Where a large number of three phase transformers can be used, it is generally advisable to install three phase units, the following advantages being in their favor as compared with single phase units:
- 1. Require less floor space than three single phase units;
- 2. Weigh less than the single phase units;
- 3. Simpler connections, as only three primary and three secondary leads are generally brought out;
- 4. Transformer presents a symmetrical and compact appearance.
Ques. What is the character of the construction of three phase transformers?
Ans. The three phase transformer is practically similar to that of the single phase, except that somewhat heavier and larger parts are required for the core structure.
| COMPARISON OF AIR BLAST, WATER COOLED, AND OIL COOLED TRANSFORMERS | ||
|---|---|---|
| Air blast type | Water cooled type | Oil cooled type |
| [8] NOTE.—No special foundations are necessary for any type of transformer other than a good, even floor, having sufficient strength to support the weight. | ||
| 1. COST | ||
| A. First cost | ||
| Necessarily more expensive than the water cooled type of similar rating. | Least expensive of all types. | Necessarily more expensive than the air blast and water cooled type of similar rating. |
| B. | ||
| The installation is extremely simple. Moisture that may have collected on the surfaces during transportation or storage should be thoroughly dried out. | Being heavier than the air blast type, these transformers, as a rule, require heavier apparatus for installing. Both transformer and tank should be thoroughly dried out before being filled with oil. The oil is usually supplied in 50 gal. hermetically sealed steel barrels to minimize possibility of moisture during transportation. | Being heavier than the air blast and water cooled type, these transformers require heavier apparatus for installing. Both transformer and tank should be thoroughly dried out before being filled with oil. |
| C. Auxiliary apparatus | ||
| A duct, or chamber, of considerable size is required under the transformers in order to conduct the cooling air to them. A blower outfit for supplying air is required. | In most cases, cooling water may be obtained with sufficient natural head. However, there are frequent cases in which it can be obtained only by the use of pumps. A system of piping for the cooling water and oil drainage is required, the cost of which depends, of course, on the station layout. | Do not require cooling water or blower. |
| D. Maintenance | ||
| An occasional cleaning, for which a supply of compressed air at about 20 lb. pressure is recommended. The blower outfit requires no more care than any other similar apparatus. | A water pumping outfit would possibly require a trifle more attention than a blower outfit in which there are no valves or piping. | No air or water circulation to demand attention. |
| 2. FLOOR SPACE | ||
| Always requires space for cooling apparatus. | Extra space only required when auxiliary pumping apparatus is necessary. | Only require space for the transformer as no extra apparatus is necessary |
| 3. LOCATION | ||
| As the transformers are open at the top they should not be located where there is much dust or dirt nor where water from any source is liable to fall on them. The blower should be so situated as to obtain clean dry air of a temperature not greater than 77° Fahr. | Transformers are completely enclosed but location should be such that no water will fall on leads or bushings. Location of auxiliary apparatus will depend on the station layout. | Transformers are completely enclosed but location should be such that no water will fall on leads or bushings. The building should be well ventilated. There is no auxiliary apparatus. |
| 4. GENERAL APPEARANCE | ||
| Terminal leads may be located in the base and the air chamber may be used for conducting and distributing the connecting wiring. The absence of overhead wiring aids in simplifying the appearance of the station. | Leads are brought out of the top of the transformers. Water cooling pipes are connected at the top in most cases. | Leads are brought out of the top of the transformers. |
| 5. OPERATION | ||
| Equal reliability in all three types. While full load efficiencies are practically equal in the three designs, it is necessary to change the proportion of iron and copper losses somewhat as the copper loss of the air blast transformer is a smaller part of its total loss than of the water cooled and oil cooled types. As a result, the regulation of the air blast transformer is a trifle better. | ||
| 6. GENERAL | ||
| The above information regarding selection of type is not applicable to air blast transformers for circuits materially in excess of 33,000 volts. On account of the great thickness of the solid insulation needed and the consequent difficulty in radiating heat from the copper, it is impracticable to design the air blast type for more than this voltage. The oil immersed designs are therefore recommended for transformers above 33,000 volts. Both oil cooled and water cooled types are available for all voltages, being restricted in this respect only by the limitations of transmission facilities.[8] | ||
Figs. 2,004 to 2,011.—Connections of standard transformers. All stock transformers are wound for some standard transformation ratio, such as 10 to 1, but various leads are brought out by means of which ratios of 5, 10 and 20 to 1 may be obtained for one transformer. The figures show the voltage combinations possible with a standard transformer.
Ques. How are transformers connected for four wire three phase distribution?
Ans. When the secondaries of three transformers are star connected, a fourth wire may be run from the neutral point, thus obtaining the four wire system.
