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The moisture content has a great influence on the electric insulation strength of oil‐pressboard. Therefore, it is very important to assess the moisture content in power transformers. There is lack of research about the effect of DC electric field on moisture migration. Here, the authors analyse the moisture migration characteristics of oil‐pressboard under DC electric field. The DC electric field has a greater influence on the moisture content in the oil. Compared with no DC electric field, the moisture content in oil is reduced by 22.91 mg/L at 50°C, while it is reduced by 17.63 mg/L at 70°C. Under the same moisture content in the oil, the moisture content in the pressboard which affected by the DC electric field is higher than the Oommen curve, the maximum difference is 0.88% at 50°C, and 0.05% at 70°C. This result can provide a reference for the revision of existing moisture assessment methods.

The design of the oil–pressboard/paper insulation structure is the key to maintain the safe and reliable operation of power transformers, which mainly relies on simulation and verification test undertaken with simple or equivalent models, falling short of the physical entity verification undertaken in the large-scale structure. Based on the Kerr electro-optic effect and the optical fiber technology, a novel electric field measuring sensor was developed in this paper for the application test in the large-scale oil–pressboard structure. The sensor is contained of collimator, total reflection prism, coupler, and customized optical elements, whose appearance is a miniature capsule-shaped configuration. The key technical performance parameters of sensor were obtained, for instance, and the effect of imported sensor on the electric field under measurement is an average 1.17% reduction in intensity; the sensitivity of designed system is 162.84 V/mm under ac voltage and 40.43 V/mm under dc voltage. Under dc voltage, the measuring accuracy of the designed system is greater than 97.42%, while in the context of ac voltage, it could reach 97.14%. The application test was conducted to capture the dynamic process of the electric field in oil in the large-scale outlet device model, which has three uniformly spaced oil spacings divided by pressboards, under ac voltage, dc voltage, and polarity reversal voltage. The experimental results verify that the developed electric field sensor could be effectively and reliably applied in the large-scale oil–pressboard insulation, and it could be confidently inferred that the larger the scale of insulation structure, the better is the performance of sensor.

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 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.

Under AC, DC and switching impulse voltages, the surface charges accumulated on the basin insulators of Gas Insulated Substation (GIS) or Gas Insulated Metal Enclosed Transmission Line (GIL) could distort local electric field and even reduce the surface flashover voltage. In order to quantitatively measure the influence of surface charge accumulation on flashover voltage of insulator in actual GIS/GIL apparatus, a high-precision 3D surface charge measurement platform and a reasonable model were established for flashover tests in reference to the insulation design and flashover properties of actual 220 kV GIS basin insulator. The surface charge accumulation dynamics and corresponding flashover voltages of the built models under different conditions were captured and analyzed. The results indicate that, as the density of accumulated surface charges increases under either AC, DC or switching impulse voltage application, the surface flashover voltage would decrease from 62.1 kV to 55.7 kV, namely a margin of 10.3% under AC voltage application. In the context of DC voltage, there appears a 22.8% flashover voltage drop, from 61.0 kV to 47.1 kV. While for the case of switching impulse voltage, the charge accumulation reduced the flashover voltage to 52.9 kV with a 13.6% decline from 61.2 kV. Based on the simulation and theoretical calculations, the ‘enhanced electric field zone’ was identified to justify the influencing mechanism of surface charges on flashover voltage. The research results could contribute to optimal insulation design and fault diagnosis of basin insulator in GIS/GIL.

