A comprehensive review on inverter topologies and control strategies for grid connected photovoltaic system Zeb, Kamran; Uddin, Waqar; Khan, Muhammad Adil; Ali, Zunaib; Ali, Muhammad Umair; Christofides, Nicholas; Kim, H. J. Published in: Renewable and Sustainable Energy Reviews
Digital Object Identifier DOI: 10.1016/j.rser.2018.06.053
Publication date: 2018
Free Download link to Journal Copy (valid until 5 September 2018): https://www.sciencedirect.com/science/article/pii/S136403211830491X
Citation for published version (APA): Zeb, K., Uddin, W., Khan, M. A., Ali, Z., Ali, M. U., Christofides, N., Kim, H. J. (2018). A Comprehensive review on Inverter Topologies and Control Strategies for Grid Connected Photovoltaic System. Renewable and Sustainable Energy Reviews, 94, 1120-1141. DOI: 10.1016/j.rser.2018.06.053
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A Comprehensive review on Inverter Topologies and Control Strategies for Grid Connected Photovoltaic System Kamran Zeb1, 2, W. U. Din1, M. A. Khan1, Zunaib Ali3, Muhammad Umair Ali1, Nicholas Christofides3, H. J. Kim1 School of Electrical Engineering, Pusan National University, Pusandaehak-ro 63 beon-gil 2, Geumjeong-gu, Busancity-46241, South Korea 2 School of Electrical Engineering and Computer Science, National University of Sciences and Technology, Islamabad-44000, Pakistan 3 Department of Electrical Engineering, Frederick University, Nicosia, Cyprus Email:
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Abstract: The application of Photovoltaic (PV) in the distributed generation system is acquiring more consideration with the developments in power electronics technology and global environmental concerns. Solar PV is playing a key role in consuming the solar energy for the generation of electric power. The use of solar PV is growing exponentially due to its clean, pollution-free, abundant, and inexhaustible nature. In gridconnected PV systems, significant attention is required in the design and operation of the inverter to achieve high efficiency for diverse 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 inverters 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 comprehensively presented. Keywords: Grid connected photovoltaic system, Inverters, Control system, DC-DC converter, Multilevel inverter
1. Introduction Research towards improving photovoltaic efficiency and increasing installation of residential rooftops PV systems is a clear indication that the distribution generation (DG) in upcoming years will be dominated by PVs. The desire to limit conventional energy sources and their use due to environmental concerns has also played an important role towards increased DG utilization. Furthermore, the electricity bills for the consumers having PV rooftop systems are drastically decreased (for example, in countries with netmetering system installed), realized as benefit by the consumers. Renewable Energy (RE) sources are the best solution to provide green energy to overcome the global energy issues. Furthermore, the use of RE sources is increased during the last decade through the advancement in the grid integration technologies [1]. Solar PV energy is one of the extensively emerging RE source. PV has the proficiency of generating the electricity in a reliable, clean, and noiseless way. Worldwide, around 75 GW of solar capacity was installed until 2016 and its capacity increased drastically to 303 GW at the end of the year [2]. Now PV is the third most important RE after hydro, and wind in terms of globally installed capacity. PV systems can be categorized into two main groups, that are, the standalone (off-grid) PV systems and the grid-connected (on-grid) PV systems [3]. The standalone system operates independent of the utility grid. On the other hand, the grid-connected applications employ PV system in conjunction with the grid.
Currently, in comparison to the standalone PV systems, the use of grid-connected PV is widely adopted in my practical applications [4-7]. A typical configuration of the grid-connected system is presented in Fig 1, consisting of a PV system and number of peripheral modules, such as the filters, transformers and the conversion technologies. The conversion technologies includes the DC/DC and DC/AC power electronics based converters. As opposed to the off-grid PV systems, the grid-connected PV does not require storage system as they operate in parallel with the electric utility grid. In addition, they supply power back to the utility grid when the generated power is greater than the load demand. A DC/DC converter together with a Voltage Source Inverter (VSI) or a Current Source Inverter (CSI) are typically used to connect the PV system to the grid. For DC to AC inversion purposes, the use of VSI in the grid-connected PV system is gaining wide acceptance day by day. Thus, the high efficiency of these inverters is the main constraint and critical parameter for their effective utilization in such applications [8]. The proper operation of the grid-connected PV mainly depends on the fast and accurate design of the VSI control system. A proper VSI controller is, therefore needed for the effective tracking of the desired reference command and achieving a good performance of the PV system. In a grid-connected PV system, the injected currents are controlled by the inverter, and thus, maintains the DC-link voltage to its reference value and regulates the active and the reactive power delivered to the grid [9]. The design of the appropriate control system for enabling the injection of controlled PV power into the grid is very critical for the effectiveness of the system. The active power from the PV is controlled with the temperature and incident solar irradiance of the PN junction diode. Considering the voltage regulation scheme and the system rating, the output reactive power reference is designed based on the method discussed in [10]. It is worth mentioning that the generated output power from the PV array is