Positive Switching Impulse Research Articles

Simulation of breakdown voltage associated with phase-to-phase air gap for ultra high voltage transmission lines under switching impulse voltage

Abstract Knowledge of the breakdown voltage of air gaps when subjected to positive switching impulse voltage is fundamental to the design of insulation coordination for power transmission systems. However, the length of phase-to-phase air gaps in ultra-high voltage power transmission lines has reached more than 30 m. It is very difficult, costly, and impossible to conduct full-scale test experiments to obtain the breakdown voltage of these gaps. Here, the numerical simulation method based on the self-consistent leader inception and propagation model was first used to simulate the air gap discharge process at four different scales and to obtain the gap breakdown voltage. The simulated data was compared with the measured data to verify the validity of the model. Then, the numerical simulation method was used to simulate the discharge process of phase-to-phase air gap for ultra-high voltage transmission lines with 30.3 m in length. The current and the development trajectory of the leader tip were obtained for the discharge process. More importantly, the breakdown voltage, which could not be obtained by full-scale testing, was obtained by simulation.

An Analysis of Positive Switching Impulse Voltage and negative streamer growth in point-sphere gap towards Valve Hall

An Analysis of Positive Switching Impulse Voltage and negative streamer growth in point-sphere gap towards Valve Hall

Experimental Investigation on the Formation Mechanism of the Lowest Breakdown Voltage Area in a Long Air Gap Containing a Floating Conductor Under Positive Switching Impulses

Experimental Investigation on the Formation Mechanism of the Lowest Breakdown Voltage Area in a Long Air Gap Containing a Floating Conductor Under Positive Switching Impulses

Discharge characterization and altitude correction of shielding sphere-plate gap at an altitude of 3400 m

Southeast Tibet in China is rich in clean energy such as water energy, light energy, and wind energy. As the key means of large-scale clean energy transmission in southeast Tibet, the ultra-high voltage (UHV) power transmission projects will be constructed at an altitude of nearly 4000 m. To accurately acquire the external insulation strength of electrical equipment operating at different altitudes, the standard positive switching impulse voltages were applied to a rod electrode and a 1.1 m diameter shielding sphere at the altitude of 3400, 2100, and 80 m. The switching impulse discharge characterization and altitude correction factors of the 1.1 m diameter shielding sphere-plate gap under different clearances at different altitudes were obtained. The discharge voltage of the rod-plate gap at the altitude of 80 m was corrected to the altitude of 3400 m using altitude correction methods recommended by GB 311.1, GB/T 16927.1, and DL/T 2305. Further comparison was made between the calculation results using the three altitude correction methods mentioned above and the test results at an altitude of 3400 m. The discharge characterization and the minimum clearance of the end fittings of the electrical equipment in the valve hall at the altitude of 3800 and 4500 m were obtained by fitting and extrapolating the test data. The research results provide important reference significance for the external insulation configuration of UHV converter stations constructed in areas with an altitude of 3000 m and above.

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.

Influence of Rain on the Support Insulation Electrical Strength

The article presents the results of studying the influence of artificial rain on the electrical strength of an extra high voltage support insulation column containing insulators of different heights when subjected to positive polarity switching impulses. It has been found that, in general, rain has an adverse effect on the support insulation discharge characteristics, which is associated with the occurrence of cascading overlaps from the screen fittings along the insulation structure components. The adverse effect caused by rain tends to aggravate with increasing the column upper element structural height, the switching impulse front time, and rainwater conductivity. As a result, the support insulation electrical strength may drop by as much as 40%. The reason for such a significant effect is the external electric field redistribution along the columns. On the one hand, the presence of a relatively high conductivity water film on the insulator surface when subjected to impulses with a front time of several milliseconds leads to an almost uniform distribution of voltage across insulation structural elements. On the other hand, sharp field strength spikes of the opposite polarity arise on the water droplets and trickles formed at the edges of neighboring sheds, which facilitates the occurrence of spark overlaps across a number of shed spacing distances, weakening of air insulation, and a decrease of structural discharge voltage. Measures to increase the support insulation electrical strength under rain conditions are proposed.

