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Moist polluting substances on high-voltage insulator surfaces can cause power-line failures by triggering electric arcs. There are at present no effective methods of measuring insulator pollution levels during normal operations. In this work, we attempt to estimate insulator pollution leakage current (PLC) as an indirect method of measuring deposits in a 30 month period of simultaneously recording leakage current and related environmental variables in substation insulators. We analyzed the relationship between raw leakage current and environmental variables. We canceled out the influence of relative humidity on leakage current by adaptive filtering and then obtained the PLC by filtering out the anomalous peaks in the recording. The proposed method considerably reduces the correlation between leakage current and relative humidity (0.826 vs 0.019). The resulting signal was only slightly correlated with other environmental variables (

The flashover performance of insulating materials plays an important role in the development of high-voltage insulation systems. In this paper, silicone rubber (SIR) is modified by CF4 radio frequency capacitively coupled plasma (CCP) for the improvement of surface insulation performance. The discharge mode and active particles of CCP are diagnosed by the digital single-lens reflex and the spectrometer. Scanning electron microscopy and x-ray photoelectron spectroscopy are used for the surface physicochemical properties of samples, while the surface charge dissipation, charge accumulation measurement, and flashover test are applied for the surface electrical characteristics. Experimental results show that the fluorocarbon groups can be grafted and the surface roughness increases after plasma treatment. Besides, the surface charge dissipation is decelerated and the positive charge accumulation is inhibited obviously for the treated samples. Furthermore, the surface flashover voltage can be increased by 26.67% after 10 min of treatment. It is considered that strong electron affinity of C–F and increased surface roughness can contribute to deepening surface traps, which not only inhibits the development of secondary electron emission avalanche but also alleviates the surface charge accumulation and finally improves the surface flashover voltage of SIR.

In extreme weather, the line trip-out events caused by the wind swing of the transmission line occur frequently, severely affecting the normal operation of the power system. Since the structure of the operating transmission line is difficult to be improved, wind swing flashover risk assessment is of great significance for the operating transmission lines. In this paper, a transmission line wind swing flashover risk assessment model based on the matter-element extension model is proposed. The analytic hierarchy process (AHP) and entropy weight method are applied to comprehensively calculate the weight value of the selected assessment index. This model can assess the early warning level of a certain tower unit in the transmission line. Finally, the feasibility and effectiveness of the proposed assessment model are verified by practical examples.

The high voltage (HV) bushing design of insulation core, umbrella skirt, lead type, and production process were proposed according to the technical requirements of the 10 kV high temperature superconducting (HTS) cable system of Shenzhen Power Supply Bureau. The design took full consideration of the withstanding voltage, impulse voltage, partial discharge, thermal shock, and anti-condensation level. According to the proposed design, a verification prototype was built and tested. It subjected to five slow 77 K-300 K thermal shock cycles and five severe 77 K-300 K thermal shock cycles to verify its performance. The sensitive electrical quantities were tested after each cycle. After the 10 cycles, no degradation of performance and no breakdown or flashover occurred were found in the prototype. The results show that the design is qualified for the expected purpose.

In this paper, based on fxbw4-10/100 insulators, the flashover dynamic characteristics of iced insulators are studied. The insulators are artificially iced and flashover experiments are carried out with water of different conductivity in the laboratory, and different flashover voltages are obtained. The dynamic changes of insulator temperature field, electric field and potential are simulated by using finite element software. Finally, the influence of different physical parameters on insulator flashover characteristics is studied.

Conductive particles in the gas insulated transmission lines (GIL) induce the breakdown between electrodes or the flashover along insulators. To solve the problem of particle moving and realize the particle-moving regulation, particle trajectories should firstly be determined in air and SF6. This paper presents the results of particle trajectories and its charging behaviors at vacuum and SF6, respectively. Metal particles with different materials and sizes were introduced and the charge quantity was calculated. The results showed that the particle lift-off electric field in SF6 was higher than that in air under the same gas pressure, that is, the charge on particle in SF6 was about 0.789 times lower than that in air under the same condition. Besides, the lift-off electric field of particle increased with the pressure increase of SF6. The charge on particle was affected by the concentration of electric field near particle and the electrical negative features of SF6. The work provided the data support for development of DC GIL in particle defect suppression.

