Sphere-plane Gaps Research Articles

Modification of streamer-to-leader transition model based on radial thermal expansion in the sphere-plane gap discharge at high altitude

Historically, streamer-to-leader transition studies mainly focused on the rod-plane gap and low altitude analysis, with limited attention paid to the sphere-plane gap at high altitude analysis. In this work, sphere-plane gap discharge tests were carried out under the gap distance of 5 m at the Qinghai Ultra High Voltage (UHV) test base at an altitude of 2200 m. The experiments measured the physical parameters such as the discharge current, electric field intensity and instantaneous optical power. The duration of the dark period and the critical charge of streamer-to-leader transition were obtained at high altitude. Based on radial thermal expansion of the streamer stem, we established a modified streamer-to-leader transition model of the sphere-plane gap discharge at high altitude, and calculated the stem temperature, stem radii and the duration of streamer-to-leader transition. Compared with the measured duration of sphere-plane electrode discharge at an altitude of 2200 m, the error rate of the modified model was 0.94%, while the classical model was 6.97%, demonstrating the effectiveness of the modified model. From the comparisons and analysis, several suggestions are proposed to improve the numerical model for further quantitative investigations of the leader inception.

Breakdown characteristics and voltage calculations of large-size sphere-plane long air gaps

Breakdown characteristics and voltage calculations of large-size sphere-plane long air gaps

Experimental study on streamer-leader transition characteristics of sphere-plane gap in valve hall under positive switching impulse voltage

Since a large number of spherical shielding balls are adopted in valve hall, investigating discharge characteristics of sphere-plane gap is of great significance for insulation design in HVDC projects. In order to study the discharge physical process of sphere-plane gap, this paper conducts sphere-plane gap experiment with gap distance set to 1.0 m and 1.5 m and electrode diameter set to 0.2 m, 0.3 m and 0.4 m under positive switching impulse voltage. The experimental comparative analysis of the discharge process of streamer-leader transition, and the physical parameters such as leader velocity and the amount of injected charge are calculated. The results show that the streamer to leader transition process is accelerated as the spherical electrode diameter increases. The leader velocity of sphere-plane gap is almost consistent with that of rod-plane gap. And the charge injected into initial streamer is much high than that of rod-plane gap.

Surface charge distribution characteristics on insulating materials under negative corona in sphere-plane gaps

The accumulation of space charge on the insulation surface can affects the distribution of the total electric field. The surface charge distribution characteristics of dielectrics under negative corona have been studied using spherical plate gaps in this paper. In order to measure the surface potential of three insulating materials under the influence of a DC corona by vibrating electrostatic probe. The experiment tested the surface potential of silicone rubber circular specimen and two umbrella-type disks with different curvatures under negative corona in a spherical gap. Afterwards, a new inversion algorithm was proposed in order to convert the surface potential to a surface charge density. Potential spots were found on the surface of silicone rubber, and the degree of charge clustering of umbrella-type disks was positively correlated with curvature. This experiment will serve as a foundation for future research on the surface charge distribution characteristics of insulation in HVDC corona equipment. Meanwhile, it can also be applied to optimize the insulation design.

Photo electric properties during streamer-to-leader transition in a long positive sphere–plane gap

In this work, a combined photoelectric detecting system is established using devices such as a photomultiplier, an electric field sensor based on optoelectronic integration, and a high-speed camera. The system is used to measure the instantaneous optical power, spatial electric field strength, and optical image during discharge of a 3-m-long sphere–plane gap under standard switching surge voltage of positive polarity. The test results show that the initial streamer of the 3-m-long sphere–plane gap can be divided into strong and weak ionization according to the rate of rise of optical power and electric field strength. Within the dark discharge period in the sphere–plane air gap, the intensity of the spatial electric field slightly jumps when the streamer-to-leader transition is completed, with an increase of about 162.59 kV/m. It is proposed to take the time when the intensity of the spatial electric field jumps in the dark discharge period as the criterion for the streamer-to-leader transition. The measured duration ta of the streamer-to-leader transition is calculated according to instantaneous optical power and spatial electric field strength. Based on the thermodynamic equilibrium equation, a Gallimberti simulation model for this streamer-to-leader transition is established. The duration ts for completing the streamer-to-leader transition is obtained according to the quantity Q of injected electric charges. By comparing the measured duration ta with the calculated duration ts of the streamer-to-leader transition, the rationality of the criterion is verified. The research results provide a basis for establishing an improved model for the inception of a leader in a long air gap.

