Free Metal Particles Research Articles

Effect of aluminum particles on the insulating properties of C4F7N/CO2 mixed gas

The C4F7N/CO2 mixed gas is most expected to replace SF6 in high-voltage power equipment, and the generation of free metal particles in gas-insulated equipment is unavoidable and highly susceptible to inducing insulation failures. In this paper, the effects of flaky, spherical, and mixed shape aluminum particle defects on the breakdown characteristics of C4F7N/CO2 mixed gas are investigated and compared to those of SF6 under the same conditions. The experimental results show that the effect of particle defects on the insulating properties of C4F7N/CO2 gas mixtures is visualized by the fact that particle defects reduce the breakdown voltage of C4F7N/CO2 gas mixtures, increase their stochasticity, and reduce the enhancement of the insulating properties of C4F7N/CO2 gas mixtures by increasing the air pressure. The greater the number of particles, the greater is the degree of influence. Comparison of the SF6 and C4F7N/CO2 gas mixtures reveals that, in the case of flake defects, increasing the air pressure improves the insulation performance of the SF6 gas better than that of the C4F7N/CO2 gas mixture. Under the condition of the same number of particles, the larger the proportion of spherical particles in the hybrid particles, the smaller is the influence of metal particles on the C4F7N/CO2 mixed gas and on SF6 gas.

Numerical Simulation of the Negative Streamer Propagation Initiated by a Free Metallic Particle in N2/O2 Mixtures under Non-Uniform Field

Under atmospheric pressure, partial discharge initiated by free metallic particles has consistently been a significant factor leading to failures in high-voltage electrical equipment. Simulating the propagation of negative streamer discharge in N2/O2 mixtures contributes to a better understanding of the occurrence and evolution of partial discharge, optimizing the insulation performance of electrical equipment. In this study, a two-dimensional plasma fluid dynamics model coupled with the current module was employed to simulate the evolution process of negative streamer discharge caused by one free metallic particle under a suspended potential at 220 kV applied voltage conditions. Simulation results indicated that the discharge process could be divided into two distinct stages: In the first stage, the electron ionization region detached from the electrode surface and propagated independently. During this stage, the corona discharge on the negative electrode surface provided seed electrons crucial for the subsequent development of negative corona discharge. The applied electric field played a dominant role in the propagation of the electron region, especially in the electron avalanche region. In the second stage, space charge gradually took over, causing distortion in the spatial field, particularly generating a substantial electric field gradient near the negative electrode surface, forming an ionization pattern dominated by ionization near the negative electrode surface. These simulation results contribute to a comprehensive understanding of the complex dynamic process of negative streamer discharge initiated by free metallic particles, providing essential insights for optimizing the design of electrical equipment and insulation systems.

Distributed Characteristics of Free Metallic Particles in Flowing Transformer Oil Under DC and AC Voltages

Transformer oil is inevitably contaminated by metallic particles during manufacturing and operation, which will accumulate in the oil gap and poses a serious threat to the oil insulation in power transformer. However, the distributed characteristics of particles in oil channels will be influenced by oil flow. In this paper, a circulation flow device of transformer oil was established and the distributed characteristics of metallic particles in flowing oil were obtained under DC (direct current) and AC (alternative current) voltages respectively. It was found that particle counts between parallel plate electrodes and rod-plane electrodes would decrease monotonously with increasing flow velocities under DC application, whereas the concentration between spherically capped electrodes would increase slightly first and then decline. When the electrodes were energized by AC voltage, the reduction of the particle concentration was more significant with the addition of flow velocities. What’s more, the temporal changes of particle amount under different conditions were obtained. Then the impacts of flow velocities and applied voltages on particle distribution were discussed based on the force analysis on metallic particles.

Accumulation characteristics of free metallic particles in flowing transformer oil under uniform and non-uniform electric field

Transformer oil is easily polluted by metal particles. Once the metal particles accumulate in the oil channel, the insulation performance of the equipment will be greatly reduced. In this paper, an oil circulation device was built and the accumulation characteristics of free metal particles in flowing transformer oil were carried out under uniform and non-uniform electric field. The images of particle accumulation process were captured by a high-speed camera. Simultaneously, we analysed the force conditions of particles in oil. And the accumulation mechanisms under different electric fields were discussed combined with the particle trajectories. It was reported that particle accumulation could be observed in static oil under both electric fields. The particle concentration between electrodes decreased gradually with time. However, when the transformer oil was flowing, the accumulation concentration of particles decreased significantly with the increase of flow velocity. The accumulation of metal particles was seriously damaged, even no longer occurred.

