Single DC-link Cascaded H-bridge Multilevel Multistring Photovoltaic Energy Conversion System

with Inherent Balanced Operation S. Koura, C. Fuentes, M. Perez, and J. Rodriguez

Department of Electronic Engineering

Universidad Tecnica Federico Santa Maria

Valparaio, Chile

Email: samir.kouro@ieee.org

Abstract-Large-scale photo voltaic energy conversion systems (LS-PECS) are currently well into the megawatt range. Therefore multilevel converters have been proposed as an attractive solution due to medium voltage operation, higher efficiency, power quality, and possibility to connect PV strings separately to each dc-link. However, the latter advantage makes the inverter susceptible to severe voltage unbalances due to the inherent power unbalance produced by the individual maximum power point tracking (MPPT) of the PV strings. This is particularly the case of the cascaded H-bridge inverter (CHB) where power cell and phase unbalances occur. In this work a new single dc-bus collector bus bar CHB is proposed for a multistring LS-PECS configuration. The operation with a single dc-bus is enabled by adding an isolation dc-de converter stage to each power cell of the CHB. This allows inherent balanced operation of the CHB while fully decoupling the multistring PV-system from the grid tie inverter. This also greatly extends the operating range of the system compared to previous solutions. Simulation results for a 3.3kV 7-level CHB tested under dynamic conditions are presented and serve as a preliminary validation of the configuration and control schemes.

I. INTRODUCTION

Photovoltaic energy conversion systems (PECS) are the

fastest growing energy source of the last 5 years [ 1]. Par­

ticularly large-scale power plants (> > 10 MW) have now

become a reality, with more than 50 plants over 30 MW

installed capacity, with the largest currently in operation rated

at 2 14 MW [2]. However, despite the accelerated growth of

large-scale PECS (LS-PECS), the power converter interfaces

commercially available have remained concentrated in two

main types: the centralized configuration with a three-phase

2-level voltage source grid tie inverter [3], and the multistring

configuration with dc-dc converters for each string and a three­

phase 2-level voltage source grid tie inverter [4].

The centralized configuration concentrates a great amount of

PV strings (composed of series connected PV modules to reach

the desired voltage), which are paralleled to reach the power

rating of the plant. Hence, the centralized inverter can only

provide a single MPPT for the whole system, reducing energy

generation in case of partial shading, temperature differ­

ence and module mismatch [3]. The multistring configuration

overcomes this disadvantage of the centralized topology by

introducing a dc-dc stage (usually a boost converter) between

the PV strings and the PV dc-bus collector bar of the grid

tie inverter. This enables independent MPPT of each string,

increasing energy conversion efficiency [5].

In practice, both configurations operate at low voltage (690

V ), which limits the use of a single three-phase 2-level voltage

source inverter up to 0.8MW, with current semiconductor tech­

nology. Hence, paralleling or using multiple power converters

is required to cover a multi-megawatt plant. However, using

several converters to share the total power of the plant does

not bring any additional benefit in terms of power quality

and efficiency. For this and other reasons multilevel converters

have been proposed for PV applications, most of which are

based on the neutral point clamped (NPC) inverter [6- 12] and

the cascaded H-bridge (CHB) inverter [ 13- 19] topologies.

In [6-1 1] the NPC multilevel inverter is used to interface

a single PV string per dc-link capacitor of the corresponding

NPC topology. This approach does not need a dc-dc stage like

the multistring topology and effectively doubles the operating

voltage, and enables MPPT of two strings. However, this

solution is not suitable for LS-PECS, since it allows only

two string connections for a commercial three-level NPC (3L­

NPC). This severely limits the size of the PV power plant

managed by the multilevel converter. This has been addressed

by using the NPC in a multistring configuration in [ 12], where

the dc-dc stage enables independent MPPT tracking of a great

number of strings and elevates the voltage for the grid tie NPC

inverter to operate at medium voltage.

On the other hand, the CHB multilevel converter has been

continuously reported for PV applications, by exploiting a

feature that is considered a drawback when operating as

inverter in motor drive applications, which is the need of

an isolated dc source for each power cell. However, in PV

systems, the strings naturally provide the isolated dc-sources.

This allows the inclusion of several strings with independent

MPPT and provides inherent voltage elevation due to series

connection of the H-bridges. However, this advantage comes

along with a great drawback, which is the inherent power

imbalance among cells in each converter leg, and consequently

power imbalance among the different phases. This explains

978-1-4673-2421-2/12/$31.00 ©2012 IEEE 4998

why most of the contributions in the literature are focused in

the single-phase solution [13- 18]. In [ 19] the phase unbalance

issue is addressed by introducing a zero sequence in the

reference signals with a feed-forward compensation of the

per-phase unbalance, shifting the neutral of the converter

while keeping balanced line to line voltages, which solves the

problem. Nevertheless, the operating range and performance

can be limited when operating closer to rated conditions, since

the balancing mechanisms are introduced in the modulation

stage, which could enter in over-modulation.

