• No results found

Reconfigurable Solar Converter for a Single-Stage Power Conversion PV-Battery System

N/A
N/A
Protected

Academic year: 2022

Share "Reconfigurable Solar Converter for a Single-Stage Power Conversion PV-Battery System"

Copied!
13
0
0

Loading.... (view fulltext now)

Full text

(1)

Rajani.G, IJRIT 751 International Journal of Research in Information Technology (IJRIT)

www.ijrit.com ISSN 2001-5569

Reconfigurable Solar Converter for a Single-Stage Power Conversion PV-Battery System

Rajani.G, M. Tech scholar, Lakireddy Balireddy College of Engineering, Vijayawada

Miss. Chandrakala, M.Tech , Assistant Professor , Lakireddy Balireddy College of Engineering, Vijayawada

ABSTRACT

The conventional solar PV systems generally need two converters in the process of conversion of solar energy into electrical energy. One is dc/dc converter and another is dc/ac converter. Therefore there are two stages in the conventional solar PV system. This project introduces a new converter called reconfigurable solar converter (RSC) for photovoltaic (PV)-battery application, particularly utility-scale PV-battery application. The main concept of the new converter is to use a single-stage three phase solar PV converter to perform both dc/ac and dc/dc operations.

This converter solution is appealing for PV-battery application, because it minimizes the number of conversion stages, thereby reducing cost, weight and volume.

I. INTRODUCTION

Renewable energy is the energy which comes from natural resources such as sunlight, wind, rain, tides and geothermal heat. These resources are renewable and can be naturally replenished. Therefore, for all practical purposes, these resources can be considered to be inexhaustible, unlike dwindling conventional fossil fuels.

Apart from the rapidly decreasing reserves of fossil fuels in the world, another major factor working against fossil fuels is the pollution associated with their combustion. Contrastingly, renewable energy sources are known to be much cleaner and produce energy without the harmful effects of pollution unlike their conventional counterparts.

With increasing concern of global warming and the depletion of fossil fuel reserves, many are looking at sustainable energy solutions to preserve the earth for the future generations. Other than hydro power, wind and photovoltaic energy holds the most potential to meet our energy demands. Alone, wind energy is capable of supplying large amounts of power but its presence is highly unpredictable as it can be here one moment and gone in another. Similarly, solar energy is present throughout the day but the solar irradiation levels vary due to sun intensity and unpredictable shadows cast by clouds, birds, trees, etc. The common inherent drawback of wind and photovoltaic systems are their intermittent natures that make them unreliable. Therefore, solar PV electricity output significantly varies. From an energy source stand point, a stable energy source and an energy source that can be dispatched at the request are desired. As a result, energy storage such as batteries for solar PV systems has drawn significant attention and the demand of energy storage for solar PV systems has been dramatically increased, since, with energy storage, a solar PV system becomes a stable energy source and it can be dispatched at the request, which results in improving the performance and the value of solar PV systems. The integration of renewable energy sources and energy-storage systems has been one of the new trends in power-electronic technology.

There are different options for integrating energy storage into a utility-scale solar PV system. Specifically, energy storage can be integrated into the either ac or dc side of the solar PV power conversion systems which may consist of multiple conversion

(2)

Rajani.G, IJRIT 752

Fig. 1 Different scenarios for PV generation and load supply sequence.

stages. Every integration solution has its advantages and disadvantages. Different integration solutions can be compared with regard to the number of power stages, efficiency, storage system flexibility, control complexity, etc.

This project introduces a novel single-stage solar converter called reconfigurable solar converter (RSC). The basic concept of the RSC is to use a single power conversion system to perform different operation modes such as PV to grid (dc to ac), PV to battery (dc to dc), battery to grid (dc to ac), and battery/PV to grid (dc to ac) for solar PV systems with energy storage. The RSC concept arose from the fact that energy storage integration for utility-scale solar PV systems makes sense if there is an enough gap or a minimal overlap between the PV energy storage and release time. Fig.1 shows different scenarios for the PV generated power time of use. In case (a), the PV energy is always delivered to the grid and there is basically no need of energy storage. However, for cases (b) and (c), the PV energy should be first stored in the battery and then the battery or both battery and PV supply the load. In cases (b) and (c), integration of the battery has the highest value and the RSC provides significant benefit over other integration options when there is the time gap between generation and consumption of power.

