Current Controlled Buck Converter based … Controlled Buck Converter based Photovoltaic Emulator...

Current Controlled Buck Converter based

Photovoltaic Emulator

Ankur V. Rana C. G. Patel Institute of Technology, Bardoli, Surat, India

Email: [email protected]

Hiren H. Patel Sarvajanik College of Engineering & Technology, Surat, India

Email: [email protected]

Abstract—The output characteristics of the Photovoltaic (PV)

modules, and hence, the array greatly depends on the

environmental factors. Therefore, it is difficult to reproduce

and maintain the same environmental conditions for testing

and comparing the performance of PV power conditioning

systems. A PV emulator, which usually is a power electronic

converter, can reproduce the desired output characteristics

irrespective of the environmental conditions. It gives

opportunity to test and analyze different PV systems in

intended controlled environment. The aim of the work is to

design a current-controlled buck type dc-dc converter based

PV emulator. In order to confirm the effectiveness of the

emulator in reproducing the PV module(s), the performance

of PV emulator is evaluated with different types of loads

(linear and non-linear loads) and is compared with the

results that would have been obtained if the loads were fed

from the real PV source. Extensive simulation results

obtained in MATLAB are included to show that the PV

emulator system behaves electrically similar to a real PV

source.

Index Terms— Photovoltaic, emulator, buck converter

I. INTRODUCTION

Several factors like depletion of the conventional

sources, increase in the cost of electricity, increased

concern about the environment, government policies and

incentives for renewable energy generation, etc. have

drawn more attention of the researchers towards the

renewable or non-conventional energy sources. One of

the most promising renewable energy sources is solar

photovoltaic (PV). Due to number of benefits like direct

solar to electric energy conversion, no operating cost, no

moving parts, modularization, no constraints in terms of

site location etc., the number of PV systems (isolated or

grid-connected) has increased greatly over the past few

years [1]

However, PV systems do have some limitations[2], [3].

These include low efficiency, higher initial cost,

interaction with the other systems connected in parallel,

etc. Also, the effect of the partial shading may lead to

decrease in the output power of the PV array, difficulty in

Manuscript received December 31, 2012; revised January 25, 2013.

tracking the maximum power point (MPP), increase in

harmonics, poor THD etc. Large number of PV systems

in the existing electrical power systems network may also

cause problems like congestion, voltage instability,

resonance etc. Such issues demand investigations in

understanding the behavior of the PV systems. Hence, it

is essential to test such systems prior to their design

and/or installations.

The objective can be achieved with the help of an

experimental set up that is capable of reproducing the

characteristics similar to that of a PV array. Such

experimental set-up is called an emulator [4]-[7]. Some of

the reasons, which suggest the need for an emulator to

reproduce the characteristics of PV array are as under:

The cost of actual PV array is very high.

1) The commissioning of actual PV array requires a

large area. Also, to study the characteristics for

different array configurations one has to reconnect

the PV modules differently, which is a laborious

task and consumes time.

2) It is difficult to emulate a PV array by simply

having either a Current Source (CS) or a Voltage

Source (VS).

3) It gives the liberty to carry out the experimentation

even at the night times when sun is not available or

under cloudy conditions and low insolation

conditions.

4) It is difficult to reproduce and maintain the similar

characteristics with the PV array as the insolation

and other environment conditions do not remain

same.

5) Such emulator can reproduce the different desired

characteristics, within no time and no extra cost, by

just making some minor changes in the algorithm of

the controller.

PV emulators based on op-amp circuits or DC-DC

converters have been proposed over the years [4]-[7]. To

emulate a PV module, a single reference solar cell and a

current amplifier is used as a reference in [7], while Lee

et.al [8], used a look-up-table with discrete values of the

solar panel’s output current and voltage. The system with

solar cell as reference is prone to inaccuracies in case the

solar cells have some defects and/or shading while the

system based on look up table relies on interpolation.

91

Journal of Industrial and Intelligent Information Vol. 1, No. 2, June 2013

©2013 Engineering and Technology Publishingdoi: 10.12720/jiii.1.2.91-96

Also, different look-up tables are required for different

modules. Further, in most of these studies the

performance of the emulator is demonstrated, mostly with

linear resistive load.