In this work both the per-cell and per-phase power unbal­

ance issue in the CHB are eliminated by concentrating the

whole PV generated power in a single dc-bus collector bar.

The isolation required for the proper operation of the CHB

is achieved by introducing an isolated dc-dc conversion stage,

between the dc-bus collector bar and the dc-link capacitor of

each H-bridge cell. The dc-dc converter used in this work is the

flyback converter, mainly due to its simplicity, low cost and its

buck-boost characteristic which allows to extend the operating

range and flexibility of the system. Nevertheless, the concept

of the proposed single dc-bus bar CHB can be implemented

with other isolated dc-dc converters. The analysis to determine

the optimal islated dc-dc stage for this particular configuration

is matter of future work and is not addressed in this paper.

The proposed LS-PECS configuration greatly simplifies the

control of the system since the PV MPPT, dc-power and ac­

power control are fully decoupled. In fact, traditional Voltage

Oriented Control (VOC) can be used for the grid tie CHB

MPPT boost

converters

/ I

I I I I I I I

+

u � co .c

11.

�

I vee I

inverter to control the grid currents and dc-bus collector bar

voltage without need of any balancing algorithm. This is

possible thanks to the single dc-collector and the inherently

balanced power distribution among cells. In addition, possible

small voltage drifts or unbalances that could occur due to dif­

ferences in the capacitors among the cells or due to differences

in initial conditions are easily compensated by the duty cycle

of the intermediate isolation dc-dc converter.

The paper covers the description of proposed LS-PECS

configuration, the different control schemes used for the MPPT

converters, the isolation dc-dc converters, and the grid tie

inverter. Simulation results for a three-cell per phase CHB (7-

level) connected to a 3.3kV medium voltage grid are included.

II. CONFIGUR ATION DESCRIPTION

The proposed large-scale photovoltaic energy conversion

system is shown in Fig. 1. The system is essentially a mul­

tistring configuration with several PV strings connected to a

common dc-bus bar through MPPT dc-dc converters, followed

by a centralized grid tie inverter. The MPPT dc-dc converters

decouple the grid tie inverter from the PV system, and are

controlled independently ensure MPPT of the corresponding

string. Therefore, the PV plant can combine different MPPT

dc-dc converters and PV strings, rated at different voltages and

power, provided the MPPT dc-dc converter can work at a duty

cycle relating input and output dc voltages. This is possible

since the dc-bus bar voltage is fixed (controlled by the grid­

tie inverter). One of the most commonly used MPPT dc-dc

Strings

,---------------------Cascaded H-bridge multilevel converter with single dc-link

-----N-----------------

Fig. 1. Single dc-bus collector bar 7 -level cascaded H-bridge multilevel multi string photovoltaic energy conversion system.

4999

topologies is the boost converter, shown in Fig. 2a, which is

the one considered in this work. In addition, PV modules and

strings can be of different type.

The grid tie inverter is a cascaded H-bridge multilevel

converter with a single dc input, in this case the dc-bus

collector bar. In order to enable proper operation of the CHB,

the input to each H-bridge must be an isolated dc-source,

therefore a medium frequency isolation transformer power cell

(isolated dc-dc converter) is inserted between each H-bridge

and common dc-bus bar. Since all the cells are connected to the

same dc-bus bar, the power can be evenly distributed among

the H-bridges. This feature eliminates all power unbalances

produced by individual MPPTs of the strings experienced in

other multilevel converter based PV systems both single- and

three-phase [ 13-15], [ 19]. As a result the operational range of

the system is extended, increasing power output and improving

performance. The dc-dc isolation converter can be selected

among a wide variety of switching mode dc-dc converters. In

this work the flyback converter, shown in Fig. 2b, has been

selected for sake of simplicity, lower cost and simple control.

Nevertheless, other isolation dc-dc converter may be used.

