flying capacitor multilevel topology for grid … · flying capacitor multilevel topology for grid...

FLYING CAPACITOR MULTILEVEL TOPOLOGY FOR GRID CONNECTED PV POWER SYSTEM

ABINADABE S. ANDRADE1, EDISON R. DA SILVA2,3, MONTIÊ VITORINO2

1Post-Graduate Program in Electrical Engineering - PPgEE – COPELE 2LEIAM, DEE, Federal University of Campina Grande

3Federal University of Paraiba

Av. Aprígio Veloso 882, Bloco CH, Campina Grande, PB – CEP 58429-900 E-mails: [email protected], [email protected], montiê[email protected]

Abstract This paper proposes a configuration that allows connecting a PV panel to the power grid. It is composed of a boost

converter, a fly-back converter, and a four-level flying capacitor (FC) inverter. Differently from other possibilities, the floating

capacitor voltage is regulated independently from the multilevel converter DC-link voltage. This simplifies the tasks of the in-verter control. Current control is used together with a Maximum Power Point Tracking (MPPT) algorithm in order to have max-

imum power transfer from the PV string to the grid. The system also controls the grid current for unity power factor operation.

Simulation and experimental results confirm the feasibility of the proposed system.

Keywords PV power system, flying capacitor multilevel inverter, power electronics, renewable energy.

1 Introduction

Among the renewable energy sources, wind and

solar ones became very popular ones. In special, the

sun furnishes more energy to the earth in one hour

than the global consumption in an entire year. In

recent years PV isolated or grid-connected systems

have been more and more attractive since costs have

been reduced. Also, the photovoltaic (PV) industry is

having an annual growth of 40% per year for the last

decade (Kroposki et al., 2009). In general, a DC–DC

boost converter is connected between the PV array

and the load or the energy storage element for ex-

tracting the maximum power point tracking (MPPT)]

from the PV array (Miyatake et al., 2011). It is also

able to regulate the voltage and load current, and the

power flux when the system is connected to the grid.

For AC application, inverters are necessary and dif-

ferent topologies have been employed in PV conver-

sion. Multilevel converters are proper alternatives for

medium and high power applications. Neutral Point

Clamped (NPC), cascaded H-Bridges, Flying Capaci-

tor (FC) converters, multilevel boost converter, t dual

inverter and other, have been used in PV systems

(Kjaer, 2005),(Li, Wolfs, 2008),(Ozdemir et al.,

2009),(Kouro et al., 2010),(Khrishnamoorthy et al.,

2013), (Mousa et al., 2009), (Trabelsi and Brahim,

2011), (Safiyi et al., 2012), (Baradani et al., 2011).

All topologies have proper advantages but also

disadvantages. For example, in NPC, the voltage

clamping capability of the clamping diodes varies

with the number of levels. The cascaded H-bridge

configuration is scalable but has the disadvantage of

using multiple insulated dc sources. In FC converters

the number of floating capacitors increases with the

number of levels. Although less studied for use in PV

systems, the Flying Capacitor (FC) converter has

been shown to be suitable for that application (Tra-

belsi and Brahim, 2011), (Safiyi et al., 2012), (Bara-

dani et al., 2011). One advantage of this converter it

is able to have four-level operation but with the same

structure of the three-level inverter. For this dc-link

and the storage capacitor voltages must have differ-

ent values (Kou et al., 2002).

In the conventional FC inverter, the load current

and capacitor voltages must be jointly controlled. In

(Safiye et al., 2012) a boost converter is used as a

first stage to boost the PV voltage to the grid level; at

second stage the FC inverter performs the MPPT

function, and also controls the grid current for unity

power factor. In (Baradani et al., 2011) the stages are

the same except that the boost converter controls the

PV voltage and step it up to the requested constant dc

link voltage and the FC converter only converts the

proper dc voltage to grid synchronous AC voltage. In

(Baradani et al., 2011) the MPPT control is similar to

that in (Safiyi et al., 2012). A problem with these

control approaches is the complicated control strate-

gy to regulate the floating capacitor voltages.

