Rod-plane Air Gap Research Articles

Breakdown mechanisms of rod-plane air gaps with a dielectric barrier subject to lightning impulse stress

The complexity of gas-insulated substations makes it difficult to predict withstand voltages. Modeling interaction between dielectric surfaces and electrical discharges is a key challenge. In this study, 60 mm rod-plane air gaps with a dielectric barrier 20 mm below the rod are stressed with lightning impulses of both polarities. The discharge mechanisms are investigated with a high-speed camera, a photomultiplier tube and a current measurement system. The discharge development and current-velocity relationship is leader-like. With positive polarity applied, a leader propagates from the upper parts of the rod to ground. Negative impulses are characterized by positive leader development from the ground plane to the rod. For both polarities, the discharge starts with streamers propagating from the rod to the barrier. Positive streamers typically reach the opposite electrode without causing breakdown directly. The findings imply that empirical breakdown prediction models for short air gaps should involve conditions for positive leader initiation and development. The results also show that dielectric barriers increase the breakdown voltage by impeding leader development. The barriers increase the shortest discharge path and shift the point of leader inception further up on the rod.

Effect of earth discontinued to the electrical field distribution in rod-plane air gaps under lightning impulse

This paper, will present some results of investigations carried out on electric field distribution on the surface of a discontinuous plane of short rod-plane air gap arrangement. For this purpose, we have used different results obtained by an experimental model and a numerical method for computation used the finite elements methods (FEM). The effect of interface on the field stress has also been studied, As a result, the interface locally reinforces the electric field, and then the electric field increases at the interface may lead to a discharge between the high voltage rode and the interface. In the vicinity of the interface, we have observed a kind of discontinuity in the evolution of the electric field intensity. This latter becomes greater than the value obtained in the case of gaps with homogeneous plane earth. The electric field distribution, obtained by means of the computational procedure, is in good agreement with the experimental results.

Study on the streamer inception characteristics under positive lightning impulse voltage

The streamer is the main process in an air gap discharge, and the inception characteristics of streamers have been widely applied in engineering. Streamer inception characteristics under DC voltage have been studied by many researchers, but the inception characteristics under impulse voltage, and particularly under lightning impulse voltage with a high voltage rise rate have rarely been studied. A measurement system based on integrated optoelectronic technology has been proposed in this paper, and the streamer inception characteristics in a 1-m-long rod-plane air gap that was energized by a positive lightning impulse voltage have been researched. We have also measured the streamer inception electric field using electrodes with different radii of curvature and different voltage rise rates. As a result, a modified empirical criterion for the streamer inception electric field that considers the voltage rise rate has been proposed, and the wide applicability of this criterion has been proved. Based on the streamer inception time-lag obtained, we determined that the field distribution obeys a Rayleigh distribution, which explains the change law of the streamer inception time-lag. The characteristic parameter of the Rayleigh distribution lies in the range from 0.6 to 2.5 when the radius of curvature of the electrode head is in the range from 0.5 cm to 2.5 cm and the voltage rise rate ranges from 80 kV/μs to 240kV/μs under positive lightning impulse voltage.

Breakdown in short rod-plane air gaps under positive lightning impulse stress

Prediction of withstand voltages in air-insulated systems are made on the basis of empirical models that are not sufficiently accurate for complex geometries. Better understanding of the spatiotemporal development of electrical discharges is necessary to improve the present models. Discharges in lightning impulse stressed 20–100 mm rod-plane gaps are examined using a highspeed camera, photo-multiplier tubes (PMTs) and a highbandwidth current measurement system. The images and measurements of gaps larger than 20mm show a fast initial streamer discharge with a current rise time of some tens of ns, followed by a dark period of a few μs and a propagation of a slower leader-type channel leading to breakdown. The breakdown mechanisms in the shortest gaps are faster and geometry dependent, probably occuring by heating of initial streamer channels. Different light filters used with the PMTs indicate that all parts of the leader-type discharge development emit light over a spectrum from UV to IR. The initial discharges emit low amounts of warm light and IR compared to the leader-type channel. Finally, it is suggested that empirical breakdown voltage prediction models should be interpreted in light of the leader-type breakdown mechanism.