Power system comprises of various over voltages which are transient in nature namely lightning and switching phenomenon under abnormal conditions which cause flashover in Gas Insulated Bus-duct (GIB) if not protected properly. So, there is a necessity for GIS to have withstanding capability of lightning and switching. Otherwise they may cause shut down of the system. In this work, 1050 kV Lightning Impulse and 750 kV Switching Impulse is superimposed on normal frequency voltage class of 100 kV, 132 kV and 145kV and are given to 1-ΦGIB for finding peak radial movement for Al and Cu particles. The movement patterns for both lightning and switching over voltages are compared. All the simulation results are shown in this paper.

Aiming at the current situation of serious economic losses caused by lightning shielding on the collection lines of wind farms in mountainous areas. First, this paper uses the horizontal section location map of the collection line to extract the terrain parameters, Then use the catenary equation to determine the spatial position of the conductor and ground of any section of the collector line along the pitch direction, and obtain the height of the conductor and the ground to the ground. Finally, complete the modeling calculation analysis of the three-dimensional electrical geometry model (EGM) of the entire collection line. The calculation results show that, when the thunderstorm day, the arrangement of the collection line and the structure of the tower are the same, the SFFOR of the collection line of the wind farm in mountainous terrain is 0.487 times/100 km/year, and the plain terrain is 0.167 times/100 km/year, that is, mountainous area is 2.9 times of plain terrain; It is recommended to use double lightning protection poles with small protection angles on the collection lines of wind farms in mountainous areas, or install lightning arresters for poles that are prone to flashover to meet the design requirements of line lightning protection.

Research on aging characteristics of epoxy resin (EP) under repetitive microsecond pulses is important for the design of insulating materials in high power apparatus. It is because that very fast transient overvoltage always occurs in a power system, which causes flashover and is one of the main factors causing aging effects of EP materials. Therefore, it is essential to obtain a better understanding of the aging effect on an EP surface resulting from flashover. In this work, aging effects on an EP surface were investigated by surface flashover discharge under repetitive microsecond pulses in atmospheric pressure. The investigations of parameters such as the surface micro-morphology and chemical composition of the insulation material under different degrees of aging were conducted with the aid of measurement methods such as atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Results showed that with the accumulation of aging energy on the material surface, the particles formed on the material surface increased both in number and size, leading to the growth of surface roughness and a reduction in the water contact angle; the surface also became more absorbent. Furthermore, in the aging process, the molecular chains of EP on the surface were broken, resulting in oxidation and carbonisation.

This paper analyzes the cause of a 500kV gapless MOA fault in a substation. The appearance inspection and disassembly analysis of the arrester are carried out. Combined with the protection action, the fault cause of the arrester is analyzed. It is found that the fault is caused by the side flashover failure of the local resistor piece of the line arrester due to the steepness of the lightning invasion wave is large. When the line is reclosed, the lower section bears the over-voltage, resulting in the damage of some resistors, and then causes the pressure release. Based on the analysis the paper puts forward some corresponding operation and maintenance measures.

In this paper, the solar spectrum selective endothermic coating with CoCuMnOx as the light absorbing pigment is prepared to absorb sunlight efficiently. After optimizing the thickness of the coating, the endothermic coating material with absorptivity of 0.928 and emissivity of 0.198 is obtained. The roughness value of the coating is 0.64 μm, which ensures that the coating has a higher sunlight absorption rate without the need for an antireflection layer. Icing flashover tests were carried out on insulators coated with endothermic highly hydrophobic anti-icing materials. The ice lightning voltage of ceramic insulators (3 pieces) was increased by 13.68%, and that of glass insulators (2 pieces) was increased by 22.96%. Compared with ceramic insulators, glass insulators coated with anti-icing materials have better protection effect.

The dynamic characteristic of the surface charge on insulating material is an important electrical characteristic, which can effectively reflect the insulation performance of materials. The phenomenon of surface charge accumulation leads to surface discharge and surface flashover. In this paper, the surface charge accumulation and dissipation of epoxy resin under different pulse parameters were studied. The experimental results showed that the increase of pulse frequency intensified the phenomenon of surface charge accumulation and inhibited the process of charge dissipation. Meanwhile, the increase of the number of pulses led to the increase of surface charge accumulation first and then saturation. At the same time, it inhibited the process of surface charge dissipation.