The electric field and space charge dynamics characteristics in oil under impulse voltage are key factors to the transformer insulation design, which are mainly obtained through simulation, falling short of experimental verifications. Applying the Kerr electro-optic method, the present paper carried out an actual measurement for the electric field characteristics in transformer oil under impulse voltage and managed to obtain their correlations with an electrode material, voltage amplitude, and wave-front time, respectively. First, it was found that, under impulse voltage and in pure transformer oil, the aluminum electrode had the largest amount of charge injection, followed by stainless steel and then copper, which was attributed to the different work functions of metal elements in electrodes. Second, with the increase of voltage amplitude, the effect of the space charge on the intermediate electric field of the oil gap was first enhanced, then weakened, and eventually enhanced. Third, as the wave-front time was prolonged, the peak of electric field in transformer oil showed a downward trend. For instance, the peak value of the electric field was reduced by 17.6 % when the wave-front time was increased from 0.5 μs to 40.0 μs. The reason is that when the wave-front time is longer, the amount of injected charge grows larger and the weakening effect becomes stronger. Fourth, a charge dynamic motion model in transformer oil under impulse voltage was established to demonstrate its spatial-temporal effects on the electric field. Besides, this paper has clarified the source of the negative and positive charges in the transformer oil under impulse voltage by using the theories of field emission and field ionization. Moreover, the variation of electron mobility over-voltage amplitude, wave-front time, and spatial distribution was worked out to present the quantitative results of its motion process.

The space electric field distribution in oil under impulse voltage is very important for the design of transformer insulation. The existing research only focused on the characteristics of the electric field at a certain point in oil-pressboard insulation structure. To in-depth study of the space electric field distribution in oil under impulse voltage changes with time and space, the measurement platform for the space electric field in oil under impulse voltage was built. The space electric field distribution in transformer oil under lightning impulse voltage was measured, and its changes with time and space were analysed. The results indicate that: As the applied voltage increases, the measured field strength begins to be lower than the applied, and the difference between the maximum measured and applied field strength increase. The space electric field distribution in oil in the wavefront time stage gradually becomes uniform with the increase of time. In the tail time stage, the weakening effect of the space charge on the electric field in oil gradually weakens with the increase of time and the applied field strength is first higher than the measured and then gradually lower than it.

The insulation state of the converter transformer is closely related to the properties of the oil-paper insulation material. The study of the partial discharge characteristics of the wrinkled of the insulation paper has practical engineering significance for the fault diagnosis of the converter transformer. According to the actual operating environment inside the converter transformer, a set of oil-paper-insulated partial discharge measurement platform under the action of AC and DC composite voltage was built. The test results show that insulation paper wrinkles under the action of AC/DC composite voltage will cause insulation damage due to higher surface electric field and partial discharge; the discharge pulse repetition rate and discharge volume increase in the initial stage, development stage and final dangerous stage of discharge. The partial discharge caused by paper wrinkled significantly increases the CH4 content in the dissolved gas in the oil. This conclusion provides a certain basis for the realization of converter transformer fault diagnosis and condition monitoring.

The Gas Insulated Switchgear (GIS) equipment observes surface charge accumulation under DC or AC voltage in real operation, which could distort the electric field and incur flashover eventually. With a 3D high-resolution surface charge measurement system established on actual 220 kV GIS basin insulator, the present research captured the surface charge distribution characteristics of the insulator under DC and AC voltage through high-precision probe scanning on the insulator surface. The test results show that: 1) Surface charge demonstrates obvious polarity effect under DC voltage. Under positive DC voltage, there accumulated mainly positive surface charges. Under negative DC voltage, the polarity of accumulated surface charges is all negative. Regardless of charge polarities, the surface charge density increases as the voltage amplitudes mounts and the voltage application duration prolongs. 2) Surface charge accumulation also takes place under AC voltage application. All charges are negative and distribute in a uniformed manner but in smaller density comparing to the DC scenario. As the voltage amplitudes increase, the surface charge density first increases, then decreases and finally reaches a near saturation state. As voltage application goes on, the surface charge accumulation becomes more obvious. Above all, the insulation issues in relation to surface charge accumulation require due attention in the application and maintenance of GIS equipment, and the present research attempts to provide useful reference for the optimal design of GIS/GIL insulators.