Breakdown in short rod-plane air gaps under positive lightning impulse stress

<p>Prediction of withstand voltages in air-insulated systems are made on the basis of empirical models that are not sufficiently accurate for complex geometries. Better understanding of the spatiotemporal development of electrical discharges is necessary to improve the present models. Discharges in lightning impulse stressed 20–100mm rod-plane gaps are examined using a highspeed camera, photo-multiplier tubes (PMTs) and a highbandwidth current measurement system. The images and measurements of gaps larger than 20mm show a fast initial streamer discharge with a current rise time of some tens of ns, followed by a dark period of a few μs and a propagation of a slower leader-type channel leading to breakdown. The breakdown mechanisms in the shortest gaps are faster and geometry dependent, probably occuring by heating of initial streamer channels. Different light filters used with the PMTs indicate that all parts of the leader-type discharge development emit light over a spectrum from UV to IR. The initial discharges emit low amounts of warm light and IR compared to the leader-type channel. Finally, it is suggested that empirical breakdown voltage prediction models should be interpreted in light of the leader-type breakdown mechanism.</p>

Study on AC withstand characteristic of air gaps in switchgear at low air pressure condition

The bare conductor in switchgear constitutes complicated air gaps, and their AC withstand characteristic will obviously affect switchgear insulation performance. In this paper, AC withstand tests of various types of air gap were carried out. The air gap samples were rod-plane air gap and 4 typical types of air gap from the bus chamber of 40.5 kV metal-enclosed switchgear. The samples were tested in climate chamber where high altitude environment was simulated. And correction of withstand voltage for switchgear air gaps at low air pressure were conducted. Test results indicate that, the withstand characteristic of 4 typical types of air gap in switchgear are different from that of rod-plane air gap. The relationship between withstand voltage Uw and air gap distance d is nonlinear. The influencing characteristic exponent n of air pressure on Uw is related with air gap type. Linearity value m and air pressure index n are independent to some degree. An equation was proposed to express the relationship of Uw with air gap distance d and altitude H, and this equation can be used to calculate the safety distance of air gaps in switchgear under low air pressure situations. The results are significant for the design and selection of switch devices.

Energy storage features and a predictive model for switching impulse flashover voltages of long air gaps

It has been a long sought goal to determine the critical flashover voltages of long air gaps by mathematical calculations instead of experiments. In this paper, a predictive model is proposed for 50% discharge voltage (U50) prediction of long air gaps subjected to switching impulses. This model is developed on the basis of the physical thought that air breakdown can be viewed as out-of-limit of the capacitive energy stored in the electric field. Two groups of features are defined to characterize the energy storage status of an air gap, from the perspective of electric field distribution and impulse voltage waveform, respectively representing the space distribution and time accumulation of the energy. The predictive model is established by support vector machine (SVM), and the input data are the energy storage features. Giving an estimated applied voltage range, the predicted result of the critical flashover voltage is searched by the golden section method. Trained by 6 known experimental data, this model is used to predict the U50 of rod-plane, rod-rod and rod-conductor long air gaps subjected to negative switching impulse voltages. The mean absolute percentage errors of 5 groups of test samples are respectively 6.0%, 5.8%, 6.3%, 3.2% and 1.7%, which are in an acceptable range in the view of engineering applications. The proposed method might be useful to predict the U50 of long air gaps with various geometries and under different voltage waveshapes, therefore reducing the required test work for insulation design of high voltage equipments.

Discharge Simulation of Typical Air Gap Considering Dynamic Boundary and Charge Accumulation

The simulation of typical air-gap discharge process is of great significance to the long air-gap discharge mechanism and UHV transmission project design. A positive fractal dynamic streamer discharge model of a rod-plane air gap was established. On the basis of the original fractal streamer model, this new model proposed an electric field calculation method based on a dynamic boundary condition, as well as a new calculation method of the time parameters based on the charge accumulation process. In the model, the electric field and the development time of the discharge step were obtained by solving the electric field distribution and charge accumulation equations. The processes of streamer starting, streamer development, charge accumulation, and discharge extinguishing were all contained in the model. Discharge process of various voltage amplitudes (235 and 595 kV), the waves of which were lighting impulses (2/50 $\mu \text{s}$ ), was simulated by this model in the 1-m rod-plane air gap. As shown in the results, for a gap under 235 kV, the length of the streamer is 200 mm, the streamer development time is $5.02~\mu \text{s}$ , the velocity is $3.98 \times 10^{{\mathrm {4}}}$ m/s, and the total charge accumulation in the streamer channel is 23.2 $\mu \text{C}$ , while for a gap under 595 kV, the gap is broken down, and the streamer development time is $9.92~\mu \text{s}$ , its velocity is $1.01 \times 10^{{\mathrm {4}}}$ m/s, increasing to $1.60 \times 10^{{\mathrm {5}}}$ m/s before discharge, and the total charge accumulation is $258.3~\mu \text{C}$ . The model matched the test results well in the dynamic processes of the length of discharge channels, the electric field waveform, and charge accumulation. This model is meaningful to research into long air-gap streamer discharge mechanisms and leader discharge processes.

