experimental investigation of a grid-connected

Experimental Investigation of a Grid-Connected Photovoltaic/Wind Energy System

Mamadou Lamine Doumbia, Member, IEEE, and Kodjo Agbossou, Senior Member, IEEE

Abstract – DC bus voltage is used as the main control variable in many industrial power electronic devices for renewable energy applications with energy storage. However, when the dc voltage varies rapidly, the performances of the system can be affected. This paper presents theoretical analysis and experimental investigation of a hybrid grid-connected photovoltaic/wind energy system in which the dc bus voltage level is used to control the power exchange between the system’s components. The system’s operation was monitored for different operating modes (Float, Silent, Sell, Low battery transfer). The power transfer is analyzed and the performances are presented. Index terms -- Photovoltaic, wind turbine, renewable energy, power transfer, voltage control, state of charge.

I. INTRODUCTION he wind and photovoltaic power systems have made a successful transition from small stand-alone sites to large

grid-connected systems [1]. The utility interconnection brings a new dimension in the renewable power economy by pooling the temporal excess or the shortfall in the renewable power with the connecting grid. The grid supplies power to the local loads when needed, or absorbs the excess power from the system when available. For many years, the Hydrogen Research Institute (HRI) has developed a renewable photovoltaic/wind energy system based on hydrogen storage [2], [3]. The system consists of a wind turbine generator (WTG) and a solar photovoltaic (PV) array as primary energy sources, a battery bank, an electrolyser, a fuel cell stack, different power electronics interfaces for control and voltage adaptation purposes, miscellaneous sensors for measurement and monitoring the system’s performances. The wind and the solar radiation are stochastic by nature. So, when designing systems with random variables, one of the major problems encountered is that the primary energy quantity to be converted into electricity cannot be predetermined [4], [5]. Initially, the renewable energy system (RES) operated in stand-alone mode. For grid-connected operation mode, different control strategies can be developed. Currently, the main industrial power control interfaces used on the HRI test bench, are operated based on dc bus voltage as control variable to manage energy transfer in the system.

This paper presents the Hydrogen Research Institute (HRI) grid-connected renewable energy system (RES) (Fig. 1). The system’s main components i.e. photovoltaic array, wind

This work has been supported by the Natural Sciences and Engineering Research Council of Canada. Authors are with the Department of Electrical and Computer Engineering and the Hydrogen Research Institute, Université du Québec à Trois-Rivières, Canada (email: [email protected], [email protected]).

turbine generator, and batteries, are described individually and their operating characteristics are presented. The power transfer study in the interconnected system is presented. The integrated grid-connected system is investigated and experimental data are provided for different operating modes. The state of charge according to different operating modes is analyzed for optimization of power transfer. Such a detailed study is useful for understanding, optimal dimensioning and effective control of renewable energy systems [6], [7], [8].

II. SYSTEM COMPONENTS Fig. 1 shows the partial block diagram of the Hydrogen Research Institute (HRI) grid-connected photovoltaic-wind energy system. The system consists of two primary renewable energy sources: a 10 kW permanent magnet wind turbine generator (WTG), and a 1 kW solar photovoltaic (PV) array, a 5 kW grid-connected reversible single phase inverter which is used to convert 48V dc voltage into alternating current (ac) with 115V or conversely from ac to dc power. The output of the WTG is connected to a three phase half-controlled rectifier. The PV array is connected to a charge controller. The outputs of the rectifier and the charge controller are connected to a 48V dc bus which is linked with the battery bank. The inverter has two separated ac outputs which are connected to the utility grid and local ac load. The Hydrogen Research Institute’s renewable energy system (RES) was investigated in grid-connected operation mode. The main power control devices (three phase rectifier, charge controller, single phase bidirectional inverter) operate using dc bus voltage as control variable to manage the energy transfer in the RES. In grid-connected mode the bidirectional inverter (a Trace Engineering SW4458) can be operated in four modes: Float, Silent, Sell and Low Battery Transfer.

