Simulation and Experimentation of Voltage Source ...

Simulation and Experimentation of Voltage Source Inverter (VSI) for a Photovoltaic System (Full text in English)

BAALI Kheira1, SAAD Salah1, ZAAMOUCHE Fares1 1

LSELM Laboratory, Badji-Mokhtar University, Annaba, Algeria

Abstract In this paper, the simulation and real time implementation of a space vector pulse-width modulation (SVPWM) control scheme to demonstrate the high performance of the photovoltaic inverter. The design and the simulation of electric operation of a photovoltaic (PV) system controlled by an intelligent method enabling to track the maximum power produced by the photovoltaic generator are described. The simulation was performed in the Simulink/MATLAB environment and the PV output voltage is represented by a dc power supply of the inverter. Furthermore, tests are performed on experimental platform to implement the developed algorithms of SVPWM inverter used to convert the produced dc voltage to a variable ac voltage. The experimental waveforms such as ac output voltages, current and total harmonic distortion are presented and analysed. Keywords: Photovoltaic energy, modelling, dSPACE, MPPT control, PV inverter Received: March, 23, 2016

1. Introduction

At present, energy sources and the increase of the greenhouse effect are the present issues. Many research works are carried out on the use of renewable energies to overcome these problems. Among these energies, photovoltaic (PV) energy produced by photovoltaic panels. The main concern is the production of electric energy by this technique and panels (PV) optimum operation. This has led to the elaboration of electronic devices enabling to use efficiently these alternative sources. Devices such as inverters designed to convert a dc voltage to a desired ac voltage and current with variable frequency become a very important element of energy conversion system a general overview of different types of PV inverter is given [1, 2 3]. The efficiency of these converters depends on the control strategy of the switches used to generate the desired output voltage and current with less harmonic distortion There are various electronic controllers used to control the inverters switches, such as analogue and digital controllers, microcontrollers and microcomputers [1, 4, 5, 6, 7]. These controllers can be used to improve inverter performances to avoid photovoltaic (PV) system failure. A dSPACE system is recently applied to control the inverter switches and can be extended to photovoltaic systems [8, 9, 10, 11, 12, 13]. In this paper, the PV system mathematical model describing the operation of each element is given. The developed model of photovoltaic panels, dc-dc converter and dc-ac inverter along with MPPT control algorithm are simulated. Moreover, the simulation and real time implementation of a space vector pulse-width modulation (SVPWM) control scheme is performed to demonstrate the high performance of the photovoltaic inverter when dSPACE is used as controller. The simulations and experiments are carried out in Simulink/MATLAB environment and dSACE1104 in real time implementation respectively.

In this work SVPWM inverter is used, replacing the conventional two-level sinusoidal pulse width-modulation (SPWM) inverter because it offers great advantages, such as improved output waveforms, lower electromagnetic interference, less harmonic distortion (THD). The results of the open loop operation have showed that SVPWM technique has high performances in harmonics current reduction, torque and speed time response. The experimental waveforms such as ac output voltages, current, total harmonic distortion, and speed curve are presented and analysed. 2. Photovoltaic generator model

The so-called photovoltaic generator is the energetic sub-system located at the beginning of the system producing a dc electric energy by photovoltaic conversion of illumination. The conventional equivalent circuit of the solar cell represented by a courant source in parallel to one is shown in Figure 1.

Figure 1. Cell equivalent circuit of two diodes circuits

The model is developed based on a circuit composed from a photocurrent source, a diode in parallel to the source, a series resistor and a shunt resistor and additional diode for better adjustment of the curves. This model requires the knowledge of four parameters at standard conditions of illumination and temperature. These parameters are given usually by the manufacturer or obtained by tests. Solar cell equivalent circuit used in this study is simplified by neglecting the shunt resistor Figure 2. The relation of the current in function of voltage can be expressed approximately by equations (1) and (2):

ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ (EEA), vol. 64 (2016), nr. 3

[

)]

(

I = I CC 1 − C1 eV C2Vco − 1

(

C 2 = Vmp

(−Vmp

C2Vco

)

