Energy and Power Engineering, 2013, 5, 205-208
doi:10.4236/epe.2013.54B040 Published Online July 2013 (http://www.scirp.org/journal/epe)
Study of Control Strategy of the Converter in Solar
Photovoltaic Power System
Jin Liu, Yigong Zhang
School of Electric and Electronics Engineering, North China Electric Power University, China, Beijing
Email: liujin@ncepu.edu.cn, zhangyigong@ncepu.edu.cn
Received February, 2013
ABSTRACT
Solar photovoltaic (PV) generation is one of the most important renewable energy resources, which is a green energy
too. The operation and control of the solar PV generation system are important for its application. This paper studies the
control strategy of the converter used in solar energy photovoltaic (PV) power generation, which can connect with the
power grid. The simulation model of the PV generation syste m is set up to verify the feasibility of the con trol strategy.
And some experiments have been done in the renewable experiment kit of TI Company.
Keywords: Converter; DSP; MATLAB; Control; Power
1. Introduction
With the demand of the energy increases, many renew-
able energy sources such as wind and solar energy are
studied to generate electric power. And solar energy
photovoltaic power generation will be widely used, be-
cause of the green electric power can be supplied by it.
The control algorithm of the converter is one of the
most important points for the use of photovoltaic power
generation. And the solar PV generation system can sup-
ply power to the custom in a distributed generation (DG)
system or to the isolated custom’s load, such as street
lamp. The control algorithm is different for the isolated
system and the system connected with the power grid [1].
This paper studies the control strategy of the converter
used in solar energy photovoltaic (PV) power generation,
which can connect with grid. The simulation model of
the PV generation system, which is built with MAT-
LAB/Simulink, is set up to verify the feasibility of the
control strategy [2].
Solar energy PV power generation system develop-
ment kit of TI Company is used to do the control ex-
periments, and the grid connection control strategy is
realized in the experiment plat [3].
2. Structure of the System
The solar photovoltaic power generation system is con-
sisted of photovoltaic array, DC/DC boost circuit, power
inverter, filter, PLL circuit and other parts [4]. The
structure of solar photovoltaic power generation system
is shown in Figure 1.
The solar photovoltaic array produces direct current
(DC) power. An d the PV g e neratio n system conv er ter th e
DC power into single phase or three phase alternating
current (AC) power (220/380 V, 50 Hz), and to supply
for the AC load or connect with the power grid.
The DC-DC boost converter stage could step up the
input DC voltage to a required voltage value, which is
the next input of the inverter. This stage also provides
voltage regulation control on the intermediate DC bus
that is not available at the source terminals, which can
regulate the DC voltage into the required value [5].
The function of PLL (Phase Locking Loop) circuit is
to trace the AC line voltage of the power grid and give
the reference voltage for the inverter output voltage
which has to be synchronized with the line voltage, if the
converter is connected with the power grid.
The inverter converts the DC voltage into single ph ase
or 3-phase AC voltage, which is based on the inverter
structure and the control algorithm. SPWM control algo-
rithm (Sine Pulse Width Modulation) is used in the in-
verter control and the control signals are calculated by
Figure 1. Block diagram of PV power system.
Copyright © 2013 SciRes. EPE
J. LIU, Y. G. ZHANG
206
the Micro-controller with some feedback signals from the
circuit, such the DC Bus voltage, the inverter output
voltage and the PLL signal. And the whole control sys-
tem is a close loop with voltage regulation [6].
In order to get high quality output AC voltage, there is
a filter circuit at the terminal of the output of the inverter,
which can filter the high numbers harmonics produced
by the inverter, even though the voltage THD (Total
Harmonic Distortion) is very small with SPWM control
algorithm.
There is a human-machine Interface in PC, which is
communicated with the controller card through SCI (Se-
rial Communication Interface) and programmed with C#
language. The functions of the interface software are to
initialize and set the control parameters of the control
system; setup the control models; monitor the p arameters
get from controller card and give out the sine waveform
of the power grid voltage, voltage waveform of the out-
put of the inverter[7].
With the interface, it is convenience to change the pa-
rameters of control and observe the different results
caused by different parameters. And using the interface
software can give quickly starting to study the control
algorithm and understand the operation principle of PV
power generation system more easily. It is a good assis-
tant to control and monitor the operation of the DC-DC
Boost circuit, PLL circuit and the inverter circuit, and
can store the data of operation in the PC [8].
3. Simulation Model
The solar photovoltaic power generation system simula-
tion model is established with the MATLAB/Simulink.
The simulation model structure is shown in Figur e 2.
The PV array produces variable DC voltage according
to the sun light and temperature.
The DC/DC Boost block step up the DC voltage into a
required DC Bus voltage, which is the voltage of the ca-
pacitor in Figure 2. And the DC Bus voltage should
great than the peak value of the AC side voltage if the
active power wants to be transmitted from the DC side to
the AC side [9].
