Control of Single-Phase Cascaded H-Bridge Multilevel Inverter with Modified MPPT for Grid-Connected Photovoltaic Systems

Chaiyant Boonmee Department of Electrical Engineering

Faculty of Engineering, Chiang Mai University Chiang Mai, Thailand

chaiyant.bm.aj@gmail.com

Yuttana Kumsuwan Department of Electrical Engineering

Faculty of Engineering, Chiang Mai University Chiang Mai, Thailand

yt@eng.cmu.ac.th

AbstractThis paper presents a control technique of cascaded H-bridge multilevel voltage source inverter (CHB-MLI) for a grid-connected photovoltaic system (GCPVS). The proposed control technique is the modified ripple-correlation control maximum power point tracking (MRCC-MPPT). This algorithm has been developed using the mean function concept to continuously correct the maximum power point (MPP) of power transferring from each PV string and to speedily reach the MPP in rapidly shading irradiance. Additionally, It can reduce a PV voltage harmonic filter in the dc-link voltage controller. In task of injecting the quality current to the utility grid, the current control technique based-on the principle of rotating reference frame is proposed. This method can generate the sinusoidal current and independently control the injection of active and reactive power to the utility grid. Simulation results for two H-bridge cells CHB-MLI 4000W/220V/50Hz GCPVS are presented to validate the proposed control scheme.

Keywords grid-connected photovoltaic systems; single-stage grid-connected inverter; maximum power point tracking.

I. INTRODUCTION Grid connected photovoltaic (PV) systems have had an

enormous increase in their market share over the last decade [1]. However the efficiency of commercial PV panels is around 15-25%. Therefore, it is very important that the power produced by these panels is not wasted, by using an efficient grid-connected inverter. Grid-connected inverter technologies for PV systems consist of 4 types: 1) centralized, 2) string, 3) multi-string, and 4) AC-module technology. The string technology is mostly used commercially and is more efficiency than the centralized but it cannot solve the mismatch losses and high voltage stress in power switches. The multi-string technology is developed by adding many strings, each interfaced with its own dc/dc converter, each string can be independently controlled to operate at the maximum power point (MPP) of the string, and is very flexible. However the multi-string has two states of power processing that cause more power losses in power switches, and copper losses in dc wires due to connecting many PV strings [2]. The transformerless cascaded H-bridge multi-level voltage source inverter (CHB-MLI) merges the benefits of

string and multi-string technology. It combines the improved maximum power point tracking (MPPT) independently for many strings connected in a series and operating as a single-stage power processing inverter. There are numerous advantages of using the CHB-MLI for grid-connected PV systems, such as low THD, small filter size, high efficiency, low stress voltage of power switches, and reduced losses from mismatch effect and partial shading. Additionally the CHB-MLI has many H-bridge cells series-connected, each H-bridge cell needs isolated dc-power sources, which can be easily supplied by PV modules or strings. From these features, the CHB-MLI topology is suitable for grid-connected PV systems (GCPVS) [3]-[4].

Fig. 1. The power scheme of single-phase CHB-MLI for GCPVS. The CHB-MLI topology is designed for supporting the

independent MPPT for each H-bridge cell. Furthermore, the characteristic of PV power is nonlinear and time varying caused by changing the atmospheric conditions [5]. Using the suitable MPPT algorithm can increase the efficiency of MPPT and GCPVS. Numerous MPPT techniques have been proposed such as hill-climbing, fractional open-circuit voltage control, P&O, incremental conductance (IncCond), fractional short-circuit current control, fuzzy logic control, neural network, ripple-correlation control and several others [6]. The ripple-correlation control MPPT (RCC-MPPT) is the one which is convenient for GCPVS with the following features: very fast convergence to reach MPP, parameter-insensitive MPPT of PV systems, several straightforward circuit implementations and well developed theoretical basis. It would be suitable for a