(

(2)

where: a: defined as a current coefficient of temperature variation

)

and

I new = I ref + ∆I

Vnew = Vref + ∆V

(4) charctéristic current - voltage

(8)

charactéristic power - voltage

5

800 G=200 w/m2 G=400 w/m2 G=600 w/m2 G=800 w/m2 G=1000 w/m2 G=1200 W/m2

4.5 4 3.5

700

G=200W /m2 G=400W /m2 G=600W /m2 G=800W /m2 G=1000W/m2 G=1200W/m2

600 500

3

power (W )

current (A)

(6)

The effect of illumination on current-voltage and power - voltage characteristics of PV panel at T=25°C are presented in Figures 2 and 3 respectively.

(V − I ) on the basis of equations (4) and (8).

∆T = T − Tref

(5)


: defined as a coefficient of voltage variation in relation to the temperature [V/°C].

, I ref ) of the curves of references (I − V ) to a new

point

(5)

(3)

temperatures T (°C ) , the model shifts any point ref

)

∆I = α G G ref ∆T + G G ref − 1 I cc

∆V = − β ∆T − R S I

 I  Vco − 1 ln1 − mp  I cc  

For other illuminations intensity G W m 2

(V

(

(1)

where:

C1 = (1 − I mp I cc )e

)

29

2.5 2

400 300

1.5 200 1 100

0.5 0

0

20

40

60

80

100 120 voltage (V)

140

160

180

0

200

Figure 2. Effect of illumination on current-voltage (I-V) characteristic of PV panel at T=25 °C

0

20

40

60

80

100 120 voltage (V)

140

160

180

200

Figure 3. Effect of illumination on power-voltage (P-V) characteristic of PV panel at T=25°C (power produced by a panel)

3. Modelling and control of dc-dc converter

The dc-dc converter is used to elevate the voltage produced by PV cells for the application requiring high voltages. Thus, this converter is called Boost converter and is illustrated in Figure 4.a.

Vout = V pv 1 − α (V pv 1 − α )

(12)

K = ton T

(13)

The obtained output voltage Vout of the boost converter is 400 V. 4. Maximum Power Point Tracking (MPPT) control

Figure 4. Synoptic diagram of dc-dc boost converter

The operation of the dc-dc boost converter is as follows: If, S = 0 the diode is on and the IGBT is off S=1 the diode is off and the IGBT is on The following equations can be derived:

(

)

V pv t on = V out − V pv t off

(9)

6Vout = t on + t off t off V pv

(10)

T = ton + toff

(11)

The main problem of the PV generation systems is that the amount of electric power generated by the solar arrays is always changing with weather conditions. An MPPT method or algorithm [7], [13], which has quick response characteristics and is able to make good use of the electric power generated in any weather conditions, is required to solve this issue. Various MPPT control methods have been studied and discussed. In this paper, the perturb-and-observe algorithm is used because of its flexibility to extract maximum power from the PV panels and deliver it to the converter. The feedback controller used for the inverter is the PI controller. This algorithm is based on system fluctuation by increasing or decreasing

Vref or by acting directly on the

duty cycle of dc − dc converter, then observing the effect on the output power in order to adjust this duty cycle. If the value of generator power P

(k ) is greater than

ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ (EEA), vol. 64 (2016), nr. 3

30

(

)

the previous value P k − 1 thus, the same direction of the previous fluctuation is kept other ways the fluctuation of the previous cycle is inversed. This algorithm is based on the fact that the derivative of the output power

V pv

in relation to panel voltage

Ppv

is equal to zero at the

maximum point of power [6],[13] as illustrated in Figure 5.

dc-dc converter and is composed from an inductance (L1), (IGBT) commutation circuit and their MPPT control and a diode. It appears at capacitor (C) terminals and acts as inverter input voltage. The voltage calculation is explained by the expressions presented below:

