Surface Flashover Phenomena and Contributory Parameters in Oil‐Pressboard Composite Insulation System Under DC Voltage

The safe operation of converter transformer, whose major insulating structures is comprised of oil-pressboard composite insulation, could be the important guarantee for the reliable power transmission of HVDC system. Due to the presence of DC voltage operating condition, the interface charge accumulation could fail the validity of RC (Resistivity-capacitance) simulation model in the insulation design and optimization. Thus, there has been arising the research focus for the electric field and interface charge characteristics of oil-pressboard insulation structures. Kerr electro-optic effect method, benefiting from the non-contacting laser measuring approach, has been widely adopted for the electric field measurement. However, the common experiment platforms by using Kerr electro-optic effect are mainly based on single-point photodetector indicating. The present paper developed a high-sensitivity photodetector array to accomplish the synchronous and regional measurement for electric field distribution in oil-pressboard insulation with a parallel-plain electrode under DC voltage. The electric fields in oil show a quite uneven distribution characteristics when voltage is applied, and the maximum deviation from different measuring photodiodes could reach 16.6%. With the time prolonging, the electric field in oil tended to decrease, but there witnessed different field strength reduction in different positions in oil, the maximum rate could reach 41.1% with a saturation time of 2960s, while the minimum one is 32.1% and 2530s, respectively. Such witnessed great differences in both distribution and time-dependent characteristics of electric field in different measuring photodiodes were explained by the non-uniformity and anisotropic of resistivity of pressboard. Hopefully, the experiment results in this paper could provide meaningful technical supports for the insulation design of converter transformer.

The distortion of the electric field in the oil–pressboard composite insulation caused by the accumulation of the interface charge is detrimental to both the insulation design and operation of converter transformers. The influence of moisture content on the surface charge accumulation of oil–pressboard insulation under DC voltage was studied in this study. In accordance with the Kerr electro‐optic effect, the electric field strengths in transformer oil and the surface charge density were acquired after applying the positive and negative DC voltages in three oil–pressboard insulation models with different moisture content, respectively. The resistivities of the oil and pressboard in three models, namely Model 1# with 3.8–4.2 ppm moisture in oil and 0.35–0.37% moisture in pressboard, Model 2# with 7.6–7.9 ppm moisture in oil and 0.79–0.82% moisture in pressboard and Model 3# with 14.9–15.4 ppm moisture in oil and 1.39–1.42% moisture in pressboard, was also measured. The results indicate that: (i) as negative charges in oil accumulated on the pressboard surface in a much greater speed than the positive ones, the electric field in transformer oil under negative DC voltage decreases more rapidly with time than that under positive DC voltage; (ii) the increase of the moisture content in both oil and pressboard, under either positive or negative DC voltage, leads to the decrease of both the electric field strength in transformer oil and the charge density with time; and (iii) the increase of moisture content could not only decrease the resistivity of both oil and pressboard, but also the ratio of the resistivity between the pressboard and the oil. On the basis of the Maxwell–Wagner theory, the decrease of the ratio between the pressboard and oil could lead to the decrease of the interfacial charge density, leading to the slow transient process of the electric field in transformer oil under DC voltage.

Accurate measurement and effective analysis of the space electric field and charge characteristics within the oil–pressboard insulation structure of converter transformer are essential for achieving precise insulation structure design. Presently, most electric field measurement methods relying on the Kerr electro‐optical effect are limited to single‐point measurements. Given the complexity of converter transformer insulation structures, including their large scale and the use of various materials, existing technologies and findings struggle to provide comprehensive electric field observations within the oil–pressboard region. After leveraging the Kerr electro‐optic effect, a high‐precision 32‐unit photodetector array has been developed to achieve regional measurements of spatial‐temporal electric field in oil–pressboard insulation. The array boasts a measurement spatial resolution of 1.4 mm2 and a sensitivity of 0.15 kV/mm, ensuring measurement accuracy exceeding 96.50%. Through practical measurements of the spatial electric field distribution within the oil–pressboard insulation under direct current voltage, the non‐uniform distribution of the electric field is effectively captured in oil. Notably, the maximum deviation in field strength at different positions within the transformer oil can reach 18.5%. Moreover, as the applied voltage increases, the unevenness coefficient of the field strength gradually rises, peaking at 1.19, which signifies a progressive expansion in the area affected by electric field distortion. By conducting tests to assess the dispersion of volume resistivity in samples from various positions within large sheets of pressboard, the variation in volume resistivity of materials is posited as one of the contributing factors to the non‐uniform distribution of electric field within the oil. The detailed accomplishments aim to provide essential technical and theoretical support for optimising insulation structure design of converter transformers.