Optimized Denoising Method for Weak Acoustic Emission Signal in Partial Discharge Detection

Acoustic emission (AE) technology can predict the occurrence of partial discharge (PD) faults, which is used to improve the safe operation level of gas-insulated switchgear (GIS) equipment. However, the strong noise interference from the production site is still the main factor affecting the identification accuracy. In this study, a simplified model is designed to approximate the accumulation of free metal particles on the surface of the GIS internal insulation structure, and white noise of various intensities is added to the collected PD-induced AE signals to simulate the background interference. The results prove that the proposed denoising method can achieve a better denoising effect in various signal-to-noise ratio (SNR) conditions. In particular, in the case of low SNR, the recognition accuracy of the accumulation degree of metal particles has been improved by more than 15%.

Acoustic Method for Multiple Free Metallic Particle Recognition in GIS/GIL

Free metallic particles are the main factor inducing flashover incidents and threatening the safety of gas-insulated switchgear (GIS) and gas-insulated transmission line (GIL) devices. Acoustic diagnosis is widely used to detect free metal particles due to its high sensitivity, but it generally fails under multiple particle conditions because of the overlap of acoustic signals. This paper proposes a new method of acoustically recognizing multiple free metallic particles. The pattern of t-Normalized N that can be used to distinguish single/multi-particles is determined first. Second, each particles neighboring collision signals are extracted from raw data by the established physical relation between adjacent collision signals. Finally, the free metal spherical double-particles mass and density values are calculated as per the numerical connection between adjacent collision signals. The proposed method is validated in a scale concentric hemisphere platform. The results of this work may provide a workable reference for GIS free metal multi-particle monitoring.

Motion behaviours and PD characteristic of free metallic particles in flowing transformer oil with paper‐covered electrodes

The introduction of solid dielectrics to electrode surfaces will change the motion behaviours of free metallic particles and in turn affect partial discharge (PD) characteristics in transformer oil. To estimate the influence, the metallic particles movement in flowing oil was compared between different cases with and without insulating paper covering electrodes here. When the electrodes were bare, particles executed up-down movements repeatedly in the vertical direction. As the electrodes were paper-covered, the particles would settle on the paper surface for a period of time. Simultaneously, the PD characteristics caused by metallic particles were measured. When the electrodes were bare, the discharges concentrated in 0–90° and 300–360° and the PD magnitude reached 390 pC. When the double electrodes were paper-covered, the PD magnitude decreased by 67% and the concentration phases of PDs would migrate to 45° and 160°. In order to analyse the experimental results, a simulation model with a paper-covered boundary was constructed. Simulated trajectories of particles were in accord with the experiment results. Base on the model, the particle motion characteristics were obtained and the electric field distribution between the particle and electrode was calculated. Finally, the influence of paper-covering on PD characteristics was discussed.

Effect of Copper and Iron Particles on the Dielectric Strength of R410A Gas as an Alternative of SF 6 in GIS

The Gas Insulated Switchgears (GIS) used in high voltage transmission and distribution networks are reliable, compact, and maintenance-free. During the manufacturing process, some metal particles remain inside GIS which affects the breakdown strength of insulating gas and hence deteriorates the performance. The breakdown performance of R410A gas in the presence of free metal particles is presented in this paper. Iron and copper particles of various sizes and shapes are used in the research work. The impact of these free particles on the characteristic of R410A gas at various gas pressures and gap distances is elaborated. Power frequency breakdown voltage test in the presence of metal particles and self-recoverability test are performed in a high voltage lab. The effect of various dimensions of metal particles on the dielectric strength of R410A gas under the quasi-uniform field in alternating current is examined. The impact on the breakdown voltage of R410A is analyzed by increasing the pressure and gap distance. Experimental results are discussed in the paper. As metal particles in between them can disrupt the R410A reaction barrier. According to the results of the experiments conducted, the breakdown voltage was affected by 30% by the addition of iron particles and 46.89% by the addition of copper particles, and it is concluded that iron particles perform better at different gaps distances.

Surface Charge Accumulation on a Real Size Epoxy Insulator With Bouncing Metal Particle Under DC Voltage

Free metal particle can affect surface charge accumulation on epoxy insulator installed in gas-insulated transmission line (GIL) or gas-insulated switchgear (GIS), which results in reduction of surface flashover voltage thus threatening the safe operation of the power equipment. This article reports on the charge accumulation behavior on a real size epoxy insulator with bouncing metal particle under DC voltage. A coaxial electrode system was designed for the test under -30 kV, a metal particle-launching system was employed to inject the bouncing linear particle to the insulator surface. The charge distribution was measured by a Kelvin-type probe, and the influence of the particle on the charge accumulation was analyzed with the help of electric field simulation. The results showed that there were mainly two moving patterns of the bouncing particle: the charges were accumulated with speckle shape and the charged particle that hit the edge of electrode had more remarkable influence on the charge accumulation than other particles. It is suggested that the charge distribution feature is dependent upon the collision point and the trajectory of the bouncing particle.

