THERMAL TRANSFER CHARACTERISTICS Cooling Method

Insulating Oil

Dielectric Constant

tan S (%)

Mineral oil

2.18

0.02

5-Chlorobiphenyl

4.25

0.03

3-Chlorobiphenyl

5.20

0.04

Chloroalkylene

5.05

<0.01

Trixylenyl phosphate

6.00

1.0

Diethylhexylphthalate

4.55

0.10

Castor oil

4.00

0.30

Alkylbenzene

2.17

0.005

Alkyl naphthalene

2.48

0.01

Alkylbiphenylethane

2.51

0.10

Silicone oil

2.52

0.008

Insulating liquids (oils) have not only excellent dielectric characteristics but also significant cooling effects. Insulating oils are important coolants in apparatus in which much heat is produced. Nevertheless, external cooling systems must sometimes be added, depending on the capacity of the appara­tus and the load.

In the case of transformers, there are several types with different cooling methods, such as the oil-immersed self­cooled type, forced-oil self-cooled type, direct-oil-flow self­cooled type, forced-oil forced-air-cooled type, direct-oil-flow forced-air-cooled type, oil-immersed forced-water-cooled type, and forced-oil forced-water-cooled type. Oil-immersed self­cooling is the simplest method. In this method transformers are cooled by natural convection. In other methods heated in­sulating oils are cooled by forced air, forced oil, or water using coolers or heat exchangers. In high-voltage and high-power transformers the latter methods are widely used.

Heat Transfer Characteristics of Insulating Oils

When insulating oil cools insulating solids that cover a heat source, such as the paper on transformer windings, the heat flux through the solid to the oil is expressed in (2)

In the expression for Nu, Re always appears in the form Re" (n > 0). Therefore, generally speaking, the higher the ve­locity is, the higher is a. For instance, in the case of a flat plane exposed to forced convection of liquid

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *