DMC490 Microgrid Controller Software Configuration Guide
10
Where:
T
m
is the mechanical torque provided by the prime mover
T
e
is the electrical torque exerted by the loading of the power system
J
is the inertia constant of the machine, and
ω
is the rotational speed.
Solving for dω/dt,
the result is:
Eq. 2
Multiplying by the top and bottom of this equation by rated speed, ω0,
the result is:
Eq. 3
Speed is
constant (dω/dt = 0) whenever mechanical power matches electrical power. This fact allows us to
control a generator to supply the required load by regulating the speed of the generator at a fixed value. This
method is known as isochronous control and is shown in Figure 4.
Figure 4 Isochronous control
Assume that electrical and mechanical power are initially equal and speed is equal to the reference speed.
When the loading of the system increases, then P
e
becomes greater than P
m
. As seen in the previous equation,
the machine speed drops. The governor takes a measurement of machine speed and compares it with a
reference speed (nominal speed). The difference is an error signal that is applied to a transfer function (typically
a proportional/integral regulator). The output of the governor drives an actuator (for instance a valve in the
case of a steam turbine). This acts to increase the flow of steam to the turbine, increasing the mechanical
power to balance the electrical power.
The operating characteristic of an isochronous generator is shown in the plot of frequency (proportional to
mechanical speed) and power; see Figure 5.
Summary of Contents for DMC490
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