Recent studies have shown that the modifications to nanofluid-impregnated pressboards by different shapes of nanoparticles are different To investigate the effect of nanoparticles shapes on creeping flashover characteristics at the interface of nanofluid-impregnated pressboard (NP), creeping flashover behavior of spherical nanoparticle fluid/pressboard (NSFP) and nanorod fluid /pressboard (NRFP) were compared to that of oil/pressboard (OP) which was used as a reference Creeping flashover experiments under AC and positive and negative impulse voltages were carried out Results show that the modifications by spherical and rod nanoparticles on creeping flashover strength are distinct and different: negative creeping flashover voltages of NSFP and NRFP are increased up to 32 and 15% respectively, and the positive ones are separately improved as high as 15% and 5% Whereas, experiments under AC voltage reveal quite a different trend: creeping flashover voltage of NSFP and NRFP are almost increased to the same extent, nearly 18%, while partial discharge inception voltage improvement in the increment of the former is 56 times larger than that of the latter Charge behaviors at oil/pressboard interfaces were displayed by thermally stimulated depolarization current method Space charge accumulation at the surface of NSFP is much less than that of NRFP and the dissipation rate of the former is faster, due to a larger trap density and a lower trap energy level, which weakens electric field distortion and results in the improvement of creeping flashover strength The underlying mechanism is that spherical nanoparticles have a specific surface area 13 times as large as that of nanorods at the same concentration, and it is a source of shallow traps

Effect of nanoparticle shapes on creeping flashover characteristics at the interface of nanofluid-impregnated pressboard

Surface flashover fault is one of the most challenge issues in oil-cellulose insulation pressboard system used in power transformer. In this study, a comparative study of the AC surface flashover properties of the novel 3-element mixed oil-cellulose insulation pressboard (3EMO-IP) and mineral oil-cellulose insulation pressboard (MO-IP) was performed under needle-plate and finger-finger electrode, respectively, measurement including dielectric property, surface flashover voltage, damage of cellulose pressboard surface and the gas generation behaviors after multiple surface flashover. Results show that the cellulose insulation pressboard immersed in the novel 3-element mixed insulation oil (3EMO) has higher relative permittivity and dielectric loss factor at 50 Hz, and also has slightly lower surface resistivity. The AC surface flashover voltage of the 3EMO-IP is higher than that of MO-IP under needle-plate and finger-finger electrode (electrode distance 5 mm, 10 mm, 15 mm and 20 mm). Compared to MO-IP, the lower electric field intensity at the oil-pressboard interface, as well as more difficult for surface charge accumulation of 3EMO-IP and the higher breakdown voltage of 3EMO lead to the higher AC surface flashover voltage of 3EMO-IP. Moreover, the carbonization of fibers in 3EMO-IP is slightly less. After multiple surface flashover, C2H2 and total hydrocarbon gases are the main differences between 3EMO-IP and MO-IP, which is more marked with the increase of flashover times. This study offers a reference for improving the surface flashover property of oil-pressboard insulation system by using 3EMO.

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AC Surface Flashover and Gas Generation Difference of the Cellulose Insulation Pressboard Immersed in Novel 3-Element Mixed Oil and Mineral Oil

This paper studies the use of circular boundary and square boundary configurations for analyzing fractal char- acteristics of creeping discharges over solid dielectric materials. A two dimensional stochastic dielectric breakdown model is used to simulate creeping discharges. The square boundary and the circular boundary represent a square shaped dielectric sample and a circular shaped dielectric sample respectively. These configurations are used to analyze the effect of using different shaped samples for creeping discharge experiment in a laboratory environment. Fractal dimension of the simulated patterns, calculated by box counting method and mass radius relation method are used to analyze the effect of circular shaped and square shaped dielectric samples for creeping discharge analysis.

Effect of the Shape of the Insulator on Fractal Characteristics of Creeping Discharges

Nanostructured dielectric composite has been considered as a promising manner in improving the flashover performance of oil-paper which has been widely used in power systems. In this paper, plasma-enhanced chemical vapor deposition (PECVD) is used to deposit SiO2 on the ceramic fiber-reinforced insulating paper. Scanning electron microscope images show a large number of SiO2 nanoparticles with diameters of 100 nm - 250 nm uniformly attached to the fiber surface after the plasma deposition. The surface flashover voltage of the insulating paper was tested in the air and the transformer oil, respectively. Results show that the corresponding DC surface flashover voltages increased by 15.1% in the air and breakdown between liquid and solid interface increased by 24.6% after the PECVD. It is believed that nanoparticles constructed in ceramic fibers change the electron injection barrier which inhibits the injection of negative charges and hinders the accumulation of charges in the dielectric. Nanoparticles can capture electric charges formed in the transformer oil which affects the generation and development of streamers, resulting in an increased dielectric strength. This study provides a new method to comprehensively improve the surface insulating property which has the prospect of promoting other dielectric materials.

https://iopscience.iop.org/article/10.1088/1361-6528/abdf8b/pdf

Nano-sized composite improving the insulating performance of insulating paper using low-temperature plasmas.

