Fuel cell power plant control to prevent reactant starvation during islanded mode of operation

1. A controller for a fuel cell power plant, the controller comprising:

at least one processor and memory associated with the processor, the processor being configured to control operation of the fuel cell power plant during an islanded mode of operation wherein the fuel cell power plant provides output power to a load, the processor being configured to control the operation of the fuel cell power plant in the islanded mode by

adjusting a droop gain of the controller to change the output power of the fuel cell power plant in response to a change in demand from the load while:

maintaining a portion of the demand from the load met by the output power of the fuel cell power plant within a predetermined allocation of islanded mode load sharing assigned to the fuel cell power plant,

maintaining a ramp up rate of the output power of the fuel cell power plant within a predetermined maximum ramp up capability of the fuel cell power plant, and

maintaining a frequency of the output power of the fuel cell power plant within a predetermined range.

2. The controller of claim 1 , wherein the processor is configured to control a rate at which the droop gain changes to maintain stability of the fuel cell power plant.

3. The controller of claim 2 , wherein the processor includes a filter that controls the rate at which the droop gain changes.

4. The controller of claim 3 , wherein the filter prevents a change in the droop gain during a predetermined time lag in response to the change in the load demand.

5. The controller of claim 1 , wherein

the processor is configured to determine a droop gain adjustment factor for adjusting the droop gain to maintain the portion of the demand from the load met by the output power of the fuel cell power plant within the predetermined allocation; and

the processor is configured to determine the droop gain adjustment factor by

determining an instantaneous ramp up rate of the output power of the fuel cell power plant, and

setting the droop gain adjustment factor equal to: (the instantaneous ramp up rate)×(a predetermined frequency droop gain)/(a predetermined maximum ramp up rate of the output power of the fuel cell power plant).

6. The controller of claim 5 , wherein the processor is configured to determine the instantaneous ramp up rate by determining a derivative of the current output power.

7. The controller of claim 6 , wherein

the determined derivative is zero during a steady state condition of the demand of the load; and

the processor applies the predetermined fixed frequency droop gain during the steady state condition of the demand of the load.

8. The controller of claim 1 , wherein the processor is configured to maintain the ramp up rate of the output power of the fuel cell power plant within the predetermined maximum ramp up capability of the fuel cell power plant by

determining an instantaneous ramp up rate of the output power of the fuel cell power plant; and

adjusting the droop gain by a droop gain adjustment factor when the instantaneous ramp up rate is greater than the maximum ramp up capability,

or

setting the droop gain adjustment factor to zero when the instantaneous ramp up rate is below the maximum ramp up capability.

9. The controller of claim 1 , wherein the processor is configured to maintain the frequency of the output power of the fuel cell power plant within the predetermined range by

setting a maximum droop gain adjustment factor to be equal to: (a predetermined maximum frequency droop)/(a current output power of the fuel cell power plant)−(a predetermined fixed frequency droop gain); and

adjusting the droop gain using a droop gain adjustment factor that is less than or equal to the maximum droop gain adjustment factor.

10. A fuel cell power plant comprising at least one cell stack assembly including a plurality of fuel cells and the controller of claim 1 .

11. A method of controlling a fuel cell power plant, the method comprising:

determining that the fuel cell power plant is operating in an islanded mode wherein the fuel cell power plant provides output power to a load; and

adjusting a droop gain to change an output power of the fuel cell power plant in response to a change in demand from the load while:

maintaining a portion of the demand from the load met by the output power of the fuel cell power plant within a predetermined allocation of islanded mode load sharing assigned to the fuel cell power plant,

maintaining a ramp up rate of the output power of the fuel cell power plant within a predetermined maximum ramp up capability of the fuel cell power plant, and

maintaining a frequency of the output power of the fuel cell power plant within a predetermined range.

12. The method of claim 11 , comprising controlling a rate at which the droop gain changes to maintain stability of the fuel cell power plant.

13. The method of claim 12 , comprising using a filter that controls the rate at which the droop gain changes.

14. The method of claim 13 , wherein the filter prevents a change in the droop gain during a predetermined time lag in response to the change in the load demand.

15. The method of claim 11 , comprising

determining a droop gain adjustment factor for adjusting the droop gain for maintaining the portion of the demand from the load met by the output power of the fuel cell power plant within the predetermined allocation; and

determining the droop gain adjustment factor by

determining an instantaneous ramp up rate of the output power of the fuel cell power plant, and

setting the droop gain adjustment factor equal to: (the instantaneous ramp up rate)×(a predetermined frequency droop gain)/(a predetermined maximum ramp up rate of the output power of the fuel cell power plant).

16. The method of claim 15 , comprising determining the instantaneous ramp up rate by determining a derivative of the current output power.

17. The method of claim 16 , wherein

the determined derivative is zero during a steady state condition of the demand of the load; and

the method comprises applying the predetermined fixed frequency droop gain during the steady state condition of the demand of the load.

18. The method of claim 11 , wherein maintaining the ramp up rate of the output power of the fuel cell power plant within the predetermined maximum ramp up capability of the fuel cell power plant comprises

determining an instantaneous ramp up rate of the output power of the fuel cell power plant; and

adjusting the droop gain by a droop gain adjustment factor when the instantaneous ramp up rate is less than the maximum ramp up capability

or

setting the droop gain adjustment factor to zero when the instantaneous ramp up rate exceeds the maximum ramp up capability.

19. The method of claim 11 , wherein maintaining the frequency of the output power of the fuel cell power plant within the predetermined range comprises

setting a maximum droop gain adjustment factor to be equal to: (a predetermined maximum frequency droop)/(a current output power of the fuel cell power plant)−(a predetermined fixed frequency droop gain); and

adjusting the droop gain using a droop gain adjustment factor that is less than or equal to the maximum droop gain adjustment factor.