Positive DC Corona Discharge Research Articles

Numerical Simulations of Ionic Wind Induced by Positive DC-Corona Discharges

This paper analyzes ionic wind production and propulsive force in various electrode configurations under atmospheric conditions. By considering the aerodynamic forces in addition to previously considered electric ones, new predictions for steady-state forces and ionic wind flow velocity are successfully compared with experimental measurements, providing convincing quantitative evidence of the predictive capabilities of drift-diffusion modeling associated with one-way Coulomb forcing of Navier–Stokes equations for ionic wind generation. Furthermore, various electrode configurations are analyzed, some of them streamlined, reducing wakes downstream collectors on the one hand and providing additional thrust on the other. The quantification of these additional thrusts is analyzed, physically discussed, and explored in various configurations.

Identification of corona discharges based on wavelet scalogram images with deep convolutional neural networks

This paper presents a new wavelet-based approach to automate the identification of positive and negative DC corona discharge current pulses. Two electrode systems with variable gap spacings formed corona discharges, and two commercial sensors (a high-frequency current transformer (HFCT) and a shunt resistor) captured transient corona discharge currents. The proposed method employs a continuous wavelet transform to generate time-frequency representations of corona discharge pulse currents, called scalogram images. The effects of sampling interval, data acquisition time, data shifting, and external noise components in the signals on the scalogram images were examined. The well-known pre-trained convolutional neural network models, AlexNet, MobileNet, and ShuffleNet, were tailored, and their ensemble structure was generated to discriminate scalogram images of discharge pulses. A framework was constructed to increase the generalization ability of the study. The results demonstrate that the scalogram images are robust candidates for corona discharge identification.

On the different regimes of positive DC corona discharges as a function of electrical and geometrical parameters

This experimental study aims to better understand the electrical and optical properties of a positive DC corona discharge and to highlight the influence of applied voltage and electrode gap on the discharge regime. When the high voltage is increased, three successive regimes have been observed, including one that has never been reported in the literature to the best of our knowledge. This is certainly due to the fact that the tungsten needle used in these experiments is a commercial needle, with a tip of perfect surface finish. In fact, after the ignition voltage, the mean current slowly increases, and it perfectly follows the theoretical Townsend's law; this is the glow regime. When the voltage reaches a first threshold, a few small current pulses appear. However, this is not the breakdown streamer regime because photographs and intensified camera visualizations highlight that the propagation of streamers toward the grounded plate is limited to half of the gap. In this regime, the time-averaged current becomes higher than that of Townsend's law. Each current pulse is composed of only one bump, and the pulse frequency is higher (approximately 20 kHz) than that of the breakdown streamers. When the voltage reaches a second threshold, the magnitude of current pulses suddenly increases, but their frequency decreases (a few kHz); this is the transition toward the breakdown streamer regime. In this regime, each current pulse is composed of two bumps: the first one corresponds to the arrival of a primary streamer at the plate and the second one corresponds to a secondary streamer. Finally, we investigated the effects of the electrode gap. The magnitude and frequency of the current pulses increase when the gap decreases. Consequently, the offset between Townsend's curve and the experimental I–V characteristics increases when the gap decreases, because the pulse component of the total discharge current increases.

Capacitance Calculation Model in Corona Discharge Case

The (I-V) characteristic pattern of the corona discharge case is very different from the pattern of ordinary electric circuits, so it is interesting to investigate. Several previous studies involved the concept of Maxwell's equations on several physical case models such as coaxial cylinder, electrohydrodynamic, and the electric wind. In this study, we use a capacitance calculation model for positive dc corona discharge in air, especially in calculating the (I-V) current-voltage characteristics of an electrode configuration model, often referred to as capacitively coupled plasma (CCP). The configuration model comprises active and passive electrodes, with the active electrode in the form of a pentagonal with the sharp end (in the middle) facing downwards in an upright position. The passive electrode under the active electrode has a large rectangular shape in a lying place. This configuration model is named The Chisel Eye and Midpoint-Plane (CEM-P). The analytical calculation of the (I-V) characteristics uses the geometric properties of the active electrode, which will produce a large corona current flow at the pointed electrode. These properties in analytical calculation manifest with the emergence of the corona flow multiplication factor at the sharp active electrode’s integration boundary condition called the shape sharpness factor k. The Python GUI Programming simulation program makes graphic simulations, Standard Deviation (SD), t-tests, and calculating the factor k (fitting curve value) between numerical calculations and research data. The values of the SD, the t-tests, and the Percentage of tangent points meet the requirements for a high level of accuracy for the four CEM-P configuration models of the (I-V) characteristics simulation graph with the graph has a relatively large percentage of tangent points values (82.35% – 94.44%).