In this proposed paper, a PV panel fed multilevel

dc-dc boost converter regulates directly the floating

capacitor voltage of the multilevel FC converter, thus

simplifying the tasks of the inverter control. Current

control is used together with a MPPT algorithm in

order to furnish the grid the energy generated from a

PV string. The system also controls the grid current

for unity power factor operation. Simulation and

experimental results confirm the feasibility of the

proposed system.

2 Proposed System

The simplified configuration of the proposed system

for interconnecting the PV string and the voltage grid

is introduced in Figure 1. It consists of a PV source

in series with a dc-dc converter composed of a non-

isolated boost converter in series with a fly-back

converter, here simply referred as boost and fly-back,

respectively. The boost converter controls the extrac-

tion of the maximum possible power (MPPT) from

the PV string. Its output capacitor (C1) feeds both the

fly-back converter and the floating capacitor of the

single-phase FC converter. The output capacitor of

Anais do XX Congresso Brasileiro de Automática Belo Horizonte, MG, 20 a 24 de Setembro de 2014

973

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the fly-back converter corresponds to the capacitors

(C2+C3) thus determining the dc-link voltage, VDC.

The dc-link voltage is then converted to ac voltage of

which the number of levels is obtained with the help

of the capacitor C1 voltage and an adequate modula-

tion control (Kou et al., 2002). The inverter also

controls the line current for unity power factor opera-

tion. The floating capacitor voltage is traditionally

regulated at 21 DCC VV resulting in 3-level opera-

tion, but this through the inverter PWM control tech-

nique. In this paper, that voltage can be independent-

ly regulated at 3/1 DCC VV for 4-level operation

using the same three-level structure and a simpler FC

inverter PWM control. In the following, details are

given on both the boost and the fly-back converters

control and on the PWM technique used for the FC

converter.

Figure 1. The proposed topology for connecting the PV-

string to the grid.

A. Modulation for Flying Capacitor Inverter

A hybrid PWM strategy based in (Oliveira et al.,

2004) was used to generate the switching pulses for

the converter operation. Table 1 shows the possible

switching states for a classical three-level FC con-

verter ( 21 DCC VV ) (Lee et al., 2003) and for 4-

level ( 3/1 DCC VV ) operation. Note that in the 3-L

case, in which 21 DCC VV , both states ‘2’ and ‘3’

result in 0aoV .

Table1. Switching States.

State S1 S2 Vao (3L) Vao (4L)

1 1 1 VDC/2 VDC/2

2 1 0 0 VDC/6

3 0 1 0 -VDC/6

4 0 0 -VDC/2 -VDC/2

The principle of the 4-L modulation PWM is pre-

sented in Figure 2, where Ts is the switching interval.

When 2/6/ *

DCaoDC VVV , the pole voltage will

vary between states ‘1’ and ‘2’. Considering that the

ON state of a switch is represented by the digital

number 1 and the OFF state by 0, then 11S and

12 S during the interval of time aT and 02 S

during the interval of time bT . From similarity of

triangles, it can be seen that *

2 aoDC

a VV

P ,

s

DC

aa T

VP

T *)2(

3 and

aSb TTT as shown in

the figure). For 6/6/ *

DCaoDC VVV , the pole

voltage varies between states ‘2’ and ‘3’. In this case

11S and 02 S during the time interval aT , and

01S and 22 S during bT . For the interval

6/2/ *

DCaoDC VVV , the pole voltage varies

between states ‘3’ and ‘4’. In this case 01S ,

12 S during the time interval aT , and 02 S dur-

ing bT .

B. PV panel and MPPT algorithm

The PV voltage (Vpv) is determined according to

the switching duty-cycle of the boost converter.

