Modelling and Analysis of Photo Voltaic Cell Fed Seven Level Multi-String Inverter

Modelling and Analysis of Photo Voltaic Cell Fed Seven Level Multi-String Inverter

A. Vinothkumar1, S. Selvakumar 2, M. Vigneshkumar3

1. Assistant Professor, Department of Electrical and Electronics Engineering, P.A college of Engineering and Technology, Pollachi.

2. M.E (Power Electronics and Drives) student, P.A college of Engineering and Technology, Pollachi.

3. Assistant Professor, Department of Electrical and Electronics Engineering, P.A college of Engineering and Technology, Pollachi.

Abstract –There is strong trend in the photovoltaic (PV) inverter technology to use transformer less topologies in order to acquire higher efficiencies. This paper presents a single-phase seven level grid connected PV inverter with two reference signals that were identical to each other with an offset that was equivalent to the amplitude of the triangular carrier signal were used to generate PWM signals for the switches. Multistring inverter based system gives better voltage regulation and efficiency compare to the multilevel inverters. Seven level multistring inverter consists of two auxiliary switches and diodes. The inverter produces output voltage in seven levels Vdc, Vdc/3, 2Vdc/3, 0,-Vdc/3,-2Vdc/3,-Vdc. The validity of the propose inverter is verified through simulation.

Keyword – Pulse Width modulation (PWM), Photo Voltaic (PV) Source, Maximum Power Point (MPP).

1. INTRODUCTION

   PV inverter, which is the heart of a PV system, is used to convert dc power obtained from PV modules into ac power to be fed into the grid. Improving the output waveform of the Inverter reduces its respective harmonic content and, hence, size of the filter used and the level of the Electromagnetic Interference (EMI) generated by switching operation of the inverter. In recent years, multilevel inverters have become more attractive for researchers and manufacturers due to their advantages over conventional three level PWM Inverters. They offer improved output waveforms, smaller filter size and lower EMI, lower Total Harmonic Distortion (THD).

2. CIRCUIT DESCRIPTION OF THE PROPOSED SYSTEM

   The proposed single-phase seven-level multi-string inverter circuit is shown in Fig 1. It consists of three strings of DC – DC step up converter connected to common dc bus, an auxiliary circuit, and conventional full-bridge inverter configuration. Input sources, PV strings are connected to the inverter via dc-dc boost converters. The dcdc boost converters are used to track the Maximum power point

   tracking (MPPT) independently and to step up inverter output voltage. The multi-string approach is adopted because each dcdc converter can independently perform MPP tracking (MPPT) for its PV strings. This will compensate for mismatches in panels of like manufacture, which can be up to 2.5%. It further offers the advantage of allowing panels to be given different orientations and so open up new possibilities in architectural applications. Another advantage of multi-string conguration is that the mixing of different sources becomes possible, i.e., existing PV panel strings could be extended by adding new higher output panels without compromising the overall string reliability or performance [1]. Depending on the design, each converter module may be able to isolate its connected power source so that the wiring of series or parallel connection of these strings can be performed safely. The power-source converter connection is a safe low-voltage connection. The dcdc boost converters are connected in parallel to avoid high dc-bus voltage, which will eventually increase the size of the capacitors and the inverters cost. Therefore, only three capacitors with equal capacitance rating are used as the dc bus, and the other dcdc boost converters are connected to this dc bus, as shown in Fig.1. A ltering inductance Lf is used to lter the current injected into the grid. The injected current must be sinusoidal with low harmonic distortion. In order to generate sinusoidal current, a sinusoidal PWM is used because it is one of the most effective methods.

   A sinusoidal PWM is obtained by comparing a high- frequency carrier signal with a low-frequency sinusoidal signal, which is the modulating or reference signal.

   The carrier has a constant period; therefore, the switches have constant switching frequency. The switching instant is determined from the crossing of the carrier and the modulating signal.

