Zhicheng Wu

Alternating streamer propagation in mineral oil under bipolar oscillating impulse voltage

This study aimed to clarify the basic process of streamer propagation in mineral oil at bipolar oscillating impulse voltage. Shadow images and light signals of streamers showed that under bipolar oscillating impulse, positive and negative streamers propagated in an alternating manner: after polarity reversal, new streamers with opposite polarity were initiated and propagated first through the gaseous channels left behind by former streamers and then toward the ground electrode. The velocity of positive streamers was found nearly an order of magnitude higher than that of negative ones; thus, positive streamers are primarily responsible for the insulation failure of mineral oil. Negative streamers played the role of maintaining gaseous channels and facilitating positive streamers initiation due to their strong heat effect. High oscillation frequency and large damping factor decreased the durations and amplitudes of positive peaks, which restrained positive streamer propagation and further resulted in the increase in the breakdown voltage. Experiments on dielectric behavior of mineral oil were conducted to verify above inferences.This study aimed to clarify the basic process of streamer propagation in mineral oil at bipolar oscillating impulse voltage. Shadow images and light signals of streamers showed that under bipolar oscillating impulse, positive and negative streamers propagated in an alternating manner: after polarity reversal, new streamers with opposite polarity were initiated and propagated first through the gaseous channels left behind by former streamers and then toward the ground electrode. The velocity of positive streamers was found nearly an order of magnitude higher than that of negative ones; thus, positive streamers are primarily responsible for the insulation failure of mineral oil. Negative streamers played the role of maintaining gaseous channels and facilitating positive streamers initiation due to their strong heat effect. High oscillation frequency and large damping factor decreased the durations and amplitudes of positive peaks, which restrained positive streamer propagation and further resulted in the in...

On-site standard lightning impulse test for 1,100-kV gas-insulated switchgear with large capacitance

Gas-insulated metal-enclosed switchgear (GIS) was developed in the mid-1960s [1]-[5]. Compared with traditional air-insulated switchgear, the GIS concentrates the bus, isolating switch, circuit breaker, lightning arrester, transformer, and many other devices in an enclosed grounding metal case. It has many advantages such as reliability, compactness, long maintenance cycle, and low environmental impact, which have led to it being widely used in electrical power systems [6]-[16]. However, increased operating voltages and more extensive power grids have given rise to increased GIS failure rates [17], [18].

Numerical Simulation of Airflow Pressure Distributions along Insulator Surface for Electric High-speed Locomotive

The aerodynamic effects around the electric high-speed locomotive could cause the airflow pressure variations along the surface of pantograph supporting insulator, and thus would influence the surface dielectric behavior of insulator. Based on the computational fluid dynamics governed compressible Navier-Stokes equation, numerical simulations were carried out by finite element method. The airflow pressure distributions around the insulator at four different airflow velocities have been studied. The results showed that the airflow pressure increased near the windward of insulator but decreased near the sideward of insulator, and that the variations of airflow pressure were closely associated with the shapes of insulator shed. The airflow pressure distribution around and/or along the surfaces of two insulator models with the different shapes have also been presented comparatively. According to the distributions of low airflow pressure region, the possible discharge or flashover path of insulator for electric high-speed locomotive was suggested finally

Note: A phase synchronization photography method for AC discharge

To research discharge physics under AC voltage, a phase synchronization photography method is presented. By using a permanent-magnet synchronous motor to drive a photography mask synchronized with a discharge power supply, discharge images in a specific phase window can be recorded. Some examples of discharges photographed by this method, including the corona discharge in SF6 and the corona discharge along the air/epoxy surface, demonstrate the feasibility of this method. Therefore, this method provides an effective tool for discharge physics researchers.