Fig. 1 Block diagram of the HRI’s grid-connected photovoltaic-wind renewable energy system

T

2009 IEEE Electrical Power & Energy Conference 978-1-4244-4509-7/09/$25.00 ©2009 IEEEAuthorized licensed use limited to: Univesity of Witswatersrand. Downloaded on July 20,2010 at 19:39:57 UTC from IEEE Xplore. Restrictions apply.

A. Photovoltaic generator The photovoltaic array is a group of several modules electrically connected in series-parallel combinations to generate the required current and voltage. Solar panels installed at IRH are composed of 16 modules, i.e. four rows of four serial connected modules. The electrical performance and the characteristic curves of the PV modules are dependent on temperature and illumination [9], [10]. The PV array is connected to a charge controller whose output is linked with the dc bus. The charge controller protects the batteries from over-discharge and over-charge conditions. It uses a three-stage (bulk, absorption and float) charging protocol to maintain battery voltage at bulk or float levels (Fig.2). The three-stage charging process results in faster charging compared to on-off relay type or constant voltage solid-state regulators. Faster recharging increases the performance of the system by storing more of the PV array’s limited output. The final float voltage setting reduces battery gassing, minimizes watering requirements and ensures complete battery recharging.

Fig. 2 Three-stage battery charging

B. Wind generator The wind turbine rotor extracts the energy from the wind and converts it into mechanical power. For steady-state calculations of the mechanical power from a wind turbine, the so called Cp(λ,β)-curve is used. An algebraic relation is assumed between wind speed and mechanical power extracted, which is described by the well-known expression [11], [12]:

( ) 32, ,

21 vCRP pmW βλρπ= (1)

The mechanical power PW,m is converted into electrical power by the generator. A three phase permanent magnet generator (PMG) is used in the HRI’s renewable energy system. The PMG is proposed as a wind turbine generator due to its property of self-excitation. The generator output ac power is converted into dc power by a three phase half-controlled rectifier bridge. In the steady state operation, the balance of power must be maintained on both ac and dc sides of the rectifier. That is, the power on the ac side must be equal to the sum of the dc load power and the losses in the rectifier circuit.

DCWDCWACWLLeW IVIVP ,,,, 3 == (2)

VLL= line-to-line voltage on three-phase ac side of the rectifier

VW,DC= Rectifier output voltage IW,DC= Rectifier output current

The half-controlled bridge can be analyzed by considering it as a phase-controlled half-wave circuit in series with an uncontrolled half-wave rectifier. The average output dc voltage is given by (3):

αππ

cos2

232

23, LLLLDCW VVV += (3)

α = angle of firing delay in the switching. In normal regulation mode (α=0), the rectifier output voltage is :

LLDCW VVπ

23, = (4)

From (3) and (4), the dc current injected by the wind generator into the DC bus is:

ACWDCW II ,, 6π= (5)

C. Battery power The battery plays the role of an energy buffer for short-term energy storage [13]. The battery is made of numerous electrochemical cells connected in a series-parallel combination to obtain the desired operating voltage and current. The battery rating is stated in terms of the average voltage during discharge and the Ah capacity it can deliver before the voltage drops below the specified limit. In our application, the battery bank is composed of eight 6V, 225 Ah Interstate batteries connected in series to provide 48V for the dc bus.

D. Bidirectional grid-connected inverter A single phase bidirectional inverter is used to interconnect dc bus with the utility grid. The inverter is able to synchronize with utility grid before connecting it to the ac load. The frequency of the grid is tracked and the inverter constantly adjusts its frequency to maintain connection with the grid. The grid-connected inverter is the main control device in the RES. In charging mode (ac to dc), the inverter uses the same three-stage (bulk, absorption and float) charging principle as PV charge controller to maintain battery voltage at bulk or float levels (Fig.2)

III. POWER TRANSFER ANALYSIS The dc bus is the central interconnection point of the RES (Fig.1). The battery bank is used to hold up the dc power differences between the supplied power from the renewable energy sources, the load power, and the power to (or from) the utility grid. The instantaneous power available at dc bus (Fig.1) can be expressed as:

( ) )()()()()( ,, tPtPtPtPtPtP loadDCbatinveWPVBus −±±+= (6)

