Page 91 - Electric Machinery Fundamentals

TRANSFORMERS 67

power systems meant that generating stations had to be small and localized and so
were relatively inefficient.
The invention of the transformer and the concurrent development of ac
power sources eliminated forever these restrictions on the range and power level
of power systems. A transformer ideally changes one ac voltage level to another
voltage level without affecting the actual power supplied. If a transformer steps up
the voltage level of a circuit, it must decrease the current to keep the power into
the device equal to the power out of it. Therefore. ac electric power can be gener-
ated at one central location, its voltage stepped up for transmission over long dis-
tances at very low losses, and its voltage stepped down again for final use. Since
the transmission losses in the lines of a power system are proportional to the
square of the current in the lines, raising the transmission voltage and reducing the
resulting transmission CULTents by a factor of 10 with transformers reduces power
transmission losses by a factor of 100. Without the transformer, it would simply
( not be possible to use electric power in many of the ways it is used today.
In a modern power system, electric power is generated at voltages of 12 to
25 kV. Transformers step up the voltage to between 110 kV and nearly 1000 kV for
transmission over long distances at very low losses. Transformers then step down
the voltage to the 12- to 34.5-kV range for local distribution and finally pennit the
power to be used safely in homes, offices, and factories at voltages as low as 120 V.

2_2 TYPES AND CONSTRUCTION
OF TRANSFORMERS

The principal purpose of a transformer is to convert ac power at one voltage level
to ac power of the same frequency at another voltage level. Transformers are also
used for a variety of other purposes (e.g., voltage sampling, current sampling, and
impedance transformation), but this chapter is primarily devoted to the power
transformer.
Power transformers are constructed on one of two types of cores. One type
of construction consists of a simple rectangular laminated piece of steel with the
transformer windings wrapped around two sides of the rectangle. This type of
construction is known as core form and is il1ustrated in Figure 2- 2. The other type
consists of a three-legged laminated core with the windings wrapped around the
center leg. This type of construction is known as shell form and is illustrated in
Figure 2-3. In either case, the core is constructed of thin laminations electrically
isolated from each other in order to minimize eddy currents.
The primary and secondary windings in a physical transfOlmer are wrapped
one on top of the other with the low-voltage winding innermost. Such an arrange-
ment serves two purposes:
(

1. It simplifies the problem of insulating the high-voltage winding from the core.
2. It results in much less leakage flux than would be the case if the two windings
were separated by a distance on the core.