Compared with the overhead transmission line, gas insulated transmission lines (gil) occupies less land area, has higher insulation reliability, and is increasingly widely used in the field of power transmission. under actual working conditions, gil will withstand a variety of voltages, such as ac, dc and impulse voltages. under such conditions, electric charges will accumulate on the surface of the gil tri-post insulator, which will enhance local electric field and affect the surface flashover voltage. based on the measurement method of electrostatic capacitance probe, a surface charge measurement device for gil tri-post insulator was developed in this paper to capture the surface charge accumulation characteristics of tri-post insulator under different voltages insf6. according to the structural characteristics of the tri-post insulator, a set of three-dimensional motion control mechanism is designed to control four probes to scan the surface of insulator in the post zone, the sphere zone and the junction zone of post and sphere with high precision. the calibration experimental results show that the system has high sensitivity and accuracy. for the side probe of post zone, the charge resolution is 0.0290c/(m2·nv) and the spatial resolution is 4.6mm2, for the 1# arc probe of post zone, the charge resolution is 0.0237c/(m2·nv) and the spatial resolution is 5.1mm2, for the 2# arc probe of post zone, the charge resolution is 0.0245c/(m2·n v) and the spatial resolution is 5.0mm2, for the sphere probe, the charge resolution is 0.0165c/(m2·n v) and the spatial resolution is 6.7mm2

The flashover phenomenon of insulator is the main cause for the insulating failure of GIS. One of the most critical factors that impacts the surface flashover voltage is the accumulation of surface charge which can even severely restrict the performance of insulators. Therefore, it is very necessary to find a way to restrain the accumulation of surface charge on the insulators. The common methods to restrain the surface charge accumulation on the insulators were reviewed in this paper. Through the reasonable comparison and analysis for those methods, the way of nano-coatings for insulators was selected to restrain the accumulation of surface charge on the basin insulators of GIS. The raw materials and formula of nano-coating materials were developed in this paper. The effective experiments were also carried out to test the suppression of nano-coatings for the surface charge accumulation on the GIS basin insulators. The experimental results indicated that, the nano-coatings can effectively restrain the accumulation of surface charge and improve the distribution of electric field on the insulator under DC and AC voltage, the maximum degree of the suppression for the surface charge under DC voltage can reach 51.2%, while under AC voltage it is 9.3%.

The Resistive-Capacitive (RC) model has been widely adopted for analysis of the electric field in oil-pressboard insulation under DC voltage. However, studies in this regard have indicated that the charge polarity effect in oil-pressboard insulation, namely the stronger adsorption of negative charges by pressboard, could not be adequately captured by the RC model. In order to analyze the electric field with due consideration of charge polarity effect, the present paper adopted the Kerr electro-optic techniques to have measured the electric field in the oil spacing under DC voltage for two experiments models with Model I having two oil spacing divided by a pressboard at the spacing ratio of 1:1, and the Model II at the spacing ratio of 1:2. As Model I is concerned, the strength of electric field in the upper oil spacing reached 1.61 times of the initial one under positive voltage and 1.60 times under negative voltage, while the field in the lower oil spacing registered as one eighth of the initial one under both positive and negative voltages. For Model II, the electric field strength in the upper oil spacing reached 2.3 times of the initial field under positive voltage and one fourth of that under negative voltage, showing much stronger charge polarity effect compared to that in Model I. In order to analyze the effect of interface charge on electric field, an electric field analysis model based on interface charge, named Interface Charge Polarity Effect (ICPE) Model, is proposed in this paper. Referring to the different adhesive abilities of positive and negative charges on pressboard, the influence of interface charge on electric field can be analyzed by the proposed model, which could make the contribution to the insulating design and proofread for the converter transformer.

The surface flashover voltage of the insulator in vacuum is much lower than that of both insulator and the vacuum gap in the same dimension. The critical factor that impacts surface flashover voltage is the accumulation of surface charge which can severely restrict the performance of the vacuum devices. In order to research the distribution of charge on the surface of the insulator under nanosecond pulse in vacuum, a nanosecond pulse generator whose output amplitude and pulse width can be adjusted flexibly and a capacitive divider which is designed for the pulse measurement have been developed in this paper. Considering the required voltage waveform and the insulation strength of test chamber, the rising edge of the nanosecond pulse designed in this paper is about 10ns, the pulse width is about 100ns. The capacitive divider whose divider ratio is 476.45:1 and bandwidth is 56.69MHz can accurately measure the output pulse voltage.