Ambient temperature has significant influence on electrical property of oil-paper insulation, which makes the surface flashover process complex. This paper aims at revealing the influence mechanism of ambient temperature on surface flashover behavior under DC voltage. In the experiments, double layer oil-papers with thickness of 0.08 mm were used, the ambient temperature was varied from 20 to $80^{\circ}\mathrm {C}$ and the amplitude of DC voltage was 3.5 kV. The results of surface potential decay (SPD) measurement show that with the increase of temperature, the initial surface potential declines and the process of SPD accelerates. The results of surface flashover indicate that it is prone to occurring surface flashover at higher ambient temperature. It is concluded that more carriers escape from deep traps to participate the procedure of surface discharge at higher temperature, which declines the surface flashover voltage.

The surface charges that accumulate on polypropylene (PP) film can induce electrical field distortion and surface flashover, which accelerate the electrical aging of a capacitor. This paper investigated the effect of combined voltage on the surface charge behaviors and flashover voltage and a modification method to inhibit the accumulation of surface charge. The carrier mobility in different conditions was calculated using a surface potential decay process to analyze the transportation of surface charges. Samples that were fluorinated for 15, 30, 45 and 60 min and unfluorinated samples were prepared prior to experiments. The samples were corona charged with a pair of needle-plate electrodes by DC voltage and a combination of DC voltage and pulse voltage. The flashover experiments were performed under DC voltage, pulse voltage and superimposed voltage in atmospheric air. The results indicate that the maximum carrier mobility and maximum flashover voltage can be obtained when the samples are surface modified for 45 min. The surface charge density decreases with an increase in the carrier mobility under DC voltage. Under a combination of DC voltage and pulse voltage, the initial surface charge density is influenced by both the carrier mobility and the pulse voltage. The results suggest that surface modification for a certain time can significantly inhibit the surface charge accumulation, improve the carrier mobility and increase the flashover voltage.

It is very important to understand the mechanism of surface flashover characteristics of the insulator materials in vacuum in order to avoid the surface discharge of sensitive components. There are two models with dominant roles and highly reorganization in academic circle. One is secondary electron emission avalanche model (SEEA) and the other is electron triggered polarity relaxation model (EPTR). However these models are still not understood clearly and there is no model which can completely explain all the phenomenon of surface flashover in vacuum, especially the reverse surface flashover phenomenon. In this paper, DC surface flashover experiments of insulator are studied in vacuum to reveal the reverse surface flashover mechanism. Results showed that there is a phenomenon in which a reverse flashover current sometimes occurs in the surface flashover process. The reverse flashover current pulse may occur in both the initial pre-flashover phase and the flashover breakdown phase, that is, when the flashover current pulse is positive, the surface flashover voltage is negative. The experiments results can be explained as follows. The electrons injection phenomenon near the cathode and holes injection phenomenon simultaneously near the anode are formed in the high electric field intensity at the triple junction (cathode end, vacuum and dielectric surface, CTJ). The electron-hole pairs migrate to the anode and cathode in the applied electric field intensity, respectively. Some of the carriers in the migration are captured by the trap center, forming the positive and negative charge trapping center. The region between the trap center and the electrodes will form an additive electric field. The negative charge trapping center weaken the local region electric field between cathode and negative charge, forming a reverse electric field to limit further injection of charge. At the same time, the trapping center enhanced the additive electric field between the anode and the trapping center. The region of the space charge electric field formed by the center of the negative charge is completely opposite to the applied electric field. When the negative charge trapping center value is sufficiently deep, the reverse additive electric field intensity will reach a critical of flashover breakdown value in the region. The partial discharge usually cannot occur due to the weakening of the high external electric field intensity. However, the applied electric field intensity of the insulator is instantaneously reduced during the surface flashover breakdown. At the same time, the electrons in the trapping center are not detrapping, so the electric field intensity of the reverse in the cathode and trapping center will rise rapidly. As a consequence of the electric field intensity reaching the critical breakdown value, a reverse partial discharge is formed immediately. Unlike the usual case, the electrons will escape from trapping center and transfer towards cathode under the action of reverse electric field intensity in the process, forming a reverse surface flashover current pulse. The reverse flashover current pulse typically occurs at a time when the applied electric field intensity value is low in the flashover process. It is suggest that the traps will play a critical role in affecting the reverse surface flashover performance under DC voltage in vacuum.