inherently unstable. With the modern developments and advancements in the power electronics, the parameters of the PV system, i.e. active (P) and reactive (Q) power can be effectively controlled to enhance the overall performance of the grid-connected system. The generation of active power in order to fulfill the load demand is the main purpose of the PV system. However, it can also be used to perform the advance functionalities of supporting the grid such as the voltage and reactive power support, fault ride through, power quality improvement, reduction in power losses and the active power filtering. The advanced functionalities can be accomplished by using diversified and multifunctional inverters in the PV system. Inverters can either be connected in shunt or series to the utility grid. The series connected inverters are employed for compensating the asymmetries of the nonlinear loads or the grid by injecting the negative sequence voltage. On the other hand, the shunt inverters are used for enabling the active power filtering function of PV by injecting the asymmetric and non-linear current locally through the PV systems at the Point of Common Coupling (PCC) [11, 12]. In some case, the series-parallel combination is carried out for providing both voltage (through series inverter) and current (through parallel inverter) support, such as in unified power quality conditioner (UPQC). Various power inverter topologies and their control structures for grid-connected PV systems are comprehensively reviewed in this paper. In recent years, the development in the solar PV is progressing day by day due to the continuous government support for RE based electricity production, cost reduction in materials, and technological improvements. In this review, the global status of PV market and classifications of power electronic based converters are focused in detail. Furthermore, various inverter topologies based on their design, classification of PV system, and the configuration of grid-connected PV inverters are discussed, described and presented in a schematic manner. A concise review of the control techniques for single- and three-phase inverters has also been demonstrated. After that, various controllers applied to grid-tied inverter are thoroughly reviewed and compared. Finally, selection of inverters and future trends are comprehensively presented. The contribution
of the proposed review study is comprehensively summarized in Table 1 by an extensive critical and analytical comparison with the various surveys already published in the literature. The rest of the paper is organized as following: an overview of the global status of PV market is demonstrated in Section 2. Section 3 categorizes the several classifications of power electronic based converters. The various topologies of inverter based on their design are elaborated in Section 4. Section 5 and Section 6 respectively investigate the classification of the PV systems and various configurations of the grid-connected PV inverters. The generic control of the grid-connected PV system is described in Section 7. Section 8 scrutinizes various control methods for the grid-connected PV systems. The selection of appropriate inverter and control method is elaborated in Section 9. Section 10 presents the future scope of the research in the grid-connected PV systems. Section 11 concludes this review with a concise summary and proposition for the future work. 2. Global Status of the PV Market The installed capacity of solar energy in 2016 is equivalent to the installation of more than 31000 solar panels every hour [34]. Considering the cumulative comparison status of the last five years, more solar PV capacity is installed in 2016. The percentage increase of the installed PV capacity in 2016 is 48 % compared to that of 2015. The capacity of solar PV installed since 2006 shows a continuous and exponential growth as depicted in Fig. 2. This increasing expansion of solar PV market is because of the rising demand for the electricity, the global urge for the reduction in carbon dioxide emission, the desire to limit the conventional energy sources, improvements and advancements in the integration technologies, advancements in the solar PV’s potentials, and increasing effectiveness of the solar PV towards supporting the electrical grid. 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 inveter in most of the cases is a power-electronics based grid side converter and can be categrorized in to two main types based on their turn-on and turn-off behaviours (commutation), that are the line communtated inverters and the self-communtaed inverters. The line communated converters depend on the circuit paramerters and the swicthes operate based on the polrity or direction of the current flow. On the other hand, the self-commutated coverters are operated with a full control over the turn–on and –off process of swicthng devices. The two main categories can further be divided in to various subtypes as illustarted in Fig. 3 and discussed as follows: 3.1. Line-Commutated Inverters In Line-Commutated Inverter (LCI) the commutation process is carried out by the parameters of the utility grid, that is, the reversal of AC voltage polarity and the flow of negative current (or zero current) initiates the commutation process. The LCI in general uses the commutating thyristors as power switching devices, which are semi-controller devices. The gate terminal of the device control the turn-on operation, whereas the turn off can not be controlled by the same mechanism as it depends on the line current or grid voltage for its turn-off. Thus, if a forced cummutation is necessary, an external circuitry is added to the semicontrolled devices to control the turn-off process as well. For example, an anti-parralel diode is added in the case of half-bridge LCI converter for enabling the process of forced commutation. The basic schemtaic diagram for a line communtaed current source inverter is shown in Fig. 4.