The cascaded H-bridge inverter is commercialized com­

monly with three power cells in series per phase to reach 3.3kV

rms line-to-Iine voltage with low voltage IGBT devices (the

six-cell CHB for 6.6kV operation is also commonly found)

[20]. Each H-bridge, shown in Fig. 2c, can generate three

voltage levels (-Vij, 0, Vij), where i stands for the phase (a, b or c) and j the cell number (1, 2 or 3). Considering a 1: 1

turn ratio in the flyback converter stage and a a steady-state

rated duty cycle of D = 0.5, the H-bridge dc-link voltages can

be considered approximately constant and equal to Vij ;::::: Vde ('Vi E {a, b,c} and j E {I, 2,3}). The output voltage of the H­

bridge cell is defined by the binary gating signal combinations

of both legs (Sl, S2), hence four switching states. As defined

in Fig. 2c, the output voltage can be expressed in terms of the

gating signals by Vij(Sl - S2 ) . Since the H-bridge cells are

connected in series at the ac side, the total converter output

voltage of a phase of the CHB is defined by

i = a, b, c ( 1)

The CHB is traditionally modulated using phase-shifted

PWM (PS-PWM) [20]. This modulation scheme is composed

of individual carrier based unipolar PWM for each H-bridge

cell, with a 60° phase shift among the carriers of all cells

of each phase to produce the characteristic multilevel stepped

waveform [2 1].The phase shift among carriers varies depend­

ing on the number of cells ( l800/k, for a CHB of k cells per

phase). The phase-shift between carries also produces a fre­

quency shift of the switching harmonics in the output voltage

to 2kfer, for a k-cell per phase CHB modulated with a carrier

frequency of fer. This improves current THD and power qual­

ity, without increasing switching frequency and losses. In fact,

this enables the reduction of the switching frequency without

compromising performance. Usually the carrier frequency, and

Fig. 2. Power circuits of the different converter blocks of the proposed configuration shown in Fig. l.

hence average device switching frequency is around 500Hz

(first switching harmonics appear around 3kHz, for a 3-cell

per phase CHB). An additional benefit of PS-PWM is the fact

that all cells process same power, and are all simultaneously

used over the whole modulation index range. For these reasons

PS-PWM has been selected in this work.

The flyback converter is basically a buck-boost dc-dc con­

verter where the inductor has been replaced by a medium to

high frequency isolation transformer. The purpose of including

an isolation dc-dc stage in the proposed configuration is to

enable a single dc-bus collector bar compared to the solution

in [19]. As can be appreciated in Fig. 2b, the flyback converter

has only one active controlled semiconductor. During the ON

state, the flux builds up in the primary of the transformer

while the diode is not conducting producing a reduction in

the output voltage. When the switch is OFF, the stored flux in

the primary produces a current in the secondary winding; the

diode conducts increasing the output voltage. The transformer

turn ratio and the duty cycle determine the output/input voltage

conversion, which in steady state is given by

(2)

where D fb is the flyback duty cycle and Nd N2 the trans­

former primary to secondary winding ratio. This ratio enables

an additional design parameter to adjust the voltage level in

addition to the boost stage and the number of H-bridge cells

in series. This allows the configuration to be very flexible and

adapted to different specifications.

III. CONTROL

One of the advantages of the proposed LS-PECS is that the

control system and control goals can be easily divided among

the converters using traditional control schemes without need

of special voltage balance mechanisms and other feedforward

or compensation methods. The following sections describe the

control scheme of each converter stage.

5000

(a)

(c)

(b)

A. Grid tie inverter control

The main function of the grid tie inverter is to control

active and reactive power, or seen differently, the grid currents

and the dc-link capacitor voltage. Among the most conunonly

used control schemes for grid tie inverters (or active front

end rectifiers) is Voltage Oriented Control (VOC) and Direct

Power Control (DPC) [22]. Since the extension of DPC for

multilevel converters is not straight forward, VOC has been

selected. However, CHB converters do not have a single dc­

link capacitor, which requires some modifications to traditional

VOC, as presented in [ 12- 15], [ 19]. In previous works a

fictitious total converter voltage (sum of dc-link voltages), or

equivalently the active power is controlled with VOC, while

the real individual dc-link capacitor voltages are controlled

within the modulation stage. This is the origin of one of the

main limitations of such solutions, since the modulation stage

can enter easily in saturation for large unbalances or when

operating close to rated values (over-modulation).

In the proposed configuration this is not longer an issue,

since all cells are connected to a COlmnon dc-bus collector bar

processing the total active power of the system. Hence, instead

of controlling the H-bridge dc-link capacitors, the VOC grid

tie inverter loop is used to control the single dc-bus collector

voltage. The corresponding control scheme is shown in Fig. 3.

The VOC scheme, as the name suggests, uses a rotational dq

reference frame transformation oriented with the grid voltage

vector to transform all ac quantities to de values to simplify

control system design. The grid current in the dq frame can

be decomposed into the real part on the d-axis (isd), which

is aligned with the grid voltage vector, hence is proportional

to active power; and the imaginary part on the q-axis (isq), perpendicular to the grid voltage vector, hence proportional

to the reactive power. Therefore, the dc-bus collector bar

voltage Vdc is controlled with isd, while reactive power is

controlled with isq, as shown in Fig. 3. The dq currents

are controlled using PI regulators, whose outputs are the dq

reference voltages. These are converted to phase values to be

modulated by the CHB inverter with PS-PWM.

