Surface charge accumulation on the spacers is one of the key issues restraining the development of HVDC GIS/GIL. The precise measurement of surface charge properties provides the basis for further study of the surface charge transport mechanism as well as the charge-induced flashover mechanism under DC voltage. In this study, the authors discuss their perspective on the current status, development needs and potential developing orientation of surface charge characterisation techniques. Different surface potential measurement methods and charge inversion algorithms are reviewed regarding the previous studies and future research needs. Drawbacks and outlooks of surface charge measurement techniques are also discussed with the background of laboratory experiment results and on-site measurements. It is hopefully that this study can serve as a useful guide reference for researchers within the same research field. More importantly, it is authors’ hope that this study can inspire some novel ideas for readers into developing of more accurate and scientific interface charge characterisation techniques.

The vacuum discharge along the dielectric surface, also called surface flashover, brings significant damages to the vacuum-solid insulation system. Here the authors implement shape-flexible, complex surface groove microstructures on the dielectric to mitigate the initiation of vacuum flashover. A particle-in-cell simulation is employed to reveal the real-time discharge development considering the blockage of the multipactor propagation as well as the space field distortion in the presence of specific surface microstructures. Electron trajectories within barriers are theoretically analysed to present how the barrier shape affects the discharge process, showing consistent results with the simulation. By analysing three parameters, namely the anode current, the surface average charge density and the flashover threshold, it is found that the effect of suppressing surface flashover remarkably augments when the groove number and depth rise up, while such effect gradually saturates when the groove depth reaches a critical value. Theoretical analyses are also provided for the decay factor as well as the optimal shape and trapezoid grooves are proved as the optimal shape distribution of microstructures. Furthermore, different secondary emission yield (SEY) distributions are considered with two kinds of structures, including the material with low-SEY inside and on the top surface of grooves.

Surface flashover of insulation systems is a basic issue in the field of high voltage and electrical insulation. To improve the surface flashover performance is of great significance for the development of advanced power transmission equipment and insulation materials. This study reviews the research progress of surface flashover in vacuum regarding to effective methods to improve the surface flashover performance in vacuum, including insulation system optimisation and material modification. The former one is beneficial to reduce the electric field distortion, and the later one is able to adjust the surface trap parameters of the material through physical and chemical methods. In addition, the ‘U-shaped’ curve is proposed to reveal the relationship between surface flashover voltage and surface trap level, discovering the synthetic effects of surface traps on surface flashover. It is expected that the ‘U-shaped’ curve will become a guidance to improve surface flashover performance through adjusting trap parameters. Moreover, several suggestions are made to build unified surface flashover model which is suitable to the range from high vacuum to high pressure.

With the increase in the voltage and capacitance of gas-insulated transmission lines (GILs), the insulation failure of GIL has attracted more and more attention. Further understanding of the partial discharge (PD) and flashover characteristics of metal particles on the insulator surface, as well as the flashover mechanism, is necessary to reduce the failure rate and improve the reliability of the equipment. In this study, an ultra-high voltage (UHV) AC insulator PD test and measurement system based on the cross-reference pulse current (PC) and ultra-high frequency (UHF) methods were established. The PD development and flashover characteristics of 5 mm-long metal particles at different positions on the surface of a UHV AC insulator, as well as the surface charge accumulation, were studied. The results show that the discharge of millimetre-scale metal particles on the insulator surface under PD test conditions is relatively low (generally lower than 2PC), and it is difficult to be detected by the conventional PC and UHF methods due to their insufficient sensitivity. Moreover, it is found that the weak PDs of the millimetre-scale metal particles will result in a charge accumulation on the insulator surface under AC voltage, which eventually will lead to the insulator flashover. The PDs of the UHV AC insulator before flashover are generally small, which will make it difficult for online monitoring systems to give an effective alarm before flashover.

Humidity has been considered as one of the main influencing factors that determine the conduction processes and electric strength of gas-insulated systems. Whereas in the past, various studies focused on the change in the partial discharge inception voltages, breakdown strength of homogeneous and inhomogeneous field arrangements, and insulator flashover voltage, recent studies have investigated the changes in ion currents measured through different gas gaps. In the framework of this contribution, a highly precise humidity control circuit has been developed to analyse the significance of humidity in the range from −25 to −5°C frost-point, which is fully applicable to operating gas-insulated devices. Using sulphur hexafluoride (SF₆) as the insulation gas at 0.45 MPa, Al₂O₃-filled epoxy resin insulators, and technically rough electrodes, the humidity was found to significantly influence the intensity of microdischarges at interfaces. Charge generation from microdischarges at the interfaces substantially increased with increasing humidity. For an electric field of 5 kV/mm that were applicable to the dimensioning of gas-insulated devices, humidity strongly influences the charge provision from technically rough interfaces and potentially contributes to the surface-charge accumulation at insulator surfaces. On the other hand, for low-field conduction phenomena, no increase in the ion currents from natural ionisation or electrophoretic conduction was observed. For the investigated range of parameters, humidity is expected to be highly relevant for the design of gas-insulated devices.

