Control Strategy for Power Flow Management in a PV System Supplying DC Loads

ABSTRACT:

The growing concern for energy saving has increased the usage of LED-based street lights, electronic chokes, compact fluorescent lamps, and inverter-fed drives. Hence, the load profile seen by the electrical grid is undergoing a notable change as these devices have to operate from a dc source. Photovoltaics (PV) being a major energy source, the aforementioned loads can be connected directly to the dc bus. A grid-connected PV system involves a power source (PV array), a power sink (load), and two power sources/sink (utility and battery), and hence, a power

flow management system is required to balance the power flow among these sources. One such system is developed for selecting the operating mode of the bidirectional converter by sensing the battery voltage. The viability of the scheme has been ascertained by performing experimental studies on a laboratory prototype. The control strategy is digitally implemented on an Altera Cyclone II Field Programmable Gate Array (FPGA) board, and the algorithm is verified for different modes of operation by varying the load. Experimental results are presented to bring out the usefulness of the control strategy.

KEYWORDS:

  1. Bidirectional converter
  2. Dc bus
  3. Photovoltaic
  4. Power flow management system (PMS)

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig.1.Grid-connected PV system with ac and dc loads.

EXPECTED SIMULATION RESULTS:

 

Fig. 2. Steady-state response. (a) Rectifier mode. (b) Inverter mode.

Fig. 3. Dynamic response for step change in load: (a) AC line current control (iL—0.5 A/div; Vdc—55 V/div; Ppv—100 W/div; io—0.5 A/div). (b) AC line current control (iL—1 A/div; Vdc—55 V/div; Ppv—100 W/div; io—0.5 A/div). (c) Proposed control (iL—1 A/div; Vdc—55 V/div; Ppv—100 W/div; ib—1 A/div; io—1 A/div; Vb—5 V/div). Dynamic response for step change in insolation: (d) AC line current control (iL—0.5 A/div; Vdc—40 V/div; Ppv—100 W/div; io—1 A/div). (e) AC line current control (iL—0.5 A/div; Vdc—40 V/div; Ppv—100 W/div; io—1 A/div). (f) Proposed control (iL—0.5 A/div; Vdc—55 V/div; Ppv—200 W/div; ib—1 A/div; io—1 A/div; Vb—5 V/div). Time: 0.025 s/div.

Fig. 4. Grid failure under (a) inverting mode and (b) rectifier mode. y-axis: Iinv: 4 A/div; Vb: 80 V/div; Ib: 4 A/div; IL: 4 A/div. x-axis: time: 50 s/div.

Fig. 5. Battery voltage, battery current, and dc current waveforms for different cases under automatic power flow control. (a) Case I. (b) Case II. (c) Case III. (d) Case IV. y-axis: Vb: 10 V/div; Ibat: 2 A/div; Iinv: 4 A/div. x-axis: time: 20 s/div.

 CONCLUSION:

A versatile control strategy for power flow management in a grid-connected PV system feeding dc loads has been presented. The importance of the scheme has been brought out by performing experimental studies on a laboratory prototype. The steady-state performance of the converter for different modes of operation has been observed, and near unity power factor has been achieved in both the rectifier and inverter modes. The transient performance of the system for step changes in load and insolation have been also illustrated. The control strategy has been digitally implemented on an Altera Cyclone II FPGA board, and the algorithm has been verified for different modes of operation by varying the load, and a good correlation between the results of computer simulation and experiments has established the validity of the PMS. The significance of the proposed scheme has been demonstrated by its effectiveness in preventing undesirable shuttling of the PV operating point and also in maintaining the THD of the injected grid current within the allowable limit of 5% by setting a minimum current reference for injection. The proposed configuration has been proved to be attractive from the perspective of providing uninterruptible power to dc loads while ensuring the evacuation of excess PV power of high quality into the grid.

REFERENCES:

[1] Yazdani and P. P. Dash, “A control methodology and characterization of dynamics for a photovoltaic system interfaced with a distribution network,” IEEE Trans. Power Del., vol. 24, no. 3, pp. 1538–1551, Jul. 2009.

[2] X. Q. Guo and W. Y. Wu, “Improved current regulation of three-phase grid-connected voltage-source inverters for distributed generation systems,” IET Renew. Power Gener., vol. 4, no. 2, pp. 101–115, Mar. 2010.

