a novel multi-string five-level

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A Novel Multi-String Five-Level PWM Inverter for

Photovoltaic Application

Nasrudin Abdul Rahim, Senior Member, IEEE, and Jeyraj Selvaraj

AbstractThis paper presents a single-phase multi-string five-level PV inverter topology for grid-connected photovoltaic (PV)systems with a novel PWM control scheme. Three PV strings arecascaded together in parallel configuration and connected to afive-level inverter to produce output voltage in five levels: zero,+1/2Vdc, Vdc,1/2Vdc and Vdc. Two reference signals identicalto each other with an offset equivalent to the amplitude of thetriangular carrier signal were used to generate PWM signalsfor the switches. DSP TMS320F2812 is used to implement thisPWM switching scheme together with a digital PI current controlalgorithm. The inverter offers much less THD and can operateat near unity power factor. The validity of the proposed inverteris verified through a prototype. The experimental results arecompared with conventional single-phase multi-string three-levelgrid-connected PWM inverter.

Index Termsgrid-connected, inverter, PI control, multi-string, five-level.

I. INTRODUCTION

As the world is concerned with fossil fuel exhaustion and

environmental problems caused by the conventional power

generation, renewable energy sources particularly solar energy

and wind energy have become very popular and demanding.

PV sources are used today in many applications as they have

the advantages of being maintenance and pollution free [1].

Solar-electric-energy demand has grown consistently by 20%-

25% per annum over the past 20 years, which is mainly due tothe decreasing costs and prices. This decline has been driven

by 1) an increasing efficiency of solar cells; 2) manufacturing-

technology improvements; 3) economies of scale; [2]. PV

inverter which is an important element in the PV system is

used to convert DC power from the solar modules into AC

power to be fed into the grid. A general overview of different

types of PV inverters is given in [3]. This paper presents a

multi-string five-level inverter for PV application. The multi-

string inverter shown in Fig. 1 is a further development of the

string inverter, where several strings are interfaced with their

own dc-dc converter to a common dc-ac inverter [4]. This is

beneficial, compared with the centralized system, since every

string can be controlled individually. Thus, the operator maystart his/her own PV power plant with a few modules. Further

enlargements are easily achieved since a new string with a

dc-dc converter can be plugged into the existing platform. A

flexible design with high efficiency is hereby achieved [3]. In

this work, a five-level inverter is used instead of a conventional

three-level PWM inverter because it offers great advantages

such as improved output waveforms, smaller filter size, lower

EMI, lower THD, and others [5][6].

A novel PWM control scheme is introduced to generate

Fig. 1. Configuration of multi-string inverters.

switching signals for the switches and to produce five output

voltage levels: zero, +1/2Vdc, Vdc, -1/2Vdc and -Vdc (assum-

ing Vdc is the supply voltage). This inverter topology uses two

reference signals instead of one to generate PWM signals for

the switches. Both the reference signals Vref1 and Vref2 are

identical to each other except for an offset value equivalent to

the amplitude of the carrier signal Vcarrier as shown in Fig.2.

Fig. 2. Carrier and reference signals.

Since the inverter is used in a PV system, a PI current con-

trol scheme is employed to keep the output current sinusoidal

and to have high dynamic performance under rapidly changing

atmospheric conditions and to maintain the power factor at

near unity. Experimental results are presented to validate the

proposed inverter configuration.

2011 IEEE International Electric Machines & Drives Conference (IEMDC)

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I I . INVERTERT OPOLOGY ANDM ODULATION

The proposed single-phase multi-string five-level inverter

topology is shown in Fig. 3. It consists of three dc-dc boost

converters connected to a common dc bus, an auxiliary circuit

and a full-bridge inverter configuration. Input sources, PV

string 1, PV string 2 and PV string 3 are connected to the

inverter via the dc-dc boost converters. Since the proposed

inverter is used in a grid-connected PV system, the utility gridis used instead of a load. The dc-dc boost converters are used

to track the maximum power point (MPP) independently as

well as to step-up the inverter output voltage Vinv to be more

than2 of the grid voltage Vg to ensure power flow from the

PV arrays into the grid [7].

In this work, multi-string approach is adopted since each

dc-dc-converter can independently perform maximum power

point tracking (MPPT) for its PV strings. This will compensate

for mismatches in panels of like manufacture, which can be

up to 2.5% [8]. It offers the further advantage of allowing

panels to be given different orientations and so open up

new possibilities in architectural applications. Furthermore,

a greater tolerance to localized shading of panels can beachieved. Another advantage of multi-string configuration is

the mixing of different sources becomes possible i.e. existing

PV panel strings could be extended by adding new higher

output panels without compromising overall string reliability

or performance. Besides that, greater safety during installation

and maintenance adds to the advantages of multi-string con-

figuration. 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 [9]. The dc-dc boost converters

are connected in parallel to avoid high dc bus voltage which

eventually will increase the size of the capacitors and theinverters cost. Therefore, only two capacitors with equal

capacitance rating are used as the dc bus and the other dc-

dc boost converters is connected to this dc bus as shown in

Fig. 3. Sinusoidal PWM is obtained by comparing a high-

frequency carrier signal with a low-frequency sinusoid signal,

which is the modulating signal 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.

