Corona Current Pulses Research Articles

The characteristics of negative corona discharge and radio interference at different altitudes based on coaxial wire–cylinder gap

The corona discharge from transmission lines in high-altitude areas is more severe than at lower altitudes. The radio interference caused thereby is a key factor to be considered when designing transmission lines. To study the influence of altitude on negative corona characteristics, an experimental platform comprising a movable small corona cage was established: experiments were conducted at four altitudes in the range of 1120‒4320 m, and data on the corona current pulse and radio interference level of 0.8-mm diameter fine copper wire under different negative voltages were collected. The experimental results show that the average amplitude, repetition frequency and average current of the corona current pulse increase with increasing altitude. The dispersion of pulse amplitude increases with increase in altitude, while the randomness of the pulse interval decreases continuously. Taking the average current as an intermediate variable, the relationship between radio interference level and altitude is obtained. The result of this research has some significance for understanding the corona discharge characteristics of ultra-high-voltage lines.

Measurement and Analysis of Corona Current Pulses in Needle-Plate Air Gap Under Negative Switching Impulse Voltages

Measurement and Analysis of Corona Current Pulses in Needle-Plate Air Gap Under Negative Switching Impulse Voltages

The characteristics of positive corona discharge at different altitudes

With the vigorous growth of UHV transmission projects, transmission lines crossing high-altitude areas are inevitable, and the degree of corona discharge in high-altitude areas is more intense. The electromagnetic environmental problems caused by corona discharge have gradually become one of the key technical problems restricting UHV transmission in high-altitude areas. To investigate how height affects the positive polarity corona’s corona characteristics, using the movable small corona cage to carry out experiments at four altitude points in the range of 1120 m–4320 m, and measures the corona current pulse and radio interference level of 0.8 mm diameter fine copper wire under different positive voltages. Several conclusions can be drawn from the data analysis, including the following: The average current pulse amplitude grows with altitude, first before decreasing, and the current pulse repetition frequency rises along with altitude. The major conclusions of this essay can serve as a starting point for the study of the corona effect.

Characterization of DC corona discharge current pulses using high-frequency measurement techniques

This paper presents corona discharge current pulse characterization under positive and negative DC voltage excitations. Two electrode systems with three different gap spacings generated corona discharges. Different corona modes in atmospheric air were observed with voltage level changes. Commercial shunt resistors and a high-frequency current transformer (HFCT) captured corona current pulses. The appropriate resistor type and value were determined for the shunt resistor measurement. BNC terminator was proposed as a shunt resistor to detect corona discharge current pulses, and the availability of the shunt resistors and HFCTs for corona pulse detection was discussed. After gathering corona discharge pulse data, features from the pulses were extracted. The best features describing corona discharge types were specified using a recursive feature elimination method based on a decision tree estimator. The corona discharges were discriminated with an accuracy rate of 0.994.

Experimental study on positive corona discharge characteristics in plane-single/double earthed rods air gaps under negative DC voltage

In order to analyze the corona discharge characteristics at the grounding object tip under the thundercloud electric field environment, scaled simulation test has been conducted on corona discharge in the plane-single/double earthed rods air gap. Five gap configurations with three different grounding rods were built during experiments. Factors as rod tip curvature, voltage and gap configuration have been considered in detail. Corona voltage, corona current and the number of ultraviolet photons were all recorded. The grounding rod tip curvature radius can affect corona discharge processes at grounding rod tip. With increasing applied negative DC voltage, five typical corona processes of dark discharge, burst streamer corona, periodic streamer corona, glow corona and brush discharge occurred at conical and spherical grounding rod tip (r=0.75cm), but under d=5cm and d=10cm no glow corona and brush corona appeared at spherical grounding rod tip (r=1.25cm). It is much easier for grounding rod tip with smaller curvature to form glow corona phenomenon. The mean value Iav of periodic streamer corona current pulse amplitude decreases with grounding rod tip curvature radius, while the mean corona current pulse width and pulse interval increases. For grounding rod with conical tip, there is little difference of average current pulse amplitude between periodic streamer corona and brush corona; for grounding rod with spherical tip (r=0.75cm), the mean value Iav of brush corona pulse amplitude increases to 3595.8 μA which has increased by a factor of about 36. The photon number at conical grounding rod tip is larger than that at spherical grounding rod tip at low voltage but the relation reverses as the applied voltage exceeds a critical value. The shielding effect of two grounding rods in plane-double rods air gaps increased with spherical grounding rod curvature radius and decreases with the lateral gap distance between two grounding rods.

