Three different comnercially avaiTable transformer oils were evaluated for their electrostatic charging behavior. One of these oils was found to have a higher charging tendency than the others. The charging behavior of this oil was moisture dependent; drying of the oil raised the charging level while addition of moisture lowered it. All the oils showed similar temperature dependence on charging behavior in a near exponential manner. Associated with high charging behavior was higher than normal electrical conducti vi ty and lower interfacial tension. The charging tendency is attributed to certain polar compounds in trace quantities, most likely surface active agents. These substances may be present in the refined oil or introduced later in the processing and handling stages.

Eelectrostatic Charging Tendency of Transformer Oils

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

It is of fundamental importance that appropriate oil purity be preserved for all applications in electrical equipment. Oil purity can be maintained if good oil handling and testing procedures are provided. Trace contaminants and very small particle contents have recently received more attention because of improved measurement techniques. As delivered, new oils require pre-treatment by the users, but most often this is only drying and filtering, although mixing and additive treatments are also used. Care has to be taken, as problems with the miscibility of the compounds can be experienced. >

Electrical insulating oils. I . Characterization and pre-treatment of new transformer oils

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

The condition of the internal cellulosic paper and oil insulation are of concern for the performance of power transformers. Over the years, a number of methods have been developed to diagnose and monitor the degradation/aging of the transformer internal insulation system. Some of this degradation/aging can be assessed from electrical responses. Currently there are a variety of electrical-based diagnostic techniques available for insulation condition monitoring of power transformers. In most cases, the electrical signals being monitored are due to mechanical or electric changes caused by physical changes in resistivity, inductance or capacitance, moisture, contamination or aging by-products in the insulation. This paper presents a description of commonly used and modern electrical-based diagnostic techniques along with their interpretation schemes.

Electrical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers

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

Probability distributions of partial discharge (PD) and dielectric breakdown voltage were investigated for the typical components of oil-filled transformer insulation.

H. Okubo

Papers

Suppression of Static Electrification of Insulating Oil for Large Power Transformers

PD and BD Probability Distribution and Equi-Probabilistic V-t Characteristics of Oil-Filled Transformer Insulation