High-voltage direct current (HVDC) and high-voltage alternating current (HVAC) cables are the most important equipment for high-voltage, large-capacity and long-distance power transmission. Electrical tree is a pre-breakdown phenomenon leading to failure of insulation materials, and it is the major issue that threatens the safe and stable operation of HVDC and HVAC cable systems. This study summarises and analyses the achievements in the research of electrical tree for HVDC and HVAC cables. The initiation mechanisms of the electrical tree, including Maxwell electro-mechanical stress, charge injection–extraction, charge trapping and electroluminescence theories, are elaborated for fully understanding the electrical degradation process in insulation materials. Then, the influences of the high electric field, high temperature and mechanical stress on electrical tree behaviours are discussed, and the relationship between charge transport and the electrical tree is analysed and illustrated. The suppression methods of the electrical tree are put forward by introducing inorganic and organic additives into insulation materials, and the suppression mechanisms are presented from the viewpoints of the structure-property and microscale–macroscale relationships. Recently, the electrical tree research studies are focused on the high-precision of initiation models, high-dependence of multi-physical fields and high-efficiency of suppression methods. The achievements provide theoretical support for improving the electrical performance of insulation materials, while it is a practical problem to explore their application feasibility in HVDC and HVAC cable.

Hexagonal boron nitride nanosheets (BNNSs) are two-dimensional nanomaterials with graphitic-like layered nanostructures, high surface areas, and large aspect ratios. Owing to their excellent thermal conductivity, electrical and mechanical strengths, BNNSs are emerging as multifunctional fillers in polymer dielectrics. In this article, the authors review the recent progress in the BN-containing polymer nanocomposites designed for high-performance film capacitors. While general synthetic approaches to BNNSs and polymer/BNNS nanocomposites are summarized, particular attention is placed on structure-property correlation and rational structural design of the composites with optimized dielectric properties and capacitive performances. In stark contrast to the polymer composites employing high dielectric constant fillers to enhance the electric displacement, a new design concept based on the utilization of BNNSs with a wide bandgap to impede electrical conduction and consequently improve breakdown strength and charge-discharge efficiency of the polymer composites, is highlighted. The significance of developing dielectric capacitors with desirable thermal conductivity and thermal stability to ensure their robust and efficient operation is emphasized. The merits and challenges regarding the existing polymer dielectrics containing BNNSs for energy storage are identified. An outlook for future research opportunities and engineering applications is also presented in this review.

With the rise of renewable energy resources, especially offshore wind power plant, the DC transmission concept has attracted more and more attention. The selection of eco-friendly insulating gas, as most critical insulating part of gas-insulated equipment is top priority for developing highly reliable system. Moreover, it is acknowledged that eco-friendly C₃F₇CN/CO₂ gas mixtures have become a potential alternative to SF₆ gas due to its excellent performance. This study reviews the basic physical properties of C₃F₇CN gas/gas mixtures and insulation properties including the gas gap breakdown and surface flashover performance at DC, as well as lightning impulse voltage. Investigation about the gas stability is presented, including the compatibility of gas with solid materials in gas-insulated transmission line (GIL), and the decomposition characteristics under long-term electrical and thermal stresses. The important progress on the charge accumulation characteristics of gas–solid interface in the novel gas environment under DC voltage stress is analysed. Finally, the key issues that need to be paid attention to and further followed through regarding the application of this novel gas in GIL are summarised and put forward. This study hopefully can provide a complete reference for the development of eco-friendly DC gas insulation equipment with C₃F₇CN/CO₂ gas mixtures.

