37 research outputs found
Currents in AC stressed liquid insulated needle plane gap
No full textCurrents and space charge phenomena in 8 different dielectric liquids have been investigated for an ac stressed needle plane gap. Applied frequencies ranged from 0.1 Hz to 100 Hz. A high resolution analogue-digital converter and an active suppression of capacitive currents were used to extract the small conductive currents. Tip radii and voltages were varied. The currents varied non-linearly with voltage. At low frequencies the perfluorpolyeter and mineral oils had almost symmetrical current in both polarities, while cyclohexane, white oil and esters had asymmetrical current with lower peak current for positive polarity. Above a certain instantaneous voltage level - within the power cycle - the currents increased with the voltage squared, fitting a space charge limited current model. Clear indications of heterocharge space charges were revealed by varying tip radii and frequencies. The heterocharge will result in an increase of the electric field. The results show that at higher voltages a purely resistive model for the liquids becomes invalid. It is also evident that the presence of space charges will influence partial discharge behavior.Currents in AC stressed liquid insulated needle plane gapacceptedVersio
The influence of pressboard parallel to the field on AC-breakdown in oil gaps
No full textpublishedVersio
Currents in AC stressed liquid insulated needle plane gap
No full textCurrents and space charge phenomena in 8 different dielectric liquids have been investigated for an ac stressed needle plane gap. Applied frequencies ranged from 0.1 Hz to 100 Hz. A high resolution analogue-digital converter and an active suppression of capacitive currents were used to extract the small conductive currents. Tip radii and voltages were varied. The currents varied non-linearly with voltage. At low frequencies the perfluorpolyeter and mineral oils had almost symmetrical current in both polarities, while cyclohexane, white oil and esters had asymmetrical current with lower peak current for positive polarity. Above a certain instantaneous voltage level - within the power cycle - the currents increased with the voltage squared, fitting a space charge limited current model. Clear indications of heterocharge space charges were revealed by varying tip radii and frequencies. The heterocharge will result in an increase of the electric field. The results show that at higher voltages a purely resistive model for the liquids becomes invalid. It is also evident that the presence of space charges will influence partial discharge behavior.Currents in AC stressed liquid insulated needle plane gapacceptedVersio
Experimental Studies of Streamer Phenomena in Long Oil Gaps
No full textIn this study, the characteristics of non-breakdown and breakdown streamers have been investigated through the effects of dissolved gases/air, carbon particles, additives, reduced pressure, liquid chemistry and voltage polarities on streamers. Two types of white oil (Marcol 52 and Exxsol D140) considered base liquids are examined. A point-plane electrode system is employed to form a high divergent field with the electrode gap of 8 cm. A “step” voltage (0.5/1700 µs) is applied to the electrode system. The voltage magnitude is varied in steps up to 540 kV maximum. Streamer characteristics are observed with stopping length, velocity, shape, breakdown and acceleration voltages. Current and light emission pulses are recorded. Both still and streak images are captured. The properties of positive streamer channel are investigated more closely. The electric field at the channel tip is determined by using the finite element method with COMSOL Multiphysics program, and mechanisms for streamer propagation are also discussed.
Dissolved gases/air has an insignificant effect on streamer propagation while carbon particles largely facilitate it. Carbon particles were seen to accelerate and markedly reduce inception, breakdown and acceleration voltages of streamers of both polarities. However, carbon particles have a stronger influence on negative streamers than positive ones. It seems that a small amount of carbon particles in the stagnant oil in the electrode gap can switch the slow 2nd mode[1] to fast 3rd mode streamers of negative polarity. Such a phenomenon cannot be observed in positive streamers.
Two kinds of additives are employed. These are a low ionization potential, N,N-dimethylaniline (DMA), and an electron scavenger, trichloroethene (TCE). DMA accelerates positive non-breakdown streamers of Marcol oil but seems to decelerate those of Exxsol oil due to the fact that DMA makes streamers either more filamentary for Marcol oil or more branched for Exxsol oil. Thus, DMA reduces the breakdown voltage of Marcol oil and increases that of Exxsol oil. DMA always decelerates positive breakdown streamers since a reduction in macroscopic field resulting from increasing shielding effect formed by more branching. Therefore, acceleration voltage is significantly increased. On the other hand, DMA does not have any significant impacts on negative streamers in both types of oil. TCE increases the velocity of both polarities because it makes streamers either less branched (positive polarity) or more filamentary (negative polarity). This leads to a decrease of both breakdown and acceleration voltages. However, TCE affects negative streamers much more than positive streamers. It is proposed that streamer propagation involves electronic processes in front of streamer channel tips.
Reduced pressure makes streamers more branched, but it still largely facilitates streamer propagation, i.e. a dramatic increase in the stopping length, thus significantly reducing the breakdown voltage. Reduced pressure seems not to influence the velocity of positive streamers while it significantly changes that of negative streamers. While slightly decreasing the acceleration voltage of positive streamers, reduced pressure decreases that of negative streamers by a factor of about two. Consequently, gaseous processes are involved in streamer propagation.
