Creeping Discharge Characteristics Research Articles

A Study on the Design of the Termination Part for a 23kV Superconducting Cable

In this paper, a study on the design of the termination part for a 23kV superconducting cable is conducted. Also, the design of a spacer is derived in consideration of electrical and mechanical safety factors. In general, the termination part of a superconducting cable is filled with gaseous nitrogen (GN₂) and gaseous sulphur hexafluoride (SF<SUB>6</SUB>), and may have a temperature gradient from cryogenic to room temperature. Therefore, a design considering the dependency of dielectric characteristics on temperature should be conducted. In this study, the criterion of electric field intensity on the creepage discharge characteristics between a solid and gaseous insulation material according to several conditions is derived through dielectric experiments. Also, overall design of the termination part for a 23kV superconducting cable is proposed considering the mechanical as well as electrical safety factors.

초전도케이블용 Stop Joint Box 설계를 위한 연면방전 특성에 관한 연구

A study on the development of a superconducting cable has been conducted actively due to its ‘0’ resistance characteristics. A superconducting cable consists of three parts: termination, core, and joint box. A joint box could be classified into normal joint box (NJB) and stop joint box (SJB) according to its structure. In this paper, a study on the development of the design technology of SJB where a bulkhead is installed to maintain a constant pressure and temperature of a cryogenic system is conducted. In an SJB system, a bulkhead is installed at a superconducting core system and impregnated with epoxy resin, and then wound with polypropylene laminated paper (PPLP) in order to insulate the superconducting core system. The creepage discharge characteristics along the surface of a solid insulation material in liquid nitrogen (LN₂) are investigated through dielectric experiments and the criterion to perform electrical insulation design of an SJB system for a distribution superconducting cable is proposed.

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

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.

Influence of Aging on Creepage Discharge Characteristics of Oil-Paper Insulation Under AC-DC Combined Voltage

There is a relative lack of research on the development of creepage discharge in aged oil-paper insulation subjected to AC-DC combined voltage in converter transformers. In this study, based on the physical and chemical properties of oil-paper, we propose a sphere-plate electrode model to study the creepage discharge parameters and characteristic spectra of aged oil-paper subjected to AC-DC combined voltage. The results show that the creepage discharge inception voltage of aged oil-paper is related to the AC voltage component of the AC-DC combined voltage. However, when the DC voltage component is 60 kV, the flashover phenomenon was found to occur in one of the severely aged samples before the AC voltage was applied. Despite being subjected to the same DC voltage, the flashover phenomenon did not occur in three other samples aged to different degrees. Large-amplitude discharges were found to appear earlier in the severely aged samples. At the same creepage discharge stage, as the aging time of the oil-paper increased, the number of large-amplitude discharges was found to rapidly increase, and the time interval between adjacent discharges was found to gradually decrease. At each stage of the creepage discharge, the small-amplitude discharge has a “hump” shaped distribution. The larger the DC voltage component, the shorter the endurance time of aged oil-paper, and the more severe the damage to the oil-paper. The DC voltage component was found to have an accelerating effect on the development of creepage discharge in aged oil-paper insulation.

Correlation between surface charge and creepage discharge on polymeric dielectrics under high-frequency high-voltage stress

In the present work, creepage discharge characteristics, i.e. amplitudes, phases, and repetitiveness, and surface charge dynamic behaviors under a 20 kHz high-frequency sinusoidal waveform high-voltage electrical stress were captured in a discharge chamber with temperature and humidity control. The results showed that the creepage discharges mostly occurred in the positive half phase, whose maximum amplitude increased with the development of discharge. The inception voltage of the creepage discharge is independent of the frequency of the external electrical stress. Once the discharge occurred, there were a large number of positive and negative particles ionized by a high electric field. Because of the much higher velocity of electrons than positive ions, the energetic discharge-produced electrons are likely to disperse away along the surface and be accumulated through adsorption, collision, and reactions. Moreover, the positive ions join the high-conductive discharge channel and disappear though the ground electrode. Thus, after high-frequency creepage discharge, only negative charges remained on the dielectric surface, as measured. Particularly, the creepage discharges mostly occurred in the positive half phase, owing to the reverse electric field induced by the accumulated negative charges. With the development of creepage discharge, some large-amplitude discharges began to occur in the positive-peak-phase region. The research concluded that the synergistic effect of negative surface charges and large-amplitude discharges eroded the dielectrics and excited the streamer to creep toward the ground electrode until flashover along the surface. Therefore, the correlation between high-frequency creepage discharge and surface charge is preliminarily revealed.