According to the comparative advantages of smaller volume, smaller flooring space, better dielectric performance and so on, Gas Insulated Switchgear (GIS) has been widely used in transformer substations. The basin insulators of GIS will withstand the switching impulse voltage as a result of complex operating conditions. The basin insulators play a critical role in determining the insulating strength of GIS, while the surface charge accumulation on that, which could not only distort the electric field but even lead the surface flashover, is the key factor for insulating properties. In order to study the effect of surface charge accumulation on the flashover voltage of GIS basin insulators in SF 6 under switching impulse voltage, this paper established a surface charge measurement system based on the electrostatic capacitance probe method and a set of flashover test platform including an effective experimental model. The experimental results indicate that, the accumulated charges on the surface of basin insulators have a remarkable impact on the surface flashover voltage of that. The more the charge accumulated on the insulator surface, the more the surface flashover voltage of insulator decreased, and the maximum amount of flashover voltage to decrease can reach 13.6%. It should be paid more attention that the accumulated surface charge indeed has influence on the surface flashover of insulators, which could do a harm to the insulating strength of GIS to some extent.

The electric field could be distorted by the accumulated surface charge on the insulator, even flashover voltage could be decreased. In vacuum, the flashover voltage of the insulator is much lower than both insulator's materials and the vacuum gap in the same dimension. In order to enhance the flashover voltage under nanosecond pulse voltage in vacuum, it is very important to take the study of the distribution of the surface charge of the insulator. A set of insulator surface charge measurement system under nanosecond pulse in vacuum has been set up in this paper. This paper adopts the electrostatic capacitance probe method to acquire the distribution of surface charge of the different samples, and compares the characteristics of surface charge accumulation among different samples. The results indicate that there is obviously charge accumulated on the surface of sample without additive, while on the surface of sample mixed with additive there is no charge accumulated or the charge accumulation amount is less than the probe sensitivity. Comparing the charge accumulation amount of all samples under three pulses to that under one pulse, there is an obvious increase.

The converter transformer is one of key equipment in the High Voltage Direct Current (HVDC) transmission system, the performance of the insulating pressboard in which can directly affect the quality of electric energy, and which could be affected significantly by the temperature and moisture. Therefore, an experimental platform was set up in this paper for the research on typical electrical parameters (mainly resistivity and permittivity) of the pressboard. The experiment results indicated that with the increase of temperature and moisture, the relative permittivity of the insulating pressboard increased gradually, and the largest increase on percentage could reach 185.9%. While at the same time, the resistivity gradually decreased, whose maximum decrease in the extent could reach 95.9%. And the properties of insulating pressboard in different manufacturers also showed significant difference. What was found in this paper could provide a reliable basis for the field evaluation of converter transformer and its optimization design.

The interface charges of oil-pressboard insulation could distort the local electric field and even weaken the insulating strength of converter transformer. This paper employs the electrostatic capacitive probe to capture the accumulation characteristics of interface charges in oil-pressboard insulation with needle-plate electrode under AC, DC and combined AC/DC voltages. The research results indicate that, under AC voltage, the polarity of interface charge is the same to that of the instantaneous value of applied voltage, and density of interface charge remains constant with the AC voltage application prolonging. In the context of DC voltage, the density of negative interface charge is about 1.2–1.5 times of that of the positive one. Under AC/DC combined voltage, the time-dependent dynamic accumulating process of interface charge behaves similarly to the waveform of AC voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. The tests for the surface flashover of oil-pressboard insulation were also conducted, aiming at discovering the impacting mechanism of interface charge accumulation on flashover voltage. The surface flashover voltage of test model under negative DC superimposed AC voltage is 1.3 times higher than that of positive DC combined with AC voltage. The built interface polarization model made an explanation for the interface charge accumulation, and the impacting mechanism of interface charge on flashover voltage, namely the positive effect of homopolar interface charges, is also proposed.

