Renewable and Sustainable Energy Reviews: A B A A C A C A

Renewable and Sustainable Energy Reviews 94 (2018) 1120–1141
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews

journal homepage: www.elsevier.com/locate/rser
A comprehensive review on inverter topologies and control strategies for T

grid connected photovoltaic system
⁎
Kamran Zeba,b, , Waqar Uddina, Muhammad Adil Khana, Zunaib Alic, Muhammad Umair Alia,
Nicholas Christofidesc, H.J. Kima
a
School of Electrical Engineering, Pusan National University, Pusandaehak-ro 63 beon-gil 2, Geumjeong-gu, Busan-city 46241, South Korea
b
School of Electrical Engineering and Computer Science, National University of Sciences and Technology, Islamabad 44000, Pakistan
c
Department of Electrical Engineering, Frederick University, Nicosia, Cyprus
A R T I C LE I N FO A B S T R A C T
Keywords: The application of Photovoltaic (PV) in the distributed generation system is acquiring more consideration with
Grid-connected photovoltaic system the developments in power electronics technology and global environmental concerns. Solar PV is playing a key
Inverters role in consuming the solar energy for the generation of electric power. The use of solar PV is growing ex-
Control system ponentially due to its clean, pollution-free, abundant, and inexhaustible nature. In grid-connected PV systems,
DC-DC converter
significant attention is required in the design and operation of the inverter to achieve high efficiency for diverse
Multilevel inverter
power structures. The requirements for the grid-connected inverter include; low total harmonic distortion of the
currents injected into the grid, maximum power point tracking, high efficiency, and controlled power injected
into the grid. The performance of the inverters connected to the grid depends mainly on the control scheme
applied. In this review, the global status of the PV market, classification of the PV system, configurations of the
grid-connected PV inverter, classification of various inverter types, and topologies are discussed, described and
presented in a schematic manner. A concise summary of the control methods for single- and three-phase in-
verters has also been presented. In addition, various controllers applied to grid-tied inverter are thoroughly
reviewed and compared. Finally, the criteria for the selection of inverters and the future trends are compre-
hensively presented.
1. Introduction the year [2]. Now PV is the third most important RE after hydro, and
wind in terms of globally installed capacity.
Research towards improving photovoltaic efficiency and increasing PV systems can be categorized into two main groups, that are, the
installation of residential rooftops PV systems is a clear indication that standalone (off-grid) PV systems and the grid-connected (on-grid) PV
the distribution generation (DG) in upcoming years will be dominated systems [3]. The standalone system operates independent of the utility
by PVs. The desire to limit conventional energy sources and their use grid. On the other hand, the grid-connected applications employ PV
due to environmental concerns has also played an important role to- system in conjunction with the grid. Currently, in comparison to the
wards increased DG utilization. Furthermore, the electricity bills for the standalone PV systems, the use of grid-connected PV is widely adopted
consumers having PV rooftop systems are drastically decreased (for in my practical applications [4–7]. A typical configuration of the grid-
example, in countries with net-metering system installed), realized as connected system is presented in Fig. 1, consisting of a PV system and
benefit by the consumers. Renewable Energy (RE) sources are the best number of peripheral modules, such as the filters, transformers and the
solution to provide green energy to overcome the global energy issues. conversion technologies. The conversion technologies includes the DC/
Furthermore, the use of RE sources is increased during the last decade DC and DC/AC power electronics based converters. As opposed to the
through the advancement in the grid integration technologies [1]. Solar off-grid PV systems, the grid-connected PV does not require storage
PV energy is one of the extensively emerging RE source. PV has the system as they operate in parallel with the electric utility grid. In ad-
proficiency of generating the electricity in a reliable, clean, and dition, they supply power back to the utility grid when the generated
noiseless way. Worldwide, around 75 GW of solar capacity was installed power is greater than the load demand.
until 2016 and its capacity increased drastically to 303 GW at the end of A DC/DC converter together with a Voltage Source Inverter (VSI) or
⁎
Corresponding author at: School of Electrical Engineering, Pusan National University, Pusandaehak-ro 63 beon-gil 2, Geumjeong-gu, Busan-city 46241, South Korea.
E-mail addresses: kamran.zeb@pusan.ac.kr (K. Zeb), waqudn@pusan.ac.kr (W. Uddin), engradilee@gmail.com (M.A. Khan), zunaib.ali@stud.frederick.ac.cy (Z. Ali),
umairali.m99@gmail.com (M.U. Ali), n.christofides@frederick.ac.cy (N. Christofides), heeje@pusan.ac.kr (H.J. Kim).
https://doi.org/10.1016/j.rser.2018.06.053
Received 10 November 2017; Received in revised form 15 June 2018; Accepted 22 June 2018
1364-0321/ © 2018 Elsevier Ltd. All rights reserved.
K. Zeb et al. Renewable and Sustainable Energy Reviews 94 (2018) 1120–1141
Fig. 1. A typical structure of off-grid system.
a Current Source Inverter (CSI) are typically used to connect the PV current (through parallel inverter) support, such as in unified power
system to the grid. For DC to AC inversion purposes, the use of VSI in quality conditioner (UPQC). Various power inverter topologies and
the grid-connected PV system is gaining wide acceptance day by day. their control structures for grid-connected PV systems are comprehen-
Thus, the high efficiency of these inverters is the main constraint and sively reviewed in this paper.
critical parameter for their effective utilization in such applications [8]. In recent years, the development in the solar PV is progressing day
The proper operation of the grid-connected PV mainly depends on the by day due to the continuous government support for RE based elec-
fast and accurate design of the VSI control system. A proper VSI con- tricity production, cost reduction in materials, and technological im-
troller is, therefore needed for the effective tracking of the desired re- provements. In this review, the global status of PV market and classi-
ference command and achieving a good performance of the PV system. fications of power electronic based converters are focused in detail.
In a grid-connected PV system, the injected currents are controlled by Furthermore, various inverter topologies based on their design, classi-
the inverter, and thus, maintains the DC-link voltage to its reference fication of PV system, and the configuration of grid-connected PV in-
value and regulates the active and the reactive power delivered to the verters are discussed, described and presented in a schematic manner. A
grid [9]. concise review of the control techniques for single- and three-phase
The design of the appropriate control system for enabling the in- inverters has also been demonstrated. After that, various controllers
jection of controlled PV power into the grid is very critical for the ef- applied to grid-tied inverter are thoroughly reviewed and compared.
fectiveness of the system. The active power from the PV is controlled Finally, selection of inverters and future trends are comprehensively
with the temperature and incident solar irradiance of the PN junction presented. The contribution of the proposed review study is compre-
diode. Considering the voltage regulation scheme and the system hensively summarized in Table 1 by an extensive critical and analytical
rating, the output reactive power reference is designed based on the comparison with the various surveys already published in the literature.
method discussed in [10]. It is worth mentioning that the generated The rest of the paper is organized as following: an overview of the
output power from the PV array is inherently unstable. With the global status of PV market is demonstrated in Section 2. Section 3 ca-
modern developments and advancements in the power electronics, the tegorizes the several classifications of power electronic based con-
parameters of the PV system, i.e. active (P) and reactive (Q) power can verters. The various topologies of inverter based on their design are
be effectively controlled to enhance the overall performance of the grid- elaborated in Section 4. Section 5 and Section 6 respectively investigate
connected system. the classification of the PV systems and various configurations of the
The generation of active power in order to fulfill the load demand is grid-connected PV inverters. The generic control of the grid-connected
the main purpose of the PV system. However, it can also be used to PV system is described in Section 7. Section 8 scrutinizes various con-
perform the advance functionalities of supporting the grid such as the trol methods for the grid-connected PV systems. The selection of ap-
voltage and reactive power support, fault ride through, power quality propriate inverter and control method is elaborated in Section 9.
improvement, reduction in power losses and the active power filtering. Section 10 presents the future scope of the research in the grid-con-
The advanced functionalities can be accomplished by using diversified nected PV systems. Section 11 concludes this review with a concise
and multifunctional inverters in the PV system. Inverters can either be summary and proposition for the future work.
connected in shunt or series to the utility grid. The series connected
inverters are employed for compensating the asymmetries of the non-
2. Global status of the PV market
linear loads or the grid by injecting the negative sequence voltage. On
the other hand, the shunt inverters are used for enabling the active
The installed capacity of solar energy in 2016 is equivalent to the
power filtering function of PV by injecting the asymmetric and non-
installation of more than 31000 solar panels every hour [34]. Con-
linear current locally through the PV systems at the Point of Common
sidering the cumulative comparison status of the last five years, more
Coupling (PCC) [11,12]. In some case, the series-parallel combination is
solar PV capacity is installed in 2016. The percentage increase of the
carried out for providing both voltage (through series inverter) and
installed PV capacity in 2016 is 48% compared to that of 2015. The
1121
Table 1
Comparative analysis of various surveys on inverter and control schemes.
Ref GS PV AG PVI C PV CI VIT RE CG PV CO PV IGCPVIC FT Focused Area
[13] ✗ ✓ ✗ ✗ ✗ ✓ ✓ ✗ ✗ ✗ Inverter and control