Authorized licensed use limited to: Univesity of Witswatersrand. Downloaded on July 20,2010 at 19:39:57 UTC from IEEE Xplore. Restrictions apply.

where PPV(t) is the power generated by the PV system, PW,e(t) is the power produced by the wind generator, Pinv(t) is the power of the inverter, Pbat(t) is the battery’s power, and PDC,load(t) is the power consumed by dc load connected to the dc bus. The battery power is negative when it operates in charging mode (energy is flowing from the dc bus to the battery bank) and positive in discharging mode (energy is flowing from the battery bank to the dc bus). The inverter power is positive when it sends energy to the dc bus and negative when it consumes energy from dc bus. For the purpose of energy balance analysis on the dc bus, the positive powers are considered to be input powers and the negative powers are considered to be output powers. So, if the sum of dc bus input powers is bigger than the sum of output powers, then the dc bus voltage will increase. On the contrary, if the sum of input powers is less than the sum of output powers, then the dc bus voltage will decrease. Based on this principle, the renewable energy system operation was investigated in different operating conditions in order to identify the optimal one.

IV. EXPERIMENTAL INVESTIGATION

A. Operation in Float mode

Float mode maintains the batteries at the float voltage level. The inverter completes a full three-stage charging cycle. After the charging stage, the ac loads are powered by the batteries until their voltage falls below the minimum set voltage value (Low battery cut out). When the batteries voltage is lower than this minimum value, then the utility grid begins to power the ac loads. If available, the photovoltaic or wind energy is used to power ac loads or charge batteries. The Float mode was used to operate the RES from 11:55 to 16:38 (April 30, 2009). Fig.3 shows the system’s operating characteristics. In the Float mode, the load is supplied by batteries as long as the dc bus voltage (Fig.3a) is greater than a pre-defined threshold (Low Battery Transfer Voltage) set to 47V in our case. When the dc bus voltage falls under 47V, the utility grid begins to feed the loads and batteries (Fig.3b) are recharged by the energy (if available) coming from PV panels (Fig.3e) and from the wind turbine generator (Fig.3f). The utility grid (Fig.3d) continues to power the loads as long as the dc bus voltage does not reach another pre-defined voltage (Low Battery Cut In voltage), set to 52V (Fig.3a) during this test. Fig.3c shows the batteries state of charge variation. Rapid dc bus voltage variations (Fig. 3a, from Time 13:15) are reflected on power transfer in the RES (Fig. 3b and Fig 3.d).

45

46

47

48

49

50

51

52

53

54

55

11:50:00 13:02:00 14:14:00 15:26:00

Time (s)

Vol

tage

(V)

-1750

-1250

-750

-250

250

750

1250

11:50:00 13:02:00 14:14:00 15:26:00

Time (s)

Pow

er (V

A)

a) DC bus voltage b) Batteries power

75

80

85

90

95

100

11:50:00 13:02:00 14:14:00 15:26:00

Time (s)

% o

f ch

arge

0

200

400

600

800

1000

1200

1400

1600

1800

11:50:00 13:02:00 14:14:00 15:26:00

Time (s)

Pow

er (V

A)

c) State of charge d) Utility power

0

50

100

150

200

250

300

350

400

11:50:00 13:02:00 14:14:00 15:26:00

Time (s)

Pow

er (V

A)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

11:50:00 13:02:00 14:14:00 15:26:00

Time (s)

Puis

sanc

e (V

A)

e) Photovoltaic power f) Wind power

Fig. 3 Grid-Connected RES operation in Float mode


B. Operation in Silent mode The renewable energy system (RES) was tested in this mode on April 20th, 2009 from 12:00 to 16:15. The load was composed of the dc load (electrolyser with 400W) and ac load (lighting 380W). At the beginning of the test, the inverter was in bulk charge mode until 12:45 (Fig.4a). During this period, the utility network (Fig.4d) tries to recharge batteries (Fig.4b) by maintaining the batteries voltage at 53.6V (Fig.4a). After the bulk charge period, the inverter works in Silent mode.

