Rongsheng Liu

Effects of nanoparticle charging on streamer development in transformer oil-based nanofluids

Transformer oil-based nanofluids with conductive nanoparticle suspensions defy conventional wisdom as past experimental work showed that such nanofluids have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. This paradoxical superior electrical breakdown performance compared to that of pure oil is due to the electron charging of the nanoparticles to convert fast electrons from field ionization to slow negatively charged nanoparticle charge carriers with effective mobility reduction by a factor of about 1×105. The charging dynamics of a nanoparticle in transformer oil with both infinite and finite conductivities shows that this electron trapping is the cause of the decrease in positive streamer velocity, resulting in higher electrical breakdown strength. Analysis derives the electric field in the vicinity of the nanoparticles, electron trajectories on electric field lines that charge nanoparticles, and expressions for the char...

A Model for the Initiation and Propagation of Positive Streamers in Transformer Oil

An electro-thermal model is presented of streamer initiation and transport that leads to electrical breakdown. The dominant charge initiation mechanism treated is electric field dependent molecular ionization. Analysis using COMSOL Multiphysics shows that molecular ionization results in fast electrons that cause a propagating electric field wave that is the dominant mechanism in the initiation of electric breakdown streamers. To model these dynamics a complete three carrier (positive ions, negative ions, and electrons) liquid-phase electric field dependent molecular ionization based streamer model was developed and solved for a positive needle voltage. The Heaviside function based on a critical energy for transformer oil boiling was used as the criterion to define regions of liquid or gas. In the gas-phase charge generation is described by Townsend ionization.

Fundamental research on the application of nano dielectrics to transformers

Nanotechnologies have potential to be used in transformer industry in enhancing material properties which may lead to a compact design of transformer and reduced manufacturing cost. In this publication, breakdown physics in nanofluids is studied for transformer insulation liquid. Several types of nanoparticles were selected and added into mineral oil. Breakdown voltage, streamer propagation velocity, oxidation stability, permittivity, electrical resistivity and dissipation factor were measured and studied. Results show that the dielectric properties of a modified liquid are strongly influenced by the type of the nanoparticles added. With the addition of one type of nanoparticles, the breakdown voltage of the liquid is much increased while the streamer propagation velocity is reduced. With the addition of another type of nanoparticles, the degree of scatter of breakdown data is improved, and thus the low probability value of breakdown is increased. The latter will increase the reliability of the insulation system or saying that there is a potential to increase the design values of the insulation system. A charging dynamic model is discussed. A trend has been seen from this study that nanofluids have potential for being used in transformer insulation liquid.

Modeling of Streamer Propagation in Transformer Oil-Based Nanofluids

Transformer oil-based nanofluids with conductive nanoparticle suspensions have been experimentally shown to have substantially higher positive voltage breakdown levels with slower positive streamer velocities than that of pure transformer oil. A comprehensive electrodynamic analysis of the processes which take place in electrically stressed transformer oil-based nanofluids has been developed and a model is presented for streamer formation in transformer oil-based nanofluids. Through the use of numerical simulation methods the model demonstrates that conductive nanoparticles act as electron scavengers in electrically stressed transformer oil-based nanofluids converting fast electrons to slow charged particles. Due to the low mobility of these nanoparticles the development of a net space charge zone at the streamer tip is hindered suppressing the propagating electric field wave that is needed to continue electric field dependent molecular ionization and ultimately streamer propagation further into the liquid. A general expression for the charging dynamics of a nanoparticle in transformer oil with infinite conductivity is derived to show that the trapping of fast electrons onto slow conducting nanoparticles is the cause of the decrease in positive streamer velocity.

Modeling the Effect of Ionic Dissociation on Charge Transport in Transformer Oil

Transformer oil is one of the most important and widely used of all dielectric liquids. Significant research into the insulating properties of transformer oil has been carried out. Much of this work has focused on the development of streamers, and how they can result in the electrical breakdown of the oil. Due to the complex nature of transformer oil no universally accepted model describing streamer initiation and propagation has emerged. In this paper, a model is developed which explores the role played by electric field dependent ionic dissociation in the initiation and propagation of streamers in transformer oil. This model couples the electrodynamics of charge transport to thermal enhancement in transformer oil. Using the COMSOL Multiphysics software application and the finite element method, the model is solved to show that ionic dissociation alone will not lead to a high enough temperature rise to initiate a streamer in transformer oil.