This paper presents an investigation on DC flashover voltage of silicone rubber (SiR) improved by dielectric barrier discharge (DBD) plasma treatments under ambient atmospheric pressure air. DC surface conductivity, surface potential decay (SPD), DC surface flashover voltage, partial discharge magnitude, Fourier transform infrared (FT-IR) spectrograms, and surface water contact angles are measured to analyze the influence of plasma treatment on the SiR. It is found that the speed of SPD increase consistently with the plasma modification time. The tendency of flashover voltage is increasing at first and then decreasing with the increased time of the plasma treatment. The magnitude and number of partial discharge pulses increase apparently with the increased plasma treatment time. Physicochemical measurements indicate that more amount of polar groups appear on surface after the DBD plasma modification, whereas the surface water contact angles decline continuously with the increased plasma modification time. However, the hydrophobicity is recovered after 30 d exposure in the air. It is demonstrated that the SPD is accelerated significantly due to the increased surface conductivities and density of shallow traps. However, the reduction of flashover voltage after longer time of the plasma treatment is attributed to the increased mobility of charge carriers on the sample surface.

In this paper, we studied the effect of filler conductivity on the surface insulation of composites. We proposed using different valence element doping as a strategy to prepare filler with gradient conductivity. The pristine BiFeO3 (BFO), Bi0.95Ca0.05FeO3, and Bi0.95Sm0.05FeO3 nanoparticles were prepared to fill into epoxy resin. The Rietveld refinement of the x-ray diffraction patterns was performed to investigate the transition of the lattice structure. According to the Kubelka–Munk function, the energy bandgap was calculated to verify the regulation of the BFO conductivity. Subsequently, we measured the flashover voltage of the composites with three types of BFO in ambient and vacuum atmosphere, respectively. We found the vacuum flashover is more sensitive to filler conductivity, and BFO with higher conductivity as filler is more conducive to surface insulation, whether in the air or vacuum. Finally, the optimal BFO bandgap of 2.1819 eV is determined for enhancing surface insulation of composites. Our research supplies a novel method for constructing high insulation composites for more wide applications and provides a scientific basis on flashover mechanism from the perspective of filler characteristics.

Surface flashover properties of alumina/epoxy spacers, involving surface charge accumulation process, are critical for the safe and reliable operation of high voltage direction current (HVDC) gas-insulated transmission line (GIL). This study reports surface charging behavior and flashover performance on alumina/epoxy spacers with different surface conductivity graded coating (SCGC) schemes in SF6/N2 mixtures under DC stress. Four kinds of SCGC schemes, including localized coating near high voltage (HV-coating), near grounded electrode (GND-coating), at the middle of spacer surface (SPM-coating), and near both high voltage and grounded electrode (HV-GND-coating), are designed by partially spraying SiC/epoxy composites on spacer surface. Surface charge distribution patterns exhibit varied features with different SCGC schemes. HV-coating and GND-coating scheme lead to aggravated homo-charge and hetero-charge accumulation respectively whereas in SPM-coating scheme surface charge shows a multi-tier distribution patterns with alternating polarity. A transition of surface charge dominant mechanism from bulk conductivity model to surface conductivity model with the increase of conductivity on coated area is found. Flashover performance differs a lot with different SCGC schemes, where HV-coating and HV-GND-coating schemes increase flashover voltage while SPM-coating and GND-coating schemes degrade flashover voltage. The optimal surface insulation strength is achieved in HV-coating scheme with coating width of about 10mm. The impact mechanism of different SCGC schemes on flashover performance is revealed based on the electric field analysis by considering the effect of surface charges.

Surface flashover characteristic of epoxy resin composite material in SF6/CF4 mix gas were studied through experimental method, and the influence of several key parameters were analyzed, such as gas pressure, SF6 mix ratio and voltage polarity. It is shown that, when the mix ratio of SF6 in the mixed gas is 40%, the flashover voltage of epoxy resin insulator can reach over 60% of the flashover voltage in SF6 with the same pressure. The synergistic effects in the needle-plate electrodes system have significant difference between positive and negative polarity. The synergistic effect for negative polarity is weaker than that in negative polarity. Comprehensive environmental requirements and surface flashover characteristics, the results provide a potential SF6 alternative for GIS equipment.