The outer voltage loop is also controlled with a PI regulator,

which outputs the reference d-axis current. The q-axis current

reference can be arbitrarily adjusted. Normaly, i;q is set to

zero, although during voltage dibs the grid operator might

request injecting reactive power to help support the grid, as is

currently the case in wind power plants.

For feedback purposes the three phase currents (ia, ib, i c) are measured and transformed to dq values. A proper grid

synchronization is required in order to have the dq frame

correctly aligned with the grid voltage vector. In this work

a PLL has been considered, for which the grid phase voltages

are measured.

Note that the VOC loop shown in Fig. 3 includes feed­

forward crossed compensation components for decoupling

between the dq current loops. This can explained using the

equivalent space vector circuits in static and rotational frame

Multistring : dc-bus bar:

Single dc-link

CHB

L,

Fig. 3. Voltage oriented control of the 7-level cascaded H-bridge grid tie converter.

Fig. 4. Space vector equivalent circuit of grid tie converter: a) stationary frame, b) synchronous rotating frame.

shown in Fig. 4a and b respectively.

From Fig. 4b the grid side voltage equation can be obtained:

. R L dis . L ' (3) Vs = Is s + sTt +Jws sIs +vc'

Then, decomposing (3) into the d and q components yields:

Note that Vsq is zero, since the dq frame is aligned with

the voltage space vector v s, hence there is no projection over

the q axis. From (4) and (5) it is clear there is a coupling

between the dq currents. In order to reduce the effects of one

variable over the other, the coupling terms can be feedforward

since the parameters are known and the involved variables are

already measured.

The active and reactive power are given by

3 - 3 P = '2�{vsis} = '2Vsdisd

Q _ 3o{ .. } _ 3 . - '2'0 VsIs - -'2Vsdtsq

(6)

(7)

Note that (7) can be used to compute the q-axis current

reference (i ;q) from the desired reactive power reference Q*,

which is usually zero.

The fact that VOC controls the dc-bus collector bar voltage

and that PS-PWM is used should ideally evenly distribute the

5001

power among the H-bridges of the converter and operate in

balanced conditions. However, since the capacitance of the

dc-links is not exactly the same in reality, and even initial

charge conditions may vary, there could be minor voltage drifts

in the capacitor voltages. However, this is not the case of

the proposed configuration, since the dc-dc isolation converter

stage is located between the dc-bus collector bar and the H­

bridge dc-link capacitors. Hence, the dc-dc isolation converter

(in this case f\yback) control scheme regulates this minor

differences and keeps the H-bridges fed with a constant and

isolated dc-source.

B. Flyback isolation converter control The main function of the dc-dc isolation converter stage

is to provide isolated dc-sources for the eHE. However, the

inclusion of this stage in the system configuration enables the

use of the extra control degree to improve system performance.

The active power regulated by the eHB grid tie inverter

controls the total dc-bus collector bar voltage Vdc, which is

in turn the input voltage for all the f\yback converters. Hence,

each f\yback converter can be used to control its corresponding

output voltage, or what is the same, the H-bridge dc-link

voltage.

This is an important advantage in performance of the

proposed configuration, since there is a double and decoupled

control stage of dc-power, namely the total dc-bus collector

bar voltage achieved by the eHB and the individual dc-link

voltages by the f\yback converters. In this way, perturbations

in the dc-bus collector bar voltage produced due to dynamic

changes in MPPT of the PV strings are not passed to the dc­

link voltages of the eHB and hence to the ac side, improving

grid-tie inverter performance (mainly power quality). In the

same way, perturbations in the grid voltage and grid harmonics

have reduced effects on Vdc and the PV-strings MPPT control.

The control scheme for each f\yback converter is shown in

Fig. 5. The eHB dc-link voltage error is controlled through

a PI regulator which adjusts directly the duty cycle of the

f\yback converter. Since Vdc is a controlled voltage, it can be

assumed as a fixed value. Hence, in steady state, the input and

output voltage for power cell ij will be related by (2).

C. PV-string MPPT boost converter control The main function of the boost converters is to perform

the MPPT of the PV strings. A secondary role of the boost

converter is voltage elevation to reduce series connection of

PV modules. The boost converter has only one active switch,

hence two switching states to control the MPP. The MPP of a

PV module or PV string can be either tracked by controlling

the PV current or the voltage. In this work the voltage MPPT

is considered. In order to perform voltage MPPT, the total

string voltage, or equivalently the the boost input capacitor

voltage needs to be controlled. However, the boost converter

switching device controls the boost inductor current, therefore

a traditional cascaded voltage and current control loop like the

one shown in Fig. 6 is used. The outer control loop regulates

the capacitor voltage Vst using a PI controller through the

y

L-__ �y I\� ____ �y� ____ �

Fig. 5.