III. OPERATIONAL P RINCIPAL OFM ULTI-LEVEL

INVERTER

Combinations of PV strings are used as the input voltagesources. The voltage across the strings are known as Vpv1,

Vvp2 and Vpv3. Voltages Vpv1, Vvp2 and Vvp3 are boosted

by the dc-dc boost converters to exceed the grid voltage Vg

and the voltage across the dc bus is known as Vpv. The

operational principle of the proposed inverter is to generate

five-level output voltage i.e. 0, +Vpv/2, +Vpv,Vpv/2,Vpvas in Fig. 4. As illustrated in Fig. 3, an auxiliary circuit which

consists of four diodes and a switch S4 is used between the

dc bus capacitors and the full-bridge inverter. Proper switching

Fig. 3. Single-phase multi-string five-level inverter topology.

control of the auxiliary circuit can generate half- level of PV

supply voltage i.e. +Vpv/2 andVpv/2. [10]. Two referencesignalsVref1and Vref2will take turns to be compared with the

carrier signal at a time. IfVref1 exceeds the peak amplitude of

the carrier signal Vcarrier, Vref2 will be compared with the

carrier signal until it reaches 0. At this point onwards, Vref1

takes over the comparison process until it exceeds Vcarrier.

This will lead to a switching pattern as shown in Fig. 5.

Switches S4 S6 will be switching at the rate of the carriersignal frequency while S7 and S8 will operate at a frequency

equivalent to the fundamental frequency.

Fig. 4. Ideal five-level inverter output voltage, Vinv.

If one of the PV strings is disconnected from the dc bus,the operation of the other dc-dc boost converters will not be

affected as they are connected in parallel. As the dc-dc boost

converters is used to track the maximum power point tracking

(MPPT) point, it can be concluded that the MPPT tracking of

the PV strings is done independently.

IV. EXPERIMENTALR ESULTS

The results are verified experimentally by using a DSP

TMS320F2812. Three PV strings with different types of solar

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Fig. 5. Switching pattern for single-phase five-level inverter.

modules and locations are connected to the five-level inverter

via a common dc bus. Table 1 illustrates the PV modulescharacteristics and their location while PWM switching signals

for the switches is generated by comparing a triangular carrier

signal with two reference signals as shown in Fig. 6.

TABLE ICHARACTERISTICS OF PV MODULES

Model: SIEMENS SP75

No. of Panels 6 in series

Max Power 75W

Short circuit current, ISC 4.8A

MPPT current, IMPPT 4.4A

Open Circuit voltage, VOC

21.7V

MPPT voltage, VMPPT 17V

Location Roof Top

Model: SIEMENS SP85


Max Power 85W

Short circuit current, ISC 5.45A


Open Circuit voltage, VOC 22.2V

MPPT voltage, VMPPT 17.2V

Location GND Floor

Model: MtsAE125MF5N


Max Power 125WShort circuit current, ISC 7.90A


Open Circuit voltage, VOC 21.8V

MPPT voltage, VMPPT 17.3V

Location Roof Top

Code Composer Studio (CCS), the programming platform

for DSP TMS320F2812, programs the control algorithm for

the proposed multi-string five-level inverter. Fig. 7 illustrates

Fig. 6. PWM switching signals forS4 S8, (a) S4. (b) S5 and S6 (c) S7and S8.

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the experimental results for Vinv and Ig. It can be seen that

Vinv consists of five levels of output voltage, and Ig has been

filtered to resemble a pure sinewave. Fig 8 shows Vg and the

Ig injected into the grid. To prove that the proposed multi-

string five-level inverter has advantages over the conventional

multi-sting three-level inverter in terms of THD and power

factor, the corresponding measurements were made on both

inverters. FLUKE 43B Power Quality Analyzer was used for

this purpose. The measured results correspond to those in

Fig. 7. The conventional multi-string three-level inverter for

grid-connected PV application is shown in Fig. 9. The same

current control techniques were used to control the overall

performance of the inverter. The only difference between

both inverters is the elimination of the auxiliary circuit, and

therefore only one dc bus capacitor is used. Fig. 10 shows

the THD measurement for the multi-string five-level inverter

while Fig. 11 shows the THD measurement for the multi-

string three-level inverter. The %THD for five-level inverter is

recorded as 5.7% while the %THD for three-level inverter is

9.5%. From both illustrations the THD measurement for multi-

string five-level inverter is much lower than that of the multi-string three-level inverter. The power factor measurement is

shown in Fig. 12. It is notable that both the grid voltage Vgand the current injected into the grid Ig are in phase with a

power factor of 0.99. Efficiency measurement was carried out

to compare the efficiency of the multi-string three-level PWM

inverter with the multi-string five-level PWM inverter for PV

application. The measured efficiency of the multi-string three-

level PWM inverter is approximately 92% while the measured

efficiency for the multi-string five-level PWM inverter is 88%.

