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UBLISHING

MARCH 2015 IEEE INDUSTRIAL ELECTRONICS MAGAZINE 471932-4529/152015IEEE

Grid-ConnectedPhotovoltaic SystemsAn Overview of Recent Research and Emerging PV Converter Technology

SAMIR KOURO,JOSE I. LEON,DMITRI VINNIKOV, andLEOPOLDO G. FRANQUELO

P

hotovoltaic (PV) energy has grown at an average annual rate of 60% in

the last five years, surpassing one third of the cumulative wind energy

installed capacity, and is quickly becoming an important part of the en-

ergy mix in some regions and power systems. This has been driven bya reduction in the cost of PV modules. This growth has also triggered

the evolution of classic PV power converters from conventional single-

phase grid-tied inverters to more complex topologies to increase efficiency, power

extraction from the modules, and reliability without impacting the cost. This article

presents an overview of the existing PV energy conversion systems, address-

ing the system configuration of different PV plants and the PV converter

topologies that have found practical applications for grid-connected

systems. In addition, the recent research and emerging PV con-

verter technology are discussed, highlighting their possible advan-

tages compared with the present technology.

Solar PV energy conversion systems have had a huge growth

from an accumulative total power equal to approximately 1.2 GW in

1992 to 136 GW in 2013 (36 GW during 2013) [1]. This phenomenon has

been possible because of several factors all working together to push

the PV energy to cope with one important position today (and

potentially a fundamental position in the near future).

Among these factors are the cost reduction and

increase in efficiency of the PV modules, the

search for alternative clean energy sources

(not based on fossil fuels), increased en-

vironmental awareness, and favorable

political regulations from local gov-

ernments (establishing feed-in tariffs

designed to accelerate investment

in renewable energy technolo-

gies). It has become usual to see

PV systems installed on the roofs

of houses or PV farms next to the

roads in the countryside.

Grid-connected PV systems

account for more than 99% of the

PV installed capacity compared to

Digital Ob ject Iden tifier 10.1109/MIE.2 014.2376976

Date of publication : 19 March 2015

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48 IEEE INDUSTRIAL ELECTRONICS MAGAZINE MARCH 2015

stand-alone systems (which use bat-

teries). In grid-connected PV systems,

batteries are not needed since all of

the power generated by the PV plant

is uploaded to the grid for direct trans-

mission, distribution, and consump-

tion. Hence, the generated PV power

reduces the use of other energy sourc-

es feeding the grid, such as hydroor fossil fuels, whose savings act as

energy storage in the system, provid-

ing the same function of power regu-

lation and backup as a battery would

deliver in a stand-alone system. Since

grid-connected systems do not need

batteries, they are more cost-effective

and require less maintenance and re-

investment than stand-alone systems.

This concept together with the cost

reduction, technology development,

environmental awareness, and theright incentives and regulations has

unleashed the power of the sun.

In Figure 1, a typical configura-

tion of a grid-connected PV system is

represented [2]. In a conventional PV

system, the PV cells (arranged in a sin-

gle module, a string of series-connected

modules, or an array of parallel-con-

nected strings) generate a dc current

that greatly depends on the solar ir-

radiance, temperature, and voltage at

the terminals of the PV system. This dc

power is transformed and interfaced tothe grid via a PV inverter. Additional el-

ements include a grid connection filter,

a grid monitor or interaction unit (for

synchronization, measurements, anti-

island detection, etc.), and a low-fre-

quency transformer (which is optional

depending on local regulations, the

converter topology, and the modulation

used to control it [3]). Another option is

an intermediate dcdc power stage be-

tween the PV modules and the grid-tied

inverter. This optional stage decouplesthe PV system operating point from the

PV inverter grid control. Additionally,

it can boost the PV system dc output

voltage if required or provide galvanic

isolation and perform maximum power

point tracking (MPPT) control.

The increase in the PV installed capac-

ity has also sparked a continuous evolu-

tion of the PV power conversion stage.

Gradually, PV power converters have be-

come extremely efficient, compact, and

reliable, allowing the maximum power

to be obtained from the sun in domestic,commercial, and industrial applications

[3], [4]. The PV converter industry has

evolved rapidly from childhood to adult-

hood in the last two decades and has

become a distinct power converter cat-

egory in its own right. One of the driv-

ers behind this progress is that the PV

converter market demanded very hard-

to-meet specifications, including high

efficiency (above 98%), long warranty

periods (to get closer to PV module war-

ranties of 25 years), high power quality,transformerless operation, leakage cur-

rent minimization (imposes restrictions

on the topology or modulation), and

special control requirements such as

750

700

650

620600580560540V

oltage(V)

Voltage(V)

0.02 0.04 0.06 0.08 0.1 0.12Time (s)

0.02 0.04 0.06 0.08 0.1 0.12Time (s)

InputFilter

Output

FilterPV System dc Link PV Inverter

Optional

dc/dc dc/ac

Optional

Low-FrequencyTransformer

Grid

1.0

0

1.0

50

0

50

Current(A)

Voltage(kV)

FIGURE 1 The generic structure of a grid-connected PV system (a large-scale central inverter shown as an example). (Photos courtesy of ABB.)

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MARCH 2015 IEEE INDUSTRIAL ELECTRONICS MAGAZINE 49

the MPPT. Another driver behind this

development is the fact that, for long

time, the power converter represented a

small fraction of the cost of the whole PV

system due to high PV module prices, al-

lowing PV inverter manufacturers room

for developing higher performance and

more sophisticated topologies. These

topologies usually included more powerelectronics devices than the classic to-

pologies used in general applications,

which have added control degrees of

freedom that can be used to make the

inverter operation more efficient, as will

be addressed later in this article.

The development of new PV convert-

er topologies has also been motivated by

manufacturers search for proprietary

technology to differentiate themselves

from their competitors and achieve a

competitive advantage in a growing PVconverter market. This has led to a wide

range of new and different power con-

verter topologies specially designed for

PV applications, which will be presented

and analyzed in this article.

Usual Requirementsfor PV ConvertersSome decades ago, PV applications

were not mature, PV modules were very

expensive to be produced, and the effi-

ciency of the PV modules was very low;

the impact of the PV power integration

in the distribution grid was not percep-

tible. In addition, the safety require-

ments imposed by electric companies

and governments were not present.

Today, in some regions, PV installations

are a relatively substantial part of the

electrical market, and as PV becomes a

more relevant actor in power systems,

the corresponding requirements and

regulations for the safe and reliable use

of PV systems are being standardized.