A physical approach of the test voltage function for evaluation of the impulse parameters in lightning impulse voltages with superimposed oscillations and overshoots

The introduction of the test voltage function or k-factor in IEC 60060-1 Ed. 3 has been an important step to ensure reproducibility and traceability in high voltage tests techniques for lightning impulses with superposed oscillations and overshoots. The standard recommends computed fitting methods to extract a base curve from the oscillating recorded voltage curve and, applying the test voltage function, it is possible to calculate the impulse parameters. Testing external insulations with positive polarity lightning impulses at 50% breakdown voltage or lower level, the disruptive discharge usually arrives after ten microseconds. For these cases the test voltage function and procedures for calculating impulse parameters proposed by the standard may lead to divergent results. In this paper a new approach is proposed for fitting the base curve for metric air gaps based on the relevance of the leader propagation phase, highlighting the influence of the instantaneous voltage between the impulse crest and the time-tobreakdown. A comparison of three calculation methods for test curves with long timeto- breakdown is discussed and a new test voltage function obtained by measurements on a 1 m rod-plane air gap is proposed. Compared to the standard test voltage factor, this approach evidences a negligible influence of oscillations and the relevancy of the impulse tail voltage for testing external insulations with moderately distorted lightning impulses. Finally, a generalization of the voltage test function is proposed for several types of insulation considering mainly the average breakdown times.

Prediction of DC Corona Onset Voltage for Rod-Plane Air Gaps by a Support Vector Machine**supported by National Natural Science Foundation of China (No. 51477120)

This paper proposes a new method to predict the corona onset voltage for a rod-plane air gap, based on the support vector machine (SVM). Because the SVM is not limited by the size, dimension and nonlinearity of the samples, this method can realize accurate prediction with few training data. Only electric field features are chosen as the input; no geometric parameter is included. Therefore, the experiment data of one kind of electrode can be used to predict the corona onset voltages of other electrodes with different sizes. With the experimental data obtained by ozone detection technology, and experimental data provided by the reference, the efficiency of the proposed method is validated. Accurate predicted results with an average relative less than 3% are obtained with only 6 experimental data.

Prediction study on positive DC corona onset voltage of rod‐plane air gaps and its application to the design of valve hall fittings

Corona inception can be viewed as the release of capacitive energy stored in the electric field. A new prediction method of the corona onset voltage based on electric field features and support vector machine (SVM) is proposed in this study. This method was used to predict the positive DC corona onset voltage of the rod-plane air gap. The predicted results were compared with the experimental values and the calculated values of the existing methods. Then, the corona onset voltage of the shielding sphere used in a valve hall of ±660 kV DC converter station was measured and predicted by the SVM model and the photoionisation model. The safe values of corona inception electric field strength were obtained. By comparing the finite element numerical calculation results of the electric field strength with the safe value, it was proved that the corona would not incept on the surface of the shielding sphere under operating voltage. This method provides a new possible way to predict corona onset voltages of valve hall fittings used in HVDC transmission projects.

Experimental study of leader tortuosity and velocity in long rod-plane air discharges

Long air gap electrical discharges are of particular interest among scientists and engineers working on high voltage techniques and lightning research. In the present work we report experimental results obtained while testing a long rod-plane air gap with positive switching-like voltage impulses to study the velocity and tortuous progression of the leader discharge. Voltage and current waveforms were recorded. Two still digital cameras were used to track the leader tortuous path. By using a fast digital camera, the leader temporal evolution was recorded and its propagation velocity was estimated. Three angles were used to describe the leader tortuous progression.