Flyback output PWM voltage control

F1yback isolation converter control scheme.

________ �y--------JA------�y------�� MPPT Current control PWM

Fig. 6. PY string MPPT boost converter control scheme.

capacitor current ic. This current can be related with the boost

inductor current reference by

(8)

The inductor current is controlled using a PI controller

through the inductor reference voltage vf, which is then

modulated using PWM adjusting the duty cycle of the boost

converter.

Any voltage based MPPT algorithm may be used to generate

the outer loop reference voltage V;t. In this work, the well

known perturb and observe (P&O) algorithm is considered due

to simplicity [23,24]. The string current ist and string voltage

Vst measurements are required to compute the generated power

for the P&O algorithm. The same values are used for feedback

in other parts of the boost control loops. In addition, the boost

inductor current measurement is needed as feedback for the

boost converter control. Note that a necessary condition by

design for the proper operation of the boost converter and to

effectively control of the boost inductor current is Vdc > Vst. The output voltage of the boost converter, or equivalently

the dc-bus collector bar voltage Vdc is regulated by the eHB

grid tie inverter. Hence it can be considered as a fixed voltage

source. The boost converter control system essentially adjusts

the duty cycle to achieve the desired input voltage imposed

by the MPPT algorithm. In steady state, the boost controller

should settle the duty cycle of the boost converter Db fulfilling

the following voltage conversion condition

(9)

Note that since all PV strings are connected through boost

converters to the common dc-bus collector bar with individual

MPPTs, and because the bus voltage is fixed at Vdc, the current

injected by each boost will vary accordingly. This is essentially

the basis of the power unbalance problem for other multilevel

based PV systems with several dc-bus collectors [ 12-15], [19],

which is not the case of this proposed configuration.

5002

IV. RESULTS

The proposed LS-PECS and control scheme has been

simulated using Matlab/Simulink with PSIM for the control

algorithm and power electronics respectively. A three-phase

7-level CHB with three cells per phase connected to a 3.3kV

grid is considered for simulation results. Each H-bridge dc­

link is controlled to IIS0V. The PV module parameters where

set to emulate the SHARP NU-U23SFI PV module rated at

23SW, with approximately 8A and 30V output current and

voltage respectively, obtained for a MPP with a radiation of

1000Wm2 and panel temperature of 2SoC.

of the PV modules was changed from 700 to 1000Wm2 at

t = 0.5s. Note from Fig. 7a that due to the increase of injected

power, the dc-bus collector bar voltage increases slightly

until regulated back to its reference by the outer voltage

loop control of the CHB. To compensate for the additional

generated power the control loop increases the d-axis current

component accordingly, as shown in Fig. 7b. This effect can

also be appreciated in the three-phase ac grid currents. A

scaled version of the phase a voltage waveform of the grid

Considering the fact that the Flyback converter transformer

ratio used in this work is 1: 1 (this can be further analysed and

optimized) and that the step-up effort is only performed by

the boost converters of each string, a total of 30 PV modules

in series are considered to reach the desired H-bridge dc­

link voltage after boosting. As stated before, the size of the

string can be reduced by changing the transformer turn ratio

(this may be necessary depending on the PV module voltage

blocking insulation capability). In addition, to have a more

realistic simulation in terms of power output, 180 strings, each

with individual dc-dc boost stage have been connected to the

common dc-bus collector bar. This makes for S400 modules

rated at a total of 1.26MW. The P&O MPPT algorithm is

performed every Sms, with step changes in the PV string

voltage reference of 10V.

Figure 7 shows the dynamic performance of the grid-tie

inverter control scheme for a step change from 0.7 to 1 pu

in generated power. To achieve this transient, the irradiation

TABLE I SIMULATION PARAMETERS

Parameter Line to Line Grid Voltage

Grid Frequency Rated Power

Rated Current Input filter inductance Input filter resistance

H-bridge DC-link capacitance H-bridge DC-link voltage reference

DC-bus collector bar capacitance DC-bus collector bar reference voltage H-bridge device avge. switching freq.