Then, the dc bus voltage depends on the power balance between the input powers (photovoltaic, wind) and output powers (electrolyser and lighting). For this test, the batteries discharging rate is low (Fig. 4c) because of the presence of enough the PV (Fig.4e) and wind (Fig.4f) powers. In Silent mode, the inverter performs a bulk charge (i.e. charging at a preset voltage level) of the batteries once per day, from the grid. This mode is typically used in utility back-up applications [14].

45

46

47

48

49

50

51

52

53

54

55

12:00:00 13:12:00 14:24:00 15:36:00

Time (s)

Vol

tage

(V)

-400

-200

0

200

400

600

800

1000

1200

1400

12:00:00 13:12:00 14:24:00 15:36:00

Time (s)

Pow

er (V

A)


90

91

92

93

94

95

96

97

98

99

100

12:00:00 13:12:00 14:24:00 15:36:00

Time (s)

% o

f ch

arge

0

200

400

600

800

1000

1200

1400

1600

1800

2000

12:00:00 13:12:00 14:24:00 15:36:00

Time (s)

Pow

er (V

A)

c) State of charge d) Utility power

0

50

100

150

200

250

300

350

400

12:00:00 13:12:00 14:24:00 15:36:00

Time (s)

Pow

er (V

A)

0

200

400

600

800

1000

1200

1400

1600

1800

12:00:00 13:12:00 14:24:00 15:36:00

Time (s)

Pow

er (V

A)

e) Photovoltaic power f) Wind power

Fig. 4 Grid-Connected RES operation in Silent mode C. Operation in Sell mode The RES was tested in this mode on May 5th, 2009 from 12:00 to 16:30 (Fig.5). A 460W local ac load was connected to the inverter output. The inverter was programmed to send up to 7A (around 800VA supplementary power) to the utility grid if the dc bus voltage is higher than a pre-defined threshold voltage (48V). At the beginning of the test, as the dc bus voltage (Fig.5a) is higher than the pre-defined sell voltage, the loads are powered by batteries (Fig.5b) through the inverter; this leads to the batteries discharging (Fig.5c). When renewable PV (Fig.5e) and wind (Fig.5f) energy sources are available, the power provided by the batteries (inverter) is

reduced. When the dc bus voltage reaches the pre-defined 48V, the energy from the batteries is reduced (Fig.5b) and loads are mainly powered by the utility grid and renewable energy sources. Sell mode enables the inverter to send the excess power to the grid. This mode must have the approval of the local power utility prior to its use. In this mode, when power from photovoltaic array or wind generator is available, it will be used to power any ac load connected to the ac output of the inverter (Fig.5d). Any excess power available from the system will be sold “into” the utility grid [14].


45

46

47

48

49

50

51

52

53

54

55

11:25:00 12:37:00 13:49:00 15:01:00 16:13:00

Time (s)

Vol

tage

(V)

-1250

-750

-250

250

750

1250

1750

11:25:00 12:37:00 13:49:00 15:01:00 16:13:00

Time (s)

Pow

er (V

A)


75

80

85

90

95

100

11:25:00 12:37:00 13:49:00 15:01:00 16:13:00

Time (s)

% o

f ch

arge

400

500

600

700

800

900

1000

1100

11:25:00 12:37:00 13:49:00 15:01:00 16:13:00

Time (s)

Pow

er (V

A)

c) State of charge d) Inverter sell power

100

150

200

250

300

350

400

11:25:00 12:37:00 13:49:00 15:01:00 16:13:00

Time (s)

Pow

er (V

A)

0

500

1000

1500

2000

2500

3000

11:25:00 12:37:00 13:49:00 15:01:00 16:13:00

Time (s)