Although, the general characteristics of streamers are similar in Exxsol oil and Marcol oil, streamers of these oils still show some differences. The 2nd mode of positive streamers has multi-filament shape for Exxsol oil whereas more bushy shape can be observed in Marcol oil. In Exxsol oil, the 1st mode of negative streamers (bush-like shape) has to switch to the 2nd mode streamers (tree-like shape) to cross the electrode gap and induce breakdown. On the other hand, the 1st mode streamers (bush-like shape) can propagate across the gap in Marcol oil. The ratio of acceleration voltage to breakdown voltage is higher than for positive streamers in Exxsol oil since the content of aromatics in Marcol oil is lower.
Positive streamers are about ten times faster than negative streamers. This is due to the fact that positive streamers are more filamentary whereas negative streamers are bushier. In addition, the breakdown voltage of negative streamers is about twice of that of positive streamers, and the acceleration voltage behaves in the similar way.
It seems that a weakly ionized plasma state is present inside low conductive channels of the 2nd mode streamers of positive polarity, and critical space charges accumulate at channel tips. Gas discharge is considered to be responsible for the appearance of reilluminations. Nevertheless, reilluminations do not have any significant effects on the propagation of streamers. For the 4th mode, channels of positive streamers are highly conductive possibly resulting from highly ionized plasma state, and critical space charges are still present at the channel tips. The similar characteristics are suggested for corresponding modes of negative streamers. It is proposed that tip processes governed by electric field, formed by the space charges, will control streamer propagation. The electric field at the channel tip is calculated. The values of 2.4 MV/cm and 7-20 MV/cm are for the 2nd and 4th modes of positive streamers, respectively. The corresponding values are 1.3-2 MV/cm and 11-20 MV/cm for negative streamers. Therefore impact ionization is possibly one main mechanism producing new charges for maintaining streamer propagation.
PhD i elkraftteknikkPhD in Electric Power Engineerin
Pressure Dependent Propagation of Positive Streamers in a long Point-Plane Gap in Transformer Oil
No full textThe propagation of positive streamers in a naphtenic transformer oil in an 80 mm point-plane gap has been investigated under an impulse voltage being close to a step voltage and under pressures ranging from 0.1 to 1.7 MPa. As has previously been found in short gap experiments of 1 – 7 mm in various liquids, increasing voltage leads to shorter stopping length of non-breakdown streamers and higher breakdown voltages while the velocity is close to independent of pressure. The “acceleration” voltage from which streamer velocity rapidly increase with increasing voltage is also close to pressure independent. It is argued that this indicates that the processes determining velocity must take place in the liquid phase at the streamer head while conditions in the channel determines the stopping length of non-breakdown streamers
Centre for 3D Geoinformation
No full textThis paper will present the initial ideas behind the foundation of a Centre, which can bring together research, public authorities, and business communities in a unique environment of developing 3D GeoInformation applications, all based on new Virtual Reality technology as well as on information regarding urban and rural areas
Conduction and Electric Fields in Dielectric Liquids in Needle-to-Plane Gaps
No full textThis paper describes measurements of conduction currents in both neat and commercial insulating liquids. Conduction in thermodynamic equilibrium has been measured by applying a low ac voltage in a uniform field. High-field pre-discharge currents have been measured in a needle-to-plane configuration stressed with respectively high voltage ac and steep step high voltage pulses. There was significantly increased conduction for high field compared to low field conditions. Notably, measurements at high fields in needle-plane configurations indicate space charge limitation of currents with polarity differences. The high field conduction exhibited considerable variation across different liquids. At high applied fields, electronic processes are probably governing the charge formation. Simulations indicate that space charges may reduce the local field in front of needle tips at time scales relevant for propagating streamers.Conduction and Electric Fields in Dielectric Liquids in Needle-to-Plane GapsacceptedVersio
Prebreakdown phenomena in hydrocarbon liquids in a point-plane gap under step voltage. Part 1: behaviour at positive polarity
No full textThis study addresses the dielectric performance of nonpolar hydrocarbon liquids and mineral oils under positive polarity stress. It is of interest to improve knowledge on how functional properties of dielectric liquids vary, as new brands arrive in the market, and existing standards are unsuited for documenting the dielectric functional parameters of these new liquids. Stopping length for non-breakdown streamers, breakdown voltages and velocities for various pre-breakdown streamer modes have been studied for a selection of model liquids (cyclohexane and white oils), for a gas to liquid oil, and a refined naphthenic transformer oil. Studies of propagation modes were done using an 80 mm point to plane gap and a step voltage with a 0.5 μs rise time. Light emission and pre-breakdown currents have been recorded and instantaneous velocities have been derived from images of propagating streamers. There are clear differences in streamer stopping lengths and mode occurrence and mode velocities between these liquids. The differences seem to be influenced by molecular sizes governing evaporation energy for streamer formation and by concentration of aromatics which can be coupled to electron avalanche processes in the streamer heads
