Backstepping Control of Smart Grid-Connected Distributed Photovoltaic Power Supplies for Telecom Equipment

ABSTRACT:

Backstepping controllers are obtained for distributed hybrid photovoltaic (PV) power supplies of telecommunication equipment. Grid-connected PV-based power supply units may contain dc–dc buck–boost converters linked to single-phase inverters. This distributed energy resource operated within the self consumption concept can aid in the peak-shaving strategy of ac smart grids. New backstepping control laws are obtained for the single-phase inverter and for the buck–boost converter feeding a telecom equipment/battery while sourcing the PV excess power to the smart grid or to grid supply the telecom system. The backstepping approach is robust and able to cope with the grid nonlinearity and uncertainties providing dc input current and voltage controllers for the buck–boost converter to track the PV panel maximum power point, regulating the PV output dc voltage to extract maximum power; unity power factor sinusoidal ac smart grid inverter currents and constant dc-link voltages suited for telecom equipment; and inverter bidirectional power transfer. Experimental results are obtained from a lab setup controlled by one inexpensive dsPIC running the sampling, the backstepping and modulator algorithms. Results show the controllers guarantee maximum power transfer to the telecom equipment/ac grid, ensuring steady dc-link voltage while absorbing/injecting low harmonic distortion current into the smart grid.

KEYWORDS:

1.      Backstepping

2.       Buck–boost converter

3.      Dc/ac converter

4.      MPPT

5.      Self-consumption

6.       Smart grids

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. PV distributed hybrid self-consumption system and telecom load.

EXPECTED SIMULATION RESULTS:

Fig. 2. MPPT operation.

        

Fig. 3. Voltage and current waveforms when there is a change from inverter to rectifier.

Fig. 4. (a)Voltage and current waveforms when there is a change from inverter

to rectifier. (b) Center part zoom of (a).

Fig. 5. Voltage and current waveforms when the load requires 25 W.

Fig. 6. Voltage and current waveforms when the load requires 62 W.

Fig. 7. DC–AC converter input power.

CONCLUSION:

This paper proposes a novel backstepping controller for a PV panel feeding a buck–boost converter, and dc linked to a telecom load and a single-phase ac–dc converter connected to a smart grid, configuring a subset of a distributed hybrid photovoltaic power supply for telecom equipments within the self-consumption concept. This setup absorbs/injects nearly sinusoidal (THD = 1.6%, lower than the 3% required by the standards) grid currents at near unity power factor and the self consumption can contribute to the smart grid peak power shaving strategy.

New nonlinear backstepping control laws were obtained for the input voltage of the buck–boost converter, thus achieving MPP operation (MPPT efficiency between 98.2% and 99.9%) and for the dc–ac converter regulating the dc telecom load voltage and controlling the ac grid current. All the control laws, fixed frequency converter modulators, voltage and current sampling, and grid synchronization have been implemented using a low-cost dsPIC30F4011 microcontroller.

Obtained experimental results show the performance of the PV self-consumption system using the backstepping control method. Results show the system dynamic behavior when the dc–ac converter changes operation from inverter to rectifier to adapt itself to the telecom load requirements. The robustness of the control laws has been tested as well. Capacitance of real capacitors can vary almost ten times around the rated value, while inductances can vary from 30% to nearly 300% of the rated value.

REFERENCES:

[1] N. Femia, G. Petrone, G. Spagnuolo, and M. Vitelli, Power Electronics and Control Techniques for Maximum Energy Harvesting in Photovoltaic Systems. Boca Raton, FL, USA: CRC Press, 2013.

[2] A.Maki and S. Valkealahti, “Effect of photovoltaic generator components on the number of MPPs under partial shading conditions,” IEEE Trans. Energy Convers., vol. 28, no. 4, pp. 1008–1017, Dec. 2013.