Characteristics of long air gap discharge current subjected to switching impulse

Measuring the pre-breakdown current of long sparks in air is important for investigating the discharge mechanism. Since the breakdown of long air gaps is conducted by a series of streamer-leader processes, the corresponding current signals cover a bandwidth of 0 to more than 20 MHz. Measurement accuracy of the current from the high voltage side is affected by the displacement current and impulse electromagnetic interference. In this paper, a coaxial current sensor with a DC bandwidth of 74.45 MHz is developed. A displacement current-restrained electrode structure is proposed to reduce the equivalent capacitance between the current sensor and the ground over 30 times. Combined with the digital optical fiber synchronous acquisition unit, a current measurement system for long air gap discharge is established. For the purpose of the UHV system's external insulation optimization design, the discharge current waveform of a 6 m rod-plane air gap under positive switching impulse voltage with 250 μs and 1000 μs time to crest is obtained. Discharge images and stressed voltage are combined to analyze the continuous feature of a current waveform under critical time to crest impulse and discontinuous feature under long front duration impulse. For the purposes of a lightning protection study, the current waveform of a 10 m rod-plane air gap is subjected to negative switching impulse. Finally, the pulse characteristics of the current corresponding to the single channel and branching stepped negative leader are discussed.

Positive switching impulse discharge performance of rod‐plane short air gap under rain conditions

In China external discharge accidents caused by heavy rain is growing. However, the rain effect is not considered in atmospheric correction for external insulation design in IEC 60071-1(Edition 8.1, 2011) or domestic standard. Hence, a series of experiments are implemented with positive switching impulse, a typical kind of overvoltage of power grid, to analyse the discharge performance of rod-plane short air gap under rain conditions. The effects of rain and its intensity, rain conductivity and ambient temperature on the discharge voltage are explored, and the theory behind them, which can help in the development of discharge mechanism of air gap under rain condition, are respectively given. The results show that under the experimental rain condition, the discharge voltage can be up to −10.9, −9, −8.5, −3.2 and −2.3% lower than under dry condition for clearance distance of 0.1, 0.2, 0.3, 0.4 and 0.5 m, respectively. Besides, it decreases with increase in rain intensity, rain conductivity and ambient temperature, respectively. Among them, the rain intensity is the most significant one, next is temperature, whereas rain conductivity is the minimum and trifling. Finally, a correction formula for breakdown voltage (positive switching impulse) of rod-plane air gap under rain condition is proposed for reference.

Electric Field Measurement in Rod-Discontinued Plane Air Gaps Using Distributed Capacity Probe

The present experimental carried out investigations aimed at elucidating the effect of earth discontinuity on the breakdown voltage of short rod-plane air gap (80 to160 mm) under negative lightning impulses (-1.2/50ms).We also carried out investigations on electric field measurement on the surface of a discontinuous plane of a rod-plane air gap arrangement. For this purpose, we used a probe with distributed capacity, under negative lightning applied impulse voltage. The probe is incorporated on the same level of plane surface.The interface locally reinforces the electric field. The electric field increases at the interface may lead to a discharge between the high voltage rode and the interface. In the vicinity of the interface, we observe a kind of discontinuity in the evolution of the electric field intensity. This one becomes greater than the value obtained in the case of gaps with homogeneous plane earth. Keywords :Breakdown voltage; electric field; distributed capacity.

Calculations of breakdown voltage of rod-plane air gaps in the presence of water streams

A breakdown model of rod-plane air gaps in the presence of water streams has been established. Water streams which do not fully cross air-gaps are considered. This model involves the breakup length and surface conductivity of a water stream, the stream’s diameter, the number of the water droplets in the remaining air gap and the distance between two adjacent water droplets. The breakdown voltage is considered in two parts: one being the voltage drop along the water stream surface, and the other the voltage drop across the remaining air gap. To study the two components of voltage drop, the equivalent surface conductivity of a water stream and the voltage-current characteristics between two water droplets were investigated. Additionally, theoretical and experimental results are compared and it has been found that under the conditions considered, the breakdown voltages are predictable to within 13%. Evidence regarding the impact of radial dispersion of droplets and their distortion on the breakdown process is considered.