Equivalent output freq. per phase No. of series connected PV modules per string

No. of PV strings Boost inductor

Boost PWM carrier frequency Flyback PWM carrier frequency

Flyback transformer ratio Open circuit voltage of module

Maximum power pont voltage of module Short circuit current of module

Maximum power point current of module

Symbol

VSll Is

Pnom isnom

Ls Rs CH Vij * Cdc Vdc* Isw Isw Ns Nst Lb Ib

fIb Nl/N2

Voe Vprn Vse fprn

'> 1.4 .............................. : ................................ . .............................. . � : Vdc : � 1.2 .............................. .. :'" _ '':' . ...: :'" :..:.. . ...: :.: :"':" '':' :.. :'" _.:.....: :..: _ . _ . ...: . ':' " .... _ ... :., ...... . � : Vdc : g 1D :

. . . . 0.8�----------------------�-----------------------L----------------------�

F -0.5 :'-'··········:'-'········· ·.�

0.45 0.5 0.55 0.6 Time [sl

Value 3.3 kVRMS

50 Hz 1.2 MW

300 ARMS 2 mH

0.1 mfl 2200 f.1,F 1150 V 4700 f.1,F 1150 V 500 Hz

3000 Hz 30 180

3 mH 5 kHz 5 kHz

1 37 V 30 V 8.6 A

7.84 A

Fig. 7. System dynamic response for a step change from 0.7 to 1 pu in generated power: a) dc-bus collector bar voltage, b) grid currents and d-axis current component isd, c) CHB grid-tie inverter 7-level three-phase voltages, d) grid active P and reactive Q power.

5003

has been added as a dashed waveform to show the proper grid

synchronization achieved by the PLL. The 7-level converter

phase voltages are shown in Fig. 7c. Note that the swell in the

dc-bus collector bar voltage during transient is not transmitted

to the multilevel converter voltage steps and grid currents since

the f\yback converter regulates the H-bridge dc-link voltages

in between. This can be appreciated in Fig. 8, where all the

f\yback outputs, i.e, the three dc-link voltages of each H-bridge

cell and each phase are depicted, during the same transient

operation. The increase in ripple amplitude of the H-bridge

dc-link voltages is due to the active power increase, hence dc­

current increase, which inherently produces higher capacitor

voltage oscillations. Finally, the active and reactive power of

the system are shown in Fig. 7d. The increase from 0.7 to Ipu

in power can be clearly appreciated. Furthermore, the reactive

power is kept to a constant zero average, even during transient

thanks to the decoupled isd and isq controllers.

To evaluate the advantage of having the dc-dc f\yback stage

in the PV system configuration, a step change in the reference

voltage of the dc-bus collector bar is introduced at t = O.4s,

from 1150 to 1400V, as shown in Fig. 9a. The grid-tie inverter

voltage loop control reduces the current during the transient

to stop injecting current to the grid to increase the dc-bus

voltage, as can be seen in Fig. 9b. Note however from Fig.

9c, that the H-bridge dc-link voltages remain controlled around

1 150V, since the duty cycle of the flyback converter changes

accordingly, as shown in Fig. 9d. This is possible due to the

fact the Flyback converter can operate as buck/boost converter,

buck in this case, reducing the average duty cycle below 0.5.

This result shows how the operating range of the system can

be extended compared to previous proposals with isolated

dc-bus collector bars per H-bridge. The fact that dynamic

1600

� 1400 " � 1200 '0 > 1000

800 1600 1400 � " E 1200

'0 > 1000 800

1600 1400 � " N 1200

'0 > 1000 800 0.45

Fig. 8. phase c.

0.5 0.55 Time lsi

H-bridge ceUs dc-link voltages: a) of phase a, b) of phase b, c) of

160t ' ••••••••• =t::EL

.

.

-�':� ..

_ - ----

..

---�-...

j I :: Lf=-_ · . _ . _

•• _-_

• . -'-.. __ . __ ._ .

. --'. .•. _ ._�_. _"_"-,'L' ._. _._ , ,_. _. ',L' ._. _, ,_ .. _.

_ .. ..L .

.. _ ._ .. _. _, ,_

. ,-, __ .. _ .. _._J_.-"

i]�i�iEJ§� - M .........

.......

..

...

...

. ; � 0'52�""""'. """ ".""""".'D�;""'.""" ............... .........

.

J:::� 0.44 L-__ -'--__ --'-__ -'-__ --'-__ --'--'-'-_--' __ --' 0.37 0.38 0.39 0.4 0.41 0.42 0.43 0.44 Time [sl

Fig. 9. Step change in dc�bus bar reference voltage vdc: a) dc�bus bar voltage, b) grid currents and d-axis current component isd, c) H-bridge cells dc-link voltages, d) change in duty-cycle of flyback converters.