Influence of protrusions on the positive switching impulse breakdown voltage of sphere‐plane air gaps in high‐altitude areas

The construction of ultra-high-voltage direct-current power transmission projects in high-altitude areas calls for the research into the discharge characteristics of the air gaps in the valve hall of a power converter station in such areas. Tests on the switching impulse discharge characteristics of the sphere-plane air gap were conducted at Yangbajing test base (Tibet, China) at an altitude of 4300 m on conditions that there were burrs on the 1.3 m diameter shield balls. The discharge characteristics, under the influences of burrs with different lengths and locations, were obtained and compared with test data from low-altitude areas. The results indicate that the arrangements of burrs with different lengths and locations can significantly reduce the switching impulse discharge voltage of sphere-plane air gaps, while the voltage sag gradually decreased down with increasing burr length. Moreover, the larger the sphere-plane air gap is, the smaller the influence of such burrs have. High altitude also substantially decreased the switching impulse discharge voltage of the air gap. When the surface of shield balls was smooth, the U50 saturation of the air gap was more significant than at low altitude; however, if there were burrs on the shield balls, the saturation effect was less significant.

Experimental Study of Visual Corona under Aeronautic Pressure Conditions Using Low-Cost Imaging Sensors.

Visual corona tests have been broadly applied for identifying the critical corona points of diverse high-voltage devices, although other approaches based on partial discharge or radio interference voltage measurements are also widely applied to detect corona activity. Nevertheless, these two techniques must be applied in screened laboratories, which are scarce and expensive, require sophisticated instrumentation, and typically do not allow location of the discharge points. This paper describes the detection of the visual corona and location of the critical corona points of a sphere-plane gap configurations under different pressure conditions ranging from 100 to 20 kPa, covering the pressures typically found in aeronautic environments. The corona detection is made with a low-cost CMOS imaging sensor from both the visible and ultraviolet (UV) spectrum, which allows detection of the discharge points and their locations, thus significantly reducing the complexity and costs of the instrumentation required while preserving the sensitivity and accuracy of the measurements. The approach proposed in this paper can be applied in aerospace applications to prevent the arc tracking phenomenon, which can lead to catastrophic consequences since there is not a clear protection solution, due to the low levels of leakage current involved in the pre-arc phenomenon.

Study on the influence of rainfall on the electric field distribution and switching impulse voltage of large‐size sphere–plane gap

Study on the influence of rainfall on the electric field distribution and switching impulse voltage of large‐size sphere–plane gap

Comparative Study of AC and Positive and Negative DC Visual Corona for Sphere-Plane Gaps in Atmospheric Air

Due to the expansion of high-voltage direct current (HVDC) power systems, manufacturers of high-voltage (HV) hardware for alternating current (ac) applications are focusing their efforts towards the HVDC market. Because of the historical preponderance of ac power systems, such manufacturers have a strong background in ac corona but they need to acquire more knowledge about direct current (dc) corona. Due to the complex nature of corona, experimental data is required to describe its behavior. This work performs an experimental comparative analysis between the inception of ac corona and positive and negative dc corona. First, the sphere-plane air gap is analyzed from experimental data, and the corona inception voltages for different geometries are measured in a high-voltage laboratory. Next, the surface electric field strength is determined from finite element method simulations, since it provides valuable information about corona inception conditions. The experimental data obtained are fitted to an equation based on Peek’s law, which allows determining the equivalence between the visual corona surface electric field strength for ac and dc supply. Finally, additional experimental results performed on substation connectors are presented to further validate the previous results by means of commercial high-voltage hardware. The results presented in this paper could be especially valuable for high-voltage hardware manufacturers, since they allow determining the dc voltage and electric field values at which their ac products can withstand free of corona when operating in dc grids.