Free microparticles—An inducing mechanism of pre-firing in high pressure gas switches for fast linear transformer drivers

Microparticle initiated pre-firing of high pressure gas switches for fast linear transformer drivers (FLTDs) is experimentally and theoretically verified. First, a dual-electrode gas switch equipped with poly-methyl methacrylate baffles is used to capture and collect the microparticles. By analyzing the electrode surfaces and the collecting baffles by a laser scanning confocal microscope, microparticles ranging in size from tens of micrometers to over 100 μm are observed under the typical working conditions of FLTDs. The charging and movement of free microparticles in switch cavity are studied, and the strong DC electric field drives the microparticles to bounce off the electrode. Three different modes of free microparticle motion appear to be responsible for switch pre-firing. (i) Microparticles adhere to the electrode surface and act as a fixed protrusion which distorts the local electric field and initiates the breakdown in the gap. (ii) One particle escapes toward the opposite electrode and causes a near-electrode microdischarge, inducing the breakdown of the residual gap. (iii) Multiple moving microparticles are occasionally in cascade, leading to pre-firing. Finally, as experimental verification, repetitive discharges at ±90 kV are conducted in a three-electrode field-distortion gas switch, with two 8 mm gaps and pressurized with nitrogen. An ultrasonic probe is employed to monitor the bounce signals. In pre-firing incidents, the bounce is detected shortly before the collapse of the voltage waveform, which demonstrates that free microparticles contribute significantly to the mechanism that induces pre-firing in FLTD gas switches.

Effects of electrode chemical reactions on SF6 discharge characteristics in extremely inhomogeneous electric fields

SF6 is widely used in the gas-insulated metal-enclosed switchgear or the corona-stabilized gas spark switch applications as a gas dielectric. It is generally believed that the discharge characteristics are only related to the electric field distribution and gas molecular density; however, the electrode chemical reactions can indeed markedly affect the SF6 discharge characteristics in the extremely inhomogeneous electric fields under the steady-state voltage. In this study, we used a needle-plane electrode system to build an extremely inhomogeneous electric field and examined the discharge characteristics within it including U-p characteristics, corona appearance, and corona current. We also analyzed the micro-region characteristics of the electrode surface, including the surface morphology, elemental composition, and chemical state to fully qualitatively determine the role of the electrode chemical reactions in discharge behavior. We found that the N-shaped U-p curve widens, the filamentous leader channels disappear, and the corona current drops suddenly as the duration of the electrode chemical reactions increases. Varying the surface morphology, elemental composition, and chemical state was observed on the electrodes of different polarities through micro-region analysis. The metal fluoride or metal sulfide film on the electrode surface may serve as a resistive coating due to its low electrical conductivity, which obstructs the leader discharge while enhancing the streamer discharge, suppressing the transition from the streamer to leader discharge and altogether significantly altering the discharge characteristics. The resistive coating produced is the primary cause of the electrode chemical reaction effects on the discharge characteristics, primarily as it depresses the transition from the streamer to leader discharge. The results presented here may provide useful guidelines for further research on SF6 discharge under inhomogeneous electric fields.SF6 is widely used in the gas-insulated metal-enclosed switchgear or the corona-stabilized gas spark switch applications as a gas dielectric. It is generally believed that the discharge characteristics are only related to the electric field distribution and gas molecular density; however, the electrode chemical reactions can indeed markedly affect the SF6 discharge characteristics in the extremely inhomogeneous electric fields under the steady-state voltage. In this study, we used a needle-plane electrode system to build an extremely inhomogeneous electric field and examined the discharge characteristics within it including U-p characteristics, corona appearance, and corona current. We also analyzed the micro-region characteristics of the electrode surface, including the surface morphology, elemental composition, and chemical state to fully qualitatively determine the role of the electrode chemical reactions in discharge behavior. We found that the N-shaped U-p curve widens, the filamentous leader channel...

Simulation and motion analysis of spherical free conducting particle between coaxial electrodes

The trend of development of Gas Insulated System (GIS) recently includes: miniaturization, intelligentization and high reliability. This needs a higher insulation performance of GIS. Free conducting particle in GIS is a main reason that causes the degradation of GIS insulation. Free conducting particle will bring a high risk to GIS insulation when it approaches to the high voltage conductor or reaches the disc-type insulator surface. Particle in radial non-uniform electrical field is apt to move along the radial direction in GIS, and probably moves into the high insulation risk region. According to the distribution of the electrical field between coaxial electrode systems, the dynamic model of spherical particle in axial non-uniform electrical field under AC voltage is amended. The motion phenomenon of particle is explained by solving the dynamic equations. Gravitational force, Coulomb force, electrical gradient force and gas viscous force are considered. Improving the applied voltage can increase the motion distance in vertical direction. The criterion can explain the incremental quantity of the motion onset voltage of particle in uniform field under AC voltage than it under DC voltage. Observations indicate that voltage exerts an incremental effect on maximum height. By analyzing improved dynamic model of spherical particle, another novel criterion to judge the penetration of the motion of spherical particle in uniform field under AC voltage is brought forward, as well.