Experimental and numerical study on corona onset threshold under combined AC-DC voltages in Wire-Plane electrodes

With the large-scale application of hybrid HVDC/HVAC transmission technology, the phenomenon of corona discharge is also intensified. This paper attempts to provide a comprehensive understanding of the variation of corona onset voltage under combined AC-DC voltages. In this paper, an experimental platform for measuring corona onset voltage and current pulses of wire-plane electrode under combined AC-DC voltages was built. A new corona model of wire-plane electrode under combined AC-DC voltages was also established by using the fluid model and the ion flow field model. The corona onset voltages of wire-plane electrode under different ratios of AC and DC voltages, conductor radius and conductor spacing were measured. It turned out that the conductor radius affected the intercept of the corona onset curve, while the conductor spacing affected the slope. In addition, the type of corona only affected the intercept of the corona onset curve, not the slope. With the same fixed voltage of non-corona conductor, the corona onset voltage of the DC-side positive corona was the largest, followed by the DC-side negative corona, then was the AC-side positive half-cycle corona, and the smallest was the AC-side negative half-cycle corona. The interaction between AC voltage and DC voltage was approximately linear. The calculations of the model illustrated that the residual space charges had a blocking effect on corona discharge. Therefore, the more residual space charge was, the higher corona onset voltage would be.

Numerical Study on the Effect of Electric Field Non-uniformity on the Pulse Characteristics of Positive Corona Discharge in SF6/N2 Gas Mixtures

This paper establishes a two-dimensional axisymmetric needle-to-plane geometry model in SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> gas mixtures to simulate the effect of the electric field non-uniformity on the pulse characteristics of positive corona discharge. By changing the needle tip radii from 25 µm to 300 µm, the corona discharge process under different electric field non-uniformity is simulated. Meanwhile, the simulated results are verified by the experiments on a corona discharge experimental setup. The results show that the rising and falling edges of positive corona current pulses correspond to the development and termination of streamers, respectively. The sub-pulse will be generated during the formation of the first current pulse, and this phenomenon is closely related to the applied voltage. The generation of sub-pulse is expected to be a feature of positive corona discharge in SF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</inf> /N <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> gas mixtures entering the brush-pulse stage. With increasing the electric field non-uniformity, the onset voltages of two characteristic pulse stages, including the streamer-pulse stage and brush-pulse stage, decrease. Moreover, the first current pulse amplitude increases, and the pulse-repetition rate presents an approximate exponential increasing trend.

Dynamics of negative coronas in airflow

Recent numerical studies captured the effects of airflow on positive streamers, when subjected to a lateral wind. The positive streamers initiate from the trailing side of the anode and propagate with a clear tilting in the direction of the airflow. In this paper, we present the results of a simulation of two successive negative corona pulses and investigate their behaviour when exposed to a lateral wind. We use a three-dimensional numerical model that couples the electrical discharge dynamics with the airflow and allows the study of the discharge on electrons and ions timescales. The results show that negative coronas are not easily influenced by the airflow and tilt just slightly in the flow direction. Our simulation explains the physical mechanism that governs the formation and termination of negative corona pulses (Trichel pulses) and describes for the first time the proposed cathode-directed streamer mechanism for negative corona current pulses.