iPV1ig iC1

vPV1

vg

vab1

Lf

iC2 vPV2

iPV2 iinv2

vab2

S11

S12

S13

S14

S21

S22

S23

S24

C1

C2

vo

PV1

PV2

H-bridge1

H-bridge2

n1

n2

a1

n2

b1

a2

b2

iinv1

978-1-4799-0224-8/13/$31.00 2013 IEEE 566

12

pv1 pv102 ff p v dt 12 pv102 ff p dt

12

pv102 ff v dt

12

pv2 pv202 ff p v dt

12

pv202 ff p dt

12

pv202 ff v dt

MPPV

dV

1 pv1 pv1p p v =

pv1p 1refv

pv1v

pv1v

PLL

*refp

*2refp

gv gdV

g

*refQ

g

delay

dq

PV1i

PV2i

pv2p

pv2v2 pv2 pv2p p v =

2refv

pv2v

g

dq

*ov

*gdi

*gqi

gdi

gqifL

fL

gdV*odv

*oqv

*2refp

PV1i

PV2i

PV1v

PV2v

gi

11S

12S

13S

14S

21S

22S

23S

24S

MRCC-MPPT Current Controller

dV

PS-CBPWM

gdV

gdV

*1refp

gcos

MPPV

crv

Fig.2 Proposed MRCC-MPPT, current control and modulation control techniques for two H-bridge cells CHB-MLI for GCPVS

modular application, which uses small converters and the applications requiring a high rate of convergence [7]-[8]. However, the conventional RCC-MPPT has some disadvantages relating to the definition of suitable time constants of the filters to generate the desired output signal for correct MPPT control. It slows down reaching the MPP of PV operating, especially in the case of rapid shading irradiance. In this paper, to overcome that disadvantage, the RCC-MPPT has been modified using the mean function concept to generate the corrected ripple signal and make the MPP operating more accurate. In the ac-side of GCPVS, to ensure that grid-connected inverters have to inject the sinusoidal current into the utility grid, according to the specific standards defined by the utility in each country. The control technique for injecting power to the grid in this paper is developed using the current control, based on the principle of rotating reference frame for the proposed single-phase CHB-MLI scheme. This emphasized that the injecting active and reactive power can be controlled separately.

This paper proposed the control method of CHB-MLI for GCPVS by using the modified RCC-MPPT method to guarantee the accurate MPP operating of power transfer from each PV string, and fast MPPT strings in case of rapidly changing of irradiation, and using the current control based on the rotating reference frame theory for injecting the active and reactive power to the utility grid.

II. THE PROPOSED CONTROL METHOD Fig. 1 shows the power scheme configuration of the single-

phase single-stage CHB-MLI grid-connected PV system consisting of two H-bridge power cells which are series-connected their output terminals. Each H-bridge cell has its own PV string power supply parallel-connected with the decoupling capacitor. The output terminals of CHB-MLI are connected to the grid through the inductive filter fL to reduce the derivative and harmonic distortion of the grid current gi . The output voltage of CHB-MLI ov is synthesized by summing the two cascaded H-bridge inverter output voltages. It can generate between three and nine voltage levels depending on the selected control technique, It can be given by

1 3 PV1( ).

n

o m m mm

v S S v=

= { }1 3, , 0,1 .m mS S (1) where m and n are the order number and number of cascaded H-bridge in the PV systems , respectively. The relationships of instantaneous PV power between grid power gridP , instantaneous capacitor power Cp , and instantaneous inductor power Lp , in case of unity power factor and neglecting power losses in CHB-MLI, is given by

PV C L grid (1 cos (2 ))p p p P t= + + (2) where PVp is the instantaneous PV power.

Fig. 2. shows the proposed control scheme for two H-bridge cells CHB-MLI consisting of three main parts; the modulation control technique, the MRCC-MPPT technique and the current control technique. The information of the proposed control technique can be detailed as follow.

A. Modulation Control From the Fig.1, the power switches are controlled with the

unipolar and phase-shifted carrier-based pulse width modulation (PS-CBPWM) techniques [9]. This technique can generate the high frequency in each voltage level of output voltage of , it is double the carrier frequency crf and multiplied by the number of cascaded H-bridge H , and given by

2o crf H f= . (3)

In case of the PV input voltage of both H-bridge are equal PV1 PV2( )v v= , the waveform of output voltage ov can be

produced of five voltage levels, for which the operating principle is shown in Fig. 3.