940

j:::�. -' - .... ' . " .

o - V,t > 880 - V;t 860 "----'-__ L-_--'-__ -'-__ L-_---'-__ -'-__ -'---'

'"r V" � 6.0 !l. 5.0

4.0 ••••• I �j 0.4 0.42 0.44 0.46 0.48 0.5 0.52 0.54 Time lsi

Fig. 10. MPPT of a single PV string and generated power for a step change in solar irradiation at t = 0.5s.

changes and perturbations in the PV system are isolated by

the Flyback converter, and not transferred to H-bridge dc-link

voltages, improves overall performance, since the H-bridge dc­

link voltages are kept constant achieving a better grid current

regulation, which in turns is the one controlling the dc-bus

collector bar voltage. Hence, the incorporation of the Flyback

stage not only introduces isolation for the eHB, but adds

robustness and extends the operating range of the system.

For sake of completeness, the dynamic performance of one

of the PV strings is shown in Fig. 10. The P&O algorithm

locks the PV string voltage reference into three voltage levels,

which change to a different set of three voltage levels at t = 5s

due MPPT following the change in irradiation, as shown in

Fig. lOa. The increase of the generated power by the string

can be clearly appreciated in Fig. lOb.

5004

V. CONCLUSION

A new LS-PECS configuration based on a single dc-bus

multistring CHB grid tie inverter has been proposed. The

main characteristic is the inclusion of a isolation dc-dc stage

between the dc-bus collector bar and the H-bridge cells of

the CHB to enable an inherent balanced power share between

H-bridge cells, facilitating the grid-tie inverter control while

extending the operating range of the system.

The f\yback converter has been used due to lower cost and

simplicity, although conceptually any isolated dc-dc converter

can be used. Adding the extra dc-dc stage provides two stages

of dc-power regulation, reducing the effects of PV MPPT

transients on the ac side, and grid side perturbations towards

the MPPT control. This extends the operating range of the

system, before limited by the saturation in the modulation

stage produced by feedforward mechanisms, and increases the

availability, reliability and conversion efficiency.

Compared to commercially available 2-level low voltage

centralized or multistring configurations, the proposed systems

concentrates in a single converter power plants rated up to

several tens of MW [25], with the added benefit of medium

voltage operation (less step-up transformer requirements and

losses), reduced cable and filer size, higher efficiency and

improved power quality, among other advantages of multilevel

converters, but without the additional control challenges.

The concept of the isolation dc-dc stage can be used

for other applications of multilevel converter which require

multiple isolated dc-sources, for example single phase CHB

for small to medium scale PV systems, and the NPC-leg

based cascaded H-bridge, recently introduced commercially

for single-phase-low-power PV systems.

ACKNOWLEDGMENT

The authors gratefully acknowledge financial support pro­

vided by Fondecyt (no. 11 lO783), by Centro Cientifico­

Tecnologico de Valparaiso (CCTVal) N° FB0821 of Universi­

dad Tecnica Federico Santa Maria.

REFERENCES

[I] Renewable Energy Policy Network for the 21st Century, "Renewables 2011 global status report," available at http://www.ren2i.net!publications. 2011.

[2] pvresources.com, "Large-scale photovoltaic power plants: top 50 ranking," available at http://www.pvresources.comlPVPowerPlantslrop50.aspx.

[3] F. Blaabjerg, Z. Chen, and S. B. Kjaer, "Power electronics as efficient interface in dispersed power generation systems," IEEE Trans. Power

Electron., vol. 19, no. 5, pp. 1184-1194, Sep. 2004. [4] M. Meinhardt and G. Cramer, "Multi-string-converter: The next step in

evolution of string-converter technology," in in P roc. 9th Eur. Power

Electronics and Applications Conf, 2001. [5] l. Carrasco, L. Franquelo, l. Bialasiewicz, E. Galvan, R. Portillo,

M. Prats, l. Leon, and N. Moreno-Alfonso, "Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey," IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002-1016, August 2006.

[6] M. Calais and Y. G. AgeJidis, "Multilevel converters for single-phase grid connected photovoltaic systems-an overview," in Industrial Elec­tronics, 1998. P roceedings. iSlE '98. IEEE international Symposium on, vol. I, luI. 7-10, 1998, pp. 224-229.

[7] S. Busquets-Monge, l. Rocabert, P. Rodriguez, S. Alepuz, and l. Bor­don au, "Multilevel Diode-clamped Converter for Photovoltaic Gener­ators With Independent Voltage Control of Each Solar Array," IEEE

Trans. Ind. Electron., vol. 55, no. 7, pp. 2713-2723, luI. 2008. [8] R. Gonzalez, E. Gubia, J. Lopez, and L. Marroyo, "Transformerless

Single-phase Multilevel-based Photovoltaic Inverter," IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 2694-2702, Jul. 2008.

[9] T. Kerekes, M. Liserre, R. Teodorescu, C. Klumpner, and M. Sum­ner, "Evaluation of Three-phase Transformerless Photovoltaic Inverter Topologies." IEEE Trans. Power Electron., vol. 24, no. 9, pp. 2202-2211, Sep. 2009.