Vout = Vin (1 − k ) , Vout = 150 1 − 0.375 , Vout = 400V where

Vout : load voltage or the inverter output voltage Vin : converter input voltage k: converter duty cycle (varies from 0 to 1) Then, the inverter converts to ac voltage waveform using space vector pulse width modulation as control strategy. 5. Inverter model Figure 5. PV characteristic of the PV module

(

)

The PV panel characteristic Ppv − V pv shows that that this derivative is positive at the left of maximum point of power and negative at the right. This will lead to the following equations: At the maximum point of power: dPpv dV pv = d (I pv ⋅ V pv ) dV pv = I pv + V pv dI pv dV pv = 0

(14)

At the left of maximum point of power: dPpv dV pv = d (I pv ⋅ V pv ) dV pv = I pv + V pv (dI pv dV pv )〉 0

(15)

At the right of maximum point of power dP pv dV pv = d (I pv ⋅ V pv ) dV pv = I pv + V pv (dI

pv

dV pv )〈 0

The inverter is the most important part of this work therefore, it is necessary to present a description of the inverter and the development of its model. Figure 6, illustrates the topology of a conventional three phase inverter used in this work. The switches of the same arm must not be on at the same time to avoid the source to be short-circuited. Its operation is based on SVPWM control technique to control the switches in order to obtain a waveform with a desired frequency. The PWM voltage source inverter with six IGBTs shown in Figure 6 is often used for this kind of application. The load is modelled from the line to neutral voltages Van , Vbn and Vcn , and the inverter switches are controlled as follows:

(16)

These equations can be expressed as follows: At the maximum point of power:

dI pv dV pv = − I pv V pv

(17)

At the left of maximum point of power: Figure 6. Inverter Topology

dI pv dV pv 〉 − I pv V pv

(18)

At the right of maximum point of power:

dI pv dV pv 〈− I pv V pv

(19)

These equations can be employed as a control algorithm to control the converter operating point by measuring the conductance and instantaneous conductance of the converter

dI pv dV pv and I pv V pv

respectively. Finally, it can be deduced that the maximum power will be obtained when the derivative of the power in relation to the voltage

(dP

pv

dV pv )

will be equal to

zero. The input voltage is 150 V representing the PV output voltage

(Vout ) . This voltage is elevated to 400 V by

Considering Ki1 , and Ki2 as transistors of each arm with i= a, b and c defining the phases. If

Ki1 = 1 , thus Ki1 is on and Ki2 is off Ki2 = 0 , thus Ti1 is off and Ti2 is on

If The line-to-line voltages are obtained from the inverter output and can be expressed as follows:

Vab = Van 0 − Vbn 0  V = V − V  bn 0 cn 0   bc Vca = Vcn 0 − Van 0 

(20)

Lines to neutral voltages of the load derived from the line-to-line voltages have a sum equal to zero thus:

ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ (EEA), vol. 64 (2016), nr. 3

Van = (1 3)[Vab − Vca ] V = (1 3)[V − V ] bc ab   bn Vcn = (1 3)[Vca − Vbc ]

(21)

They can be obtained from the inverter output voltages by introducing the neutral to line voltage of the load in relation to reference point no [6,7].

Van + Vnn 0 = Van 0  V + V = V  nn 0 bn 0   bn Vcn + Vnn 0 = Vcn 0 

31

v  1 − 1 2 − 1 2   an  2 3   v bn   0 − 3 2 − 3 2   v   cn 

 vα  v  =  β

(28)

From the combination of the three state variables (Ca, Cb and Cc), the inverter has eight switching states. It can generate eight different vectors of voltage output. These eight space vectors define the limits of the six sectors in the (α, β) frame as illustrated in Figure 7.

(22)

Therefore, it can be concluded:

Vnn 0 = (1 3)[Van 0 + Vbn 0 + Vcn 0 ]

23)

The switches are considered as ideal then if (i = a, b and c) thus,

Vin 0 = Ki ⋅ E − E 2

(24)

Figure 7. Inverter voltage vectors representation

The following matrix is obtained:

Van 0 = (Sa − 0,5)E  V = (Sb − 0,5)E   bn 0  Vcn 0 = (Sc − 0,5 )E 

Two of these vectors equal to zero (25)

(26)

Substituting (22) in (23) the following expression is obtained:

Van   2 − 1 − 1  S a  V  = (1 3)E − 1 2 − 1  S   bn    b  − 1 − 1 2   S c  Vcn 

r and V7(111) .