[14] ✗ ✓ ✓ ✓ ✓ ✓ ✗ ✗ ✗ ✗ Inverter topologies
[15] ✗ ✗ ✓ ✗ ✓ ✓ ✓ ✗ ✓ ✗ Grid-connected PV
[16] ✗ ✗ ✗ ✓ ✓ ✓ ✓ ✗ ✓ ✗ Grid-connected PV
[17] ✗ ✗ ✓ ✗ ✗ ✓ ✗ ✗ ✗ ✗ Solar energy
[18] ✗ ✗ ✓ ✗ ✗ ✓ ✗ ✗ ✗ ✗ Solar energy
[19] ✗ ✗ ✓ ✓ ✓ ✓ ✗ ✗ ✗ ✗ Inverter and control
[20] ✗ ✗ ✗ ✓ ✓ ✓ ✓ ✗ ✓ ✓ Grid-connected PV
[21] ✗ ✗ ✓ ✓ ✗ ✓ ✗ ✗ ✗ ✗ Solar PV system
[22] ✗ ✗ ✓ ✓ ✗ ✓ ✗ ✗ ✓ ✓ Grid-connected PVI
[23] ✗ ✗ ✗ ✗ ✓ ✓ ✓ ✗ ✓ ✗ Grid-connected PVI
[24] ✗ ✗ ✗ ✓ ✓ ✓ ✗ ✗ ✓ ✗ Grid-connected PVI
[25] ✗ ✓ ✗ ✓ ✓ ✓ ✗ ✗ ✓ ✗ Grid-connected PVI
[26] ✗ ✗ ✓ ✗ ✓ ✓ ✓ ✗ ✓ ✗ Grid-connected PVI
[27] ✗ ✗ ✓ ✗ ✗ ✓ ✗ ✗ ✗ ✗ Solar PV system
[28] ✗ ✗ ✓ ✗ ✗ ✓ ✗ ✗ ✗ ✗ Solar PV system
[29] ✗ ✗ ✓ ✗ ✗ ✓ ✗ ✗ ✗ ✗ Hybrid energy systems
[30] ✗ ✗ ✓ ✗ ✗ ✓ ✗ ✗ ✗ ✗ MPPT
[31] ✗ ✓ ✓ ✓ ✓ ✓ ✗ ✗ ✓ ✗ Grid-connected PVI
[32] ✗ ✗ ✓ ✓ ✓ ✓ ✗ ✗ ✗ ✗ Multilevel inverter
[33] ✗ ✗ ✓ ✗ ✗ ✓ ✓ ✓ ✓ ✗ Grid-connected IC
[OS] ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ ✓ Grid-connected PVI
Note: Global Status of PV market (GS PV), Advancement of Grid-Connected PV Inverter (AG PVI), Classification of PV system (C PV), Classification of Inverters (C I),
Various Inverter Topology (V I T), Renewable Energy (RE), Control of Grid-Connected PV system (CG PV), Controllers for Grid-Connected PV system (CO PV),
Industrial Grid-Connected PV Inverters Comparisons (IGCPVIC), Future Trends (F T), Focused Area (F A), Inverter Control (IC), Photovoltaic Inverter (PVI).
Fig. 2. Solar PV global capacity and annual additions, 2006–2016.
3. Classification of the power electronic based converters
For a grid-connected PV system, appropriate phase, frequency, and

voltage magnitude of the three-phase AC output signal of the PV system
is required for the fast and accurate synchronization with the grid. The
DC to AC conversion is performed by an important component of the
grid-connected PV system known as the inverter [35–45]. The inverter
in most of the cases is a power-electronics based grid side converter and
can be categorized in to two main types based on their turn-on and
turn-off behaviours (commutation), that are the line commutated in-
verters and the self-commutated inverters. The line commutated con-
verters depend on the circuit parameters and the switches operate
Fig. 3. Classifications of power electronic based converters [41].
based on the polarity or direction of the current flow. On the other
hand, the self-commutated converters are operated with a full control
capacity of solar PV installed since 2006 shows a continuous and ex- over the turn–on and –off process of switching devices. The two main
ponential growth as depicted in Fig. 2. This increasing expansion of categories can further be divided in to various subtypes as illustrated in
solar PV market is because of the rising demand for the electricity, the Fig. 3 and discussed as follows:
global urge for the reduction in carbon dioxide emission, the desire to
limit the conventional energy sources, improvements and advance-
3.1. Line-Commutated Inverters
ments in the integration technologies, advancements in the solar PV's
potentials, and increasing effectiveness of the solar PV towards sup-
In Line-Commutated Inverter (LCI) the commutation process is
porting the electrical grid.
carried out by the parameters of the utility grid, that is, the reversal of
1122
Fig. 5. Grid-connected Self-commutated VSI [48].

Fig. 4. Grid-connected Line-commutated CSI [47].
Table 2
AC voltage polarity and the flow of negative current (or zero current) Difference between the VCM and CCM of VSI [41].
initiates the commutation process. The LCI in general uses the com-
Parameter VCM CCM
mutating thyristors as power switching devices, which are semi-con-
troller devices. The gate terminal of the device control the turn-on Inverter type Self-commutated VSI Self-commutated VSI
operation, whereas the turn off can not be controlled by the same Fault short circuit High Low (Limited to rated
mechanism as it depends on the line current or grid voltage for its turn- current current)
off. Thus, if a forced commutation is necessary, an external circuitry is Control parameter AC voltage AC current
added to the semi-controlled devices to control the turn-off process as

well. For example, an anti-parallel diode is added in the case of half-
Mode (VCM) and the Current Control Mode (CCM). In case of VCM, the
bridge LCI converter for enabling the process of forced commutation.
main controller variable is the PCC voltage, thus there is no control on
The basic schematic diagram for a line commutated current source in-
the line currents. On the other hand, in CCM the line currents are de-
verter is shown in Fig. 4.
livered in a control manner. The differences between the VCM and the
CCM are presented in Table 2 The VCM is recommended for the stand-
3.2. Self-Commutated Inverter alone or off-grid PV systems, as maintaining the PCC voltage magni-
tude, frequency and phase is of major importance in case of the stand-
The Self-Commutated Inverter (SCI) is the fully controlled power- alone power networks. Nevertheless, both VCM and CCM can be im-
electronic converter. The potential at the gate terminal controls both plemented for the grid-connected PV system, but CCM is most com-
the turn-off and the turn-on process of the power switching devices. The monly used method. The reason for using CCM is that the stiff electrical
transfer of current from one switching device to the other is enabled in grid dictates the PCC voltage, thus controlling the currents for deli-
a controlled manner. The devices used in the SCI include MOSFET and vering the produced PV power is more reliable and safer than the VCM
IGBT. For medium to high power application exceeding 100 kW and method with no control on currents. In case of grid disturbances, the
low-frequency range of 20 kHz, IGBTs are used. On the other hand, for a transient current suppression is possible with CCM and a high-power
high frequency typically in the range of 20–800 kHz and a low power factor can be acquired by simple control structure that is why inverters
less than 20 kW, MOSFETs are employed. For generating the output with the CCM are extensively utilized in grid-connected PV systems.
voltage waveform and for controlling the SCI, the Pulse Width Thus, the preferred inverter for a grid-connected PV system is the VSI
Modulation (PWM) switching technique is used. operated in current control mode.
For grid-connected inverter applications, high switching frequency
is required to allow the reduction in weight of the inverter, reduce the 3.2.2. Current Source Inverter
output current and voltage harmonics, and also to decrease the size of In Current Source Inverter (CSI), the input side of the inverter is
the output filter [46]. The SCI is a fully controller power electronic connected to a DC current source and hence, the polarity of the input
converter, thus it controls both inverter output current and voltage current remains the same. The polarity of the input DC voltage, how-
waveform. Furthermore, it is highly robust to the utility grid dis- ever, determines the direction of average power flow through the in-
turbances, suppress the current harmonics and improves the grid power verter. An AC current waveform of a variable width and a constant
factor. Nowadays, SCI is preferred over LCI for grid-connected PV sys- amplitude can be obtained at the output side. As opposed to VSI, a large
tems due to the advancement made to the control system for SCI and in inductor that upholds the stability of the current is attached in series to
addition, due to the evolution of advance switching devices similar to the input side of the CSI. A comparison summary between the VSI and
that of the power IGBTs and MOSFETs. The SCI can further be divided the CSI is presented in tabular form in Table 3.
in to voltage source converters and current source converters.
4. Various inverter topologies
3.2.1. Voltage Source Inverter
In Voltage Source Inverter (VSI), the DC voltage source is at the Based on the configuration and types of components used, inverters
input side of converter, thus the polarity of the input voltage remains can be classified into different categories. These division of categories is
the same. However, the polarity of the input DC current determines the based on various factors, such as, number of power processing stages
direction of average power flow through the inverter. At the output i.e. single stage and multi-stage, transformer and transformerless con-
side, an AC voltage waveform of a variable width and a constant am- figurations, number of levels involved in the design and the type of
plitude can be obtained. A tie-line inductor is used along with the VSI to switching used. Each category is briefly discussed and described as
limit the current flow from the inverter to the utility grid. Furthermore, follows:
a relatively large capacitor, similar to a voltage source is connected in
parallel with the input DC side of VSI. The self-commutated VSI con- 4.1. Inverters based on number of power processing stages
figuration is shown in Fig. 5.
The VSI can be operated in two modes that are the Voltage Control The inverters based on the power processing stages are classified
1123
Table 3
Dissimilarity between the VSI and CSI [49].
Parameters VSI CSI
Dependency on load The output voltage amplitude is independent of load. On the other hand, The output current amplitude is independent of load. On the other hand, the
the output current magnitude and waveform are dependent on the nature output voltage magnitude and waveform are dependent on the nature of load
of load
Power Source A DC voltage source with lesser or insignificant impedance is the input of Changeable current from a DC voltage source having high impedance is the
VSI. input of a CSI
Related The total power loss is low because of low conduction loss and high The total power loss is high because of high conduction loss and Low
loss switching loss. switching loss.
Input A constant input voltage is maintained. In parallel to the input DC side of The input current is continuous however changeable. In series to the input
parameter a VSI, a capacitor is connected. Whereas DC capacitor is efficient, cheap, DC side of a CSI, an inductor is connected. Whereas, DC inductor contributes
and small energy storage. more losses, expensive, and bulky.
multiple-stage buck-boost inverter topology is presented, where a high-

