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

Figure 4.3 Fuel Cell Stack Voltage as a Function of the Output Current.
The bottom line is that the voltage changed of about 20V as the output current went from zero to
50A, out of a 68V no-load voltage. This characteristic implies that the output voltage of the DC
stack needs to be interfaced with a DC/DC converter with the DC bus voltage. Without such an
interface when the fuel cell output voltage becomes smaller than the voltage of the DC bus the
power would flow in the opposite direction towards the stack.
4.2 DC Bus and Storage Ratings
The DC bus has two ratings, one is on the voltage, the other is on the amount of energy that can
be stored in it. The DC bus voltage rating is the voltage level from the prime mover.
Microturbines have a 3 Volts range on a 760V, while fuel cells need to be interfaced with a
DC/DC converter to obtain such a small range of change in the output voltage as the loading
conditions change. The voltage from a 35kW fuel cell would require a series of 10 stacks like the
one showed on Figure 4.3 reaching a no load voltage output in the range of 700V before the
DC/DC converter.
Since the prime mover does not have instantaneous tracking capability of the power command,
the DC bus needs to store enough energy to be able to supply the load immediately while the
prime mover ramps up. The worse case is during island operation when the grid is not available
to supplement that transient energy. The amount of rated stored energy should be enough to
guarantee tracking load requests in the face of the slow dynamics of the prime mover. This
implies that the ratings on the storage are dependent on the technology of the prime mover, for
instance, a microturbine that takes ten seconds to ramp up output power should have less storage
than a fuel cell that takes over a minute to ramp up.
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