The Current-Voltage Characteristics for Electrode Geometry Model of Positive DC Corona Discharge in Air

Research on corona plasma discharges from plasma reactors has been carried out using the Point and Semicircular-Plane (PSC-P) electrode configuration in the air. The purpose of this study is to compare the suitability level of the current-voltage (I-V) characteristics curve from the results of experimental data and numerical simulation calculations of the electrode geometry function based on the variation of three electrode configuration sizes (small plate, medium plate, and large plate) and two variations of the distance between the two electrodes in each electrode configuration size. The results of the study prove that there is a high degree of conformity between numerical calculations with experimental data through calculations from the Python GUI Programming by taking a fitting value from the certain k-shape sharpness factor. The calculation location of the k-shape sharpness factor lies on the electrode tip sharp surface which will produce the largest plasma flow in the plasma reactor equipment.

Materials degradation in non-thermal plasma generators by corona discharge

Atmospheric corona discharge devices are being studied as innovative systems for cooling, sterilization, and propulsion, in several industrial fields, from robotics to medical devices, from drones to space applications. However, their industrial scale implementation still requires additional understanding of several complex phenomena, such as corrosion, degradation, and fatigue behaviour, which may affect final system performance. This study focuses on the corrosive behaviour of wires that perform as a high-voltage electrode subject to DC positive corona discharge in atmospheric air. The experiments demonstrate that the non-thermal plasma process promotes the growth of the oxidative films and modifies the physicochemical properties of the materials chosen as corona electrodes, hence affecting device operation. Surfaces exposed to this non-thermal plasma are electrically characterized by negative exponential decay of time-depend power and analysed with SEM. Implications on performance are analysed and discussed.

Effect of Gas-Mixture Ratio on the Characteristics of Positive DC Corona Discharge in SF6/N2 Gas Mixtures

The characteristics of positive DC corona discharge in SF6/N2 gas mixtures under different gas-mixture ratios are investigated with a needle-to-plane electrode configuration. With increasing applied voltage, a discharge-stage transition process including the Townsend stage, streamer-pulse stage, brush-pulse stage and breakdown stage is observed. The variation in current-pulse characteristics, including pulse-time parameters, mean pulse amplitude and pulse-repetition rate, is investigated under different gas-mixture ratios. The pulse-time parameters are not affected by gas-mixture ratio. The pulse-repetition rate increases with the increase of SF6 concentration and mean current, ranging from hundreds of kilohertz to thousands of kilohertz. The streamer-pulse amplitude increases with increasing SF6 concentration. With increasing mean current, the small-amplitude brush pulses become dominant, thus the mean pulse amplitude decreases. In order to further explore the influence mechanism of gas-mixture ratio on positive corona discharge, a fluid model of positive corona discharge is developed and relevant simulations show the formation process of current pulse, and verify the validity of the pulse characteristics changing with gas-mixture ratio.

The Research on the Microscopic Mechanism and Dynamical Characteristics of DC Positive Corona Discharge in Pure O₂

We systemetically research the microscopic physical mechanism of positive corona discharge and comprehensively analyse the regular and irregular pulsed oscillations of discharge outputs. Especially, we discover the formation mechanism of irregular pusled discharge phenomenon for the positive corona and give a profound explanation. Accorinding to the physical process of discharge, we propsoe a new dynamical model which can describe the Trichel pulsed discahrge outputs and the characteristics of waveforms. Based on nonlinear dynamics method, we predict and analyse differernt stages of discharge. In particular, the experimentally observed irregularly pusled discharge on positive corona is reproduced by the presented theory. Our study reveals that the positive ions as current carriers predominate in the total current and the photons occurring can be treated as the feedback signal in the nonlinear process of discharge system. We find that the photo-ionization is an indispensable role as the source forming electron avalanches for sustaining the positive corona discharge. Moreover, according to quantum mechanics we demonstrate the high frequency photons with energy higher than the ionization energy may photo-ionize oxygen molecules in pure oxygen. We point out that the recombination radiation is competent to photo-ionize neutral molecules producing the seed electrons. Besides, we analyze the characteristics of diversely pulsed outputs and further explain the microscopic menchnism generating the symmetric and asymmetric sharp jagged pulses. Furthermore, we discover the intrinsic physical relations between positive ion and surface electric field, and the significant influnces of the attributes of different particles on the discharge behaviors.