When 1aS , Lpv VV and when 0aS ,

1CLpv VVV , where pvV is the PV voltage,

LV is

voltage across the inductor, and 1CV is the voltage

over the floating capacitor C1. Then,

for 1aS , L

V

L

V

dt

di pvLL , (1)

Ts/2 Ts/2

PaVao*

0

01

01

S2

S1

TaTb

Vdc/2

Vdc/6

-Vdc/6

-Vdc/2

(a) 2/6/ *

DCaoDC VVV

Ts/2 Ts/2

PaVao*0

01

01

S2

S1Ta

Tb

Vdc/2

Vdc/6

-Vdc/6

-Vdc/2

(b) 6/6/ *

DCaoDC VVV

Ts/2 Ts/2

PaVao*

0

01

01

S2

S1

Ta

Tb

Vdc/2

Vdc/6

-Vdc/6

-Vdc/2

(c) 6/2/ *

DCaoDC VVV

Figure 2. Principle of the 4-level PWM strategy.


974

S1

S2

ALoSa

SbO

S1

S2

{+

-+

-

+

-

+

-

{ { CA {

PLLRd PImod PWM

+-

+-

S1 S2VDC

*

ILO cos(Ɵ )*

Vao*

++Vc1

Vc1*

Rc

MPPT

+-

Ra +-

-+

Rb

VV

VV

VpvIpv

Vpv*

Ic* IL*

IL

VL*

+ -

MPPT Control Flying Capacitor

Control DC-Link and Factor Power Control

VV

VV

+ -

d*

VC1 VC2

VC3

+

-

VDC

+ -

L

VDC

iLO* iLO

VL

Ic

Figure 3. Proposed system including the constituent parts of the control system.

for 0aS 1

1Cpv

LL VVLL

V

dt

di . (2)

From (2) and considering that pvV is always posi-

tive, the state 1aS always results in an increment

in the current, since 0dt

diL . Instead, from (1)

and when 1CL VV , a decrement of current does

occur since 0dt

diL . The inductor current can be

used to control either the output voltage or the

boost input voltage. Since the output voltage 1CV is

controlled by the fly-back through Sb, the current is

then used to control the input voltage pvV . The

voltage reference comes from a MPPT algorithm

that defines the voltage reference that produces the

maximal power transfer (Kroposki et al., 2009.

Figure 3 shows the control scheme for the boost

converter. For achieve the MPPT, the method used

is that of the incremental conductance resulting in

the voltage reference *

pvV . The PV voltage error is

controlled by a PI controller. Note that the capaci-

tor voltage of the capacitor connected to the PV can

be controlled through the current Ci . The PI con-

troller output is the current reference *

Ci . Since the

control is achieved through the inductor current,

and not by the the capacitor current, this relation-

ship can be found through Kircchoff’s law, that is,

CpvL iii (3)

Finally, the PWM reference is the output of an-

other PI controller fed by the inductor current error.

C. Proposed system control

In the proposed system control the FC inverter

capacitor voltage 1CV is controlled by the boost

converter through switch Sb. It is the input voltage

of the flyback converter, of which the output volt-

age is the inverter dc-link voltage DCV . Since

switch Sa controls the PV voltage, control of DCV is

achieved through the FC inverter current control.

The output voltage of the flyback converter is

given by d

d

N

N

V

V

C

DC

11

2

1

, in which d is the duty-

ratio defined by the switch Sb, In this work

12 NN . Also, voltage 1CV is compared to the

capacitor voltage reference value and the error

generates the duty-cycle required through the con-

troller Ra to force the actual capacitor voltage to its

reference value.

The dc-link voltage DCV is compared to its ref-

erence *

DCV and that voltage error originates the

required current reference through the controller

Rd, that generates the amplitude of the grid current

error *

0LI . The error of load current reference is

synchronized with the grid voltage in order to im-

pose the power factor close to unity. This is

achieved with the help of PLL (phase-locked-loop)

that furnishes the co-sine of the power angle re-

quested to generate the requested synchronized

current error. This error is processed by the control-

ler PImod. in order to generate the reference voltage

that defines the PWM switching of the FC convert-

er to regulate the dc-link voltage.

