Yuanxiang Zhou

A Compact Repetitive Unipolar Nanosecond-Pulse Generator for Dielectric Barrier Discharge Application

Dielectric barrier discharge excitated by pulsed power is a promising approach for producing nonthermal plasma at atmospheric pressure, but pulsed power generators vary widely in performance and should be chosen according to application requirements. In this paper, a repetitive unipolar nanosecond-pulse generator is constructed using resonant charging and one-stage magnetic compression circuits, where IGBT and magnetic switches are the key units, respectively. The generator is capable of providing repetitive pulses with a voltage of up to 30 kV and duration of 70 ns at a 300 resistive load. Output pulse voltage can be adjusted by varying ac input voltage or trigger pulse-width. This compact and convenient generator has been used successfully to produce stable dielectric barrier discharge.

Surface Treatment of Polyethylene Terephthalate Films Using DBD Excited by Repetitive Unipolar Nanosecond Pulses in Air at Atmospheric Pressure

Dielectric barrier discharge (DBD) is proved to be an effective method for surface treatment of polymers over the past 20 years and is now widely used in the fields of surface modification. In this paper, surface treatment of polyethylene terephthalate (PET) films for improving the hydrophilicity using DBD excited by unipolar nanosecond pulses is presented. Two typical discharges exhibiting homogeneous and filamentary modes are obtained under certain experimental conditions, and then, they are used to modify the surface of PET films. The microstructure and properties of the film surface before and after treatment are characterized with water contact angle measurement, scanning electron microscope, atomic force microscope, and X-ray photoelectron spectroscope. The results show that, with the treatment power density of 192 and 158 mW/cm2 in filamentary and homogeneous modes, respectively, surface morphology and property after treatment are significantly changed, and the physical etching and introduction of oxygen-containing polar functional groups account for the decease of surface contact angle and the increase of surface roughness. Compared with the filamentary DBD treatment, the homogeneous DBD (HDBD) is more effective in surface treatment, such as lower contact angle, higher roughness, and more oxygen-containing function groups after treatment. It is attributed to the fact that the HDBD can achieve a uniform treatment and can make the physical and chemical interaction much sufficient.

Nanosecond-pulse gliding discharges between point-to-point electrodes in open air

In this paper, gliding discharges with a point-to-point electrode geometry were produced by a repetitively pulsed power supply with a rise time of ?100?ns and a full-width at half-maximum of ?200?ns. The characteristics of such discharges were investigated by measuring their voltage?current waveforms and taking photographs of their discharge images. Experimental results showed that once the breakdown occurred, the nanosecond-pulse gliding discharges went into a stable stage at all air gaps, behaving in a mode of repetitive sparks. Under certain conditions, a non-stable stage would appear some time after the discharge went into the stable stage, in which the gliding discharges transitioned from repetitive sparks to diffuse discharges. Furthermore, several factors (gap spacing, pulse repetition frequency (PRF) and gas flow rate) influencing the discharge characteristics were investigated. It was observed that both the breakdown voltage and ignition voltage increased with the gap spacing, and a diffuse discharge was absent when the gap spacing was less than 6?mm. The breakdown voltage decreased with the increase in the PRF and its decrease ratio was larger in large gap spacing than in small gap spacing. Discharges would transit from repetitive sparks to diffuse discharges as the flow rate increased. Furthermore, a comparison of nanosecond-pulse and ac gliding discharges was conducted with respect to the power supply. The consumption and energy, the relationship between the power supply and the load, and the time interval between two pulses were three main factors which could lead to different characteristics between the nanosecond-pulse and ac gliding discharges.

Detection of x-ray emission in a nanosecond discharge in air at atmospheric pressure

Measurement of x-ray emission is an important parameter to investigate runaway behavior of fast electrons produced in nanosecond-pulse gas discharge. An online detection system of x rays is described in this paper, and the system consists of an x-ray detector with NaI (Tl) scintillator and photomultiplier tube, and an integrated multichannel analyzer. The system is responsible for detecting x-ray emission signal, processing the detected signals, and scaling the energy distribution. The calibration results show that every channel of the detection system represents a given x-ray energy and various x rays can be divided into different energy ranges between 10 and 130 keV. For a repetitive nanosecond-pulse breakdown between highly nonuniform gaps in open air, an energy distribution is obtained using the online detection system. It shows that the x-ray emission is a continuous spectrum and the x rays of above 60 keV dominate in the detected energy distribution.

Partial discharge characteristics in oil-paper insulation under combined AC-DC voltage

Heretofore, unlike HVAC transformers, there is not much experience in both insulation design guides and test methods for ultra HV converter transformers. This special insulation phenomenon is very prominent due to the complication of voltage applied on valve-side windings of converter transformers. There is a severe lack of empirical research on partial discharge in oil-paper insulation under the combined ACDC voltage. The study in this paper provides a timely research into the issues. Nine possible configurations, which intent to simulate different defects in the transformer, have been investigated. The PD inception voltages varied ratios of combined AC-DC voltages were conducted. The discharge magnitude and its relation with the applied voltage for selected four common configurations were obtained during voltage increasing and decreasing respectively, which presented the PD initiating process and also reflected the insulation behavior of dielectrics. Besides, the Finite Element Analysis Soft was used to propose the effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on PD characteristics, which is very lacking in the previous references. The roles of DC and AC component in PD characteristics were discussed in the context of the relevant work in literatures.