3.2. Self-Commutated Inverter The Self-Commutated Inverter (SCI) is the fully controlled power-electronic converter. The potential at the gate terminal controls both the turn-off and the turn-on process of the power switching devices. The transfer of current from one switching device to the other is enabled in a controlled manner. The devices used in the SCI include MOSFET and IGBT. For medium to high power application exceeding 100 kW and lowfrequency range of 20 kHz, IGBTs are used. On the other hand, for a high frequency typically in the range of 20-800 kHz and a low power less than 20 kW, MOSFETs are employed. For generating the output voltage waveform and for controlling the SCI, the Pulse Width Modulation (PWM) switching technique is used. For grid-connected inverter applications, high switching frequency is required to allow the reduction in weight of the inverter, reduce the output current and voltage harmonics, and also to decrease the size of the output filter [46]. The SCI is a fully controller power electronic converter, thus it controls both inverter output current and voltage waveform. Furthermore, it is highly robust to the utility grid disturbances, suppress the current harmonics and improves the grid power factor. Nowadays, SCI is preferred over LCI for grid-connected PV systems due to the advancement made to the control system for SCI and in addition, due to the evolution of advance switching devices similar to that of the power IGBTs and MOSFETs. The SCI can further be divided in to voltage source converters and current source converters. 3.2.1.
Voltage Source Inverter
In Voltage Source Inverter (VSI), the DC voltage source is at the input side of converter, thus the polarity of the input voltage remains the same. However, the polarity of the input DC current determines the direction of average power flow through the inverter. At the output side, an AC voltage waveform of a variable width and a constant amplitude can be obtained. A tie-line inductor is used along with the VSI to limit the current flow from the inverter to the utility grid. Furthermore, a relatively large capacitor, similar to a voltage source is connected in parallel with the input DC side of VSI. The self-commutated VSI configuration is shown in Fig. 5. The VSI can be operated in two modes that are the Voltage Control Mode (VCM) and the Current Control Mode (CCM). In case of VCM, the main controller variable is the PCC voltage, thus there is no control on the line currents. On the other hand, in CCM the line currents are delivered in a control manner. The differences between the VCM and the CCM are presented in Table 2 The VCM is recommended for the stand-alone or off-grid PV systems, as maintaining the PCC voltage magnitude, frequency and phase is of major importance in case of the stand-alone power networks. Nevertheless, both VCM and CCM can be implemented for the grid-connected PV system, but CCM is most commonly used method. The reason for using CCM is that the stiff electrical grid dictates the PCC voltage, thus controlling the currents for delivering the produced PV power is more reliable and safer than the VCM method with no control on currents. In case of grid disturbances, the transient current suppression is possible with CCM and a highpower factor can be acquired by simple control structure that is why inverters with the CCM are extensively utilized in grid-connected PV systems. Thus, the preferred inverter for a grid-connected PV system is the VSI operated in current control mode. 3.2.2.
Current Source Inverter
In Current Source Inverter (CSI), the input side of the inverter is connected to a DC current source and hence, the polarity of the input current remains the same. The polarity of the input DC voltage, however, determines the direction of average power flow through the inverter. An AC current waveform of a variable width and a constant amplitude can be obtained at the output side. As opposed to VSI, a large inductor that
upholds the stability of the current is attached in series to the input side of the CSI. A comparison summary between the VSI and the CSI is presented in tabular form in Table 3. 4. Various Inverter Topologies Based on the configuration and types of components used, inverters can be classified into different categories. These division of categories is based on various factors, such as, number of power processing stages i.e. single stage and multi-stage, transformer and transformerless configurations, number of levels involved in the design and the type of switching used. Each category is briefly discussed and described as follows: 4.1. Inverters based on number of power processing stages The inverters based on the power processing stages are classified into two main types, which are the single stage inverters and the multiple stage inverters, as presented in Fig. 6. 4.1.1. Single stage inverter The single stage inverter performs various functions, such as the control of injected grid currents, the function of voltage amplifications and the process of maximum power point tracking. The design of the single stage inverter handles the double peak power according to the equation presented below 𝑝𝑔𝑟𝑖𝑑 = 2𝑃𝑔𝑟𝑖𝑑 𝑠𝑖𝑛2 (𝜔𝑔𝑟𝑖𝑑 𝑡) where, 𝜔𝑔𝑟𝑖𝑑 is the grid frequency and 𝑃𝑔𝑟𝑖𝑑 is the peak grid power. In single stage inverter, the use of line frequency transformer (operating at low frequency) adds a large amount of weight to the inverter as well as contribute to the peak efficiency losses of 2 % [52]. The use of high-frequency transformer or transformer-less converter design on the other hand are the most efficient, cost effective and lighter in weight. They are increasingly replacing the line frequency transformers. Various inverter topologies such as the buck-boost or the boost converter designs are presented in [53-66] with certain merits and demerits. The DC to AC conversion and MPPT voltage amplification in [53-66] take place in a single stage. In these topologies, either an inductor is used as the energy storage element or a high-frequency transformer performing the functions of isolation and energy storage. The key characteristics of the buck-boost single stage inverter is the elimination of line frequency transformer. However, single stage inverters frequently suffer from a low range of input DC voltage, low power quality, and reduced power capacity. Furthermore, the current stresses on the power switching devices increase with the increase of power capacity. Consequently, the single stage inverters are avoided in certain application where wide input voltage range, high power quality, and high distribution capacity is required. Consequently, for such applications, multiple stage inverters are preferred. 