Characteristics of the propagation of partial discharge ultrasonic signals on a transformer wall based on Sagnac interference

The acoustic emission(AE)method has been widely recognized for the detection of incipient insulation fault phenomenon(partial discharge,PD)in power transfomiers,nevertheless,the installation and placement of AE sensors should be taken into full consideration.In this manuscript,a three-dimensional multiphysics model was established and simulated to research the characteristics of the propagation in the transformer wall.Furthermore,a piezoelectric transducer was used to detect PD ultrasonic signals and verify the simulation results in the laboratory.To ensure the accuracy of the detection,an optical fiber sensor based on the Sagnac interference principle was designed and adopted.The variation of the amplitude of the ultrasonic signal with distance reveals the characteristics of the ultrasonic signal propagating in the transformer wall.The distribution of sound pressure on the upper and lower surfaces of the simulation model proved that ultrasonic waves propagate in the form of symmetrical Lamb waves in the transformer wall.Moreover,the amplitude of the AE attenuates due to absorption and refraction loss,and local fluctuations on account of acoustic interference.Thus,a stable signal detected by an AE sensor does not represent the absence of PD in a transfomier.To improve the reliability of AE detection,it is proposed in this manuscript that repeated movement of the AE sensor is necessary to obtain a suitable measurement position.Similarly,it is necessary to adjust the position of the AE sensor in order to locate the PD source well.In addition,this study is expected to provide a theoretical analysis and a fiber sensor to address the problem of sensor placement in AE detection.

A low-cost and high-performance self-trigger method for high-voltage transient measurement

Stable triggering of acquisition devices is critical to accurately measure electromagnetic transients,especially for very fast transient overvoltages(VFTOs).A highly stable and lowcos t self-trigger method for transient measurement is proposed.An integrated low-voltage arm module was developed to synchronously realise the triggering and VFTO measurement.The trigger branch was developed using a high-pass filter.This component obtained the high-frequency component of the VFTO to trigger the oscilloscope.The trigger branch was integrated with the measurement branch through an impedance converter.An attenuator was implemented before the impedance converter to avoid overvoltage damage to the chip.The circuit parameters of the attenuator were optimised to alleviate the disturbance of the measurement branch.The calibration results demonstrated that the±1 dB bandwidth of the measurement branch in this system ranged from 11.8 Hz to 105 MHz.The low cutoff frequency of the trigger branch was 710 Hz.The VFTO waveforms generated by the operation of a disconnector in a real 252 kV gas insulated switchgear were measured using the proposed system.The test results verified that the self-trigger measurement system can provide stable trigger signals and accurately measure the VFTO waveforms.

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.

High‐performance ZnO varistor ceramics prepared by arc‐induced flash sintering with low energy consumption at room temperature

The development of ultra‐high voltage transmission lines requires a ZnO arrester with excellent electrical response and high voltage gradient.Compared with conventional preparation methods,flash sintering allows fast production of novel high‐performance ZnO varistor ceramics with low energy consumption and controlled grain growth.Herein,ZnO varistor ceramics were prepared using the flash sintering method at an air pressure of 21 kPa for 60 s at 25℃.The results showed that the flash‐sintered samples had fine grains and reduced loss of volatile elements.Moreover,the voltage gradient,leakage current density,and non‐linear coefficient of the flash‐sintered varistor ceramics were significantly improved compared with those of conventionally sintered samples.Owing to the short preparation time and absence of a heating process,the energy consumption of the proposed flash sintering method was significantly reduced to 10%of that by conventional sintering.The proposed method is a promising approach for the preparation of ZnO arresters with excellent electrical properties and serves as an effective method that realises energy saving and emission reduction.

Fault diagnosis of the bushing infrared images based on mask R‐CNN and improved PCNN joint algorithm

Bushings are served as an important component of the power transformers;it's of great significance to keep the bushings in good insulation condition.The infrared images of the bushing are proposed to diagnose the fault with the combination of image segmentation and deep learning,including object detection,fault region extraction,and fault diagnosis.By building an object detection system with the frame of Mask Region convolutional neural network(CNN),the bushing frame can be exactly extracted.To distinguish the fault region of bushings and the background,a simple linear iterative clustering‐based pulse coupled neural network is proposed to improve the fault region segmentation performance.Then,two infrared image feature parameters,the relative position and area,are explored to classify fault type effectively based on the K‐means cluster technique.With the proposed joint algorithm on bushing infrared images,the accuracy reaches 98%,compared with 44%by the conventional CNN classification method.The integrated algorithm provides a feasible and advantageous solution for the field application of bushing image‐based diagnosis.

Optical sensors for power transformer monitoring:A review

Power transformers play a vital role in power transmission systems from the energy source to the consumption center.In situ diagnostics of the performance of power transformers provides a variety of benefits to ensure reliable electricity transmission.Immunity to electromagnetic interference,high sensitivity,high insulation as well as small dimensions of optical sensing make it very attractive for applications in power transformer monitoring.This paper presents a critical review of various diagnostic methods for power transformers such as partial discharge,dissolved gases,temperature and other important sensing,and optical detection.The advantages and disadvantages of different monitoring methods are carefully discussed and assessed.Finally,the existing technical barriers and future prospects of optical monitoring methods for power transformers are presented.

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 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.

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 flash sintering with strong electric field

Flash sintering is a novel field‐assisted sintering technology for ceramics that allows a dramatic reduction in processing time and temperature.Since 2010,when flash sintering was first reported,it has been a focus of research interest in the field of materials science.Recent study results have confirmed that the sintering temperature decreases with the strength of the applied electric field;for some ceramics,the sintering temperature can be lowered even to room temperature.This represents an innovative breakthrough for ceramic sintering at ultra‐low temperatures.However,once the electric field strength in flash sintering is increased,new questions and challenges arise,such as whether or not partial discharge occurs in the green body if under electro‐thermal coupling stress,how to quantitatively analyse the impact of discharge on sintering,and,moreover,whether the sintering mechanism under a strong electric field is consistent with that under a weak electric field.These research questions require knowledge of partial discharge detection,dielectric theory,and other topics that are beyond the scope of materials science.To address this need,this review summarises the work carried out in flash sintering with a strong electric field from the perspective of high voltage and insulation technology.First,the flash sintering process and mechanism are briefly introduced.Then,the published literature on flash sintering with a strong electric field for various ceramic materials is summarised in depth.In addition,experimental phenomena that are observed in flash sintering with a strong electric field,such as flash and blackening,are discussed.Finally,some suggestions for future work are presented.It is anticipated that the knowledge gap between different areas of study can be filled by this review.

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.