there is still a lack of effective methods to predict the corona effects of HVDC transmission line based on cage tests. This paper presents a method of using the corona cage to evaluate the radio interference of HVDC transmission lines. First, the condition under which the tests of a corona cage can be equivalent to those of an HVDC transmission line is discussed theoretically. It shows that the key is to make the electric-field distribution around the conductors in the corona cage the same as that around the transmission line. Then, a numerical approach and the Kaptzov's assumption are used to obtain the applied voltage on the conductors in a corona cage in order to set up the same electric-field distribution as that around the transmission line. Reduced-scale tests both in time domain and in frequency domain show that the method is effective. The method can also be extended to investigate the dc corona and its other effects based on cage tests.

Self-powered wireless sensor networks based on autonomous wireless sensor nodes have been increasingly attractive due to the outstanding merits of maintenance-free and self-sustainability. In spite of the explosive growth of the wireless sensor network techniques, the corresponding power supply methods with miniaturized, low-cost, flexible, reliable, and efficient designs are still limited. In this paper, a novel magnetic energy harvester based on nonlinear resonance is presented, which has been verified to be capable of scavenging energy from the power-frequency (50 or 60 Hz) magnetic field distributed around the transmission line. We have theoretically and experimentally demonstrated the greatly enhanced harvesting properties in both power density and enlarged bandwidth, which provides great robustness to frequency fluctuations of magnetic field. A prototype of this brand new design is fabricated and tested. Experimental results show that the prototype is capable of harvesting a power as high as 13.5 mW at merely 2.4 Oe (RMS) power-frequency AC magnetic field. The high power density (0.5625 mW/cm3 at 2.4 Oe) and the wide working bandwidth (2.6 Hz) promise its application prospects as a sufficient and reliable power supply for sensors in the smart grid.

Extruded polymeric HVDC cables have drawn great attention in modern power systems. This paper reviews the history and development of the polymeric HVDC cables and summarizes the key technical problems in extruded HVDC cables. It is pointed out that two key issues should be solved. One is the electric field inversion within HVDC cable insulation which is caused by the temperature and electric field dependent DC volume resistivity of the polymeric insulation materials. The other is the space charge behavior under multi-fields coupling, including charge injection, transportation, accumulation and dissipation characteristics. The following aspects need to be particularly concerned in the future: the interaction between temperature, electric field, space charge and DC volume resistivity under multi-fields coupling; mechanisms of nanoparticles doping on enhancing the properties of polymeric insulation material; the long-term operation characteristics of nanodielectrics; collaborative optimal regulation methods and theories on the properties of polymeric insulation material; and recyclable insulation materials for future HVDC cables.

To study the surface charge distribution of high-voltage direct-current (HVDC) spacers for gas insulated lines (GIL), a surface charge inversion algorithm based on surface potential measurements is necessary. However, previous studies on inversion calculation of surface charge density only considered the surface potential of one side, neglecting the effect of residual surface charge on the other side of the spacer; this leads to an inaccurate inverted surface charge distribution. In this paper, the inversion algorithm methods used to determine surface charges over the past few decades were summarized, and their advantages and disadvantages were discussed. Based on the previous charge inversion algorithms, an improved surface charge inversion algorithm considering the surface potential of both sides of a cone-type model spacer was developed. The improved method was verified experimentally using a cone-type model spacer. Compared with that obtained by previous charge inversion algorithms, the charge distribution derived from the improved algorithm was closer to the situation determined from theoretical analysis. This work has practical relevance for the surface charge measurement of cone-type spacers and offers a new orientation in the surface charge analytical algorithm.

