Performance Recovery of Voltage Source Converters with 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.

Modeling, Analysis and Testing of Autonomous Operation of an Inverter-Based Microgrid

ABSTRACT:  

The analysis of the small-signal stability of conventional power systems is well established, but for inverter based microgrids there is a need to establish how circuit and control features give rise to particular oscillatory modes and which of these have poor damping. This paper develops the modeling and analysis of autonomous operation of inverter-based microgrids. Each sub-module is modeled in state-space form and all are combined together on a common reference frame. The model captures the detail of the control loops of the inverter but not the switching action. Some inverter modes are found at relatively high frequency and so a full dynamic model of the network (rather than an algebraic impedance model) is used. The complete model is linearized around an operating point and the resulting system matrix is used to derive the eigenvalues. The eigenvalues (termed “modes”) indicate the frequency and damping of oscillatory components in the transient response. A sensitivity analysis is also presented which helps identifying the origin of each of the modes and identify possible feedback signals for design of controllers to improve the system stability. With experience it is possible to simplify the model (reduce the order) if particular modes are not of interest as is the case with synchronous machine models. Experimental results from a microgrid of three 10-kW inverters are used to verify the results obtained from the model.

KEYWORDS:

1.      Inverter

2.      Inverter model

3.      Microgrid

4.      Power control

5.      Small-signal stability

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Typical structure of inverter-based microgrid.

 EXPECTED SIMULATION RESULTS:

Fig. 2. Active power (filtered) response of micro-sources with 3.8 kW of step

change in load power at bus 1.

Fig. 3. Reactive power exchange between the micro sources with 3.8 kW of

step change in load power at bus 1 (Initial values: Q1 =0, Q2 = 􀀀200, Q3 =

+200; Final values: Q1 = +600, Q2 = 􀀀300, Q3 = 􀀀200).

Fig. 4. Active power (filtered) response of micro-sources with 16.8 kW and

12 kVAR RL load step change at bus 1.

Fig. 5. Reactive power (filtered) response of micro-sources with 16.8 kW and

12 kVAR RL load step change at bus 1.

Fig. 6. Output voltage (d-axis) response with 27 kW of step change in load

power at bus 1.

Fig. 7. Inductor current (d-axis) response with 27 kW of step change in load

power at bus 1.

CONCLUSION:

In this paper, a small-signal state-space model of a microgrid is presented. The model includes inverter low frequency dynamics dynamics, high frequency dynamics, network dynamics, and load dynamics. All the sub-modules are individually modeled and are then combined on a common reference frame to obtain the complete model of the microgrid.

The model was analyzed in terms of the system eigenvalues and their sensitivity to different states. With the help of this analysis the relation between different modes and system parameters was established. It was observed that the dominant low-frequency modes are highly sensitive to the network configuration and the parameters of the power sharing controller of the micro sources. The high frequency modes are largely sensitive to the inverter inner loop controllers, network dynamics, and load dynamics.

Results obtained from the model were verified experimentally on a prototype microgrid. It was observed that the model successfully predicts the complete microgrid dynamics both in the low and high frequency range.

Small signal modeling has had a long history of use in conventional power systems. The inverter models (and the inclusion of network dynamics) illustrated in this paper allow microgrids to be designed to achieve the stability margin required of reliable power systems.

 REFERENCES:

[1] R. H. Lasseter, “Microgrids,” in Proc. Power Eng. Soc.Winter Meeting, Jan. 2002, vol. 1, pp. 305–308.

[2] A. Arulapalam, M. Barnes, A. Engler, A. Goodwin, and N. Jenkins, “Control of power electronic interfaces in distributed generation microgrids,” Int. J. Electron., vol. 91, no. 9, pp. 503–523, Sep. 2004.

[3] R. Lassetter, “Integration of Distributed Energy Resources: The CERTS Microgrid Concept,” CERT Rep., Apr. 2002.

[4] M. S. Illindala, P. Piagi, H. Zhang, G. Venkataramanan, and R. H. Lasseter, “Hardware Development of a Laboratory-Scale Microgrid Phase 2: Operation and Control of a Two-Inverter Microgrid,” Nat. Renewable Energy Rep., Mar. 2004.

[5] Y. Li, D. M. Vilathgamuwa, and P. C. Loh, “Design, analysis and realtime testing of a controller for multibus microgrid system,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1195–1204, Sep. 2004.

Grid to Vehicle and Vehicle to Grid Energy Transfer using Single-Phase Bidirectional ACDC Converter and Bidirectional DC – DC converter

 ABSTRACT:

In this paper, a configuration of a single-phase bidirectional AC-DC converter and bidirectional DC-DC converter is proposed to transfer electrical power from the grid to an electrical vehicle (EV) and from an EV to the grid while keeping improved power factor of the grid. In first stage, a 230 V 50 Hz AC supply is converted in to 380V dc using a single-phase bidirectional AC-DC converter and in the second stage, a bidirectional buck–boost dc-dc converter is used to charge and discharge the battery of the PHEV (Plug-in Hybrid Electric Vehicle). In discharging mode, it delivers energy back to the grid at 230V, 50 Hz. A battery with the charging power of 1.2 kW at 120V is used in PHEV. The buck-boost DC-DC converter is used in buck mode to charge and in a boost mode to discharge the battery. A proportional-integral (PI) controller is used to control the charging current and voltage. Simulated results validate the effectiveness of proposed algorithm and the feasibility of system.