Partial discharge is the flashover of sparks that occur in an insulating part. as a result of the high potential difference in the insulation. Partial discharge can occur in solid insulation materials. It is play an important role in the electrical power system to separate two or more conductor of electric voltage. So, between these conductors do not occur flashover. The material was chose as the insulation material for high voltage equipment is polymer material, it is because in addition to having a high breakdown voltage. Polyethylene is one of the materials included in the polymer group, generally better known as plastic because the material is thermoplastic. There are several methodologies before conduct this research, that includes literature studies. Literature Studies is a research method by finding material from various libraries on topics that will be discussed in this research. Do Partial Discharge testing. And then, record the data obtained. From the experiments that have been done, the results show that a good Partial Discharge value is below 10 pC, if the cable has more than doubled from the normal value, the cable have been damaged or have been consumed by the age of the cable.

One important component in the high voltage overhead lines placed on the transmission tower is the insulator. When polymer insulators are installed in tropical climates, there will be two conditions occurs, such as: i) the changes from wet to dry will cause the pollutant particles in the air will attach to the surface of the insulator. This condition will cause the sedimentation on the surface of the insulator; ii) the changes from dry to wet will form a conductive layer that leads to the leakage current flow. The flow of leakage current leads to the formation of dry-band on the surface. The formation of a dry band creates an electric arc or PD and can eventually cause a flashover insulator. Laboratory experiments were carried out at different levels of pollution and RH. PD signals in the form of leakage current flowing to the ground are transformed by HFCT. PD patterns are collected through an innovative PD detection system. PD signals measured in the form of PRPD patterns are evaluated using the concept of equivalent band-width and time-length equivalent to understand the pollution performance of polymer insulators. The width of the frequency field produced from the experiment can be the basic of analysing the severity of the surface pollution of outdoor polymer insulators.

Surface flashover voltage on solid insulators in gas cooled superconducting power devices is studied. The relationship between the surface flashover voltage and the dielectric strength of the gas media is established. Analysis of the data on surface flashover voltage measurements on cylindrical samples made of high-pressure fiberglass laminate (G10) in three different gas media with varying dielectric strength and pressure showed a positive correlation between the surface flashover voltage and the dielectric strength of the gas both at room temperature and 77 K. The positive relationship, however, is not linear. The results suggest that using gas media with higher dielectric strength, employing higher pressure, and lowering temperature in HTS power devices leads to higher surface flashover voltages and higher operating voltages because surface flashover is usually the limiting voltage in devices that use solid support structures and possess voltage gradients.

Epoxy (EP) resin based nano-composites are widely used in high voltage direct current (HVDC) high temperature superconducting (HTS) power cable. In this paper, the DC surface flashover characteristics of ZnO/EP composites were studied at both room temperature and 77 K. The samples were made by dispersing ZnO nano-particles into EP resin with various weight percentages of 0%, 1%, 3%, 6% and 10%, respectively. The experiments were carried out with a cryogenic system in which DC high voltages ranging from 0 kV to 100 kV were supplied. The results showed that the surface flashover voltages increased with the increase of ZnO content at both room temperature and 77 K, and the surface flashover voltages at 77 K were higher than that at room temperature for composites with the same ZnO content.

Surface discharge in vacuum occurs under intense external field, also called flashover. The flashover threshold over which surface flashover occurs is of vital importance in vacuum-solid interface. It is widely assumed that flashover is initiated by single surface multipactor and develops as a plasma discharge along dielectric surface, namely secondary electron emission avalanche (SEEA). Yet SEEA model is not versatile for all practical vacuum-solid interfaces and there remain experiment results contradicting theory predictions without resolute answer. Here we further expand previous theories and consider conventional SEEA model under arbitrary field distributions. An example of conical insulator is then given to test the validity of SEEA model under arbitrary field. It is found that obtained flashover threshold is only consistent with experiment at low cone angle where field near cathode triple junction is strong, while remarkable discrepancies appear when anode field is significantly enhanced. The discrepancies can be understood if a different discharge model, i.e. anode-initiated flashover (AIF) is included. Both particle-in-cell simulation and optical diagnostics are employed to visualize this unique discharge, which are then formalized on theoretical ground to elucidate the early stage of AIF. Transition between two flashover phases are then clarified. Plasma-surface interaction and sheath dynamics during discharge are analyzed theoretically, showing distinct sheath evolutions in two flashover models. Additionally, a new factor reflecting field distortion is introduced to estimate discharge threshold under SEEA framework.