   The table 2.1 gives the switching operation of the Seven Level inverter. The voltage levels in the truth table are Vdc, Vdc/3, 2Vdc/3, 0, -Vdc/3, -2Vdc/3, -Vdc. The switching operation of the above voltage levels for the switches will be on (1) and off (0) according to the input need to be given. In Pulse width modulation technique the input can be given as in

   the table 1. The on and off can be done according the output required.

   Fig.2. Switching pattern for the single phase seven level inverter

3. PWM MODULATION

   Modulation index given as,

   M a for a seven-level PWM inverter is

   Fig.1. Circuit diagram of Multistring inverter

   Where,

   M a

   Am

   2 Ac

   (1)

   Ac is the peak-to-peak value of carrier

   Voltage Levels	S1	S2	S3	S4	S5	S6
   Vdc	0	0	1	0	0	1
   Vdc/3	0	1	0	0	0	1
   2Vdc/3	1	0	0	0	0	1
   0	0	0	1	1	0	0
   -Vdc/3	0	0	0	1	1	0
   -2Vdc/3	0	1	0	0	1	0
   -Vdc	1	0	0	0	1	0

   Am is the peak value of voltage reference Vref .

   As two reference signals are identical to each other, (1)

   can be expressed in terms of the amplitude of carrier signal

   Vc by replacing

   Ac withVc , and

   Am Vref 1 Vref 2 Vref

   then,

   M Vref

   2Vc

   (2)

   If M 1, higher harmonics in the phase waveform is obtained. Therefore, M is maintained between zero and one. Here two reference signals Vref 1 and Vref 2 are compared with

   the carrier signal at a time. If Vref

   exceeds the peak amplitude

   Table 2.1 Truth table for the 7-level multistring inverter

   of carrier signal Vcarrier , then Vref 2 will be compared with the carrier signal until it reaches zero. At this point onward, Vref 1

   takes over the comparison process until it exceeds Vcarrier . This will lead to a switching pattern, as shown in Fig 2. Switches S4S9 will be switching at the rate of the carrier signal frequency, while S7 and S8 will operate at a frequency that is equivalent to the fundamental frequency. Table 1 illustrates the level of Vinv during S4S9 switch on and off.

   Section 4 illustrating the Simulink model of the PV cell is given in the appendix.

4. CHARACTERISTICS OF PV CELLS

   1. Characteristics of PV cell at constant temperature

      degr cent	ee igrade
      10	00 W/m2
      80 60 40	0 W/m2 0 W/m2 0 W/m2
      20	0 W/	m2

      160

      From the above characteristics (Fig.4, Fig.5, Fig.6, Fig.7) curves the power generation continuously varies along with two main factors, which are known as cell temperature and irradiance. In this work MPPT technique is used for finding

      140

      120

      OUTPUT POWER

      100

      80

      60

      40

      20

      0

      Tc=25

      0 10 20 30 40 50 60 70 80

      OUTPUT VOLTAGE

      the maximum output at various instant of time.

5. MAXIMUM POWER POINT TRACKING CONTROL (MPPT)

   In order to operate the PV cells at the maximum power point, several techniques has been suggested in the literature

   .Some of them are,

   1. Look up table Method

      Fig.4. Power and voltage waveform at constant temperature for PV cells

      100	0 W/m2				T	C=25 DEG	CENTIGRA
      80	0 W/m2
      60	0 W/m2
      400	W/m2
      200	W/m2

      3

      DE

      2.5

      OUTPUT CURRENT

      2

      1.5

      1

      0.5

      0

      0 10 20 30 40 50 60 70 80

      OUTPUT VOLTAGE

      Fig.5. Current and voltage waveform at constant temperature for PV cells

   1. Characteristics of PV cell at constant irradiance

   I	RRADIAN	CE = 1000	W/m2					25 DE	G. CENTI
   0	DEG CEN	TIGRADE
   								50 DEG	CENTIGR
   								75 DEG	CENTIGR

   180

   160

   1. Perturbation and Observation methods

   2. Model based computation methods

   3. Artificial intelligence techniques

      Model based Computation methods are of two types they are,

      1. Voltage based MPPT (VMPPT)

      2. Current Based MPPT (CMPPT)

   In this work focus is made on Current Based MPPT (CMPPT).The CMPPT method simplifies the entire control structure of the power conditioning system and uses an inherent current source characteristic of solar cell arrays. Therefore it exhibits robust and fast response under rapidly changing environmental conditions.

