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Hollow porcelain insulators in substations are frequently confronted with rain flashovers under extreme rainfall. This study aims to investigate the spatial and temporal variation of electric fields between hollow porcelain insulator sheds affected by dynamic deformation of pendant water drops and effects of the dimensionless number of fluids, the Weber number (We) and the electric Bond number (BoE), on it for influencing factors' analysis. Flow‐electric field coupling simulations were carried out to compute the magnitude and the position of AC electric fields between the sheds. The results show that the maximum electric field at a point in time (Etmax) increases significantly after the breakup of the pendant water drop, and its position alters accordingly. For low We and BoE, the global maximum electric field (Egmax) increases with increasing We and BoE, respectively. It is much closer to the adjacent sheds and occurs later than Etmax at the breakup of the pendant water drop (Ebmax). By contrast, Egmax decreases in different degrees at high We and BoE, respectively. There is little difference between Egmax and Ebmax in the position and the occurring time. The influence mechanism on the maximum electric field and discharges and the relationship between discharges induced by the pendant water drop and rain flashover are discussed.

The flashover performance of insulators can be improved by BS (booster sheds) in the rain, which is mainly attributed to the reasons that BS break up long cascades of water and block connections of arcs. However, surface rainwater characteristics and arc characteristics of bushing have not been quantitatively studied under heavy rainfall. In this article, the artificial rain tests were conducted on a 500 kV transformer high-voltage bushing equipped with and without BS under the rainfall intensity of 10 mm/min. X (the total length of water column) and Larc (the critical length of arc) on the bushing surface were taken as the feature parameters of surface rainwater characteristics and arc characteristics, respectively. The effects of BS on Eh (the rain flashover voltage gradient along the insulation height), X and Larc were investigated, respectively. Furtherly, the relationships were studied among Eh, X and Larc. Results indicate that Eh has a sharp rise as the number of BS (NBS) is from one to two, however the rise of Eh gradually decreases when NBS exceeds two. X decreases while Larc increases with the rise of NBS, however both the change ranges of them continually fall. Furthermore, Larc presents remarkable negative correlation to X because of the effect of the electric field. Eh rises nonlinearly with the decrease of X, which is due to the change of the wetting uniformity on the bushing surface and the potential redistribution along air gaps in the presence of the local arc.

Rain is one of the dominant meteorological factors threatening the outdoor insulation performance of ultra-high voltage (UHV) power stations. Discharge occurring on the polluted surface of post insulators in rain has always been a major concern of power stations. Previous studies have shown that suspended water droplets on the shed edge play an important role in the rain flashover process. In this work, artificial contamination rain flashover tests were carried out in a laboratory on a UHV DC (Ultra-High Voltage Direct Current) large-diameter composite post insulator, which had a rod diameter of 625 mm and alternating sheds (105 mm and 75 mm for larger and smaller shed overhang, respectively). The discharge mechanism was analyzed base on the observation of discharge phenomenon of suspended water-drops on the shed edge. Moreover, simulation models by COMSOL Multiphysics were established to investigate the electric field around sheds and suspended water-drops, as well as the shape change of water droplets on the insulation surface, especially at the edge of the shed. Results show that the shape parameters of water-droplets changed continuously under the combined action of gravity, surface tension, and capillary tension. Suspended water-droplets on the shed edge showed a great influence on the electric field distribution, and the resulting discharge lead to the bridging between sheds. This work paves a new way to revealing the contamination rain flashover mechanism on post insulators and provides critical knowledge for power stations on preventing flashover accidents.

When it comes to safety accidents in power system, pollution flashover often causes more economic loss because of its large impact area and long impact time. To understand the contamination characteristics of insulators and predict the state of pollution, a typical insulator's contamination model in wind tunnel was established with Multiphysics simulation software, and a feature particle tracking method was proposed based on sample survey and feature particle analysis. The unitary pollution characteristics of insulator can be judged from the weighted average tracking results of sample particles with single particle size and charge amount. Based on this foundation, the effects of AC and DC electric fields were taken into consideration to analyze the influence of different kind of electric field on insulator contamination. The simulation results show that DC electric field has a significant effect on insulator fouling under the condition of weak wind and large particle size.

