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.

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