Insulation Design Rule for a Spacer in SF_(6)/N_(2)-filled DC Gas Insulated Apparatus

A recurring challenge of a DC SF_(6)/N_(2)-filled GIS/GIL apparatus is the charge accumulation at DC stress.The conventional design rules and knowledge of AC spacers may not be applicable for this new type of apparatus.A novel design rule is proposed considering the effect of accumulated charge on the threshold of electric field strength being resistant to the superposed voltage.A surface charge accumulation simulation model is introduced,and the key parameters in the simulation model are measured.In addition,an experimental platform for a 100 kV spacer flashover test is established.Spacer flashover tests under superimposed voltage with opposing polarities are carried out,and the withstanding voltage of the spacer is obtained.Finally,based on the proposed model,the threshold of the surface electric field strength(tangential component)on the DC spacer in SF_(6)/N_(2) mixed gases is discussed.For a reliable insulation design of a DC GIS/GIL apparatus filled with 0.7 MPa SF_(6)/N_(2),the threshold of surface electric field strength on the DC spacer is 12 kV/mm.The insulation design rule can be referenced in the design of a high-voltage DC SF_(6)/N_(2)-filled GIS/GIL apparatus.

Intelligent overheating fault diagnosis for overhead transmission line using semantic segmentation

The strain clamps and leading wires are important components that connect conductors on overhead transmission lines and conduct current.During operation,poor contact between these components can cause abnormal overheating,leading to electric failures and threatening power system reliability.Recently,the use of unmanned aerial vehicles equipped with infrared thermal imagers for strain clamp and leading wire maintenance has become increasingly popular.Deep learning-based image recognition shows promising prospects for intelligent fault diagnosis of overheating faults.A pre-treatment method is proposed based on dynamic histogram equalisation to enhance the contrast of infrared images.The DeepLab v3+network,loss function,and existing networks with different backbones are compared.The DeepLab v3+network with ResNet101 and convolutional block attention module added,and the Focal Loss function achieved the highest performance with an average pixel accuracy of 0.614,an average intersection over union(AIoU)of 0.567,an F1 score of 0.644,and a frequency weighted intersection over union of 0.594 on the test set.The optimised Atrous rates has increased the AIoU by 12.91%.Moreover,an intelligent diagnosis scheme for evaluating the defect state of the strain clamps and leading wires is proposed and which achieves a diagnostic accuracy of 91.0%.

Flashover Behavior of Cone-type Spacers with Inhomogeneous Temperature Distribution in SF_(6)/N_(2)-filled DC-GIL Under Lightning Impulse with DC Voltage Superimposed

Before designing DC gas insulated transmission lines(GIL),the insulation characteristics of spacers under a lightning impulse(LI)voltage with a superimposed DC voltage must be clarified.Many experiments have been performed to guide the development of the DC spacer.However,the impacts of thermal stresses on their insulation performance under combined voltage are always neglected,and little attention has been given to the insulation characteristics of spacers in an eco-friendly gas filled DC-GIL.To solve these problems,an experiment setup with the abilities to load the thermal field and the DC voltage with LI voltage superimposed on the cone-type spacer are built.After that,the insulation characteristics of spacers under the combined voltage of DC with lightning impulse at 0.1 MPa SF_(6)/N_(2) gas mixtures are investigated,under different thermal stress conditions.The experiment’s results indicate that:1)The flashover voltage of spacers under the same polarity superimposed voltages is slightly higher than that under LI alone.Conversely,the flashover voltage under reverse polarity superimposed voltages is lower than that under LI alone.2)The longer the duration of the foregoing DC voltage,the differences between the flashover voltage under superposed voltage and that under LI alone are larger.3)The combined flashover voltages are reduced under thermal stress.To ensure the insulation performance of newly designed DC spacers,the insulation test is suggested to be applied under thermal stress and combined voltage of the DC with lightning impulse,especially for superimposed reverse polarity.