frequency transformer is used and works with a low DC voltage. In both
the aforementioned topologies, the rectified sine wave current obtained
in the first stage is converted into the full wave sinusoidal current at the
line-frequency switching by the second-stage current source inverter.
Two other multiple-stages inverters are proposed by [69]. One consists
Fig. 6. PV inverter types (a) Single stage inverter, (b) Two stage inverter [67]. of VSI associated in parallel with pseudo DC-link, and the other one
consists of a CSI coupled in series with a comparatively bulky inductor
into two main types, which are the single stage inverters and the in the last stage. The second one is designed by General Electric Com-
multiple stage inverters, as presented in Fig. 6. pany (GEC).
For passing the DC component of the input PV source and filtering
out the voltage spikes the process of power de-coupling is required in
4.1.1. Single stage inverter
single and multiple stage inverters. A bulky electrolytic capacitor
The single stage inverter performs various functions, such as the
having high capacitance is utilized to accomplish this decoupling. The
control of injected grid currents, the function of voltage amplifications
capacitor can be placed in two different ways, that are, in between the
and the process of maximum power point tracking. The design of the
two converter stages as a DC link or in parallel with the PV modules, as
single stage inverter handles the double peak power according to the
illustrated in Fig. 7. In general, the main objective of the inverter is to
equation presented below
convert the DC power into the AC power at the high switching fre-
pgrid = 2Pgridsin2 (ωgrid t ) quency. However, operating at such high switching frequency results in
undesired switching transients. Thus, the input side of the PV system is
where, ωgrid is the grid frequency and Pgrid is the peak grid power. protected with a DC-link capacitor, which blocks the flow of these
In single stage inverter, the use of line frequency transformer (op- transients from moving in a backward direction. These capacitors,
erating at low frequency) adds a large amount of weight to the inverter however, have several disadvantages, for example, at high operating
as well as contribute to the peak efficiency losses of 2% [52]. The use of temperatures their lifetime is lower in comparison to the other devices
high-frequency transformer or transformer-less converter design on the utilized in the inverter circuits. In addition, they are costly, and bulky in
other hand are the most efficient, cost effective and lighter in weight. size. Furthermore, in practical cases, these capacitors produce various
They are increasingly replacing the line frequency transformers. Var- significant problems. Their reliability and power conversion efficiency
ious inverter topologies such as the buck-boost or the boost converter are low. Because of these concerns, a prominent research is progressing
designs are presented in [53–66] with certain merits and demerits. The day by day to reduce or eliminate the capacitance of electric capacitor
DC to AC conversion and MPPT voltage amplification in [53–66] take and to utilize the small film capacitors as an alternative [50,51]
place in a single stage. In these topologies, either an inductor is used as
the energy storage element or a high-frequency transformer performing
4.2. Transformer and transformerless inverters
the functions of isolation and energy storage. The key characteristics of
the buck-boost single stage inverter is the elimination of line frequency
Another classification of the inverters, as per the existing literature,
transformer. However, single stage inverters frequently suffer from a
is made based on the existence or absence of the transformer. In other
low range of input DC voltage, low power quality, and reduced power
words, this classification can also have the single or multiple power
capacity. Furthermore, the current stresses on the power switching
stages but the main categorization in this case is based on the trans-
devices increase with the increase of power capacity. Consequently, the
former. In general, on the basis of transformer, the grid-connected PV
single stage inverters are avoided in certain application where wide
inverter topologies are categorized into two groups, i.e., those with
input voltage range, high power quality, and high distribution capacity
transformer and the ones which are transformerless.
is required. Consequently, for such applications, multiple stage in-
Line-frequency transformers are used in the inverters for galvanic
verters are preferred.
isolation of between the PV panel and the utility grid. The isolation
transformer helps in eliminating the problem of DC current injection
4.1.2. Multiple stage inverter
An inverter with more than one power processing stage is referred
to as the multiple stage inverter as presented in Fig. 9(b). In this type of
inverter, the last stage performs the function of DC to AC conversion
while the starting one (and the intermediate) stages achieve the voltage
amplification and in some cases the function of galvanic isolation. A
multiple stage buck-boost inverter is presented in [68]. The input DC
voltage range for converter is very low, it is the non-isolated type be- Fig. 7. Power de-coupling capacitor different positions for single stage and
cause no transformer is used in this case [68]. In [68] another isolated multiple stage Inverter [14].
1124
Fig. 8. (a) Placement of the Line-frequency transformer between the inverter and the grid. (b) HF-link grid-connected ac/ac inverter. (c) High-frequency transformer
is embedded in a dc-link PV-module-connected dc-dc converter [77].
from the PV system into the utility grid. Since, line frequency trans- limited especially in the low power applications (e.g., AC-module in-
formers are heavy in weight and bulky in size increasing in this way the verters), which thus will affect the overall efficiency. The double-stage
overall cost of PV system, so therefore the line-frequency transformer PV technology can solve this issue since it consists of a DC–DC con-
are considered as the problematic component of the inverter. An al- verter that is responsible for amplifying the voltage of the PV module to
ternative solution to this is to utilize the high-frequency transformer a desirable level for the inverter stage.
embedded in the inverter or DC/DC converter, which reduces the size The most commonly used transformer-based topologies of single-
and weight of the system, and thus decreases the overall cost. phase grid-connected inverters are half H-bridge, full H-bridge, HERIC,
Considering this, some inverter topologies are presented in Fig. 8. H5, H6, NPC, active NPC, flying capacitor, and Coenergy NPC.
The transformer-less inverter in comparison with the transformer Recently, in the market there are many manufacturers for transformer-
topologies are cost-effective solutions and present higher efficiency. less PV inverters e.g.: REFU, Danfos solar, Ingeteam, Conergy, Sunways,
However, for addressing the problem of DC current injection, they re- and SMA, offering the maximum efficiency of up to 98% and high
quire extra circuitry to be installed. Another problem related to trans- European efficiency (> 97%). The transformer-less inverters can be
former-less topologies is that there is no galvanic isolation between the single stage or multiple stages. A two stages grid-connected high-fre-
utility grid and the PV array. Furthermore, it may cause voltage fluc- quency transformer-based topologies is discussed in [78], where a
tuations between the PV array and the ground, depending upon the 160 W combined fly-back and a buck-boost based two-switch inverter is
inverter circuit. A virtual capacitor formed between the surface of PV presented. Similarly [79], presents a High Efficient and Reliable In-
array and the installed ground, this fluctuating voltage contributes to verter (HERIC) grid-connected transformer-less topology. The HERIC
energizing the capacitor. Depending on the structure of PV panel and topology increases the efficiency by including the zero voltage with the
the weather parameters, the capacitor may have values up to 1 μF / kWp help of an AC bypass to the performance of full-bridge with bi-polar
for thin-film cells and typically lies in the range of 50 and 150 nF / kWp modulation. Furthermore, five switching devices based H5 topology is
[70]. An electrical hazard may cause if a person standing on the ground presented in [80]. Various other topologies are proposed with their
touches the PV array due to the capacitive current flow in his body. promising features in [79–84] as: (a) one topology is full bridge to-
Another problem related to the PV array is the generation of electro- pology with DC bypass comprising of two diodes and six switching
magnetic interference, which is caused due to the voltage fluctuations. devices, (b) another topology is H6-type configuration, which is made
However, according to different research studies [71–76], the electro- up of two split inductors as a low-pass filter, two freewheeling diodes
magnetic interference of transformer-less topologies is negligible and and six power switching devices, this topology is well suited for non-
hazardless. However, to prevent unsafe current levels (above 10 A) in isolated module integrated inverter, (c) one of the topologies is flying
the design of transformer-less grid-connected inverter, certain re- capacitor type topology with midpoint clamping to the neutral wire of
commendation should be followed. the power grid due to three level output voltage it provides a low filter
The differences between transformer-less and transformers-based inductor current ripple, (d) another topology is called Karschny (flying
inverter are presented in Table 4. The line frequency transformers are inductor) that eliminate any voltage oscillations due to direct connec-
bulky in size, expensive and reduce the system efficiency because of tion of the negative terminal of the PV array and the output neutral.
power losses in the transformer windings. Transformer-less inverter
topologies are introduced for PV application to overcome these issues.
It can improve the system efficiency by 1–2%. Furthermore, they are 4.3. Multilevel inverters
lighter, smaller and lower in cost. Transformer-less inverters can be
single stage or multiple stages. A major drawback of the single-stage PV Today, the decrease in the overall cost of the grid-connected PV
topologies is that the output voltage range of the PV panels/ strings is system is due to the improvement in the existing grid-connected in-
verter technologies. In comparison to the simple two-level inverters,
Table 4
Differences between transformer based and transformer-less inverters [72].
Inverter Line-frequency transformer based inverter High-frequency transformer based Transformer-less inverter
Inverter
Advantages Safer due to galvanic isolation, high reliability, safer due to galvanic isolation, high efficiency, lightweight, compact high efficiency, light
simpler design and simpler design weight, compact, Complex design
Disadvantages High volume and weight, low efficiency Costly technology, and complex Additional safety measures
essential
1125
Fig. 10. Full-bridge single leg switch clamped inverter [93].
Fig. 9. Three-level half-bridge diode clamped inverter [95].
multilevel grid-connected inverters offer numerous benefits. The mul-