Conceptual Design of a Spacer for a 154 kV HTS Apparatus

The termination part of a high temperature superconducting apparatus such as a superconducting fault current limiter or a superconducting cable is usually insulated by pressurized SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas for its excellent dielectric strength. However, SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> condenses into LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> because of its high boiling point (approximately 209 K) during operation. The condensed SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> into LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> may result in problems such as dielectric degradation and system pressure decline. In order to prevent the condensation of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> into LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the termination of an HTS apparatus is divided into two parts by using a spacer: an upper part pressurized by SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> with room temperature and a lower part pressurized by GN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> with cryogenic temperature. The termination part should be operated in a high-pressure condition in order to ensure safe dielectric characteristics of HTS apparatuses. In this paper, dielectric characteristics as well as mechanical characteristics of a spacer under pressurized gas are conducted. It is well known that electrical breakdown may occur in the form of creepage discharge along the surface of a solid material rather than in the form of penetration sparkover through a solid or gaseous material. Experiments on the creepage discharge characteristics according to several solid materials under various pressures are conducted and a computer simulation using a finite element method is performed to analyze electric field distribution and mechanical strength of a spacer. Finally, the specifications of a spacer for 154 kV superconducting fault current limiters such as diameter, thickness, shape, and material, are suggested according to various safety factors.

Effects of temperature on creepage discharge characteristics in oil-impregnated pressboard insulation under combined AC–DC voltage

Due to the complexity of the valve side winding voltage of the converter transformer, the insulation characteristics of the oil-impregnated pressboard (OIP) of the converter transformer are different from those of the traditional AC transformer. The study on effect of temperature on the creeping discharge characteristics of OIP under combined AC–DC voltage is seriously inadequate. Therefore, this paper investigates the characteristics of OIP creepage discharge under combined AC–DC voltage and discusses the influence of temperature on creepage discharge characteristics under different temperatures from 70 °C to 110 °C. The experimental results show that the partial discharge inception voltage and flashover voltage decrease with increasing temperature. The times of low amplitude discharge (LAD) decrease and amplitude of LAD increases. Simultaneously, the times of high amplitude discharge (HAD) gradually increase at each stage of creepage discharge with higher temperature. The analysis indicates that the charge carriers easily accumulate and quickly migrate directional movement along the electric field ahead of discharging. The residual charge carriers are more easily dissipated after discharging. The ‘hump’ region of LAD moves to the direction of higher discharge magnitude. The interval time between two continuous discharges is shortened obviously. The concentration of HAD accelerates the development of OIP insulation creepage discharge. The temperature had an accelerating effect on the development of discharge in the OIP under applying voltage.

Effect of UV radiation aging on creepage discharge characteristics of high temperature vulcanized silicon rubber at high altitude

The physicochemical properties and creepage discharge characteristics of aged high temperature Vulcanized (HTV) silicone rubber materials were investigated by ultraviolet radiation (UV) aging method in this study. The experimental results show that as the aging time increases, the creepage discharge flashover voltage increases first and then decreases. But the aging time has little effect on the creepage discharge inception voltage. With the aging time prolonged, the discharge endurance time of HTV silicone rubber is shortened, and the creepage discharge development velocity is accelerated. In the short time of applying voltage to aging material, the magnitude of discharge increases rapidly. According to the partial discharge characteristic parameters of creepage discharge, the whole creepage discharge process is partitioned into four stages. Compared with unaged HTV silicone rubber, the aged HTV silicone rubber has less fluctuation in performance parameters and a clear trend. The study found that UV aging not only affects the physicochemical and hydrophobic properties of the HTV silicone rubber, but also accelerates the development of creepage discharge under AC voltage.

Analysis of creeping discharges on oil-impregnated pressboard under combined AC and DC voltages

This paper presents a comparative analysis of the creeping discharge characteristics on mineral oil-impregnated pressboard (OIP) under combined AC and DC voltages. The creeping discharge behavior at various voltage levels to failure were analyzed using partial discharge (PD) measurements, camera observations, and electric field simulations. It was found that the creeping discharge distribution was significantly influenced by the DC component, and it was difficult to induce failure even on OIP with high moisture under combined AC and DC voltage. The flashover along the free-oil boundary layer above the OIP during creeping discharge occurred much more easily under combined AC and DC than under AC voltage, and the quality of the oil was important to avoid flashover. These differences were attributed to the combined influence of the electric field distribution under DC and space charge accumulation.

