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:

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Figure 1 Simulation model for GRID connected applications

SIMULATION RESULTS

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Figure 2. Power response for 50kW of load

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Figure 3. Current response for 50kW of load

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Figure 4. Power response for 50kW of load

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Figure 5. Current response for 30kW of load

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Figure 6. Power response for 70kW of load

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Figure 7. Current response for 70kW of load

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Figure 8. Response of power for step change in load

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Figure 9. Response of current for step change in load

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Figure 10. Response of power flow during faults in load

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Figure 11. Response of current flow during faults in load

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Figure 12. Response of Reactive Power Flow of 200 VAR

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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

  1. Padulles, G. W. Ault, and J. R. McDonald, “An Approach to the Dynamic Modeling of Fuel Cell Characteristics for Distributed Generation Operation,” IEEE- PES Winter Meeting, vol. 1, Issue 1, pp. 134-138, January 2000.
  2. Pasricha, and S. R. Shaw, “A Dynamic PEM Fuel Cell Model,” IEEE Trans. Energy Conversion, vol. 21, Issue 2, pp. 484-490, June 2006.
  3. R. Pathapati, X. Xue, and J. Tang, “A New Dynamic Model for Predicting Transient Phenomena in a PEM Fuel Cell System,” Renewable Energy, vol. 30, Issue 1, pp. 1-22, January 2005.
  4. Wang, and M. H. Nehrir, “Dynamic Models and Model Validation for a PEM Fuel Cells Using Electrical Circuits,” IEEE Trans. Energy Conversion, vol. 20, Issue 2, pp. 442-451, June 2005.
  5. J. Hall, and R. G. Colclaser, “Transient Modeling and Simulation of a Tubular Solid Oxide Fuel Cell,” IEEE Trans. Energy Conversion, vol. 14, Issue 3, pp.749-753, September1999.

Distributed Generation System Control Strategies in Microgrid Operation

 

ABSTRACT:

Control strategies of distributed generation (DG) are investigated for different combination of DG and storage units in a microgrid. This paper develops a detailed photovoltaic (PV) array model with maximum power point tracking (MPPT) control, and presents real and reactive power (PQ) control and droop control for DG system for microgrid operation. In grid-connected mode, PQ control is developed by controlling the active and reactive power output of DGs in accordance with assigned references. In islanded mode, DGs are controlled by droop control. Droop control implements power reallocation between DGs based on predefined droop characteristics whenever load changes or the microgrid is connected/disconnected to the grid, while the microgrid voltage and frequency is maintained at appropriate levels. This paper presents results from a test microgrid system consisting of a voltage source converter (VSC) interfacing with a DG, a PV array with MPPT, and changeable loads. The control strategies are tested via two scenarios: the first one is to switch between grid-connected mode and islanded mode and the second one is to change loads in islanded mode. Through voltage, frequency, and power characteristics in the simulation under such two scenarios, the proposed control strategies can be demonstrated to work properly and effectively.

KEYWORDS:

  1. Distributed generation
  2. PV
  3. Microgrid
  4. Droop control
  5. PQ control

 SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

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Fig. 1. Schematic of the microgrid.

CONTROL SYSTEM:

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Fig. 2. Schematic of the PQ control.

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Fig. 3. Schematic of the droop control.

 EXPECTED SIMULATION RESULTS:

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Fig. 4. PQ control under grid-connected mode.

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Fig. 5. Droop control for switching modes.

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Fig. 6. Droop control for varying load.

 

CONCLUSION:

In this paper a detailed PV model with MPPT, and PQ and droop controllers is developed for inverter interfaced DGs. The use of PQ control ensures that DGs can generate certain power in accordance with real and reactive power references. Droop controller is developed to ensure the quick dynamic frequency response and proper power sharing between DGs when a forced isolation occurs or load changes. Compared to pure V/f control and master-slave control, the proposed control strategies which have the ability to operate without any online signal communication between DGs make the system operation cost-effective and fast respond to load changes. The simulation results obtained shows that the proposed controller is effective in performing real and reactive power tracking, voltage control and power sharing during both grid-connected mode and islanded mode. To fully represent the complexity of the microgrid, future work will include the development of hierarchical controllers for a microgrid consisting of several DGs and energy storage system. The function of primary controller is to assign optimal power reference to each DG to match load balances and the secondary controllers are designed to control local voltage and frequency.

