Page 72 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 4
TRANSFORMER

COMPONENTS AND

MAINTENANCE

INTRODUCTION

Electric power is generated most economically at 14 to 25 kV. System loads such as
motors, lights, etc., require a voltage source of 440, 220, and 110 V. Transformers are
needed to change the voltage level. They are also used for these reasons:
1. To reduce transmission losses between power plants and the load by stepping up the
voltage. (When the voltage is stepped up, the current will be stepped down. This results
in reduction of transmission losses which are proportional to the square of the current.)
2. Reduction in the diameter of the transmission line (amount of copper or aluminum) due
to a reduction in the current flowing in the line.
Typical transmission voltages are 13.2, 22, 66, 230, 345, and 500 kV. In general, the longer
the distance, the higher the voltage used.

CLASSIFICATION OF TRANSFORMERS

The two types of transformers are air-cooled (dry type) and oil-filled. The transformer rat-
ing increases with improved cooling methods. These are the typical ratings for various
types of transformers:

Dry Transformers
The two types of dry transformers are self-air-cooled and forced-air-cooled. The heat is
removed by natural convection in self-air-cooled transformers. In forced-air-cooled, it is
removed by blowers. The rating of dry-type transformers used to be limited to less than 1
MVA. However, modern technology pushed this rating to 20 MVA. This was mainly due to
improvements in the quality of insulation and mechanisms of heat removal from the trans-
former.

Oil-Immersed Transformers
In this type of transformer, the windings and core are immersed in oil. The main types of
oil-immersed transformers are
4.1
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