Page 262 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS GENERATORS

SYNCHRONOUS GENERATORS 12.25

SYNCHRONOUS GENERATOR RATINGS

There are limits to the output power of a synchronous generator. These limits are known as
ratings of the generator. Their purpose is to protect the generator from damage caused by
improper operation. The synchronous generator ratings are voltage, frequency, speed,
apparent power (kilovoltamperes), power factor, field current, and service factor.

The Voltage, Speed, and Frequency Ratings

The common system frequencies used today are 50 Hz (in Europe, Asia, etc.) and 60 Hz (in
the Americas). Once the frequency and the number of poles are known, there is only one
possible rotational speed.
One of the most important ratings for the generator is the voltage at which it operates.
Since the generator’s voltage depends on the flux, the higher the design voltage, the higher
the flux. However, the flux cannot increase indefinitely because the field current has a
maximum value.
The main consideration in determining the rated voltage of the generator is the break-
down value of the winding insulation. The voltage at which the generator operates must not
approach the breakdown value.
A generator rated for a given frequency (say, 60 Hz) can be operated at 50 Hz as long as
some conditions are met. Since there is a maximum flux achievable in a given generator,
and since E K , the maximum allowable E must change when the speed is changed.
A A
For example, a generator rated for 60 Hz can be operated at 50 Hz if the voltage is derated
to 50/60, or 83.3 percent, of its design value. The opposite effect will occur when a gener-
ator rated for 50 Hz is operated at 60 Hz.

Apparent Power and Power Factor Ratings
The factors that determine the power limits of electric machines are the shaft torque and the
heating of the windings. In general, the shaft can handle larger power than the machine is
rated for. Therefore, the steady-state power limits are determined by the heating in the
windings of the machine. The windings that must be protected in a synchronous generator
are the armature windings and the field windings.
The maximum allowable current in the armature determines the maximum apparent
power for the generator. Since the apparent power S is given by

S 3V I
A
if the rated voltage is known, the maximum allowable current in the armature determines
the rated apparent power of the generator.
The power factor of the armature current does not affect the heating of the armature
windings. The stator copper loss (SCL) heating effect is
2
P 3I R
SCL A A
These effects are independent of the angle between the I and V . These generators are
A
not rated in megawatts (MW), but in megavoltamperes (MVA).
The field winding copper losses are
2
P RCL I R F
F

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