Review on the Characteristics,Heating Sources and Evolutionary Processes of the Operating Composite Insulators with Abnormal Temperature Rise

This paper reviews the research progress on abnormal temperature rise(ATR)of composite insulators.The ATR of composite insulators can be divided into two types,point-form temperature rise(PFTR)and bar-form temperature rise(BFTR).The composite insulators with PFTR only show significant temperature rise at high relative humidity(RH)(>70%),and the temperature rise is located in the area that is 20 cm above the metal end-fitting.In a low humidity environment(<30%),there is little temperature rise(<1.0 K).The polarization loss on the surface of the silicone rubber housing under an AC electric field after moisture absorption is the main heating source.Corona discharge in high RH causes surface degradation of the silicone rubber.The composite insulators with BFTR shows significant temperature rise at both high(>70%)and low(<30%)RH.The temperature rise could reach more than 10◦C and the temperature rise area is wider,extending from the high-voltage end to several shed units at the lowvoltage side.And the glass fiber reinforced plastic(GRP)core in the composite insulator is found to be corroded.The heating energy is supplied by both conductance loss and polarization loss of the corroded GRP core.The decay-like degradation of the GRP core is caused by the combination of damp conditions,high electric field,discharge,mechanical load,et al.and may evolve into a decay-like fracture of the composite insulator.The preventive methods concerning quality control,structure optimization,material modification and operational strategy are presented.It is suggested that when PFTR is detected on the composite insulator,the inspection period of the insulator should be properly shortened.The composite insulator should be replaced as soon as the BFTR was detected.

Feasibility of C_(3)F_(7)CN/CO_(2)gas mixtures in high-voltage DC GIL:a review on recent advances

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 C_(3)F_(7)CN/CO_(2) gas mixtures have become a potential alternative to SF_(6) gas due to its excellent performance.This study reviews the basic physical properties of C_(3)F_(7)CN 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 C_(3)F_(7)CN/CO_(2) gas mixtures.

Study on the influencing factors to reduce the recovery time of superconducting tapes and coils for the DC superconducting fault current limiter applications

Superconducting fault current limiters(SFCLs)are important in improving the stability of electrical power systems.However,the existing resistive-type DC SFCL has a long re-covery time,which cannot meet the requirements of power system auto-reclosure.Thus,it is necessary to investigate the methods of reducing recovery time of SFCLs.In the simulations on superconducting tapes,there is a lack of investigation about the effects of design factors of superconducting tapes and coils such as material,thickness,coil radius,coil winding method on recovery characteristics.The objective of this study is to obtain the effects of material and geometric properties of superconducting tapes and coils on recovery characteristics and obtain methods to reduce the recovery time.The recovery times under different conducting currents are also compared by simulations and exper-iments to verify simulation accuracy.The maximum error is below 40%and most of the cases have an error below 10%.Besides,by increasing tape thickness,the recovery time can be reduced by 22%.Tape materials and coil winding methods can be designed ac-cording to the auto-reclosure and current-limiting requirements of the power systems.The research provided directions for the design of superconducting tape and super-conducting coil for resistive-type DC SFCLs.

Intelligent Breakage Assessment of Composite Insulators on Overhead Transmission Lines by Ellipse Detection Based on IRHT

With the development of unmanned aerial vehicle(UAV)technology,visible images are playing an important role in the maintenance of power systems.To achieve the shed breakage evaluation of composite insulators by UAV visible images,an intelligent fault assessment method is proposed.First,the composite insulators in visible light images are identified by Faster-RCNN.After image preprocessing,the image is enhanced and the noise is removed.Then,a canny operator is used to extract the edge of the sheds.An Improved Randomized Hough Transform(IRHT)is used to detect the ellipses in the edge image.The parameters of the detected ellipse,length of major axes and minor axes,center coordinates and deflection angle of major axes,are used to realize the segmentation of the composite insulator.Finally,the number of pixel points in the ellipse and the distance between the points and the ellipse boundary are used to judge whether there are breakage or cracks on the sheds.The area ratio of the breakage to the whole shed is calculated based on the number of pixel points inside the broken area.This method can be realized without a large amount of training dataset of the specific fault type and provides a technical basis for the online fault assessment of a composite insulator on overhead transmission lines.

Technique for Order Preference by Similarity to an Ideal Solution-based comprehensive health assessment of composite insulators for overhead transmission lines

There are a lot of quantitative performance characterisation methods for composite insulators.However,how to apply the characterisation methods with different dimensions to comprehensively assess the health state of composite insulators remains a big problem for operators and researchers.A comprehensive health assessment model for composite insulators based on the Technique for Order Preference by Similarity to an Ideal Solution(TOPSIS)method is proposed in this study.Fourteen composite insulators from four regions in Yunnan Province,China,and five quantitative characterisation indicators of the insulators,namely hardness,hydrophobicity,loss of hydrophobicity,recovery of hydrophobicity,and trap charge,are selected to verify the model.The results show that the Pearson correlation coefficient of the TOPSIS-based comprehensive health score with operating time is 0.72,which is better than the other single indicator.By introducing the operating time into the TOPSIS-based model,we can also obtain the ageing resistance performance of the composite insulator.This work provides a new idea to assess the health state and ageing performance of composite insulators.