Influences of surface tips of a shield ball on the discharge characteristics of a long sphere‐plane air gap under positive switching impulses

The study established a test platform for a long sphere-plane air gap in the test hall of the ultra-high voltage (UHV) direct current(DC) Testing Site. Under the sphere-plane gaps with 3, 5 and 7 m distance, the study carried out the experiment on discharge characteristics under switching impulses on a shield ball on which surface tips had different lengths at different locations. By comparing with the test data obtained under conditions with a smooth surface of the shield ball and the same gap distance, the discharge laws under the influence of surface tips on the shield ball are shown as follows: the presence of the tips greatly decreased the discharge voltage of the sphere-plane gap with the maximum decrease amplitude of 39.0%. Moreover, the decrease degree was increasingly significant with the growing length of tips. The influence of the same length of tips on the discharge voltage of the gap gradually declined with the growth of the gap distance. The presence of tips significantly reduced the discharge voltage, especially in the perpendicular line of the shield ball. The results indicate that it's necessary to strictly control the machining and installation to avoid the occurrence of tips in UHV DC engineering.

Feature extraction of electric field distribution and its application in discharge voltage prediction of large sphere-plane air gaps

The electric field calculations cannot effectively guide the insulation design of high voltage equipment since the insulation strengths of dielectrics cannot be predicted according to the electromagnetic analysis. In this paper, some features related to the electric field distribution such as the electric field strength, electric field gradient, path length, and the electric field integral are defined and extracted from finite element calculations of air gaps. The electric field features are taken as the input parameters of a grey correlative model established by support vector machine (SVM), so as to establish the multi-dimensional nonlinear relationships between these features and the discharge voltage of an air gap. The grid search (GS) method was used to search the optimal parameters of the SVM. This model was applied to predict the 50% discharge voltages of large sphere-plane air gaps in UHVDC valve halls. The predicted results were obtained after multiple iterative calculations, using the golden section search method. Compared to experimental data, the relative errors of the predicted results are within an acceptable range in view of engineering applications, which indicates the validity of the proposed method. This paper achieves the discharge voltage of air gaps by numerical computations instead of costly experiments.

Electric Field Distribution and Switching Impulse Discharge under Shield Ball Surface Scratch Defect in an UHVDC Hall

The dimension and surface state of shielding fittings in ultra high voltage direct current (UHVDC) converter station valve halls have a great influence on their surface electric field and switching impulse characteristics, which are important parameters confirming the air gap distance in the valve hall. The characteristics of impulse discharge under different lengths, dent degrees and burrs around the scratches of Φ1.3 m shield balls with a 2 m sphere-plane gap length were tested, in the UHVDC testing base of the Hebei Electric Power Research Institute. The discharge characteristics under the influence of the surface scratches of the shield ball were obtained. The results demonstrate that the discharge voltage of sphere-plane gap decreases obviously when there are unpolished scratches on the surface of the shield ball. However, when the scratches are polished, the discharge voltage has no significant impact. At the same time, a 1:1 full-scale impulse test model was established based on the finite element method. The electric field intensity and the space electric field distribution of the shield ball were obtained under the influence of scratches with or without burrs. The results of the simulation show that when the surface of the shield ball is smooth, the electric field distribution around it is even. The electric field intensity on the surface of the shield ball increases obviously when there are burrs around the scratches. When there is no burr around the scratches, the length and depth of the scratches have no obvious effect on its electric field distribution. Meanwhile, calculation results are consistent with test results. The results can provide an important basis for the design and optimization of shielding fittings, and technical support for its localization.

