M.J. Lee

Tandem-chamber charge density monitor

A device that is capable of measuring the charge density carried by an insulating fluid is described. The monitor relies on the principle of charge relaxation in two tandem chambers in such a way that the measurement is independent of residence and relaxation times. It is robust, has no moving parts, and is also able to monitor the conductivity of the fluid in applications for monitoring the streaming electrification in power transformers. Performance, accuracy, and applications of the device are outlined. >

The influence of electrohydrodynamic motion on the breakdown of dielectric liquids

A mathematical model for the transient electrohydrodynamic (EHD) motion in liquid dielectrics is used to examine the behaviour of dielectric fluids flowing at various velocities between high-voltage electrodes. The resulting flow patterns and space charge distribution profiles are correlated with observed variations in transformer-oil breakdown properties, and explanations for these variations are proposed.

Flow-induced electrification and partial discharge measurements in transformer duct structures

Physical models which are representative of the materials, construction, and dimensions of power transformer duct structures have been fabricated for both core- and shell-form designs. Electrification characteristics have been documented as a function of the parameters of practical interest and the results obtained have the general character of those found in earlier studies. However, partial discharges have only been detected under worst-case conditions for the blocking structure associated with the shell-form configurations which generate more charge than the corresponding conditions in the core-form construction. The studies have quantified electrification in realistic models characteristic of actual transformers and could be used as a guide to provide estimates of charge levels in energized transformer structures. >

Optical studies of electrokinetic processes in dielectric fluids

The behavior of transformer oil and other fluids used for the cooling and insulation of power system equipment is significantly influenced by enforced motion. Spatially resolved optical emission which illustrates the changes brought about by increased velocity in a stressed oil duct is presented. Schlieren records taken under the same circumstances permit the examination of concomitant hydraulic instabilities and their dependence on electric field through EHD vorticity generation. Optical data to show the influence of dielectric interfaces in the stressed gap and the impact of upstream charge injection on prebreakdown light emission are also presented. The results indicate that the modification of flow patterns and formation of a stagnant area are important aspects in determining the events leading to liquid breakdown, and have indicated the importance of oil motion per se in electrokinetic effects in liquid-cooled power system equipment. >

Dielectric integrity associated with circulating insulating fluids

The behaviour of transformer oil and other fluids used for the cooling and insulation of power system equipment is significantly influenced by enforced motion. Not only can charges generated by streaming electrification1 accumulate to prejudice dielectric integrity, but the dielectric strength of the fluid is also altered per se2 by the actions of the flow in a complex, but predictable, manner. Both these aspects need to be understood to permit informed design and operation of power equipment and appropriate countermeasures.

The determination of charge behavior at a liquid-solid interface

A methodology has been developed to provide information on the way in which charges distribute and relax on cellulose surfaces and on their impact on the breakdown of oil-cellulose insulation. Using this approach, not only can the charge be monitored in time, but its spatial distribution can be ascertained. The method has further been extended to apply an interrogating impulse voltage across a charged surface which permits the impact of surface charge on creep breakdown to be determined. Field plots which allow the calculation of a power frequency (capacitive) distribution in the presence of a static charge reveal that the impact of accumulated surface charge on the dielectric integrity of transformer structures is significant and should be considered in design criteria.<<ETX>>

Influence of enforced liquid motion on breakdown properties

It has been established by a number of investigators [1]–[4] that the statistics of breakdown of liquid dielectrics change with forced liquid motion. More specifically, the distribution of liquid breakdown voltage shifts with increasing velocity from the extreme value distribution that characterizes stationary fluids to a Gaussian distribution [2],[3]. This is quite significant from the practical point of view, since the long tail of the distribution at lower voltages is eliminated.

Flow-induced electrification and partial discharge measurements in energized transformer duct structures

Physical models which are representative of the materials, construction, and dimensions of power transformer duct structures have been fabricated for both core- and shell-form designs. Electrification characteristics have been documented as a function of the parameters of practical interest and the results obtained have the general character of those found in earlier studies. However, partial discharges have only been detected under worst-case conditions for the blocking structure associated with the shell-form configurations which generate more charge than the corresponding conditions in the core-form construction. The studies have quantified electrification in realistic models characteristic of actual transformers and could be used as a guide to provide estimates of charge levels in energized transformer structures.<<ETX>>

The generation of electrostatic charges in flowing dielectric fluids subjected to alternating fields

Several instances of anomalous failure in power system equipment have prompted investigations particularly in Japan [1] and in the US [2] to try to quantify and understand the streaming electrification process in dielectric liquids. Such studies have been successful in identifying the major parameters influencing the process but have usually been conducted by examining the charge separation and relaxation phenomena at an interface in a moving fluid without consideration of a super imposed electric fieid.

Optical studies of electrokinetic processes in flowing oil

Dielectric liquids subjected to enforced motion in narrow ducts exhibit anomalous but reproducible breakdown behavior[1]. Furthermore the velocity dependent dielectric integrity of such an arrangement is characteristically different for sustained and impulse electric stress[2-4]. The physical mechanisms responsible for electrical breakdown in these circumstances are complex since it is known that both electrohydrodynamic (EHD) phenomena and streaming electrification are present to influence prebreakdown events [2].