In general, two groups of require-

ments can be considered when a PVinstallation has to be designed, built,

tested, and commercialized. These two

groups are the performance require-

ments and the legal regulations that

the PV converters and the PV installa-

tions have to meet.

Performance Requirements

of PV Converters

Efficiency

The losses of the PV inverters have re-duced in time, and efficiency achieves

values above 97% (see, for instance, Sun-

nyBoy 5000TL by SMA for domestic ap-

plications below 5.25 kW) and even more

for central inverters (see, for instance,

SunnyCentral 760CP XT by SMA, a cen-

tral inverter with nominal power up to

850 kW with 98% of efficiency) [5]. There-

fore, it can be affirmed that the PV in-

verter efficiency for state-of-the-art brand

products stands around 98%. However,

it has to be noticed that the efficiency is

expected to achieve higher values when

silicon carbide (SiC) and gallium nitride

(GaN) devices are vastly used as the ba-

sic power semiconductors of the PV in-

verters in the next decade [6].

Power Density

Power density is always important,

but it is becoming critical mainly for

domestic and commercial appli-

cations (below 20 kW). In this way,

several solutions have been recently

presented such as the ABB PVS300

inverter, which is based on a neutral-

point-clamped (NPC) topology, achiev-ing a very compact solution with very

high power density [7].

Installation Cost

Figure 2 shows the evolution of each

cost component of a PV system in cen-

tral Europe [8]. Comparing the figures

from the year 2000 to the figures in

2012, there is an important reduction

in the total cost (68%), but the most

important reduction (78%) is due to

the cost of the PV modules. The invert-er cost has been reduced by 68%, and

the installation-related costs have had

the smallest reduction in the entire

group (56%). Installation costs may

vary greatly from one country or re-

gion to another as the land, labor, and

other local factors may have a great

influence on the total cost.

Minimization of Leakage Current

The leakage current appears because

of the high stray capacitance between

the PV cells and the grounded metallic

frame of each module and the high-fre-

quency (HF) harmonics caused by the

modulation of the power converter. Gal-

vanic isolation can help to interrupt the

14

($)

(%)

12

10

8

6

4

2

02000 2012 2000 2012

100

9080

70

60

50

40

30

20

10

0

Planning and Fees

Labor

Construction MaterialsInverter

PV Modules

(a) (b)

FIGURE 2 The evolution of the cost distribution of PV systems (in the range of 250 kWp) [8]: (a) the costs in U.S. dollars and (b) the distributionof each component cost.

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TABLE 1 A SUMMARY OF THE TYPICAL INTERNATIONAL CODES AND STANDARDS FOR PV APPLICATIONS.

CODES AND STANDARDS SCOPE AND CONTENT OF THE STANDARD

Grid connected IEC 60364-7-712, IEC 61727,IEC 61683, IEC 62093, IEC 62116,IEC 62446, UL 1741

Installations of buildingsUtility interface and measuring efficiencyInterconnected PV invertersSystem documentation, commissioning tests, and inspectionUse in independent power systems

Off grid IEC 62509, IEC 61194 IEC 61702,IEC/PAS 62111, IEEE Standard 1526,IEC 62124

Battery charge controllersStand-alone systemsRating of direct-coupled pumping systemsSpecifications for rural decentralized electrification

Rural systems IEC/TS 62257 Small renewable energy and hybrid systemsProtection against electrical hazardsSelection of generator sets and batteriesMicropower systems and microgridsHousehold lighting equipment

Monitoring IEC 61724, IEC 61850-7, IEC 60870 Measurement, data exchange, and analysisCommunication networks and systems for power utility automationDistributed energy resources and logical nodes

Electromagneticcompatibility (EMC)/electromagnetic

interferenceemissions

EN 61000,FCC Part 15

European Union EMC directive for residential, commercial, light industrial, and industrial facilitiesU.S. EMC directive for residential, commercial, light industrial, and industrial facilities

leakage path, but the use of a transform-

er presents drawbacks such as higher

cost and additional losses, leading in

general to a reduction of the efficiency.

Nevertheless, the transformer is manda-

tory in some countries because of local

regulations. If the transformer is not

mandatory, as a second solution, sev-

eral power converter topologies havebeen specifically designed to minimize

the effect of the HF harmonics on the

leakage currents [9].

Legal Requirements of PV Systems

Galvanic Isolation

One important requirement for PV sys-

tems is galvanic isolation for safety

reasons. This feature is required only

in some national codes, such as RD-

1699/2011, which applies to the connec-tion of PV systems to the low-voltage

(LV) distribution grid in Spain. This re-

quirement means that the PV topologies

are not standardized and that they have

to be specifically designed to fulfill this

galvanic isolation requirement, which is

usually achieved by introducing a trans-

former (high or low frequency).

Anti-Islanding Detection

The islanding phenomenon for grid-

connected PV systems occurs when

the PV inverter does not disconnect

after the grid has tripped and continues

to provide power to the local load [10].

In the conventional case of a residential

electrical system cosupplied by a roof-

top PV system, the grid disconnection

can appear as a result of a local equip-

ment failure detected by the ground

fault protection or of an intentional dis-

connection of the line for servicing. Inboth situations, if the PV inverter does

not disconnect, some hazardous situa-

tions can occur, such as

retripping the line with an out-of-

phase closure, damaging some

equipment

a safety hazard for utility line work-

ers who assume that the lines are

de-energized.

To avoid these serious situations,

safety measures and detection methods

called anti-islanding requirements havebeen required in standards. In IEEE 1574,

it is defined that after an unintentional is-

landing where the PV system continues

to energize a portion of the power sys-

tem (island) through the point of com-

mon coupling, the PV system shall detect

the islanding and stop to energize the

area within 2 s [11].

Other Codes and Standards

Since PV applications are becom-

ing more and more important, codes

and standards are continuously being

defined by international and nation-

al committees and governments to

achieve a safe, high-quality, and normal-

ized operation. International standards

are normally defined by the Internation-

al Electrotechnical Commission, the Eu-

ropean Committee for Electrotechnical

Standardization, and the IEEE. Usually,

the governments define their specificcodes based on international standards

but take into account local factors

such as the geography, grid structure,

and ratio between the renewable en-

ergy and the total installed power. For

instance, VDE-AR-N 4105 is applied in

Germany as a local code defining the

power curtailment, frequency and volt-

age support, and dynamic grid support

(ride-through capability). It can be no-

ticed that a national code can become

an international standard if it is suc-cessfully accepted by the international

commissions [12].