Puis

sanc

e (V

A)

e) PV power f) Wind power

Fig. 5 Grid-Connected RES operation in Sell mode D. Operation in Low Battery transfer mode The “Low battery transfer mode” allows a system to switch automatically between utility connected and stand alone battery operation. In this mode, the inverter will power the loads from the battery and other energy sources until the battery voltage drops to the pre-defined “low battery transfer voltage” value. It will then connect to the utility grid and charge the battery. The loads will be powered by the utility until the battery voltage reaches the “Low battery cut in voltage” value. The inverter will then disconnect the utility and power the loads from the battery or any other connected dc power source. This mode is often used instead of the Sell mode because approval from the utility is not required - no

power will be sent into the utility distribution system when this mode is selected [14]. The RES was tested in Low battery transfer mode on May 11th, 2009 from 11:45 to 16:33 (Fig.6). Around 1200W local ac load was connected to the inverter output. At the beginning of the test, as the dc bus voltage (Fig.6a) is higher than the pre-defined threshold voltage (47V), the loads are powered by batteries (Fig.6b) through the inverter; this leads to the batteries discharging (Fig.6c). When the dc bus voltage reaches the pre-defined 47V, the utility grid begins to power the local load (Fig.6d). At all time, if renewable PV (Fig.6e) or wind (Fig.6f) energy source is available, the power provided by the batteries or by the utility grid is reduced.


46

47

48

49

50

51

52

53

54

11:45:00 12:57:00 14:09:00 15:21:00 16:33:00

Time (s)

Vol

tage

(V)

-1500

-1000

-500

0

500

1000

1500

2000

11:45:00 12:57:00 14:09:00 15:21:00 16:33:00

Time (s)

Pow

er (V

A)


75

80

85

90

95

100

11:45:00 12:57:00 14:09:00 15:21:00 16:33:00

Time (s)

% o

f ch

arge

Inverter power

0

200

400

600

800

1000

1200

1400

1600

1800

11:45:00 12:57:00 14:09:00 15:21:00 16:33:00

Time (s)

Pow

er (V

A)

c) State of charge d) Utility grid power

0

50

100

150

200

250

300

350

400

450

500

11:45:00 12:57:00 14:09:00 15:21:00 16:33:00

Time (s)

Pow

er (V

A)

0

500

1000

1500

2000

2500

3000

11:45:00 12:57:00 14:09:00 15:21:00 16:33:00

Time (s)

Pow

er (V

A)

e) PV power f) Wind power

Fig. 6 Grid-Connected RES operation in Low battery transfer mode

V. CONCLUSION The renewable energy system was investigated experimentally in four grid-connected operation modes (Float, Silent, Sell, Low battery transfer). For all these modes, the system’s operating conditions were monitored and power transfer was analyzed. In Float mode, the inverter completes a full three-stage charging cycle. After the charging stage, the ac loads are powered by the batteries until their voltage falls below the minimum set voltage value (Low battery cut out). When the batteries voltage is lower than this minimum value, then the utility grid begins to power the ac loads. In Silent mode, the inverter performs a bulk charge (i.e. charging at a preset voltage level) of the batteries once per day, from the grid, and then goes totally silent. In Sell mode, when the batteries voltage is higher than a predefined value, the inverter sends the excess energy to the grid. In this mode, when power from photovoltaic array or wind generator is available, it will be used to power any ac load connected to the ac output of the inverter. In Low battery transfer mode, if the system is not properly sized, frequent transfers from the battery to the utility and result in poor performance of the system and excessive energy

consumption from the utility. The daily output of the renewable power source (solar, wind etc.) should be able to meet the daily power requirements of the loads being operated under typical conditions. The renewable energy system has shown better operating performance in the Silent and Sell modes. Moreover, to improve the system performances in all operating modes, new control method based on the state of charge of batteries must be developed. This is our next research direction to improve the operating performances of the grid-connected renewable energy system in all operation modes.

VI. REFERENCES [1] Mukund R. Patel, Wind and Solar Power Systems, CRC Press, 1999. [2] K. Agbossou, M. Kolhe, J. Hamelin, and T. K. Bose, “Performance of a

Stand-Alone Renewable Energy System Based on Energy Storage as Hydrogen,” IEEE Transactions on energy Conversion, vol. 19, no. 3, pp. 633-640, 2004.

[3] Doumbia, M.L., Agbossou, A., Granger, E. “Simulink Modelling and Simulation of a Hydrogen Based Photovoltaic/Wind Energy System”, EUROCON, 2007. The International Conference on "Computer as a Tool" 9-12 Sept. 2007 Page(s): 2067 – 2072.