High voltage power cables play a critical role in global electricity transmission and distribution. The currently used power cables cannot fulfil the green and sustainable requirement of modern society because of the thermoset nature of cable insulation and shields. This study is aimed at developing thermoplastic shields for high voltage power cable, which is one bottleneck restricting the development of environmental-friendly cables. Using carbon black (CB) as the main conductive component and a small amount of carbon nanotubes (CNTs) or graphene as the second filler, highly conductive polypropylene based composite materials were prepared for potential shield applications. It was found that, at a fixed conductive filler loading, the replacement of a small amount CB by CNTs can significantly enhance the electrical conductivity and suppress its temperature dependence. However, when CB was replaced by graphene, only limited enhancement of electrical conductivity could be achieved and the electrical conductivity is still highly dependent on temperature. Dissipative particle dynamics simulations demonstrated that the enhanced conduction property in the CNTs-containing composites could be understood by the shorter average distance between CB and CNTs. Finally, the coordination between the newly developed conductive composites and the environmental-friendly thermoplastic polypropylene insulation was evaluated via high voltage direct current measurements, and the results revealed that the CNTs-containing composites showed excellent suppression effect on the space charge injection and accumulation in the insulation. This research paved a new way for developing environmental-friendly high voltage power cable shields.

Generally, the electrical properties of nanocomposite are affected by the type, size, filling concentration and surface treatment process of the nanoparticle. In this study, nanocomposites of polyethylene (PE) with varying filling contents of nano-alumina particles were prepared by the melting blending method and three different kinds of coupling agents were applied for surface modification properties of the nanoparticles. Two of them were silane based and the other was titanate based. The effect of different coupling agents on the dielectric properties was studied. Fourier-transform infrared spectroscopy and thermo-gravimetric analysis were used to verify their compositions. Scanning electron microscope and polarised optical microscopy were used for morphology study. Dielectric permittivity, direct current (DC) volume resistivity and DC breakdown strength characterised their improved insulation performance with nano-alumina as filler. Thermal stimulated current results revealed that adding nano-alumina particles into low-density PE could provide more deep traps and increase DC resistivity.

Polymer dielectrics are widely used in electrical and electronic apparatus and devices because of their capability to insulate conductors, withstand high fields and suffer negligible conductive losses. Their near-to-zero conductivity has been explained in terms of long-accepted theories of electronic and ionic transport that lead to the accumulation of local net charge regions at high electric fields. Here the authors describe a previously unknown conduction mechanism consisting of small bipolar ultra-fast charge pulses crossing the polymer with the mobility, as large as, 4 to 5 orders of magnitude greater than that of the previously known. The authors show that this motion is a consequence of molecular relaxation processes triggered by the electric field locally enhanced by the pulses themselves. Pulse accumulation at the electrodes increases interface field and thus contributes substantially towards premature failure in insulating dielectrics in DC fields.

In recent years, small-size, low-weight aerospace propulsion systems have developed rapidly for space exploration, on-orbit scientific instruments and extra space missions, while additional electron emission devices are commonly required in those propulsion systems. Carbon nanotube (CNT) based field emission cathodes exhibit extraordinary field emission properties and are regarded to be an ideal alternative of conventional thermionic or hollow cathodes. In this study, the authors give an overview of present status of researches on CNT-based electron emission cathodes for utilising as neutralisers or electron sources particularly in space electric propulsion systems, the theory and characteristics of CNT are also illustrated. Furthermore, challenges, problems and possible solutions before actual applications of CNT in a space mission are discussed accordingly.

With the continuous expansion of power grid capacity, the problem of short-circuit current exceeding the standard is becoming increasingly serious. Fault current limiter is a promising solution and is gradually becoming a research hotspot. In this study, fault current limiters are classified into four categories. And saturated-core fault current limiters are emphatically introduced, including its working principle and comparison with the other three categories. Saturated core fault current limiters are divided into four branches according to the ways leading the core saturated. A comprehensive review of the research activities and emerging technologies of saturated-core fault current limiters for AC power systems is presented in this study. The working principle and typical structure of DC-biased-, permanent-magnet-, superconducting- and hybrid-type saturated-core fault current limiters are introduced. The advantages and disadvantages of four types of saturated core fault current limiters are compared in detail from the aspects of current-limiting performance, iron core size and DC magnetomotive force. Real grid application examples of some types of devices are presented, as well as new progress in the techniques, are covered and discussed in detail. One may find the content of this study helpful as a detailed literature review or as practical technical guidance.

