3. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)
A THREE-PHASE GRID-CONNECTED PHOTOVOLTAIC SYSTEM
WITH REACTIVE POWER CONTROL
Le Minh Phuong, Phan Quoc Dzung, Nguyen Minh Huy
Faculty of Electrical & Electronic Engineering, HCMC University of Technology.
ABSTRACT
This paper presents the control model of three phase grid connected photovoltaic generation
system with a new approach for reactive power regulation. The model contains a detailed
representation of the main components of the system that are the solar panels, DC/DC
converter, DC-link, a grid side three phase voltage source inverter (VSI) and output filters to
reduce harmonic distortion of line current. In this paper, a complex control scheme including
two PI controllers and cooperated with MPPT is proposed to stabilize DC voltage. A three
phase grid connected voltage source inverter synchronizes to the grid by a robust phase-
locked loop (PLL). The proposed model is simulated in Matlab/Simulink Toolbox and
implemented using DSP TMS320F2812. Simulation and experimental results show the high
stability and high efficiency of this three-phase grid-connected PV system. It also proves the
excellent performance control units as improving a flexible regulation of power factor for the
interval (0.5-1).
KEYWORDS: three phase Grid-connected inverter; maximum power point tracking
(MPPT); photovoltaic (PV); solar energy.
1. INTRODUCTION
Nowadays fossil fuel is the main energy
supplier of the worldwide economy, but the
recognition of it as being a major cause of
environmental problems makes the
mankind to look for alternative resources in
power generation [1].
The control strategies applied to distributed
systems become of high interest. And how
to improve the performance of grid inverter,
increase the switching frequency and power
density to meet power quality requirements
has become a research hotspot in recent
years [2].
In this paper, authors focus on two main
problems, as a control of dc-link voltage
and a reactive power control. This paper
also proposed an improved MPPT control
algorithm for photovoltaic under rapidly
changing solar radiation. The simulation
results on Matlab/Simulink show that
detection MPPT quickly in conditions of
changes in radiation and temperature.
Experiments are programmed on DSP
TMS320F2812 and applied solar system
consists of 5 PV panels connected in series,
the rate parameters of each panel is 50Wp,
22V open circuit voltage. The simulation
and experiment results show good response,
high stability and high efficiency of this
three-phase grid-connected PV system
when the grid voltage and load changes.
The main circuit block diagram of the PV
grid connected system is shown in Figure 1,
including:
DC-DC unit: DC boost converter is
used to boost the PV array voltage and
track the maximum solar power.
DC-AC unit: The three phase inverter
with bridge topology converts DC to
sinusoidal AC and provides to load.
Filter circuit: AC-side filter is
composed of R-L-C to ensure the
quality of grid current.
4. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)
PV
DC/DC DC/AC Filter
AC AC AC
MPPT
DSP
+
-
Figure 1. General diagram of grid connected
photovoltaic system
2. PROPOSED CONTROL
STRUCTURE FOR GRID
CONVERTER
The proposed control structure of three-
phase grid-connected PV system consists of
PV panels is presented in Figure 2. In this
structure, the DC_Link voltage is controlled
accordingly to the necessary output power.
Its output is the reference for the active
current controller and the reactive current
must be imposed to the system to control
the power factor. The dq control structure is
normally associated with PI controllers
since they have a satisfactory behavior
when regulating dc variables. Since the
controlled current has to be in phase with
the grid voltage, the phase angle used by
the abc to dq transformation module has to
be extracted from the grid voltages. As a
solution, filtering of the grid voltages and
using arc-tangent function to extract the
phase angle can be a possibility [3],[4]. In
addition, the Phase-Looked Loop (PLL)
technique [5] became a state of the art in
extracting the phase angle of the grid
voltages. Moreover, to control power factor
the reactive power control loop is presented
in the structure, where the actual power
factor (PF) is calculated using the
conventional instantaneous power
definition in ‘abc’ systems.