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 because no transformer is used in this case [68]. In [68] another isolated multiple-stage buck-boost inverter topology is presented, where a highfrequency 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 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 in the last stage. The second one is designed by General Electric Company (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 single and multiple stage inverters. A bulky electrolytic capacitor having high capacitance is utilized to accomplish this decoupling. The capacitor can be placed in two different ways, that are, in between the two converter stages as a DC link or in parallel with the PV modules, as illustrated in Fig. 7. In general, the main objective of the inverter is to convert the DC power into the AC power at the high switching frequency. However, operating at such high switching frequency results in undesired switching transients. Thus, the input side of the PV system is protected with a DC-link capacitor, which blocks the flow of these transients from moving in a backward direction. These capacitors, however, have several disadvantages, for example, at high operating temperatures their lifetime is lower in comparison to the other devices utilized in the inverter circuits. In addition, they are costly, and bulky in size. Furthermore, in practical cases, these capacitors produce various significant problems. Their reliability and power conversion efficiency are low. Because of these concerns, a prominent research is progressing day by day to reduce or eliminate the capacitance of electric capacitor and to utilize the small film capacitors as an alternative [50, 51] 4.2. Transformer and transformer less inverters Another classification of the inverters, as per the existing literature, is made based on the existence or absence of the transformer. In other words, this classification can also have the single or multiple power stages but the main categorization in this case is based on the transformer. In general, on the basis of transformer, the grid connected PV inverter topologies are categorized into two groups, i.e., those with transformer and the ones which are transformer less. Line-frequency transformers are used in the inverters for galvanic isolation of between the PV panel and the utility grid. The isolation transformer helps in eliminating the problem of DC current injection from the PV system into the utility grid. Since, line frequency transformers are heavy in weight and bulky in size increasing in this way the overall cost of PV system, so therefore the line-frequency transformer are considered as the problematic component of the inverter. An alternative solution to this is to utilize the high-frequency transformer embedded in the inverter or DC/DC converter, which reduces the size and weight of the system, and thus decreases the overall cost. Considering this, some inverter topologies are presented in Fig. 8. The transformer-less inverter in comparison with the transformer topologies are cost-effective solutions and present higher efficiency. However, for addressing the problem of DC current injection, they require extra circuitry to be installed. Another problem related to transformer-less topologies is that there is no galvanic isolation between the utility grid and the PV array. Furthermore, it may cause voltage fluctuations between the PV array and the ground, depending upon the inverter circuit. A virtual capacitor formed between the surface of PV array and the installed ground, this fluctuating voltage contributes to energizing the capacitor. Depending on the structure of PV panel and the weather parameters, the capacitor may have values up to 1 𝜇𝐹/𝑘𝑊𝑝 for thin-film cells and typically lies in the range of 50 and 150 𝑛𝐹/𝑘𝑊𝑝 [70]. An electrical hazard may cause if a person standing on the ground touches the PV array due to the capacitive current flow in his body. Another problem related to the PV array is the generation of electromagnetic interference, which is caused due to the voltage fluctuations. However, according to different research studies [71-76], the electromagnetic interference of transformer-less topologies is negligible and hazardless. However, to prevent unsafe current levels (above 10 A) in the design of transformer-less grid-connected inverter, certain recommendation should be followed.
The differences between transformer-less and transformers-based inverter are presented in Table 4. The line frequency transformers are bulky in size, expensive and reduce the system efficiency because of 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 lighter, smaller and lower in cost. Transformer-less inverters can be single stage or multiple stages. A major drawback of the single-stage PV topologies is that the output voltage range of the PV panels/ strings is limited especially in the low power applications (e.g., AC-module inverters), which thus will affect the overall efficiency. The double-stage PV technology can solve this issue since it consists of a DC–DC converter that is responsible for amplifying the voltage of the PV module to a desirable level for the inverter stage. The most commonly used transformer-based topologies of single-phase grid-connected inverters are half H-bridge, full H-bridge, HERIC, H5, H6, NPC, active NPC, flying capacitor, and Coenergy NPC. Recently, in the market there are many manufacturers for transformer-less PV inverters e.g.: REFU, Danfos solar, Ingeteam, Conergy, Sunways, and SMA, offering the maximum efficiency of up to 98% and high European efficiency (>97%). The transformer-less inverters can be single stage or multiple stages. A two stages gridconnected high-frequency transformer-based topologies is discussed in [78], where a 160 W combined flyback and a buck-boost