The distribution law of acid rain in China is counted, and the operation of power grid equipment in acid wet deposition area is investigated. It is found that southern China has the strongest acidity of precipitation in the country. The ratio of SO4 2-, N03 -, NH4+, Ca2+ concentration in acid rain is higher. Acid rain causes serious corrosion of transmission line and transformation equipment in Shandong, Sichuan, Shanxi and Henan provinces of China. The acid rain and corrosion products aggravate the dissolution and chemical reaction of the original dry deposition pollution on the external insulation surface of power grid, which may lead to corrosive flashover.

The flashover characteristics of polluted insulators under lightning impulse and switching impulse are simulated in laboratory. It is found that when the gray density value is fixed, the flashover voltage begins to saturate under the shielding failure voltage while the salt density value reaches 0.1∼0.15mg/cm2 And the salt density saturation point under the counterattack voltage waveform is earlier than that under the counterattack voltage waveform, around 0.02mg/cm2. When the gray density value is fixed, under the counterattack voltage waveform, the flashover voltage decreases slightly in the gray density of 0.05mg/cm2 to 0.10 mg/cm2 range, and the flashover voltage begins to rise with the increase of gray density. Under the shielding failure waveform, the flashover voltage always decreases in the gray density of 0.05mg/cm2 to 0.20mg/cm2range, and begins to rise gradually after the gray density exceeds 0.20mg/cm2.

In order to obtain the elemental compositions of the projectile and target materials during 2A12 aluminum projectile shot on a natural dolomite plate, three kinds of experiments have been conducted using a spectral acquirement system established on a two-stage light gas gun for impact velocities ranging from 2.20 km/s to 4.20 km/s, at the same projectile incidence angle of 30°. Experimental results show that the elemental compositions of the projectile and target materials in the strong shock experiments have a good agreement with the original elemental compositions of the projectile and target. In addition, the relations between spectral radiant intensity and elemental compositions of the projectile and target materials have been obtained for different impact velocities, in which the spectral radiant intensity of the main elements in the material increases with increasing impact velocity, and more elements appear with increasing impact velocity since more energy would result from a higher velocity impact.

In electrical devices poured by epoxy resin, there are a lot of interfaces between epoxy resin and other solid dielectrics, i.e. solid-solid interfaces. Experiments were carried out to study the flashover characteristics of two typical solid-solid interfaces (epoxy-ceramic and epoxy-PMMA) under steep high-voltage impulse for different electrode systems (coaxial electrodes and finger electrodes) and different types of epoxy resin (neat epoxy resin, polyether modified epoxy resin and polyurethane modified epoxy resin). Results showed that, the flashover of solid-solid interface is similar to the breakdown of solid dielectric, and there are unrecoverable carbonated tracks after flashover. Under the same distance of electrodes, the electric stress of coaxial electrodes is lower than that of finger electrodes; and after the flashover, there are more severe breakdown and larger enhanced surface conductivity at interface for coaxial electrodes, as compared with the case of finger electrode. The dielectric properties are also discussed.

Pseudo-spark switch (PSS) is one of the most widely used discharge switches for pulse power technology. It has many special characteristics such as reliability in a wide voltage range, small delay time, as well as small delay jitter. In this paper, the measuring method for the initial plasma of ZnO surface flashover triggering device of PSS is studied and the results of the measurement show that the electron emission charge is mainly influenced by trigger voltage, gas pressure and DC bias voltage. When the bias voltage increases from 2 kV to 6 kV with the gap distance fixed at 3 mm, the electron emission charge changes from 2 μC to about 6 μC. When the gap distance changes from 3 mm to 5 mm with the bias voltage fixed at 2 kV, the electron emission charge increases from 1.5 μC to 2.5 μC. When the gap distance is 4 mm, the hold-off voltage of PSS is 45 kV at gas pressure of 2 Pa, the minimum operating voltage is less than 1 kV. So, the operating scope is from 2.22% to 99% of its self-breakdown voltage. The discharging delay time decreases from 450 ns to 150 ns when the trigger pulse voltage is 1 kV and the discharging voltage is changed from 1 kV to 12 kV. When the trigger pulse voltage is 6 kV, the discharging delay time is less than 100 ns and changes from 100 ns to 50 ns, and the delay jitters are less than 30 ns.