Computational and Experimental Study of Time-Averaged Characteristics of Positive and Negative DC Corona Discharges in Point-Plane Gaps in Atmospheric Air

The use of stationary solvers instead of approximate solution methods or time-dependent solvers, which are standard tools in gas discharge modeling, allows one to develop a very fast and robust numerical model for studying the time-averaged characteristics of dc corona discharges. Such an approach is applied to dc corona discharges in point-plane gaps in ambient air. A wide range of currents of both voltage polarities and various gap lengths are investigated, and the simulation results are validated by comparing the computed current–voltage characteristics and spatial distributions of the radiation intensity with experimental results. Specific features of the numerical and experimental results at both polarities are discussed.

Electrohydrodynamic ionic wind, force field, and ionic mobility in a positive dc wire-to-cylinders corona discharge in air

Ionic wind refers to the acceleration of partially ionized air between two high-voltage electrodes. We study the momentum transfer from ions to air, resulting from ionic wind created by two asymmetric electrodes and producing a net thrust. This electrohydrodynamic (EHD) thrust, has already been measured in previous studies with digital scales. In this study, we provide more insights into the electrohydrodynamic momentum transfer for a wire-to-cylinder(s) positive dc corona discharge. We provide a simple and general theoretical derivation for EHD thrust, which is proportional to the current/mobility ratio and also to an effective distance integrated on the surface of the electrodes. By considering various electrode configurations, our investigation brings out the physical origin of previously obtained optimal configurations, associated with a better tradeoff between Coulomb forcing, friction occurring at the collector, and wake interactions. By measuring two-dimensional velocity fields using particle image velocimetry (PIV), we are able to evaluate the resulting local net force, including the pressure gradient. It is shown that the contribution of velocity fluctuations in the wake of the collecting electrode(s) must be taken into account to recover the net thrust. We confirm the proportionality between the EHD force and the current/mobility ratio experimentally, and evaluate the ion mobility from PIV measurements. A spectral analysis of the velocity fluctuations indicates a dominant frequency corresponding to a Strouhal number of 0.3 based on the ionic wind velocity and the collector size. Finally, the effective mobility of charge carriers is estimated by a PIV based method inside the drift region.

Revisiting the positive DC corona discharge theory: Beyond Peek's and Townsend's law

The classical positive Corona Discharge theory in a cylindrical axisymmetric configuration is revisited in order to find analytically the influence of gas properties and thermodynamic conditions on the corona current. The matched asymptotic expansion of Durbin and Turyn [J. Phys. D: Appl. Phys. 20, 1490–1495 (1987)] of a simplified but self-consistent problem is performed and explicit analytical solutions are derived. The mathematical derivation enables us to express a new positive DC corona current-voltage characteristic, choosing either a dimensionless or dimensional formulation. In dimensional variables, the current voltage law and the corona inception voltage explicitly depend on the electrode size and physical gas properties such as ionization and photoionization parameters. The analytical predictions are successfully confronted with experiments and Peek's and Townsend's laws. An analytical expression of the corona inception voltage φon is proposed, which depends on the known values of physical parameters without adjustable parameters. As a proof of consistency, the classical Townsend current-voltage law I=Cφ(φ−φon) is retrieved by linearizing the non-dimensional analytical solution. A brief parametric study showcases the interest in this analytical current model, especially for exploring small corona wires or considering various thermodynamic conditions.

Contribution to the Chemistry of Plasma-Activated Water

Plasma-activated water (PAW) was prepared by exposure to nonthermal plasma produced by a positive dc corona discharge in a transient spark regime. The activation of water was performed in atmosphere of various surrounding gases (air, nitrogen, carbon dioxide, and argon). This PAW retains its biological activity, measured on the mouse neuroblastoma cells culture, even after storage for more than one year. The highest hydrogen peroxide content was found for PAWs prepared in the atmospheres of argon or carbon dioxide, whereas the PAWs prepared in air and nitrogen exhibited lower hydrogen peroxide content. The acidity of PAWs mediated by nitric and nitrous acid formation displayed an opposite trend. It is concluded that the long-lasting biological effect of PAW is mediated by hydrogen peroxide in acid milieu only, whereas other possible active components decompose rapidly.