The paper presents a current-controlled buck converter

as a PV emulator, which can exhibit the characteristics of

the PV panels. The functioning of the emulator relies on

the PV model[9] and so can emulate different modules

easily with minor modifications. The performance of the

emulator is compared with that of the actual

characteristics and the simulation results are also

presented.

II. PV MODEL

Figure 1. Equivalent Circuit representing a one-diode model of a PV Cell [9]

Fig. 1 shows an equivalent circuit to model a PV cell.

A single diode model is considered [9] along with the

series resistance Rs to take into account the internal

electrical losses. Shunt resistance Rp is generally very

large and hence, ignored. The equation expressing the

relation between output current (Ipv) and its terminal

voltage (Vpv) under given solar radiation (G ) and

temperature (T) is

– ( ( )

) (1)

where,

Io is the diode saturation current [A];

n is the diode quality factor;

q is the electronic charge(1.6 ×10-19

C);

k is the Boltzmann constant (1.6×10-23

J/K);

T is cell temperature [°C];

IL is the photo current generated by PV cell;

The non-linear transcendental equation (1) is difficult

to solve and hence, some numerical technique need to be

used to solve it. The approach suggested by Walker et.al,

[9] is used for solving (1). The characteristic for a

particular module can be obtained using (1)-(8). However,

some data for that module are required, which can be

obtained from the datasheet of the PV module or a priori

from experiments done on the module. Hence, to solve (1)

the previously known values of open circuit voltage (Voc)

and short circuit current (Isc) at two different temperatures

T1 and T2 are used. The subscript ‘1’ and ‘nom’ refers to

the standard conditions (Gnom = 1000W/m2, T1 = 25°C).

( ) ( ⁄ )

((

) (

–

))

(2)

( ) ( )

( ( )

– )

⁄ (3)

where, Vg is the energy gap of the material of the cell.

( ) ( ( ) ) (4)

( ) ( ) ( )⁄ (5)

( ( ) ( ) )

( )⁄ (6)

where Ko is the temperature coefficient of Isc (A/K). The

series resistance is computed using following equations

⁄ (7)

( ) ⁄

( )

(8)

III. SYSTEM CONFIGURATION

Fig. 2 shows the system configuration for a PV

emulator which comprises a buck type (step-down) dc-dc

converter, sensors, conditioning circuits, controller a gate

drive circuit. Vin represents a DC source of 100V.

Hysteresis (or bang-bang) control is used to provide

controlled output current. The reference current for the

hysteresis controller is derived using the PV model. The

value of inductor L and filter capacitor C are 25mH and

2000µF, respectively.

Figure 2. System configuration of a PV emulator

As shown in Fig. 2, the output voltage Vo and the

output current Io of the converter are sensed, filtered and

fed to the controller, which controls the converter to

behave like a PV module i.e. to act as a PV emulator. It is

evident that a PV module (or an array) operates at

different values of Vpv and Ipv depending on values of G,

T and the load connected across it (Rload). Hence, to

control the buck converter to operate at the voltage and

current in accordance to the values at which a PV

module operate under given conditions, the controller is

fed with G, T, Vo and Io. The controller employs the PV

model discussed in Section II. Using (1)-(8) and the

parameters G,T, Vo and Io, the controller computes the

value of Rpv and Rload. Rpv is defined as the ratio of Vpv and

Ipv. Depending on the difference in the value of Rload and

Rpv, the controller takes the corrective action to force the

operating point where, difference in Rload and Rpv is zero

or is negligible. The operating principle and the algorithm

for controlling the dc-dc converter as the PV emulator, is

discussed in the next section.