The electrical strength of insulator in vacuum is much lower than either the insulator itself or the vacuum gap in the same dimension, and one of the considerable factors that could affect the electric field or even the surface flashover is the surface charge accumulation. In order to capture the surface charge characteristics and its impacting mechanism on electric field of ceramic insulator under nanosecond voltage in vacuum, this paper employed electrostatic capacitive probe to develop a surface charge measurement system. The accumulating and dissipating characteristics of surface charge on four Al2O3 ceramic insulators with different manufacturing conditions were captured, indicating that, 1) the surface charge mainly accumulates near the anode, but there presents various distribution patterns of different samples; 2) the charge density of different samples also shows different increasing trend with the voltage application counts increasing, and the maximum difference could reach 78.07 %; 3) within 1 hour after the voltage cutting off, the maximum density of dissipated surface charge on all samples is less than 7.0 %, ensuring the reliability and effectiveness of experimental results. The surface micro-morphology and surface traps characteristic take the joint actions to make the explanations for the surface charge accumulation and migration mechanisms. Finally, based on surface charge measurement results and field simulation results, the quantitative impact of surface charge on surface electric field were proposed in this paper, aiming at providing meaningful references for the insulation design.

Attributed to the high reliability and high safety, gas insulated switchgear (GIS) is playing an important role in the power transmission system, its reliability is exerting more influence on the entire power grid. Under different operation conditions, the surface charges can accumulate on the surface of insulators of GIS, which is one of the main factors causing surface flashover. Surface charges not only enhance the local electric field intensity but also provide charges for creeping discharge, which finally leads to flashover. In order to study the effect of surface charge accumulation on flashover voltage of GIS insulator in SF6 under DC and AC voltage, this paper establishes a surface charge measurement system which is based on electrostatic capacitance probe method and a set of flashover test platform. It can be seen from the experimental results that no matter under DC or AC voltage, the more charge accumulated on the insulator surface, the more surface flashover voltage of insulator decreased, and under DC voltage the surface flashover could decreased by 23% maximum, and that under AC voltage is 10.2%.

According to the advantages of smaller volume, smaller flooring space, and better dielectric performance and so on, Gas Insulated Switchgear (GIS) has been widely used in transformer substations. The disc insulators play a critical role in determining the insulating strength of GIS, while the accumulated surface charge on the insulator is the key factor for its insulating properties. The insulators of GIS will withstand the switching impulse voltage as a result of complex operating conditions. The present paper adopts electrostatic capacitance probe to develop a surface charge measurement system which can measure the surface charge on the actual disc insulators of GIS. Through the structure optimization, the charge leakage of designed probe is very small. This paper studied the effect of amplitude and pulse number of switching impulse voltage on the accumulation characteristic of surface charges on GIS insulator in SF6. The experimental results indicate that, under the switching impulse voltage, no matter positive or negative, the surface of insulator accumulates both positive and negative charges, but the amount of negative charges is larger than that of the positive charges. With the increase of amplitude and pulse number of positive switching impulse voltage, there is an obvious increase on the average amount of surface charge accumulation. However, under the negative voltage, the average amount of surface charge accumulation firstly increases and then decreases with the increase of amplitude, and which could rarely be influenced by the increase of pulse number of voltage.

Converter transformer is one of the important apparatus of high voltage direct current (HVDC) transmission system. Because of the complex structures and operating conditions, the winding of valve side of converter transformer will withstand DC voltage, combined AC-DC voltage and even polarity reversal voltage, and the insulating properties of which under polarity reversal voltage has drawn great attention. Under the special condition, the interface charge will be accumulated on the oil-pressboard interface, which can directly affect the electric field distribution of the converter transformer dielectric and even decrease the flashover voltage. Therefore, it is necessary to study the characteristics of interface charges and their influence on flashover voltage. The oil-pressboard insulation test platform is designed in this paper, including the interface charge of oil-pressboard insulation measurement platform and polarity reversal test platform. The minimum spatial resolution of interface charge measurement platform is about 1mm2, and its measuring sensitivity can reach 0.129pC/mm2. This paper adopts a needle-plate test model for measurement. The reversal time of the polarity reversal test apparatus is on the level of millisecond that conforms to the actual operation of the converter transformer. The experimental results under polarity reversal voltage indicated that the interfacial charge could greatly reduce the flashover voltage of polarity reversal, and the surface potential produced by charges can be up to 40% of polarization voltage. The changes of oil-pressboard interface charge density before and after the polarity reversal have been measured. It is found that the electric field produced by interface charge can strengthen the maximum synthesis field intensity of the needle electrode after polarity reversal, thus greatly decreases the flashover voltage.