B. The Modified RCC-MPPT The most of MPPT algorithms are based on the tracking of

the maximum power operating point, which is the minimum derivative or partial derivative of the change in PV output power

p at the change in PV voltage v , it can be written as / 0p v = . The value of power derivative /p v is an

567

Fig. 3. Phase-shifted carrier-based PWM for a 2-H-bridge CHB-MLI with amplitude modulation ratio 0.9am = and frequency modulation ratio 7fm =

Fig. 4. The typical of p-v, i-v curves and the simulation result of power derivative /p v and the change of PV power p of a PV string. important variable to define the reference PV voltage refv in the dc-link voltage controller for reaching the MPP.

Fig. 4 shows the three curves of PV characteristic, i-v curve, p-v curve and power derivative /p v curve, where MPP is the top of p-v curve and the power derivative / 0p v = . The MPPT algorithms force the PV systems to track the MPP by regulating the PV voltage PVv .

The RCC-MPPT method uses the instantaneous PV power ripple PVp and the instantaneous PV voltage ripple PVv , which are the inherent alternative components in single-phase GCPVS, to find out the power derivative /p v .This method needs to know the oscillation frequency fos which is double the grid frequency f1, fos = 2f to correct the MPPT algorithm operating. The mean function concept is used for finding instantaneous PV voltage ripple and instantaneous PV power ripple as

PV PV PV v v v= (4) PV PV PV p p p= (5) where PVv and PVp are the dc components of PV voltage and PV power, respectively.

The power derivative /p v is given by

PV PV1

2 PV PV

PV PVPV PV

12

2

.2

t

t ft

t f

f p v dtp vp

v v vf v v dt

(6)

This paper proposes the modified RCC-MPPT (MRCC-MPPT) by using the mean function as the main process in order to overcome the complicacy to define the suitable time constant of using the 1st order HPFs and 1st order LPFs in the conventional technique [8]. It also generates a fast response for the desired output signal in case of rapidly shading irradiance.

From the simulation results in Fig. 4, the curve of power derivative /p v and the curve of the upper term of power derivative PV PV.p p v = can be met at the zero point at the same PV voltage, at the MPP. It indicates that the curve of p can be used to find the MPP correctly as the curve of power derivative /p v .

The block diagram of the proposed MRCC-MPPT and dc-link voltage controller are shown in Fig. 2. In this case, it has two sets of MRCC-MPPT dc-link voltage controllers for two H-bridge cells, which have the same operation. For example of H-bridge1, the MRCC - MPPT only uses the mean function to find out the mean term of instantaneous PV power

PV 1p and mean term of instantaneous PV voltage P V 1v . They are subtracted from the instantaneous PV power PV 1p and instantaneous PV voltage PV1v to keep their ripples and multiply them to get the product ripple PV1 PV1p v . The mean function is used again to find the output of p . This method still uses the sign function to generate +1 if the PV1 PV1 0p v > or -1 if the PV1 PV1 0p v

(a)

(b)

(c)

(d)

Fig. 5. The waveform of PV system during a case 1 irradiance profile, using the proposed MPPT method and PQ control scheme. (a) The shaded irradiance profile of both PV strings, (b) Power variables of PV system, (c) The PV voltages PV1 PV2 , v v and PV currents PV1 PV2 , i i of both PV strings, and (d)

Output voltage of CHB-MLI ov , grid voltage vg, and grid current .gi in stationary reference frame as

fg go

g go

i Vv dLi Vv dt

= +

(7)

where ,o ov v are the output voltage of CHB-MLI, ,g gV V are the grid voltage, and ,g gi i are grid current.

In the dq rotating reference frame, the active power p and the reactive power q are given by

1 21 2

gd gd

gd gq

p V i

q V i

=

=

(8)

where ,gd gqV V are the grid voltages , and ,g d gqi i are the grid currents in dq rotating reference frame.