IV. ALGORITHM FOR THE CONTROL OF CONVERTER

G

T

Io

RSIpv

Vpv

IL

Voltage

conditioning

Circuit

Current Sensing

& Conditioning

Circuit

PV

MODEL

Vo / Io

TG

Bang – Bang

Control

Gate

Drive

L Io

Vo

Iref

Vin

Load

Io

Vo

Io

Rload

Vo

C

DSP Implementation

92


©2013 Engineering and Technology Publishing

Figure 3. Operating principle of PV emulator demonstrating the controlled shift of the operating point on the characteristics of a desired

PV module.

Fig. 3 depicts the operating principle of a system

shown in Fig. 2. It shows a V-I characteristic of a module

to be emulated and load line corresponding to a fixed

resistive load (Rload) on the same V-I plot. For the given

load, if the converter operates at the intersection point of

V-I characteristic of a module and the load line Rload, the

converter is able to behave as an emulator. Fig. 3 shows

that as the load line Rload intersects the V-I characteristic

of module at point c, the converter can act as an emulator

if its steady-state output voltage and current are Vpv3 and

Ipv3, respectively. At this point Vpv3/Ipv3=Rpv3=Rload.

Let ‘a’ be initial operating point. So the converter

outputs voltage V1 at current I1. The V-I characteristic of

the module shows that an actual PV module can generate

current Ipv1 when operating at the voltage Vpv1=V1. The-

corresponding load resistance is Rpv1 (=Vpv1/Ipv1), which is

less than Rload. Thus, to force the converter to act as an

emulator Rpv (ratio of Vpv and Ipv) should be increased till

it equals Rload. This can be achieved by increasing Vpv and

decreasing Ipv. Fig. 3 shows that operating point moves

from a to c (path a-c), as converter output voltage Vo is

increased from V1=Vpv1 to V3 =Vpv3 in steps. This results

into the increase in Rpv from Rpv1 to Rpv3 and the converter

outputs Vo and Io that the PV module would generate with

Rload.

Figure 4. Flowchart for the algorithm to control dc-dc converter as PV emulator.

Fig. 4 shows the flowchart for the algorithm which the

controller employs to control the converter as a PV

emulator. In respect to the PV module to be emulated,

the ‘initialization’ step includes passing of known values

(from the datasheet) of some parameters (T1, ISC(T1),

V0C(T1) , T2, ISC(T2), V0C(T2), G(nom), n, k, q, ISC(T1,nom)). The

next step is to pass on G and T for which the performance

of PV module is desired. Based on these parameters

photocurrent IL and diode saturation Io current are

obtained with (2) and (5). As the converter has to behave

similar to the PV module, the output voltage Vo of the

converter should be the same as the voltage Vpv that the

PV module would generate when operating under the

given conditions. Hence. Vo is fed to the controller and

along-with IL and Io computed in the earlier step, the PV

module’s output current is obtained using (1).

As (1) is a non-linear equation, Newton-Raphson

method is used to compute the current Ipv that a PV

module would generate under the given conditions and is

used as the reference current Iref for controlling the output

current of the converter. Rpv is then computed as the ratio

of Vpv and Ipv and compared with Rload which is obtained

from Vo and Io. If Rpv is less then Rload, (1) is then

computed with Vpv = Vpv+ΔVpv and then the new value of

Ipv is stored as Iref and used as reference current for the

bang-bang controller (hysteresis controller). Alternatively,

if Rload is less then Rpv, (1) is computed with Vpv = Vpv-

ΔVpv. If the difference in ΔR between Rpv and Rload is

within acceptable level the same value of Vpv is used for

computing Ipv. Values of ΔR and ΔVpv decide the

performance of the emulator and hence, should be

judiciously selected in context to the rating of the PV

emulator. Smaller the value of ΔR better is the accuracy

of the converter in emulating the characteristic of the PV

module (or an array). Larger the value of ΔVpv lesser is

the time to reach to the final steady-state operating point.

V. SIMULATION RESULTS

The section discusses the performance of PV emulator

(Fig. 2) with different types of loads: linear load and non-

linear loads. The control algorithm shown in Fig. 3 is

adopted with ΔR = 0.2Ω and ΔVpv=0.01V. A variable

resistor is considered for a linear load, while dc-dc

converter feeding a resistive load is considered as a non-

linear load. The simulation results presented in this

section are carried out in MATLAB/Simulink.