based two-switch inverter is presented. Similarly, [79] presents a High Efficient and Reliable Inverter (HERIC) grid-connected transformer-less topology. The HERIC topology increases the efficiency by including the zero voltage with the help of an AC bypass to the performance of full-bridge with bi-polar modulation. Furthermore, five switching devices based H5 topology is presented in [80]. Various other topologies are proposed with their promising features in [79-84] as: (a) one topology is full bridge topology with DC bypass comprising of two diodes and six switching devices, (b) another topology is H6-type configuration, which is made up of two split inductors as a low-pass filter, two freewheeling diodes and six power switching devices, this topology is well suited for non-isolated module integrated inverter, (c) one of the topologies is flying capacitor type topology with midpoint clamping to the neutral wire of the power grid due to three level output voltage it provides a low filter inductor current ripple, (d) another topology is called Karschny (flying inductor) that eliminate any voltage oscillations due to direct connection of the negative terminal of the PV array and the output neutral. 4.3. Multilevel Inverters Today, the decrease in the overall cost of the grid-connected PV system is due to the improvement in the existing grid-connected inverter technologies. In comparison to the simple two-level inverters, multilevel grid-connected inverters offer numerous benefits. The multilevel inverters result in the AC voltage at the inverter’s output terminal, 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 initiated the term of multilevel [86]. Consequently, a few multilevel converter typologies have been produced in [86-94]. In the subsequent subsection, some multilevel inverter topologies are described. 4.3.1. Half-bridge Diode Clamped Inverters A schematic diagram of the half bridge diode clamped three-level inverter, which is an important part of the single-phase transformer-less grid-connected PV systems is presented in Fig. 9 [95, 96]. At the output terminal of the inverter, a positive voltage can be achieved by simultaneous switching of the switches S1 and S2. A negative voltage is obtained by switching of S3, and S4, whereas a zero voltage is created by
turning-on both S2 and S3 at the same time. In order to allow the transfer of power from PV to the utility grid, the DC bus voltage must always be more than the grid voltage amplitude. The midpoint of the PV array is grounded, and this reduces the electromagnetic interference and eliminates the capacitive earth current, which are the advantages of this inverter topology. 4.3.2. Full-bridge Single Leg Clamped Inverters A full-bridge single leg clamped inverter, for residential PV systems is described in Fig. 10 [93]. In addition to conventional full bridge switches S6, S5, S4, and S3, bidirectional switches S1 and S2 along with the diodes D1 and D2 are added. This allows the proper control of current flowing to and from the midpoint of DC bus. With this topology, the minimum size of the inverter for a transformer-less PV system is approximately 1.5 kW. 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 topology connects in series the AC output of two full-bridge configurations in order to increase number of voltage levels. This is because each individual bridge can produce at its AC output with three dissimilar voltage levels. Thus, resulting in an inclusive five-level AC output voltage. The modular and scalable feature of the cascaded inverter is the key advantages, as it can be extended to achieve even higher number of levels just by cascading the basic three-level modules. For a transformer-less PV system, with small input DC voltage on the input side (i.e. 40 V each), more than two full bridge configurations can be connected in series, as suggested in [87]. Furthermore, in [92, 98] cascaded inverters are presented for high power applications. 4.4. Soft/Hard Switching Inverters The inverters can also be categorized based on the type of switching employed. In this case, the inverters are categorized based on the type of switching employed and not on the number of power stages. In general, there exist two types, the hard and soft switching inverters. Thus, both hard and soft switching inverters can be comprises of one or more than one power stages. Nowadays, the grid-connected PV inverters are designed using the soft switching technique in order to achieve high power density, high efficiency, and better performance. Serious EMI problems and switching losses are caused by abrupt variation in switch currents and voltages, especially in the high-frequency switching inverter [99, 100]. This abrupt switching of the devices at a random instance is referred to as the hard switching and thus, causes various problems in the switching process. Due to the stray inductances and parasitic capacitances of the power electronic devices, the high current or voltage spikes occurs during the abrupt switching transients. Passive components based high-frequency resonant networks such as the capacitor-inductor tank, the powerswitching devices and the auxiliary diodes are combined with the traditional hard-switching PWM circuits to form the soft-switching topology. There are two types of soft-switching: zero-voltage switching (ZVS) for reducing dv/dt and zero-current switching (ZCS) for reducing di/dt. The traditional PWM based buck-boost inverter topologies have several disadvantages such as, (a) highfrequency harmonic components causing EMI, (b) large leakage current due to the intrinsic high-frequency common mode voltage at the output terminals, (c) low efficiency at high switching frequency (d) increases the size and weight of the converter if designed to operate at low switching frequency and high efficiency. These limitations are overcome by the resonant soft switching techniques, the voltage across or the current through the switching device is ensured to be zero at the instance of switching. This minimizes the switching losses of the power switching devices. Various soft-switching inverter topologies are discussed in the literature. The work 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 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-frequency