   A. Current Based Maximum Power Point Tracking Control

   140

   GRADE

   The main idea behind current based MPPT is that the

   POWER OUTPUT

   120

   100

   ADE

   current at the maximum point

   Imp has a strong relationship

   80 with the short circuit current Isc . Isc can either be measured

   ADE

   60

   40

   on line under different operating conditions or can be computed from a validated model.

   20

   0

   0 10 20 30 40 50 60 70 80 90

   OUTPUT VOLTAGE

   3

   2.5

   Model Response Curve Fitting

   Fig.6. Power and voltage waveform at constant irradiance for PV cell

   y = 0.948*x

   Current at Maximum Power (Imp)

   2

   						0 c	entigrade
   						25	centigra	de
   10	00 W/m2					50 ce	ntigrade
   						75 centi	grade

   3

   2.5

   OUTPUT CURRENT

   2

   1.5

   1

   0.5

   1.5

   1

   0.5

   0

   0.5 1 1.5 2 2.5 3

   SHORT CIRCUIT CURRENT(Isc)

   Fig.8. Current based control

   Fig.8 represents the current based control scheme which

   0

   0 10 20 30 40 50 60 70 80 90

   OUTPUT VOLTAGE

   Fig.7.Current and voltage waveform at constant irradiance for PV cells

   gives the linear relationship between short circuit current and current at maximum power Imp using curve fitting.

   Isc

6. SIMULINK MODEL OF SEVEN LEVEL INVERTER

   The feasibility of the proposed approach is verified using computer simulations. A model of the seven-level inverter is constructed in MATLAB-Simulink software. A new strategy with reduced number of switches is employed. Cascaded H bridge 7 level inverter requires 12 switches to get

7. RESULT AND DISCUSSION

   A. Switching Pattern for the Single Phase seven-level inverter

   seven level output voltage and with the proposed topology requirement is reduced to 6 switches. The new topology has the advantage of its reduced number of switching devices (switches) compared to conventional cascaded H-bridge multilevel inverter, and can be extended to any number of levels. The simulink model of seven level multi-string inverter is represented in the Fig.9

   A. Simulation diagram of multistring inverter

   Carrier With Two References

   Voltage in Volts

   2

   0

   -2

   0 0.005 0.01 0.015 0.02

   Control Signal to Switch 2

   Voltage in Volts

   2

   0

   -2

   0 0.005 0.01 0.015 0.02

   Control Signal to Switch 4

   Voltage in Volts

   2

   0

   -2

   0 0.005 0.01 0.015 0.02

   Control Signal to Switch 1

   Voltage in Volts

   2

   0

   -2

   0 0.005 0.01 0.015 0.02

   Control Signal to Switch 3

   Voltage in Volts

   2

   0

   -2

   0 0.005 0.01 0.015 0.02

   Control Signal to Switch 5

   Voltage in Volts

   2

   0

   -2

   0 0.005 0.01 0.015 0.02

   Fig.10. Switching pattern for seven level inverter

   Fig.10 represents the switching pattern for seven level inverter. Here two reference signals Vref 1 and Vref 2 will take turns to be compared with the carrier signal at a time. If Vref exceeds the peak amplitude of carrier signalVcarrier , then Vref 2 will be compared with the carrier signal until it reaches

   zero. At this point onward, Vref 1

   takes over the comparison

   Fig.9. Simulink model of Seven level multi- string inverter

   The above simulink model in Fig.9 could recreate the characteristic curves shown in Fig.6 and Fig.7 of section V when correctly inserted into the progam.