2. PROPOSED RECONFIGURABLE SOLAR CONVERTER 2.1 Introduction

The schematic of the proposed RSC is presented in Fig.2 The RSC has some modifications to the conventional three-phase PV inverter system. These modifications allow the RSC to include the charging function in the conventional three phase PV inverter system.

Fig.2 Schematic of the proposed RSC circuit

(3)

Rajani.G, IJRIT 753 Assuming that the conventional utility-scale PV inverter system consists of a three-phase voltage source converter and its associated components. Optional inductors are included if the ac filter inductance is not enough for the charging purpose.

2.2 Operation modes of the RSC

All possible operation modes for the RSC are presented in Fig. 4.In Mode 1, the PV is directly connected to the grid through a dc/ac operation of the converter with possibility of maximum power point tracking (MPPT) control and theS1andS6switches remain open. In Mode 2, the battery is charged with the PV panels through the dc/dc operation of the converter by closing theS6 switch and opening theS5 switch. In this mode, the MPPT function is performed; therefore, maximum power is generated from PV. There is another mode that both the PV and battery provide the power to the grid by closing theS1 switch. This operation is shown as Mode 3. In this mode, the dc-link voltage that is the same as the PV voltage is enforced by the battery voltage; therefore, MPPT control is not possible. Mode4 represents an operation mode that the energy stored in the battery is delivered to the grid. There is another mode, Mode 5 that the battery is charged from the grid. This mode is not shown in Fig .4 .

Fig. 3 Example of different system operation modes of a RSC-based solar PV power plant

Fig:4 All operation modes of the RSC. (a) Mode 1—PV to grid. (b) Mode2—PV to battery. (c) Mode 3—PV/battery to grid. (d) Mode 4—battery to grid

2.3 System benefits of solar PV power plant with the RSC concept

The RSC concept allows not only the system owners to possess an expandable asset that helps them to plan and operate the power plant accordingly but also manufacturers to offer a cost-competitive decentralized PV energy storage solution with the RSC. The technical and financial benefits that the RSC solution is able to provide are more apparent in larger solar PV power plants. Specifically, a large solar PV power plant using the RSC scan be

(4)

Rajani.G, IJRIT 754 controlled more effectively and its power can be dispatched more economically because of the flexibility of operation. Developing a detailed operation characteristic of a solar PV power plant with the RSC is beyond the scope of this paper. However, different system controls as shown in Fig 3 can be proposed based on the requested power from the grid operator Preq and available generated power form the plant gen. These two values being results of an optimization problem (such as unit commitment methods) serve as variables to control the solar PV power plant accordingly. In other words, in response to the request of the grid operator, different system control scheme scan be realized with the RSC-based solar PV power plant as follows:

1) system control 1 for Pgen>Preq ; 2) system control 2 for Pgen<Preq;

3) system control 3 for Pgen=Preq;

4) system control 4 for charging from the grid (Operation Mode 5).

2.4 RSC control

2.4.1. Control of the RSC in the dc/ac operation Mode

The dc/ac operation of the RSC is utilized for delivering power from PV to grid, battery to grid, PV and battery to grid. The RSC performs the MPPT algorithm to deliver maximum power from the PV to the grid. Like the conventional PV inverter control, the RSC control is implemented in the synchronous reference frame. The synchronous reference frame proportional-integral current control is employed. In a reference frame rotating synchronously with the fundamental excitation, the fundamental excitation signals are transformed into dc signals.

As a result, the current regulator forming the innermost loop of the control system is able to regulate ac currents over a wide frequency range with high bandwidth and zero steady-state error. For the pulse width modulation (PWM) scheme, the conventional PWM scheme is utilized. Fig 5 presents the overall control block diagram of the RSC in the dc/ac operation.

2.4.2. Control of the RSC in the DC/DC Operation Mode

The dc/dc operation of the RSC is also utilized for delivering the maximum power from the PV to the battery. The RSC in the dc/dc operation is a boost converter that controls the current flowing into the battery.