High-voltage direct current circuit breakers (HVDC CB) are one of the key technologies of multi-terminal DC systems and DC grids. Different from other equipment that use a large number of power electronic devices, the HVDC CB cannot obtain its power from the primary system at high potential, making the power supply of the complex multiple electrical potential equipment in HVDC CB a challenge. In this study, the authors propose an HV isolated power supply system based on the multi-output high-frequency isolated power supply unit and the cascaded isolation transformer network. This power supply system can support the 500 kV CB to turn-off 25 kA current with 800 kV overvoltage.

During operation of high voltage (HV) motors, different types of discharges, e.g. surface discharge, corona discharge and bar-to-bar discharge can occur at the same time, increasing the difficulty of partial discharge (PD) sources determination and PDs pattern identification. In this study, the end-winding of a 10 kV motor coil was artificially aged and the related PD was measured. The initiation and variation of different PDs under multi-factor stresses were studied. The localised fingerprints coexisting with multiple PDs were identified and analysed. The results confirm that the end-winding discharge process was significantly influenced by the experiment relative humidity (RH) and temperature. The bar-to-bar discharge was easily recognised at a lower voltage since the identification of bar-to-bar discharge pattern would be affected by the corona discharge pattern at a higher voltage. It is shown that it is more difficult for the corona discharge to be detected when RH exceeded 80%, while the surface discharge dominated at the higher RH. In addition, the PDs were more easily identified with the rise of the temperature. This study can provide a reference in PD identification test and be useful for the PD online monitoring of HV motors.

A simple procedure to estimate series capacitance of a uniform transformer winding from its measured frequency response analysis (FRA) and shunt capacitance is presented. Unlike previously published approaches, this method does not involve any cumbersome and time-consuming curve-fitting nor running optimisation/search algorithms, and neither does it require data of winding geometry. The procedure relies on a property that is observable in the impedance function of a lossless winding, viz., the ratio of the product of squares of open circuit natural frequencies to the product of squares of short circuit natural frequencies bears a special relation to impedance function coefficients. Its feasibility was initially verified by simulation, and then by experiments on small-sized continuous-disk and interleaved-disc windings, followed by a large-sized 33 kV, 3.5 MVA continuous-disc winding, and finally on a 315 kVA 11/13.8 kV transformer. After measuring FRA, the process involves just finding roots of a polynomial, from which the initial impulse voltage distribution constant and series capacitance can directly be determined. Given these attractive features, authors believe that this method is implementable on existing FRA instruments, so that, along with routinely measured FRA, these two important constants of a winding can be displayed.

In the long-running of sulphur hexafluoride (SF₆)-insulated equipment, SF₆ inevitably decomposes to various decomposition products under electric discharge, including SOF₂ and SO₂F₂. In this work, single Pt modified molybdenum disulphide (Pt-MoS₂) monolayer, and double Pt modified molybdenum disulphide (Pt₂-MoS₂) monolayer are proposed to analyse its adsorption and sensing properties to SOF₂ and SO₂F₂ with single and double gas molecules adsorption based on density functional theory. The adsorption energy, density of states, and molecular orbit theory are employed to analyse the adsorption and sensing mechanism. It turns out that the Pt-MoS₂ and Pt₂-MoS₂ present outstanding adsorption capacity to gas molecules. Specifically, double SOF₂ adsorption on Pt₂-MoS₂ shows the best adsorption performance, and the conductivity of the adsorption system changes the most in the adsorption process. Overall, both Pt-MoS₂ and Pt₂-MoS₂ perform as an excellent gas sensor. This study provides a theoretical basis to develop Pt-MoS₂ and Pt₂-MoS₂ based materials for SOF₂ and SO₂F₂ detection in SF₆-insulated equipment.