Influence of trace SF 6 on discharge characteristics of SF 6 /N 2 gas mixture under lightning impulse

As one kind of synergistic effect gas, the discharge characteristics of SF<inf>6</inf>/N<inf>2</inf> gas mixtures would be influenced significantly by trace amounts of SF<inf>6</inf>. That is, the discharge characteristics of pure N2 is quite different from that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures with relatively low content of SF<inf>6</inf>. For the application of SF<inf>6</inf>/N<inf>2</inf> gas mixtures in power system equipment, this paper studied on the discharge characteristics of SF<inf>6</inf>/N<inf>2</inf> gas mixtures with low SF<inf>6</inf> mixing ratio in extremely non-uniform electric field under positive and negative lightning impulse. Meanwhile, the characteristic of SF<inf>6</inf>/N<inf>2</inf> gas mixtures were compared with that of pure N<inf>2</inf> and SF<inf>6</inf>. The experimental results indicate that the 50% breakdown voltage increases linearly with the rise of gas pressure under negative lightning impulse, and the rising rate of breakdown voltage with pure N2 is greater than SF<inf>6</inf>/N<inf>2</inf> gas mixtures even greater than pure SF<inf>6</inf> gas. At high gas pressure, the 50% breakdown voltage of pure N2 is higher than that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures, that is to say, the negative synergistic effect appeared. Under positive lightning impulse, the influence of gas pressure on 50% breakdown voltage is weaker, but the N-curve characteristic appeared because of the effect space charge. Meanwhile, the negative synergistic effect also appeared under positive lightning impulse at high gas pressure. The experimental results indicate that the breakdown time delay of pure N2 is much longer than that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures with 1% SF<inf>6</inf> mixing ratio because of the obvious difference between the formative time delay of N2 and SF<inf>6</inf>/N<inf>2</inf> gas mixtures. The phenomenon of breakdown time delay demonstrates that the discharge form of N<inf>2</inf> is different from that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures, that is to say, the streamer propagation changes to streamer-leader propagation.

Evaluation of the Martin empirical formulae for transformer oil: Statistical meaning of the time parameter

To study the statistical meaning of the time parameter t63% in the Martin empirical formula, we took Kunlun 25# transformer oil as the liquid insulating dielectric and utilized an impulse voltage generator, the impulse width of which could range from 50 ns to 1000 µs, along with the corresponding measuring equipment. The relationships between the breakdown voltage, the breakdown time lag and the discharge probabilities were acquired through experiments under impulses with quasi-uniform field. The results show that the discharge probabilities of different voltage levels under the fixed impulse waveform and the breakdown time lag under the same voltage level both could be well fitted to the three-parameter Weibull distribution functions, whose shape parameter was a constant only correlated to the electrode geometry. Meanwhile, on the basis of the Weibull distribution functions and corresponding impulse waveforms, we defined the effective duration and show the statistical meaning of the time parameter t63%.

Flashover characteristics of basin-type insulator with metallic particles on its surface under standard lightning impulse

The basin-type insulator is one of the most important parts in SF6 gas insulated switchgear (GIS). Flashover of basin-type insulators will cause serious harm to the electrical system. The fact that metallic particle attached to the surface cannot be detected in the delivery test under the standard lightning impulse (SLI) will pose potential threat to GIS. This paper studied flashover characteristics of the basin-type insulator with metallic particle contamination attached to its high voltage electrode in SF6 under SLI. An 1800 kV lightning impulse generator and a specialized pulse-voltage divider were used in the study. The results show that the flashover voltage drops rapidly when the metallic particle is attached to its surface. At high pressure, the basin-type insulator will be more sensible to the metallic particle. The metallic particle may cause danger to the system even its size is small. The reduction of flashover voltage will become slow with the increase of the metallic particles length. The gas pressure has little effect on the flashover voltage rate when the metallic particle exceeds a certain length. The flashover voltage under negative polarity of SLI is higher than that under positive polarity. Polarity effect will be obvious when the gas pressure is low. The discharge development process of basin-type insulator has been studied on the aspect of space charge accumulation at last.