KEYWORDS:

1.      Plug-in Hybrid Electric Vehicle (PHEV)

2.      Bidirectional AC-DC Converter

3.      DC-DC Converter

4.      Vehicle to grid (V2G)

5.      Electric drive vehicle (EDVs)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig.1 Proposed configuration for V2G and G2V Energy transfer

EXPECTED SIMULATION RESULTS:

Fig.2 Charging and discharging of PHEV battery (Full profile)

                                                                                    

Fig.3 Charging and discharging of PHEV battery (in large view)

Fig.4. Discharging and Charging of PHEV battery demonstrating unity

Power factor operation

Fig.5 Waveform and harmonics spectrum of the discharging grid current

Fig.6 Waveform and harmonics spectrum of the Charging grid current

CONCLUSION:

The proposed converter has delivered the AC current to/and from the grid at unity power factor and at very low current harmonics which ultimately prolongs the life of the converter and the battery and minimizes the possibility of distorting the grid voltage. It also enables V2G interactions which could be utilized to improve the efficiency of the grid.

REFERENCES:

[1] Young-Joo Lee, Alireza Khaligh, and Ali Emadi, “Advanced Integrated Bidirectional AC/DC and DC/DC Converter for Plug-In Hybrid Electric Vehicles,” IEEE Trans. on Vehicular Tech. vol. 58, no. 8, pp. 3970-3980, Oct, 2009.

[2] Bhim Singh, Brij N. Singh, Ambrish Chandra, Kamal Al-Haddad, Ashish Pandey and Dwarka P. Kothari, “A review of single-phase improved power quality ac–dc converters,” IEEE Trans. Industrial Electronics, vol. 50, no. 5, pp. 962-981, Oct. 2003.

[3] M.C. Kisacikoglu, B. Ozpineci and L.M. Tolbert, "Examination of a PHEV bidirectional charger system for V2G reactive power compensation," in Proc. of Twenty-Fifth Annual IEEE Applied Power Electronics Conference and Exposition (APEC), 2010, 21-25 Feb.2010, pp.458-465.

[4] M.C. Kisacikoglu, B. Ozpineci and L.M. Tolbert, “Effects of V2G reactive power compensation on the component selection in an EV or PHEV bidirectional charger," in Proc. of Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, 12-16 Sept. 2010, pp.870-876.

[5] W. Kempton and J. Tomic, “Vehicle-to-grid power fundamentals: Calculating capacity and net revenue,” J. Power Sources, vol. 144, no. 1, pp. 268–279, Jun. 2005.

Single-Phase AC/AC Buck-Boost Converter with Single-Phase Matrix Topology

 ABSTRACT:

This paper deals with a new family of single-phase AC/AC buck-boost converter based on singlephase matrix topology. The proposed converter provides a wider range of output AC voltage in which the output voltage can be bucked and in-phase/out-of-phase with the input voltage; and the output voltage can be boosted and in-phase/out-of-phase with the input voltage. A commutation strategy is employed to realize snubberless operation. The operating principles, circuit analysis and experimental results based on TMS320F2812 DSP are presented.

KEYWORDS:

1.       Z-source converter

2.       Single-phase matrix converter (SPMC)

3.      PWM AC/AC converter

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1: Proposed topology.

 EXPECTED SIMULATION RESULTS:

(a) Buck in-phase

                                                                                (b) Buck out-of-phase

Fig. 2: Experimental results for buck mode with Vi = 70 Vrms/60Hz, D = 0.3. (a) Region (I) (buck inphase).

(b) Region (III) (buck out-of-phase). Top: vi; Bottom: vo. (x-axis: 10ms/div., y-axis: 100V/div)

(a) Boost in-phase

(b) Boost out-of-phase

Fig. 3: Experimental results for boost mode with Vi = 50 Vrms/60Hz, D = 0.7. (a) Region (II) (boost

in-phase). (b) Region (IV) (boost out-of-phase). Top: vi; Bottom: vo. (x-axis: 10ms/div., y-axis:

100V/div)

CONCLUSION:

In this paper a new family of single-phase Z-source AC/AC buck-boost converter based on singlephase matrix converter topology has been presented. The proposed converter has following fertures:the output voltage can be bucked-boosted and in-phase with the input voltage; the output voltage can be bucked-boosted and out-of-phase with the input voltage. In order to provide a continuous current path, the safe-commutation strategy is employed. Steady-state analysis and experimental results were illustrated. By duty-ratio control, the proposed converter becomes “solid-state transformers” with a continuously variable turn ratio. The proposed converter can be used as dynamic voltage restorer (DVR) to compensate voltage sags and swells in AC/AC line conditioning without any energy-storage devices requirement. The feature which the output voltage is bucked-boosted and in-phase with the input voltage is used for voltage sag compensation. The feature which the output voltage is bucked boosted and out-of-phase with the input voltage is used for voltage swell compensation.

REFERENCES:

[1] Fang X. P., Qian Z. M., Peng F. Z.: Single-phase Z-source PWM AC-AC converters, IEEE Power Electronics Letters, vol. 3, no. 4, 2005, pp. 121-124.

[2] Tang Y., Xie S., Zhang C.: Z-source AC-AC converters solving commutation problem, IEEE Trans. Power Electronics, vol. 22, no. 6, 2007, pp. 2146-2154.

[3] Kwon B. H., Mim B. D., Kim J. H.: Novel commutation technique of AC-AC converters, IEE Proc. Electr. Power Appl., vol. 145, no. 4, July 1998, pp. 295-300.

[4] Wheeler P. W., Rodriguez J., Clare J. C., Empringham L., Weinstein A.: Matrix converter: a technology review, IEEE Trans. on Ind. Electronics, vol. 49, no. 2, April 2002, pp. 276-288.

[5] Nguyen M. K., Jung Y. G., and Lim Y. C.: Single-phase Z-source buck-boost matrix converter, in proc. Of IEEE Applied Power Electronics Conference and Exposition, APEC’09, pp. 846-850, 2009.