The DC Bus voltage regulation control subsystem
block is showed in Figure 3. It is consisted of PLL block,
abc to dq0 converter block, PI controller block, dq0 to
abc inverter block and the PWM pulses generator block.
The inverter circuit is a three phase full bridge voltage
source converter, which uses the IGBT as switches, and
there are Diodes reverse parallel connected with the
IGBTs. The function of Diodes is to provide the flow
path for the reactive power flowing from the AC side to
the DC side. SPWM control model is used to generate
the PWM pulses to drive the switches of the inverter and
converter the DC voltage into AC voltage. The SPWM
control pulses for IGBT1 and IGBT2 are showed in Fig-
ure 4. The width of pulse is changed as a sinusoidal
waveform. And the pulses of IGBT1 and IGBT2 are op-
posite because they are connected in one bridge arm. The
dead time of the IGBTs is not considered in the simula-
tion model, which is considered in the reality control
system.
Because the inverter is non-linear load and it can pro-
duce high number harmonics, which frequency are inte-
ger times of the switches’ frequency. So, there is a filter
circuit at the output of the inverter, which function is to
filter the harmonics. And the filter is the LCL filter,
which has better character than LC filter.
The simulation output waveform is showed in Figure
5. And the above line is the output voltage waveform of
inverter with SPWM control, and is called as SPWM
waveform, which is consist of the line frequency
Figure 2. Simulation model structure.
Figure 3. Simulation control subsyste m bloc k.
0
0.2
0.4
0.6
0.8
1
V1-Pusle
00.01 0.02 0.03 0.04 0.05 0.06
0
0.2
0.4
0.6
0.8
1
V2-Pulse
Figure 4. SPWM control pulses of IGBT1&2.
-500
0
500
S
Vab inverter
00.01 0.02 0.03 0.04 0.050.06 0.07 0.08 0.090.1
-400
-200
0
200
400
S
Vab_load
Figure 5. Simulation output voltage waveform without filter;
voltage waveform with fi lter.
Copyright © 2013 SciRes. EPE
J. LIU, Y. G. ZHANG 207
Figure 6. Control card of TMS320F28035.
Figure 7. Control pulses for the switches of Inverter.
Figure 8. Output voltage waveform of Inverter.
waveform and high numbers harmonics waveform. The
under two waveforms are the voltage and current wave-
form of the load, and the high numbers are filtered by the
filter circuit. It can provide high quality inverter output
voltage waveform.
4. Experiments
Solar photovoltaic power generation systems experiment
platform is the TI company PV power system develop-
ment package. This system is mainly composed by the
PV array, DC/DC (Boost) circuit, inverter circuits, filter
circuit, the sampling circuit, TMS320F28035 controller
etc [10].
TMS320F28035 controller is a high-efficiency 32-Bit
CPU (TMS320C28x™) of TI Company. It is only single
3.3V supply and 60MHz device. It has two internal
Zero-pin Oscillators and up to 45 Multip lexed GPIO Pins
and three 32-Bit CPU Timers [11].
An internal voltage regulator allows for single rail op-
eration. Analogy comparators with internal10-bit refer-
ences have been added and can be routed directly to con-
trol the PWM outputs. The ADC converts from 0 to
3.3-V fixed full scale range and supports ratio-metric
VREFHI/VREFLO references. The ADC interface has
been optimized for low overhead/latency.
The control card of TMS320F28035 controller is
showed in Figure 6.
And it communicates with the PC through SCI. The
control algorithm is progressed in the control card, and
the control signals (SPWM pulses) are sent to the main
circuit to drive the inverter to converter the DC voltage
into the required AC voltage.
The frequency of the inverter switches is 25 kHz. And
the four SPWM control pulses of the two arms of the
inverter are showed in Figure 7. Each two pulses are
complemented, and there is dead time zone setup be-
tween these two pulses to prevent the short circuit hap-
pened in the one arm of the inverter. And the dead time
zone can be changed through the human-machine inter-
face software in the PC, or be changed in the control
program according to the requirement of control [12].
The experiments of DC/DC booster circuit and single
phase inverter SPWM control algorithm are finished in
the experimental platform. The experiment output wave-
forms are showed in Figure 8 below. The yellow line is
the DC Bus voltage, the blue line is the power grid volt-
age, the green line is the output voltage waveform of the
inverter, and the purple line is the output current wave-
form of the inverter, which load is a resister. And the
output voltage of the inverter is same phase and same
frequency with the power grid voltage. And the synchro-
nization between the output voltage and the power grid
voltage is realized by the PLL circuit and the close loop
control [13 ] .
5. Conclusions
The control syste m of power converter is one of th e most
important parts of the solar photovoltaic power genera-
tion system. This paper completes the simulation of the
control system and realized the control strategy in ex-
periment system. It gives the test and verify of the simu-
lation control algorithm in the experiment platform, and
more detail research experiments can be development in
the platform.
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