The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.

50 years in the development of insulating liquids

Streaming electrification of large power transformers is discussed here from the standpoint of characteristics of insulating oils used in them.

Suppression of Static Electrification of Insulating Oil for Large Power Transformers

Streaming electrification in transformers occurs at the interface between the insulating oil and the insulating material as the oil circulates in the transformer. As a result, the insulating oil and the surface of the solid materials become charged and dielectric breakdown is possible within the oil or at the interface of insulating materials.

Suppression of static electrification of insulating oil for large power transformers

Since December of 1983, there have been several failures of large power transformers attributed to the static electrification phenomena that have been brought to the attention of Texas Power & Light Company. Two of these failures were ours. Prior to December of 1983, numerical statistics describing failures from static electrification are apparently not readily accessible. IEEE and other reference literature addressed the phenomena, but the magnitude of the problem cannot be quantified in terms of numbers of failures.

The static electrification phenomena in power transformers

A Couette charger (CC) facility has been built to simulate flow electrification processes in transformers where transformer oil fills the annulus between coaxial cylindrical electrodes. Transient measurements with a step change in temperature have shown the charge density to change from an initial value to a new steady-state value, including cases of polarity reversal, both values dependent on the equilibrium moisture levels in oil and pressboard. Through a model that accounts for diffusion of charge from the paper-oil interface into the bulk of the oil, these measurements are used to deduce the equilibrium wall charge density. To help understand the effects of energization and to scale laboratory measurements at low voltages and low frequencies to operating transformers, a charge injection model was refined to examine the migration of double layer and injected charge in the imposed sinusoidally time-varying electric field. Good fits between this model and measurements are achieved. >

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Fluid electrification measurements of transformer pressboard/oil insulation in a Couette charger

In this paper, the streaming electrification in core type forced oil cooled power transformers is investigated, using a full-scale model transformer and various experimental models. It has been clarified that streaming electrification in transformers is influenced mainly by flow rate, oil temperature, surface conditions of insulating materials and characteristics of insulating oil. Electrostatic discharge occurs eventually when the amount of local charge exceeds a certain limit.

H. Murata

Papers

Electrostatics in Power Transformers