DC
DC
SOLAR
PV
MODULES
PWM
MPPT
(Incremental
Conductance)
Vpv Ipv
Three phase
Grid
abc
dq
abc
dq
PLL
DC
AC
SVPWM
PI
PI
-wL
wL
ab
dq
Buck/Boost
converter
PI
Power
Factor
Control
Cosf Ref
Vdc Ref
Vq
Vd
+
+
+
+
+
+
+
-
+
-
+
- Vd*
Vq*
DC_Link
Id Iq
Vd Vq
Vdc
Va Vb Vc
Ia Ib Ic
Figure 2. Proposed control structure of three phase grid-connected PV system.
2.1 The MPPT Method
The Incremental Conductance MPPT is
implemented in this control structure.
Figure 3 shows a P-V characteristic of PV
panel and principle of Incremental
conductance algorithm.
Figure 3. Principle of Incremental
conductance algorithm.
5. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)
The differential of power (dP) is expressed
following:
0
0
0
dP
At MPPT
dV
dP
Left of MPPT
dV
dP
Right of MPPT
dV
Where
( )dP d IV dI I
I V I V
dV dV dV V
Replace (2) to (1), we can get:
(3.1)
(3.2)
(3.3)
dI I
At MPPT
dV V
dI I
Left of MPPT
dV V
dI I
Right of MPPT
dV V
Figure 4. Flowchart of Incremental
Conductance algorithm.
The Incremental conductance algorithm for
MMPT is simulated on Matlab-Simulink.
This PV system includes 20 solar panels get
into two parallel rows; each row consists of
10 panels. The rated parameter of each
panel is Pn = 50Wp, an open-circuit voltage
Voc = 21.42 V, and short-circuit current Isc
= 3.11A. Figure 5 shows the characteristics
P –V of the system and it’s MPP
corresponding to the different solar
radiation (from 0.2kW/m2 to 1kW/m2),
which are found by proposed Incremental
conductance.
Power(W)
Voltage (V)
P-V Characteristic
Figure 5. P-V characteristic of PV system.
Figure 6 shows the output power of system
when solar insolasion is 1kW/m2
then it
reduces till 0.2kW/m2 and output DC
voltage of the solar system.
Power(W)-Voltage(V)
Figure 6: Simulation of tracking process (Step
reduce of insolation )
Simulation results show that the system can
remain stable in case of a 40% step change
of isolation. The high stability of the MPPT
method will also ensure the high efficiency
of the system by drawing the maximum
power from the PV panels under different
insolation conditions.
2.2 Dc-Link Control
The DC voltage controller is used to
produce the reference current value for the
current controller..
A PI controller is used for the DC voltage
and its output is feed-forwarded to the
output of PF controller to obtain the
(1)
(2)
(3)
6. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)
reference for the active current id and the
reactive current iq
2.3 Reactive Power Control
For three-phase power systems with
sinusoidal voltages and sinusoidal currents,
quantities such as active power, reactive
power, active current, and reactive current
are conventionally defined on the average
concept. But for systems with unbalanced
and distorted currents, average concept is
not suitable [7].
In this grid-connected PV system, the
effective method is provided to calculate
and compensate the reactive power for
three-phase systems. The actual powers P
and Q are calculated using the conventional
instantaneous power definition in ‘abc’
systems by (4) and (5).
a a b b c cP v i v i v i (4)
1
( )
3
bc a ca b ab cQ v i v i v i (5)
2.4 Three Phase Pll Structure
The PLL is used in order to determine the
phase angle θ and the frequency of the grid
and in this paper, the conventional
synchronous reference frame PLL. The
block diagram of the used PLL is further
shown in Figure.7. A regulator PI is used to
control this variable and the output of this
regulator is the grid frequency. In this way,
the phase angle of grid voltage is detected
and this is the output of the algorithm.
