Abstract—This paper will outline the simulation of a

photovoltaic emulator. A photovoltaic emulator is a DC-DC converter which has similar electrical characteristics to a photovoltaic panel. A photovoltaic emulator allows PV systems to be analyzed in a controlled environment where using actual photovoltaic panels would produce inconsistent results for the PV system, due to variability in outside temperature and weather conditions.

Index Terms-- photovoltaic, irradiance, emulator, buck

converter.

I. INTRODUCTION ITH increasing interest in renewable energies large amounts of money and effort have been put into

research and development for photovoltaic (PV) systems. The larger interest in PV systems has increased demand for appropriate equipment to test PV systems.

In this paper a circuit for a photovoltaic emulator will be simulated. A photovoltaic emulator has similar electrical characteristics to a PV panel. A photovoltaic emulator essentially is a power supply with similar current and voltage characteristics as a PV panel. They however provide a controlled environment, which is not affected by external factors such as temperature and weather. This allows repeatable conditions to test PV equipment such as inverters and provides a controlled environment to test an overall PV system.

II. BACKGROUND

A. Need For PV Emulator Having a controlled environment to test PV equipment is

difficult since consistent repeatable conditions are impossible to reproduce. A PV panel’s electrical characteristics will change based on a variety of factors including the amount of irradiance received, temperature of the panel, and the material

D. S.L Dolan is with the Electrical Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93407, USA (e-mail: dsdolan@calpoly.edu).

J. Durago and J. Crowfoot are with the Electrical Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93407, USA.

Taufik is with the Electrical Engineering Department, California Polytechnic State University, San Luis Obispo, CA 93407, USA (e-mail: taufik@calpoly.edu).

used to make the PV panel. A PV emulator will simulate the current and voltage characteristics of a photovoltaic panel under these various conditions. Having consistent electrical characteristics will allow easier analysis and optimization of PV systems [2].

Fig. 1: Implemented PV Emulator Hardware Example [1]

A huge variety of solar panels exist. It would be prohibitively expensive to purchase each type of panel and test individually. A PV emulator would be able to simulate many different types of solar panels, under various temperature and weather conditions.

B. History There are several PV emulators commercially available as

well as some PV emulators created by universities for educational purposes. Some of these emulators are modular so that they can be connected in series or in parallel to further simulate a whole PV array system. Commercial emulators are prohibitively expensive to serve as laboratory learning tools for students and thus in house creation of the emulators allows for a tailor made solution.

A large amount of PV emulators are used to test the photovoltaic systems on space satellites. [3] Due to the extreme conditions in outer space, designers would like to quickly and accurately simulate how their PV system will behave in space. Thus PV emulators are their best and most practical option.

Simulation of A Photovoltaic Emulator

Dale Dolan, Member, IEEE, Joseph Durago, Joe Crowfoot, Taufik, Senior Member, IEEE

W

C. Other Topologies Used Others that have made PV emulators have used several

different DC-DC converter topologies. To simulate a whole array of PV Panels with a maximum output power of 4.4KW a full-bridge architecture was implemented [4]. Others emulators that were able to mimic the electrical characteristics of PV panels used half-bridge, buck, and buck-boost topologies [5][6]. There are several topologies that can be used to implement a PV emulator, where topology selection is mainly based on the desired maximum output power.

Fig, 2: Commercially Available PV Emulator [3]

III. PV PANEL CHARACTERISTICS A PV panel can be modeled by the ideal diode equation

given below. The goal of the photovoltaic emulator is to have an output which follows the ideal diode equation as close as possible [7]:

(1) Where K is the Boltzman constant = 1.38x10-23 J/K; q is the electronic charge = 1.6x10-19 C; T: is the cell temperature; Is: is the saturation current . The ideal diode equation serves to provide a guide for us to create the control scheme for the buck converter that would adequately model a PV panel. For a given load current, there is a corresponding voltage

IV. DC-DC CONVERTER DESIGN

A. Input Stage A common concern in building power supplies is electromagnetic interference to and from the power supply however an input filter is unnecessary because this is a piece of testing equipment. Also it will not be used for commercial or in vital applications therefore the PV emulator does not

need to be extra durable. Therefore no complex filtering will be needed at the input.

Rp

Rs

DIph

Fig. 3: Equivalent Circuit for a Solar Cell

The PV emulator will be used for off-line applications. A full bridge rectifier shall be used to provide a DC input for the buck converter. For simulation purposes a 108 Vrms DC source was used to model the input voltage which was calculated from the formula below:

B. Power Stage A buck converter topology was chosen as the power

stage of the PV emulator. The buck converter allows for a wide range of output power. To emulate various PV panels the output power range should be between 0W and 300W. This wide output power range will reflect the various power that different panels can produce as well as emulate different irradiance values.

The buck converter will be designed to operate in continuous conduction mode under all load conditions and the ripple on the output voltage cannot exceed 3%.

1, Buck Converter Topology

The Buck Converter has a very basic function of stepping

down a variable input DC voltage, to create a constant DC output voltage. This output voltage remains constant regardless of load or input fluctuations. This section will review the derivation of the Buck Converter’s transfer function, as well as the derivation for rated values for all components of the Buck converter described in this paper. The basic buck converter circuit is shown in Figure 4.