tilevel inverters result in the AC voltage at the inverter's output term-
inal, which comprises of several staircase voltage levels. The staircase
sinusoidal waveform resulting from the multilevel inverter is close to an
actual and pure sinusoidal wave with low total harmonic distortion.
Thus, the filter requirement is reduced and the harmonic distortion is
low. Various DC voltage levels can be easily produced due to the
modular structure of PV arrays; therefore, multilevel topologies are
principally suitable for the PV systems. Since 1975 [85], the idea of the
multilevel converter has been presented and three-level converter in-
itiated the term of multilevel [86]. Consequently, a few multilevel Fig. 11. Cascaded inverter [97].
converter typologies have been produced in [86–94]. In the subsequent
subsection, some multilevel inverter topologies are described.
4.3.3. Cascaded inverters
For DC-AC conversion, a cascaded inverter used in a transformer-
less grid-connected PV system is illustrated in Fig. 11 [97]. This to-
4.3.1. Half-bridge diode clamped inverters
pology connects in series the AC output of two full-bridge configura-
A schematic diagram of the half-bridge diode clamped three-level
tions in order to increase number of voltage levels. This is because each
inverter, which is an important part of the single-phase transformer-less
individual bridge can produce at its AC output with three dissimilar
grid-connected PV systems is presented in Fig. 9 [95,96]. At the output
voltage levels. Thus, resulting in an inclusive five-level AC output vol-
terminal of the inverter, a positive voltage can be achieved by si-
tage. The modular and scalable feature of the cascaded inverter is the
multaneous switching of the switches S1 and S2. A negative voltage is
key advantages, as it can be extended to achieve even higher number of
obtained by switching of S3, and S4, whereas a zero voltage is created by
levels just by cascading the basic three-level modules. For a trans-
turning-on both S2 and S3 at the same time. In order to allow the
former-less PV system, with small input DC voltage on the input side
transfer of power from PV to the utility grid, the DC bus voltage must
(i.e. 40 V each), more than two full bridge configurations can be con-
always be more than the grid voltage amplitude. The midpoint of the
nected in series, as suggested in [87]. Furthermore, in [92,98] cascaded
PV array is grounded, and this reduces the electromagnetic interference
inverters are presented for high power applications.
and eliminates the capacitive earth current, which are the advantages
of this inverter topology.
4.4. Soft/hard switching inverters
The inverters can also be categorized based on the type of switching

4.3.2. Full-bridge single leg clamped inverters
employed. In this case, the inverters are categorized based on the type
A full-bridge single leg clamped inverter, for residential PV systems
of switching employed and not on the number of power stages. In
is described in Fig. 10 [93]. In addition to conventional full bridge
general, there exist two types, the hard and soft switching inverters.
switches S6, S5, S4, and S3, bidirectional switches S1 and S2 along with
Thus, both hard and soft switching inverters can be comprises of one or
the diodes D1 and D2 are added. This allows the proper control of
more than one power stages. Nowadays, the grid-connected PV in-
current flowing to and from the midpoint of DC bus. With this topology,
verters are designed using the soft switching technique in order to
the minimum size of the inverter for a transformer-less PV system is
achieve high power density, high efficiency, and better performance.
approximately 1.5 kW.
Serious EMI problems and switching losses are caused by abrupt
1126
Note: PD: Power De-coupling, LIEC: Large Input Electrolytic Capacitor, IDI: Intermediate DC-link Inductor, SFC: Small Film Capacitor, ToTI: Type of Transformer Interconnection, T-L: Transformer-Less, H-FT: High
variation in switch currents and voltages, especially in the high-fre-
Four switching devices based Single stage buck-boost inverter topology
Series resonant dc-dc converter with bang-bang dc-ac converter [118]

quency switching inverter [99,100]. This abrupt switching of the de-
Novel selective dual-mode timesharing sine wave controlled soft-

vices at a random instance is referred to as the hard switching and thus,
Single-stage inverter based on buck-boost configuration [110]

causes various problems in the switching process. Due to the stray in-
Zeta-Cuk configuration derived Single-stage inverter [108]

ductances and parasitic capacitances of the power electronic devices,
Half-bridge diode clamped three-level inverter [115]

the high current or voltage spikes occurs during the abrupt switching
Full bridge single leg switch clamped inverter [116]

Two-stage non-isolated buck-boost inverter [111]
transients. Passive components based high-frequency resonant net-
works such as the capacitor-inductor tank, the power-switching devices
Two-stage isolated buck-boost inverter [111]

Single-stage isolated flyback inverter [109]
and the auxiliary diodes are combined with the traditional hard-
switching PWM circuits to form the soft-switching topology. There are
Two-switch flyback inverter [114]

two types of soft-switching: zero-voltage switching (ZVS) for reducing
Single stage boost inverter [107]

dv/dt and zero-current switching (ZCS) for reducing di/dt.
The traditional PWM based buck-boost inverter topologies have
switching inverter [113]
Cascaded inverter [117]

several disadvantages such as, (a) high-frequency harmonic compo-
inverter by GEC [112]

Multiple stage boost
nents causing EMI, (b) large leakage current due to the intrinsic high-
frequency common mode voltage at the output terminals, (c) low effi-
Topology type
ciency at high switching frequency (d) increases the size and weight of
the converter if designed to operate at low switching frequency and
[106]
high efficiency.
These limitations are overcome by the resonant soft switching
techniques, the voltage across or the current through the switching
EC
M
M
M
M
M
H
H
H
H
H
H
device is ensured to be zero at the instance of switching. This minimizes
ELT
the switching losses of the power switching devices. Various soft-
M
M
M
M
M
M
M
M
M
M
M
M
L
switching inverter topologies are discussed in the literature. The work
H-FT
H-FT
H-FT
H-FT
H-FT
H-FT
ToTI
T-L
T-L
T-L
T-L
T-L
T-L
T-L
T-L
in [101] presents a series-resonant DC-DC converter with bang-bang
DC-AC inverter. It is a two-stage inverter and the advantage of this
LIEC & IDI

LIEC & IDI
topology is that no in-rush current flows when the inverter is attached
to the grid for the first time. The authors in [102] give the idea of high-
LIEC
LIEC
LIEC
LIEC
LIEC
LIEC
LIEC
LIEC
LIEC
LIEC
LIEC
SFC
PD
frequency link series resonant soft-switching inverter in which the

Diode
switching devices are operated under ZCS. The study in [103] presented
Four
Four
Two
Two
Two
Two
Two
Two
Two
One
a single stage soft switching fly-back inverter based on capacitive id-
Six
ling. The authors in [104] proposed the LLCC resonant inverter with
Switch
Three
Eight
Eight
ZVT-PWM boost converter, the LLCC resonant inverter includes a par-
Four
Four
Four
Four
Four
Five
Five
Six
Six
Six
Six
allel-resonant tank and a series-resonant tank that provide the AC
output voltage with low THD. The work presented in [105] designed
Power rating
High power
Frequency Transformer, ELT: Expected Life Time, L: Low, M: Moderate, H: High, EC: Expected Cost.
application
ZVS-ZCS-PWM inverter with ZVT-PWM boost converter. This topology
500-3 kW
500-3 kW
>1.5 kW
consists of three stages, the first stage is a ZVT-PWM boost converter,
500 W
160 W
250 W
>3 kW
>3 kW
>3 kW
2 kW
2 kW
4 kW
5 kW
the second stage is a ZVSZCS- PWM buck converter and the third stage
is a line-frequency full bridge inverter. A detailed comparison and
Inverter using electrolytic capacitor of low capacitance or using film capacitor in place of a
benchmarking evaluation of the aforementioned inverter topologies is

presented in Table 5.
5. Classification of photovoltaic system
The PV system is categorized into two main types that are, the
stand-alone PV systems and the grid-connected PV systems. This clas-
sification is based on the component configuration of PV systems, their
functional and operational requirements and their connections to the
other power sources and loads. The standalone system operates in self-
sustained mode, independent of the utility grid. On the other hand, the
grid-connected applications employ PV systems in conjunction with the
utility grid. In general, the grid-connected PV systems are able to pro-
vide AC and/or DC power services to the grid as well as the connection
Evaluation of different inverter topologies.
to other alternate Energy Storage (ES) devices. Due to the low cost and
maintenance requirements, as well as the environmental friendly
High-frequency transformer inverter
nature, the grid-connected PV systems with ES are frequently adopted

large electrolytic capacitor
in many practical applications. It is worth mentioning that, without the

storage, the PV system has to be shut down during night-time or cloudy
day. The grid-connected systems with ES have several features and
Multiple-stage inverter
Soft-switching inverter
Single-stage inverter
characteristics, such as, 1) the charging of the battery during off-peak