Prediction model of voltage–time characteristics for SF 6 long gap under VFTO and lightning impulse voltage

Voltage–time (V–t) characteristics of the SF6 long gap under very fast transient overvoltage (VFTO) draw wide attention at the high-voltage transmission systems. In this research, a generating system of VFTO and lightning impulse (LI) up to 2.5 MV was set up. The V–t characteristics for SF6 gaps with hundreds of millimetres length under negative LI and VFTO were researched. Experimental results indicated the rate of rising of V–t curve for the sphere-plane gap under VFTO was moderate, and V–t curve under LI was rapid, resulting in the intersection of the two types of V–t curves. However, for the rod-plane gap, the V–t curve under VFTO was below that under LI at 0.5 MPa. The shape of V–t curve had many relationships with discharge propagation velocity in the SF6 gap, which was determined by the gap distance, field non-uniformity, gas pressure and applied voltages. Based on analysis of discharge process and physical background of V–t curve, the semi-empirical formulae were derived and the prediction model was established. It was demonstrated the predicted V–t curves could accurately predict breakdown characteristics of long SF6 gas gaps

Hybrid prediction of the power frequency breakdown voltage of short air gaps based on orthogonal design and support vector machine

This paper attempts to establish the grey correlation between the breakdown voltage of air gap and its energy storage status before discharge inception. A new method is presented for the breakdown voltage prediction of short air gaps. This method is based on an orthogonal design array and support vector machine (SVM). A hybrid model is established to predict the power frequency breakdown voltage of three typical air gaps including sphere-sphere, rod-plane, and rod-rod gaps. The input data are fifty parameters extracted from the static electric field distribution of the air gap, and the output of the model is whether the gap will breakdown under a given voltage. This model is trained by 9 samples selected by L 9 (33) orthogonal array and applied to predict the breakdown voltage of 90 test samples. With effective feature dimension reduction and parameter optimization of the SVM model, the predicted results coincide well with the experimental data, and the mean absolute percentage error (MAPE) is only 3.9%. The SVM model is also applied to predict the breakdown voltage of some atypical short air gaps including sphere-plane gaps, rod-sphere gaps, sphere-sphere gaps with different diameters, and rod-plane-sphere gaps. The MAPEs are respectively 4.3, 3.2, 4.2, and 6.4%, which verifies the validity and generalization performance of the proposed method. This model provides a new way to gain the critical breakdown voltage of air gaps, and therefore contributes to reducing the required test work.

A prediction method for breakdown voltage of typical air gaps based on electric field features and support vector machine

Breakdown voltage of the air gap is of vital importance for the design of the external insulation in high-voltage transmission and transformation projects. In this paper, a new prediction method for the breakdown voltages of typical air gaps based on the electric field features and support vector machine (SVM) was proposed. According to the finite element calculation results of static electric field distribution, the electric field values in the whole region, discharge channel, surface of the electrode and the shortest path were extracted and post-processed, which constituted the electric field features characterizing the gap structure. Then, the breakdown voltage prediction model of the air gap was established by using electric field features as the input parameters to SVM, and whether the gap breakdown would happen as the output parameters of SVM, which changing the regression problem to a binary classification problem. This model was applied to predict the power frequency breakdown voltages of different short air gaps including sphere-sphere gaps, rod-plane gaps, sphere-plane gaps and sphereplane- sphere gaps. The power frequency breakdown voltages of longer air gaps which are affected by corona, and the 50% positive switching impulse breakdown voltages of long sphere-plane gaps and rod-plane gaps were predicted as well. The predicted results agree well with experimental values and simulated results of the published models, which validate the effectiveness of the proposed model. This method supplies a new possible way to obtain the breakdown voltage of air gaps.