A summary of the current interna-

tional standards for PV applications is

included in Table 1. For large-megawatt

PV power plants, the grid-connection re-

quirements are in line with wind power

parks, connected to either distribution

or transmission levels. These codes

are mainly recommendations, and each

country adapts them to the specific

national operation and regulations. So

manufacturers slightly change the final

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design of their products to fulfill the re-

quirements of each country. A clear ex-

ample of this issue can be found in the

galvanic isolation requirement existing

in some European countries.

PV System ConfigurationsGrid-connected PV power generation

systems can be found in different sizesand power levels for different needs and

applications, ranging from a single PV

module from around 200 W to more than

a million modules for PV plants over

100 MW [13]. Therefore, the generic PV

energy conversion systems structure,

shown in Figure 1, can vary significantly

from one plant to another. For simplicity,

grid-connected PV systems can be sub-

divided depending on their power rat-

ing: small scale from a few watts to a few

tens of kilowatts, medium scale from afew tens of kilowatts to a few hundreds,

and large scale from a few hundreds of

kilowatts to several hundreds of mega-

watts, as shown in Figure 3. In addition,

PV systems can be further classified de-

pending on the PV module arrangement:

a single module, a string of modules,

and multiple strings and arrays (parallel

connected strings) [14]. The PV module

arrangement also gives the inverter con-

figuration its name: ac-module inverter,

string inverter, multistring inverter, or

central inverter, as shown in Figure 3

and Table 2.

The ac-module configuration uses a

dedicated grid-tied inverter for each PV

module of the system [15]. Therefore,

this configuration is also known as a

module-integrated inverter and microin-

verterbecause of the small size and low

power rating of the converter. The LV

rating of PV modules (generally around

30 V) requires voltage elevation for

grid connection. This is why ac-module

inverters are only found with an addi-

tional dcdc stage, usually with an HF

transformer to provide galvanic isola-

tion and elevate voltage. Because of the

additional dcdc stage and HF isolation,

this is the configuration with the low-

est power converter efficiency, which is

compensated somehow by the highest

MPPT accuracy due to the dedicated

converter. This configuration is useful

for places with lots of partial shading,complex roof structures, small systems,

or combinations of different roof orien-

tations. The small size of the converter

allows a very compact enclosure design

that can be attached to the back of each

PV module, hence the name module

integrated inverter. Because of their LV

operation, metaloxidesemiconductor

field effect transistor (MOSFET) devices

are most commonly found in these to-pologies. Nevertheless, this concept

might benefit from new, faster, and

more efficient semiconductor devices

(SiC and GaN) and therefore gain more

importance in the future.

String inverters interface a single

PV string to the grid [16]. They can be

subdivided into single- and two-stage

conversion topologies, depending on

the addition (or not) of a dcdc stage

used to adapt the dc voltage output

from the PV string to the dc side volt-age of the grid inverter. In addition,

the dcdc stage decouples the MPPT

control from the grid side control (ac-

tive and reactive power) by enabling

a fixed voltage at the inverter dc side.

Furthermore, grid inverters can be

found with or without galvanic isola-

tion. Isolation can be introduced at

the grid side with low-frequency trans-

formers (large and heavy) or within

the dcdc stage with a high-frequency

transformer (light and compact but

with additional losses from several

dcdc converter semiconductors).

The different combinations between

single or two stage, with transformer

or transformerless string inverters,

has led to a wide range of different

configurations as shown in Figure 3.

Compared to ac-module inverters,

the string inverter has a less accurate

MPPT of the PV systems and, under

partial shading, would reduce the en-

ergy yield. However, for a PV system

of the same power rating, the string

inverter has a lower cost per watt and

is more efficient. The string inverter

is very popular for small- to medium-

scale PV systems, particularly for resi-

dential rooftop PV plants.

To add more flexibility to the string

inverter and improve the MPPT per-

formance of the PV system, the mul-

tistring concept was developed [17].

The strings are divided into smallerpieces (fewer modules in series) and

connected through independent MPPT

dcdc converters to the grid-tied in-

verter. The dcdc stage also boosts the

voltage of the smaller strings. The addi-

tional dcdc stages are a cost-effective

solution compared to having several

string inverters. As can be seen in Fig-

ure 3, multistring inverters can also be

found with or without isolation. Sincethey reduce partial shading and mis-

matching, they are suitable not only

for rooftop PV systems but also for me-

dium- and large-scale plants.

Finally, the central inverter interfaces

a whole PV array to the grid through a

single inverter [2]. The array is com-

posed of parallel-connected strings. A

blocking diode in series to each string is

necessary to prevent them from acting as

load when partial shading or mismatch

occurs. Because the whole array is con-nected to a single inverter, this configu-

ration can only provide a single MPPT

operation, leading to the lowest MPPT

efficiency of all configurations. Never-

theless, it provides a simple-structure,

reliable, and efficient converter, making

it one of the most common solutions

for large-scale PV plants. Since they op-

erate at an LV (11,000 V), the limit of

insulated-gate bipolar transistor (IGBT)

technology enables converters of up to

850 kW. To increase the power rating,

some manufacturers commercialize two

central inverters connected through a

12-pulse transformer, with a rating up

to 1.6 MW. Nevertheless, very large PV

plants can currently reach several hun-

dreds of megawatts. Therefore, several

hundreds of dual central inverters are

needed in large PV farms.

Industrial PV InvertersThe evolution in power converter tech-

nology for PV applications, driven by

the growth in the PV installed capacity

and the search for the ultimate PV in-

verter, has led to the existence of a wide

variety of power converter topologies

used in practice. Figure 3 shows several

industrial PV inverter topologies for cen-

tral, string, multistring, and ac-module

configurations, which will be analyzed in

this section. Table 3 summarizes some

of the characteristics of some commer-

cial power converter topologies forthese PV inverter applications.

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String Inverter Topologies

The most common string inverter to-

pology is the full- or H-bridge inverter.

Several modified and enhanced versions

have found their way into the market

[18]. The H-bridge with a grid-side low-

frequency transformer features a simple

power circuit, galvanic isolation, and

voltage elevation provided by the trans-

former, which enables a larger range of

input voltages. This converter can be

controlled with three-level carrier-based

PV String

PV String

H-Bridge H-Bridge

H-Bridge H-Bridge

H-Bridge

H-Bridge

H-BridgeMultiple PV Strings

H-BridgeSingle

PVModule

Interleaved

FlybackConverters

LFTransformer

Boost with Bypass

Boost Stage PVBypassSwitch

AsymmetricCHB

2vdcFloating

dc Source

4vdc

vdc

Floatingdc Source

BypassSwitch

H5

H6D1

H6D2

T-Type

Three-Level (3L)-NPC

5-Level-NPC

HERIC

HF Transformer

+

+

+

++

+

+

+ +

+ +

+

+ ++

+

+

+

+

+

+

+

+

+

+ + +

+

+

+

+

+

+

LV MV

LV MV

LV MV

PV

Array3L-NPC

3L-NPC

3L-T-Type

2L-VSI

2L-VSI

+

+

+

+

++

+

+

+

aN

Grid PVInverters

CentralInverter

String

Inverter

Multistring

Inverter

acModule

Inverter

FIGURE 3 Industrial PV inverter topologies for central, string, multistring, and ac-module configurations. (MV: medium voltage; 2L-VSI: two-level voltagesource inverter.)