Fig. 4: Circuit of Buck Converter

a. Buck Converter Transfer Function When the Switch is ON, the Diode turns off, and ideally the switch becomes a short, as shown in Fig. 5. Using the Ideal Inductor Equation:

(2) substituting VL1 = V1 – VR1 into (2) yields (3)

(3) Substituting dt = DT, where D = Duty Cycle and T = Switching Period yield (4)

(4) Now with the switch OFF, the same process can be used to derive (5) Using Fig. 6 as an equivalent circuit.

(5) These two equations represent two halves of one period of current through the inductor. One can sum them and set them equal to zero to solve for VR1. (4) +(5) yields (6)

= 0 (6) Solving for VR1 yields (7)

(7) This equation shows that the output voltage will always be less than the input voltage, since duty cycle must always be less than 1.

b. Inductance Derivation We can use equation (5) or (6) to design the size of the

inductor. For the purpose of this application, a frequency of 100 kHz will be used such that the transient speed of the emulator is fast enough to reproduce expected transients. Since our current values will need to be very low, we require that the maximum change in inductor current be 0.01 A. We will also assume the worst case duty cycle so that the minimum inductor value will be calculated. Given these values, the minimum inductor value was calculated to be 1mH, as shown below. Note that for simulation purposes a much higher inductor value was used:

c. Output Capacitance Derivation

The output capacitance value can be calculated by using the charge equation for a capacitor: Q = VC. Because there is a volt second balance for a charging and discharging capacitor, you can solve for the capacitance in terms of ripple voltage.

Q = C (8)

Where: ` (9) Substituting (9) into (8) and solving for C:

(10)

Fig. 5: Buck Converter Circuit When Switch Is ON

Fig. 6: Buck Converter Circuit When Switch Is OFF

VR1 = Vout = DV1

Fig. 7: Circuit of PV Emulator

Capacitor Value was chosen to accommodate an output voltage ripple that would satisfactorily replicate the output voltage of a solar cell. This ripple value can be estimated to be approximately 0.01V. Therefore a capacitor value of was chosen to be 1 uF.

d. Diode and Switch Ratings Diode and Switch Ratings can be found by finding peak

inverse voltage, and average current for both components. For the diode, the peak inverse voltage occurs when the switch is ON. This value will equal the input voltage, which in this case is 108V. The diode will only conduct current when the switch is off. Therefore the current through the diode will equal the maximum output current times the time the diode is conducting. The time the diode conducts is opposite of when the switch is on, so the output current is multiplied by (1-D). Because the duty cycle changes on feedback, we will use the worst case scenario for duty cycle. This yields a current rating of 5.58 A .

The switch only conducts during the duty cycle of the switch, therefore the switch current rating equals output current times duty cycle, however since duty cycle varies with feedback, we will assume worst case scenario for duty cycle. This results in a current rating of 5.58A.

C. Control A closed loop system is used to control the pulse-

width-modulation (PWM) for the switch. A look-up table is used to provide the various data points on the I-V curve such that the simulation will match the anticipated hardware implementation. When a change in load occurs the controller senses the change in current. The reference voltage for the PWM control will change accordingly which adjusts the overall output voltage of the PV emulator. The control system currently has only proportional control. A proportional-integral controller (PI) would provide a faster response with less steady state error however design of a PI controller is out

of scope of this paper but will be added to the simulation at a later date [8].

Fig. 8: Theoretical and Simulated PV I-V Curve

V. SIMULATION RESULTS Initially our first test was a single load. As seen from

Fig. 9 the settling time was 8.13 ms for a 9.49 Ω resistor. In steady state the reference voltage and output voltage are equal to one another.

Fig. 10 shows an increasing step change in load. We are stepping down the current by increasing the load and therefore we can see how this effects the response time of the emulator. As the output current is stepped down closer to zero, specifically in the final step in Fig. 10, and the first step in Fig. 11, we see there is worse settling time and percent overshoot. This is because the circuit was designed for a a 0.01 A ripple, and at these very low loads, the inductor current goes into discontinuous mode. This mode of operation creates a bad transient response for any change in output. As can be seen in Fig. 11, the output voltage never settles for the first plateau. This is a characteristic of many PV emulators as they are good at emulating behavior in between short circuit current and open circuits voltage states, yet cannot adequately model them by themselves. Fig. 6 Depicts the I-V curve of the experimental and theoretical values for the PV emulator. The emulator does a good job recreating the curve in steady state. The only errors exist at points very close to open circuit voltage and short circuit current. This is because of the factors mentioned in the previous paragraph. Fig. 8 highlights how well this topology functions in steady state. Whereas Fig. 10 and 11 highlight some problems with transient response. These errors in transients are mostly due to the control scheme used. It is expected that these errors will be minimized once the PI controller is implemented.