The surface flashover voltage of the insulator in vacuum is much lower than that of both insulators and the vacuum gap in the same dimension. The critical factor that effects surface flashover voltage is the accumulation of surface charge which can severely restrict the performance of the vacuum devices. Owing to the excellent electrical properties and mechanical properties, Cross-linked Polystyrene (XPS) is applied more and more widely as insulator material in military and aerospace industry. In order to study the characteristics of surface charge and its effect on surface flashover voltage on the XPS insulator under nanosecond pulse voltage in vacuum, a set of insulator surface charge measurement system and surface flashover voltage test platform under nanosecond pulse in vacuum have been set up in this paper. This paper adopts the electrostatic capacitance probe method to acquire the distribution of surface charge on XPS insulator, studies the effect of amplitude and pulse number of voltage on the characteristics of surface charge accumulation, and analyzes the effect of surface charge on surface flashover voltage under nanosecond pulse voltage in vacuum. The experimental results indicate that, with the increase of voltage amplitude and pulse number, there is an obvious increase on the amount of surface charge accumulation. The accumulated charges on the surface have marked effect on surface flashover voltage, and the maximum amount of flashover voltage to decrease is 15.5%. Under the same conditions, the amount of surface charge accumulation of XPS insulator is much less than that of ceramic insulator.

Under AC/DC combined electric field, the oil-pressboard insulation of the converter transformer accumulates space charges to generate a partially distorted non-uniform field and thus affects the electric strength distribution of the insulation system. The present research uses the needle-plate electrode to simulate the distorted non-uniform electric field fostered by the space charges and adopts the electrostatic capacitive probe method to record the changing patterns of the interface charges in the AC/DC combined electric field, and studies the impact of interface charges on flashover voltage, aiming at providing concrete references for the design of converter transformer. The research results indicate that, under AC/DC combined electric field where the ratios between AC voltage equivalent value and the DC voltage are recorded as 1∶1 and 1∶5, the interface charges accumulated at the upper layer of the pressboard in the non-uniform electric field possess the same polarity as that of the DC proportion in the applied voltage complex. The charge density under the combined electric field superimposed by AC on negative DC voltage appears larger than that in the context of the positive DC voltage. The interface charge density increases with the duration of voltage application and also fluctuates with the AC instantaneous field in the similar manner as power frequency voltage. The larger the DC proportion in the combined electric field, the less the fluctuation appears. Due to the reverse effect of the homopolar charges to the electric field, the surface flashover voltage in the combined electric field superimposed by AC voltage on the negative DC voltage is 1.3 times greater than that in the scenario of positive DC voltage.

Surface charge accumulation can incur changes in electric field distribution, involved in the electron propagation process, and result in a significant decrease in the surface flashover voltage. The existing 2D surface charge measurement fails to meet the actual needs in real engineering applications that usually adopt the 45° conical frustum insulators. The present research developed a novel 3D measurement platform to capture surface charge distribution on solid insulation under nanosecond pulse in a vacuum. The results indicate that all surface charges are positive under a positive pulse and negative under a negative pulse. Surface charges tend to accumulate more near the upper electrode. Surface charge density increases significantly with the increase in pulse counts and amplitudes. Accumulation of surface charge results in a certain decrease of flashover voltage. Taking consideration of the secondary electron emission for the surface charge accumulation, four materials were obtained to demonstrate the effects on surface charge. Combining the effect incurred by secondary electron emission and the weighty action taken by surface charge accumulation on the flashover phenomena, the discharge mechanism along the insulator surface under nanosecond pulse voltage was proposed.