Recalling that: Ci = 1: Upper switch is on and the lower is off Ci= 0: Upper switch is off and the lower is on Reference

r

Substituting (23) in (25) the equation below is obtained:

Van = 2 3Van 0 − 1 3Vbn 0 − 1 3Vcn0  Vbn = − 2 3Van 0 + 1 3Vbn 0 − 1 3Vcn0 − V = 2 3V − 1 3V + 1 3V an 0 bn 0 cn 0  cn

r V0( 000 )

voltage vector Vref can be defined in the (α, β) frame by:

r Vref = vα + j v β

(29)

ϕ = 2πf ϕ f

: The instantaneous angle of the reference vector

: Fundamental frequency of inverter output parameters Then, the reference vector

(27)

In recent years, several pulse width modulation (PWM) techniques were developed and studied. Two techniques were given a great interest mainly; sinusoidal pulse width modulation (SPWM) [14, 15, 16] and space vector pulse width modulation (SVPWM), in this work a SVPWM is used for its efficiency and simplicity.

V ref is approximated

r r during one sampling period by adjacent vectors V j , V j +1

and

r V0

r

(or V7 ) (j=1,…, 5) corresponding to the eight

possible inverter switches - states. The symmetrical threephase system enables to limit the study to a general case where the reference vector V ref is located in the sector1 of

π 3

radians as shown in Figure 8.

6. Theory of SVPWM technique

The SVM technique was developed for space vector electric machines control. Its principle is to rebuild the r reference vector Vref from different voltage vectors [17, 18, 19, 20, 21, 22]. Each vector corresponds to a combination of a three phase inverter switches states. The space vector pulse width modulation (SVPWM) technique processes the signals directly on the diphase frame of the Concordia transformer. Thus, the line to neutral voltages van, vbn and vcn at the inverter output are represented in the (α, β) frame by the following equations:

Figure 8. Projection of reference vector in sector I

Thus, the following equation is obtained:

(

Tp V ref = T1V 1 + T2 V 2 + T0 V 0 orV 7

)

(30)

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32

V 0 and V 7 are the zero vectors V 0 = V 7 = 0 , Tp : Modulation period, Tp = 1 fp fp : modulation frequency,

T1 : Time attributed to vector V 1 , T2 :

Time attributed to vector

T0 : Time shared by V 0

and

V7.

(α , β )

gives the equations

T1 = 2 (T pVref E )sin((π 3) − ϕ )

T2 = 2 (T pVref E )sin ϕ T0 = 1 2 (T p − T1 − T2 )

(36)

7. Computer simulation and experiments

V2,

Solving equation (30), after the decomposition on two axes of complex frame presented below:

d a = Ta T p  d b = Tb T p  d c = Tc T p

(31)

In order to validate the developed algorithms computer simulation and experimental tests were conducted on a two level inverter feeding 1.5 KW induction motor. Simulation parameters are given as follows, the modulation index is M= 0.86, the torque is Cr = 4 N. m and the commutation frequency is 5 Khz. The experimental test rig designed to validate and confirm the theoretical developments and evaluate at the same time the performances of SVPWM control technique is illustrated in Figure 10.