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pulse-width modulation (PWM) tech-

niques since the common mode voltages

cannot generate a leakage current due

to isolation. The bypass switching state

(zero voltage level) prevents a reactive

current flow between the filter inductor

and the dc-link capacitor. Nevertheless,

the bulky transformer has several disad-

vantages (low power density and lower

efficiency), making this topology less

popular with time.

The transformerless H-bridge, also

known as anH4 inverter(shown in a two-

stage configuration with a boost dcdc

stage), gets rid of the low-frequency

transformer by splitting the grid induc-

tor into the phase and neutral wires of

the systems and using a bipolar PWM

(two-level) to solve the issues of the

switched common-mode voltage and

leakage currents and by using a boost

stage for a wider input voltage range.

The downside is that the two-level mod-

ulation reduces the power quality at the

grid connection and lowers the efficien-

cy since there is a reactive current flow

between the passive elements of the

circuit at zero voltage through the free-

wheeling diodes as the dc-link capacitor

is not isolated from the grid at any time.

The H-bridge with the HF isolated dc

dc stage is composed of a MOSFET full-

bridge inverter, an HF transformer, and a

diode full-bridge rectifier. This approach

greatly reduces the size of the converter,improving the power density compared

to low-frequency transformer-based to-

pologies. However, the additional con-

verter stages introduce higher losses.

To overcome the problem of the re-

active current transfer between the grid

filter and the dc-link capacitor in transfor-

merless H-bridge string inverters during

freewheeling, several proprietary solu-

tions have been introduced by different

manufacturers [18][20]. The H5 string in-

verter by SMA adds an additional switch

between the dc-link and the H-bridge in-

verter to open the current path between

both passive components, increasing

the efficiency and reducing the leakage

current. The highly efficient and reliable

inverter concept (HERIC) introduced

by Sunways uses instead a bidirectional

switch that bypasses the whole H-bridge

inverter, separating the grid filter from the

converter during freewheeling. The H6 to-

pology introduced by Ingeteam [21] adds

an additional switch in the negative dc

bar to the H5 topology. Two versions were

introduced: one with a diode connected

in parallel to the dc side of the H-bridge

of the H6 topology, called the H6D1, and

the H6D2, which adds two auxiliary free-

wheeling diodes instead of one. Both

allow freewheeling without interaction

between passive components while en-

abling a unipolar output compared to the

H5. The difference between the H6D1 and

the H6D2 is that in the former, the addi-

tional switches block the total dc voltage,while in the latter, they only block half.

The three-level NPC inverter

(3L-NPC) also has several modified and

enhanced versions for PV string invert-

ers [22]. The advantage of the 3L-NPC

over the H-bridge is that it provides a

three-level output without a switched

common-mode voltage since the neu-

tral of the grid is grounded to the same

potential as the midpoint of the dc link.

This enables transformerless opera-

tion without the problem of the leakage

currents and modulation methods that

do not use the potential of the con-

verter. The main drawback compared

to the H-bridge is that it requires a total

dc link of double the voltage to connect

to the same grid. Hence, more modules

need to be connected in series or an

additional boost stage is required.

A full-bridge of two 3L-NPC legs was

introduced by ABB, resulting in the 5L-

HNPC inverter [23]. As with the H-bridge,

this converter also requires a symmetri-

cal grid filter distributed between the grid

phase and grid neutral wires. A special

modulation technique can achieve a line-

frequency common-mode voltage; hence,

no leakage currents are generated while

enabling transformerless operation.

The T-type or three-level transistor

clamped string inverter was introduced

by Conergy. The converter can clamp the

phase of the grid directly to the neutral

to generate the zero voltage level using a

bidirectional power switch. For the samereason as the 3L-NPC, it can operate

TABLE 2 A GRID-CONNECTED PV ENERGY CONVERSION SYSTEMS CONFIGURATIONS OVERVIEW.

SMALL SCALE MEDIUM SCALE LARGE SCALE

AC MODULE STRING MULTISTRING CENTRAL

Power range

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transformerless. The main difference is

that it does not require the two additional

diodes of the 3L-NPC. The bidirectional

switches block each half of the voltage

blocked by the phase-leg switches.

The asymmetric cascaded H-bridge

was introduced by Mitsubishi [24]

and features three series-connected H-

bridge cells operating with unequal dc

voltage ratios (1:2:4). The PV system

is connected through a boost dcdc

stage to only one of the H-bridge cells,

which is the only one processing active

power to the grid. The other two cells

use floating dc links for power quality

improvement through the generation of

13 voltage levels. This enables a reduc-

tion of the switching frequency without

compromising the power quality. The

topology requires a bidirectional bypass

switch connected to the large cell to

reduce the changing potential between

the PV system and the ground to reduce

the possibility of leakage currents andenable transformerless operation.

Multistring Topologies

The main difference between the mul-

tistring and string configuration is that

multistring is exclusively a two-stage

system composed by more than one

dcdc stage [17]. Hence, all inverter

topologies in the String Inverter To-

pologies section could be used in a

multistring configuration. Like with

string inverters, the same combina-

tions of isolated and transformerless

configurations apply with or without

symmetric grid filters.

One of the first multistring invert-

ers introduced in practice was the half-

bridge inverter with boost converters in

the dcdc stage by SMA [17]. Other to-

pologies that have followed include the

H-bridge, the H5, the three-phase two-lev-

el voltagesource inverter (2L-VSI), the

3L-NPC, and the three-phase three-level

T-type converter (3L-T) [18]. Figure 3

shows some examples of practical mul-

tistring configurations. The most com-mon dcdc stages used for multistring

configurations are the boost converter

and the HF isolated dcdc switch mode

converter based on an H-bridge, HF

transformer, and diode rectifier.