Influence of voltage reversal on space charge behavior in oil-paper insulation

The presence of space charge is a serious threat to the reliability of insulation material under voltage polarity reversal. The results of space charge evolution in oil-paper insulation during external voltage polarity inverse at room temperature were presented in this paper. A mirror image effect charge was observed in the steady state, but the formation of it was affected by the first polarizing, which could cause a change of electrical property. The results showed that the mirror image effect was independent of the involved nature, type and dynamics of charge. Experiments of different reversal polarities and periods indicated that the slow decay of heterocharge retained from previous homocharge injection was the cause of electric field enhancement. And the reversal period therefore had an effect on voltage polarity reversal, namely the shorter the reverse period, the higher the stress enhancement. The reliability of oil-paper insulation could be influenced by the polarity reversal because charge injection and distribution were unpredictable, and space charge modified and in return was affected by the field as well.

A Comparative Study of Water Electrodes Versus Metal Electrodes for Excitation of Nanosecond-Pulse Homogeneous Dielectric Barrier Discharge in Open Air

Atmospheric pressure low-temperature plasmas produced by dielectric barrier discharge (DBD) provide a promising approach for civilian application of pulsed power technology. In this paper, repetitive nanosecond pulses were generated using a magnetic compression solid-state pulsed power generator, and the rise time and pulse duration of the nanosecond pulse are ~30 and 70 ns, respectively. The DBD in open air is created using two kinds of electrodes, i.e., water and metal electrodes. The electrical, luminous, and optical characteristics of the DBDs under these two electrodes are studied and compared. The experimental results show that no filaments are observed and the discharge is homogeneous when water electrodes are used. The DBD still behaves in a filamentary mode when the discharge gap is extended to 4 cm in the case of metal electrodes. The results are validated by fast images taken by an intensified charge-coupled device camera. In addition, some discussion about the experimental results is presented. Improvement of discharge uniformity is due to the effect of resistive stabilization using water electrodes.

Space charge dynamics at the physical interface in oil-paper insulation under DC voltage

Space charge has received much attention recently because an understanding of space charge in oil-paper insulation is useful for the design of converter transformers. This paper presents a study of space charge dynamics at the physical interface using the pulsed electroacoustic (PEA) method. After sample inversion, space charge in the vicinity of semiconductor electrode could be clearly observed when the sample is under depolarization. When an unexposed sample was placed next to a previously polarized one, negative charge appeared at the dielectric interface. These results indicated that there were other reasons in addition to acoustic loss that led to space charge distribution distortion. As regards the dielectric interface, it was found that an electrical double layer was formed at the interface, and the space charge sign was dependent upon the polarity of applied voltage. Space charge behavior at the interface depended on several aspects of the status of the interface, and during voltage polarity reversal, the interface was a weak point of the insulation.

A study on electric conduction of transformer oil

In this paper, a high sensitivity electrometer is adopted to measure DC conduction currents across the oil gap. The effect of morphology of electrode surface (3 dimensional roughness is approximately 0.03 mm) on conduction current is considered adequately, which is very lacking in the previous references. Based on the Fower-Nordheim theory, three stages will be clearly distinguished with the increase of the electric field strength. At low electric field stage (E<;0.44 kV/mm), the current through the oil I is directly proportional to the electric field E, which meets the Ohms law. Oil conductivity mainly depends on impurity ions with only low mobility. At medium electric field stage (0.44 kV/mm<;E<;1.33 kV/mm), ln (I/E2) appears to be proportional to E-1, which meets the field emission equation. At high electric field stage (E>1.33 kV/mm), the obtained I-V2 characteristics is linear, which can be explained by the space charge limited current. Moreover, main factors affecting the conduction current, including oil gap, electrode geometry and materials, polarity effect, temperature, hydrostatic pressure and moisture, have been analyzed and the causes are interpreted as well.

Space charge behavior evolution with thermal aging of oil-paper insulation

Space charge is a threat to insulation materials and oil-paper insulation gradually degrades in service. The degradation of cellulose affects space charge formation, accumulation and dissipation, and the presence of space charge affects the performance of dielectric. In this paper, the effect of thermal aging on space charge behaviors and other properties including permittivity, conductivity, and tensile strength was investigated. Oil-paper samples were aged at a hot spot temperature (180°C). During thermal aging, space charge distribution was measured at a regular interval at room temperature. And the pulse electro-acoustic (PEA) method was used for space charge measurement. The results showed that the thermal aging at hot spot temperature caused increase of conductivity and tensile strength degradation. The permittivity increased at the beginning but then it decreased, while dissipation factor showed an upward trend. Besides these, trap distribution and carriers characteristics were also affected. The formations of space charge of unaged and aged oil paper were consequently different under low electric field. Due to the generation of traps by the degradation of cellulose, more charge, especially positive charge was trapped, but the increased traps mostly were shallow traps. Based on the Schottky model, a correlation between space charge injection and the permittivity was drawn. This, together with the appearance of positive bulk charge could be used for oil paper aging status diagnosis.