CONTROL OF SOLID OXIDE FUEL CELL (SOFC) SYSTEMS IN STAND-ALONE AND GRID CONNECTED MODES

ABSTRACT

As energy consumption rises, one must find suitable alternative means of generation to supplement conventional existing generation facilities. In this regard, distributed generation (DG) will continue to play a critical role in the energy supply demand realm. The common technologies available as DG are micro-turbines, solar, photovoltaic systems, fuel cells stack and wind energy systems. In this project, dynamic model of solid oxide fuel cell (SOFC) is done. Fuel cells operate at low voltages and hence fuel cells need to be boosted and inverted in order to connect to the utility grid. A DC-DC converter and a DC-AC inverter were used for interfacing SOFC with the grid. These models are built in MATLAB/SIMULINK. The power characteristics of the fuel cell, DC-DC converter, DC-AC inverter are plotted for reference real power of 50kW for standalone applications. The power characteristics of the DC-AC inverter are plotted for 30kW, 50kW, 70kW of load and also for step change in load for grid connected applications.

KEYWORDS:

1.      Distributed Generation

2.      DC-DC Converter

3.      Solid Oxide Fuel Cell (SOFC)

SOFTWARE: MATLAB/SIMULINK

SIMULATION MODEL:

Figure 1 Simulation model for GRID connected applications

 SIMULATION RESULTS

Figure 2. Power response for 50kW of load

Figure 3. Current response for 50kW of load

Figure 4. Power response for 50kW of load

Figure 5. Current response for 30kW of load

Figure 6. Power response for 70kW of load

Figure 7. Current response for 70kW of load

Figure 8. Response of power for step change in load

Figure 9. Response of current for step change in load

Figure 10. Response of power flow during faults in load

Figure 11. Response of current flow during faults in load

Figure 12. Response of Reactive Power Flow of 200 VAR

Figure 13. Response of Reactive power Flow for step change

CONCLUSION

A dynamic model of the solid oxide fuel cell (SOFC) was developed in this project in MATLAB environment setup.

A DC-DC boost converter topology and its closed loop control feedback system have been built. A three phase inverter has been modeled and connected between the SOFC-DC-DC system on the one side and the utility grid on the other side. A control strategy for the inverter switching signals has been discussed and modeled successfully.

The fuel cell, the converter and the inverter characteristics were obtained for a reference real power of 50kW.The slow response of the fuel cell is due to the slow and gradual change in the fuel flow which is proportional to the stack current. The interconnection of the fuel cell with the converter boosts the stack voltages and also regulates it for varying load current conditions. The fuel cell stack voltage drops to zero for discontinuous current and the system shuts down. The fuel cell unit shuts off for real power above the maximum limit. Additional power at the converter is provided by the inductor, connected in series with the equivalent load which acts as an energy storage. The inductor can be replaced by any energy storage device such as a capacitor or a battery for providing additional power during load transients.

The inverter control scheme uses a constant power control strategy for grid connected applications and a constant voltage control strategy for standalone applications to control the voltage across inverter and current flowing through the load. The characteristics for the system have been obtained. The inverter voltage, current, power waveform have been plotted. The real power injection into the grid takes less than 0.1s to reach the commanded value of 50kW. The reactive power injection has been assumed to be zero and was evident from the simulation results. The maximum power limit on the fuel cell is 400kW. For any reference power beyond this limit, the fuel cell loses stability and drops to zero. This limit has been set by the parameters considered for the fuel cell data. Higher power can be commanded by either increasing the number of the cells, increasing the reversible standard potential or by decreasing the fuel cell resistance.

The system was then subjected to a step change in the reference real power from 40 to 80kW.The fuel cell, the converter and the inverter responses were obtained. The characteristics of the fuel cell (voltage, current and power) have a slower gradual change at the instant of step change. The DC link voltage was maintained at the reference value by the closed loop control system. Step change in the reference power from 40 to 80kW has been considered in order to observe the sharing of power from inverter to grid and from grid to the load of the fuel cell. The reactive power was zero until the step change and after the step change, oscillations were observed in the reactive power as well. Voltage, current, power characteristics of inverter, load and grid as been plotted for various conditions of load.

REFERENCES

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