Journal of Industrial and Intelligent Information Vol. 1, No. 2, June 2013
Current Controlled Buck Converter based
Photovoltaic Emulator
Ankur V. Rana
C. G. Patel Institute of Technology, Bardoli, Surat, India
Email: ankur.rana@utu.ac.in
Hiren H. Patel
Sarvajanik College of Engineering & Technology, Surat, India
Email: hiren.patel@scet.ac.in
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. 
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.
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.
©2013 Engineering and Technology Publishing
doi: 10.12720/jiii.1.2.91-96
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Journal of Industrial and Intelligent Information Vol. 1, No. 2, June 2013
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.
(
(
)
(
(
)
⁄
)
(
)
)
⁄
(
(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
II.
PV MODEL
IL
G
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.
Ipv
RS
Io
Vpv
T
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
–
(
(
)
)
L
Load
Vo
Gate
Drive
(1)
(
( ⁄
(
)
((
)
) ( –
(
(
)
–
Io
Io
Iref
PV
MODEL
Voltage
conditioning
Circuit
Vo
Vo / I o
Rload
Vo
T
G
DSP Implementation
Figure 2. System configuration of a PV emulator
))
)
⁄
Current Sensing
& Conditioning
Circuit
Bang – Bang
Control
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.
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).
)
C
Vin
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;
(
Io
(2)
(3)
)
where, Vg is the energy gap of the material of the cell.
(
)
(
(
©2013 Engineering and Technology Publishing
))
IV. ALGORITHM FOR THE CONTROL OF CONVERTER
(4)
92
Journal of Industrial and Intelligent Information Vol. 1, No. 2, June 2013
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 = VpvΔVpv. 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-I Characteristics of
a PV module
RPV2
Current (A)
RPV1
IPV1
IPV2
IPV3
I2
RPV3 = RLoad
c
= I3
Load Line
b
I1
a
R Load
V1
V2
V3
= VPV1
=VPV2
= VPV3
Voltage (V)
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. Thecorresponding 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.
V. SIMULATION RESULTS
The section discusses the performance of PV emulator
(Fig. 2) with different types of loads: linear load and nonlinear 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 nonlinear load. The simulation results presented in this
section are carried out in MATLAB/Simulink.
START
Initialization
I ref = 0.001 A
Read
G, T, VO , IO , VPV = VO
Calculate I PV Using
Equation no. ( 1 )
IPV = Iref
If
IPV <=0.001
Yes
No
TABLE I. PV MODULE SPECIFICATION
Open Circuit Voltage
Voc
21
Short Circuit Current
Isc
3.74
Voltage at MPP
Vm
17.1
Current at MPP
Im
3.5
Maximum Power
Pm
59.9
RPV = V PV / I PV
If
Yes
¦R P V- Rl oa d¦< ? R
No
If RPV < Rload
No
Yes
VPV = VPV +
VPV
VPV = VPV -
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
VPV
Figure 4. Flowchart for the algorithm to control dc-dc converter as PV
emulator.
©2013 Engineering and Technology Publishing
V
A
V
A
W
93
Journal of Industrial and Intelligent Information Vol. 1, No. 2, June 2013
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.
model. It is observed that the characteristic is non-linear
and is difficult to obtain it by just a CS or a VS.
4
2.
63
Ω
B
5Ω
=
R
=
2
R
Current ( A )
A
3
R=
10 Ω
1
C
R = 50 Ω
D
0
0
5
10
20
15
Voltage ( V )
Figure 5. V-I characteristics of PV Module
20
R=5 Ω
10
0
0.04
0.12
0.16
Voltage ( V )
Current ( A )
0.2
Time ( S )
Current ( A )
R=2.63 Ω
0.04
4
R=2.63 Ω
3
0.08
A’
0.12
Time ( S )
R=5 Ω
0.16
B’
(c)
C’
R=50 Ω
D’
1
0
0.2
R=10 Ω
2
0
4
8
12
16
20
Voltage ( V )
Figure 6. PV emulator response when feeding a resistive load: (a)
Output voltage (b) Output current and (c) output voltage versus output
current
0.2
0.25
0.3
(b)
1.934 A
2
1
0.05
0.1
0.15
0.2
Time ( s )
0.25
0.3
(c)
3
R= 10.204Ω
2
1
0
2
4
6
8
10
12
Voltage ( V)
⁄
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`)
©2013 Engineering and Technology Publishing
0.15
Time ( s )
14
16
18
20
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.
R=50 Ω
1
0 0
Current ( A )
R=10 Ω
2
0.1
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
(b)
R=5 Ω
0.05
4
4
3
0
3
0
0.08
(a)
5
00
(a)
19.72 V
10
4
R=2.63 Ω
0
15
0
R=50 Ω
R=10 Ω
20
Current ( A )
Voltage ( V )
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).
(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
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Journal of Industrial and Intelligent Information Vol. 1, No. 2, June 2013
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 nonlinear load is directly connected to the PV module.
Voltage ( V )
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.
20
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.
20.64 V
15
(a)
10
5
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Time ( S )
4
Current ( A )
VI. CONCLUSIONS
3
(b)
2
0.6881 A
1
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
REFERENCES
Current ( A )
Time ( S )
4
2
R = 30 
1
0
[1] S. Ahmed, A. Jaber, and R. Dixon, “Renewable 2010 global status
(c)
3
0
2
4
6
8
10
12
14
16
18
[2]
20
[3]
Voltage( V )
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
[4]
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
(
)
[5]
[6]
[7]
[8]
[9]
(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
©2013 Engineering and Technology Publishing
report,” Renewable Energy Policy Network for the 21st Century
(REN21), pp. 15, 2010.
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.
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.
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.
D. Dolan, J. Durago, J. Crowfoot, and Taufik, “Simulation of a
photovoltaic emulator,” North American Power Symposium, pp. 16, 2010.
J. Ollila, “A medium PV-powered simulator with a robust control
strategy,” in Proc. IEEE Conference on Control Applications,
1995, pp. 40.
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.
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.
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.
95
Journal of Industrial and Intelligent Information Vol. 1, No. 2, June 2013
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.
©2013 Engineering and Technology Publishing
96