PI
1/S
αβ
abc
Va
Vb
Vc
Loop Filter
Ud*
VOC
w
wff
Ud
Uq
-
+ + +
ab
dq
Figure 7. Control structure of PLL
3. SIMULATION OF THE
PROPOSED CONTROL SCHEME
The proposed control scheme is
implemented in Matlab/Simulink Toolbox
and shown in Figure.8. The simulation
results demonstrate the excellent
performance of the proposed control
scheme. The DC/DC converter combines
MPPT algorithm can boost voltage up to
600V and three phase grid connected
DC/AC inverter, a rated grid parameters are
380V, 50Hz. This PV system includes 20
solar panels get into two parallel rows; each
row consists of 10 panels. The rated
parameter of each panel is Pn = 50Wp, an
open-circuit voltage Voc = 21.42 V, and
short-circuit current Isc = 3.11A.
Figure 8. Simulation waveforms of the
proposed three-phase grid-connected PV
system.
Simulations are started with the conditions:
an ínsolasion is 1kW/m2
and a temperature
is 25o
C, at t = 0.27s the output voltage of
the boost DC/DC reachs to 580V this three
phase inverter is synchronous with the grid;
at time t = 0.4s the insolasion is reduced till
0.6 kW/m2. The power factor is controlled
and is given 1 during the simulation.
Simulation results are presented in Figure 8,
shows the system is connected to the grid at
time t = 0.27s and are always kept stable
even when radiation decreased 40%.
4. HARDWARE IMPLEMENTATION
USING DSP 2812
Based on the earlier theoretical analysis, the
experimental system was designed and
7. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)
implemented on DSP TMS320LF2812. It’s
installed at Power Electronics Research
Lab, Ho Chi Minh city University of
Technology. Experimental results are
measured by using Tektronix TDS2024B
oscilloscope and Fluke 345 Power Quality
Clamp Meter. The PV system includes 5
PV panel 50 Wp in series and connects to
50 - 120V three phase grid via the three
phase Staco Variable Transformers.
Figure 9: Experimental models
Figure 9 presents photographs of the
inverter and PV panels in the proposed PV
system that was installed at Electronics
Research Lab, Ho Chi Minh city
University, Viet Nam. The experiment is
implemented in the actual conditions of
Vietnamese climate with the temperature
30°C for the three following cases study:
4.1 Case study 1:
The Power factor (PF) is controlled and
given 1. The phase voltage waveform and
three phase currents are shown in Figure
10.
Figure 10: Experimental waveforms of voltages
and currents (PF=1)
4.2 Case study 2:
The Power factor (PF) is controlled and
given 0.9. The phase voltage waveform and
three phase currents are shown in Figure
11.
Figure 11: Experimental waveforms of
voltages and currents (PF=0.9)
8. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012)
4.3 Case study 3:
The Power factor (PF) is controlled and
given 0.9. The phase voltage waveform and
three phase currents are shown in Figure 12
Figure 12: Experimental waveforms of
voltages and currents (PF=0.5)
5. CONCLUSION
The implementation of a three-phase grid-
connected PV system is presented in this
paper. The new control approaches are
applied in the system can remarkably
improve system stability during rapidly
changing process of insolation. Due to its
improvement on the dynamic response, the
DC link voltage is kept almost constant, it
allow the inverter to synchronize to the grid
and stabilize the system even when an
insolation is reduced till 40%. After a step
change of insolation, the controller can
maintain the dc-link voltage and keep it
close to the MPP. Output reactive power of
the system can be controlled. The actual
values of PF are very close to the reference.
It indicates the efficiency of this proposed
control system.
6. REFERENCES
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power plant performance and
economics,” IEEE Trans. Energy
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Jun. 1990.
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Dougal, “Power controller design for
maximum power tracking in solar
installations,” IEEE Trans. Power
Electron., vol. 19, no. 5, pp. 1295–
1304, Sep. 2004.
[3] F. Blaabjerg, R. Teodorescu, M. Liserre
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Proceedings of IAS'06, pages 1015-
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Proceedings of PESC'06, pages 1-7,
2006
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[7] H. Akagi, Y. Kanazawa, and A. Nabae,
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