[10] E. Ozdemir, S. Ozdemir, and L. M. Tolbert, "Fundamental-frequency modulated Six-level Diode-clamped Multilevel Inverter for Three-phase Stand-alone Photovoltaic System," IEEE Trans. Ind. Electron., vol. 56, no. 11, pp. 4407-4415, Nov. 2009.

[11] L. Ma, X. Jin, T. Kerekes, M. Liserre, R. Teodorescu, and P. Rodriguez, "The PWM strategies of grid-connected distributed generation active NPC inverters," in Energy Conversion Congress and Exposition, 2009. ECCE. IEEE, Sep. 20-24, 2009, pp. 920-927.

[l2] S. Kouro, K. Asfaw, R. Goldman, R. Snow, B. Wu, and J. Rodriguez, "NPC multilevel multi string topology for large scale grid connected photovoltaic systems," in 2010 2nd IEEE International Symposium on Power Electronics for Distributed Generation Systems (PEDG 2010), 2010, pp. 400-4005.

[l3] O. Alonso, P. Sanchis, E. Gubia, and L. Marroyo, "Cascaded h-bridge multilevel converter for grid connected photovoltaic generators with independent maximum power point tracking of each solar array," in Power Electronics Specialist Conference, 2003. PESC'03. 2003 IEEE 34th Annual, vol. 2, June 2003, pp. 731-735 vol.2.

[l4] E. Villanueva, P. Correa, 1. Rodriguez, 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. 4399-4406, Nov. 2009.

[l5] S. Kouro, A. Moya, E. Villanueva, P. Correa, B. Wu, and l. Rodriguez, "Control of a cascaded h-bridge multilevel converter for grid connection of photovoltaic systems," in 35th Annual Conference of the IEEE Industrial Electronics Society (iECON09), 2009, pp. 1-7.

[16] J. Negroni, F. Guinjoan, C. Meza, D. Biel, and P. Sanchis, "Energy­sampled data modeling of a cascade h-bridge multilevel converter for grid-connected pv systems," in International Power Electronics Congress, 10th IEEE, oct. 2006, pp. 1-6.

[17] c. Cecati, F. Ciancetta, and P. Siano, "A multilevel inverter for photo­voltaic systems with fuzzy logic control," Industrial Electronics, IEEE Transactions on, vol. 57, no. 12, pp. 4115-4125, dec. 2010.

[l8] G. Brando, A. Dannier, and R. Rizzo, "A sensorless control of h-bridge multilevel converter for maximum power point tracking in grid con­nected photovoltaic systems," in Clean Electrical Power, 2007. ICCEP '07. International Conference on, may 2007, pp. 789-794.

[19] S. Rivera, S. Kouro, J. I. Leon, B. Wu, J. Rodriguez, L. G. Franquelo. "Cascaded H-Bridge Multilevel Converter Multistring Topology for Large Scale Photovoltaic Systems". 20th IEEE International Symposium on Industrial Electronics (ISlE 2011), Gdansk, Poland, 27-30 June, 2011.

[20] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo, B. Wu, J. Rodriguez, M. A. Perez, and J. I. Leon, "Recent advances and industrial applications of multilevel converters," iEEE Trans. indo Electron., vol. 57, no. 8, pp. 2553-2580, August 2010.

[21] J. Rodriguez, L. G. Franquelo, S. Kouro, J. I. Leon, R. C. Portillo, M. A. M. Prats, and M. A. Perez, "Multilevel Converters: An Enabling Technology for High-power Applications," P roc. IEEE, vol. 97, no. 11, pp. 1786-1817, Nov. 2009.

[22] M. Malinowski, M. P. Kazmierkowski, and A. M. Trzynadlowski, "A comparative study of control techniques for pwm rectifiers in ac adjustable speed drives," IEEE Trans. on Power Electron., vol. 18, no. 6, pp. 1390-1396, Nov. 2003.

[23] D. P. Hohm and M. E. Ropp, "Comparative study of maximum power point tracking algorithms," P rogress in P hotovoltaics: Research and Applications, vol. II, no. I, pp. 47-62, 2003.

[24] N. Femia, D. Granozio, G. Petrone, G. Spagnuolo, and M. Vitelli, "Predictive & adaptive mppt perturb and observe method," IEEE Trans. on Aerospace and Electron. Syst., vol. 43, no. 3, pp. 934-950, 2007.

[25] S. Kouro, J. Rodriguez, B. Wu, S. Bernet, M. Perez, "Powering the Future of Industry: High-Power Adjustable Speed Drive Topologies," IEEE Industry Applications Magazine , vol. 18, no. 4, pp. 26-39, July­Aug, 2012.

5005