TABLE I. PV MODULE SPECIFICATION

Open Circuit Voltage Voc 21 V

Short Circuit Current Isc 3.74 A

Voltage at MPP Vm 17.1 V

Current at MPP Im 3.5 A

Maximum Power Pm 59.9 W

For the simulation study, the Solarex MSX60 60W PV

module is considered. The specifications of the PV

module corresponding to 25°C temperature and

1000W/m2 solar irradiation level are shown in the Table I.

Fig. 5. shows that the characteristic obtained for the

module with the mathematical model discussed in

section-I. It matches with that obtained with real PV

V-I Characteristics of

a PV module

a

bLoad Line

R Load

= VPV1 =

V1

c

VPV2

V2

= PV3

V3

V

PV1I

PV2I

PV3I

PV1R PV2

R

PV3R = RLoad

Voltage (V)

Cu

rrent

(A)

1I

2I

3= I

START

ReadG, T, VO, IO, VPV = VO

Calculate IPV Using

Equation no. ( 1 )

IPV = Iref

RPV = VPV / IPV

VPV = VPV + VPV

VPV = VPV - VPV

Initialization

I ref = 0.001 A

If IPV <=0.001

If

¦RPV- Rload¦< ? R

If RPV < Rload

No

No

Yes

Yes

Yes

No

93



model. It is observed that the characteristic is non-linear

and is difficult to obtain it by just a CS or a VS.

Figure 5. V-I characteristics of PV Module

The operating point on the characteristic; may it be on

CS region, non-linear region, or VS region; is dependent

on the load. If Rload =R =10Ω, the operation will be at a

point where the PV module’s output voltage and current

are such that their ratio is equal to 10Ω. This is achieved

at point C, where the output voltage and current are

19.69V and 1.969A, respectively (19.69/1.969=10Ω).

Thus, depending on the load matching, there exists a

unique point on the characteristic where the operation is

possible. Hence, as the Rload changes, the output voltage

and current of the PV array change, unlike an ideal CS or

a VS where only one of these either output voltage or

output current changes. Some reference points A

(R=2.63Ω), B (R=5Ω), C(R=10Ω) and D(R=50Ω) are

marked, which shows that the Rpv decreases as the

operation moves from Point A (in the VS region),

towards the point D (in the CS region).

Figure 6. PV emulator response when feeding a resistive load: (a) Output voltage (b) Output current and (c) output voltage versus output

current

Fig. 6 shows performance of the PV emulator (Fig. 2),

when the resistance is varied (corresponding to points A`

to D`). Rload at time t=0s is 2.63Ω . The step change in the

resistances are applied at t=0.05s (2.63Ω to 5Ω), t=0.1s

(5Ω to 10Ω), and t=0.15s (10Ω to 50Ω). Fig. 6(a) and (b)

show the variation in the dc-dc converter’s output voltage

and current respectively, corresponding to these

resistances. The steady state operating points (A`B`C`D`)

obtained with the PV emulator are also highlighted in Fig.

6(c). It can be observed from Figs. 5 and Figs. 6(c) that

the steady state operating points for both the cases; (i)

resistive load directly fed from PV module (Fig. 5) and (ii)

resistive load fed from the converter (Fig. 6(c)); have a

close match. Thus, the dc-dc buck converter is controlled

as a PV emulator to behave similar to that of the PV

module. The response time of the emulator when a step

change in resistance is applied is also very small, less

than 0.01s. Unlike the results shown in Fig. 5, ripple in Io

(about 0.1A) and Vo (about 0.0007V) is observed in Fig.

6.However, the ripple is quite small and has not much

significance when the PV emulator is used in place of PV

module for testing the PV systems. In fact, the ripple in Io

and Vo can be minimized, if desired, by increasing the

buck inductance and filter capacitance.

Figure 7. PV emulator response when feeding a non-linear load

employing a buck type load converter: (a) output voltage (b) output current and (c) output voltage versus output current

Fig. 7 depicts the performance of the emulator when

buck emulator is feeding a non-linear load. A dc-dc step

down converter with a fixed resistance of 5Ω at its output

port is considered as a non-linear load. Thus, the entire

set-up consists of two step-down (buck) dc-dc converters.

