Surface Charging Phenomena on HVDC Spacers for Compressed SF_(6) Insulation and Charge Tailoring Strategies

Surface flashover of spacers is a key factor limiting the application of HVDC GIS/GIL,while the charge accumula-tion on the surface of the spacer could have a potential adverse effect on the surface flashover voltage.This paper discusses the laws regarding distribution patterns of surface charges and the related mechanisms.The field-dependent property is discussed in detail to comprehensively illustrate the charge transport mecha-nism and explain the research differences regarding different surface charge patterns obtained by previous researchers.In addition,the main surface charge control methods for epoxy resin are summarized and discussed.The potential research directions of charge control methods and key points in manufacturing of spacers used in HVDC GIS/GIL are also explored.

Insulating materials for realising carbon neutrality:Opportunities,remaining issues and challenges

The 2050 carbon-neutral vision spawns a novel energy structure revolution,and the construction of the future energy structure is based on equipment innovation.Insulating material,as the core of electrical power equipment and electrified transportation asset,faces unprecedented challenges and opportunities.The goal of carbon neutral and the urgent need for innovation in electric power equipment and electrification assets are first discussed.The engineering challenges constrained by the insulation system in future electric power equipment/devices and electrified transportation assets are investigated.Insulating materials,including intelligent insulating material,high thermal conductivity insulating material,high energy storage density insulating material,extreme environment resistant insulating material,and environmental-friendly insulating material,are cat-egorised with their scientific issues,opportunities and challenges under the goal of carbon neutrality being discussed.In the context of carbon neutrality,not only improves the understanding of the insulation problems from a macro level,that is,electrical power equipment and electrified transportation asset,but also offers opportunities,remaining issues and challenges from the insulating material level.It is hoped that this paper en-visions the challenges regarding design and reliability of insulations in electrical equipment and electric vehicles in the context of policies towards carbon neutrality rules.The authors also hope that this paper can be helpful in future development and research of novel insulating materials,which promote the realisation of the carbon-neutral vision.

Gas–solid interface charge characterisation techniques for HVDC GIS/GIL insulators

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.

Effect of long-term fluorination on surface electrical performance of ethylene propylene rubber

To investigate the effect of fluorination on surface electrical performance of ethylene propylene rubber(EPR),four pieces of EPR specimens are prepared and fluorinated for different duration ranging from 120 min to 480 min.The surface morphology and element compositions of experimental specimens are tested.The surface potential decay and complex permittivity are measured.The tracking discharge property and the erosion properties after tracking test are investigated.The surface charge transport and electrical property tailoring mechanism are discussed.The results show that when the fluorination duration is from 120 min to 360 min,the surface morphology gradually becomes flat and compact with time,and the surface resistance to electrical tracking is improved.While an excessive fluorination with the duration of 480 min leads to a significant increase in surface conductivity and a weakening in the resistance to electrical tracking,which is mainly due to a change in surface morphology.The recommended fluorination duration for an optimised surface property is between 240 and 360 min for EPR specimen.

Advanced insulating materials contributing to“carbon neutrality”:Opportunities,issues and challenges

The vision of 2050 worldwide carbon-neutral ignites a revo-lution in all industries.Enormous innovations in both electrical power equipment and electrified transportation assets are shaping our future energy structure.Insulating materials,as the core element of energy infrastructure,are facing unprece-dented challenges that stand in the pathways to global carbon neutrality.However,researchers from both academic in-stitutions and industrial sections are working to turn the challenges into opportunities.

Ageing evaluation of cable insulations subjected to radiation ageing:Application of principal component analyses to Fourier Transform Infra-Red and dielectric spectroscopy

The evolution of properties of crosslinked polyethylene(XLPE)-based insulating mate-rials under radio-chemical ageing conditions is presented and discussed.In total,seven different polymer compounds were tested,characterised by an increasing quantity of additives,for example,antioxidants(AOs)and fillers as flame retardants.Samples were analysed using different testing techniques,that is,Fourier Transform Infra-Red and dielectric spectroscopy.This allowed a broad characterisation of the polymer,bringing to good correlations between the arising of new chemical groups caused by ageing and the modifications on the complex permittivity.Phenol-based AOs are found to be the most efficient in terms of protection against oxidation.On the contrary,flame retardants are concluded not to have any impact on the material ageing.Acquired data were then treated using the principal component analysis.The data treatment successfully distinguished the material composition(with and without additives)and the ageing environments.The results confirm the relevance of this analysis in understanding the impact of irradiation on the chemical and functional properties of the polymer materials studied.

Development and pattern identification of end-winding discharge under effect of relative humidity and temperature for HV motors

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.

Guest Editorial:Interface Charging Phenomena for Dielectric Materials

Interface charge accumulation,leading to unpredictable insulation breakdown,restricts the development and industrialisation of high voltage direct current(HVDC)power equipment.Understanding the mechanism of interface charge transport and interface charge triggered dielectric breakdown is,therefore,of particularly high interest and vital importance.For example,the surface charging of aircraft in space brings damage to board electronic instruments as well as interference to sensing systems.Moreover,the randomly distributed charge clusters on the spacer surface,as shown in the cover page of this Special Issue,is considered as a potential threat to the surface flashover,while the accumulation of space charge within the insulating material poses a threat to the reliability of DC power cables.However,currently we still know very little about the development of charge dynamics in different interfaces,and the related breakdown mechanism of dielectrics induced by charge accumulation is also not very clear.