Time-domain performance of audible noise for positive dc corona: Numerical simulations and measurements

Corona-generated audible noise of HVDC transmission lines is a paramount parameter for design consideration. An acoustic source model and a computational technique are proposed to simulate the sound pressure pulse of positive dc corona discharge. A reduced-scale experimental platform with artificial defeat conductor is built up to verify this numerical method and used to study the time-domain characteristics of the audible noise of the single, three and five corona sources, respectively. The measured and computational results show that calculations of sound pressure are reasonable agreement with the measurements. The repetition frequency of the audible noise pulses increases more significantly than their amplitude, as the number of the corona discharge points increase. Furthermore, with the increase of the applied voltage, the repetition frequency and amplitude of the audible noise pulses increases.

Positive direct current corona discharges in single wire-duct electrostatic precipitators

This paper is aimed to study the characteristics of the positive dc corona discharges in single wire-duct electrostatic precipitators. Therefore, the corona discharges were formed inside dry air fed single wire-duct reactor under positive dc voltage at the normal atmospheric conditions. The corona current-voltage characteristics curves have been measured in parallel with the ozone concentration generated inside the reactor under different discharge conditions. The corona current-voltage characteristics curves have agreed with a semi empirical equation derived from the previous studies. The experimental results of the ozone concentration generated inside the reactor were formulated in the form of an empirical equation included the different parameters that were studied experimentally. The obtained equations are valid to expect both the current-voltage characteristics curves and the corresponding ozone concentration that generates with the positive dc corona discharges inside single wire-duct electrostatic precipitators under any operating conditions in the same range of the present study.

Improvement in ionization efficiency of direct analysis in real time-mass spectrometry (DART-MS) by corona discharge.

Herein it is shown that a combination of direct analysis in real time (DART) with a corona discharge system consisting of only a needle electrode easily improves DART ionization efficiency. Positive and negative DC corona discharges led to a formation of abundant excited helium atoms as well as the reactant ions H3O(+)(H2O)n and O2˙(-) in the DART analyte ionization area. These phenomena resulted in an increase in the absolute intensities of (de)protonated analytes by a factor of 2-20 over conventional DART. The other analyte ions detected in this corona-DART system (i.e., molecular ions, fragment ions, oxygenated (de)protonated analytes, dehydrogenated deprotonated analytes, and negative ion adducts) were quite similar to those obtained from DART alone. This indicates a lack of side reactions due to the corona discharge. The change in the relative intensities of individual analyte-related ions due to the combination of a corona discharge system with DART suggests that there is no effect of the abundant excited helium in the analyte ionization area on the fragmentation processes or enhancement of oxidation due to hydroxyl radicals HO˙. Furthermore, it was found that the corona-DART combination can be applied to the highly sensitive analysis of n-alkanes, in which the alkanes are ionized as positive ions via hydride abstraction and oxidation, independent of the type of alkane or the mass spectrometer used.

Influence of water conductivity on particular electrospray modes with dc corona discharge — optical visualization approach

The effect of water conductivity on electrospraying of water was studied in combination with positive DC corona discharge generated in air. We used a point-to-plane geometry of electrodes with a hollow syringe needle anode opposite to the metal mesh cathode. We employed total average current measurements and high-speed camera fast time-resolved imaging. We visualized the formation of a water jet (filament) and investigated corona discharge behavior for various water conductivities. Depending on the conductivity, various jet properties were observed: pointy, prolonged, and fast spreading water filaments for lower conductivity; in contrast to rounder, broader, and shorter quickly disintegrating filaments for higher conductivity. The large acceleration values (4060 m/s2 and 520 m/s2 for 2 μS/cm and 400 μS/cm, respectively) indicate that the process is mainly governed by the electrostatic force. In addition, with increasing conductivity, the breakdown voltage for corona-to-spark transition was decreasing.

