Lecture 8
Effect of power imbalance on system frequency
Consider the analogy of a mechanical system, where a car is traveling with a velocity
v
,
there is mechanical power, and friction, as depicted in
Figure 1
.
Figure 1  Mechanical analogy
The kinetic energy of the car with mass m is
2
1
2
e
km
v
=
If P
mech
< P
friction
then the car slows down. Alternatively if we push the pedal then the car
speeds up, i.e.
v
decreases, as well as the kinetic energy.
A similar situation occurs in rotating machines. A generator is a rotating device
connected to a turbine with a moment of inertia
I
.
Figure 2  Power Conversion
If
P
mech
= P
elec
then the speed of rotation (
ω
m
) is constant and the electric frequency (
ω
)
as well.
This is called the steady state mode of operation. Hence if P
elec
increases, analog to
P
friction
, then
ω
m
decreases and the input of steam or water should be increased, but this
response is slow (couple of minutes).
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View Full DocumentTherefore if P
mech
cannot keep up with the change on P
elec
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 Spring '09
 FRANCISCODGALIANA
 Energy, Kinetic Energy, Mass, Pmech

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