VI. NOTES ON HARDWARE IMPLEMENTATION When the PV emulator will be manufactured some

design issues need to be addressed. At the control stage a proportional-integral control shall be used. This will allow for a faster transient response for the PV emulator [9][10].When designing the PI controller the gain of the proportional and

integral values need to be set to appropriate values so as to provide an adequate transient response as well as ensure that the system will not become unstable. Design of the PI controller is out of the scope of this paper, however stability issues will have to be addressed when creating the PV emulator. The control stage will most likely be provided by a DSP board to provide PWM control of the switch as well as the logic needed to create the I-V curve of a solar panel. A program such as LabView or Matlab/Simulink will be used in order to have a graphical user interface (GUI) to control some parameters of the PV panel and outside conditions. These parameters will include the short circuit current, open circuit voltage, rated current, rated voltage, and ambient temperature. Together these parameters will be put into an algorithm which will create an appropriate I-V curve to emulate.

VII. CONCLUSION This paper gives an overview of simulating a PV

emulator using a buck converter. This will emulate a single solar panel for the purpose of studying control schemes and power electronic interfaces for single panels. The idea of using a DC-DC converter to emulate a PV panel can be extended so that high output powers can be achieved. This will allow a whole array of PV panels to be emulated.

VIII. REFERENCES [1] Cirrincione, M.; Di Piazza, M.C.; Marsala, G.; Pucci, M.; Vitale, G

“Real Time Simulation of Renewable Sources”; Industrial Electronics, 2008. ISIE 2008. IEEE International Symposium

[2] Athena Energy Corporation, 1000W PV Emulator Brochure [3] Agilent Technologies, E4360 Modular Solar Array Simulators [4] Martin-Segura, G. ; Lopez-Mestre, J. ; Teixido-Casas, M. ; Sudria-

Andreu, A., “Development of a Photovoltaic Array Emulator System Based on a Full-Bridge Structure” Electrical Power Quality and Utilisation, 2007. EPQU 2007. 9th International Conference

[5] Ollila, J., “A medium power PV array simulator with a robust control stategy” Control Applications, 1995., Proceedings of the 4th IEEE Conference

[6] Tsai, H.; Tu, C.; Su, Y, “Development of Generalized Photovoltaic Model Using MATLAB/SIMULINK” World Congress on Engineering and Computer Science 2008

[7] National Instruments, “Photovoltaic Cell I-V Characterization Theory and LabVIEW Analysis Code,”. Available: http://zone.ni.com/devzone/cda/tut/p/id/7230 [Accessed May 30, 2010]

[8] Duran, E.; Sidrach-de-Cardona, M.; Galan, J.; Andujar, J.M., “Measurement Method for PV Modules based on DC-DC Converters with Microcontroller” Power Electronics and Applications, 2009. EPE '09. 13th European Conference

[9] Zhaoxia Leng; Jian Liu; Qingfeng Liu; Huamin Wang; “A Digital Optimal Control Method for DC-DC Converters Based on Simple Model”Power Electronics and Intelligent Transportation System, 2008. PEITS '08. Workshop

[10] Kurokawa, F.; Mizoguchi, T.; Sukita, S.; Osuga, H., “A new digital control for forward type multiple-output dc-dc converter“Telecommunications Energy Conference, 2009. INTELEC 2009. 31st International

IX. BIOGRAPHIES

Dale S.L. Dolan (M’2001) is an Assistant Professor of Electrical Engineering at Cal Poly with experience in renewable energy projects, education, power electronics and advanced motor drives. He received his BSc in Zoology in 1995 and BEd in 1997 from the University of Western Ontario. He received the BASc in Electrical Engineering in 2003, MASc. in Electrical Engineering in 2005 and PhD in Electrical Engineering in 2009 all from the University of Toronto. He is the chair on the board

of directors of Windy Hills Caledon Renewable Energy, past chair of the OSEA (Ontario Sustainable Energy Association) Board and was an executive chair of the 7th World Wind Energy Conference 2008 (WWEC 2008). He is currently a member of the management committee for the Ontario Green Energy Act Alliance in the midst of implementation of the most progressive renewable energy policy in North America. His research interests involve sustainable/renewable energy generation, wind power generation, smart grid technology, power systems, electromagnetics, power electronic applications for distributed generation, grid connection impacts of renewable generation, energy policy promoting widespread implementation of sustainable power generation, sustainable energy project economics and sustainability of technologies. Taufik (M’1999, SM’2007) was born and raised in Jakarta, Indonesia. He received his BS in Electrical Engineering with minor in Computer Science from Northern Arizona Univ. in

1993, MS in Electrical Engineering from Univ. of Illinois Chicago in 1995, and Doctor of Engineering in Electrical Engineering from Cleveland State University in 1999. He then joined the Electrical Engineering department at Cal Poly State University in 1999 where he is currently a Professor. He is a Senior Member of IEEE and has done consulting work for several companies

including Capstone Microturbine, Rockwell Automation (Allen-Bradley), Picker International, Rantec, San Diego Gas & Electric, APD

Time

0s 5ms 10ms 15ms 20ms 25ms 30msV(Vfb)

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Fig. 9: Response to a single load

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Fig. 10: Stepped increase in load resistance

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Fig. 11: Stepped decrease in load resistance