Differences in streamer initiation and propagation in ester fluids and mineral oil

The properties of ester fluids and mineral oils differ in many cases in a qualitative way. This means that these liquids may look like very similar in some experiments while in others they behave very differently. These differences must be understood and taken into account in the design of ester filled transformers to avoid unexpected failures. Classic models such as the two-parameter Weibull statistics of breakdown voltage are in some respects too simple to properly catch the underlying physics. We discuss the importance of segregating initiation and propagation processes to properly interpret results of experiments with esters and mineral oil. In transformer design it is in some parts more important to avoid initiation while in others it is more important to stop propagating streamers. Furthermore, to obtain low probability statistics from high probability breakdown experiments, both average and spread of breakdown voltages are required. The difference in spread of initiation and propagation voltages is discussed. Esters and mineral oils behave in a qualitatively similar way regarding initiation while they differ substantially in the propagation process. In this paper we compare some different experiments in terms of initiation and propagation.

Geometry impact on streamer propagation in transformer insulation liquids

Lightning impulse voltage breakdown tests were made in needle/plane, needle/sphere and U-type electrode systems and compared between ester fluids and mineral oil. Results show that the geometry of electrode has significant impact on both breakdown voltage and the velocity of streamer. The more divergent the electric field, the lower the breakdown voltage. The lowest breakdown voltage was found in needle/plane geometry having the most divergent field. In needle/sphere geometry the breakdown voltage was twice as high. In the U-type geometry the breakdown voltage was further increased. At breakdown voltage, streamer propagation velocity in needle/plane geometry was about 2 km/s and in the U-type geometry, the streamer velocity was about 10 km/s. Less difference was found between ester fluids and mineral oil in the U-type geometry for both streamer velocity and the values of breakdown voltage at an oil gap length of 35 mm, which is closer to the field distribution in a real transformer. The impact of electrode geometry dominated over the difference between liquids, which made the ester fluids and mineral oil more similar in the more homogeneous electric field of the U-type electrode system.

Modeling streamers in transformer oil: The transitional fast 3 rd mode streamer

This paper presents an electro-thermal hydrodynamic model that explains the development of different streamer modes in transformer oil. The focus is on the difference between the slow 2nd and fast 3rd mode streamers discussed in the literature. Through the use of numerical methods the model demonstrates that streamer modes arise in transformer oil due to the electric field dependent molecular ionization of different families of hydrocarbon molecules (i.e., aromatic, naphthenic, and paraffinic) at increasing electric field levels (or applied voltages). Ionization of the low number density aromatic molecules, that generally have lower ionization energies than naphthenic/paraffinic molecules, leads to the propagation of 2nd mode streamers with velocities on the order of 1 km/s. As the applied voltage is increased, the ionization of the main hydrocarbon molecules in transformer oil, high number density naphthenic/paraffinic molecules, dominates producing high electric field levels and space charge at the streamer tip. This ultimately leads to the propagation of the 3rd mode streamer with velocities on the order of 10 km/s.

Streamer propagation in composite oil/cellulose insulation under LI voltages

A needle/plane geometry was used to study fundamental properties of streamer propagation in composite transformer oil/cellulose insulation under LI (lightning impulse: 1.2/50 /spl mu/s) voltages. The plane electrode was either an aluminium plate (bare oil gap) or covered with laminated oil-impregnated paper of different thickness. Breakdown voltage and streamer propagation speed were seen to differ at different polarities and dependent on the gap length of the needle/plane geometry. A ratio of oil-gap length to electrode cover thickness seems to exist, below which, the cover thickness dominates the breakdown of the composite insulation. Above that, length of the oil gap dominates the breakdown.

Space Charge Distribution in an Extruded Cable Aged in Tap Water for 3 Years

Moisture is one of the parameters which might have an effect on the space-charge distribution in an extruded insulation system under DC voltage. Investigation into their relationship is of importance for gaining insight into the conduction and breakdown mechanisms of insulation materials. In this study, a specially manufactured lab sample in the form of an extruded cable has been aged in tap water for 3 years under DC voltage. Space charge measurements were made using the PEA (pulsed electro-acoustic) method. These measurements were performed before, during and after the wet ageing. Limited hetero-charge was observed in the vicinity of insulation screen / insulation interface. Both negative and positive DC voltages have been examined. The cable survived the 3-year-long wet ageing without breakdown.