A new coaxial pulsed plasma thruster (PPT) laboratory model is designed and employed in this study. A Teflon sleeve is connected with the anode, which is shaped as a nozzle, and a cathode is mounted in the cavity of the Teflon sleeve and kept in close contact with it. A thread is then designed in the internal surface of the Teflon sleeve, and because of the strong field strength of the cathode triple junction (CTJ), vacuum flashover occurs and a plasma jet is acquired behind the anode. The electric field distribution of the designed coaxial PPT laboratory model is simulated by MAXWELL 3D simulation software, and the plasma density and thrust are measured by a Langmuir probe and a piezoelectric thin-film sensor, respectively. Through a series of comparative experiments, we discuss the impact of optimal designs, such as the thread and the nozzle-shaped anode, on the discharge characteristics of the coaxial PPT. The experimental and simulation results indicate that the designed coaxial PPT laboratory model presents better discharge characteristics in view of its higher plasma density and greater thrust.

Traditional detection techniques are affected by surrounding factors and cannot accurately determine the location of the fault point. In order to solve the problem, a short-term flashover fault intelligent detection method in the paper is proposed for high-power main network power system. The short-term flashover fault feature of the high-power main network power system is extracted by the wavelet-type wavelet packet energy moment. The CPRS networking mode is used to intelligently detect the short-term flashover fault of the high-power main network power system. Experiments show that the proposed technique is more accurate than traditional detection techniques.

In this work, we measure directional patterns of charge flow of ions and mass flow of neutrals in particle beam generated by vacuum surface flashover in linear configuration at voltage of 70 kV. We used a generator, which provide a discharge current of 2.8 kA. The samples used are polymethylmethacrylate, polytetrafluorethylene and polyethylene. We found that the directional patterns of mass and charge flows in linear configuration have distinct axial asymmetry. The corresponding angular distributions are significantly wider in the plane which is normal to the discharge than in the plane which contains the discharge. Angular distributions of charge flow are wider than distributions of neutral mass flow in the same planes. Besides, the asymmetry in ion current amplitudes at the cathode side and at the anode side is observed. We found that the share of low-velocity ions in ion current drops drastically in the plane which contains the discharge at the angles of 15 degrees and larger for all the tested materials.

In this work we study plasma beams generated by vacuum surface flashover discharge. We measured ionic current, velocity of ions at values of discharge current 2.8 kA and 5.5 kA for polymethylmethacrylate (PMMA), polytetrafluorethylene (PTFE) and polyethylene (PE). The aim of the work is to compare parameters of particle flows at different values of discharge current and unchanged voltage. We used generators with coaxial and disk-shaped glycerol-filled pulse-forming lines. Maximum voltage is 70 kV; voltage pulse duration is 20 ns; width of a current pulse first half wave is 30 ns. Average velocity of the ions for all materials is within the range ∼ 100−150 km/s and doesn’t change significantly as the current increases from 2.8 kA to 5.5 kA. Ion current of the beam increase approximately linearly at higher discharge current. We estimate the density of electrons in plasma as a function of distance from the discharge. The applicability of this type of discharge for creating plasma for neutralizing high-power ion beams in inertial confinement fusion facilities is discussed.

In an energy saving building fire, the high temperature and pressure of the indoor smoke or flame break the window glass, and even forms ejecting fire that the flame spills out of the window. It is extremely dangers that the ejecting fire flame to the upper floor might contribute to the unpredicted hazard of the fire in energy saving building with amounts of insulation material. Therefore, the aim of this paper is proposing a method of protecting external windows based on the water sprinkler system. The system can suppress the spread of fire on the facade of the building, and prevent the formation of three-dimensional combustion of the building. This paper design and carried the two groups full-scale experiments to check the suppression effect of water spray system on the glass of the window under the different heat release rate (HRR) and thermal radiation from the fire. The conclusions describe the thermal radiation attenuation characteristics and temperature attenuation characteristics inside and outside the window. Consequently, we find that the sprinkler system can effectively protect the integrity of the glass and control the outdoor temperature within a safe range under the conditions of fire source power of 0.6 MW, 1.5 MW, and 3 MW. After the sprinkler system was turned on, the outdoor heat flux density rapidly decreased from a peak of 197 W/m2 and stabilized at 80 W/m2, which was 59% lower than the peak value. The research provides the deep insight into the temperature suppression phenomenon under the spray, and the protection method of ejecting fire for the external window is helpful to reduce the possibility of flashover in the indoor spray coverage. Due to the large range of heat release rates set in this paper, the experimental results provide a data reference for the fire protection design of future energy saving buildings.