link series resonant soft-switching inverter in which the switching devices are operated under ZCS. The study in [103] presented a single stage soft switching fly-back inverter based on capacitive idling. The authors in [104] proposed the LLCC resonant inverter with ZVT-PWM boost converter, the LLCC resonant inverter includes a parallel-resonant tank and a series-resonant tank that provide the AC output voltage with low THD. The work presented in [105] designed ZVS-ZCS-PWM inverter with ZVT-PWM boost converter. This topology consists of three stages, the first stage is a ZVT-PWM boost converter, the second stage is a ZVSZCS- PWM buck converter and the third stage is a line-frequency full bridge inverter. A detailed comparison and 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 gridconnected PV systems. This classification 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 gridconnected PV systems are able to provide AC and/or DC power services to the grid as well as the connection to other alternate Energy Storage (ES) devices. Due to the low cost and maintenance requirements, as well as the environmental friendly nature, the grid-connected PV systems with ES are frequently adopted 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 characteristics, such as, 1) the charging of the battery during off-peak 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 the power management targets. The standalone PV systems on the other hand are not new [119] as they are supplying electrical power to the remote areas for decades without any connection to the utility grid. The 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 electrical power to the well sized AC or DC loads. Until 1995, the standalone PV systems were more commonly used as compared to the gridconnected systems, as presented in Fig. 12 [120]. Later on, after 1995, the grid-connected systems become more dominant, contributing in this way to the overall operation of the power system. In both standalone or grid-connected PV systems, power electronic based inverter is the main component that converts the DC power to AC power, delivering in this way the power to the AC loads or electrical grid. Usually, the output power of the PV system is optimized by the Maximum Power Point Tracker (MPPT), which is a kind of DC-DC converter and is interconnected between the load and the PV array. The grid-connected PV systems are heavily employed these days, as can be seen from Fig. 2. However, this increasing penetration presents numerous challenges to the power system. Their undesirable impacts to the distribution grid involve the reliability and stability issues. The major challenges are: (a) voltage fluctuations at the PCC, (b) frequency variations, (c) overvoltage in the distribution feeder because of the reverse power flow, (d) intermittent power generation of the PV systems, (e) current and voltage harmonics generated by the inverters, and (f) low power factor operation of the distribution transformer [121]. 6. Configurations of the grid-connected PV inverters The grid-connected inverters undergone various configurations can be categorized in to four types, the central inverters, the string inverters, the multi-string inverts and the ac module inverters. The four types are shown in Fig. 13 and explained below with their design characteristics, advantages and limitations
6.1. Central Inverters In this design, a large number of series interconnection of the PV modules is enabled in order to increase the voltage rating of inverter and to avoid the further amplification of the system connected to the grid [122125]. This series interconnection is commonly referred to as the string. On the other hand, in order to increase the power level a parallel interconnection of these strings is developed by employing the string diodes [125]. The central inverter topology, however, has several restrictions such as: (a) the losses in the string diodes, losses as a result of voltage mismatch, losses among PV modules, and centralized MPPT power losses, (b) interconnection of the PV modules and inverter requires a high voltage DC cables, (c) the line-commutated thyristors usually used in this topology produces poor power quality and current harmonics, (d) non-modular and non-flexible design, and (e) in some cases failure of the PV plant because of the central inverter [126-131]. 6.2. String Inverters Nowadays, string inverters are the most commonly used grid-connected inverters [132,133]. In a string inverter, a single string of the PV module is attached to the inverter. It is a reduced version of the central 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% to 3% higher in comparison to the central inverter. The mismatch and partial shading are also reduced in this topology [135]. 6.3. Multi-string inverter In multi-string inverter, many strings are connected to their individual DC to DC converter, with a separate MPP tracking system. All the strings are then connected to a common DC to AC converter. Basically, it is a further modification of the string inverter. This topology is preferred over central inverter as every string is controlled individually. It is a hybrid topology that combines the advantageous feature of central and string topologies. It is modular in structure and can be easily expanded by adding a new string to the existing one [136, 137]. In multi-string topology, Insulated Gate Bipolar Transistors (IGBTs) are utilized for high power and low switching frequency whereas, Metal Oxide Field Effect Transistors (MOSFETs) are used for high switching and low power. 6.4. AC Modules In this topology, the integration of inverter and PV module is carried 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 modules are eliminated in this topology [138]. It has a modular design and can be easily expanded. The optimal adjustment of the inverter and the PV module is supported by this topology. Nowadays, the AC modules employ the self-commutated converter topology as the DC-AC inverter [139]. As mentioned, all the functions including DC to AC conversion, MPPT, and voltage amplification are performed in a single module, and thus, it makes the circuit more complex and increase the price per wattage. A detailed comparison and benchmarking of the four converter topologies are summarized in Table 6. The comparison is performed on the basis of advantages, disadvantages, costing, and rating. The PV Technology characteristics are described in Table 7. 7. Control of grid-connected photovoltaic system