Central Topologies

Central inverter configurations are main-

ly used to interface large PV systems

to the grid. The most common inverter

topology found in practice is the 2L-VSI,

composed of three half-bridge phase

legs connected to a single dc link. The

inverter operates below 1,000 V at the dc

side (typically between 500 and 800 V),

limited by the PV modules insulation,

which prevents larger strings. Grid con-

nection is done through a low-frequency

transformer to elevate the voltage al-

ready within the collector of the power

plant to reduce losses. More recently, the

three-phase 3L-NPC and the three-phase

3L-T converter have been also used for

this configuration, as shown in Figure 3.

The characteristics, advantages, and dis-advantages analyzed for the single-phase

TABLE 3 A SUMMARY OF CHARACTERISTICS AND EXAMPLES OF SELECTED INDUSTRIAL PV INVERTER TOPOLOGIES.

TOPOLOGY 2L-VSI HERIC 3L-NPC H-NPC 1:2:4-CHB H5 HFH-BRIDGEDC-DC

HF FLYBACKDC-DC

Pros Simple androbustLarge capacity

No freewheelingcurrent lossesTransformer less

Constant CMvoltageLow THD

Low THDTransformer less

High powerqualityTransformerless

No freewheelingcurrent lossesTransformer less

Small andcompactEasyinstallation

Small andcompactEasyinstallation

Cons Higher THDLarge trafo.Poor MPPT

Bidirectional bypassswitch

HF isolationHighnumber ofdevices

No 5 levelwaveformHigh number ofdevices

ComplexmoduleComplexcontrol

Special PWMmodulation

High input-outputvoltage rat ioSoftswitching

High input-output voltageratio Lessefficient HFtrafo concept

Commercialexample

Configuration Central String String String String Multistring AC module AC module

Input voltage 550850 V 900 V 600 V 900 V 380 V 750 V 60 V 45 V

Rated acpower

1.5 MW 4.8 kW 4.8 kW 8 kW 4 kW 5,250 W 200 and300 W

190260 W

Gridconnection

Three phase Single phase Single phase Single phase Single phase Single phase Single phase Single, threephase

Efficiency 98.5% 97.8% 97.3% 97% 97.5% 97% 96.5% 96.3%

Isolation LF transformer Transformerless HFtransformer

Transformer less Transformer less Transformer less HFtransformer

HFtransformer

Number ofIndependentMPPT

Two arrays One string One string One string One string Two string One module One module

Brand /model

Satcon PrismPlatform Equinox

Sunways NT 5000 Danfoss DLX4.6

ABB PVS 300 TL8000

Mitsubishi PV-PN40G

SMA Sunny Boy5000TL

Power OneAuroraMICRO-0.3-I

SiemensMicroinverterSystem

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versions of these topologies for PV string

systems also hold for the central invert-

er version.

AC-Module Topologies

A commercial ac-module topology is the

interleaved flyback converter, developed

by Enphase Energy [25] and current-

ly commercialized by Siemens, shownin Figure 3. The flyback converter per-

forms MPPT and voltage elevation and

provides galvanic isolation while the

H-bridge inverter controls the dc-link

voltage (output voltage of the flyback),

grid synchronization, and active/reac-

tive power control. Several flyback con-

verters are connected in parallel, which

enables a higher switching frequency,

resulting in a further reduction of the HF

transformer and, hence, a very compact

inverter. It also allows for a reduction inthe current ripple both at the input and

output of the dcdc stage due to the

phase-shifted carrier modulation, extend-

ing the life span of the capacitors.

Another commercial ac-module in-

tegrated converter, shown in Figure 3,

includes a resonant H-bridge stage with

an HF isolation transformer and a diode

bridge rectifier as a dcdc converter in-

stead of the flyback developed by Enec-

sys [26]. The H-bridge dcdc stage has

better power conversion properties com-

pared to the flyback.

Recent Advances onGrid-Connected PV InvertersThe last decade has seen marked prog-

ress in the research and development

of new power converter topologies for

PV applications. The main research ef-

forts have concentrated on the highest

possible efficiency, power density, and

reliability of the converter to further

increase the overall performance of

the PV installation. In the majority of

cases, the newly emerged topologies

are the full-power converters in which

the whole amount of the PV panel (or

PV string) power has to be processed.

Advances in DCAC Converters

for PV Systems

As shown in Figure 1, in a typical PV in-

verter, the two-stage power conversion

is currently the most common approachto cope with a wide input dc voltage

range produced by the PV panel. In that

case, the PV power conditioning systemconsists of the front-end dcdc convert-

er for the MPPT and the inverter to feed

the power to the ac load or grid [27]

[29]. However, this multiple-stage power

conversion system could lower the ener-

gy efficiency and reliability of the PV in-

stallation. To overcome these problems,

in 2003, the novel family of single-stage

buckboost inverters was introduced

by Prof. Peng [30], with the most prom-

ising topology being the quasi-Z-source

inverter (qZSI) [31]. This buckboostinverter is a combination of the two-port

passive quasi-impedance network with a

2L-VSI [Figure 4(a)]. The distinctive fea-

ture of the qZSI is that it can boost the

input voltage by using an extra switch-

ing statethe shoot-through state. The

shoot-through state is the simultaneous

conduction of both switches of the same

phase leg of the inverter. This operation

state is forbidden for the traditional VSI

because it causes the short circuit of

the dc-link capacitors. In the qZSI, the

shoot-through state is used to boost the

magnetic energy stored in the inductors

of the quasi-Z-source network without

short-circuiting the dc capacitors. This

increase in inductive energy, in turn, pro-

vides the boost of the voltage across the

inverter during the traditional operating

states (active states). The qZSI has the

input inductor that buffers the source

current, which means that during the

continuous conduction mode, the input

current never drops to zero, thus featur-

ing the reduced stress of the input volt-

age source. Moreover, the properties of

the qZSI allow the energy storage (typi-

cally the battery) to be connected in

parallel with one of the capacitors of the

quasi-Z-source network [32]. The state of

charge of the battery is then controlled

by varying the shoot-through duty cycle

of the inverter switches. Therefore, the

simple energy storage system for cover-

ing the peak power demands could beused in the qZSI without any additional

circuits. The two-level qZSI could be eas-

ily extended to the multilevel topology,as presented in Figure 4(b). The three-

level NPC qZSI has similar advantages

as the two-level topology; moreover, it

could be used with single or multiple PV

sources [33]. As in the case of two-level

qZSI, the short-term energy storage (bat-

tery) can be connected in parallel either

with the external or internal capacitors

of the quasi-Z-source-network. Thanks

to all of these advantages, the qZSI is re-

ferred to as one of the most promising

power conversion approaches for futurePV power conditioners.