Therefore, from the control point of view, it is just sufficient to control only the inductor current. Like the dc/ac operation, the RSC performs the MPPT algorithm to deliver maximum power from the PV to the battery in the dc/dc operation. Fig 6 shows the overall control block diagram of the RSC in the dc/dc operation.

2.5 Design Considerations and Modifications to the Conventional Three-Phase PV Converter

One of the most important requirements of the project is that a new power conversion solution for PV- battery systems must have minimal complexity and modifications to the conventional three-phase solar PV converter system. Therefore, it is necessary to investigate how a three-phase dc/ac converter operates as a dc/dc converter and what modifications should be made. It is common to use a LCL filter for a high-power three-phase PV converter and the RSC in the dc/dc operation is expected to use the inductors already available in the LCL filter.

There are basically two types of inductors, coupled three phase inductor and three single-phase inductors that can be utilized in the RSC circuit. Using all three phases of the coupled three-phase inductor in the dc/dc operation causes a significant drop in the inductance value due to inductor core saturation. The reduction in inductance value requires inserting additional inductors for the dc/dc operation which has been marked as “optional” in Fig 2. To avoid extra inductors, only one phase can perform the–dc/dc operation. However, when only one phase, for instance phase B, is utilized for the dc/dc operation with only either upper or lower three insulated-gate bipolar transistors (IGBTs) are turned OFF as complementary switching, the circulating current occurs in phases A and C through filter capacitors, the coupled inductor, and switches, resulting in significantly high current ripple in phase B current. To prevent the circulating current in the dc/dc operation, the following two solutions are proposed:

(5)

Rajani.G, IJRIT 755 1) All unused upper and lower IGBTs must be turned OFF

2) The coupled inductor is replaced by three single-phase inductors.

While the first solution with a coupled inductor is straightforward, using three single-phase inductors makes it possible to use all three phase legs for the dc/dc operation. There are two methods to utilize all three phase legs for the dc/dc operation: 1) Synchronous operation 2) Interleaving operation.

Fig. 5 Overall control block diagram of the RSC in the dc/ac operation

Fig 6 Overall control block diagram of the RSC in the dc/dc operation.

In the first solution, all three phase legs can operate synchronously with their own current control. In this case, the battery can be charged with a higher current compared to the case with one-phase dc/dc operation. This leads to a faster charging time due to higher charging current capability. However, each phase operates with higher current ripples. Higher ripple current flowing into the battery and capacitor can have negative effects on the lifetime

(6)

Rajani.G, IJRIT 756 of the battery and capacitors. To overcome the aforementioned problem associated with the synchronous operation, phases B and C can be shifted by applying a phase offset. For the interleaving operation using three phase legs, phases B and C are shifted by 1200and 2400, respectively.

3. MATLAB/SIMULATION RESULTS 3.1 Simulink model for MODE-1(PV to grid)

Fig 7 shows the simulink model of dc/ac control in mode1.The voltage on the dc side of the inverter is 200v, current is 5A peak for the frequency of 50HZ .Here PV is directly connected to the grid through a dc/ac operation of the converter.

Fig. 7 Simulink model of dc/ac control in mode-1

Fig. 8 PV voltage

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0

50 100 150 200 250

time(t)

voltage(v)

PV voltage

(7)

Rajani.G, IJRIT 757

Fig. 9 Output voltage in Mode-1

Fig .10 Output current in mode-1

3.2 Simulink model for MODE-2(Battery to grid)

Fig.11 shows simulink model of dc/ac control in Mode-2.The voltage on the dc side of the inverter is 118v, which is the battery voltage. The average battery charging current is 1.8A,with the frequency of 50HZ. In this operation mode , the energy stored in the battery is delivered to the grid.

Fig. 11 Simulink model of dc/ac control in Mode-2

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 -300

-200 -100 0 100 200 300

time(t)

voltage(v)

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 -8

-6 -4 -2 0 2 4 6 8

time(t)

current(A)

(8)

Rajani.G, IJRIT 758

Fig. 12 Battery output voltage

Fig .13 Output voltage in Mode-2

Fig. 14 Output current in Mode -2

3.3 Simulink model for MODE-3(PV to battery)

In mode- 3, the three phase inverter is used as a dc/dc converter. In mode-3 battery is charged with the PV panels through dc/dc operation of the converter. Here generally consider 3 cases as explained before:

a)With coupled inductor using only single phase b) With single inductor using only single phase.

c) Interleaving operation using all the three phases.