In this study, a simulation model of surface charge accumulation has been established. The model considers three accumulation ways, i.e. electrical conduction within the gas, through insulator volume and along the insulator surface. The generation, diffusion, drift and recombination of charge carriers are also taken into account. Based on it, the influence of polarity reversal, reversal time on surface charge and electric field distribution on a basin-type insulator are studied. The polarity of the surface charges and the direction of the electric field change after the voltage polarity reversal. When the preload voltage is equal to reversal voltage, the surface charge and the electric field distributions at steady state before and after voltage polarity reversal are all the same with opposite sign, and not affected by the reversal time. However, the time to reach the steady state varies with different reversal time. The steady-state surface charge and electric field increased with the rise of reversal voltage. The transient normal and tangential electric field would not exceed the value of the steady state, which means voltage polarity reversal has no additional influence on insulation performance. This research can provide guidance to the design and manufacture of DC GIS/GIL.

Charge accumulation has always been a problem for the safe operation of gas insulated switchgear (GIS)/gas insulated transmission line (GIL) both under AC and DC. It is of great significance to investigate the behaviour of surface charge under high voltage. In this study, the charge distribution characteristics and accumulation mechanism on insulator surface in air and heptafluorobutyronitrile/carbon dioxide (C₄F₇N/CO₂) mixtures under AC voltage are studied via the means of surface potential measurement and inversion algorithm, combined with the improved method of controlling the truncated phase of AC voltage. The results show that under the needle–plate electrode structure, charge distribution on the insulator surface presents a three-tier concentric circle structure both in air and C₄F₇N/CO₂ gas mixtures, and the charge composition of the innermost circle is closely related to the truncated phase of AC voltage. Under the same amplitude of voltage, the range of charge distribution on insulator surface in C₄F₇N/CO₂ mixtures is smaller than that in air, with the negative charges domination. It is suggested that the characteristic of charge distribution in C₄F₇N/CO₂ mixtures is related to the larger electron attachment cross-section of C₄F₇N gas in a wide range of electron energy distribution.

A surface discharge non-equilibrium plasma model of air–polyimide under pulsed electrical stress is established, by considering the reaction of charged particles on the dielectric surface and the secondary electron emission caused by the condition that high-energy particles bombard the material surface. The model defines the chemical reaction of air discharge by using simplified set of reactions, which greatly reduces the complexity of the model. To avoid the negative value of particle density in the process of solution, the logarithmic finite-element method is used to solve the model established, so as to implement the dynamic simulation of the surface discharge process. Also, the temporal and spatial evolution of the microparameters such as charge and electric field distribution during discharge are obtained, and the reliability of the model is verified by experiments in terms of discharge development pattern and surface charge accumulation. By comparing the development process of surface discharge under single pulse and repetitive pulses, it can be seen that surface discharge develops from needle electrode to ground electrode under both repetitive pulses and single pulse stress, but the relationship between the discharge propagation time under repetitive pulses and pulse repetition rate is a ‘u’ curve, and the inflection point moves to higher repetition rate region with the increase of voltage.

The Maxwell–Wagner model and bipolar charge transport model both aim at describing the charge transport behaviours in dielectrics. The Maxwell–Wagner model performs well in calculating the polarisation current and electric field distribution, but it is too macroscopic to describe the microscopic behaviours of space charges. The bipolar charge transport model can well simulate the space charge accumulation, but it cannot quantitatively relate the microscopic transport behaviours with the macroscopic conductivities of dielectrics. Considering the shortages of the above two models, an improved charge transport model, named carrier dynamic equilibrium model, was proposed in this study to simulate the polarisation process of the low-density polyethylene/ethylene propylene diene monomer bi-layer dielectric by introducing a source term of carrier dynamic equilibrium. Effects of carrier mobility and non-equilibrium carrier lifetime on the simulation results were explored, and a comparison among the results of measurement and different models was also made. Compared with the Maxwell–Wagner model and bipolar charge transport model, the improved model has the best coincidence with measurements, which can provide an accurate reference for the design of high-voltage direct current insulation systems.

Dielectric response analysis, performed in time domain and frequency domain is one of the interesting methods used for condition assessment of high voltage equipment. In this article, the degradation of epoxy-mica composite insulation which is widely used in rotating machines has been investigated through frequency domain spectroscopy measurements. Artificial ageing of the carefully prepared stator bar insulation sample was performed through simultaneous application of high temperature and electric field in a designated environmental chamber for specific durations. To study the effect of temperature, the measurements were performed at temperatures ranging from 20 to 60°C at different ageing status. Following experimental work, the measurement results were modelled using Havriliak–Negami model and ageing sensitive parameters were extracted. To study the effect of temperature, the measurements were performed at temperatures ranging from 20 to 60°C. The modelled results matched well with the experimental measurements and certain model parameters sensitive to the ageing condition were identified. The numerical relationships between the insulation status of the stator bar and the aforementioned parameters were identified. It was observed that the nature of the numerical relationship remains identical in the investigated temperature range, although the coefficients vary to some extent.

