Control and Design of Microgrid Components - Power Systems ...

ating on the voltage output, establishing the maximum voltage level at which that the power is
produced.
Microturbines have a two pole permanent magnet generator that converts the mechanical power
of the shaft into electric power at variable frequency, depending on the speed of rotation. The
variable frequency voltage is rectified to a DC voltage that is almost independent of the variable
AC frequency.
Figure 4.2 Capstone DC Bus Voltage During Load Changes in Island Mode.
Figure 4.2, [21] shows the behavior of the DC bus voltage during load changes on a Capstone
microturbine operating in island mode. Each time the electrical load is diminished, the DC bus
voltage immediately increases due to the lower power demand: at that point the fuel valve
controller decreases the fuel to the microturbine to reduce its output power and regulate the DC
voltage. When the load is increased, the DC voltage decreases because the storage is providing
immediately power to the load: at this point the controller increases the fuel injection to the
microturbine to increase its output power and restore the DC voltage. DC bus voltage is 760V
and excursions in either direction never exceed 3 volts, suggesting that the range of voltages on
the DC bus is independent of the angular speed of the microturbine shaft.
The fuel cells have a different behavior on the DC voltage output as a function of the output
power. For example, Figure 4.3 [24] shows the stack voltage as the current output increases on a
3.5kW fuel cell stack: the voltage changes of almost of 1 Volt per Ampere for the first 10 A, then
it changes with a smaller slope.
51