The DC to AC inverter helps in controlling the power factor by injecting the sinusoidal current into the grid. The DC energy generated from the solar PV is converted into the AC power and is efficiently transferred to the electrical grid by the application of grid side inverter (GSI). The proper operation of the grid side inverter is ensured by designing fast and accurate control system. Thus, the control of GSI is one of the most significant part of the grid-connected PV system connected. The two main sub-classifications of the PV control system are: (a) MPP control module: The maximum power extraction from the PV module or input RE source is performed by the MPP control. (b) Inverter control module: ensures (a) a proper grid synchronization and high quality of the injected power, (b) control of the active and reactive power delivered to the grid, and (c) the control of DClink voltage. The inverter control strategy consists of two main cascaded loops. Typically, a loop which controls the grid current is a fast-internal current loop, and loop which regulates the DC-link voltage is a slow external voltage loop. The current protection and power quality issues are associated with the current control loop. The significant features required from the current controller are faster dynamic response and harmonic compensation under distorted grid conditions. For balancing the power flow in the system, the DC-link outer voltage controller is employed. Generally, the purpose of the external controller is the stability of slow dynamic system and optimal regulation. For stability purpose, the current control loop is designed with dynamic speed lower than the speed of voltage control loop (approximately 5 to 20 times greater). The designing of voltage controller does not require the transfer function of internal current control loop, since the external and internal loops can be designed in a decoupled way [144-152]. In some cases, the cascade of voltage control loop and power loop can be used as an alternative of the current loop and the injected currents are indirectly controlled. A detailed evaluation of the control structures for single-phase inverters are evaluated in Table 8. 8. Various controllers for the grid-connected PV system The overall operation of the grid-connected PV system depends on the fast and accurate control of the grid side inverter. The problems associated with the grid-connected PV system are the grid disturbances if suitable and robust controllers are not designed and thus, it results in grid instability. According to the specific operating condition and behavior of the electrical grid, the controllers of PV system are divided into 6 categories, which are the linear controllers, the non-linear controllers, the robust controllers, the adaptive controllers, the predictive controllers, and the intelligent controllers [156]. 8.1. Linear controllers These controllers are designed based on the features and dynamics of the linear system. The typical feedback control theory is used for analyzing and designing these controllers.
8.1.1. Classical controllers This family consists of the Proportional-Integral-Derivative (PID), the Proportional-Integral (PI), the Proportional-Derivative (PD), and Proportional (P) controllers. These controllers are the base of classical linear systems and control science. A few classic controllers are tabulated in table 10. 8.1.2. Proportional Resonant (PR) controllers
PI and PR controllers work in a similar manner but in two different operating frames. The PI controller allows efficient tracking of DC signals, whereas the PR controller tracks a sinusoidal signal with the frequency of sinusoid as its central frequency. The way the integration takes place in PI controller is different form the one that takes place in PR controller. As opposed to PI controller, the integrator in case of PR controller integrates the frequencies, which are close to the resonant frequency. Consequently, phase shifts or stationary errors are not involved [157]. 8.1.3. Linear Quadratic Gaussian (LQG) controllers The combination of the linear-quadratic regulator and the Kalman filter forms an LQG controller. According to separation principle, these two controllers can be designed and computed independently. LQG 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]. 8.2. Non-linear controllers These controllers have an extraordinary operation compared to the basic controllers. However, in terms of design and implementation, these controllers are complicated. 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 due to its robust and improved performance. The main advantages of this technique are insensitivity to parameter variation and load disturbances. 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 the performance of SMC is heavily dependent. The performance of the SMC is also dependent on the sampling time and suffers from distortion if an inadequate sampling time is selected. In addition, when SMC tracks a variable reference, the phenomena of chattering is observed, which is a major disadvantage of SMC, degrading in this way the overall efficiency of the PV system [159]. 8.2.2. Partial Feedback Linearization (PFL) controllers Feedback linearization is the direct way for designing the non-linear 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 to design the controller for these systems. When the non-linear system is converted into a halfway linear system, is known as Partial Feedback Linearization (PFL) and if converted into a completely linear system is known as Exact Feedback Linearization (EFL) [160].
8.2.3. Hysteresis controllers One of the non-linear controllers is hysteresis controller. An adaptive band of the controller must be created in order to attain a stable switching frequency, which is an important step for implementing hysteresis controller. Hence the output of this controller is the state of the switches, therefore consideration regarding the isolated neutral is important again [161].
