Insulating oil, like petroleum system liquids such as gasoline, toluene, and kerosene, has high volume resistivity. In such insulating liquids, ionic compounds A+B— dissociate into A+ and B—, and the ions become charge carriers:

A+B – ^ A+ + B-

There is no charging when the numbers of A+ and B— are equal. However, for example, static electrification is observed when the number of positive ions is increased by the flow of liquid. This static electrification is called streaming electrifi­cation.

Mechanisms of Streaming Electrification

The streaming electrification process arises from electric charge motion, separation, and relaxation (Fig. 5). These pro­cesses happen simultaneously:

1. Electric Charge Motion. Certain ions in the liquid are adsorbed on a solid in contact with it.

2. Electric Charge Separation. Other ions are carried off by flow, and an imbalance of positive and negative ions occurs.

3. Electric Charge Relaxation. Surplus ions are neutral­ized, and the imbalance is canceled.

Properties of Streaming Electrification

When the following three conditions coexist, large streaming electrification is observed:

1. It is easy to move an electric charge, because a solid surface is active.

2. Charges are easily separated.

3. Surplus electric charge is not easily canceled.

When any one of the three processes is hindered, stream­ing electrification can be prevented. The choice of insulating oil can also affect streaming electrification.

Polarity of Streaming Electrification

In streaming electrification between insulating paper and in­sulating oil, the oil becomes positively charged and the insu­lating paper negatively charged. A possible reason is a pecu­liarity of the oxygen of the hydroxyl group (-OH) in the insulating paper (cellulose).

Oxygen, having high electronegativity (that is, ability to attract electrons), attracts the electron of hydrogen. It thereby becomes negatively charged, and the hydrogen becomes posi­tively charged. The cellulose molecule surface is covered with positively charged hydrogen, which adsorbs negative ions in oil selectively. Therefore, the insulating oil becomes positively charged, and the insulating paper is negatively charged.

Streaming Electrification and the Deterioration of Insulating Paper

The hydroxyl group (-OH) of cellulose is changed to the alde­hyde group (-CHO) or the carboxyl group (-COOH) by oxida­tive deterioration. The extent of polarization due to electron transfer from hydrogen to oxygen, mentioned above, is in the following order:

hydroxyl group < aldehyde group < carboxyl group

Accordingly, streaming electrification increases as the insu­lating paper deteriorates.

Oil temperature (°C)

streaming electrification, and also direct observation of streaming electrification (9) in transformers have been re­ported. A good measure of the streaming electrification in a transformer is the coil leakage current.

Figure 6 shows the oil temperature dependence of the coil leakage current, and reveals that it is influenced by differ­ences in the insulating oil.

Coil leakage current is significant at all operating temper­atures in large-capacity transformers. Therefore, measure­ment of the streaming electrification is essential in trans­former design with respect to internal structure, oil flow rate, and insulating material. One may also select an oil that re­sists charging and charge separation.

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