In Fig. 2, there are two current feedback control loops in the current control scheme for the accurate control of grid currents gdi and gqi .To generate the proper grid voltage orientation, the voltage and current in (8) have to be transformed to the two-phase voltage and current in dq rotating

(a)

(b)

(c)

(d)

Fig. 6. The waveform of PV system during a case 2 irradiance profile, using the proposed MPPT method and PQ control scheme. (a) The shaded irradiance profile of both PV strings, (b) Power variables of PV system, (c) The PV voltages PV1 PV2 , v v and PV currents PV1 PV2 , i i of both PV strings, and (d)

Output voltage of CHB-MLI ov , grid voltage vg, and grid current .gi reference frame in order to generate the desired active and reactive power components. The reference output voltages of CHB-MLI *odv ,

*oqv produced by the controller are given by

* * *

f f* * *gdod gd gd

ggqoq gq gq

Vv i idL LVv i idt

= + + (9)

The modulation control signal of H-brigde1 is the reference output voltage *ov , which can be generated by transforming the output voltages reference of CHB-MLI *odv ,

*oqv in the dq

rotating reference frame to be the reference output voltages of CHB-MLI in stationary reference frame.

III. SIMULATION RESULTS The PV system structure and control scheme shown in Fig. 1

and Fig. 2 have been implemented in MATLAB/SIMULINK in order to verify the behavior of the proposed control scheme. The model of the PV module is the single-diode model, the characteristic of the cells and equations were implemented according to [10]. The two sets of 10 series-connected of PV panels supplied to each H-bridge in the CHB-MLI. The parameters of each PV panel and the parameters of the PV system are shown in Table I.

60

1 2 of PV , PVS 500

1000

PVt p invt P g1 P

C1 P

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2000

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P V 1 P V 2, p p inv1 inv2 , P P

PV1 PV2 , i i

vo

ig

300

300

266 V

0

66

0

60

W

2W/m

8 4

A

1.45 1.55 1.65 1.75 time (s)

vg V A

3000

1000

PV1 PV2 , vv PV1 vPV1 i

PV2 i

PV2 v266

V

66

60

vo

ig

300

300

0 0

60

1.45 1.55 1.65 1.75 time (s)

vg V A

8 4

A

of PV 2S of PV 1S

500

1000

PVt p

invt P g1 P

C1 P

4000

2000

C2 P

PV1 p PV2p

inv1 P

inv2 P

W

2W/m

1000

0

3000

569

A. MPPT with Rapidly Changing Irradiance In order to verify the effect of using the proposed MRCC-

MPPT technique to control the two H-bridge cells CHB-MLI for GCPVS in the two situations of rapidly shading irradiance. In case 1, both PV strings, PV1 and PV2, were shaded from irradiance in the same time and power from the irradiation as shown in Fig. 5(a). In case 2, only the PV string of H-bridge2 PV2 was under shading conditions and the PV1 was operated in constant irradiance as shown in Fig. 6(a). In both cases, the irradiance change was operated in 0.05 s period for the decreasing and increasing ramps. The irradiance change starts from 1000 W/m2 to 500 W/m2, waits at this level for 0.15 s, and increases again from 500 W/m2 to 1000 W/m2, with a constant slope. The temperature was considered 25c during the simulation.

Fig. 5 and Fig. 6 show the simulation results of the proposed control method in case 1 and case 2 of irradiance profiles, respectively. From the features of the PV system, the maximum total PV power generated by PV1 and PV2

PVtp was 4000 W in case of without shading, 2000 W in case 1and 3000 W in case 2.

Fig 5(b) and Fig 6(b) show the behaviors of the following variables in the condition of case 1 and case 2 respectively: the summing of both instantaneous PV powers PVtp , the summing of mean input power of two H-bridge inverters invtP , the active power to grid g1P , the instantaneous PV power of each PV string PV1 PV2,p p , the mean input power of H-bridge1 and H-bridge2 inverters inv1 inv2,P P , the mean power of capacitors C1 C2,P P . The power transferred to utility grid

g1P came from the summing of the input power of both H-bridge inverters, multiplied to the function of CHB-MLI and subtracted the power of inductor Lp . From the results, the PV system produced the maximum power for 4000W, 3000W and 2000W in case of without shading, case 1 and case 2 irradiation shading, respectively. Although, during the PV1 and PV2 could not receive the same value of power as case 2, the CHB-MLI and the proposed active and reactive power controller continuously injected the maximum power to the utility grid.

