Superposition of DC voltage and submicrosecond impulses for energization of electrostatic precipitators

This paper discusses the development of an impulsive microelectrostatic precipitation technology, which uses superposition of submicrosecond high-field pulses and dc electric field. Short impulses allow the application of higher voltages to the ionization electrodes of a precipitation system without the initiation of breakdown. These higher levels of electric field generate higher ionic concentrations, resulting in more efficient charging of the airborne particles, and can potentially improve precipitation efficiency. This work is focused on the analysis of the behavior of impulsive positive corona discharges in a coaxial reactor designed for precipitation studies. The efficiency of precipitation of coarse and fine particles has been investigated using different dc and impulse voltage levels in order to establish optimal energization modes

Micro-electrostatic precipitation for air treatment

Particulate matter suspended in the atmosphere is a major contaminant and is prevalent in urban environs, reducing the quality of air in the places that the majority of humans reside. Medical research has labelled PM2.5 as a potential risk to human health. To combat this issue, new legislation regarding PM2.5 has been passed. Electrostatic precipitators exhibit a drop in efficiency at ~(0.1-1) μm PM diameter. Therefore, the present work is focused on an investigation of microelectrostatic precipitation technology, for improvement of indoor air quality.;Initial work included investigation of impulsive positive energisation, in a specially designed single stage, coaxial reactor, utilizing 250 ns impulses superimposed on dc voltage. Precipitation efficiency for coarse and fine powders has been investigated for various levels of superimposed impulsive and dc energisation in order to identify optimal energisation conditions.;Further steps were taken to decouple charging from collection stages in order to optimize the air cleaning process to a greater extent. Precipitation experiments were conducted using ambient air and cigarette smoke. Maximum precipitation efficiency was achieved when both stages were energised, under impulsive and dc energisation in each stage respectively. Analytical work regarding PM charging has also been conducted.;Lastly, the coaxial precipitator reactor was scaled-up for possible indoor air cleaning applications. Similarly, impulsive energisation combined with dc voltage at the different stages has been used and proved to increase precipitation efficiency. Test fluids used were beeswax candle fumes and ambient air. Simulations have also been conducted to optimize the ESP process.;In conclusion, it has been shown that impulsive energisation of ESPs is highly efficient,100% for particles greater than 250 nm, for PM2.5 in concentrations found in indoor environments. This could potentially help in increasing indoor air quality, with all the corresponding health, working efficiency and ultimately state economic benefits it could achieve.Particulate matter suspended in the atmosphere is a major contaminant and is prevalent in urban environs, reducing the quality of air in the places that the majority of humans reside. Medical research has labelled PM2.5 as a potential risk to human health. To combat this issue, new legislation regarding PM2.5 has been passed. Electrostatic precipitators exhibit a drop in efficiency at ~(0.1-1) μm PM diameter. Therefore, the present work is focused on an investigation of microelectrostatic precipitation technology, for improvement of indoor air quality.;Initial work included investigation of impulsive positive energisation, in a specially designed single stage, coaxial reactor, utilizing 250 ns impulses superimposed on dc voltage. Precipitation efficiency for coarse and fine powders has been investigated for various levels of superimposed impulsive and dc energisation in order to identify optimal energisation conditions.;Further steps were taken to decouple charging from collection stages in order to optimize the air cleaning process to a greater extent. Precipitation experiments were conducted using ambient air and cigarette smoke. Maximum precipitation efficiency was achieved when both stages were energised, under impulsive and dc energisation in each stage respectively. Analytical work regarding PM charging has also been conducted.;Lastly, the coaxial precipitator reactor was scaled-up for possible indoor air cleaning applications. Similarly, impulsive energisation combined with dc voltage at the different stages has been used and proved to increase precipitation efficiency. Test fluids used were beeswax candle fumes and ambient air. Simulations have also been conducted to optimize the ESP process.;In conclusion, it has been shown that impulsive energisation of ESPs is highly efficient,100% for particles greater than 250 nm, for PM2.5 in concentrations found in indoor environments. This could potentially help in increasing indoor air quality, with all the corresponding health, working efficiency and ultimately state economic benefits it could achieve

Plasma closing switches

The 19900 Individual MSc project is a direct continuation of the EE928 Case study, thus objectives are already set and most theoretical parts of the work conducted has been already discussed. However, it was deemed necessary to include some of these chapters, in much more detail thought, as a way to justify the course taken in the experimental work and also as an introduction to the project report, so as not to have as prerequisite the study of the case study report. As a consequence, the first two chapters, namely 3 and 4 , placed after a brief introduction and the contents page, make a detailed description about pulsed power in general, both from historical and application perspective, and explain some basic physical processes occurring in plasma switches, namely the corona effect and the spark discharge. In the 5th chapter a description of the experiments conducted, the topologies under test, together with the experimental results are presented in detail. Important information extracted from the experiments is combined in the 6th chapter in order to propose an analytical model, based on which the design of corona stabilised plasma closing switches can be performed. Finally, in the conclusions and discussion section, the main project points and some important experimental results are summarised, and some possible future continuation of the work done is presented. The report ends with the bibliography section in ISO 690 format