Category of inverter
Multilevel inverter
hours, 2) buying power from the grid in case PV and battery power is
not available, and 3) selling the excess of produced power to the grid
during peak load hours. The PV system with ES addresses the issues of
meeting the peak load demand and contributes in this way flexibly to
Table 5
the power management targets. The standalone PV systems on the other

hand are not new [119] as they are supplying electrical power to the
1127
Fig. 12. Ratio of off-grid versus grid-connected solar PV distribution between 1993 and 2012.
remote areas for decades without any connection to the utility grid. The 6.1. Central inverters
standalone PV systems operate independent of the utility grid. They are
usually powered by a PV array or by a hybrid PV system and supply In this design, a large number of series interconnection of the PV
electrical power to the well sized AC or DC loads. Until 1995, the modules is enabled in order to increase the voltage rating of inverter
standalone PV systems were more commonly used as compared to the and to avoid the further amplification of the system connected to the
grid-connected systems, as presented in Fig. 12 [120]. Later on, after grid [122–125]. This series interconnection is commonly referred to as
1995, the grid-connected systems become more dominant, contributing the string. On the other hand, in order to increase the power level a
in this way to the overall operation of the power system. In both parallel interconnection of these strings is developed by employing the
standalone or grid-connected PV systems, power electronic based in- string diodes [125]. The central inverter topology, however, has several
verter is the main component that converts the DC power to AC power, restrictions such as: (a) the losses in the string diodes, losses as a result
delivering in this way the power to the AC loads or electrical grid. of voltage mismatch, losses among PV modules, and centralized MPPT
Usually, the output power of the PV system is optimized by the Max- power losses, (b) interconnection of the PV modules and inverter re-
imum Power Point Tracker (MPPT), which is a kind of DC-DC converter quires a high voltage DC cables, (c) the line-commutated thyristors
and is interconnected between the load and the PV array. usually used in this topology produces poor power quality and current
The grid-connected PV systems are heavily employed these days, as harmonics, (d) non-modular and non-flexible design, and (e) in some
can be seen from Fig. 2. However, this increasing penetration presents cases failure of the PV plant because of the central inverter [126–131].
numerous challenges to the power system. Their undesirable impacts to
the distribution grid involve the reliability and stability issues. The 6.2. String inverters
major challenges are: (a) voltage fluctuations at the PCC, (b) frequency
variations, (c) overvoltage in the distribution feeder because of the Nowadays, string inverters are the most commonly used grid-con-
reverse power flow, (d) intermittent power generation of the PV sys- nected inverters [132,133]. In a string inverter, a single string of the PV
tems, (e) current and voltage harmonics generated by the inverters, and module is attached to the inverter. It is a reduced version of the central
(f) low power factor operation of the distribution transformer [121]. inverter [134]. The power range is low due to a single string (typically
up to 5 kW). A distinct MPPT is applied to each string and also the string
diode losses are eliminated. Thus, the overall efficiency is around 1–3%
6. Configurations of the grid-connected PV inverters higher in comparison to the central inverter. The mismatch and partial
shading are also reduced in this topology [135].
The grid-connected inverters undergone various configurations can
be categorized in to four types, the central inverters, the string in-
6.3. Multi-string inverter
verters, the multi-string inverts and the ac module inverters. The four
types are shown in Fig. 13 and explained below with their design
In multi-string inverter, many strings are connected to their in-
characteristics, advantages and limitations ( Fig. 14 and 15).
dividual DC to DC converter, with a separate MPP tracking system. All
Fig. 13. Configurations of grid-connected PV inverter [125,152].
1128
Fig. 14. (a) Single phase inverter with DC/DC converter. (b) Single phase inverter without DC/DC converter. (c) Single phase inverter with PCSP.
the strings are then connected to a common DC to AC converter. topologies are summarized in Table 6. The comparison is performed on
Basically, it is a further modification of the string inverter. This to- the basis of advantages, disadvantages, costing, and rating. The PV
pology is preferred over central inverter as every string is controlled Technology characteristics are described in Table 7.
individually. It is a hybrid topology that combines the advantageous
feature of central and string topologies. It is modular in structure and
7. Control of grid-connected photovoltaic system
can be easily expanded by adding a new string to the existing one
[136,137]. In multi-string topology, Insulated Gate Bipolar Transistors
The DC to AC inverter helps in controlling the power factor by in-
(IGBTs) are utilized for high power and low switching frequency
jecting the sinusoidal current into the grid. The DC energy generated
whereas, Metal Oxide Field Effect Transistors (MOSFETs) are used for
from the solar PV is converted into the AC power and is efficiently
high switching and low power.
transferred to the electrical grid by the application of grid side inverter
(GSI). The proper operation of the grid side inverter is ensured by de-
6.4. AC modules signing fast and accurate control system. Thus, the control of GSI is one
of the most significant part of the grid-connected PV system connected.
In this topology, the integration of inverter and PV module is carried The two main sub-classifications of the PV control system are:
out in a single electrical device. It is a “plug and play” device and does
not require expertise for its installation. The mismatch losses of the PV (a) MPP control module: The maximum power extraction from the PV
modules are eliminated in this topology [138]. It has a modular design module or input RE source is performed by the MPP control.
and can be easily expanded. The optimal adjustment of the inverter and (b) Inverter control module: ensures (a) a proper grid synchronization
the PV module is supported by this topology. Nowadays, the AC mod- and high quality of the injected power, (b) control of the active and
ules employ the self-commutated converter topology as the DC-AC in- reactive power delivered to the grid, and (c) the control of DC-link
verter [139]. As mentioned, all the functions including DC to AC con- voltage.
version, MPPT, and voltage amplification are performed in a single
module, and thus, it makes the circuit more complex and increase the The inverter control strategy consists of two main cascaded loops.
price per wattage. Typically, a loop which controls the grid current is a fast-internal cur-
A detailed comparison and benchmarking of the four converter rent loop, and loop which regulates the DC-link voltage is a slow
1129
Fig. 15. (a) Block diagram of dq control strategy. (b) Block diagram of αβ-control strategy. (c) Block diagram of abc control strategy.
external voltage loop. The current protection and power quality issues controllers, and the intelligent controllers [156].
are associated with the current control loop. The significant features
required from the current controller are faster dynamic response and 8.1. Linear controllers
harmonic compensation under distorted grid conditions. For balancing
the power flow in the system, the DC-link outer voltage controller is These controllers are designed based on the features and dynamics
employed. Generally, the purpose of the external controller is the sta- of the linear system. The typical feedback control theory is used for
bility of slow dynamic system and optimal regulation. For stability analyzing and designing these controllers.
purpose, the current control loop is designed with dynamic speed lower
than the speed of voltage control loop (approximately 5–20 times 8.1.1. Classical controllers
greater). The designing of voltage controller does not require the This family consists of the Proportional-Integral-Derivative (PID),
transfer function of internal current control loop, since the external and the Proportional-Integral (PI), the Proportional-Derivative (PD), and
internal loops can be designed in a decoupled way [144–152]. In some Proportional (P) controllers. These controllers are the base of classical
cases, the cascade of voltage control loop and power loop can be used as linear systems and control science. A few classic controllers are tabu-
an alternative of the current loop and the injected currents are in- lated in Table 10.
directly controlled. A detailed evaluation of the control structures for
single-phase and three-phase inverters are evaluated in Table 8 and
8.1.2. Proportional Resonant (PR) controllers
Table 9, respectively.
PI and PR controllers work in a similar manner but in two different
operating frames. The PI controller allows efficient tracking of DC
8. Various controllers for the grid-connected PV system signals, whereas the PR controller tracks a sinusoidal signal with the
frequency of sinusoid as its central frequency. The way the integration
The overall operation of the grid-connected PV system depends on takes place in PI controller is different form the one that takes place in
the fast and accurate control of the grid side inverter. The problems PR controller. As opposed to PI controller, the integrator in case of PR
associated with the grid-connected PV system are the grid disturbances controller integrates the frequencies, which are close to the resonant
if suitable and robust controllers are not designed and thus, it results in frequency. Consequently, phase shifts or stationary errors are not in-
grid instability. According to the specific operating condition and be- volved [157].
havior of the electrical grid, the controllers of PV system are divided
into 6 categories, which are the linear controllers, the non-linear con- 8.1.3. Linear Quadratic Gaussian (LQG) controllers
trollers, the robust controllers, the adaptive controllers, the predictive The combination of the linear-quadratic regulator and the Kalman
1130
Table 7
Cost is low when compared to string inverter but at

1. In case of faults the replacement of inverter is not
6. Modularized nature can produce mass production
2. Deals with the problem of partial shading and
PV technology characteristics [140–143].
4 Elimination of bulky electrolytic capacitor
5. Individual modules failure detection and

PV technology Parameter's details
5. Longer average life (around 25 years)

1. No mismatch losses between modules
significant features
3. Flexible and expandable in design
high levels it may presents high cost

Fuel used Solar power
that is why it is more appropriate
Operating range 1 kW up to 300 MW

Efficiency of PV cells 6–7% organic cells, 11–14% for thin film, and
12–16% for crystalline silicon
Application types Utility-scale, commercial, and residential
Benefits Low maintenance and operating costs, sustainable and
debugging is easy
up to500– 600 W
emission-less technology, and modular type.
Environmental impact No direct CO2, CO, NOx emissions
AC Module
Maintenance & 0.004 USD/kWh for utility scale generation and 0.07
simple operation annual USD/kWh (AC) for grid-connected residential.
costs
Installation costs 600 − 1300 (USD/kW)
Drawbacks Fluctuating output power due to the deviation in
2. The reliability of the system decreased as all the
weather patterns, higher installation costs, require

4. Separate current control and MPPT is utilized
3. The DC-DC converter can be used for voltage
electronic & mechanical tracking devices and back up

1. Surplus losses inside the DC-DC converter
storage for maximum efficiency.

2. Partial shading energy loss is reduced
strings are coupled to a single inverter

1. String diode losses are eliminated
filter forms an LQG controller. According to separation principle, these

two controllers can be designed and computed independently. LQG
In comparison to centralized
Topology its cost is higher
controller is suitable for both time-varying and time-in varying systems.