Effect of TiO 2 coating on the surface condition and corona characteristics of positive DC conductors with particle matters

Persistent severe smog and dust weather exists in many regions of China. Particles accumulate on the surface of high-voltage transmission lines, which affects the surface morphology of the transmission line and induces corona discharge. This work mainly studies the influence of electric field and photocatalysis on the corona characteristics of transmission lines. In this work, plasma spraying is used to prepare a dense, smooth and excellent photocatalytic TiO2 coating on the surface of transmission wires. An electron microscope and a white light interference profilometer is used to study the surface condition of the samples. The variation in the surface condition of the samples is observed under different fouling, light time and electric field intensity. The corona characteristics of power transmission lines under different surface conditions are studied. The results show that light and applied electric field promote the photocatalytic effect of TiO2, which helps to decompose the contamination on the surface of power transmission wires and thus reduce the corona discharge of transmission lines. The increase in corona inception voltage reduces the corona current pulse repetition rate and the ion current density during corona discharge.

Observations of corona point discharges from grounded rods under thunderstorms

We investigated features of corona point discharges when a grounded conductive rod is subject to a high background electric field. In the laboratory, high voltage is applied to an upper plate in a plane-to-point experiment, and the discharges are observed from a grounded rod using an ultraviolet camera and a photosensor. The current is measured using a shunt resistor, a current transformer, and a high impedance ammeter. We have found that corona current pulses have fast rise time (tens of nanoseconds) and slow decay (hundreds of nanoseconds). For the initial stage of the discharges, above a certain threshold, the frequency of pulses increases with the voltage applied. In the field, one current sensor coupled to a grounded rod, 1.5 m above a roof, was installed in a site located at an altitude of 2525 m above sea level. The same pulses observed in the laboratory are recorded during periods of electric field enhancement caused by the movement of charged clouds or lightning activity near the sensor. For that specific installation, the frequency of pulses is correlated with the ambient electric field measured 250 m away. To the best knowledge of the authors, this is the first time in which such correlation is made for outdoor measurements, seeing that other works correlate the electric field with the average current flow. Pulses of positive corona were no longer observed when the electric field magnitude was lower than 1.8 kV/m and pulses of negative corona were more atypical and presented a higher threshold, of about 3.8 kV/m. This study is relevant for understanding the production of corona and space charges in high structures, such as wind turbine blades, towers, and buildings in general.

Modeling the dependence of the negative corona current density on applied voltage rise time

In this paper, a numerical simulation is used to investigate the influence of applied voltage rise time on negative corona current characteristics in SF6 at atmospheric pressure. There were 23 particle species and 67 kinds of reactions considered in plasma chemical reactions. The influence of different rise times of the applied voltage is investigated. The spatial distributions of the radial electric field and total density of electrons and positive and negative ions, at and after the inception time of the corona current pulse, are used to explain the role of the rise time of the applied voltage on the mechanism of corona current pulse formation. It is found that the corona inception time, inception voltage, peak value of current pulse, number of sub-current pulses, and the time between the sub-pulses are strongly affected by the movement of negative ion clouds produced by the attachment. The quantitative analysis of the negative ions shows that the light ions preceded the heavy ions in their motion far away from the cathode and that this motion is governed by the rise time of the applied voltage.

HVAC Corona Current Characteristics and Audible Noise During Rain

Corona Audible Noise is a major factor regarding the public acceptance of overhead lines. In this article, 2-conductor bundle is used to study the influence of rain on AC corona discharge. Rainfall is simulated by a large rain dispenser, which can be adjusted over a wide range. Using this experimental setup, the transient AC audible noise and corona current pulse characteristics under rainfall condition were studied. It was found that there are three different stages of audible noise level during rain, in which the audible noise level during the steady state stage is the highest. This transient characteristic could be explained by two different types of corona discharges for discharges above and below the conductor. Furthermore, the steady state audible noise level was studied under a wide range of rain rates and applied surface electric fields. It was found that the sensitivity of the audible noise level to the rain rate decreases with the applied electric field. This finding is of vital importance as the rain intensity is regarded as one of the most important factors influencing the HVAC audible noise, and a higher rain rate is usually considered to cause a higher audible noise level. A possible explanation of this phenomenon is attributed to the effect of a “critical number of corona sources”, which is well known from DC corona and possible evidence is provided based on long-exposure photos.