As expected, the efficiency of the multi-string five-level PWM

inverter is lower compared to the conventional multi-string

three-level PWM inverter. The main reason is the addition

of the auxiliary circuit between the dc-dc boost convertersand the full-bridge inverter configuration. Switching losses of

switch S4 in the auxiliary circuit caused the efficiency of the

multi-string five-level PWM inverter to be approximately 4%

less than the multi-string three-level PWM inverter. However,

simulation and experimental results show that the THD of the

proposed inverter is lower as compared with the conventional

three-level PWM inverter which is an important element for

grid-connected PV systems.

V. CONCLUSION

A single-phase cascaded PV string multilevel inverter has

been presented in this paper. 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 analysed in detail. Experimental results

indicate that the THD of the multi-string five-level inverter

is much less than that of conventional multi-string three level

inverter.

Fig. 7. Experimental Result ofVinv and Ig .

Fig. 8. Experimental Result ofVg and Ig .

REFERENCES

[1] N. A. Rahim, Saad Mekhilef, Implementation of Three- Phase gridConnected Inverter for Photovoltaic Solar Power Generation SystemProceedings IEEE. PowerCon 2002. Vol 1, pp. 570 573., Oct 2002

[2] J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galvan, R. C.PortilloGuisado, M. A. M. Prats, J. I. Leon, N. Moreno-Alfonso, Power-Electronic Systems for the Grid Integration of Renewable Energy Sources:A Survey,IEEE Trans. on Industrial Electronics,vol. 53, no. 4, pp. 1002-1016, Aug 2006.

[3] S. B. Kjaer, J. K. Pedersen, F. Blaabjerg, A Review of Single-Phase Grid

Connected Inverters for Photovoltaic Modules, IEEE Trans. Ind. Appl.,vol. 41, no. 5, pp. 12921306, Sept/Oct. 2005.

[4] M. Meinhardt, G. Cramer, Past, Present and Future of Grid-ConnectedPhotovoltaic and Hybrid-Power-Systems, in Proc. IEEE-PES Summer

Meeting, vol. 2, pp. 1283-1288, July 2000.[5] S. J. Park, F. S. Kang, M. H. Lee, C. U. Kim, A New Single-Phase

Five-Level PWM Inverter Employing a Deadbeat Control Scheme, IEEETrans. Power Electronics., vol. 18, no. 18, pp. 831-843, May. 2003.

[6] J. Selvaraj, N. A. Rahim, Multilevel Inverter for Grid-Connected PVSystem Employing Digital PI Controller, IEEE Trans. on Industrial

Electronics, vol. 56, no. 1, pp. 149- 158, Jan 2009.[7] M. Calais A Cascaded Inverter for Transformerlss Single Phase Grid-

Connnected Power Electronics Specialists Conference, 2000. PESC 00.

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Fig. 9. Conventional multi-string three-level PWM inverter for PV applica-tion.

Fig. 10. THD result of multi-string five-level PV inverter.

2000 IEEE 31th Annual Volume 3, 18-23 June 2001 Pages: 1173 - 1178Vol.3.

[8] T. Noguchi, S. Togashi, R. Nakamoto, Short-Circuit Pulse-BasedMaximum-Power-Point Tracking Method for Multiple Photovoltaic-and-Converter System, IEEE Trans. on Industrial Electronics, vol. 49, pp.217- 223, Feb 2002

[9] R.Walker, P. C. Sernia, Cascaded DC-DC Converetr Connection ofPhotovoltaic Mudules, IEEE Trans.on Power Electronics. , vol. 19, no.4, pp. 1130-1139 , July 2004.

[10] S. Kouro, J. Rebolledo, J. Rodriguez, Reduced Switching-Frequency-Modulation Algorithm for High-Power Multilevel Inverters,IEEE Trans.on Industrial Electronics, vol. 54, no. 5, pp. 2894-2901, Oct. 2007.

Fig. 11. THD result of multi-string three-level PV inverter.

Fig. 12. Grid Voltage Vg and Grid Current Ig at near-unity power factor.

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