Epoxy nano–micro composite specimen prepared with micro silica and ion trapping nanoparticle, by shear mixing process, was exposed to gamma radiation and its performance for space charge and charge trap characteristics were analysed. The threshold for space charge accumulation of epoxy nanocomposites reduces and rate of space charge accumulation increases with an increase in dosage of gamma irradiation. The average growth of space charge density during poling and charge decay rate during depoling are relatively higher for gamma-irradiated specimens than the virgin specimen. The initial surface potential has a marginal reduction with increase in the dosage of gamma radiation, but the surface potential decay rate has increased significantly. Trap distribution characteristics indicate more number of shallow traps and increase in charge mobility after irradiation. The relative permittivity and loss tangent of the specimens have high impact due to gamma irradiation. The activation energy calculated from DC conductivity by Arrhenius law reduces with increment in radiation dose. Laser-induced breakdown spectroscopy reflected no change in elemental composition with gamma-irradiated specimen. The variation in plasma temperature and ion line to atomic line intensity ratio with dosage of gamma radiation have direct correlation to the Vickers hardness number of the specimens.

Determining the voltage ratio change is one of the core issues in the traceability of the DC voltage divider. Basing on the previous research results, this study proposes an improved DC voltage summation method to evaluate the voltage division ratio error of 1000 kV DC resistance divider. The principle of the method is to calibrate the voltage divider with rated voltage 2U by using two auxiliary voltage dividers which are with rated voltage U, wherein the high-voltage (HV) arm and the low-voltage arm of the auxiliary voltage divider can be separated. Research results show that compared with the conventional method, the method can reduce one measurement variable when determining the divider's ratio change, thus simplifying the calibration process. The voltage ratio of 100 kV measured by the method of this study was well-verified by the calibration results from the National Institute of Metrology (NIM, China) and Physikalisch-Technische Bundesanstalt (PTB, Germany). Using the proposed method, the ratio change of DC voltage divider at an applied voltage of 1000 kV was effectively obtained and the uncertainty of 2.5 μV/V was achieved. Research results can provide technical guarantee for the accurate measurement of HVDC magnitude.

Thermal aging of natural ester oil shows drastic reduction in partial discharge inception voltage (PDIV) and a significant variation is observed only above a certain aging time, under AC, DC, high frequency AC voltages and with harmonic voltages with different total harmonic distortion. Weibull distribution studies on PDIV measurements indicate a reduction in scale parameter (α) with increase in thermal aging temperature. A characteristic reduction in breakdown voltage was observed with the thermally aged ester oil, under AC, DC and standard lightning impulse voltage. The breakdown voltage variation with aged ester oil follows normal distribution. Ultraviolet (UV) analysis of ester oil thermally aged at 160°C has revealed a regular shift of the derived absorbance parameter to longer wavelengths. The interfacial tension and turbidity exhibits an inverse relationship with the thermally aged ester oil. Gas chromatography/mass spectrometric analysis of the thermally aged ester oil predicted the formation of more carboxylic acids and ketones with aging duration. The steady-state fluorescence on thermally aged ester oil exhibits a shift in its emission profile, which is in tandem with the UV absorption spectroscopic analysis. Fluorescence analysis can be adopted as a real-time monitoring tool in transformers, to understand the condition of liquid insulation. The viscosity dependence on the wavelength of derivative absorption maxima follows a direct relationship with the thermally aged natural ester oil.

Second generation (2G) high-temperature superconductor (HTS) (RE)Ba₂Cu₃O _x (REBCO) shows a great potential in building high field magnets beyond 23.5 T. The electromagnetic modelling is vital for the design of HTS magnet, however, this always suffers the challenge of huge computation for high field magnets with large number of turns. This study presents a novel electromagnetic modelling based on T-A formulation for REBCO magnets with thousands of turns. An equivalent turn method is proposed to reduce the number of turns in calculation, so that the computation cost can be reduced significantly, and meanwhile the key electromagnetic behaviour of HTS magnet can be simulated with enough accuracy. The ramping operation of a fully HTS magnet with 12,000 turns are analysed using both the original T-A model with actual turns and improved T-A model with equivalent turns. The two models show a good agreement on the key electromagnetic behaviours of the magnet: distribution of current density, magnetic fields, screen current induced field and magnetisation loss, so that this improved T-A model using equivalent turns is validated. The T-A modelling of REBCO magnet is a powerful tool for the electromagnetic analysis of industry-scale high field magnets.