[4] Francois Giraud, and Zyiad M. Salameh, “Steady-State Performance of a Grid-Connected Rooftop Hybrid Wind–Photovoltaic Power System with Battery Storage”, IEEE Transactions on Energy Conversion, Vol. 16, No. 1, March 2001.


[5] Roger Jacobson, et al. “Renewable Hydrogen Systems Integration and Performance Modeling”, Proceedings of the 2001 DOE Hydrogen Program Review, NREL/CP-570-30535.

[6] Yaow-Ming Chen, Chung-Sheng Cheng, and Hsu-Chin Wu, “Grid-Connected Hybrid PV/Wind Power Generation System with Improved DC Bus Voltage Regulation Strategy”, IEEE Transactions on Industrial Electronics, Vol. 55, No. 4, April 2008.

[7] Seul-Ki Kim, Jin-Hong Jeon, Chang-Hee Cho, Jong-Bo Ahn, and Sae-Hyuk Kwon, “Dynamic Modeling and Control of a Grid-Connected Hybrid Generation System With Versatile Power Transfer”, IEEE Transactions on Industrial Electronics, Vol. 55, No. 4, April 2008.

[8] S. R Vosen., J.O. Keller, “Hybrid energy storage systems for stand-alone electric power tems: optimization of system performance and cost through control strategies”. International Journal of Hydrogen Energy, Vol. 24, Page(s): 1139-1156, 1999.

[9] J. A. Gow, C.D. Manning, “Development of a photovoltaic array model for use in power-electronics simulation studies”, IEE Proc.-Electr. Power Appl., Vol. 146, No. 2, March 1999.

[10] M. L. Doumbia, K. Agbossou, “Modelling and Simulation of a Hydrogen Based Photovoltaic/Wind Distributed Generation System”, Intech Renewable Energy", ISBN 978-953-7619-X-X, July 2009.

[11] R. Cardenas, R. Pena, “Sensorless Vector Control of Induction Machines for Variable-Speed Wind Energy Applications”, IEEE Trans. On Energy Conversion, Vol. 19, No. 1, March 2004.

[12] J. G. Slootweg, S. de Haan, h. Polinder, and W. Kling, “General Model for Representing Variable Speed Wind Turbines in Power System Dynamics Simulations”, IEEE Trans. On Power Systems, Vol. 18, No. 1, February 2003.

[13] A. Chérif, M. Jraidi and A Dhouib, “A battery ageing model used in stand alone PV systems”, Journal of Power Sources, vol.112, Issue 1, Page(s): 49-53, 2002.

[14] Trace Engineering Company Inc. (Xantrex), SW Series Inverter/Chargers Owner’s Manual, 1999.

VII. BIOGRAPHIES Mamadou L. Doumbia received the M.Sc. degree in electrical engineering from Moscow Power Engineering Institute, Russia, in 1989, the M.Sc. in Industrial Electronics from the Université du Quebec à Trois-Rivières, Canada, in 1994 and Ph.D. degree in electrical engineering from the Ecole Polytechnique de Montréal, Canada, in 2000. He worked as Research engineer at the CANMET Energy Technology Centre and currently, he is a Professor at the Université du Quebec à Trois-Rivières (UQTR), Canada. His research interests

include power electronics, electric drives, renewable energy and distributed generation systems.

Kodjo Agbossou received his B.S. (1987), M.S. (1989) and Ph.D. (1992) in Electronic Measurements from the Université de Nancy I, France and post-doctoral research at the Electrical Engineering Department of the Université du Québec à Trois-Rivières (UQTR), Canada. He was Project Manager and Research Professional (1995-1998) at UQTR. Since 1998, he is professor and actually Full Professor in the Electrical and Computer Engineering Department of UQTR. Since 2007 he is the Director of the Department of

Electrical and Computer Engineering. Presently, he is doing research in the area of renewable energy, integration of electrical energy system, control and measurements. He is the author of more than 70 publications and has 3 patents.


experimental investigation of a grid-connected

Documents