Initially a coupled three phase inductor is used and only phase B is utilized for the dc/dc operation as shown in Fig 15.

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 117

117.5 118 118.5 119

time(t)

voltage(v)

battery voltage

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 -300

-200 -100 0 100 200 300

time(t)

voltage(v)

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 -8

-6 -4 -2 0 2 4 6 8

time(t)

current(A)

(9)

Rajani.G, IJRIT 759

Fig. 15 Simulink model for the performance of the RSC with single-phase operation in the dc/dc mode with coupled inductor

Fig.16 Battery voltage

Fig.17 PV voltage

When only phase B is utilized for the dc/dc operation ,the circulating current occurs in phase A and C through filter capacitors ,the coupled inductors and switches resulting in significantly high current ripple in phase B current as shown in fig.19.

Fig.18 Battery current

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 117

117.5 118 118.5 119

time(t)

voltage(v)

battery voltage

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0

50 100 150 200 250

time(t)

voltage(v)

PV voltage

0 50 100 150 200

-4 -3 -2 -1 0 1 2

time(t)

current(A)

(10)

Rajani.G, IJRIT 760

Fig.19 Phase B current in dc/dc mode due to coupled inductors

Fig.20 is the simulink model for performance of the RSC in the dc/dc mode with single inductor. Here coupled inductor is replaced by single inductor, phase current present in this is much lower ripple as shown in Fig 21.

Fig. 20 Simulink model for performance of the RSC in the dc/dc mode with single inductor

Fig 21 Phase B current in dc/dc mode due to single inductor

In the above (Fig. 20) the simulink model only single inductor is used for dc/dc operation, due to this the ripple current is decreased, but the other two phases are wasted. By using interleaving operation, we can use three phases. Using three single phase inductors makes it possible to use all the three phase legs for the dc/dc operation.

And also the ripple current reduced. Here phase B and phase C are shifted by 1200and 2400 respectively. The below Fig 22 simulink model shows the interleaving operation.

0 50 100 150 200 250 300

5 10 15 20 25 30 35 40 45 50 55 60

time(t)

current(A)

0 50 100 150 200 250 300 350 400 450 500

-2 -1 0 1 2 3 4 5 6 7 8

time(t)

current(A)

(11)

Rajani.G, IJRIT 761

Fig. 22 Simulink model for three phase interleaving operation

Fig:23 Capacitor current

Fig.24 Battery voltage

50 100 150 200 250 300 350

-6 -4 -2 0 2 4 6

time(t)

current(A)

0 200 400 600 800 1000 1200

0 20 40 60 80 100 120 140

time(t)

voltage(v)

(12)

Rajani.G, IJRIT 762

Fig .25 Battery current

Fig.26 Three phase currents in the dc/dc mode with interleaving operation

4. CONCLUSION

This project introduced a new converter called RSC for PV-battery application. The basic concept of the RSC is to use a single power conversion system to perform different operation modes such as PV to grid (dc to ac), PV to battery (dc to dc), battery to grid (dc to ac) for solar PV systems with energy storage. The proposed solution requires minimal complexity and modifications to the conventional three-phase solar PV converters for PV-battery systems. Therefore, the solution is very attractive for PV-battery application, because it minimizes the number of conversion stages, thereby reducing weight and volume. Simulation results have been presented to verify the concept of the RSC and to demonstrate the attractive performance characteristics of the RSC. These simulation results confirm that the RSC is an optimal solution for PV-battery power conversion systems.

REFERENCES

[1] U.S. Department of Energy, “Solar energy grid integration systems-energy storage (SEGIS-ES),” May 2008.

[2] D. Bundestag, “Gesetzzur Neuregelung des Rechts der erneuerbaren Energien im Strombereich und zur¨

Anderung damit zusammenh¨ angenderVorschriften,” Bundesgesetzblatt, 2008.