The design of linear feedback controllers for the control of nonlinear
and uncertain systems is provided by the application of LQG control to
linear time-invariant (LTV) systems [158].
amplification
Multi-String
8.2. Non-linear controllers

50 kW
These controllers have an extraordinary operation compared to the

5. Since each PV module contribute AC signal to a common AC
basic controllers. However, in terms of design and implementation,

these controllers are complicated.
bus that is why it provides improved safety and stability
Its cost is higher in comparison to centralized inverter
8.2.1. Sliding mode controllers

For the regulation of the output voltage of the PWM inverters,
Sliding Mode Controller (SMC) technique have been used extensively
2. Partial shading energy loss is reduced
due to its robust and improved performance. The main advantages of

1. String diode losses are eliminated
this technique are insensitivity to parameter variation and load dis-

3. Flexible in structure and design
1. Suitable for low power ratings
turbances. Hence, in the ideal case, an invariant steady-state response

can be accomplished by the application of SMC to the PV system. On the
other hand, it is difficult to locate a legitimate sliding surface, to which
4. Decent consistency
the performance of SMC is heavily dependent. The performance of the

SMC is also dependent on the sampling time and suffers from distortion
1–5 kW /string
if an inadequate sampling time is selected. In addition, when SMC

tracks a variable reference, the phenomena of chattering is observed,
String
which is a major disadvantage of SMC, degrading in this way the overall

efficiency of the PV system [159].
Historical overview of grid-connected PV inverter [14,234].
4. The working of solar module is interrupted

3. Power feeding to the utility grid is cut off
Lower cost in comparison to string inverter

2. DC losses due to high voltage DC cables
1. Mismatch in PV modules, string diodes,
8.2.2. Partial Feedback Linearization (PFL) controllers

1. Central inverter presents lower cost
Feedback linearization is the direct way for designing the non-linear

and centralized MPPT power losses
controllers, as a non-linear system is converted to fully or partially

linear system. By cancelling the non-linearities within the system makes
this possible. So, the linear controller design approaches can be utilized
in case of inverter failure
5. Non-flexible in design
to design the controller for these systems. When the non-linear system is
under partial shading
converted into a halfway linear system, is known as Partial Feedback

6. Low reliability
Linearization (PFL) and if converted into a completely linear system is

known as Exact Feedback Linearization (EFL) [160].
1–50 MW
Central
8.2.3. Hysteresis controllers

One of the non-linear controllers is hysteresis controller. An adap-
Disadvantages
tive band of the controller must be created in order to attain a stable

Power Rating
Advantages
switching frequency, which is an important step for implementing

Topology
Costing
hysteresis controller. Hence the output of this controller is the state of

Table 6
the switches, therefore consideration regarding the isolated neutral is

important again [161].
1131
Table 8 control scheme has high computational complexity [164]. Table 10

Control configurations for single-phase inverters [153]. comprehensively describes few adaptive control schemes.
Topologies Advantage Inconvenient Figures
8.5. Predictive controllers
Single phase • Fast Dynamic • Complex 14(a)
inverter with
DC/DC
• Instantaneous
current control
hardware
circuit
The future behavior of controlled parameters is predicted by using
converter • No full control
of power factor
system model in predictive controllers. Based on a predefined optimi-
zation criteria, to obtain the optimal actuation the controller utilizes
Single phase
inverter
• Simplicity of the
conversion system
• Complex
hardware
14(b) this information. It can be applied to different systems while a multi-
without DC/DC • Instantaneous circuit
variable case can be considered, because of its non-linearities, con-
converter current control • No full control straints that can be simply incorporated, and very fast dynamic re-
• Fast Dynamic of power factor sponse. It has also easy implementation. The comparison of classic
Single phase • Simplicity • No full control 14(c) controller and this controller comparison shows, that an excessive
inverter with • Less circuitry of current number of calculations is required in the predictive controller.
PCSP • Few resources • No fast
• Reactive
controlled
power dynamics
8.5.1. Deadbeat controllers
The dynamic response of the controlled system which is controlled
by the differential equation is discretized and derived, in the deadbeat
8.3. Robust controllers control theory. Centered on these equations, at the end of the sampling
period for the state variables to reach the reference values the control
A robust control theory approach is utilized in order to design a signal is calculated, at the start of the individually sampling period.
controller with concern to uncertainties. The goal of these methods is to
acquire stability in the occurrence of partial modelling errors as well as 8.5.2. Model Predictive Controller (MPC)
robust performance. In the robust control, bounds, clear description, In the MPC, a cost function is defined from a flexible criteria, to
and good criteria must be defined. This controller can promise robust select optimal actions that should be minimized. In this strategy, a
performance and stability of the closed-loop systems, even in the mul- model of the system is utilized for predicting the response of the vari-
tivariable systems [162]. ables till a precise time. In design stage of the controller, the MPC
simply includes system constraints and non-linearities.
8.3.1. H-infinity controllers
To use H-infinity methods, the control problem is represented as an 8.6. Intelligent controllers
optimization problem by the control designer and then solves it.
Multivariable system problems are solved by H-infinity techniques. But, Intelligent controllers obtain automation through the imitation of
it needs a good model of the system to be controlled and has high biological intelligence. Furthermore, the way biological systems trou-
computational complexity. Additionally, non-linear constraints are not bleshoot problems are borrowed for the idea. Then, that is utilized to
well handled. solve the control problem.
8.3.2. Mu-synthesis controllers 8.6.1. NN controllers

The effect of unstructured and structured uncertainties on the per- Neural Network (NN) is inspired from the human nervous system. It
formance of the system is considered by the mu-synthesis approach. On is a connection of a biological brain system that is stimulated by a
the notion of an organized singular value, the design of the controller is number of artificial neurons. It has the ability to obtain a higher fault
based, in this method. In the power and energy domain, the previous tolerance and estimate an optional function mapping [165]. When NN
application of this method can be found in [163]. is used in control system, it can train either off-line or online.
8.4. Adaptive controllers 8.6.2. Repetitive controllers

The basic idea of the Repetitive Controllers (RC) is derived from the
In adaptive control methods, depending on the operating conditions principle of the internal model. Good tracking/rejection can be
of the system the control action is automatically adjusted. With high achieved, in the closed-loop path if the model of any disturbance/re-
performance, the accurate system parameters are not required. This ference is injected. The basic structure of the RC controllers is described
Table 9
Control structures for three-phase inverters [154,155].
Topologies Advantage Inconvenient Figures Controller Type
dq control • Simplicity • The steady-state error is not removed 15(a) PI

• Controlling and filtering can be easier accomplished • Compensation
poor
capability of the low-order harmonics is very
αβ -control • The steady-state error is removed • Complex Hardware circuit 15(b) PR

• Around
acquired
the resonance frequency, a very high gain is • No complete control of power factor
• High dynamic
abc control • The transfer function is complex 15(c) PI
• Simple transfer function • More complex than hysteresis and Deadbeat PR
• High dynamic • High complexity of the control for current regulation. Hysteresis
• Rapid development
• High dynamic. • Implementation in high frequency microcontroller Dead-Beat
• Simple control for current regulation.
• Rapid development
1132
Table 10
Main features of the proposed controllers in literature.
Ref. Reference Frame I M F CP F A Controller
[167] Three-phase, αβ A, D PWM S-L V, P LCL DG Classic