[3] Epia Org. (2013, Jul.). Self-consumption of PV electricity—Position paper. [Online]. Available:http://www.epia.org/fileadmin/user_upload/Position_Papers/Self_and_direct_consumption_-_position_paper_-_final _version.pdf

[4] SunEdison. (2011, Nov.). Enabling the European consumer to generate power for self-consumption. [Online]. Available: http://www. sunedison.com/wps/wcm/connect/35bfb52a-ec27-4751-8670-fe6e807e8063/SunEdison_PV_Self  consumption_Study_high_resolution_%2813_ Mb%29.pdf?MOD=AJPERES

[5] A. Nourai, R. Sastry, and T.Walker, “A vision & strategy for deployment of energy storage in electric utilities,” in Proc. IEEE Power Energy Soc. Gen. Meeting, 2010, pp. 1–4.

Modular Multilevel DC/DC Converters with Phase Shift Control Scheme for High Voltage DC-Based Systems

ABSTRACT

In this paper, by investigating the topology derivation principle of the phase shift controlled three-level DC/DC converters, the modular multilevel DC/DC converters, by integrating the full-bridge converters and three-level flying-capacitor circuit, are proposed for the high step-down and high power DC-based systems. The high switch voltage stress in the primary side is effectively reduced by the full-bridge modules in series. Therefore, the low-voltage rated power devices can be employed to obtain the benefits of low conduction losses. More importantly, the voltage auto-balance ability among the cascaded modules is achieved by the inherent flying capacitor, which removes the additional possible active components or control loops. In additional, zero-voltage-switching (ZVS) performance for all the active switches can be provided due to the phase shift control scheme, which can reduce the switching losses. The circuit operation and converter performance are analyzed in detail. Finally, the performance of the presented converter is verified by the simulation results.

KEYWORDS

1.      Modular multilevel DC/DC converter

2.      Phase shift control scheme

3.      Input voltage auto-balance

4.      Zero voltage switching

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig.1. Proposed modular multilevel DC/DC converter with input voltage auto-balance ability.

SIMULATION RESULTS

Fig.2. Simulation waveforms: (a) Input voltage without flying capacitor and (b) Input voltage with flying capacitor.

Fig.3. Simulation result of primary voltage and current.

      

Fig.4. Simulation result of ZVS operation: (a)ZVS operation for S11 and (b) ZVS operation for S14.

Fig.5. Simulation result of input voltage sharing.

CONCLUSION

In this paper, a novel phase shift controlled modular multilevel DC/DC converter is proposed and analyzed for the high input voltage DC-based systems. Due to the inherent flying capacitor, which connects the input divided capacitors alternatively, the input voltage is automatically shared and balanced without any additional power components and control loops. Consequently, the switch voltage stress is reduced and the circuit reliability is enhanced. By adopting the phase shift control scheme, ZVS soft switching performance is ensured to reduce the switching losses. The modular multilevel DC/DC converter concept can be easily extend to N-stage converter with stacked full-bridge modules to satisfy extremely high voltage applications with low voltage rated power switches.

REFERENCES

[1]         H. Kakigano, Y. Miura and T. Ise, “Low-Voltage Bipolar-Type DC Microgrid for Super High Quality Distribution,” IEEE Trans. Power Electron., Vol. 25, No. 12, pp. 3066-3075, Dec 2010.

[2]         S. Anand and B. G. Fernandes, “Reduced-Order Model and Stability Analysis of Low-Voltage DC Microgrid,” IEEE Trans. Ind. Electron., vol. 60, No. 11, pp. 5040-5049, Nov 2013.

[3]         S. Anand and B. G. Fernandes, “Optimal voltage level for DC microgrids,” IEEE Conf. Ind. Electron. (IECON), pp. 3034-3039, 2010.

[4]         D. Salomonsson, L. Soder and A. Sannino, “An Adaptive Control System for a DC Microgrid for Data Centers,” IEEE Trans. Ind. Appl., vol. 44, No. 6, pp. 1910-1917, Nov./Dec. 2008.

[5]         K. B. Park, G. W. Moon and M. J. Youn, “Series-Input Series-Rectifier Interleaved Forward Converter With a Common Transformer Reset Circuit for High-Input-Voltage Applications,” IEEE Trans. Power Electron., vol. 26, No. 11, pp. 3242-3253, Nov 2011.

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

[1]         J. 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]         S. 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]         P. 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]         C. 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]         D. 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.