Impulsive corona discharges for fine particles precipitation in a coaxial topology

Air-borne micrometer and submicrometer particles produced by anthropogenic sources contaminate atmospheric air, especially in large cities where both population and industrial activities are higher leading to a reduced air quality. Recent research has pointed out particles less than 2.5 μm in diameter (PM2.5) as a potential health hazard. To address this issue, stricter legislation has been put into force to reduce PM2.5 emissions. This paper is focused on the development of an impulsive microelectrostatic precipitation technology for charging and removal of fine air-borne particles in an economically feasible way. In this paper, a compact coaxial precipitator has been developed for possible indoor air cleaning applications. High-voltage impulses together with dc voltage have been used for energization of the reactor as it has been shown to enhance the precipitation efficiency. This precipitation system has been used for removal of fumes and fine air-borne particles from ambient air. In addition to the experimental part, analytical work has been conducted to optimize the electrostatic precipitation process and to reduce its power consumption

Effect of corona stabilisation in atmospheric air in DC and impulse conditions

Corona discharges in electro-negative gases can generate space charge which re-distributes the electric field in the inter-electrode gap. In the case of negative energisation of the sharp high-voltage (HV) electrode in atmospheric air, negative space charge reduces the ideal Laplacian electric field in the vicinity of the cathode, which may result in an increase in the breakdown voltage (corona stabilisation effect). Such stabilisation can be used in repetitive plasma closing switches to maintain the same voltage level between HV impulses generated by pulsed power systems. Positive energisation of the HV electrode in air demonstrates lower breakdown voltage due to less pronounced space charge effects, resulting in a reduced field-grading effect. The present paper reports on an investigation into breakdown characteristics in point-plane electrode topologies stressed with positive and negative DC voltages, and HV impulses produced by a Marx generator with maximal voltage of 100 kV and rise time of ~800 ns. It has been shown that the corona stabilisation effect strongly depends upon the polarity of HV energisation and electrode topology: radius of curvature of the point electrode and inter-electrode distance. In the present paper stainless steel electrodes with three different curvatures have been used: 0.115 mm, 1.0 mm and 2.5 mm. The gap width has been varied from 2 to 20 mm. At shorter distances the stressed air gap demonstrated the same values of positive and negative breakdown voltages. However, the critical gap could be identified at which the negative breakdown voltage becomes higher than the positive one, and with an increase in the inter-electrode distance this difference in the breakdown voltages continues to grow. This identification of the critical gap width and corresponding critical breakdown voltage allows the development of switch topologies with well pronounced corona stabilisation effect. In the case of impulse stresses, the critical breakdown time has been obtained as a maximum pre-breakdown time, which is identical for impulses of both polarities. With further increase in the gas spacing, the pre-breakdown time for negative impulses increases significantly as compared with the time to breakdown for positive impulses (~3 fold increase has been observed for the point electrode with the radius of 2.5 mm, from ~500 ns to ~1500 ns). This systematic study provides valuable information on corona stabilisation effect as a function of the electrode curvature, inter-electrode distance and HV polarity. Based on the obtained results, an algorithm for the development of corona stabilised electrode topologies has been established. This may help in the design of advanced plasma closing switches for pulse power systems which require a high level of operational stability

Investigation of impulsive corona discharges for energisation of electrostatic precipitation systems

Various industrial and domestic processes as well as developing nano-technologies generate micron and sub-micron particles. This phenomenon is more prevalent in large cities where population density and industrial activities are much higher, meaning that a large percentage of the world population is being exposed to everyday inhalation of particulate matter (PM). This may result in negative health effects, many of which are not investigated fully yet. The current research project is focused on the development of a small scale impulsive micro- electrostatic precipitator (IMP) for the removal of PM at homes or in public environments, being small in contrast with the industrial ones. This IMP will implement superimposed DC and sub-microsecond electric fields in order to charge and remove PM efficiently. As the impulse breakdown voltage in a gap is much greater than the DC one, the IMP will also avoid operating close to DC breakdown voltage levels. The designed IMP system composes of a plasma-generation and particle-collection electrodes. For the former, threaded rods of 3 and 6mm have been used as well as a smooth 1.5mm one, while the latter consists of a stainless-steel tube of 28mm internal diameter. The rods were placed coaxially into the tube, with the particle laden air flowing homogenously from the top to the bottom of the reactor. The transmission line based pulse generator developed is able to produce 270ns pulses with frequency of up to 100Hz. The efficiency of precipitation of micron sized particles was evaluated for different DC and impulse voltage levels by measurements of mass of collected particles. Breakdown voltage, corona initiation voltage and parameters of impulse coronas have been obtained under different energisation regimes. Precipitation experimental results showed that the positive or negative charging regimes play an important role in the system efficiency. The ultimate objective of this research project is to investigate precipitation levels of PM2.5, which constitutes a range of lower precipitation efficiency for available ESPs, as well as potential microbiological decontamination efficiency of impulsive non-thermal plasmas

Frequency Dependence of the Electric Field Grading of End-Winding of Generator Bars