Creepage Discharge Characteristics of Solid Insulation Materials for Superconducting Power Cable

Solid insulation materials need to be robust in terms of both mechanical and electrical performance for application in a high-voltage superconducting apparatus. It is known that creepage discharge characteristics of solid insulation materials are inferior to their electrical breakdown characteristics with penetration. Therefore, electrical breakdown accidents in a superconducting apparatus could occur as a form of creepage discharge rather than electrical breakdown with penetration. In this study, we conduct dielectric experiments on the penetration and creepage discharge characteristics of epoxy resin and polypropylene laminated paper under cryogenic temperature by using three kinds of electrode systems. The first is for creepage discharge experiments along the surfaces of solid insulation materials in liquid nitrogen. The second is for creepage discharge experiments across the interface between two different solid insulation materials making contact in liquid nitrogen. The third is for experiments of electrical breakdown with penetration on solid insulation materials in liquid nitrogen. Dielectric experiments are conducted with an absolute pressure of 0.1 MPa under alternating current and lightning impulse voltages. Experimental results for electrical breakdown characteristics of solid insulation materials with penetration are compared with the creepage discharge characteristics on the surface and interfaces between solid insulation materials. Moreover, the criteria for electric field intensities at creepage discharge voltages in various conditions are deduced as empirical formulas.

Influence of hydrostatic pressure on creeping discharge characteristics over solid/liquid insulating interfaces under AC and DC voltages

This study deals with the influence of hydrostatic pressure on discharges propagating at solid/liquid insulating interface submitted to a divergent electric field under AC and DC voltages. The investigated insulator samples are of phenoplast resin (bakelite) and the considered liquid is mineral oil. The experimental results show that the hydrostatic pressure and the waveform, amplitude and polarity of voltage greatly influence the characteristic parameters of creepage discharges. The stopping (or final) length, L f, which is the maximum extension of these discharges increases with voltage; it is reduced when applying a hydrostatic pressure. L f increases quasi-linearly with the voltage and decreases quasi-linearly when the hydrostatic pressure is increased, whatever the voltage waveform. These results are of great interest especially for design and dimensioning of insulation systems in high voltage oil-filled apparatus and especially non-breathing components.

Creeping Discharge Characteristics of Nanofluid-Impregnated Pressboards Under AC Stress

This paper presents experimental research on creeping discharge characteristics of TiO2 nanofluid-impregnated pressboards (NPs) under sustained ac voltage. It was found that NPs possess higher resistance to creeping discharge than that of pure oil-impregnated pressboard. The results of permittivity and surface charge accumulation and decay characteristics reveal that the modification of nanoparticles can not only make the electric field more uniformly distributed at the interface of nanofluid/pressboard due to a better matching in permittivity but also significantly increase the surface charge decay rate via a trapping and detrapping process in shallower traps. Thus, the creeping discharge characteristics of NPs are greatly improved by the modification of TiO2 nanoparticles.

Creepage Discharge Characteristics Along the Interface Between Two Solid Insulation Materials in GN2

Glass-fiber-reinforced plastic (GFRP) and MC Nylon are widely used in superconducting apparatuses, such as in superconducting fault-current limiters, due to their excellent mechanical and dielectric performance. Many superconducting apparatuses are operated in a high-voltage condition. Thus, an unexpected dielectric accident can occur with damage to a superconducting magnet. Therefore, dielectric experiments on the creepage discharge characteristics of GFRP and MC Nylon were conducted for developing a high-voltage superconducting coil system. For this paper, the creepage discharges along the surfaces of solid insulators in gaseous nitrogen (GN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), and along the interface between two pieces of solid insulation in contact, were experimented on and analyzed, with respect to ac and dc voltages, at 300 K, under the absolute pressure of 0.1 MPa.

Influence of grounded back electrode on AC creepage breakdown

Solid insulation using back electrodes has been adopted in various types of electrical equipment. This paper investigates the impact of back electrode on solid insulation AC creepage discharge characteristics. Breakdown voltages at different gap distances are obtained via experiments, and electric field distribution is calculated using computer software. We compare the results between 'with' and 'without' use of back electrodes, and investigate the internal relationship between breakdown properties and electric field distribution. It is observed that back electrodes result in concentration of the electric field near the high voltage electrode. A larger gap distance results in an increase in the maximum tangential electric field strength. When tangential electric field strength exceeds a certain critical value, partial discharge streamers are generated. Such streamers help to lower the breakdown voltage and make the maximum electric field strength less than expected value.

