Performance Recovery of Voltage Source Converterswith Application to Grid-connected Fuel Cell DGs

ABSTRACT:  

Most common types of distributed generation (DG) systems utilize power electronic interfaces and, in particular,  three-phase voltage source converters (VSCs) which are mainly  used to regulate active and reactive power delivered to the grid. The main drawbacks of VSCs originate from their nonlinearities, control strategies, and lack of robustness against uncertainties. In this paper, two time-scale separation redesign technique is proposed to improve the overall robustness of VSCs and address the issues of uncertainties. The proposed controller is applied to a grid-connected Solid Oxide Fuel Cell (SOFC) distributed generation system to recover the trajectories of the nominal system despite the presence of uncertainties. Abrupt changes in the input dc voltage, grid-side voltage, line impedance and PWM malfunctions are just a few uncertainties considered in our evaluations. Simulation results based on detailed model indicate that the redesigned system with lower filter gain (_) achieves more reliable performance in compare to the conventional current control scheme. The results also verified that the redesigned controller is quite successful in improving the startup and tracking responses along with enhancing the overall robustness of the system.

KEYWORDS:

  1. Power converters
  2. Solid oxide fuel cell (SOFC)
  3. Distributed generation (DG)
  4. Time-scale separation redesign

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

 Fig. 1. Schematic diagram of a grid-connected SOFC power plant with redesigned controller.

 EXPECTED SIMULATION RESULTS:

 Fig. 2. Active (top) and reactive (bottom) output power in case 1 (input dc

voltage) uncertainty using PI and redesigned controller.

Fig. 3. Output voltage (top) and current (bottom) of each SOFC array in case

1 (input dc voltage) uncertainty using PI and redesigned controller.

Fig. 4. Active (top) and reactive (bottom) output power in case 2 (grid-side

voltage) uncertainty using PI and redesigned controller.

Fig. 5. d-axis (top) and q-axis (bottom) currents of the VSC in case 2 (gridside

voltage) uncertainty using PI and redesigned controller.

Fig. 6. Active (top) and reactive (bottom) output power in case 3.1 (line

resistance) uncertainty using PI and redesigned controller.

Fig. 7. Active (top) and reactive (bottom) output power in case 3.2 (line

inductance) uncertainty using PI and redesigned controller.

Fig. 8. Additive random Gaussian noises on duty ratio of phase a (top), b

(middle), and c (bottom) of the VSC.

Fig. 9. Active (top) and reactive (bottom) output power in case 4 (duty

ratio) uncertainty using PI and redesigned controller.

 CONCLUSION:

 This paper presents a new control technique based on two time-scale separation redesign for the VSC of a grid connected SOFC DG system. A three-phase VSC is used to regulate active and reactive power delivered to the grid. In addition, variations in the input dc voltage, line impedance, grid-side voltage and duty ratio are mathematically formulated as additive uncertainties based on the nonlinear model of the VSC. As a result, the proposed controller is able to address the issues of robustness and further enhance the system stability in the presence of uncertainties. The redesigned controller also presents a fast and accurate startup response and delivers superior decoupling performance as compared to the conventional PI controller. Moreover, the redesigned controller significantly reduces the maximum overshoot in the output power while the system with a conventional controller exhibits deterioration in the output response which leads to excessive current and voltage variations in the FC arrays.

REFERENCES:

[1] P. Kundur, Power System Stability and Control. New York, NY, USA:McGraw-Hill, 1994.

[2] R. Seyezhai and B. L. Mathur, “Modeling and control of a PEM fuel cell based hybrid multilevel inverter,” International Journal of Hydrogen Energy, vol. 36, pp. 15029-15043, 2011.

[3] T. Erfanmanesh and M. Dehghani, “Performance improvement in gridconnected fuel cell power plant: An LPV robust control approach,”

International Journal of Electrical Power & Energy Systems, vol. 67, pp. 306-314, 2015.

[4] S. A. Taher and S. Mansouri, “Optimal PI controller design for active power in grid-connected SOFC DG system,” International Journal of Electrical Power & Energy Systems, vol. 60, pp. 268-274, 2014.

[5] R. Teodorescu, M. Liserre, and P. Rodriguez, Grid Converters for Photovoltaic and Wind Power Systems. Hoboken, NJ, USA: John Wiley & Sons, 2011.

A PLL Based Controller for Three Phase Grid Connected Power Converters

A PLL Based Controller for Three Phase Grid Connected Power Converters

 ABSTRACT

The current control of three-phase grid-connected converters is typically carried out by using a proportional resonant controller or synchronous reference frame proportional integral regulator. The implementation of these controllers often requires knowledge of the grid voltage frequency/phase angle, which is typically provided by a synchronization unit. It implies that dynamics and possible inaccuracies of the synchronization unit have a considerable impact on the current controller performance. The aim of this letter is to design an adaptive current controller by using a conventional synchronous reference frame phase-locked loop (SRF-PLL). In this way, the current controller and synchronization part are merged into a single unit, which results in a simpler and more compact structure. The effectiveness of the proposed controller is verified using experimental results.

KEYWORDS:

  1. Current control
  2. Distributed generation (DG) systems
  3. Phase-locked loop (PLL)
  4. Power converters
  5. Synchronization
  6. Three phase grid

 SOFTWARE: MATLAB/SIMULINK

CONTROL SYSTEM CIRCUIT DIAGRAM:

Three-Phase Grid

Fig. 1. Power stage of a three-phase VSC with the proposed PLL-based controller and a harmonic/imbalance compensator.

EXPECTED EXPERIMENTAL RESULTS:

PLL Based Controller

Fig. 2. Experimental results for the test 1.

Three Phase Grid

Fig. 3. Experimental results for the test 2.

Three Phase Grid

Fig. 4. Experimental results for the test 3.

 CONCLUSION

In this letter, a PLL-based controller for grid-connected converters was proposed. This controller, which is realized by adding a positive feedback loop to the conventional SRFPLL, eliminates the need for a dedicated synchronization unit and, therefore, results in a more compact structure. To enhance the harmonic/imbalance rejection capability of the suggested controller, multiple complex integrators tuned at low-order disturbance frequencies is employed. To simplify the tuning procedure, a simple yet accurate linear model describing the frequency estimation dynamics of the proposed controller was was verified using some experimental results. The main contribution of this letter is not the proposed controller. It is actually demonstrating the possibility of making a frequency-adaptive controller from a standard PLL. The importance of this contribution will be more evident when we notice that there are a large number of advanced PLLs which can be explored for the controller design.

REFERENCES

  • M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvan, R. C. P. Guisado, M. A. M. Prats, J. I. Leon, and N. Moreno-Alfonso, “Powerelectronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002–1016, Jun. 2006.
  • Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1184–1194, Sep. 2004.
  • K. Bose, “Power electronics and motor drives recent-progress and perspective,” IEEE Trans. Ind. Electron., vol. 56, no. 2, pp. 581–588, Feb. 2009.
  • Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.
  • Zeng and L. Chang, “An advanced SVPWM-based predictive current controller for three-phase inverters in distributed generation systems,” IEEE Trans. Ind. Electron., vol. 55, no. 3, pp. 1235–1246, Mar. 2008.