A computational discharge model for sphere–plane long air gap under switching impulse voltage

There are lots of sphere–plane air gaps in valve halls in extra-high-voltage and ultra-high-voltage converter stations. Accurate prediction for discharge characteristics of sphere–plane gaps is of great significance for the selection of shielding structure and determining the dielectric strength of the valve hall. In this paper, based on the physical process of corona inception and continuous leader inception, a computational model for calculating the discharge voltage of a sphere–plane air gap under positive switching impulse voltage is proposed, and the leader characteristics and discharge voltage are analyzed. Then, the switching impulse discharge test of the sphere–plane gap with 0.15, 2, and 0.3 m radius spherical electrodes is carried out to verify the correctness of the model. The results show that the discharge voltage calculated by the proposed method is consistent with the test results, and the error is within 7.3%. The leader inception voltage and inception time increase with the increasing spherical electrode radius at the same gap distance, and an identical spherical electrode require higher leader inception voltage and faster leader inception time with the increasing gap distance.

Multiple discharges before leader inception in long air gaps under positive switching impulses

Multiple discharges before leader inception in long air gaps under positive switching impulses

Experimental Investigation of Discharge Path Selectivity of a Long Air Gap Containing a Floating Conductor

Live-line maintenance is an effective means to ensure the reliability of the power system. When linemen float between the high-voltage conductors and the ground potential towers, the discharge characteristics of air gaps are significantly affected. In this article, simplified experiments are carried out to simulate the linemen by a floating conductor. The discharge characteristics of air gaps when the floating conductor is in different positions are studied. An experimental platform with a long air gap containing a floating conductor is used to obtain the breakdown voltage and the discharge path of air gaps under a positive switching impulse. The experimental results show that the existence of a floating electrode affects the air gap breakdown voltage and the discharge path. When the transverse displacement is 80 cm, almost all the discharge paths directly connect the high voltage electrode and plane electrode. Furthermore, the influencing factors of discharge path and breakdown voltage are analyzed, and the fitting equations of breakdown voltage and discharge path probability considering spatial position coordinates are established. Finally, the easy breakdown region considering discharge path is proposed, which provides a theoretical reference for quantitative analysis of the path of linemen entering high potential safely, and to improve the working efficiency of live-line maintenance.

Surface Dielectric Characteristics of GFRP and PTFE in Cryogenic Environment Under the Switching Impulse Superimposed on DC Voltage

For insulation materials utilized in DC superconducting fault current limiter, surface discharge along the interface is more damaging to the insulation relative to penetration breakdown. Therefore, for the reliable and economical insulation design of DC superconducting fault current limiter, it is very important to investigate an insulation materials and study the surface dielectric characteristics. Additionally, DC superimposed switching impulse can cause critical stress to the insulation of DC superconducting fault current limiter and can potentially result in breakdown. Therefore, the insulation materials of DC superconducting fault current limiter must have sufficient surface distance to prevent surface discharge under DC superimposed switching impulse. In this paper, the surface breakdown tests were performed by applying DC superimposed switching impulse to glass fiber reinforced polymer (GFRP) and polytetrafluoroethylene (PTFE) specimen in cryogenic environment. Surface dielectric characteristics according to positive and negative polarities of DC and switching impulse were analyzed. In addition, the surface dielectric characteristics were analyzed by selecting the gap distance. From the experimental results, the surface dielectric strength of GFRP was higher than that of PTFE, but the difference in surface dielectric strength between GFRP and PTFE decreased as the gap distance increased. The surface breakdown voltage of PTFE was lowest at the superposition of positive DC and positive switching impulse. However, GFRP has the lowest surface breakdown voltage when negative DC superimposed positive switching impulse. Consequently, it is possible to design the optimal surface distance of solid insulation materials in DC superconducting current limiter by considering the superposition of DC and switching impulse polarity.

Assessment of the Effect the Height and Relative Position of Support Insulation Components Have on the Insulation Electrical Strength