REFERENCES:

Barsali, S., Ceraolo M., Pelacchi, P., and Poli, D. (2002). Control techniques of dispersed generators to improve the continuity of electricity supply. IEEE Conf., Power Engineering Society, vol.2, pp.789-794.

Cai, N., and Mitra J. (2010). A decentralized control architecture for a microgrid with power electronic interfaces. IEEE conf., North American Power Symposium, pp. 1-8.

Chen, X., Wang, Y.H., and Wang, Y.C. (2013). A novel seamless transferring control method for microgrid based on master-slave configuration. IEEE Conf., ECCE Asia, pp. 351-357.

Cho, C. H., Jeon, J.H., Kim, J.Y., Kwon, S., Park, K., and Kim, S. (2011). Active synchronizing control a microgrid. IEEE Trans., Power Electron., vol. 26, no. 12, pp. 3707-3719

Choi, J.W. and Sul, S.K. (1998). Fast current controller in three-phase AC/DC boost converter using d-q axis crosscoupling. IEEE Trans., Power Electron., vol.13, no.1, pp. 179-185.

A New Control Strategy for a Multi-Bus MV Microgrid Under Unbalanced Conditions

 

ABSTRACT:

This paper proposes a new control strategy for the islanded operation of a multi-bus medium voltage (MV) microgrid. The microgrid consists of several dispatchable electronically-coupled  distributed generation (DG) units. Each DG unit supplies a local load which can be unbalanced due to the inclusion of singlephase  loads. The proposed control strategy of each DG comprises a proportional resonance (PR) controller with an adjustable resonance frequency, a droop control strategy, and a negative-sequence impedance controller (NSIC). The PR and droop controllers are, respectively, used to regulate the load voltage and share the average power components among the DG units. The NSIC is used to effectively compensate the negative-sequence currents of the unbalanced loads and to improve the performance of the overall microgrid system.Moreover, the NSIC minimizes the negative-sequence currents in the MV lines and thus, improving the power quality of the microgrid. The performance of the proposed control strategy is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment.

KEYWORDS:

  1. Distributed generation
  2. Medium voltage (MV)
  3. Microgrid
  4. Negative-sequence current
  5. Power sharing
  6. Unbalance load
  7. Voltage control

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

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 Fig. 1. MV multi-bus microgrid consisting of two DG units.

EXPECTED SIMULATION RESULTS:

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Fig. 2 Unbalanced load changes in feeder F1 (a) instantaneous real, and (b)

reactive power components.

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Fig. 3. Amplitude of (a) positive- and (b) negative-sequence currents of the feeders.

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Fig. 4. Instantaneous voltages at DG terminals during unbalanced load changes in feeder F1, (a) DG1and (b) DG2 .

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Fig.5. Frequency of islanded microgrid during unbalanced load changes.

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Fig. 6. (a) Negative-sequence output impedance of each DG, and (b) amplitude of negative-sequence current of DG units.

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Fig. 7. Dynamic response of DG units to unbalanced load changes in feeder F1: (a) real power, and (b) reactive power components of DG units.

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Fig. 8. Unbalanced load changes in feeders F3 and F2 (a, b) instantaneous real and reactive power of feeders.

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Fig. 9. Amplitude of (a) positive and (b) negative-sequence currents of the feeders.

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Fig. 10. (a) Negative-sequence output impedance, and (b) amplitude of negative- sequence current for each DG.

CONCLUSION:

This paper presents a new control strategy for amulti-bus MV microgrid consisting of the dispatchable electronically-coupled DG units and unbalanced loads. The negative-sequence current of a local load is completely compensated by its dedicated DG. However, the negative-sequence current of the nonlocal loads is shared among the adjacent DGs. The proposed control strategy is composed of a PR controller with non-fixed resonance frequency, a droop control, and a negative-sequence impedance controller (NSIC). The PR and droop controllers are, respectively, used to regulate the load voltage and to share the average power among the DG units. The NSIC is used to improve the performance of the microgrid system when the unbalanced loads are present. Moreover, the NSIC minimizes the negative- sequence currents in the MV lines, and thus, improving the power quality of the microgrid. The performance of the proposed control strategy is investigated by using digital time-domain simulation studies in the PSCAD/EMTDC software environment. The simulation results conclude that the proposed strategy:

  • robustly regulates voltage and frequency of the microgrid;
  • is able to share the average power among the DGs;
  • effectively compensates the negative-sequence currents of local loads; and
  • shares the negative-sequence current of the nonlocal loads such that the power quality of the overall microgrid is not degraded.