The current study aims to develop polyvinyl chloride (PVC) nanocomposites with enhanced electrical and mechanical properties by incorporating titanium oxide (TiO₂) nanoparticles within PVC chains. Different loading of nanoparticles and different nanoparticle surface states were considered. The surface states are unfunctionalised, functionalised using vinyl silane and functionalised using amino silane. The choice of a most suitable surface state was a critical factor that guarantees a good dispersion of nanoparticles and consequently enhances the compatibility between TiO₂ and PVC matrix. The process followed in the PVC/TiO₂ nanocomposites preparation, loaded with different wt.% of TiO₂ nanoparticles, was the solvent method. The dielectric properties measured here were the relative permittivity (ɛ _r), dielectric loss (tanδ), breakdown strength (AC and DC under uniform field) and the internal partial discharges (PDs) within insulation cavity. All measurements have been performed under room temperature and at frequency ranged from 20 to 1.0 MHz. Furthermore, the mechanical properties of the samples like elongation, elasticity modulus and tensile strength were also studied. Vinyl silane showed better improvements in both electrical and mechanical performances compared to the amino silane, especially in cases of high weight fractions of TiO₂. This is because of the improvement in the PVC-TiO₂ interfacial region arise from the similarity of polarity and surface tension values of vinyl silane with that of PVC matrix and TiO₂ nanoparticles.

Basin-type insulators are made by epoxy resin composite materials. Curing defects are easily formed in basin-type insulators, and degrade their insulating performances. High curing exothermic rates of epoxy resin composite materials have deemed as the main cause of curing defects formation. In this study, methyl tetrahydrophthalic anhydride (Me-THPA) was used to extend the molecular chain of liquid epoxy resin to prepare epoxy resin composite materials with low curing exothermic rates. Curing exothermic properties, molecular weight distribution, curing stress, microstructures, and surface flashover characteristics of the composite materials were investigated. The results showed that chain-extended epoxy resin had low curing exothermic rates. The curing stress of the chain-extended epoxy resin composite materials was small. The curing defects forming in the composite materials were inhibited. Negative DC surface flashover characteristics of these composite materials were improved. Furthermore, variation of functional groups of the composite materials was studied before and after surface flashover tests. Results showed that the content of carbon–oxygen single bonds (–C–O) in the chain-extended epoxy resin composite materials was observed to decrease. The –C–O bonds also affect the voltage withstand capability of the epoxy resin composite materials.

Saturable reactor insulation is currently stressed by an exponential decay pulse voltage under normal operating conditions. The partial discharge (PD) characteristics of epoxy resin under an exponential decay pulse voltage were studied here and were compared at 25 and 110°C. In addition, this study compares these PD characteristics with those under a sinusoidal voltage to better measure the insulation design margin of the saturable reactor under an exponential decay pulse voltage. Finally, this study explains the PD mechanism based on the three-capacitor circuit model and space charge accumulation. Compared with the sinusoidal voltage, a higher amplitude, a higher inception voltage and fewer PDs are obtained under the pulse voltage. The reason may be related to the accumulation of space charge. Due to the duality of the space charge effect, the promotion effect of space charge accumulation on the PD under the pulse voltage is dominant, and an increase in temperature will weaken the promotion effect. In contrast, the inhibitory effect of space charge accumulation on the PD under the sinusoidal voltage is dominant. The experimental results can provide a basis for the optimal design of saturable reactor insulation under an exponential decay pulse voltage.

As the surface conditions play a significant role on corona discharge and its related effects of the conductors, the influence of fine particulate matter on positive-polarity, direct-current conductors was studied experimentally in this study. The surface morphologies of the conductor could be discovered from the experiments. The typical morphologies are the parallel chains of particles. To evaluate the surface condition quantitively, the surface roughness of the conductors is measured. It is found that the applied voltage and testing time have a great influence on the surface condition. After that, the corona characteristics of conductors are tested. It reveals that the total ground level electric field and ion flow density increases with the surface roughness growing.