Another hot topic regarding recent

PV inverters is the use of different

multilevel converter topologies to en-

able medium-voltage (MV) grid con-

nection. Most commercial topologies

shown in Figure 3 are in fact multilevel

converters (e.g., three-level H-bridge,

three-level NPC, three-level T-type,

and their derivatives). However, all of

these converters connect to LV grids

since PV strings cannot surpass the

1,000-V limit due to the module insu-

lation standard. Therefore, to be able

to connect to MV grids, the multilevel

converters must be able to support

several individual strings at the dc

side and connect them somehow in

series through the converter power

stages to the output. Several alterna-

tive topologies have been introduced

to achieve a high number of levels and

reach MV operation [34][41]. An ad-

vantage of using multilevel converters

as PV inverters is related to the output

waveforms high quality, which reduc-

es the grid connection filter require-

ments, leading to a compact design

for low-power applications (usually

domestic rooftops). In addition, the

use of multilevel converters can lead

to avoidance of the additional boost

converter in the input or the step-up

transformer in the output, eliminat-

ing the additional power conversionstages and improving the efficiency of

The last decade has seen marked progress in the

research and development of new power converter

topologies for PV applications.

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the system. On the other hand, some

multilevel converter topologies, such

as the cascaded H-bridge converter or

the modular multilevel converter, can

take advantage of splitting the PV ar-

ray system to achieve higher efficien-

cy values using independent MPPT

algorithms. This could be interesting

for central inverters of PV medium-

and high-power plants [37][41].

Another research focus involves de-

veloping PV inverters with additional

energy storage capability usually based

on batteries. These hybrid systems

present the advantages of improving

the frequency and voltage regulation,

storing the energy if it is not demand-

ed by local loads, and supplying this

energy when required, increasing the

overall system operation (usually called

peak load shaving). These systems are

mainly focused for stand-alone sys-

tems, household applications, or weak

grid-connected applications of large PV

plants [42][45]. The use of hybrid PV

batteries can already be found as a com-

mercial product for household applica-

tions (see, for instance, the Sunny Boy

3600/5000 Smart Energy by SMA) [46].

This trend emerges as an important field

of development for the future.

On the other hand, it is important

to note that the high penetration of PVsystems has led to the consideration of

future regulations following the path

already written by the wind energy

sector. In this way, future regulations

about LV ride-through and reactive

power compensation could be also ap-

plied to medium and large PV systems

[47], [48]. This issue will become par-

ticularly important for large PV plants

normally using conventional two-level,

three-phase central inverters. Future

regulations may force the power con-

version stage to upgrade and motivate

the introduction of more advanced

multilevel converters and include ener-

gy storage to meet grid codes and pro-

vide the system operational flexibility.

Advances in DCDC Converters

for PV Systems

As was introduced in Figure 1, the dc

dc conversion stage is usually intro-

duced to adapt the voltage range of the

PV array to the dc bus of the PV invert-

er, and it simultaneously develops the

MPPT control.

Related to this issue, another topic

of growing interest in the PV topologies

has emerged in the field of module inte-

grated converters (MICs). Generally, an

MIC is a self-powered, high- efficiency,

step-up dcdc converter with galvanic

isolation that operates with autono-

mous control and is integrated to thePV panel for tracking the maximum

power point locally. The galvanic iso-

lation is essential to reduce ground

leakage currents and grid current to-

tal harmonic distortion [49]. As in the

case of the previously mentioned PV

inverters, the research trends here are

directed toward the highest possible

power conversion efficiency and pow-

er density. According to our research

survey, the resonant power conversion

with the maximum possible utilization

of the parasitic elements of the circuit

and the wide-bandgap semiconductors

is the most popular approach for MIC

performance improvement.

Generally, MICs can be categorized

as topologies either with a double- or

single-stage power conversion. In the

first case, the auxiliary boost converter

steps up the varying voltage of the PV

panel to a certain constant voltage level

and supplies the input terminals of the

isolated dcdc converter. In that case,

the primary inverter within the dcdc

converter operates with a near-constant

duty cycle, thus ensuring better utili-

zation of an isolation transformer. In

[50], the combination of a synchronous

boost converter with a series resonant

dcdc converter (SRC) was presented

[Figure 5(a)]. The SRC offers the ad-

vantages of high efficiency as it can

operate without switching losses dueto zero voltage switching (ZVS) and a

+

VPV

+

VPV

+

+

+

+ +

Battery Option 1

Battery Option 1

Battery Option 1

BatteryOption 2

BatteryOption 2

Filter

Filter

Passive Quasi-Z-SourceNetwork

Passive Quasi-Z-SourceNetwork

VSI NPC Inverter

(a) (b)

+

FIGURE 4 Generalized topologies of the most popular single-stage buck-boost inverters: (a) two-level and (b) 3-L NPC quasi-Z-source inverters.

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high power density because of its bi-

directional core excitation. The two-

stage structure could be simplified by

the replacement of a boost converter

with a passive impedance network

[Figure 5(b)] [51]. The impedance net-

work is a two-port passive circuit that

consists of capacitors, inductors, and

diodes in a special configuration. A spe-cific feature of the impedance network

is that it can be short-circuited, which,

in turn, will lead to the voltage boost

across the input terminals of the main

converter [52]. Thus, the varying output

voltage of the PV panel is first preregu-

lated by adjusting the shoot-through

duty cycle (simultaneous conduction

of both switches of the same phase leg

of the inverter); afterward, the isolation

transformer is being supplied a voltage

of constant amplitude value. The imped-

ance source dcdc converter [53] ex-

tended by the series resonant network

can minimize the switching frequency

range of the traditional SRC and willlead to a high converter efficiency over

a wide input voltage and load variation

range. Moreover, because of inherent

short-circuit immunity, the reliability

can be enhanced substantially.

In a single-stage power converter,

the primary inverter operates within

the wide input voltage range and the ef-

ficiency optimization could become an

issue. Here, different approaches were

recently studied. For example, in [54],

a highly efficient multiresonant dcdc

converter was proposed [Figure 5(c)].

Despite its complex structure, the con-

verter has a minimal number of external

discrete components in the design of theresonant tank: the series and parallel in-

ductances are realized by using the leak-

age and magnetizing inductances of the

isolation transformer, respectively. The

parallel capacitance is mostly formed

by the sum of the parasitic capacitances

of the rectifying diodes and the isolation

(a) (b)

(c) (d)

(e) (f)

(g) (h)

FIGURE 5 The new emerged topologies of the PV module integrated dcdc converters: (a) a two-stage topology consisting of a synchronousboost and a series-resonant converter, (b) a quasi-Z-source series resonant converter, (c) an LLCC series-parallel resonant converter, (d) a series

resonant converter with a bidirectional ac switch, (e) a dual-mode resonant converter, (f) a half-bridge LLC resonant converter, (g) a parallel reso-nant current-fed push-pull converter, and (h) a multiresonant two-inductor boost converter with a nondissipative regenerative snubber.