 REFERENCES:

[1] N. Hatziargyriou, H. Asano, R. Iravani, and C. Marnay, “Microgrids,” IEEE Power Energy Mag., vol. 5, pp. 78–94, Jul.–Aug. 2007.

[2] A. G. Madureira and J. A. P. Lopes, “Coordinated voltage support in distribution networks with distributed generation and microgrids,” IET Renew. Power Gener., vol. 3, pp. 439–454, Sep. 2009.

[3] IEEE Recommended Practice for Monitoring Electric Power Quality, IEEE Std. 1159, 2009.

[4] IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, ANSI/IEEE Std. 141, 1993.

[5] R. Lasseter, “Microgrids,” in Proc. IEEE Power Eng. Soc. Winter Meeting, 2002, pp. 305–308.

 

 

An Integrated Hybrid Power Supply for Distributed Generation Applications Fed by Nonconventional Energy Sources

ABSTRACT

A new, hybrid integrated topology, fed by photovoltaic (PV) and fuel cell (FC) sources and suitable for distributed generation applications, is proposed. It works as an uninterruptible power source that is able to feed a certain minimum amount of power into the grid under all conditions. PV is used as the primary source of power operating near maximum power point (MPP), with the FC section (block), acting as a current source, feeding only the deficit power. The unique “integrated” approach obviates the need for dedicated communication between the two sources for coordination and eliminates the use of a separate, conventional dc/dc boost converter stage required for PV power processing, resulting in a reduction of the number of devices, components, and sensors. Presence of the FC source in parallel (with the PV source) improves the quality of power fed into the grid by minimizing the voltage dips in the PV output. Another desirable feature is that even a small amount of PV power (e.g., during low insolation), can be fed into the grid. On the other hand, excess power is diverted for auxiliary functions like electrolysis, resulting in an optimal use of the energy sources. The other advantages of the proposed system include low cost, compact structure, and high reliability, which render the system suitable for modular assemblies and “plug-n-play” type applications. All the analytical, simulation results of this research are presented.

 

INDEX TERMS: Buck-boost, distributed generation, fuel cell, grid-connected, hybrid, maximum power point tracking (MPPT), photovoltaic.

 

SOFTWARE: MATLAB/SIMULINK

 

BLOCK DIAGRAM

image001   Fig. 1. Various HDGS configurations. (a) Conventional, multistage topology using two H-bridge inverters [4], [6]. (b) Modified topology with only one H-bridge inverter [4]. (c) Proposed topology. λ denotes solar insolation (Suns).

  

SIMULATION RESULTS

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Fig. 2. Simulation results of the integrated hybrid configuration showing transition from mode III to mode II and then to mode I. T1 and T2 denote the transition between mode III to mode II and mode II to mode I respectively.

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Fig. 3. Simulation results of the integrated hybrid configuration operating in electrolysis mode (mode I to mode III and then to mode I). T1 and T2 denote the transition between mode I to mode III and mode III to mode I respectively.

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Fig.4. Performance comparison of the proposed HDGS system with and without an FC source in parallel with the PV source.

 

CONCLUSION

A compact topology, suitable for grid-connected applications has been proposed. Its working principle, analysis, and design procedure have been presented. The topology is fed by a hybrid combination of PV and FC sources. PV is the main source, while FC serves as an auxiliary source to compensate for the uncertainties of the PV source. The presence of FC source improves the quality of power (grid current THD, grid voltage profile, etc.) fed into the grid and decreases the time taken to reach theMPP. Table IV compares the system performance with and without the FC block in the system. A good feature of the proposed configuration is that the PV source is directly coupled with the inverter (and not through a dedicated dc–dc converter) and the FC block acts as a current source. Considering that the FC is not a stiff dc source, this facilitates PV operation at MPP over a wide range of solar insolation, leading to an optimal utilization of the energy sources. The efficiency of the proposed system in mode-1 is higher (around 85% to 90%) than mode 2 and 3 (around 80% to 85%).