Numerical Simulation of the Characteristics of Heavy Particles in Bar-Plate DC Positive Corona Discharge Based on a Hybrid Model

An improved, multicomponent, and 2-D hybrid model is presented in detail for the simulation of bar-plate dc corona discharge in dry air (O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> =1:4). The model is based on plasma hydrodynamics, and chemical models in which 12 species (e, O, O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> , O <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> , N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) and 32 collision reactions between such species are considered. In addition, the photoionization and secondary electron emission effects are also incorporated within the model. The simulation is established with a bar-plate electrode configuration with an interelectrode gap of 5.0 mm, where the positive dc voltage applied to the bar is 3.0 kV, the pressure in air discharge is fixed at 1.0 atm, and the gas temperature is assumed to be a constant (300 K). Using individual discharge current waveform and V-I curve, the effectiveness of this developed model is validated by experimental results. With this hybrid model, electric field distribution and net space charge distribution at four representative time points during a pulse are discussed. Moreover, the composition and distribution of particles are analyzed emphatically. The obtained results show that, among all reactions, the reaction rate of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , which is the collision reaction between electron and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is maximal, whereas the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> density is significantly less than O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> . O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> has the largest number of negative ions and thus restrains the electron collision process significantly. In addition to N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , O is the major neutral particle, but owing to the small quantity of neutral particles, the effect is relatively weak. The obtained results will provide valuable insights into the physical mechanism of positive corona discharge in air.

Simulation of Expansion of Thermal Shock and Pressure Waves Inducaed by a Streamer Dynamics in Positive DC Corona Discharges

This paper is devoted to the simulation of the thermal shock and the induced pressure-waves expansion, generated by a dc pin-to-plan corona discharge in the air at ambient temperatures and under atmospheric pressure. The positive dc voltage applied to the tip generates a monofilamentary streamer that crosses the gap from the tip toward the plan. The simulation models are based on the coupling of a 2-D dynamics streamer model with the hydrodynamics conservation equations of a compressible gas. The source term for the gas dynamics equations takes into account the fast-energy relaxation from excited molecules to the random thermal energy. The simulation shows that the streamers generate a thermal shock near the anodic tip, which induces high pressure gradients and finally the gas expansion. The thermal shock is located just in front of the anodic tip, where the injected energy density is the highest. After 0.3 μs, the mean gas temperature increases up to around 800 K in a small volume just in front of the anodic tip while the maximum temperature reaches 1200 K. In addition, two pressure waves, a spherical and a cylindrical one, are induced with a propagation velocity of 370 m s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> i.e., close to the speed of sound in air.

Comparison of styrene removal in air by positive and negative DC corona discharges

This communication discusses styrene removal in air by positive and negative DC corona discharges. Experiments were performed with a wire-plate reactor and under a gas flow rate of 305 m3/h. In terms of averaged applied voltage and corona current, it is observed that the maximum negative corona current is always at least two times larger than the positive one at the same voltage level. At the same corona discharge energy density, however, the positive corona discharge produces around 2–6 times more ozone in comparison with the negative corona. For styrene removal, the positive corona processing is also around 2–6 times more effective than the negative corona. Humidity, an important and variable component of ambient air, affects the positive corona processing significantly. But it exerts a moderate effect on the negative corona. The differences between positive corona and negative corona discharges are attributed to their different discharge properties.

Detection of SF6 decomposition products generated by DC corona discharge using a carbon nanotube gas sensor

This paper describes application of carbon nanotube (CNT) gas sensor to detection of SF6 decomposition products generated by positive or negative DC corona discharge. The present authors have demonstrated CNT gas sensor-based partial discharge (PD) detection, aiming to develop a new diagnosis method of gas-insulated switchgear (GIS) filled with high pressure SF6 gas. In our previous studies, PD was generated by ac high voltage because most of GIS are designed and operated for conventional high voltage ac (HVAC) power transmission lines. In recent years, however, the electrical power industry has shown a trend shifting from HVAC to high voltage dc (HVDC) power transmission, which has many advantages such as higher power capacity and lower power loss. This technological trend motivated us to explore and expand the application of CNT gas sensor to detection of SF6 decomposition products generated by dc corona discharges. It was found that the CNT gas sensor exhibited significant response to dc corona discharge depending on the voltage polarity. In order to elucidate the mechanism of the polarity effect, SF6 decomposition products were analyzed by Fourier transformation infrared (FTIR) spectroscopy as well as using a gas detection tube. Based on comparison between the polarity effects on the CNT sensor response and the decomposition products, a possible contribution of HF to the CNT gas sensor response was suggested.