(32) (33)

0≤ϕ ≤π 3 The same rules were applied for others sectors, from II to VI. The following relations are obtained:

T1 = 2 (T pVref E )sin((π 3) − ϕ + (k − 1 3)π )  T2 = 2 (TpVref E )sin ϕ − (k − 1 3)π  T0 = 1 2 (T p − Tk − Tk +1 )

(34) Figure 10. Experimental test rig

Thus, the sequence (or mode) of commutations during a period of modulation Tp is represented as follows:

The real-time applications on the dSPACE DS1104 are carried out using Real-Time Interface in MATLAB /Simulink environment. This test rig is composed from the following elements: − IGBTs voltage source inverter commercialized by SEMIKRON with a DC source of 400 V − A 1.5 KW three phase induction motor − Powder brake used to provide accurate load torque − A dSPACE 1104 card (controller Board) was integrated in a PC enabling to generate the required pulses to control the inverter switches. − Current and voltage sensors to measure the output voltages and currents. The visualization of system waveforms is realized through CONTROL DESK software, enabling to control the signals from Simulink dSPACE schemes. The modulation frequency is 5 KHz and the torque applied to the motor shaft is 4 Nm.

V 0 ⇒V1 ⇒V 2 ⇒V 7 ⇒V 7 ⇒V 2 ⇒V1 ⇒V 0

8. Results and discussions

k : the sector number (k=1,…,6) Figure 9 shows a chronogram of pulses when the reference vector

V ref is located in the first sector.

Figure 9. Chronogram of pulses of sector Ia

The waveforms obtained by simulation and experimental tests are presented in Figure 11 and Figure 12 respectively for comparison as shown below.

Thus;

Ta = T1 + T2 + T0  Tb = T2 + T0 T = T 0  c

(35)

Ta, Tb and Tc are commutation times of upper switches of inverter arms a, b and c respectively. Therefore, the duty cycles to generate the signal of space vector modulation are given by:

ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ (EEA), vol. 64 (2016), nr. 3

a)

b)

c)

d)

33

Figure 11. Simulation tests: a) Line to line voltage vab(v), b) Phase current Ia(A), c) Current frequency spectrum THD (%), d) Speed curve in transient and steady states

a)

b)

c)

d)

Figure 12. Experimental tests: a) Line to line voltage vab(v), b) Phase current Ia(A), c) Current frequency spectrum THD (%), d) Speed curve in transient and steady states

The main characteristics of PV system enabling to supply a system in an isolating area where no energy source is available are presented. A PV generator delivering a dc voltage to static power converter tracking the operating optimum point is discussed. The electric model of the system is simulated in Matlab environment for different temperatures and solar illumination and their influence on (I-V) and (P-V) characteristics. This model showed that the optimization is very important at low illumination as stated by [2, 3, 13]. Inversely with temperature which lowers the optimal operating point. As the inverter is the most important device of the system, an investigation was carried out to improve its performances. The line to line (Vab) voltages is shown in Figure 11a. It can be observed that, the three-phase inverter produces two line to line voltages (Vab), (-E and E). It is also noted that the frequency spectrum of the phase currant Ia (Figure 11.b), has showed a reduced harmonic distortion (THD=1,81 %) as presented in Figure 11 c. At t=1.8s a torque of Tr=4 Nm is applied to the motor shaft, In steady state the torque ripples are reduced

(observed at the phase current Figure 11.b). This means that the SVPWM provides an improved electromagnetic torque therefore, better speed regulation is obtained with a time response less than 0.5 s as shown in Figure 11 d. However, Figure 12.a illustrates the line-to-line voltage Vab at the inverter output. It can be observed that the shape of this wave is similar to those obtained by simulation. Figure 12.b shows the inverter phase current Ia waveform, it can be remarked from its frequency spectrum illustrated in Figure 12.c that the waveform SVPWM technique has high quality spectrum with a THD = 3,3 % as it has been demonstrated by simulation. Figure 12.d. According to IEEE standard, the obtained THD is well below the norms and in concordance with the results of the literature [8, 10, 12]. The speed versus the time curve is measured in transient and steady state showing a time response at 0.5s demonstrating a good speed regulation. 9. Conclusions

The space vector pulse width modulation implementation has been developed in MATLAB/Simulink

34

ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ (EEA), vol. 64 (2016), nr. 3

programming environment and in dSPACE 1104 kit system. The performance analysis of this control technique from the point of view of output voltage and currents has been presented in this paper. The results of the open loop operation have showed that SVPWM technique has high performances in harmonics current reduction, torque and speed time response. Therefore, this technique as inverter control strategy is more suitable for PV System. The SVPWM technique is one of the means to develop to improve inverter performances in order to avoid photovoltaic (PV) system failure.