[168] Three-phase, dq A, D SVM M-L V, C LC DG Classic
[169] Three-phase, αβ A, D SPWM M-L V, C LC DG Classic
[170] Three-phase, dq A, D VLUT S-L V L DG Classic
[171] Single-phase A, D SPWM M-L V, C LCL G Adaptive
[172] Three-phase, αβ D PWM S-L C L G DB
[173] Three-phase, dq D SVM M-L C LC APF Adaptive, Repetitive
[174] Three-phase, dq D PWM M-L C LCL G DB
[175] Three-phase, αβ D SVM M-L C LCL DG Adaptive, MPC
[176] Three-phase, dq A PWM M-L C, P L DG Classic
[177] Three-phase, αβ A PWM M-L C LC APF Classic
[178] Single-phase A PWM M-L C L APF Classic
[179] Three-phase, αβ A PWM S-L C L DG Classic
[180] Three-phase, dq A PWM M-L V, P LC PV Classic
[181] Single-phase A, D PWM M-L C LCL G Classic, PR
[182] Three-phase, αβ A SVPWM M-L C, P L DG Classic, PR
[183] Three-phase, αβ A SVM S-L V, C LCL DG PR
[184] Three-phase, dq A PWM S-L C LCL G Classic, PR
[185] Single-phase A PWM S-L C LCL PV PR
[186] Three-phase, αβ A SVM S-L C LCL PV PR
[187] Single-phase A PWM M-L C LCL G PR
[188] Single-phase A SPWM M-L C LCL G Classic, PR
[189] Three-phase, dq D PWM S-L C LCL G LQG
[190] Three-phase, αβ D SVPWM S-L C L PV PR,LQG
[191] Single-phase A PWM M-L V LC UPS SMC, Fuzzy
[192] Three-phase, dq A PWM S-L C L PV SMC
[193] Three-phase, dq A PWM S-L V, P LCL PV PFL
[194] Single-phase A PWM M-L V, C LC G Classic, Hysteresis
[195] Single-phase A PWM M-L C L G Hysteresis
[196] Three-phase, αβ D PWM S-L C LCL G Hysteresis, MPC
[197] Three-phase, dq A PWM S-L C LC G H∞ , Repetitive
[198] Three-phase, αβ A PWM M-L C LC G H∞
[199] Three-phase, dq A PWM M-L P LC DG Adaptive
[200] Three-phase, dq A SVPWM S-L V LC DG Adaptive
[201] Three-phase, dq A SVPWM S-L V LC DG Adaptive
[202] Single-phase D SPWM S-L C L General Adaptive, Repetitive
[203] Three-phase, αβ D PWM M-L C L General Adaptive
[204] Three-phase, dq D SVM S-L V LC UPS Predictive
[205] Three-phase, αβ D PWM M-L P L PV, APF Fuzzy, Predictive
[206] Three-phase, αβ D PWM M-L P L PV, APF SMC, Predictive
[207] Three-phase, dq D SVM S-L C L General DB
[208] Three-phase, dq D PWM S-L C L General Adaptive, DB
[209] Single-phase D PWM M-L V, C LC UPS DB
[210] Three-phase, dq D SVPWM S-L C L DG DB
[211] Three-phase, αβ D PWM S-L V LC UPS DB, Repetitive
[212] Three-phase, abc A PWM S-L P LC DG MPC
[213] Three-phase, abc A PWM S-L V, P LCL,LC DG MPC
[214] Three-phase, abc D PWM S-L V, C LCL General MPC
[215] Three-phase, αβ D PWM S-L C L General MPC
[216] Three-phase, dq D SVPWM S-L C L General MPC
[217] Three-phase, dq D SVM S-L C L PV MPC
[218] Three-phase, αβ A PWM M-L C, P LC DG Classic, Repetitive
[219] Three-phase, dq D SVM S-L C L PV Classic, Repetitive
[220] Single-phase D PWM S-L V LC General Repetitive
[221] Three-phase, abc D PWM S-L V, C L General Classic, Repetitive
[222] Three-phase, dq D SPWM M-L V, C LC UPS Repetitive
[223] Single-phase A, D PWM S-L C LCL General RC
[224] Three-phase, abc D PWM M-L P L PV Fuzzy, NN
[225] Three-phase, abc A, D PWM M-L P L PV Classic, NN
[226] Three-phase, αβ A PWM M-L V, C, P LCL DG Autonomous
Note: I: Implementation, A: Analog, D: Digital, F: Feedback Loop, S-L: Single Loop, M-L: Multi Loop, Control Parameter: CP, C: Current, V: Voltage, P: Power, F: Filter,
M: Modulation, A: Application, G: General, DG: Distributed Generated.
in [166]. During a period the error signal should be stored, in order to 8.6.3. Fuzzy Logic Controllers
determine the reduction or elimination of the error in other periods. In Fuzzy Logic Controllers (FLC), the knowledge of smart human
Hence, for periodic non-linear loads, RC has been used. The dynamic being is defined and implemented to control the dynamics of a system.
response of this controller is not desirable although its performance is The architecture of FLC method consists of (a) Fuzzification, (b) Rule
appropriate for periodic nonlinear loads. In order to tackle this issue, base, (c) Inference mechanism, and (d) De-Fuzzification. In fuzzifica-
RC by parallel or cascaded structure can be joined with extremely dy- tion, a set of crisp data is converted into fuzzified data, in rule bases
namic response controllers. certain rules are defined according to the requirement of application to
be controlled, in inference mechanism the rules are evaluated and the
decision is made according to the defined rules, in de-fuzzification the
1133
fuzzified data is converted back to crisp data and thus, a proper control transformers (low or high frequency) or using switch (in case of
action is achieved. transformerless inverters). In Spain under grid code RD 1699/2011,
this feature is required for the connection of PV to low-voltage
8.6.4. Autonomous controllers distribution system and is also adopted in the other countries of the
To perform the complex control tasks independently, autonomous world. Thus, depending on application, the selection of galvanic
controllers are used. By adding intelligence to the procedure of refining isolation can be made.
autonomy, to acquire an advance level of automation, engineers di-
rectly try to automate the human's technology and knowledge. 9.1.2.2. Anti-islanding detection. Islanding is the process in which the
PV system continues to supply power to the local load even though the
9. Selection of inverters and control methods power grid is cutoff [231]. A safety feature is to detect islanding
condition and disable PV inverters to get rid of the hazardous
9.1. Requirements for PV inverters conditions. The function of inverter is commonly referred to as the
anti-islanding. Some of the hazardous conditions are (a) damaging the
A few decades ago, the efficiency of PV module was very low as they equipment, re-tripping the line with an out of phase closure and (b) a
were expensive to produce and its applications were not fully devel- safety hazard for utility line workers who assume that the lines are de-
oped. There were no selection and safety requirements imposed by the energized. The feature of anti-islanding protection is required under the
government and electric companies. Today, with the advancement of standard IEEE/UL1741 1547 [232].
PV and power electronic technologies, the regulations and requirements
for the PV systems are being standardized. In general, for manu- 9.2. Ideal features for standalone inverters
facturing, testing, designing and commercialization two groups of re-
quirements and guidelines should be considered i.e. (a) performance Ideally, the standalone inverters should have the following features
requirements and (b) legal regulations. In addition, this section presents [233], (a) sinusoidal output voltage, (b) low radio frequency and audio
the ideal features required from standalone or grid connected inverters, noise, (c) disconnection under low DC-link voltage, (d) output voltage
followed by a comparative assessment of industrial inverters. and frequency within permissible limits, (e) low idling and no-load
losses, (f) cable to withstand large fluctuation in the input voltage, (g)
9.1.1. Performance requirements output voltage regulation, (h) high efficiency at light loads, (i) surge
9.1.1.1. Efficiency. Efficiency is an important factor for selecting an handling capacity, (j) low THD generation by the inverter, (k) protec-
appropriate inverter. With the passage of time the advancements made tion against under/overvoltages and frequency variations, short circuit,
to the inverter technology reduces the power losses and the efficiency etc., and (l) handling of overloading for a short period of time due to
reaches to 97% (for residential applications with power levels below higher starting currents from refrigerators, pumps, etc.
5.25 kW i.e. SunnyBoy 5000TL by SMA) and 98% (for applications up
to 850 kW, such as the central inverter i.e. SunnyBoy 760CP XT by 9.3. Ideal features for grid-connected inverters
SMA) [227]. In the next decade, there are still chances that the
efficiency will improve further when gallium nitride (GaN) and The characteristics of the grid-tied inverters are as follows [233]: (a)
silicon carbide (SiC) devices will be used as the power devices [228]. faster dynamic response, (b) power factor should be close to unity, (c)
Thus, selection of inverter heavily dependent on the efficiency of adequate frequency control, (d) low harmonic output, (e) efficient
inverter topology. synchronization with the grid, (f) tolerance to fault currents, (g) DC
current injection, and (h) protection to under/over frequency and
9.1.1.2. Power density. Power density is the amount of power that can under/over voltage.
be handled per unit volume. The power density is always important and
critical for both commercial and domestic application below 20 kW. To 9.4. Comparative assessment of industrial inverters
overcome this problem several solutions has been proposed such as ABB
PVS300 inverter based on neutral point-clamped topology [229]. The evolution in the power electronic converter technology for PV
applications, the growth in the PV installed capacity and the search for
9.1.1.3. Leakage current minimization. The high frequency (HF) the ultimate PV inverter have led to the existence of a wide variety of
harmonics caused by the modulation of the power converters, and the power converter topologies used in practice. Fig. 16 shows several in-
high stray capacitance between the grounded metallic frame of each dustrial PV inverter topologies for central, string, multistring, and ac-
module and the PV cells causes the flow of leakage current. The leakage module configurations [234]. Several features of these inverters
path is interrupted by the galvanic isolation provided by the topologies are presented in Section 6. The basic control structures for
transformer, however additional losses in transformer reduces the both single- and three-phase systems are detailed in Section 7. Ac-
efficiency. Several, transformerless inverter topologies are used to cording to HIS report 2015, an SMA German company has the highest
minimize the effect of HF harmonics on the leakage currents [230]. share of 14% on the basis of revenue earning from the PV inverter,
Thus, there is a trade-off among the cost, efficiency and elimination of followed by Huawei (9%) and small percentages for Sungrow, ABB, and
HF harmonics. SolarEdge inverter manufactures. For different countries, the inverter
specifications are different as each country has their own standards and
9.1.1.4. Installation and manufacturing cost. The installation and grid codes. A comparative assessment for grid-connected PV inverters is
manufacturing costs of inverter are important factors in selecting an carried out in Table 11 for various inverter supplier companies
appropriate inverter. The manufacturing cost is a trade-off between the [235–244].
power quality and the performance capabilities of inverter. However,
the installation cost various from one country to another country as it 10. Future scope of the research
depends on the labor, land and other local factors that influence the
total cost. To meet the future energy demand, the major focus nowadays is to
further increase the penetration level of renewable energy sources.
9.1.2. Legal requirements However, a major disadvantage is the uncertain nature of these source
9.1.2.1. Galvanic isolation. Galvanic isolation is one of the significant in terms of reliability, system security and system stability. Thus, for the
requirements for the safety reasons. Galvanic isolation is achieved using robust and accurate integration of solar energy to the utility grid, there
1134
Fig. 16. Industrial inverter topologies for String, Multistring, Central, and ac Module configuration. (2L-VSI: two-level voltage–source inverter; MV: medium voltage)
[234].
is need to examine the modern power electronics converters to meet the In the last decade, a progressive research is carried out on the de-
requirement for new grid codes specified by the utility operators and to velopment of new topologies for grid connected power converters. The
result a high-quality output with minimum harmonic content. reliability, power density, highest possible efficiency, and overall per-
Furthermore, there is a need to advance the design procedure of the PV formance of the power converters are the areas where research is
arrays in order to obtain higher efficiencies. Thus, a continuous re- headed. Few of the booming research topics in transformerless con-
search is need towards improving the PV efficiency by introducing new verters are: (a) utilizing transformerless multilevel converter to enable
and advanced materials that can be used for the fabrication of PV pa- medium voltage for grid connection, (b) qausi-Z-source-network for
nels. future power conditioners, and (c) developing power converters with
1135
K. Zeb et al.
Table 11
Comparative assessment of industrial grid-connected PV inverters [235–244].
Model ULX Indoor SunnyTripower 25000TL HPC-004SL Frontius Zigar Solar Outs Vista Save RM-1000 POM 500 Sunny Boy 8000- SG100K3
International SS1000TL US
GmbH
Country Denmark Germany Korea Austria Spain Australia Italy India Germany India
Company name Danfoss A/S SMA Solar Technology AG Hyundai Heavy Fronius Agilo Zigor Cooperation Gista Sunnyenrgy Tilsstems S. r. l Power Micro SMA Solar Neowatt
Industries CO., Ltd 75.0–3 Pty Ltd system Pvt. Technology AG Power
Ltd Solutions
Pvt Ltd
Max. DC power 5.85 25.22 390 78.1 5.8 1.1 – 550 8.6 110
(kW)
Max. DC Voltage 450 1000 250 950 – 550 400 900 600 880
Nominal DC 310 600 – 460 5.5 – 150 – 345 –
Voltage
Min. DC Voltage to 125 188 – 475 – 100 70 – 365 –
start feed in
Max. DC Current 30 33 25 170 20 8.5 11 1200 30 250
MPP Voltage 180–350 390–800 100–380 460–820 235–750 135–500 80–180 450–850 300–480 450–820
Range
No. of MPP 3 2 – 1 – 1 – 1 1 –
Trackers
DC Inputs – 6 – 4 – 1 – – 4 –
1136
Max. AC Power 4.5 25 4 75 5 1 1.2 550 8 100
Output AC Voltage 208.5–251.5 160–280 177.76–222.2 170–270 – 180–270 210–275 229.5–310.5 211–305 310–450
Range
Nominal AC 230 220,230,240 202 230,400 230 230 270, 315 240–277 400
Voltage
Max. AC Current 23 36.2 – 29 5 1 1176 32
Frequency 50 50,60 50,60 50,60 50 50 50 50,60 60 50, 60
Power Factor 0.97 1 0.95 – 0.99 0.99 1 1 1 0.99
No. of feed-in 1 3 1 3 1 1 3 1 3
Phase
Max. Efficiency 94.3 98.3 95 97.3 – 97 97 98.7 96.8 94
Euro Efficiency 93.4 98.1 94.5 96.7 94 96.5 98.2 – –
Power < 0.2 1 – – – < 0.1 < 10 0.1 –
Consumption
at Night
THD <5 <3 < 2.5 <3 <4 <3
Transformer Yes No No Yes – No No – LF T/F –
Humidity – 0–100 – 0–95 0.90 – – 0–95 0–100 –
Interface RS 485 RS 485 – RS 485, RS 485 RS 232, RS 485 – RS 485, RS 485, Bluetooth –
Ethernet,WLAN Ethernet
Standards IEC62109 IEC 62109 IEC 62109, IEC UL 1741 – – – –
61727,
IEC 62116
Protection Grid Monitoring, Grid Monitoring, Grid Overtemperature DC load Overcurrent Ground Fault Reverse Polarity Reverse Polarity
Features Isolation Fault Monitoring, Protection, Overvoltage Disconnector, Protection, Monitoring, Protection, Protection,
Monitoring, Over Residual Current Device, Protection, Anti- Overload Overvoltage Residual Current Residual Current Overvoltage
load Protection, Over Voltage Protection, islanding Protection Protection Protection, Short- Device, Overvoltage Device, Anti- Protection, Short
Anti-Islanding Short Circuit Protection circuit Protection, Protection, Anti- islanding Circuit Protection
Protection Anti-Islanding islanding Protection Protection
Protection
Renewable and Sustainable Energy Reviews 94 (2018) 1120–1141
additional power storage and low voltage ride through capabilities. In case of hard-switching PWM converters. On the other hand, at the ex-
near future, it is anticipated that the PV market will be captured by penses of high current and voltage on power switching devices, the
newly developed power converter based on SiC semiconductor devices. resonant converters utilizes ZCS and ZVS soft-switching technique that
The next-generation GaN PV converters along with these new SiC can greatly minimize the switching losses and also increases efficiency.
power converters will enable new era of grid-connected PV system by To conclude, some soft-switching inverter topologies i.e. modified time-
enhancing efficiency and performance of the power converters. sharing dual mode controlled soft switching inverter, series-resonant
dc-dc converter with bang-bang dc-ac inverter, some multilevel con-
11. Conclusion and future work cepts i.e. cascaded inverter, full-bridge single-leg switched clamped
inverter, and half-bridge diode clamped inverter, and some trans-
Solar PV has gained exceptional importance as one of the emerging former-less topologies i.e. HERIC, H6, H5 are considered as attractive
technologies to overcome the increasing demand for the electricity and regarding high efficiency, compact structure, higher lifetime, and low
the need for the reduction of carbon dioxide emissions and depleting cost.
fossil fuels. In this paper global energy status of the PV market, clas- It is also discussed in this paper that the proper operation of grid
sification of the PV system i.e. standalone and grid-connected topolo- connected PV system is ensured by the fast and accurate design of its
gies, configurations of grid-connected PV inverters, classification of control system. The control structures for single-phase grid-connected
inverter types, various inverter topologies, control procedures for single inverters are mostly classified into three categories: (1) control struc-
phase and three phase inverters, and various controllers are in- ture for single-phase inverter with DC-DC converter, (2) control struc-
vestigated, reviewed, and described in a schematic manner. ture for single-phase inverter without DC-DC converter, and (3) control
Considering the configurations of grid-connected PV inverters, cen- structure based on Power Control Shifting Phase (PCSP). The methods
tralized inverters, string inverters, multiple string inverters, and AC used to control the three-phase inverters are the synchronous reference
module integrated inverters are discussed and described. According to frame control, the stationary reference frame control, and the natural
Table 2, the power rating of the centralized inverter is 1–50 MW sui- abc-control. Consequently, six categories of several controllers’, that are
table for commercial applications. The power rating for string inverter the linear controllers, the non-linear controllers, the robust controllers,
is 1–50 kW and is utilized for commercial and residential applications. the adaptive controllers, the predictive controllers, and the intelligent
Similarly, the power rating for module integrated inverters is 500 W controllers are critically and analytical investigated.
and are suitable in grid-connected, street-lightning, and residential In the near future, it is expected that overall performance of the
applications. grid-connected solar PV system will be improved and the cost will be
Furthermore, in this review, the classifications of inverter categories minimized. According to the specific power requirements, location, and
consisting of line commutated and self-commutated inverters, current capacity for grid connection, this review study will help the engineers
source and voltage source inverters, the commonly used switching de- in selecting the most suitable and appropriate control technique and
vices, and the current and voltage control modes for VSI converter are inverter topology. It is also anticipated that this survey will be ad-
comprehensively reviewed. Nowadays, inverters are mostly using either vantageous to the engineers, researchers, manufacturers, and users
power IGBTs or MOSFETs. Power MOSFETS are used for high frequency working in the field of solar energy for enhancing the harnessing of
and low power switching operations, whereas IGBTs are employed solar energy and its grid integration. In addition, it will also help them
when high power and low-frequency operations is required. Between in selecting appropriate topology for their particular application.
the CCM and VCM mode of VSI, the CCM is preferred selection for the
grid-connected PV systems. References
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Renewable and Sustainable Energy Reviews 94 (2018) 1120–1141