 

REFERENCES

[1] J. Kabouris and G. C. Contaxis, “Optimum expansion planning of an unconventional generation system operating in parallel with a large scale network,” IEEE Trans. Energy Convers., vol. 6, no. 3, pp. 394–400, Sep. 1991.

[2] P. Chiradeja and R. Ramakumar, “An approach to quantify the technical benefits of distributed generation,” IEEE Trans. Energy Convers., vol. 19, no. 4, pp. 764–773, Dec. 2004.

[3] Y. H. Kim and S. S. Kim, “An electrical modeling and fuzzy logic control of a fuel cell generation system,” IEEE Trans. Energy Convers., vol. 14, no. 2, pp. 239–244, Jun. 1999.

[4] K. N. Reddy and V. Agarwal, “Utility interactive hybrid distributed generation scheme with compensation feature,” IEEE Trans. Energy Convers., vol. 22, no. 3, pp. 666–673, Sep. 2007.

[5] K. S. Tam and S. Rahman, “System performance improvement provided by a power conditioning subsystem for central station photovoltaic fuel cell power plant,” IEEE Trans. Energy Convers., vol. 3, no. 1, pp. 64–70.

 

A Unified Control Strategy for Three-Phase Inverter in Distributed Generation

ABSTRACT:
This paper presents a unified control strategy that enables both islanded and grid-tied operations of three-phase inverter in distributed generation, with no need for switching between two corresponding controllers or critical islanding detection. The proposed control strategy composes of an inner inductor current loop, and a novel voltage loop in the synchronous reference frame. The inverter is regulated as a current source just by the inner inductor current loop in grid-tied operation, and the voltage controller is automatically activated to regulate the load voltage upon the occurrence of islanding. Furthermore, the waveforms of the grid current in the grid-tied mode and the load voltage in the islanding mode are distorted under nonlinear local load with the conventional strategy. And this issue is addressed by proposing a unified load current feedforward in this paper. Additionally, this paper presents the detailed analysis and the parameter design of the control strategy. Finally, the effectiveness of the proposed control strategy is validated by the simulation results.

KEYWORDS:
1. Distributed generation (DG)
2. Islanding
3. Load current
4. Seamless transfer
5. Three-phase inverter
6. Unified control

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:
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Fig. 1. Overall block diagram of the proposed unified control strategy.

EXPECTED SIMULATION RESULTS:
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Fig. 2. Simulation waveforms of load voltage vC a , grid current iga, and inductor current iLa when DG is in the grid-tied mode under condition of the step down of the grid current reference from 9 A to 5 A with: (a) conventional voltage mode control, and (b) proposed unified control strategy.

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Fig. 3. Simulation waveforms of load voltage vC a , grid current iga, and inductor current iLa when DG is transferred from the grid-tied mode to the islanded mode with: (a) conventional hybrid voltage and current mode control, and (b) proposed unified control strategy.

CONCLUSION:
A unified control strategy was proposed for three-phase inverter in DG to operate in both islanded and grid-tied modes, with no need for switching between two different control architectures or critical islanding detection. A novel voltage controller was presented. It is inactivated in the grid-tied mode, and the DG operates as a current source with fast dynamic performance. Upon the utility outage, the voltage controller can automatically be activated to regulate the load voltage. Moreover, a novel load current feed forward was proposed, and it can improve the waveform quality of both the grid current in the grid-tied mode and the load voltage in the islanded mode. The proposed unified control strategy was verified by the simulation results.
REFERENCES:
[1] R. C. Dugan and T. E. McDermott, “Distributed generation,” IEEE Ind. Appl. Mag., vol. 8, no. 2, pp. 19–25, Mar./Apr. 2002.
[2] R. H. Lasseter, “Microgrids and distributed generation,” J. Energy Eng., vol. 133, no. 3, pp. 144–149, Sep. 2007.
[3] C. Mozina, “Impact of green power distributed generation,” IEEE Ind. Appl. Mag., vol. 16, no. 4, pp. 55–62, Jul./Aug. 2010.
[4] IEEE Recommended Practice for Utility Interface of Photovoltaic(PV) Systems, IEEE Standard 929-2000, 2000.
[5] IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard 1547-2003, 2003.