[17]

[18]

[19] [20]

10. Acknowledgements The authors gratefully acknowledge the Algerian General Directorate of Research for providing the facilities and the financial funding of this project.

[21]

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[22]

inverters”, Inter. Journal of Power and Energy Systems, vol. 21, no. 2, pp. 74-80, 2001. B. Lang, M. Miao, W. Liu and G. Luo, “Simulation and experiment study of space vector pulse width modulation”, presented at the 9th Inter. Conf. on electronic measurement and Instruments ICEMI’09, pp. 1-408/1-412, Aug 2009, Beijing. R. Gregor, F. Barrero, S.L. Toral and M.J. Dura “Predictive-space vector PWM current control method for asymmetrical dual threephase induction motor drives” IET Electr. Power Appl., 2010, Vol. 4, Iss. 1, pp. 26–34. Z. Yu, Space-Vector PWM with TMS320C24x/F24x Using Hardware and Software Determined Switching Patters, Application Report SPRA524, Texas Instruments, March 1999. G. Narayanan and V. T. Ranganathan, “Extension of operation of space vector PWM strategies with low switching frequencies using different overmodulation algorithms”, IEEE Trans. on Power Electronics, vol. 17, no. 5, pp. 788-798, Sept 2002. L. Michels, R.F. de Camargo, F. Botter !on, H.A. Gr.udling and H. Pinheiro “Generalized design methodology of second-order filters for voltage-source inverters with space-vector modulation” IEE Proc.-Electr. Power Appl., Vol. 153, No. 2, March 2006. K. M. Rahman, M. A. Choudhury, M. R. Khan, M. A. Kashem and M. R. Yusoff, “Frequency modulated PWM for voltage source inverters”, Inter. Journal of Power and Energy Systems, vol. 21, no. 2, pp. 74-80, 2001.

12. Biography Kheira BAALI was born in Berrahal Annaba, Algeria, on December 20, 1983. She received the degrees of Magister on electromechanical Engineering from University of Badji-Mokhtar Annaba Algeria in 2010. She is a Ph.D student at University of Badji-Mokhtar Annaba, Algeria. Her research interests are mainly in the area of electric drives, power electronics and renewable energies. LSELM Laboratory, Université Badji Mokhtar, Annaba, B.P.12 Annaba, 23000 Algeria; [email protected] Salah SAAD was born in Batna, Algeria in 1958. He received the degree of Engineer in electromechanical applied to mining fields from Badji-Mokhtar Annaba University Algeria and Ph.D degree from Nottingham University UK in 1983 and 1988 respectively. Since 1988 he worked as lecturer, senior lecturer then professor at Badji-Mokhtar Annaba University Algeria.He has supervised many graduated and post-graduate student thesis. He has also conducted many researches projects in power electronics applications, electric ac and dc drives as well as diagnosis and faults detection in ac machines. His research interests are mainly in the area of power electronics such as harmonics elimination by active filters, PWM and Space vector modulation control, multilevel inverters and new converter topologies. He has author and co-author of many journal and conference papers. He co-authored a book in the field of signal processing published in Algeria in 1992. LSELM Laboratory, Université Badji Mokhtar, Annaba, B.P.12 Annaba, 23000 Algeria; [email protected] ZAAMOUCHE Fares was born in Annaba, Algeria, on Fabruary 10, 1984. He received the degrees of Magister on electromechanical Engineering from the University of Badji-Mokhtar Annaba Algeria in 2004. He is a lecturer at University of Laarbi Tebessi.His research interests are electric drives, power electronics, energy quality, power systems, non-linear and intelligent control. He has authored and co-authored many journal and conference papers. LSELM Laboratory, Université Badji Mokhtar, Annaba, B.P.12 Annaba, 23000 Algeria; [email protected]