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TABLE 4 SPECIFICATIONS OF SOME EMERGING DCDC CONVERTER TOPOLOGIES FOR PV APPLICATIONS.

FIGURENUMBER

PMAX(W) VPV(V) VDC-BUS(V) INPUTVOLTAGEGAIN RANGE

FSW(KHz) EFFICIENCY(%)

NUMBER OFSWITCHES

NUMBER OFDIODES

TYPE OFRESONANCE

5(a) 275 1545 400 8.926.7 210 (boost)350 (SRC)

97 Six Two Series

5(b) 500 50150 100 0.72 49.3 96 Four Three Series

5(c) 244 2035 700 2035 215268.5 96 Four Two LLCC

5(d) 300 1555 320 5.821.3 130 97.4 Six Two Series

5(e) 240 2240 400 1018.2 76185 96.5 Three Four LLC

5(f) 200 2448 380 7.915.8 100 95.4 Two Two LLC

5(g) 250 2040 400 1020 50100 96.6 Two Two Parallel

5(h) 210 26.6 350 13.2 100 93.6 Two Four LLC

transformer. In this converter, the care-

fully optimized resonant tank leads to

high efficiency within a wide specified

input voltage range.

The new resonant converter topol-

ogy shown in Figure 5(e) can operate in

two resonant modes adaptively depend-

ing on the panel operation conditions,

thus maintaining a high efficiency with-in a wide input range at different output

power levels [55]. As in the case of the

previous topology, the specific proper-

ties of the circuit components, such

as parasitic capacitances of MOSFETs,

leakage, and the magnetizing induc-

tance of the transformers, were used as

snubbers or elements of the resonant

network. One of distinctive features of

this novel topology is a half-wave recti-

fier added to the secondary side of the

auxiliary transformer. When the half-wave rectifier is enabled, together with

the voltage doubler rectifier of the main

circuit, it will provide the output voltage

equal to their summed output voltages.

The converter features ZVS for primary

side switches and zero current switch-

ing (ZCS) for rectifying diodes for both

resonance modes and achieves the max-

imum efficiency close to 97%.

Another approach to the high-effi-

ciency resonant converter for PV MIC

applications is presented in Figure 5(d).

With the simple addition of a bidirec-

tional ac switch across the secondary

winding of the isolation transformer, the

highly efficient series resonant converter

is combined with both a phase-shift mod-

ulated full-bridge buck converter and a

pulse-width modulated boost converter

to provide input voltage regulation over

a wide input voltage and output power

range [49]. The converter features the

ZVS and/or ZCS of the primary sideswitches and the ZCS of the rectifying

diodes, which finally results in a high ef-

ficiency within a wide operation range

of the converter. In all of the previously

mentioned dcdc converter topologies,

special attention was paid to the reduc-

tion of circulation energy to further in-

crease the power conversion efficiency.

One of the significant advantag-

es of the high-efficiency half-bridge

LLC dcdc converter [Figure 5(f)] is

a reduced number of primary sideswitches and, therefore, a more simple

structure than the previous MICs. As

in the previous cases, the leakage and

magnetizing inductances of the isola-

tion transformer together with the ex-

ternal resonant capacitor form the LLC

resonant network. As a result, the main

power switches can achieve ZVS and

the output diodes can realize ZCS in

a wide input and load range [56]. Tra-

ditionally, with the help of the voltage

doubler rectifier, the high voltage gain

is realized with the optimal turns ratio

of the isolation transformer.

The soft-switching current-fed push

pull converter presented in Figure 5(g)

is another realization possibility of a

simplified-structure MIC. [57]. It has

the advantages of a traditional current-

fed pushpull converter, such as low

input current stress, high voltage gain,

and low conduction losses of switches.

Moreover, thanks to the parallel reso-nance between the secondary leakage

inductance of the isolation transformer

and a resonant capacitor, the transistors

are turned on and off at the zero-voltage

and zero-current conditions. The diodes

of the voltage-doubler rectifier are also

turned off at zero current.

An interesting topology of the

low-cost MIC was presented in [58].

The classical two-inductor isolated

boost converter was further improved

by use of a nondissipative regenerativesnubber along with a hysteresis con-

troller and constant duty cycle control

[Figure 5(h)]. Furthermore, a multireso-

nant tank formed by the magnetizing in-

ductance of the transformer, its leakage

inductance, and the external resonant

capacitor was introduced. As a result of

all of these modifications, the proposed

converter features a low input current

ripple, ZCS conditions for the input

switches and output rectifying diodes,

and improved light-load behavior.

Table 4 summarizes the most impor-

tant specifications of the experimental

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prototypes of the discussed dcdc con-

verter topologies for PV applications. (It

has an indicative character just to high-light the recent advances in the technol-

ogy; more detailed information can be

found in [49], [50], [53], and [58].)

Advances in Power Semiconductors

for PV Systems

Power converters for PV systems are

slowly taking advantage of the good

characteristics of wide-band-gap (WBG)

devices. WBG devices are built using

nonsilicon materials such as SiC or GaN.

The main characteristics that make

WBG devices attractive as an alterna-

tive to silicon devices are as follows:

high voltage ranks that help to

build converters with larger nomi-

nal power

very fast commutation, enabling

high switching frequency and reduc-

ing the size, volume, weight, and

cost of passive reactive components

while reducing switching losses

high maximum working tempera-

ture, leading to an important reduc-

tion of size, volume, weight, and

cost of auxiliary thermal manage-

ment devices.

As an alternative to silicon devices,

the cost of the WBG devices must be

taken into account. The silicon in-

dustry takes advantage of 60 years of

expertise (the first silicon bipolar tran-

sistor was presented in 1954), and con-

sequently, the production processes

are very mature and well optimized.On the other hand, the new processes

are still far from maturity as they are

in their first steps; however, they are

a very promising technology as, nodoubt, the cost reductions will come

gradually. It is important to highlight

that the extra cost of the devices may

be compensated by important reduc-

tions in the costs related to the pas-

sive reactive components and heat

management. For critical applications,

the reduction in size and the increase

in performance can make these devic-

es very attractive [59][61].