Due to the wide application of high-order resonant and nonlinear electric equipment in the UHVDC converter station, the electromagnetic transient process is a complex issue during the short-circuit fault transient of the system caused by lightning strike or other reasons. In this study, firstly, an operating ±800 kV DC line is artificially short-circuited to simulate the flashover fault process. The instantaneous voltages and currents and the electrical fields are measured by advanced sensors and high sampling rate recording devices at different points in the converter station, in order to comprehensively study the high-frequency characteristics of the UHVDC system. Then, the portal impedances of the major UHVDC equipment as well as the stray parameters are systematically measured in an actual ±800 kV station. Based on the data measured, the accurate high-frequency characteristics of the ±800 kV converter station are analyzed. The results of the study can be used to guide the EMC and relay protection designs of the UHVDC converter station, so that the stability of the system can be improved.

An inversion method for evaluating lightning current waveforms from measured electromagnetic field data based on time series neural network (TSNN) is presented in this paper. The back-propagation neural network (BPNN) is also adopted to evaluate the channel-base current using measured electromagnetic field data, and comparisons of inversion results between TSNN and BPNN are presented. The inversion results are in good agreement with corresponding measured channel-base currents. The proposed method can evaluate the channel-base current in areas with complex terrain, and it is useful for studies on lightning-protection in power systems and lightning characteristics

Polypropylene (PP) has shown great potential as recyclable HVDC cable insulation material. This paper aims to investigate the effect of different nanoparticles on tuning the electrical properties of PP. Surface modified MgO, TiO 2 , ZnO and Al 2 O 3 nanoparticles with various contents were melt blended with PP. The microstructural morphology, dielectric properties, DC volume resistivity, space charge behavior and DC breakdown strength of the nanocomposites were investigated. It was found that the dielectric permittivity all increases with the increase of nanoparticle content, only the dielectric loss of TiO 2 /PP nanocomposites increases. For MgO and TiO 2 nanocomposites, the DC volume resistivity increases with the increase of nanoparticle content and then decreases, while it continues to increase for Al 2 O 3 nanocomposite. TiO 2 /PP nanocomposite with 5 phr nano-TiO 2 shows lower volume resistivity than PP. Space charge suppression and DC electric breakdown strength show similar variation trend, both increase with the increase of nanoparticle content and then decrease. MgO and TiO 2 nanocomposites show the most obvious space charge suppression and TiO 2 nanocomposites exhibit the highest DC breakdown strength, which is 43% higher than that of pure PP. Considering the electrical properties investigated, the optimal content for MgO, TiO 2 , ZnO and Al 2 O 3 nanoparticles is about 3, 1, 1 and 1 phr, respectively. Among these four kinds of nanoparticles, MgO and TiO2 nanoparticles are more capable than ZnO and Al 2 O 3 nanoparticles to modify the electrical properties of PP and more potential to be used as recyclable HVDC cable insulation material.

In this work, we studied multiple doping of ZnO varistors with Ga 2 O 3 and In 2 O 3 and determined its effect on electrical properties, characterized by using scanning electron microscopy, current–voltage testing, capacitance–voltage testing, and X-ray diffraction. At a given gallium concentration, the residual voltage ratio of the sintered varistors decreased clearly with the indium dopant concentration increased. Additionally, with increasing indium dopant concentration, the average grain size decreased slightly, and the voltage gradient was improved. The varistors containing 0.045 mol% indium and 0.4 mol% gallium showed optimal performance, exhibiting a residual voltage ratio of 1.54, as well as a voltage gradient of 462.5 V/mm, a leakage current of 1.30 µA/cm 2 , and nonlinear coefficient of 72.3. Such co-doped ZnO varistors can provide better protection when used in surge protectors.

The ion mobility is the one of the basic parameters in the study on gas discharge. There are many environmental parameters affecting the average mobility of ions in air. Among them pressure and humidity are important factors. Until now there is no uniform average mobility of ions in air in practice. This paper presents the results of a series of experiments on the influence of pressure and humidity on the average mobility of ions in air. Firstly, an improved analysis method to deduce the average air ion mobility in air is presented with the help of an apparatus for generating parallel-plane ion flow field. Then, the mobility is measured in a climatic test cabinet. The positive and negative mobilities in the air with the pressure from 80 to 110 kPa and the relative humidity from 20% to 80% at the room temperature are provided. Finally, the rule of mobility-pressure-humidity variation curve is discussed and the specific formula for average mobility of ions in air with respect to pressure and humidity is then raised.

