Study of Control Strategy of the Converter in Solar Photo-voltaic Power System

doi:10.4236/epe.2013.54B040

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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: [email protected], [email protected]

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.

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.

0.2

0.4

0.6

0.8

V1-Pusle

00.01 0.02 0.03 0.04 0.05 0.06

0.2

0.4

0.6

0.8

V2-Pulse

Figure 4. SPWM control pulses of IGBT1&2.

-500

500

Vab inverter

00.01 0.02 0.03 0.04 0.050.06 0.07 0.08 0.090.1

-400

-200

200

400

Vab_load

Figure 5. Simulation output voltage waveform without filter;

voltage waveform with fi lter.

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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