Solar power converters benefit

greatly from the good characteristics

of new WBG devices. A large number

of researchers have presented new

developments based on SiC and GaN

devices, and the comparison with their

counterparts made of silicon is clearly

favorable to the new devices [49], [59]

[67]. It seems that efficiencies of more

than 99.5% can be reached with SiC de-

vices for converters in the range of 10

100 kW. Converters as small as 100 W

(suitable for the panel integrated with

the inverters) have been demonstrated

to reach 1 MHz of switching frequency

[64]. A summary of some of the power

modules with SiC diodes already on the

market is shown in Table 5.

ConclusionsThe PV market has experienced expo-

nential growth in the last decade, be-

coming an important alternative and a

clean energy source in many countries.

Along with the decrease in price and theincrease in efficiency of the PV modules,

the PV converter topologies have been

continuously changing, following more

demanding requirements and standards.These regulations are being adapted to

a new power system scenario where

renewable energy sources are an impor-

tant part of the energy mix. Today, and

meeting these legal requirements, PV

converter topologies deal with issues

such as high efficiency, high power den-

sity, grid code compliance, reliability,

long warranties, and economic costs.

A good number of PV converter to-

pologies can be found on the market

for string, multistring, central, and ac-

module PV applications. Among all of

these converter topologies, it can be af-

firmed that one of the most important

appearances has been the multilevel

converters, mainly the NPC, the T-type,

and the H-bridge, not only for high-pow-

er applications but also for residential

applications in the kilowatt and LV range.

In the near future, it is expected that

a completely new family of PV convert-

ers will be developed based on SiC pow-

er semiconductors. (There are some

commercial PV converters but only

with SiC diodes.) These new SiC-based

PV converters and the next-generation

GaN PV converters will reduce the com-

promise between performance and effi-

ciency, enabling the next generation of

grid-connected PV systems.

AcknowledgmentThe authors would like to acknowledge

the support provided by the Solar En-ergy Research Center (SERC-Chile,

TABLE 5 COMMERCIAL POWER SEMICONDUCTOR MODULES FOR GRID-CONNECTED PV INVERTERS.

TOPOLOGY H-BRIDGE + BOOST NPC T-TYPE 2L-VSI

Ratings 650 V @ 50 A 600 V @ 150 A 1.2 kV @ 75 A 650 V @ 300 A

Configuration String Multistring AC module

Central String Multistring

Central String Multistring

Central

Circuit

Commercial example Infineon F4-75R07W2H3_B51 Semikron SK150MLI066T Infineon F3L75R12W1H3_B27 Semikron SK300GB063D

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CONICYT/FONDAP/15110019) and the

Advanced Center for Electrical and Elec-

tronics Engineering (AC3E, CONICYT/

FB0008). We would also like to acknowl-

edge the financial support provided by

the Andalusian government under proj-

ect P11-TIC-7070, the Spanish Ministry

of Education and Science under project

ENE2012-36897, and the Estonian Minis-try of Education and Research (project

SF0140016s11).

BiographiesSamir Kouro ([email protected])

received his M.Sc. and Ph.D. degrees in

electronics engineering from the Uni-

versidad Tecnica Federico Santa Maria

(UTFSM), Valparaso, Chile, in 2004 and

2008, respectively. He joined UTFSM in

2008 as an associate researcher and

became an adjunct professor in 2014.From 2009 to 2011, he was a postdoc-

toral researcher with the Department

of Electrical and Computer Engineering,

Ryerson University, Toronto, Canada.

His research interests include power

electronics, control, and renewable en-

ergy power conversion systems (pho-

tovoltaic and wind). He is a principal in-

vestigator of the Solar Energy Research

Center, Chile, and a titular researcher of

the Advanced Center of Electrical and

Electronics Engineering at UTFSM. He

has coauthored one book, four book

chapters, and more than 70 refereed

journal and conference papers. He has

served as a guest editor of special sec-

tions inIEEE Transactions on Industrial

Electronics and IEEE Transactions on

Power Electronics.He received the IEEE

Power Electronics Society Richard M.

Bass Outstanding Young Power Elec-

tronics Engineer Award in 2012, the

IEEE Industry Applications Magazine

first-prize paper award of 2012, the IEEE

Transactions on Industrial Electronics

Best Paper Award of 2011, and the IEEE

Industrial Electronics Magazine Best

Paper Award of 2008. He is a Member

of the IEEE.

Jose I. Leon([email protected]) re-

ceived his B.S., M.S., and Ph.D. degrees

in telecommunications engineering

from the University of Seville, Spain, in

1999, 2001, and 2006, respectively. He is

currently an associate professor withthe Department of Electronic Engineer-

ing, University of Seville. His research

interests include electronic power sys-

tems, modulation and control of power

converters, and renewable energy sys-

tems. He was a recipient as a coauthor

of the 2008 IEEE Industrial Electronics

Magazine Best Paper Award and the

2012 IEEE Transactions on Industrial

ElectronicsBest Paper Award. He wasthe recipient of the 2014 IEEE Indus-

trial Electronics Society Early Career

Award and is currently serving as an

associate editor of IEEE Transactions

on Industrial Electronics. He is a Senior

Member of the IEEE.

Dmitri Vinnikov (dmitri.vinnikov@

ieee.org) received his Dipl.Eng., M.Sc.,

and Dr.Sc.techn. degrees in electrical en-

gineering from Tallinn University of Tech-

nology, Estonia, in 1999, 2001, and 2005,

respectively. He is currently the head ofthe Power Electronics Research Group

at the Department of Electrical Engineer-

ing, Tallinn University of Technology. He

has authored more than 150 published

papers on power converter design and

development and holds several patents

and utility models in this field. His re-

search interests include switch-mode

power converters, modeling and simula-

tion of power systems, applied design of

power converters and control systems,

and application and development of

energy storage systems. He is a Senior

Member of the IEEE.

Leopoldo G. Franquelo(lgfranquelo

@us.es) received his M.Sc. and Ph.D. de-

grees in electrical engineering from the

University de Seville, Spain, in 1977 and

1980, respectively. He is currently with

the Department of Electronics Engineer-

ing, University of Seville. His current

research interests include modulation

techniques for multilevel inverters and

its application to power electronic sys-

tems for renewable energy systems. He

has been a Distinguished Lecturer since

2006 and an associate editor of IEEE

Transactions on Industrial Electronics

since 2007 and co-editor-in-chief since

2014. He has been a member at large of

the Industrial Electronics Society (IES)

AdCom (20022003), the vice president

for conferences (20042007), and the IES

president elect (20082009). He was the

IES president (20102011). He is a Fellowof the IEEE.

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