First acts as a PV emulator and second acts as a load

converter. The load converter is operated at a constant

duty cycle D = 0.7 and 6.6kHz switching frequency. In

the steady state, as effective resistance at the input port of

the step down converter is governed by the following

expression, the effective load resistance at the input port

of the converter i.e. at the output port of PV emulator is

about 10.204Ω.

⁄ (9)

Here, Rin and Rout are the resistances at input and

output ports of the dc-dc converter, respectively.

Therefore, the output voltage and the current of the PV

emulator settle at the values corresponding to resistance

of 10.204Ω (near to point C Fig. 5). The steady state

output current and voltage of the emulator are 1.934A

Cu

rre

nt (

A )

0 5 10 150

1

2

3

4

20

A B

C

D

R = 2

.63Ω

R = 50 Ω

R = 5

Ω

R = 10 Ω

Voltage ( V )

Cu

rre

nt

( A

) R=2.63 Ω

0 0.04 0.08 0.12 0.16 0.20

10

20

Time ( S )

Volt

age (

V )

0 0.04 0.08 0.12 0.16 0.20

1

2

3

4

Time ( S )

0 4 8 12 16 200

1

2

3

4

Voltage ( V )

Cu

rrent

( A

)

R=2.63 Ω

R=5 ΩR=10 Ω R=50 Ω

R=5 ΩR=10 Ω

R=50 Ω

R=2.63 Ω R=5 Ω

R=10 Ω

R=50 Ω

(a)

(b)

(c)A’ B’

C’

D’

0 0.05 0.1 0.15 0.2 0.25 0.30

5

10

15

20

Time ( s )

Vol

tage

( V

)

0 0.05 0.1 0.15 0.2 0.25 0.30

1

2

3

4

Time ( s )

Cur

rent

( A

)

0 2 4 6 8 10 12 14 16 18 200

1

2

3

4

Voltage ( V )

Cur

rent

( A

)

19.72 V

1.934 A

R = 10.204Ω

(a)

(b)

(c)

94



19.72V, respectively, which is in accordance to the output

of the PV module as observed from Fig. 5. Ripple in the

output current is about 0.1A and that in output voltage is

0.004V. The time to reach the steady state is 0.07s. As

the duty cycle of this converter varies, the effective

resistance at the input port of the converter varies

yielding different operating point on the V-I

characteristics of Fig. 5. Though the performance is not

shown for other values of duty cycles, the performance of

PV emulator for different duty cycles is in agreement

with that obtained when the non-linear load is directly

connected to the PV module.

Figure 8. PV emulator response when feeding a non-linear load employing a boost type load converter: (a) output voltage (b) output

current and (c) output voltage versus output current

The performance of the PV emulator is also tested with

another non-linear load that consists of a boost converter

feeding a resistive load. Fig. 8 depicts the performance of

the emulator (shown in Fig. 2). A fixed resistance of

120Ω is considered at the output port of boost converter

(load converter). Thus, the entire set-up, just like the

previous case, consists of two dc-dc converters. But

unlike the previous case, first converter that acts as a PV

emulator is a buck converter while the second one that

acts as a load converter is a boost converter. The load

converter is operated at a constant duty cycle D = 0.5 and

6.6 kHz switching frequency. For the boost converter,

effective resistance at the input port of the step down

converter is governed by the following expression

( ) (10)

Hence, the effective load resistance at the input port of

the converter i.e. at the output port of PV emulator is

about 30Ω. Therefore, the output voltage and the current

of the PV emulator settle at the values corresponding to

resistance of 30Ω, which lies between point C and point

D. The steady state output current and voltage of the

emulator are 0.6881A and 20.64V, respectively, which is

in accordance to the output of the PV module as observed

from Fig. 5. Ripple in the output current and in output

voltage are of the similar magnitude as that in the

previous case. The time to reach the steady state is also

0.15s. The performance of PV emulator for different duty

cycles is in agreement with that obtained when the non-

linear load is directly connected to the PV module.