[3] H. Konishi, T. Iwato, and M. Kudou, “Development of large-scale power conditioning system in Hokuto mega- solar project,” in Proc. Int. Power Electron. Conf., 2010, pp. 1975–1979.

[4] J. H. Enslin and D. B. Snyman, “Combined low-cost, high efficient inverter, peak power tracker and regulator for PV applications,”IEEE Trans. Power Electron., vol. 6, no. 1, pp. 73–82, Jan. 1991.

0 50 100 150 200

-4 -3 -2 -1 0 1 2

time(t)

current(A)

60 80 100 120 140 160 180

0 1 2 3 4 5 6

time(t)

current(A)

(13)

Rajani.G, IJRIT 763 [5] H. Ertl, J. W. Kolar, and F. Zach, “A novel multi cell dc-ac converter for applications in renewable energy systems,”IEEE Trans. Ind. Electron.,vol. 49, no. 5, pp. 1048–1057, Oct. 2002.

[6] C. Zhao, S. D. Round, and J. W. Kolar, “An isolated three-port bidirectional dc/dc converter with decoupled power flow management,” IEEE Trans. Power Electron., vol. 23, no. 5, pp. 2443–2453, Sep. 2008.

[7]M. Bragard, N. Soltau, R. W. De Doncker, and A. Schiemgel, “Designand implementation of a 5 kW photovoltaic system with Li-ion batteryand additional dc/dc converter,” inProc. IEEE Energy Converters.

Congr.Expo., 2010, pp. 2944–2949.

[8] W. Li, J. Xiao, Y. Zhao, and X. He, “PWM plus phase angle shift control scheme for combined multiport dc/dc converters,” IEEE Trans. Power Electron., vol. 27, no. 3, pp. 1479–1489, Mar. 2012.

[9] N.Benavidas and P. Chapman, “Power budgeting of a multiple-input buck boost converter,”IEEE Trans. Power Electron., vol. 20, no. 6, pp. 1303–1309, Nov. 2005.

[10] P. Barrade, S. Delalay, and A. Rufer, “Direct connection of supercapacitorsto photovoltaic panels with on-off maximum power point tracking,”IEEETrans. Sustainable Energy, vol. 3, no. 2, pp. 283–294, Apr. 2012.

[11] 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.

[12] Z. Zhao, M. Xu, Q. Chen, J. Lai, and Y. Cho, “Derivation, analysis,and implementation of a boost-buck converter-based high-efficiency PV inverter,” IEEE Trans. Power Electron., vol. 27, no. 3, pp. 1304–1313,Mar.

2012.

[13] C. Ho, H. Breuninger, S. Pettersson, G. Escobar, L. Serpa, and A. Coccia,“Practical design and implementation procedure of an interleaved boostconverter using SiC diodes for PV applications,” IEEE Trans. PowerElectron., vol.

27, no. 6, pp. 2835–2845, Jun. 2012.

[14] M. Bragard, N. Soltau, R. W. De Doncker, and A. Schmiegel, “Design andimplementation of a 5 kW PV system with Li-ion battery and additionaldc/dc converter,” inProc. IEEE Energy Convers. Congr. Expo., 2010,pp.

2944–2949.

References

Related documents

strong sense of political awareness and.. therefore it is of more than passing

Given the limited number of samples analyzed for dilutional linearity in this study and the magnitude of the differences (the slopes of ED 50 relative to spiked con- centration

- 0 3 while for the series illustrated on the right side of the panels, the &#34;conservative&#34; series, an isolate that seemed minimally altered was selected.. Thus

AGREE: Appraisal of Guidelines for Research and Evaluation; CONSORT: Consolidated Standards of Reporting Trials; GRADE: Grading of Recommendations Assessment, Development

Conclusion: The 10 item PCTB scale provides a valid and reliable means of obtaining ratings of burden from formal care teams working in nursing homes in order to evaluate

Further, utilizing two genuine social IoT applications, we exhibit that our versatile trust administration convention is able to do adaptively modifying the best trust

It’s called NextGen because it gives birth to a new Data Warehouse Architecture which clearly is an enhancement of the already existing Data Warehouse Architecture with an