SF6 Partial Discharge Decomposition Characteristics Under Different Lengths of Wire-Type Metal Particles in Direct Current Gas Insulated Equipment

Aiming at the serious defects of wire-type free metal particles, which can cause severe partial discharge (PD) in Direct current (DC) gas-insulated equipment, the experiments on the influence of four lengths (3.0, 5.0, 7.0, and 9.0 mm) of wire-type free metal materials on SF6 decomposition under DC PD were conducted. The formation rules of four characteristic decomposition components, namely, CF4, SOF2, SO2F2 and SO2, under different lengths of wire-type free metal particles, were determined using gas chromatography. Results showed that the amounts of four kinds of SF6 DC PD characteristic decomposition components are proportional to the length of wire-type free metal particles. The long length of wire-type free metal particles increases the intensity of PD and the amount and effective generation rates of SF6 DC PD characteristic decomposition components. All these findings provide an experimental basis for the diagnosis of insulation defect types in SF6 DC gas insulated equipment.

Functions of phenyl sulfide in polyimide as the electrode coating film to suppress the metal-particle motion within DC GIL

Polyimides containing diphenyl sulfide units were directly coated on the Aluminum electrodes via solution polycondensation-thermal imidization method, which is important for suppression of the free metal particle motion in the Direct Current Gas Insulation Transmission Line (DC GIL). The micro-structures were identified by Fourier-transform infrared spectroscopy (FT-IR). A self-designed experimental platform with plate-to-plate electrode configuration was constructed, by which the metal particle motion behaviors were traced and studied under high voltage DC stresses. To clarify the functionality of phenyl sulfide in the polyimide, the physicochemical properties including thermal and dielectric properties as well as adhesion works were tested accordingly. The microscopic interactive scenarios between polyimides and the electrodes were elucidated by reactive force field molecular dynamics (ReaxFF MD) simulations. The experimental and simulated results were correlated, and also compared with that from the non-sulfur contained polyimide specimen. For the spherical Aluminum particles, a significantly improved DC lifting voltage up to 21.12 kV was achieved by the sulfur-contained polyimide coating, due to the fact that sulfur in the polyimides will increase the dielectric constant as well as the adhesion strength with metal particles. Also, the changes in bulk conductivity are correlated more closely to the variation of particle-motion behaviors after being lifted in the DC electric field. The observed results show that, the inclusion of sulfur element can effectively enhance the adhesion between the polyimide film and the DC GIL Aluminum electrode, as sulfur in the diamine structure of polyimide is more likely to bind with Aluminum than oxygen.

Study on SF6 gas breakdown characteristics in the presence of a free metal particle

The presence of metal particles in power equipment may cause gas discharges and threaten insulation reliability. The discharge phenomenon in the presence of particles involves complex physical processes. To describe these processes, a numerical method is proposed to analyze the relationship between the motion of the particle and the discharge at the same time. First, processes including the generation of the first electron, the formation of a streamer corona, the transition from the streamer to the leader, and the leader propagation are considered. Voltage–time characteristics are calculated with particles of different shapes. Then, the characteristics of the particle motion driven by the Coulomb force are analyzed. Finally, the threshold field for partial discharge and the breakdown in the presence of the particle are discussed taking into account the voltage–time characteristics and the analysis of the particle motion mentioned above. The results demonstrate that there is a threshold field when breakdown occurs in the presence of a long and narrow particle at the negative electrode before the particle leaves the electrode.

Partial Discharge Detection and Discharge Type Identification of Gas Insulated Switchgear Based on Photoelectric Detection

Gas insulated switchgear (GIS) is a relatively common high-voltage substation equipment. Due to the internal complex structure and integrated switch, it is easy to cause insulation defects, burrs and partial discharge of metal particles, thereby reducing the insulation of equipment performance, which has attracted the attention of many researchers. In this study, aiming at the problem of partial discharge in GIS, a detection device of partial discharge based on photomultiplier instrument is proposed. In this device, Zemax software is used to simulate a front-end optical signal receiving module that enhances the photosensitive surface and enhances the optical signal aggregation effect. In terms of signal processing, the output voltage signal of the photomultiplier instrument is filtered, and the corresponding signal is extracted for amplification. The partial discharge detecting device designed in the experiment is used to judge the internal insulation defects of the GIS, and to discriminate the metal protrusion defects, the metal particle defects on the insulator surface, the free metal particle defects, and the air gap defects between the insulator and the high voltage conductor, respectively. The partial discharge signal ϕ-u-v spectra of different defect models are constructed. Through the image simulation of partial discharge signals corresponding to defects, it is verified that the detection device can distinguish the partial discharge caused by different defects, which is helpful to prevent the occurrence of malignant discharge accidents and maintain the stable operation of GIS equipment.