In the present study, the effect of indium doping on the micro-characteristics and electrical properties of ZnO varistors co-doped with Al2O3 and Y2O3 were determined. Scanning electron microscopy, current-voltage testing in a range from small to large current, capacitance-voltage testing, and X-ray diffraction pattern testing were conducted. The results show that both the residual voltage ratio and the leakage current of sintered ZnO varistors decrease and then increase as the indium dopant increases at a given aluminum and yttrium content. The nonlinear coefficient shows an inverse relationship. In addition, the voltage gradient of the samples increases as the indium dopant increases. The sintered ZnO varistor samples with 0.02 mol% indium, 0.2 mol% aluminum, and 0.9 mol% yttrium show the optimal performance, exhibiting a 1-mA residual voltage of 448 V/mm, a leakage current of 0.69 µA/cm2, a nonlinear coefficient of 76, and a residual voltage ratio of 1.58. This study has great significance for improving the protective effects of surge protection devices assembled with ZnO varistors and the stability of power systems.

The light emission during the growing of the electric tree has been studied before, but very little is known about it before the electric tree initiation. The aim of this work is to study the light emission before electric tree initiation in insulate materials under high AC stress. The ultra-sensitive photomultiplier tube is used to detect the photons in a darkroom and a series of optical filters are used to investigate the spectrum of the photons. The existence of photons before tree initiation is proved and the pattern of them during an AC cycle is observed. The spectrums of the light emission in different dielectric materials are acquired and provide some evidence for the interaction between free charges and polymer molecules. Although the investigation of the light emission is far from being complete, it is found that the light emission under AC voltage starts with the recombination of opposite charges and the interaction process begins under a higher electric field. Based on the experimental results, a brief charge behaviour model is built up.

This study determined the effect of indium doping on the electrical properties of Al-doped ZnO varistors. Scanning electron microscopy, current-voltage testing in a range from small to large current, capacitance-voltage testing, and X-ray diffraction pattern testing were carried out. The results show that both the voltage gradient and nonlinear coefficient of sintered ZnO varistors increase and then decrease with increasing indium dopant at a given aluminum content. Meanwhile, the leakage current and residual voltage ratio show an inverse relationship. The varistor ceramic with 0.015 mol% indium and 0.02 mol% aluminum has optimal performance. This work is helpful for improving the protective effects of ZnO varistors and increasing the safety of electrical systems.

ZnO varistors with high energy absorption capability and aging stability were fabricated. The main prescriptions of ZnO varistors include 94.09 mol%ZnO, 1.0 mol%Bi 2 O 3 , 0.75 mol%MnO 2 , 1.0 mol%Co 2 O 3 , 0.5 mol%Cr 2 O 3 , 1.0 mo.%Sb 2 O 3 , 1.2 mol%SiO 2 , 0.1 mol%Al 2 O 3 , and 0.36 mol%Ga 2 O 3 . The varistors maintained a large voltage gradient and a high nonlinearity. AC accelerated aging test performed under 0.9 U 1 mA at 115 °C with 1000 h showed that the ZnO varistors were stable. The varistors without being aged were in good condition after being struck 18 times by a 400 A square wave energy pulse. The absorption energy was more than 327 J/cm 3 and the degradation rate coefficient ( K t ) was 0.04. These values are suitable for manufacturing 1000 kV ultra-high voltage surge arresters.

The interfacial region between a nanoparticle and the polymer matrix is considered to have an important effect on the properties of nanocomposites. In this experimental study, the tuning effects of surface-modified TiO2 nanoparticles on the aggregation structure and trapping property of the nanocomposites are investigated. The addition of TiO2 nanoparticles increases the number of spherulites and decreases their sizes. It is also found that TiO2 nanoparticles can suppress the mobility of chain segments in the interfacial region, suppress crystallization, and reduce the crystallinity, depending on the surface modification and loading levels of nanoparticles. On the basis of the conclusions, the model of a spherulite and the interface is established and the trap distribution of the interface is analyzed according to the results of TSC measurements. It is assumed that there is a strong correlation between traps and the charge transport of nanocomposites, and the mechanism of charge transport is discussed wit...