The electric field distribution around the insulator is the main influencing factor of its flashover characteristics. And the electric field distribution of the insulator is affected by the external tower structure and connecting parts. Therefore, there are differences in electric field distribution of the insulators with different tower structures. And the insulator flashover characteristics vary depending on the tower structure and the suspension position of the insulators. In order to study the effect of tower structure on the electric field distribution around the insulator and its effect on flashover voltage, based on COMSOL Multiphysics, the finite element simulations on the electric field distributions around the composite three-phase insulators suspended on the umbrella-shape the drum-shape towers were conducted in this paper. The formation of the electric field distribution around the insulator suspended in the tower and the influence of the tower structure on its electric field distribution were analyzed. In addition, the flashover characteristics test of the composite insulator in the $110\mathrm {k}\mathrm {V}$ transmission line at high altitude area was carried out. The test was under a standard lightning impulse voltage of $1.2/50\mu \mathrm {s}$. Combined with the test results, the relationship between the 50% impulse discharge voltage of the three-phase insulators suspended in the tower and the uneven coefficient of electric field is further analyzed and verified.

The installation of grading rings for composite insulators on high voltage transmission lines was an effective measure to improve the distribution of electric field and prevent brittle fracture of the core bar. In this paper, the flashover characteristics of insulators with and without the installation of the grading ring on the 110 kV voltage grade cat head tower were tested and simulated. Firstly, The U 50 , volt-time characteristics and discharge images of the insulators are studied by experiments. The experimental results show that the unit length U 50 of the insulator with the grading ring will increase compared to the insulators without the grading, and the volt-time characteristic position is basically unchanged while the starting point of the discharge channel is transferred from the fitting of insulators to the grading ring and the number of leading channel branches increase. Secondly, the electric field distribution around the insulator before and after the installation of the grading ring is studied by simulation. The simulation results show that the maximum field strength around the insulator decreases when the grading ring is installed, while the unevenness coefficient decreases and the average electric field strength increases. The simulation results agrees well with the test data. The research results can be used as a reference for the optimization design of the voltage equalization loop and the lightning protection of high altitude transmission lines.

In recent years, the phenomenon of electrolytic corrosion on porcelain insulators’ iron caps has occurred in many HVDC transmission lines in China such as the Yunguang, Xiangshang, Tianguang, Gaozhao and Xingan HVDC lines. The validity of using U-shaped zinc rings to restrain iron caps’ electrolytic corrosion is tested and verified in this paper. In order to research the influence of U-shaped zinc rings on the electric field intensity of insulators, Finite Element Method (FEM) simulation software was used to calculate the surface electric field of insulators with and without a zinc ring. The influence of zinc ring size on the inhibitory effect was studied by accelerated electrolytic corrosion tests, and the reason that zinc rings of the same size exert different inhibitory effects on insulators of different tonnage is explained in terms of iron caps’ curvature radii and water pocket characteristics. Test results indicate that the installation of U-shaped zinc rings is useful to suppress iron caps’ electrolytic corrosion, and that the inhibitory effect of zinc rings of the same size on insulators with larger cap curvature radii is better than that on smaller ones. Optimal designs of the structure and size of the U-shaped zinc ring and its service life are proposed in this paper, based on the test results and the actual engineering practice.

Converter transformers play a critical role in HVDC power transmission system. Space charge is a general concern for dielectric system under DC field. In this paper, the space charge behavior in a sandwiched oil-pressboard insulation system has been investigated using the pulsed electro-acoustic (PEA) method. The presence of homo-charges in the pressboards bulk and hetero-charges at the two interfaces between oil gap and pressboards leads to the electric field enhancement in the pressboards but reduction in the oil gap. The electric field enhancement is much larger than the value estimated by the Maxwell-Wagner polarization.

The combined influence of nitrogen impurities and radial electric field on dielectric barrier discharges in atmospheric helium is investigated using a two-dimensional (2D) fluid simulation. Discharge current waveforms, 2D electron densities, distributions of surface charge, and radial and axial components of the electric field at the electrode edge are calculated for different impurity levels varying from 0 to 30 ppm. It is observed that the discharge presents the characteristic of a column in pure helium, and it gradually becomes a relatively uniform glow discharge as the impurity level is increased to 20 ppm; for the higher impurity level of 30 ppm, the discharge adopts a concentric-ring pattern discharge. Our result shows that the radial electric field at the electrode edge is approximately 0.6–1.2 kV/cm during the discharge. This radial electric field has an effect that leads to a non-uniform discharge. After doping a low level of impurities, the Penning ionizations caused by the impurities can inhibi...