Fig. 5(c) and Fig. 6(c) show the behaviors of the following variables in the condition of case 1 and case 2 shading respectively: the instantaneous PV voltage of each PV string P V 1 P V 2,v v and PV current of each PV string P V 1 P V 2,i i . In case 1 shading conditions, the behaviors of PV voltages and PV currents supplied to both H-bridges were the same, the PV voltages P V 1 P V 2,v v were changed lightly and the PV currents

P V 1 P V 2,i i were decreased and increased explicitly following the profile of irradiation. In case 2 shading condition, only PV current supplied to H-bridge2 P V2i was decreased and increased follow the received irradiation, the PV voltage PV2v was changed a little to keep the MPP of PV2 string. From the results, the power from PV1 and PV2 can be controlled

Fig. 7. The power-voltage characteristic curves of PV string PV1 under a case 1 irradiance profile (S) 1000-500-1000 W/m2 using MRCC-MPPT

(a)

(b)

(c)

Fig. 8 Results of the active and reactive power control (a) Reference Grid currents *gdi , *gqi and grid currents gdi , gqi , (b) active power pg and reactive

power qg ,and the fundamental active power 1Pg and reactive power 1Qg ,

(c) the waveform of output voltage ov , grid voltage gv and grid current gi .

separately to transfer the maximum power from each PV string to the utility grid.

Fig. 5(d) and Fig. 6(d) show the waveforms of output voltage of CHB-MLI, grid current gi ,and grid voltage gv in the case 1 and case 2, respectively. The proposed control method can control the CHB-MLI to generate the five voltage levels with sinusoidal patterns and inject the sinusoidal current which in-phase to grid voltage continuously for providing unity power factor.

*gdi

gd i

gq i

g1

= gd gd2p V i

10

10

30

2000

0

0

4000

g1

= gd gq2q V i

ig

vg vo

300

300

0

60

V

0

60

ig

vg vo

g1 sin( ) = g g1 1Q V I

g1 cos( ) = g1 g1 1P V I

A

1.0 1.1 1.2 1.3 1.4 time (s)

2000

1000

0

21000 W mS =

2=500 W mS

pv (W )p B

C

A

0 100 200 300 PV (V)v

W/Var

A

*gqi

THD 3.26%ig = THD 45.95%vo =

263

MPPV

28.8 =

570

Fig. 7 shows the relationship curve between PV1 power PV1p versus PV1 voltage PV 1v in case 1. The curves started from the minimum PV power point at the point A and track to the MPP at the point B following the line of p-v characteristic curve of the irradiance 1000 W/m2. The value of PV1 power can be controlled to run around the MPP (2000W), until the irradiance profile was reduced to 500W/m2. It caused the value of PV1 power PVp to be decreased to operate around the new MPP, at point C. The irradiance profile was waited in this level 0.15 s and move back to the last level (1000W/m2) again. The value of PV1 power PV1p was increased and run around MPP at the point B (2000W) immediately. The MPP of the PV system can be tracked immediately by the proposed MPPT method.

B. Active and reactive power control

Fig. 8 shows the simulation results of active and reactive power controlled with the proposed control technique, during constant irradiation. In Fig. 8(a), the grid current ,gd gqi i can be controlled by adjusting the grid current references of the active current *gdi and the reactive current

*gqi . The dynamic

response of the CHB-MLI was simulated to three steps changed in reactive current reference *gqi . The step settings of reactive current reference were stepped from 0A to-10A, -10A to +10A, and +10A to 0A, respectively. They caused the injected reactive power gQ changed from 0 to -1555var, -1555var to +1555var, and +1555var to 0, respectively, which corresponded to reactive power g1Q calculated in fundamental formulas, as shown in Fig. 8(b).