Effective light concentration in gold short nanosphere chain on platinum mirror for surface-enhanced Raman scattering

We studied closely spaced gold nanosphere chains on Pt mirrors exhibiting strong plasmon coupling between both horizontally and vertically oriented modes relative to the chain. Resonance modes originating from the plasmon couplings realized effective light concentration around the short nanosphere chain and showed red shifts with decreasing interparticle gap length, revealing the hybrid nature of the two plasmonic modes. Thanks to the effective light concentration, the short nanosphere chain demonstrated strong surface enhanced Raman scattering (SERS) that was not strongly affected by variations in the length of the gaps or when some of the spheres made contact with each other. Even with large gaps, the short nanosphere chain exhibited consistent SERS under a low excitation power of only 0.4 mW/μm2, owing to the geometrical robustness of the nanospheres and Pt plane supporting enhancement of the electric field in the sphere-plane gaps.

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.

Insulation characteristics of 1100 kV GIS under very fast transient overvoltage and lightning impulse

Very fast transient overvoltage (VFTO) in gas insulated switchgear (GIS) is of great concern at the higher rated voltages because the difference between the rated lightning impulse (LI) and the VFTO withstand voltage decreases. In order to clarify VFTO withstand level of 1100 kV GIS, a simulation system was designed for generation of VFTO. The breakdown characteristics of sphere-plane gap and rod-plane gap in SF6 under VFTO and LI were investigated for different gas pressures. Experimental results show that the breakdown voltages in slightly inhomogeneous field gaps under VFTO are higher than that under LI. However, for the rod-plane gaps, the breakdown voltages and voltage-time curves under VFTO fall below that of LI at higher pressure in the case of negative polarity. The polarity effect of rod-plane gaps under LI is reversed at a certain pressure. Based on the analyses, it is concluded that the different breakdown characteristics of SF6 gaps may be attribute to the difference between space charge behaviors during the discharge development under the impulse with different wave front and oscillation tail of VFTO and LI.

The impulse dielectric behavior of N2 gas in sphere-plane gap

The focus of this work has been on the pre-breakdown phenomena and the breakdown characteristics of N2 gas in a sphere-plane gap under various impulse voltages. Both electrical and optical experimental investigation methods were used. Following parameters were considered: gas pressure range from 0.2 to 0.6 MPa, electric field utilization factor of the electrode configuration 71%, positive and negative impulse waveforms with the rise time of 500 ns, 1.2 μs and 180 μs. The observed discharge processes before the breakdown through the light emission images by the ICCD camera are in good agreement with the streamer mechanism. Under both polarity stresses, discharges are initially concentrated around the tip of the sphere and later pointing towards the earth electrode. However, negative streamers are thinner and more diffuse. As expected, the breakdown voltages for negative polarity are lower than those for positive polarity regardless of the gas pressure and shape of the applied impulse voltage. The breakdown voltage is increased with shortening the rise time of pulse waveforms. As a substitute for SF6, N2 gas under pressures above 0.3 MPa can reach the standard rated withstand voltage for 24 kV C-GIS.

Design Tool for Liquid-Nitrogen Gaps in Superconducting Apparatus

For designers of high temperature superconducting equipment with liquid nitrogen as a dielectric, an expedient universal curve is sought that provides breakdown strength for a specified class of electrode shapes, with any practical sizes of electrodes and gap; thus the universal curve fills in missing experimental data. Universal breakdown strength curves at pressures of or slightly above 100 kPa, are being developed for AC, DC or impulse stress for the class with sphere-sphere, plane-plane and sphere-plane gaps, with three independent parameters: the size of each electrode and gap. A user can normalize his parameters and find the corresponding breakdown strength, even though no data were available for his exact dimensions. For AC and DC stresses the geometrical effects of stressed area/ volume are incorporated from most published AC and DC experimental data of the last 50 years, by plotting breakdown field versus new geometrical quantities, such that all data fall approximately on or near one normalized universal curve. This avoids the usual difficult task of calculating stressed area and volume effects on the breakdown values for the graph ordinate. For impulse stress a more traditional plot suffices to produce a universal curve. This suggests that area/volume effects might not be so important with impulse stress. If the method proves reliable, it may be possible to determine design parameters for a broad range of geometries, help unify seemingly disparate breakdown data in the literature, and provide easily used, practical guidance for designers.