   The life time of the PV panel depends on the environmental conditions at which the panel is installed. Aging effect is unavoidable but it can be minimized using anti-aging agents like ethylene vinyl acetate. This serve to improve the life time of PV panel to certain extent. The exact life time of the PV panel is unpredictable as it depends upon the field conditions and quality of manufacturing.

   process until it exceedsVcarrier . This will lead to a switching pattern, as shown in Fig.10. Switches S4S9 will be switching at the rate of the carrier signal frequency, while S7 and S8 will operate at a frequency that is equivalent to the fundamental frequency.

   Fig.11 Simulation results for seven level inverter output voltages

   (M 0.5)

8. CONCLUSION

   This paper has presented a single-phase multistring seven-level inverter for PV application. A novel PWM control scheme with two reference signals and a carrier signal has been used to generate the PWM switching signals. The circuit topology, control algorithm, and operating principle of the proposed inverter have been analyzed in detail. The configuration is suitable for PV application as the PV strings operate independently and later expansion is possible.

9. SCOPE FOR THE FUTURE WORK

The proposed model is simulated using MATLAB.The simulation results indicate the functioning of the proposed model. The future work is to implement the proposed model in hardware. The proposed model can be improved by increasing the level of the inverter output.

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APPENDIX SIMULINK MODEL OF PV CELL

Fig.3 Line diagram of PV cell

1. Multi-string five-level inverter with novel PWM control scheme for PV

   Application, Nasrudin A.Rahim, IEEE, and JeyrajSelvaraj, IEEE transactions on industrial electronics,vol 57,n0.6,June 2010.

   Where,

   I IL

2. ID

   (3)

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4. Kaliamoorthy. M, Sekar R.M., and Rajaram.R, A new single-phase PV fed five-level inverter topology connected to the grid IEEE Trans.communication control and computing technologies.pages:196- 203, 2010.

5. S. Vazquez, J. I. Leon, L. G.Franquelo, J. J. Padilla, and J. M. Carrasco,

DC voltage-ratio control strategy for multilevel cascaded converters

I = Output Current in Amps

I L = Photo Generated Current in Amps

I D = Diode Current in amps

By Shockley equation, current diverted through diode is,

U IR

(4)

C pv = overall heat capacity of PV cell/Module

I D Io exp s 1

nkT / q kin, pv = transmittance absorption product of PV cells

Where,

Io = Reverse Saturation Current

Kloss

Ta

= overall heat loss coefficient

= ambient temperature

n = Diode Ideality Factor k = Boltzmanns Constant T = Absolute Temperature

q = Elementary Charge

For silicon of 250C nkT / q =0.0259 volts=,

A = effective area of PV cell/Module

I I

U IRs

(5)

D o exp 1

Substituting above equation in equation (3)we get,

I I

• I exp U IRs

L o

1 (6)

where nkT / q

completion factor.

is known as Thermal voltage timing

I

Photo generated current IL is calculated by

I

L

L,ref

• I ,SC T

Tc,ref

(7)

c

Where, ref

= Irradiance (W / m2 )

ref

= reference irradiance (1000W / m2 )

IL,ref = Light current at reference condition

Tc = PV cell temperature

Tc,ref = Reference temperature

I ,SC = Temperature coefficient of the short circuit current (A/ C)

Saturation Current Io is given by

T 273 3 e N T 273 (8)

o o,ref

I I c,ref exp gap s 1 c,ref

Where,

Tc 273

qref

Tc 273

Io,ref = saturation current at the reference condition (A)

egap = band gap of the material (1.17eV for Si)

Ns = number of cells in series of the PV module

q = charge of the electron

ref = value of at the reference Condition

Thermal Model of Photovoltaic Cell is

C dTc k

U x I K

T T

(9)

pv dt

Where,

in, pv A

loss c a