To reduce the electric field stresses in the end-winding in hydropower generator bars, a field dependent grading tape is applied in the transition zone from outer corona protection to bare insulation. The design is typically optimized for 50/60 Hz with both low heat generation and optimal electric field distribution. The introduction of power electronic converters with high switching frequencies and dU/dt for controlling the generators changes the optimal design requirements. This work investigated experimentally the effect of higher voltage-frequencies on the conductivity in three field grading tapes from the same manufacturer with different field grading properties. The experimentally determined material parameters were then applied as inputs to finite element method (FEM) models to evaluate the electric field distribution at different voltage-frequencies. Measurements showed that the DC conductivity was highly nonlinear and strongly dependent on the electric field. This strong nonlinearity decreased at low electric fields as the frequency increased. The AC conductivity had a transition from a strong nonlinear material at line frequency to a less nonlinear material at 10 kHz. Simulations of the surface electric field and temperature using the DC conductivity for different voltage frequencies showed minor differences from simulations using the apparent AC conductivity. This indicates that the low field conductivity is of minor importance as it varied by two decades between DC and 10 kHz without resulting in a significant difference in either electric field, or temperature. The measured and simulated surface temperature increased linearly with applied voltage frequency. The simulated surface electric field was well distributed at line frequency, whereas it was greatly enhanced above 1 kHz.Frequency Dependence of the Electric Field Grading of End-Winding of Generator BarsacceptedVersio

Sub-microsecond impulsive corona discharges for electrostatic precipitation applications

The present paper discusses the development of the impulsive micro-electrostatic precipitation technology (μ-ESP), which uses superposition of sub-microsecond high field pulses and DC electric field. Short impulses allow the application of higher voltages to the ionisation electrodes of a precipitation system without the initiation of breakdown. These higher levels of electric field generate higher ionic concentrations, resulting in more efficient charging of the airborne particles and can potentially improve precipitation efficiency. The present work is focused on analysis of the behavior of impulsive positive corona discharges in coaxial reactor designed for precipitation studies. The efficiency of precipitation of coarse and fine particles has been investigated using the different DC and impulse voltage levels in order to establish optimal energisation modes

DC and sub-microsecond impulse energisation of electrodes in electrostatic precipitation

Airborne fine particles generated by internal combustion engines, power plants and other industrial and domestic sources pose a potential health risk, and this risk is exacerbated by the increasing use of nano-particle based technologies. Particles of size less than two-and-a-half micrometers (PM2.5) have recently started to attract attention as they can be carried long distances, can stay airborne for long periods of time and can penetrate deep into the lungs. As a result, manufacturers of air cleaning systems for domestic and commercial use are continuously looking for more effective methods to reduce the concentration of PM2.5 airborne particulate matter. The present paper discusses the development of the scientific and engineering basis for the impulsive micro-electrostatic precipitation technology, which will use superposition of sub-microsecond high field pulses and DC electric field. Such short HV impulses allow application of significantly higher voltages to the active ionisation electrodes of the precipitation system without the initiation of spark breakdown. The breakdown properties of atmospheric air show that the reduction of the impulse duration from 2 µs to 200 ns results in a 2-fold increase in the breakdown voltage. Higher levels of electric field will help to generate higher ionic concentrations, resulting in efficient charging of the airborne particles and improved removal efficiency. This paper reports initial results on the development of the micro-ESP system and analysis of its efficiency. A coaxial micro-ESP precipitation chamber composed of plasma-generation and particle-collection electrodes has been designed. The transmission-line based impulse energising system is able to generate square sub-microsecond impulses with frequency up to 100 Hz and magnitude of a few tens of kV. The efficiency of precipitation of dust micron and sub-micron sized particles has been investigated using different voltage levels and frequencies in order to establish optimal electrode topologies and energisation modes. At the present stage the precipitation efficiency has been evaluated by weighting of the collected particles; in future it is planned to use a portable particle spectrometer. Positive and negative DC and impulse energisation of the ionizing electrode has been used. It has been shown that the polarity of the high voltage stress and energisation mode has significant influence on the precipitation efficiency

The use of impulsive corona discharges for the removal of fine particles in a novel coaxial electrostatic precipitator

Summary form only given. Power plants, internal combustion engines and other sources produce micron and sub-micron particles, which contaminate the air. This problem is faced mainly in large cities where both population and industrial activities are higher leading to significantly reduced air quality. Recent research has pointed out particles less than 2.5 μm in diameter (PM2.5) as a potential health hazard. In the light of these results directives and strict legislation has been put into force in order to reduce PM2.5 emissions. This research paper is focused on an impulsive microelectrostatic precipitation technology in order to charge and remove suspended particles from the air in an economically feasible way. HV impulses together with dc voltage has been used in order to energise the reactor as it has been shown to enhance the precipitation efficiency. In the present work a compact, yet larger in scale, coaxial precipitator has been developed for possible indoor applications. This precipitation system has been tested for removal of smoke and fine airborne particles from ambient air. In addition to the experimental part, analytical work has been conducted in order to optimize the electrostatic precipitation process and reduce power consumption