Oil-paper insulation will inevitably endure impulse voltage with a superimposed dc voltage (superimposed voltage) because of introduced lightning or switching, which can induce charge accumulation and trigger flashover during the operation of a converter transformer. Temperature rise is also a nonnegligible factor that can significantly influence the performances of oil-paper insulation. This paper aims to reveal the effects of ambient temperature and superimposed voltage on surface charge and flashover behaviors in an oil-paper insulation system. Surface potential decay (SPD) experiments were performed under superimposed voltage with various polarity and amplitude combinations and at the temperature ranging from 20 °C to 80 °C. The results show that the initial surface potential and decay rate increase as the amplitude of the impulse voltage increases at 20 °C. The initial surface potential declines, and the process of SPD is accelerated at higher temperatures because of the increased concentration of shallow traps. Subsequently, flashover tests were conducted under superimposed voltage. It is found that flashover occurs after applying superimposed voltages even with a rather low dc voltage. In addition, the dc voltage component is primarily responsible for flashover. Weibull results also indicate that it is prone to trigger flashover at higher temperatures, because thermal excitation helps more carriers to escape from deep traps and participate in the process of flashover. Accordingly, the insulation characteristics of oil-paper system can be significantly influenced by superimposed voltage and temperature, which should draw great concern among the power grid.

In order to study the influence of temperature on surface flashover behavior with oil-paper interface under combined AC-DC voltage, a discharge test assembly was established to investigate the influence of temperature on developing process of surface flashover of oil-paper interface from 25 °C to 90 °C. Based on impulse current method, the process of surface flashover of oil-paper interface under combined AC-DC voltage was studied by electrodes configurations that simulated the tangential electric field and radial electric field of surface of pressboard, in which the voltage was increased step-by-step. Meanwhile the partial discharge (PD) spectrums at whole flashover stages were studied. Moreover, high speed camera was used to record the whole process of surface flashover which was divided into 4 stages: (1)Inception stage, (2)Streamer stage, (3)Intermittent breakdown stage, (4) Surface flashover stage. The experiments results shown that flashover voltage and PD inception voltage at high temperature (HT) was lower than the voltage at low temperature (LT). The average amplitudes of PD at LT were lower than at HT. The growth rate and repetition rate of PD at HT was faster than at LT. It was easier to develop the flashover at oil-paper interface under combined AC and DC voltage at HT. So the flashover voltage was lower at HT. the surface of pressboard was destructed severely at HT, the track of surface discharge was obvious at HT.

Low insulation strength at the oil–gas surface due to oil leakage and partial discharge of oil-immersed power equipment is a major threat to the safe and reliable operation of power systems. This paper investigates the initiation and development of the oil–gas surface discharge. The oil–gas surface discharge test platform was established, and discharge tests were carried out at different gap distances (1–2.5 mm). By coupling the electric field and flow field, the multi-layer dielectric discharge streamer model was built, and the characteristics of charge and electric field distribution at different gap distances were studied. The test results show that the liquid surface between the electrodes rises during the discharge process. Furthermore, the surface discharge voltage exceeds the air gap discharge voltage. With the simulation analysis, the oil–gas surface discharge is a typical streamer development process. Under 50 kV applied voltage and 2.5 mm gap distance, the average development speed of the streamer is 12.5 km/s. The larger the gap distance is, the greater the average streamer development speed is. The recording and numerical simulation of the discharge process are of great significance for exploring the mechanism of oil–gas surface discharge, optimizing the discharge process, and diagnosing partial discharges.

Changing the AC cable line to DC operation can make full use of the original transmission line corridor and improve the transmission capacity of the line. However, the electrical characteristics of insulating media in DC operation are different from those in AC operation. In order to study the electric field distribution and space charge accumulation characteristics of AC-AC-linked polyethylene (XLPE) cable under DC voltage, the temperature field, electric field and space charge characteristics of cable insulator under different temperature differences were studied based on bipolar carrier model. It is found that the electric field distribution and space charge accumulation characteristics in the cable insulation are greatly affected by the temperature gradient (temperature difference). After being pressurized for a long time, the electric field distribution in the insulator will be obviously reversed, that is, after AC voltage is changed to DC operation, there will be a serious distortion of the electric field on the outside of the insulating layer.

The design of oil-pressboard/paper insulation structure is the key to maintain the safe and reliable operation of power transformers, which mainly relies on RC model simulations and verification tests undertaken with simple or equivalent test models, neglecting the potential scale effect and falling short of the physical entity verification conducted in large-scale oil-pressboard insulation structures. Based on Kerr electro-optic effect, the electric field and interface charge characteristics in both small and large scale oil-pressboard insulation structures were obtained to present the impacting mechanism of scale effect. The coaxial large-scale model is based on a converter transformer outlet device entity, which has three layers of pressboards with the thickness of 1 mm to separate out three 5-mm-wide oil spacings, and the model has an inner diameter of 200 mm and a length of 500 mm. As for the small scale model with parallel-plate electrodes, it was scaling down at the same proportion with the large one on the basis of electric field strength equivalence principle. The experiment results indicated that: 1) in the small-scale model, as the size of pressboard increases, the difference between the measured field strength in oil and the RC model one would increase, which could be attributed to the more interface charge accumulation in larger scale model, namely the larger the scale of model, the stronger the weakening effect of more interface charge on electric field in oil; 2) The spatial-temporal distribution characteristics of electric field in the large-scale model are obtained showed that, in the inner and middle oil-spacing, the measured steady-state field strength is larger than that obtained by RC model, with a maximum increase of 77.3%, while in outer oil-spacing, it is smaller than the RC model calculating one, and the maximum decrease is 36.8%; 3) As for the large scale oil-pressboard test model with multi oil-spacings and multi interfaces, the calculation method for charge density on different interfaces was proposed to quantitatively present the impacting mechanism of scale effect on the electric field and interface charge characteristics.