Characteristics of creeping discharge along aerial insulated wire under impulse voltages with various wave front durations

AbstractWhen lightning occurs in the neighborhood of outdoor high‐voltage distribution lines, creeping discharges propagate along the wire surface from the binding wire tip just after insulator flashover. These discharges give rise to various faults on distribution lines, for instance, disconnection and melting of wire, punch‐through breakdown, and so on. We must clarify the creeping discharge characteristics associated with various inductive lightning surges from the viewpoint of safety in high‐voltage distribution systems. In our previous paper, it was reported that the lengths and aspects of the negative creeping discharges were influenced by the wave front durations of impulse voltages applied to the central line with a grounded binding wire. The present study was performed to obtain more information on such creeping discharges.This paper describes the distinctive characteristics of a creeping discharge along the insulated wire surface when impulse voltages with various wave front durations are applied to the binding wire. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 158(3): 29–37, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20430

Impulse creeping discharge characteristics in insulating oils with EHD function reinforced by added HFC gas components

The basic properties of electrohydrodynamic (EHD) pumping and dielectric strength were examined for two insulating oils, rapeseed ester oil and silicone oil, which were denatured by a bonding of hydrofluorocarbon (HFC)134a gas components decomposed using a barrier discharge reactor. In this work, the mechanism of a pure conduction pumping contributed to the evaluation of the electrical pumping in sample oils. Dielectric strength of oils was evaluated from the impulse creeping discharge characteristics and the 60 Hz breakdown voltage test. Experimental results revealed that the creation of radicals such as C-F and C-H in denatured oils led to a slight deterioration of the insulation ability compared to standard oils, but the pumping ability was reinforced significantly. Such additional EHD function may be utilized as a new cooling method in oil-filled power apparatuses in view of the dielectric strength

各種インパルス電圧による架空絶縁電線表面の沿面放電特性

When the lightning occurs at the neighborhood of outdoor high voltage distribution lines, the creeping discharges propagate along the wire surface from the binding wire tip just after the flashover of insulator. These discharges give rise to the various disasters at distribution lines, for instance, disconnection and melting of wire, punch-through breakdown etc. We must clarify the creeping discharge characteristics under various inductive lightning surges from a viewpoint of a safety in high voltage distribution systems. In our previous paper, it was reported that the lengths and aspects of the negative creeping discharges were influenced by the duration of wave front of impulse voltage applied to the central line under the grounded binding wire. The present study was accomplished in order to obtain more information on such creeping discharges.This paper describes the distinctive characteristics of creeping discharge along the insulated wire surface when the impulse voltages with various durations of wave front are applied to the binding wire.

Influence of the Groove on Barrier Surface on Creeping Discharge Characteristics in N2 Gas under .MU.s Pulse Voltage

For a needle-plane electrode system with a barrier, which establishes the electric field across the axis of a groove, creeping discharge characteristics in N2 gas under µs pulse voltage applications have been investigated. The distance h between the barrier surface and the needle tip as well as the distance M between the groove center and the needle tip were changed. In the case of h=0.3mm, when the needle tip is located near the far-side groove edge from the plane electrode (M=0.6mm), the flashover voltage has the maximum value. At that time, a growth of a corona is suppressed near the groove edge. These unique characteristics should associate with a field relaxation.

Creepage discharge characteristics over solid-liquid interfaces with grounded side electrode

The properties of streamers traveling over the surface of oil-immersed solid dielectrics were experimentally studied under lightning impulse conditions. Streamer polarity and the position of a grounded side electrode significantly affected the relationship between the streamer extension length and the applied voltage. Solid surface charging also had a large effect on the streamer propagation. However, the streamer propagation properties showed a consistent dependence on the potential at the solid-liquid interfaces. In addition, the potential drop inside the streamer channel was measured as a function of normalized streamer length. The curve revealed that the potential drop increased drastically within the region of /spl sim/20% from the streamer tip. The streamer appeared to progress with a constant mean velocity.

Influence of Creeping Distance and Gas Pressure on Negative Creeping Discharge for Composite Insulation Consisting of N2/SF6 and Solid Dielectric under Pulse Voltage

Creeping discharge characteristics on negative polarity of a composite gas/solid insulating system with N2/SF6 mixed gas have been studied by inserting a solid insulator between a needle electrode and a plane electrode. In the case of the shorter creeping distance, the flashover voltage on the negative needle increased with increasing SF6 gas content, which was a positive effect of the SF6 gas mixture. When creeping distance was increased, a considerable decrease in the flashover voltage on the negative needle under the gas pressures P of 0.2 MPa and 0.3 MPa was observed by the addition of a small amount of SF6 into N2. The effect of SF6 gas mixture for the flashover voltage on the negative needle changed from positive to negative at a threshold creeping distance. Furthermore, the threshold creeping distance depended on the gas pressure.