3 Simulation Results and Performance analysis

Simulated results have been obtained through

Matlab and PSIM. Table 2 shows the system data

used for simulation. Note that the flying capacitor

voltage is regulated at 60V (half of that the DC-

link) in case of 3-L operation and at 40V (1/2 of

that the DC-link) in case of 4-L operation. The PV

module considered has 36 PV cells and a variant

irradiation in the range from 900 to 1100 W/m2 at a

temperature or 25º C, as shown in Figure 4(a). The


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Table 2. System parameters used in simulation

Parameter Symbol Value

Boost Indutor L 7mH

Line Indutor LS 7mH

N1/N2 FlyBack a 1

Boost-FlyBack PWM carrier

freq fcc 10kHz

FC HPWM carrier freq. fs 1kHz

Boost input capacitor C 2200uF

FC DC-Linc capacitor C2,C3 4400uF

FC Flying capacitor C1 2200uF

DC-link voltage VDC = VC2 + VC3 120V

Flying cap. voltage for 3 level Vc1 60V

Flying cap. voltage for 4 level Vc1 40V

Grid voltage Vca 50V

radiation conditions produce an average power of

67 W. The power factor obtained is near unity, as

shown in Figure 4(b), for preliminary values of grid

voltage (50 V) and current (2.5 A peak-to-peak).

Figures 4(c) and 4(d) depict the three-level and

four-level FC inverter pole voltages (phase volt-

age), respectively. The THD reduces from 3.24%,

in case of the 3-L operation, to 2.88%, in case of 4-

L operation, both at 1 kHz. As expected, the system

dc-link voltage is regulated and the floating capaci-

tor ripple are maintained at low levels. Figure 4(e)

shows that the peak-to-peak ripple is 3.2 V in the

dc-link voltage (top) and 0.9 V in the floating ca-

pacitor voltage (bottom) in case of 3-L operation.

As shown in Figure 4(f), in case of 4-L operation

the peak-to-peak ripple is 0.66 V in the dc-link

voltage (top) and 0.8 V in the floating capacitor

voltage (bottom).

4 Experimental Results

Preliminary experimental results have been ob-

tained with the help of a DSP with a switching

frequency of 10 kHz. Same parameters used for

simulation have been adopted to verify the feasibil-

ity of the proposed system. The waveforms in Fig-

ure 5(a) confirms the good power factor obtained at

the grid, while Figure 5(b) verify the pole voltage

for 3-L operation. Also Figure 5(c) confirms the

results obtained with simulation in terms of DC-

link voltage (top) and flying capacitor voltage (bot-

tom) ripples are confirmed. More real results are

being obtained.

5 Conclusion

This paper proposed a new grid-connected

photovoltaic power system based on a three-level

flying capacitor (FC) inverter that can also operate

with four levels and with grid power factor control.

The system is composed by a boost converter and

(a) Solar panel: Pmax and Po.

(b) Voltage and current in the grid

(c) Pole voltage of the FC inverter to 3 Level

(e) Vdc and Vc1 for 3 Level

(d) Pole voltage of the FC inverter for 4 Level


976

(f) Vdc and Vc1 for 4 Level

(g) Vc2 and Vc3 for 4 Level

Fig. 4. Proposed system: simulation results

a flyback converter besides the FC inverter. The

system control allows to extract the maximum

power from the PV panel under irradiation condi-

tions. Instead of being regulated by the inverter

control, as in other PV systems, the floating capaci-

tor is independently regulated by the boost convert-

er. This eliminates the floating capacitor voltage

control through the inverter PWM strategy, which

is simplifies. Results have shown that the THD and

capacitor voltages ripple values are as expected.

Experimental results to validate the theretical and

simulation results.

Acknowledgment

Authors are grateful to the Coordenação de

Aperfeiçoamento de Pessoal de Nível Superior

(CAPES), the Conselho Nacional de Desenvolvi-

mento Científico e Tecnológico (CNPq) and the

Fundação de Apoio à Pesquisa da Paraíba

(FAPESQ) for funding this research.

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flying capacitor multilevel topology for grid … · flying capacitor multilevel topology for grid...

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