Influence of freezing water conductivity on the positive corona performance of soft rime ice-covered conductor

Corona performance of transmission lines in foul weather conditions, icing particularly, have attracted widely attentions recently. In this research, the influence of freezing water conductivity on positive corona performance of soft rime ice-covered conductor is studied in corona cage via energized artificial icing method. The most important finding of this research is that the maximum deviation of the corona inception voltage obtained by the compared three corona inception method could reach 14.7% in soft rime icing condition, and the quadratic law method is recommended. The formed soft rime is composed of the divergent dendritic layer and the ice layer. Based on the ultraviolet photon videos and the corona current pulses waveform, the corona development process of soft rime ice-covered conductor is divided into four stages: feeble discharge mode, positive onset streamer mode, positive glow mode, and positive pre-breakdown streamer mode. Moreover, with higher freezing water conductivity, the obtained icing weight and icing thickness are bigger, consequently, the corresponding corona loss and audible noise are also larger.

Correlation between audible noise and corona current generated by AC corona discharge in time and frequency domains

Corona-generated audible noise is induced by the collisions between space charges and air molecules. It has been proven that there is a close correlation between audible noise and corona current from DC corona discharge. Analysis on the correlation between audible noise and corona current can promote the cognition of the generation mechanism of corona discharge. In this paper, time-domain waveforms of AC corona-generated audible noise and corona current are measured simultaneously. The one-to-one relationship between sound pressure pulses and corona current pulses can be found and is used to remove the interferences from background noise. After the interferences are removed, the linear correlated relationships between sound pressure pulse amplitude and corona current pulse amplitude are obtained through statistical analysis. Besides, frequency components at the harmonics of power frequency (50 Hz) can be found both in the frequency spectrums of audible noise and corona current through frequency analysis. Furthermore, the self-correlation relationships between harmonic components below 400 Hz with the 50 Hz component are analyzed for audible noise and corona current and corresponding empirical formulas are proposed to calculate the harmonic components based on the 50 Hz component. Finally, based on the AC corona discharge process and generation mechanism of audible noise and corona current, the correlation between audible noise and corona current in time domain and frequency domain are interpreted qualitatively. Besides, with the aid of analytical expressions of periodic square waves, sound pressure pulses, and corona current pulses, the modulation effects from the AC voltage on the pulse trains are used to interpret the generation of the harmonic components of audible noise and corona current.

The detailed characteristics of positive corona current pulses in the line-to-plane electrodes

The corona current pulses generated by corona discharge are the sources of the radio interference from transmission lines and the detailed characteristics of the corona current pulses from conductor should be investigated in order to reveal their generation mechanism. In this paper, the line-to-plane electrodes are designed to measure and analyze the characteristics of corona current pulses from positive corona discharges. The influences of inter-electrode gap and line diameters on the detail characteristics of corona current pulses, such as pulse amplitude, rise time, duration time and repetition frequency, are carefully analyzed. The obtained results show that the pulse amplitude and the repetition frequency increase with the diameter of line electrode when the electric fields on the surface of line electrodes are same. With the increase of inter-electrode gap, the pulse amplitude and the repetition frequency first decrease and then turn to be stable, while the rise time first increases and finally turns to be stable. The distributions of electric field and space charges under the line electrodes are calculated, and the influences of inter-electrode gap and line electrode diameter on the experimental results are qualitatively explained.

Measurement and analysis of time-domain characteristics of corona-generated radio interference from a single positive corona source

The corona-generated electromagnetic interference commonly known as radio interference (RI) has become a limiting factor for the design of high voltage direct current transmission lines. In this paper, a time-domain measurement system is developed to measure the time-domain characteristics of corona-generated RI from a single corona source under a positive corona source. In the experiments, the corona current pulses are synchronously measured through coupling capacitors. The one-to-one relationship between the corona current pulse and measured RI voltage pulse is observed. The statistical characteristics of pulse parameters are analyzed, and the correlations between the corona current pulse and RI voltage pulse in the time-domain and frequency-domain are analyzed. Depending on the measured corona current pulses, the time-domain waveform of corona-generated RI is calculated on the basis of the propagation model of corona current on the conductor, the dipolar model for electric field calculation, and the antenna model for inducing voltage calculation. The well matched results between measured and simulated waveforms of RI voltage can show the validity of the measurement and calculation method presented in this paper, which also further show the close correlation between corona current and corona-generated RI.