High frequency surface discharge is one of the main causes of insulation failure in the high frequency power transformer. In order to extend the endurance lifetime and investigate the behaviours of surface discharge, a series of polyimide films containing phenyl thioether groups were prepared and tested in a self-built experimental platform with a high frequency voltage source. First, the surface discharge inception voltages on the films were tested under high frequency sinusoidal voltage. Then, with the application of continuous high frequency voltage, surface discharge tests were conducted to observe the entire evolution process of surface discharge including its initiation, self-sustaining, streamer and the ultimate flashover. Combining with the frequency-domain dielectric spectra and x-ray diffraction tests, effects of phenyl thioether content on the evolution process in each stage were analyzed. Experimental results and analyses indicate that the discharge inception voltage is opposite to the dielectric loss factor. The depth and distribution of traps caused by crystal and amorphous regions play the different role in various stages of surface discharge. The endurance lifetime of polyimide film containing 40% phenyl thioether was the longest and the effects of which were further discussed based on the discharge behaviour analysis. The paper demonstrates that molecular-structure modification is an effective method to improve the high frequency electrical performance of polyimide.

Due to the possible risk of flashover and explosion caused by the venting natural gas, in this paper, the PHAST software was used to determine the safe distance considering different factors. The results shows that the range of flashover is mainly depended on the venting pressure and venting, and once venting velocity within 20×104m3/h, the fire emptying is suggested. The range of flashover of natural gas in different directions were determined.

The falling of bird droppings near the insulator of overhead transmission lines will cause distortion of the electric field distribution near the insulator, resulting in air gap breakdown and flashover. Studying the area where there may be a risk of breakdown near the insulators in such a situation is of great significance for bird damage prevention of overhead transmission lines. Based on the finite element method, this paper established the 110kV transmission line model using ANSYS Maxwell finite element software and calculated the variation of the electric field distribution around the insulator when the bird droppings fall. Using the air gap average breakdown electric field strength as the criterion to judge whether breakdown occurs or not, the risk area of the air gap breakdown is determined, thereby determining the protection range of the transmission line for the flashover caused by the falling of bird droppings.

An investigation is conducted regarding the influence of surface roughness on the flashover strength of an insulator in vacuum. A series of experiments is first presented, showing the relationship between surface roughness and flashover voltage, combined with measurement of surface potential distribution. It is found that surface charging on a roughened dielectric is suppressed remarkably; also the flashover voltage threshold depicts a rise-and-fall trend with roughness increasing, exhibiting a voltage summit at a certain roughness value. In addition, optical observation is implemented to draw a parallel with an electron trajectory in vacuum. A proposal can therefore be made that multipactor expansion tends to be mitigated as electron bombardment on a dielectric occurs with a lower secondary electron yield. Then a theoretical model considering the microscopic physical process is established to explain the above phenomena, consisting of an internal charge migration layer, an interfacial electron absorption layer and an external electron obstruction layer. The validity of this theoretical model can be confirmed by obtaining the surface trap distribution, microscopic morphology and net secondary electron yield.

The statistics of tripping data of transmission lines show that external damage has become the main cause of transmission lines tripping. The foreign object cutting-out, which accounted for 61.8% of the total, was the most common external damage in transmission line. This paper focuses on the DC flashover characteristics of small-scale plastic sheds of greenhouses with distances of 1m and 2m. The results of the study showed that the surface of plastic sheds was less dirty and the average surface conductivity was lower. In the dry condition, it can be regarded as an insulator, and in the case of moisture, however, its insulation resistance is reduced to a megohm level. Under the electric field of 70 kV/m, no flashover occurred in both the wrinkled and the tiled plastic sheds. The flashover voltage per unit length of the wrinkled is about 15%-21% lower than that of the tiled, and wrinkled plastic sheds flash over more easily. As the pole pitch increases, the flashover voltage per unit length decreases, and the flashover voltage per unit length of both the wrinkled and the tiled is reduced by about 30%.

The particularity of lightning flashover failure of wind farm is analyzed, and it is pointed out that the formation mechanism of lightning overvoltage in wind farm collector system is different from that of power system distribution network. On this basis, the mechanism of lightning flashover failure of wind farm collector system is analyzed. The EMTP-RV software is used to simulate the typical lightning over-voltage formation and propagation process of the wind farm collector system. Results show that the fault mechanism of the lightning flashover fault system is analyzed. Finally, it is pointed out that the design scheme of the insulation system of the wind farm collection system should not directly copy the design scheme of the distribution network of the power system.