Effect of water streams on the AC breakdown performance of short rod-plane air gaps

The ac breakdown performance of rod-plane air gaps in the presence of water streams has been explored. Experimental results show that the ac breakdown voltage of rod-plane short air gaps in the presence of a water stream is significantly lower than in the dry condition. In addition, the ac breakdown voltage of an air gap decreases with the increase of the water stream velocity, diameter and conductivity. The relative breakdown voltage of an air gap decreases with an increase of water stream velocity from 0.75 m/s to 1.79 m/s and nozzle diameter from 1.0 mm to 3.0 mm by 2.6% / 0.1 m/s and 0.8% / 0.1 mm, respectively. High speed images of the deformation of the water stream and the breakdown process of air gap are reported show that with the increase of the applied voltage, the critical break-up length and the size of main water droplets decrease. In addition, the effect of the electrical field on the break-up length of a water stream and the size of droplets are independent of the jet velocity.

Study on the effect of artificial radiation simulating terrestrial solar ultraviolet radiation on negative DC discharge performance of air gap

It is important to reveal the effect of terrestrial solar ultraviolet radiation (TSUVR) on discharge performance of external insulation for the design and operation of a power system in regions with strong TSUVR, especially in Tibet. However, no research was carried out in this field until now. In this study, an artificial UV lamp was used to imitate TSUVR in the relevant experiments to research on the effect of negative DC discharge. It was found that artificial radiation simulating terrestrial solar UV radiation (STSUVR) does not affect appreciably the negative DC corona characteristics and breakdown voltage of short rod-plane air gaps of several electrode materials. That is because the wavelength of the STSUVR is in the range of 290–400 nm, and its intensity is less than 200 W/m2. Thus, air, Cu and Fe cannot be photoionised by STSUVR. There is also no effect of the STSUVR on arc propagation because of energy. Although a part of the photons in the TSUVR has higher energy than the work function of Al, it is very easy for the Al to be oxidised, and the oxide film (Al2O3) can prevent Al from being ionised by TSUVR. Therefore the TSUVR does not have an obvious effect on negative DC discharge performance of air gap.

Semi-Empirical Calculation Method of the Positive First Corona Space Charge Under Different Impulse Rising Rates in Long Air Gaps

In this paper, the variation of the development of positive first corona space charge in long air gaps is investigated. A 1-m rod-plane air gap under different rising rates <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$dU/dt$</tex></formula> of impulse voltages is reported. The current signals and still photographs are recorded during the first corona development. The relationship between the first corona charge <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Q_{{\rm i}}$</tex></formula> and the inception voltage <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$U_{{\rm i}}$</tex></formula> is presented. The physical mechanism of the influence of the first corona is provided. With the experiments and the single branch propagation model, a semi-empirical calcultion method for positive first corona charge under impulse in long air gaps is obtained through the calculation of the number of branches <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$N_{{\rm b}}$</tex></formula> . Using this method, the variations between <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Q_{{\rm i}}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$U_{{\rm i}}$</tex></formula> for different <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$dU/dt$</tex></formula> and electric field distributions are calculated and compared with the reported experimental results. Reasonable agreement between the calculated and experimental results is obtained. It shows that this calculation method is suitable for predicting the positive first corona charge <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Q_{{\rm i}}$</tex></formula> and the discharge current at a given <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$dU/dt$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$U_{{\rm i}}$</tex></formula> .

Switching impulse breakdown characteristics of large sphere-plane air gaps compared with rod-plane air gap

With the increase of operation voltage to 800 kV-1100 kV, the sphere-plane gaps with radius of 0.5-1 meter are widely used in the converter station of UHVDC power transmission system. In order to ensure the safety of UHVDC system and reduce the investment, it is necessary to investigate the breakdown characteristic of large radius sphere-plane gap. This paper implemented experiments on three different gap configurations: rod-plane, sphere (R=0.55 m)-plane, and sphere (R=0.75 m)-plane. The 50% breakdown voltages and the current injected into the high voltage electrode were measured. Besides, the discharge process was observed by a high speed CCD camera. Three types of current waveforms related to different streamer progression characteristics were found. With the increase of sphere radius, it can be found that the streamer gradually becomes dominant in the discharge process, which leads to higher average breakdown field, longer streamer length and much more injected charge of first streamer.