On the critical charge required for positive leader inception in long air gaps

The amount of the electric charge injected by the streamer corona bursts during the stage of leader inception determines the energy deposited to thermalize the corona stem into a leader segment. This paper is aimed at investigating the critical charge required for positive leader inception in air by using a thermo-hydrodynamic model with a detailed kinetic scheme. In order to simplify the analysis and to speed up the simulation, a reduced kinetic scheme for air is proposed. Numerical comparisons show that the reduced scheme can obtain almost the same results as the previous comprehensive kinetic scheme but with only half of the number of species and reactions. The thermo-hydrodynamic model with the reduced kinetics is then used to solve the radial dynamics of a single stem heated by current pulses typical of streamer corona bursts. The critical charge necessary for the direct transition of a first streamer corona into a leader under electrodes with large curvature radius is estimated between 0.08 and 0.5 µC per stem. Furthermore, the simulation shows that the gas heating of corona stem formed from electrodes with small curvature radius is mainly determined by the total accumulated charge injected by previous streamer corona bursts and the length of the dark periods in between the current pulses. The shape and the number of the corona current pulses in the discharge also play a role and their effects are discussed. It is suggested that the transition into a leader is triggered when a secondary streamer burst is initiated after the gas temperature is increased by the heating of previous streamers to about 1200 K. In addition, it is found that the heating produced by the charge injected by previous streamer corona bursts can be neglected if the dark period to the next burst is larger than few hundreds of µs for a corona stem with moderate initial stem radius. This indicates that the critical charge criterion obtained from laboratory experiments does not hold to evaluate the inception of positive leaders under conditions when long dark periods are present.

Dynamic corona characteristics of water droplets on charged conductor surface

The formation of the Taylor cone of a water droplet on the surface of the conductor in a line-ground electrode system is captured using a high-speed camera, while the corona current is synchronously measured using a current measurement system. Repeated Taylor cone deformation is observed, yielding regular groupings of corona current pulses. The underlying mechanism of this deformation is studied and the correlation between corona discharge characteristics and cone deformation is investigated. Depending on the applied voltage and rate of water supply, the Taylor cone may be stable or unstable and has a significant influence on the characteristics of the corona currents. If the rate of water supply is large enough, the Taylor cone tends to be unstable and generates corona-current pulses of numerous induced current pulses with low amplitudes. In consequence, this difference suggests that large rainfall results in simultaneously lower radio interference and higher corona loss.

Comparative study of two different measuring methods for corona current pulses

Based on a designed coaxial cylindrical corona discharge platform, this paper compares two methods for the measurement of corona current pulse, namely from cage side and conductor side both at low potential. It is found that the corona pulse can be obtained from both methods and the waveform of the current pulse is highly dependent on the sampling resistor. Then two simplified models are proposed to interpret the experimental results and the relationship between the two measurement methods. It shows that the key point in accurately measuring the corona current pulse is the selection of suitable sampling resistor.

Influence of AC voltage on the positive DC corona current pulses in a wire-cylinder gap View Document

In the development of hybrid HVDC and HVAC transmission lines, the study of radio interference is an important issue. Positive corona current pulses from high voltage transmission lines are the main source of radio interference. In this paper, the design of a wire-cylinder gap electrode system is presented to study the influence of AC voltage on the characteristics of positive corona current pulses. The study shows that the mode of the current pulses is different from that of either DC or AC corona discharge. Waveform parameters of the pulses, such as rise time, half wave time, duration time, repetition rates, average amplitude, and time intervals of secondary pulses are all statistically analyzed in this study. The empirical formulas for the repetition rates with different AC voltages are presented. A theoretical explanation based on an ion cloud model is given to reveal the mechanism behind the influence of AC voltage on positive corona discharge. The experimental results could provide some references for the prediction of radio interference from hybrid AC/DC transmission lines.