In Fig. 8(c), it can be seen that the phase current of CHB-MLI can operate under the in-phase, lagging and leading power factor conditions with the phase-angles 1 were 0 ,

28.8+ , and 28.8 , respectively. The injected active power

gP is almost constant at 4000 W (PMPP). However, there was

some slightly changed of active power gP while the reactive

current reference *gqi was stepped. The output voltage of CHB-

MLI ov can be generated in five voltage levels with high output frequency 6 kHzof = while the PWM carrier frequency 1.5 kHzcrf = , The total harmonic distortion (THD)

of grid current gi and the output voltage of CHB-MLI ov are 3.26% and 45.95%, respectively.

IV. CONCLUSIONS This paper has presented a control technique of CHB-MLI

for GCPVS. The MRCC-MPPT method by using the mean function to be the main process of the maximum power transferring is proposed in order to correct the MPP operating of PV string continuously and quickly reaching MPP in case of the rapidly shading irradiation. The current control technique based-on the theoretical rotating reference frame to

control the injecting of active power and reactive power independently, is proposed. From the results, they can be guaranteed that the proposed algorithm cause high accuracy and fast respond for correction the MPP in case of rapidly shading irradiance and it do not need the harmonic filter in the dc-link voltage controller. The proposed method can generate the sinusoidal grid current with low THD of grid current. Simulation results confirm the correction and reliability the developed control scheme and algorithm method with two situations, full and half PV string shading.

TABLE I MAIN DESIGNED PARAMETERS OF PV SYSTEM

Parameters Symbol ValuePV panel MPP voltage in STC VMPP 26.3 V MPP current in STC

IMPP 7.61 A PV string for each H-bridge cell Rated current Irated 7.61 A Rated MPP voltage Vrated 263 V Rated maximum power

Prated 2000 W Power and control scheme DC-link capacitor C1, C2 2000F PWM carrier frequency fcr 1.5 kHz Single-phase utility grid 2g gV V =

f1 220 VRMS

50 Hz Inductor Lf 5 mH dc link voltage controller: PI Kp, Ki 25, 200 Grid current controller: PI Kp, Ki 15, 40

Ambient Temperature T 25C

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selected IEA countries between 1995 and 2011, International Energy Agency Photovoltaic Power Systems Programme, Tech. Rep. IEA PVPS T1-21:2012, August 2012.

[2] M. Malinowski, K. Gopakumar, J. Rodriguez, and M. A. Prez, A survey on cascaded multilevel inverters, IEEE Trans. Ind. Electron., vol. 57, no. 7, pp. 21972206, Jul. 2010.

[3] J. Rodrguez, J. S. Lai, and F. Z. Peng, Multilevel inverters: A survey of topologies, controls and applications, IEEE Trans. Ind. Electron., vol. 49, no. 4, pp. 724738, Aug. 2002.

[4] E. Villanueva, P. Correa, J. Rodrguez, and M. Pacas, Control of a single-phase cascaded H-bridge multilevel inverter for grid-connected photovoltaic systems, IEEE Trans. Ind. Electron., vol. 56, no. 11, pp. 43994406, Nov. 2009.

[5] R. Kadri, J.-P. Gaubert, and G. Champenois, An improved maximum power point tracking for photovoltaic grid-connected inverter based on voltage-oriented control, IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 6675, Jan. 2011.

[6] T. Esram and P. L. Chapman, Comparison of photovoltaic array maximum power point tracking techniques, IEEE Trans. Energy Convers., vol. 22, no. 2, pp. 439449, Jun. 2007.

[7] T. Esram, J. W. Kimball, P. T. Krein, P. L. Chapman, and P. Midya, Dynamic maximum power point tracking of photovoltaic arrays using ripple correlation control, IEEE Trans. Power Electron., vol. 21, no. 5, pp. 12821291, Sep. 2006.

[8] D. Casadei, G. Grandi, and C. Rossi, Single-Phase Single-Stage Photovoltaic Generation Based on Ripple Correlation Control Maximum Power Point Tracking, IEEE Trans. on Energy Conversion, vol. 21, no. 2, pp. 562-568, June 2006

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