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

A comprehensive review on inverter topologies and control strategies for T

Fig. 1. A typical structure of oﬀ-grid system.

[13] ✗ ✓ ✗ ✗ ✗ ✓ ✓ ✗ ✗ ✗ Inverter and control

Fig. 2. Solar PV global capacity and annual additions, 2006–2016.

3. Classiﬁcation of the power electronic based converters

For a grid-connected PV system, appropriate phase, frequency, and

Fig. 5. Grid-connected Self-commutated VSI [48].

added to the semi-controlled devices to control the turn-oﬀ process as

multiple-stage buck-boost inverter topology is presented, where a high-

Fig. 10. Full-bridge single leg switch clamped inverter [93].

Fig. 9. Three-level half-bridge diode clamped inverter [95].

multilevel grid-connected inverters oﬀer numerous beneﬁts. The mul-

The inverters can also be categorized based on the type of switching

Four switching devices based Single stage buck-boost inverter topology

Series resonant dc-dc converter with bang-bang dc-ac converter [118]

Novel selective dual-mode timesharing sine wave controlled soft-

Single-stage inverter based on buck-boost conﬁguration [110]

Zeta-Cuk conﬁguration derived Single-stage inverter [108]

Half-bridge diode clamped three-level inverter [115]

Full bridge single leg switch clamped inverter [116]

Two-stage isolated buck-boost inverter [111]

Two-switch ﬂyback inverter [114]

Single stage boost inverter [107]

switching inverter [113]

Cascaded inverter [117]

inverter by GEC [112]

LIEC & IDI

frequency link series resonant soft-switching inverter in which the

benchmarking evaluation of the aforementioned inverter topologies is

5. Classiﬁcation of photovoltaic system

nature, the grid-connected PV systems with ES are frequently adopted

in many practical applications. It is worth mentioning that, without the

characteristics, such as, 1) the charging of the battery during oﬀ-peak

the power management targets. The standalone PV systems on the other

Fig. 13. Conﬁgurations of grid-connected PV inverter [125,152].

Cost is low when compared to string inverter but at

4 Elimination of bulky electrolytic capacitor

5. Individual modules failure detection and

5. Longer average life (around 25 years)

3. Flexible and expandable in design

high levels it may presents high cost

Operating range 1 kW up to 300 MW

weather patterns, higher installation costs, require

electronic & mechanical tracking devices and back up

storage for maximum eﬃciency.

strings are coupled to a single inverter

ﬁlter forms an LQG controller. According to separation principle, these

controller is suitable for both time-varying and time-in varying systems.

8.2. Non-linear controllers

These controllers have an extraordinary operation compared to the

basic controllers. However, in terms of design and implementation,

Its cost is higher in comparison to centralized inverter

8.2.1. Sliding mode controllers

due to its robust and improved performance. The main advantages of

this technique are insensitivity to parameter variation and load dis-

1. Suitable for low power ratings