This paper presents an analysis of the creeping discharge characteristics caused by a needle electrode in oil-pressboard insulation under a positive DC voltage The discharge behavior under a strongly (model I) or weakly (model II) vertical component of the electric field is studied using partial discharge (PD) detections, camera observations, and charge distribution simulations It is found that the development process of the creeping discharge can be divided into three stages according to the stepwise growth trends of apparent charge amplitude and PD repetition rate The time-resolved PD pattern and equivalent time-frequency pattern are different in the two models Most discharge pulses are unimodal with underdamped oscillations, but bimodal pulses can occur in model II The streamer in model I has a tail caused by the stranded charge A secondary streamer can occur in model II following the initial streamer, causing a bimodal PD pulse The study findings can provide suggestions for the optimization of insulation design and the promotion of converter transformer maintenance

Characteristics of Creeping Discharge Caused by a Needle Electrode in Oil-Pressboard Insulation under +DC Voltage

The propagation of surface discharge due to interfacial polarization was numerically analyzed at the oil-nanocomposite interface using fully coupled finite-element analysis incorporating the relative permittivity from experiments. To improve the insulation ability, a new nanodielectric insulating material has been proposed in which a pressboard is coated with epoxy resin mixed with silica nanoparticles; this nanocomposite material can enhance the breakdown voltage in power systems with a certain level of silica nanoparticles. To specify the electric breakdown performance of this nanocomposite material, we measured the bulk relative permittivity of epoxy resin containing different percentages of silica nanoparticles on the pressboard. Surface discharge, or creepage discharge, tends to propagate along the solid–liquid interface and then leads to flashover. The mechanism of surface discharge, therefore, is a critical issue for understanding the dielectric breakdown strength in solid–liquid interface problems. To quantitatively analyze and explain the characteristics of surface discharge, here, the fully coupled finite-element analysis technique has been applied and tested with various relative permittivity values of nanocomposite materials. This phenomenon has been simulated using the fully coupled governing equations using Poisson’s equation for electric field and charge continuity equations, including surface charge accumulation for charge transport. After verification of our numerical setup in a conventional oil-pressboard system, a needle-bar electrode system was proposed and applied to the analysis of surface discharge propagation for the new nanocomposite materials with bulk dielectric permittivity. The propagation speed at the oil-nanocomposite interface was compared with different percentages of nanosilica. Finally, the physical mechanism of surface discharge due to the interfacial polarization was analyzed with the space, bounded, and surface charge densities at the oil-nanocomposite interface based on the numerical results.

Finite-Element Analysis for Surface Discharge Due to Interfacial Polarization at the Oil-Nanocomposite Interface

This paper deals with experimental characterization of creepage discharge to improve the electrical characteristics of oil/pressboard (PB) composite insulation system by solid layer coating such as epoxy resin, and nano size silica/epoxy resin composite on PB surface under positive standard lightning impulse voltage (1.2/50 µs) using a needle/bar electrode arrangement in mineral oil. Creepage discharge properties of conventional PB are compared with solid layer coated PB, in terms of discharge final propagation length (l d ) and partial discharge inception voltage (PDIV). As a result, it is found that partial discharge inception probability (P d ) of solid layer coated PB increases at all applied impulse voltage levels compared with that of conventional PB. The result is interpreted in terms of drying effect, surface modification effect and electron suppression effect by nano/epoxy composite coating on PB surface.

Nano SiO 2 /epoxy coating effect on creepage discharge characteristics in oil/pressboard composite insulation system

Oil-immersed power transformers adopt oil/pressboard (PB) composite insulation system. To downsize the equipment, further understanding about the creepage discharge phenomena is required. In this study, we have investigated the effect of white discharge trace on the creepage discharge in oil/PB composite insulation system. Experimental results show that partial discharge inception voltage (PDIV) became lower after the white trace formation. In addition to this, the formation delay time of the discharge became shorter after the white trace formation. These results can be explained by the gaseous nature of the white trace.

Effect of discharge trace formation on creepage discharge characteristics in oil/pressboard composite insulation system under impulse voltage application

Impulse Creepage Discharge Properties of Solid Coated Pressboard/Oil Composite Insulation with Rod-plane Electrode System

This paper deals with experimental characterization of creepage discharge to improve the electrical characteristics of oil/pressboard (PB) composite insulation system by thin solid layer coating such as epoxy resin and Teflon on PB surface under positive standard impulse voltage (1.2/50 µs) using a rod-plane electrode arrangement in mineral oil. To evaluate the creepage discharge properties according to surface condition, the number of discharge branches, discharge final length (ld) and partial discharge inception voltage (PDIV) are investigated versus the type of solid layer coating for different applied voltage using a digital camera. As a results, it is found that solid layer coating of PB surface significantly influences the PDIV and creepage discharge properties.

Kyunghoon Jang

Papers

Finite-Element Analysis for Surface Discharge Due to Interfacial Polarization at the Oil-Nanocomposite Interface

Nano SiO 2 /epoxy coating effect on creepage discharge characteristics in oil/pressboard composite insulation system

Effect of discharge trace formation on creepage discharge characteristics in oil/pressboard composite insulation system under impulse voltage application

Impulse Creepage Discharge Properties of Solid Coated Pressboard/Oil Composite Insulation with Rod-plane Electrode System

Effect of thin solid layer coating on creepage discharge characteristics in oil/pressboard composite insulation system