Electromagnetic transient processes are strongly related to the parameters of transmission lines. With the ground return parameters considered, the frequency dependent parameters of transmission lines are highly dependent on the soil parameters, which include resistivity and dielectric permittivity. In this paper, with the measurement results of frequency variation of soil parameters, the influence of the frequency dependent soil parameters on the frequency dependent characteristics of the transmission line parameters were studied using the complex return plane method. According to the calculation results, the frequency characteristics of soil parameters have obvious influences on the parameters of overhead transmission lines. The influences of the humidity, temperature and particle size of the soil on the ground-return parameters were also evaluated.

Induced lightning is the major cause of interruptions in power distribution lines. To reduce lightning accidents and improve the power supply reliability, it is significant to study the characteristics of the induced lightning overvoltage by experiment and calculation. In the paper, a calculation method of lightning induced overvoltage, considering lightning leader progress, non-ideal ground corona effect, field-line coupling, was introduced. The accuracy of the program was verified by comparison between calculated results and the practical experimental results, the calculation results are close to the experimental data. A scaled experiment was carried out to obtain the basic relationship between lightning and overvoltage on distribution lines. Influence factors on overvoltage were analyzed.

Polymer dielectrics with high thermal conductivity and low dielectric loss are highly attractive for applications in electric and electronic industry. In this paper, surface modified aluminum nitride (AlN) nanoparticles were introduced into polypropylene (PP) matrix via melt blending to prepare thermoplastic dielectrics with enhanced thermal conductivity and low dielectric loss. The microstructure, DC conductivity, dielectric properties, breakdown strength, space charge behavior, thermal conductivity and thermal stability of the nanocomposites were investigated. It is found that the inclusion of surface modified AlN nanoparticles results in improved thermal conductivity and thermal stability of PP/AlN nanocomposites. The dielectric constant shows a slight increase with AlN nanofiller content, and the dielectric loss remains at very low level of below 0.003. DC conductivity of PP/AlN nanocomposites increases but shows desirable weak dependence on electric field. DC and AC breakdown strength slightly decrease with the AlN nanoparticle content but it is still high enough for dielectric applications. More importantly, the PP/AlN nanocomposites is a thermoplastic material which is potential to be recycled and has less impact on the environment.

As basic insulating material, epoxy resins are widely used as the matrix of insulators in HVDC transmission system. In most cases, alumina of micro range is blended to improve mechanical properties as well as reduce expenses, etc. However, apart from benefits, the incorporation of micro-particles could alter surface electric behaviors due to the change in material structures and the surface trap distribution under operating condition. Therefore, the surface charge behavior under dc is changed as well, which can affect the surface flashover characteristic. Direct fluorination is a method to change surface properties by introducing stable C-F structure to the surface of the dielectric material. The newly formed fluorinated layer could bring changes to electrical characteristics, thus affecting surface electrical properties under dc stress. In this paper, samples are prepared using alumina casted epoxy resin composites. The two parts are blended using the same process used to produce cone-type insulators in gasinsulated substations (GIS) in industrial application. Experiment samples were treated with F2 /N2 mixture containing 12.5% F2 in volume at 50 °C under 0.1MPa. The scanning electron microscopy (SEM) and fourier transform infrared spectroscopy (FTIR) were used to investigate surface physicochemical characteristics. Water and diiodomethane droplet contact angles were measured and surface energies were calculated. The distribution of the surface voltage potential, as well as the surface conductivity and DC flashover voltage were measured. Experiment results confirmed that the fluorination to epoxy resin and alumina composites can change the surface morphology by changing the exposed area of surface alumina particles. It also showed that fluorinated samples have higher DC flashover voltage in both air and SF6 due to the combined effect of the change in the surface morphology and the surface conductivity.