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.

Metal Particle Contamination in Gas-insulated Switchgears/Gas-insulated Transmission Lines

Metal particle contamination in a gas-insulated switchgear(GIS)or a gas-insulated transmission line(GIL)is an important factor leading to the decline of insulation performance.Exploring the deterioration mechanism and suppression measures of metal particles on insulation is a key technical problem in enhancing the dielectric strength of GIS/GIL equipment.In this paper,the charge and motion characteristics of metal particles are first introduced.The gas gap breakdown caused by free metal particles and the surface flashover caused by metal particles near or adsorbed on the insulator are then analyzed according to different particle motion patterns and spatial locations.Subsequently in terms of operation managements,the existing methods of particle detection are analyzed.In addition,the main inhibition methods of metal particles are introduced from three aspects:particle trap,insulator surface treatment and electrode coating.Finally,the prospects in the future research on particle pollution in GIS/GIL are also pointed out.

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.

Environment-friendly Insulating Gases for HVDC Gas-insulated Transmission Lines

Environment-friendly gas insulating mediums adapted to a DC gas-insulated transmission line(GIL)electric field condition is the key to the next generation of Environment-friendly HVDC GILs.In this paper,we review the literature on sulfur hexafluoride(SF6)alternatives including the scientific understanding,control,and implementation of gas-solid systems in this type of power transmission.First,the structure-activity relationship between the molecular structure and physico-chemical properties of Environment-friendly insulating gases is presented.Then,the search and prediction of important physicochemical properties of gases are summarized.Subsequently,in view of the potential of environmental friendly insulating gases,the swarm parameters of gas discharge and breakdown properties in a quasi-uniform field,inhomogeneous field,and at the gas-solid interface,that need to be taken into account with industrialized DC GILs are discussed.The latest research progress on insulation characteristics,especially the polarity effect in DC gas-solid insulation systems,the sensitivity to the electrode surface state,and the non-uniformity of the electric field,and the influence of metal particles and their variation with air pressure,is highlighted.In addition,the heat transfer characteristics of insulating gases,related to DC GIL transmission with a large current-carrying capacity and the influence of alternative gases on the heat transfer characteristics are described.Finally,aiming at solving the contradiction of low environmental impact,high dielectric strength and low liquefaction temperatures in the selection of alternative gases,an coordinated regulation model for Environment-friendly gases in DC GILs is established.Considerations for future work on this topic are also presented.

Review of Charge Accumulation on Spacers of Gas Insulated Equipment at DC Stress

Charge accumulation on the spacer surface in gas insulated equipment is very severe in the DC field,leading to easy flashover,which restricts the application of DC GIS/GIL.Therefore,knowledge about the charge accumulation characteristics of gas insulated equipment at DC stress is essential.In this paper,we reviewed the research methods and the characteristics of charge accumulation on spacers.A summary of charge measurement methods and setups are presented.And the surface charge inversion algorithms are introduced.Then the simulation model for charge accumulation in the DC field is reviewed.Subsequently,the charge accumulation mechanisms and phenomenon,the influence factor of surface charge accumulation and the influence of accumulated charge on spacer insulation characteristics are summarized.In addition,some suppression methods of surface charge accumulation are discussed.Finally,based on the understanding of charge accumulation on spacers,some suggestions on further studies are presented to aid in the design of a better spacer free from surface charges.

Effect of barrier thickness of Polytetrafluoroethylene on the Direct Current breakdown characteristics of liquid nitrogen

When high-temperature superconducting(HTS)tapes in high voltage DC HTS devices are quenched,amounts of thermal bubbles are generated in liquid nitrogen(LN_(2))that will reduce the insulation strength greatly.Especially when resistive-type superconducting fault current limiters meet a short-circuit fault,the insulation strength of the LN_(2)decreased significantly due to the thermal bubbles and electrothermal pressure.Solid insulating barriers can improve the insulation strength of the LN_(2).The objective of this study is to obtain the effect of barrier thickness on the DC dielectric breakdown characteristics of LN_(2).Insulating barrier Polytetrafluoroethylene with three different thicknesses were tested.Three types of electrodes such as rod-plane,needle-plane,and planeplane were applied.The results showed that the barriers can increase the negative and positive breakdown voltage both with and without thermal bubbles in a slightly nonuniform field(SNF)and non-uniform field(NF).For the SNF,the DC breakdown voltage is higher when the barrier is thinner.The effect of the barrier thickness in the NF is lower than that in the SNF field.For the NF,the positive breakdown voltage is higher when the barrier is thicker.