VI. CONCLUSIONS

PV array emulator based on buck converter topology is

presented. Simulation results showed that PV emulator

has good steady state response. The settling time is also

quite less. The response is compared with that obtained

with a real PV source. When tested with non-linear loads

comprising buck or boost converter, performance of the

converter as an emulator matches that of real PV source.

Even with non-linear buck type load, which consist of a

switch in series at the output terminal of emulator, the

converter is able to reproduce the characteristic of a PV

module. However, unlike the PV source, the output

voltage and current of PV emulator show some ripples

whose magnitude is dependent on the size of filter

capacitor and inductor. Future work will focus on the

implementation of PV emulator.

REFERENCES

[1] S. Ahmed, A. Jaber, and R. Dixon, “Renewable 2010 global status report,” Renewable Energy Policy Network for the 21st Century

(REN21), pp. 15, 2010. [2] N. Viet and A. Yokoyama, “Impact of fault ride-through

characteristics of high-penetration photovoltaic generation on

transient stability,” Power System Technology, pp. 1-7, 2010. [3] H. Patel and V. Agarwal, “MATLAB-based modeling to study the

effects of partial shading on PV array characteristics,” IEEE Trans.

on Energy Conversion, vol. 23, pp. 302-310, 2008. [4] Z. Ziming, Z. jianwen, S. Haimeng, W. Gang, H. Xiwen, and Z.

Shi “Research on photovoltaic array emulator system based on a novel zero-voltage zero-current switching converter,” Power and

Energy Engineering Conference, 2010, pp. 1-4.

[5] D. Dolan, J. Durago, J. Crowfoot, and Taufik, “Simulation of a photovoltaic emulator,” North American Power Symposium, pp. 1-

6, 2010. [6] J. Ollila, “A medium PV-powered simulator with a robust control

strategy,” in Proc. IEEE Conference on Control Applications, 1995, pp. 40.

[7] S. Armstrong, C. Lee, and W. Hurley, “Investigation of the

harmonic response of a photovoltaic system with a solar emulator,”

European Conference on Power Electronics and Applications, vol.

9, 2005, pp. 8-10.

[8] J. Lee, B. Min, T. Kim, et.al, “Development of a photovoltaic simulator with novel simulation method of photovoltaic

characteristics,” in Proc. 31st International Conference on Telecommunication Energy, 2009, pp. 1-5.

[9] G. Walker, “Evaluating MPPT converter topologies using a MATLAB PV model,” Journal of Electrical & Electronics

Engineering, vol. 21, no. 1, pp. 49-56, 2001.

Ankur V. Rana received the degree of B.E in

electrical engineering from Govt. Engg.

College of Bharuch, South Gujarat University,

Bharuch, India in 2009 and M.Tech from

Sarvajanik College of Engineering and Technology, Surat, in 2012. His research

interests include Photovoltaic Energy

Technology. He is Asst. Professor at Chhotubhai Gopalbhai Patel Institute of

Technology, Bardoli, India.

0 2 4 6 8 10 12 14 16 18 200

1

2

3

4

Voltage( V )

Cu

rren

t(

A)

20.64 V

0.6881 A

R = 30

(a)

(b)

(c)

Time ( S )0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0

1

2

3

4

Cu

rren

t(

A)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.350

5

10

15

20

Time ( S )

Vo

lta

ge

(V

)

95



Hiren H. Patel received the degree of B.E in electrical engineering from S.V. National

Institute of Technology, South Gujarat

University, Surat, India in 1996 and M.Tech and Ph.D degrees from Indian Institute of

Technology, Bombay, India, in 2003 and

2009, respectively. His research interests include computer-aided simulation

techniques, distributed generation, and

renewable energy, especially energy extraction from photovoltaic arrays. He is Professor at Sarvajanik

College of Engineering and Technology, Surat, India and is a certified

energy manager. He has authored several international and national research papers and is a life member of Indian Society for Technical

Education.

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