The article presents the results from studies of the effect the height and relative position of extra high voltage class support insulation components have on the insulation electrical strength when subjected to positive polarity switching impulses, at which the insulation minimum permissible overall dimensions are determined. It is shown that the highest values of the insulation flashover voltage are achieved when a leader discharge develops along purely air trajectories: screen--pedestal or screen-- plane. A similar result is obtained in the case of cascade flashovers from the screen over the column elements if the upper insulator structural height is significantly larger than that of the lower one. The reverse mutual arrangement of these insulators leads to a significant drop in the insulation flashover voltage during cascade flashovers. It is shown that at the final jump onset moment, the main contribution to the insulation flashover voltage value is introduced by the voltage drop across positive streamers, which is directly proportional to the length of these streamers. Hence it follows that the ambiguous effect of the height and mutual arrangement of the support insulation components on the insulation electrical strength during cascade flashovers is mainly due a significant difference in the lengths of the positive streamer zones at the final jump onset. With the upper insulator height equal to 2.7 m, the length of positive streamers at this moment is commensurable with the length of streamers when the flashover develops along purely air trajectories, whereas decreasing the upper insulator height by almost a half entails a significant decrease in the streamer lengths. Measures to eliminate undesirable cascade flashovers and ensure the highest electrical strength of support insulation are proposed.

Intelligent computing and analysis of breakdown voltage of rod-rod long air gap

The discharge characteristics of air gap is one of the important factors to determine the insulation level which is crucial for electrical engineering construction. In order to calculate the 50% breakdown voltage of rod-rod air gap under different meteorological conditions and different gap distances, we take the positive standard switching impulse test data of rod-rod air gap under different meteorological conditions as samples. Based on training set, a breakdown voltage prediction model of rod-rod air gap under different meteorological conditions was established by cuckoo search optimized weighted support vector regression (CS-ω-SVR). The results show that the average absolute percentage error of the test set is 3.6%, which verifies the validity of this computing method. The method can provide reference for the computation of air gap breakdown voltage under different altitudes and extreme meteorological conditions.

Effect of a floating conductor on discharge characteristics of a long air gap under switching impulse

Live working is an important means for the maintenance of transmission lines. During the process of entering the equipotential, the live working lineman is a floating conductor, which affects the discharge characteristics of the air gap of the transmission line. In order to improve the safety protection of linemen, the effect of a floating conductor on discharge characteristics of a long air gap under positive switching impulse was investigated. In this paper, an experiment platform of long air gaps with floating conductors was built, and the breakdown voltage and discharge progress were studied. The experimental results showed the effect of the rod-shaped electrodes and the electrodes with rod-shaped protrusions on the breakdown voltage of the air gaps, as well as the discharge process of the air gaps when the floating electrodes were at different positions. The research conclusions can provide a theoretical basis for discharge mechanism and live working.

Intelligent computing of positive switching impulse breakdown voltage of rod-plane air gap based on extremely randomized trees algorithm

The insulating strength of an air gap is an important basis for external insulation design of power transmission and transformation project, and both gap structure and meteorological conditions have a great influence on the breakdown voltage of air gap. The temperature, humidity and other meteorological conditions in the actual operating environment of power transmission and transformation projects may change greatly with time, so presently, there is no better method to calculate the breakdown voltage of air gaps in various meteorological conditions. In this paper, a prediction model for positive switching impulse breakdown voltage of rod-plane air gap based on extremely randomized trees is proposed. All samples are divided into training set and test set by Self-Organizing Mapping neural network and stratified sampling method. In the test set, the predicted breakdown voltage results of rod-plane air gap based on decision trees, random forest, extremely randomized trees and support vector machine are compared, with reference to the rectification results of atmospheric correction method. The result shows that the maximum absolute relative error of the model is 4.3% and the mean absolute percentage error is 1.6%. Then, the importance of features of import variables is calculated with the trained extremely randomized trees model. Finally, the breakdown voltage of rod-plane air gap in correspondence to different gap distances in four extreme meteorological conditions is calculated with the model. The method can provide reference for calculation of the breakdown voltage of air gap in various meteorological conditions.

Computation of breakdown voltage of long rod-plane air gaps in large temperature and humidity range under positive standard switching impulse voltage

The 50% breakdown voltage of the air gap is an important basis for the design of the external insulation of the high voltage power systems, and the atmospheric conditions have a large impact on the breakdown voltage of the air gap. The temperature and humidity of the operating environment of the high voltage power systems have a wide range. The breakdown voltages of the gaps obtained in the laboratory are mostly near the standard atmospheric conditions, and there is no good way to calculate the discharge voltage outside the standard atmospheric conditions. This paper established a 50% positive switching impulse breakdown voltage prediction model based on the cuckoo search algorithm optimized support vector regression for the rod-plane air gap. This calculation model is used to predict the selected prediction data. The predicted results are basically consistent with the experimental results, and the effect is better than the atmospheric correction results. Finally, the method was used to calculate the 50% positive impulse breakdown voltage of the rod-plane air gap under the extreme temperature and humidity conditions in Beijing. This method provides a new idea for obtaining the air gap breakdown voltage under different atmospheric conditions.