Polypropylene (PP) film is extensively employed as the dielectric in film capacitor because of its advantages of low loss and high dielectric strength. Surface charge that accumulated on PP film can cause electrical filed distortion and induce surface flashover when under DC and pulse voltage, which accelerate electrical aging of a capacitor. This paper investigated the surface charge accumulation and surface flashover voltage under DC voltage and pulse voltage and a modification method to inhibit the accumulation of surface charge. Samples that were fluorinated for 15, 30, 45 and 60 min and original samples were prepared prior to the experiments. Samples were corona charged with a pair of needle-plate electrodes. The transportation of surface charges was analyzed by the calculation of the carrier mobility. The flashover experiments were performed under combination of DC voltage and pulse voltage in atmospheric air. The results indicate that the minimum surface charge accumulation and maximum flashover voltage can be obtained when the samples are surface modified for 45 min.

Polypropylene (PP) film is extensively employed as the dielectric in film capacitor because of its advantages of low loss and high dielectric strength. Surface charge that accumulated on PP film can cause electrical filed distortion and induce surface flashover when under DC and pulse voltage, which accelerate electrical aging of a capacitor. This paper investigated the surface charge accumulation and surface flashover voltage under DC voltage and pulse voltage and a modification method to inhibit the accumulation of surface charge. Samples that were fluorinated for 15, 30, 45 and 60 min and original samples were prepared prior to the experiments. Samples were corona charged with a pair of needle-plate electrodes. The transportation of surface charges was analyzed by the calculation of the carrier mobility. The flashover experiments were performed under combination of DC voltage and pulse voltage in atmospheric air. The results indicate that the minimum surface charge accumulation and maximum flashover voltage can be obtained when the samples are surface modified for 45 min.

Surface flashover is a crucial issue in the field of electrical insulation, and it involves many complex physical processes. In this paper, the development process of the surface flashover was studied from different methods. The initial process of positive surface streamers in different gas environments (air, N2, CO2) was studied by photoelectric observation. The evolution of positive surface streamers in the air was described based on a 2D fluid model. The influence of the surface trap energy level on flashover development and the relationship between gas adsorption and surface trap energy level was discussed by density functional theory calculation preliminarily. The results showed that the initial ionization process of surface flashover is considered as the collision ionization between the initial electrons and gas molecules and photoionization of high-energy photons. Some of the high-energy photons can not only ionize some gas molecules but also cause the surface of the insulator to emit electrons (photoemission process), which could promote the development of the streamer. Both the ionization of the gas molecules and the photoelectric emission of the insulator surface may determine the initial development of surface flashover. During the process of the flashover, the electron density of the surface streamer (∼1021 m−3) is high and the main streamer tends to develop towards the insulator surface. The attraction of surface streamers changes with the position of the initial electron, and the positive surface charge brings the stronger ionization process, and the negative surface charge has the opposite effect. The band gap of the insulator surface is affected by the adsorption of gas molecules, which is considered as introducing shallow traps on the surface. For impulse voltage, the charge accumulation and internal charge migration may be not evident, the initial photoionization process and initial surface charge distribution affect the flashover process primarily.

In order to study the breakdown characteristics of oil-paper insulation under compound electric field, the typical oil-pressboard insulation model was adopted according to the insulation structure in converter transformer. The electric field in oil and oil-impregnated pressboard was simulated according to the equivalent circuit for the above model, and the breakdown characteristic test for typical oil-pressboard insulation, oil and oil-impregnated pressboard was carried out separately under compound electric field. To analyze the relationship between the breakdown strength and electric field, the electric field expressions for oil gap and the oil-impregnated pressboard were derived under compound voltage with different AC content. The test result shows that breakdown voltage of oil-pressboard insulation rises first and falls later under compound voltage. According to the characteristics the electric field distribution and breakdown strength of oil gap and oil-impregnated pressboard, it can be found that the breakdown strength of oil-pressboard insulation mainly depends on electric field distribution and breakdown characteristics of oil and oil-impregnated pressboard under compound voltage.