An Improved Modulated Carrier Control with On-Time Doubler for Single-Phase Shunt Active Power Filter

ABSTRACT

This paper proposes an improved modulated carrier control with on-time doubler for the single-phase shunt active power filter, which eliminates harmonic and reactive currents drawn by nonlinear loads. This control method directly shapes the line current to be sinusoidal and in phase with the grid voltage by generating a modulated carrier signal with a resettable integrator, comparing the carrier signal to the average line current and making duty ratio doubled. Since the line current compared to the carrier signal is not the peak, but the average value, dc-offset appeared at the conventional control methods based on one-cycle control is effectively addressed. The proposed control technique extirpates the harmonic and reactive currents and solves the dc-offset problem. The operation principle and stability characteristic of the single-phase shunt active power filter with the proposed control method are discussed, and experimental results with laboratory prototype under various load conditions verify its performance.

KEYWORDS

1.      Single-phase shunt active power filter

2.      Modulated carrier control

3.      Indirect control

4.      One-cycle control

5.      Harmonic and reactive currents elimination

6.      Nonlinear load.

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM

Fig. 1. Single-phase shunt active power filter with nonlinear load.

Fig. 2. Overall control structure of the proposed control method with the shunt APF.

EXPECTED SIMULATION RESULTS

Fig. 3. Measured grid voltage, line current, APF current and load current waveforms of the shunt APF system based on the proposed control method at full load condition (vin : 200 V/div, iin : 20 A/div, if : 20 A/div, i- L : 20 A/div).

Fig. 4. Measured grid voltage, line current, APF current and load current waveforms of the shunt APF system based on the proposed control method at half load condition (vin : 200 V/div, iin : 20 A/div, if : 20 A/div, iL : 20 A/div).

Fig. 5. Current controller switching mechanism.

Fig. 6. Measured dc-link voltage, line current, APF current and load current waveforms of the shunt APF system in load transient from 800 W to 1600 W (vo : 100 V/div, iin : 20 A/div, if : 20 A/div, iL : 20 A/div).

Fig. 7. Measured grid voltage, line current, APF current and load current waveforms of the shunt APF system at 110 Vrms grid voltage. (vin : 100 V/div, iin : 10 A/div, if : 10 A/div, iL : 10 A/div) Under (a) 200 W, (b) 270 W, (c) 340 W, (d) 400 W load condition

Fig. 8. Power factors of the nonlinear load system with and without the APF under various load conditions.

CONCLUSION

An improved modulated carrier control for single-phase active power filter has been proposed. The shunt APF with the proposed control method fulfills harmonic and reactive current elimination at the line current by comparing the carrier signal to the average line current and having the duty ratio doubled. On top of that, the control method totally gets rid of the dc-offset problem arisen at the conventional one based on one-cycle control and ameliorates the current control loop stability without additional ramp signal. The operation principle of power stage, the main control mechanism, and the stability characteristic of the current control loop are analyzed in detail. Experimental results with the shunt APF system under assorted conditions verify the performance of the proposed control method in steady and transient states.

REFERENCES

[1]         Elham B. Makram, E.V. Subramaniam, Adly A. Girgis, and Ray Catoe, “Hamonic filter design using actual recorded data,” IEEE Transaction on Industrial Application, vol. 29, no. 6, pp. 1176-1183, Nov. 1993.

[2]         F. Z. Peng, “Harmonic sources and filtering approaches,” IEEE Transaction on Industrial Application Magazine, vol. 7, no. 4, pp. 18-25, Jul. /Aug. 2001.

[3]         Czarnecki, L. S., Ginn, H. L., “The effect of the design method on efficiency of resonant harmonic filters,” IEEE Transactions on Power Delivery, vol. 20, no. 1, pp. 286-291, Jan. 2005.

[4]         Fakhralden A. Huliehel, Fred C. Lee, and Bo H. Cho, “Small-signal modeling of the single-phase boost high power factor converter with constant frequency control,” PESC’92 Record. 23rd annual IEEE Power electronics Specialists Conference, 1992, vol.1, pp. 475 – 482.

[5]         R. Martinez, P. N. Enjeti, “A high-performance single-phase rectifier with input power factor correction,” IEEE Transactions on Power Electronics, vol. 11, no. 2, pp. 311–317, Mar. 1996.