Experimental Assessment on Air Clearance of Multiple Valve Unit Considering Switching Impulse and DC Superimposed Switching Impulse

Multiple valve unit (MVU), which converts AC to DC and DC to AC, is one of the key elements of high voltage DC (HVDC) transmission. Therefore, the insulation design of MVU against overvoltage should be considered for the stable and reliable operation of HVDC transmission system. Especially, the air clearance of MVU should be calculated based the switching impulse, since it is fatal to MVU in terms of electrical insulation. However, the previous studies were limited to wave front, and the air clearance of the switching impulse is specified only for an ultra-high voltage (UHV) above 750 kV. As a result, it is difficult to calculate the air clearance of MVU which must endure for a switching impulse under 750 kV. In addition, when the switching impulse introduced while the MVU is in normal operation, it is superimposed to DC and creates the most severe situation, but the studies on such subjects are also insufficient. Therefore, as a fundamental step to calculate the air clearance of MVU, the dielectric characteristics of switching impulse and DC superimposed switching impulse in air have been investigated. The experiments on switching impulse showed that the critical flashover voltage was varied according to the curvature of electrode in the gap distance, up to eight times of the electrode radius. However, beyond that gap distance, the critical flashover voltage became similar, regardless of the radius of electrodes. In case of the superimposed experiment, it was performed according to DC pre-stress level and the polarities of switching impulse. The results were most severe when the positive switching impulse was superimposed on the positive DC, and the peak voltage at which flashover occurs was independent of DC pre-stress.

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 on the final jump phase in long air gap discharges

The final jump is an essential process of long air gap discharge. This paper will focus on the experimental study on the characteristics of the final jump stage under positive switching and lightning impulses in the long air gaps. To demonstrate the propagation of the leader, a high-speed camera (FASTCAM SAX2) was used to record. The lightning impulse (1.9/50 μs) that was produced by a 7.2 MV Marx generator applied to the 3 m or 5 m air gaps. By looking at the results for the gaps as mentioned above; the observation results showed that the average final jump length under lightning impulse is 1.06 m and 1.91 m, respectively. The average velocity during the final jump phase under positive switching and lightning impulses is obtaining from the observation images by fitting the measurements. According to the discharge images, the method to calculate the length and velocity during the final jump stage. Finally, the current under the lightning impulse voltage is thousands of amperes as compared to switching impulse.

On the stem diameter under positive impulses in long air gaps

The diameter of a stem, a common origin point for streamers where the transition from the streamer to the leader takes place, has a significant effect on the simulation of streamer-to-leader transition. To measure the diameter of the stem, a schlieren system was established with the temporal and spatial resolutions of 5 μs and 70 μm, respectively. Experiments were conducted under positive lightning and switching impulses; the applied voltage, discharge current, and schlieren images were recorded synchronously. A method was proposed to measure the diameter of the stem based on the schlieren images, with which the stem diameter as a function of time was obtained. Moreover, the initial diameters of the stem under different conditions were obtained through the linear fitting. At last, a relation between the initial cross-sectional area of the stem and the discharge current was given within a certain range.

Influence of front time on positive switching impulse discharge characteristics of UHVDC tower gaps

The front time of switching impulses has a significant influence on long air gap discharge performance, but most testing experiments have been conducted using standard impulses. Here, we carried out experiments investigating the discharge characteristics of a ±800 kV transmission line-tower gap (5–8 m) under positive switching impulses with different front times and obtained important physical parameters, such as the time and voltage of the leader inception, the leader propagation velocity, and the 50% breakdown voltage (U50). The results show that (1) the critical wave front time increases with the air gap distance; (2) for the tested gaps, the time and voltage of the leader inception show minimum dispersion when the front time is 250 μs; (3) an approximately U-shaped curve occurs between the front time and the average velocity of the continuous leader propagation, and the velocity is the slowest with a front time of 500 μs. The results illustrated that choosing standard switching impulses (250 μs/2500 μs) to conduct a discharge test in a ±800 kV transmission line-tower gap is both suitable and effective; however, when the operation voltage further increases, the front time should be increased appropriately when conducting switching impulse tests.