[3] H. C. Chiang, T. T. Ma, Y. H. Cheng, J. M. Chang, and W. N. Chang, “Design and implementation of a hybrid regenerative power system combining grid-tie and uninterruptible power supply functions,” IET Renew. Power Gener., vol. 4, no. 1, pp. 85–99, Jan. 2010.

[4] F. Giraud and Z. M. Salameh, “Steady-state performance of a grid connected rooftop hybrid wind–photovoltaic power system with battery storage,” IEEE Trans. Energy. Convers., vol. 16, no. 1, pp. 1–7, Mar. 2001.

[5] J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvàn, R. C. P. Guisado, M. A. M. Prats, J. I. León, and N. Moreno-Alfonso, “Power electronic systems for the grid integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002– 1016, Aug. 2006.

 

 

Full-Soft-Switching High Step-Up Bidirectional Isolated Current-Fed Push-Pull DC-DC Converter for Battery Energy Storage Applications

ABSTRACT:

This paper presents a novel bidirectional current fed push-pull DC-DC converter topology with galvanic isolation. The control algorithm proposed enables full-soft-switching of all transistors in a wide range of input voltage and power with no requirement for snubbers or resonant switching. The converter features an active voltage doubler rectifier controlled by the switching sequence synchronous to that of the input-side switches. As a result, full-soft-switching operation at a fixed switching frequency is achieved. Operation principle for the energy transfer in both directions is described, followed by verification with a 300 W experimental prototype. The converter has considerably higher voltage step-up performance than traditional current-fed converters Experimental results obtained are in good agreement with the theoretical steady-state analysis.

KEYWORDS:

  1. Current-fed dc-dc converter
  2. Bidirectional converter
  3. Soft-switching
  4. ZVS
  5. ZCS
  6. Push-pull converter
  7. Switching control method
  8. Naturally clamped

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

image001

Fig. 1. Full-soft-swithicng CF push-pull converter proposed.

EXPECTED SIMULATION RESULTS:
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Fig. 2. Experimental current and voltage waveforms of the switch S1.1

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Fig. 3. Experimental current and voltage waveforms of the switch S1.2.

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Fig. 4. Experimental current and voltage waveforms of the switch S4.

CONCLUSION:

A novel bidirectional current-fed push-pull converter with galvanic isolation was introduced. It features full-soft switching operation of all semiconductor components, while its DC voltage gain is higher than in traditional current-fed converters due to the utilization of the circulating energy for the input voltage step-up. As a result, it does not suffer from short intervals of energy transfer from the input side to the output side since at least half of the switching period is dedicated for this. Moreover, it does not require any clamping circuits, since the novel control algorithm features natural clamping of the switches at the current-fed side. Despite a relatively high number of semiconductor components, it shows the peak efficiency of 96.3%, which does not depend on the energy transfer direction for the corresponding operating point. Soft-switching operation with continuous current at the current fed side makes the converter proposed suitable for residential battery energy storage systems. Further research will be directed towards experimental verification of the converter performance with a lithium iron phosphate battery.

REFERENCES:

[1] F. Blaabjerg, and D.M. Ionel, “Renewable Energy Devices and Systems – State-of-the-Art Technology, Research and Development, Challenges and Future Trends,” Electric Power Components and Systems, vol.43, no.12, pp.1319-1328, 2015.

[2] C, Heymans, S, B. Walker, S. B. Young, M. Fowler, “Economic analysis of second use electric vehicle batteries for residential energy storage and load-levelling,” Energy Policy, vol. 71, pp. 22-30, Aug. 2014.

[3] J. Weniger, T. Tjaden, V. Quaschning, “Sizing of Residential PV Battery Systems,” Energy Procedia, vol. 46, pp. 78-87,2014.

[4] S. J. Chiang, K. T. Chang and C. Y. Yen, “Residential photovoltaic energy storage system,” IEEE Trans. Ind. Electron., vol. 45, no. 3, pp. 385-394, Jun 1998.

[5] S. X. Chen, H. B. Gooi and M. Q. Wang, “Sizing of Energy Storage for Microgrids,” IEEE Trans. Smart Grid, vol. 3, no. 1, pp. 142-151, 2012.