Characteristic Analysis of Partial Discharge of Free Metal Particle in GIS

Free metal particle is one of the main defects affecting the safe and stable operation of GIS equipment. In order to further analyze the characteristics of partial discharge produced by metal particles, an independent sealed test model which can simulate real defects is designed in this paper. By adjusting applied voltage, particle size and number of particles, PRPD and n-φ pattern of partial discharge caused by particle defects are characterized. Analysis of the causes of formation of features, especially in the light of particle motion patterns, the transfer and neutralization of charges carried by particles at various stages are elaborated, which provides a reference for the follow-up research and field detection work.

Research on Discharge Characteristic of Free Metal Particles in DC Bushing

Free metal particle was a common impurity in fault DC bushing, which was produced while its operation. This paper mainly focused on analyzing the typical fault of DC bushing, morphology and motion characteristic of mental particles. A experiment platform was built to study the discharge characteristic, and the result consistently with analysis was obtained. The mental particles would aggregate and accumulate due to the unevenly temperature distribution, and lead to distortion of electric field. The location and width of mental particles would have impact on discharge characteristic. The research result could be used as structural optimization and design of DC bushings, which was worth a further study both in academic and engineering application.

Partial Discharge Detection and Characteristics of Gas-insulated Substation Free Metal Particles Based on Acoustic Emission

Partial Discharge Detection and Characteristics of Gas-insulated Substation Free Metal Particles Based on Acoustic Emission

Research progress on metal particle contamination in GIS/GIL

The metal particle contamination in gas-insulated switchgear (GIS) or 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 firstly 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.

Internal UHF antenna for partial discharge detection in GIS

A compact ultra-high-frequency (UHF) antenna is developed in this work. First, by means of shifting symmetrical octagonal monopole into asymmetrical one, the impedance matching performance of antenna at high frequencies is improved; then, by cutting the radiation patch and ground plane in part, the antenna is reduced by 37.9% in size while maintaining its bandwidth; finally, a Γ-shaped slot on the ground plane is utilised to create an additional resonance, decreasing the low cutoff frequency without enlarging the antenna volume. The proposed antenna only occupies 124 mm × 77 mm × 1.6 mm and achieves 3:1 voltage standing wave ratio from 575 MHz to more than 4.5 GHz, along with simple structure, good radiation patterns, moderate peak gain, and strong pulse handling capability. Furthermore, the electrical dimension, ratio bandwidth, and bandwidth dimension ratio are compared with those of existing broadband antennas. The partial discharge (PD) experiments have been carried out for four typical insulation defect models in gas-insulated switchgear (GIS), and the results demonstrate that it is more sensitive to PD signals arising from floating metal, free metal particle, and void in the epoxy resin as compared to the disc coupler.

Deposition of SiOx film on electrode surface by DBD to improve the lift-off voltage of metal particles

The free metal particles pollution has been a severe problem affecting the safety operation of gas insulated transmission lines (GIL) system and causes insulation failure. In this paper, a facile insulation film deposition method by using atmospheric pressure dielectric barrier discharge (AP-DBD) plasma was introduced. The SiO x film was deposited on copper surface to improve the lift-off voltage of metal particles. The film composition, surface morphology and film thickness based on the plasma deposition time were well studied. The results showed that deposited film was composed with Si- O-Si group, Si-OH group along with small amount of inorganic groups (CH 2 and CH 3 ). The oxidation degree and film thickness were enhanced when plasma treatment time was longer. The theoretical model of particle lift-off voltage was calculated before and after surface coating. After the electrode covered with SiO x insulation film, the lifting voltage of metal particles improved by about 38%, which agreed well with the prediction value. Our results proved that SiO x film deposited by AP-DBD was a very effective method for lift-off voltage improvement.

Comparison of SF6 decomposition characteristics under negative DC partial discharge initiated by two kinds of insulation defects

To investigate the difference between the sulfur hexafluoride (SF6) decomposition characteristics initiated by two sorts of insulation defects — free metal particles and metal protrusion defects — under negative DC partial discharge (PD), SF6 decomposition data was obtained under different DC voltages on the basis of constructing a SF6 decomposition experimental platform. The differences between the characteristic components and the characteristic ratios of the two defects at the same discharge quantity and voltage were systematically analyzed. Results show that the two defects have significant differences in the type of decomposition components, the yield sizes of the components, and the changes in the component ratios. This paper proposed c(CF4 + CS2)/c(CO2) and c(CF4)/c(SO2F2) as the characteristic ratios to characterize the fault severity of the two defects, and their physical nature were revealed. The combination of the above ratios with the existing c(SO2F2)/c(SOF2 + SO2) can construct a feature quantity for identifying the two defects.