8.3. Robust controllers A robust control theory approach is utilized in order to design a controller with concern to uncertainties. The goal of these methods is to acquire stability in the occurrence of partial modelling errors as well as robust performance. In the robust control, bounds, clear description, and good criteria must be defined. This controller can promise robust performance and stability of the closed-loop systems, even in the multivariable systems [162]. 8.3.1. H-infinity controllers To use H-infinity methods, the control problem is represented as an optimization problem by the control designer and then solves it. Multivariable system problems are solved by H-infinity techniques. But, it needs a good model of the system to be controlled and has high computational complexity. Additionally, non-linear constraints are not well handled. 8.3.2. Mu-synthesis controllers The effect of unstructured and structured uncertainties on the performance of the system is considered by the mu-synthesis approach. On the notion of an organized singular value, the design of the controller is based, in this method. In the power and energy domain, the previous application of this method can be found in [163]. 8.4. Adaptive controllers In adaptive control methods, depending on the operating conditions of the system the control action is automatically adjusted. With high performance, the accurate system parameters are not required. This control scheme has high computational complexity [164]. Table 10 comprehensively describes few adaptive control schemes. 8.5. Predictive controllers The future behavior of controlled parameters is predicted by using system model in predictive controllers. Based on a predefined optimization criteria, to obtain the optimal actuation the controller utilizes this information. It can be applied to different systems while a multivariable case can be considered, because of its non-linearities, constraints that can be simply incorporated, and very fast dynamic response. It has also easy implementation. The comparison of classic controller and this controller comparison shows, that an excessive number of calculations is required in the predictive controller. 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 control theory. Centered on these equations, at the end of the sampling period for the state variables to reach the reference values the control signal is calculated, at the start of the individually sampling period. 8.5.2. Model Predictive Controller (MPC) In the MPC, a cost function is defined from a flexible criteria, to select optimal actions that should be minimized. In this strategy, a model of the system is utilized for predicting the response of the variables till a precise time. In design stage of the controller, the MPC simply includes system constraints and nonlinearities.
8.6. Intelligent controllers Intelligent controllers obtain automation through the imitation of biological intelligence. Furthermore, the way biological systems troubleshoot problems are borrowed for the idea. Then, that is utilized to solve the control problem. 8.6.1. NN controllers Neural Network (NN) is inspired from the human nervous system. It is a connection of a biological brain system that is stimulated by a number of artificial neurons. It has the ability to obtain a higher fault tolerance and estimate an optional function mapping [165]. When NN is used in control system, it can train either off-line or online. 8.6.2. Repetitive controllers The basic idea of the Repetitive Controllers (RC) is derived from the principle of the internal model. Good tracking/rejection can be achieved, in the closed-loop path if the model of any disturbance/reference is injected. The basic structure of the RC controllers is described in [166]. During a period the error signal should be stored, in order to determine the reduction or elimination of the error in other periods. Hence, for periodic non-linear loads, RC has been used. The dynamic response of this controller is not desirable although its performance is appropriate for periodic nonlinear loads. In order to tackle this issue, RC by parallel or cascaded structure can be joined with extremely dynamic response controllers. 8.6.3. Fuzzy Logic Controllers In Fuzzy Logic Controllers (FLC), the knowledge of smart human being is defined and implemented to control the dynamics of a system. The architecture of FLC method consists of (a) Fuzzification, (b) Rule base, (c) Inference mechanism, and (d) De-Fuzzification. In fuzzification, a set of crisp data is converted into fuzzified data, in rule bases 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 fuzzified data is converted back to crisp data and thus, a proper control action is achieved. 8.6.4. Autonomous controllers To perform the complex control tasks independently, autonomous controllers are used. By adding intelligence to the procedure of refining autonomy, to acquire an advance level of automation, engineers directly try to automate the human’s technology and knowledge.
9. Selection of Inverters and Control Methods 9.1. Requirements for PV Inverters A few decades ago, the efficiency of PV module was very low as they were expensive to produce and its applications were not fully developed. There were no selection and safety requirements imposed by the government and electric companies. Today, with the advancement of PV and power electronic technologies,
the regulations and requirements for the PV systems are being standardized. In general, for manufacturing, testing, designing and commercialization two groups of requirements and guidelines should be considered i.e. (a) performance requirements and (b) legal regulations. In addition, this section presents the ideal features required from standalone or grid connected inverters, followed by a comparative assessment of industrial inverters. 9.1.1. Performance requirements
Efficiency
Efficiency is an important factor for selecting an appropriate inverter. With the passage of time the advancements made to the inverter technology reduces the power losses and the efficiency reaches to 97% (for residential applications with power levels below 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 SMA) [227]. In the next decade, there are still chances that the efficiency will improve further when gallium nitride (GaN) and silicon carbide (SiC) devices will be used as the power devices [228]. Thus, selection of inverter heavily dependent on the efficiency of inverter topology.
Power density
Power density is the amount of power that can be handled per unit volume. The power density is always important and critical for both commercial and domestic application below 20 kW. To overcome this problem several solutions has been proposed such as ABB PVS300 inverter based on neutral point-clamped topology [229].
Leakage current minimization
The high frequency (HF) harmonics caused by the modulation of the power converters, and the high stray capacitance between the grounded metallic frame of each module and the PV cells causes the flow of leakage current. The leakage path is interrupted by the galvanic isolation provided by the transformer, however additional losses in transformer reduces the efficiency. Several, transformerless inverter topologies are used to minimize the effect of HF harmonics on the leakage currents [230]. Thus, there is a trade-off among the cost, efficiency and elimination of HF harmonics.
Installation and manufacturing cost
The installation and manufacturing costs of inverter are important factors in selecting an appropriate inverter. The manufacturing cost is a trade-off between the power quality and the performance capabilities of inverter. However, the installation cost various from one country to another country as it depends on the labor, land and other local factors that influence the total cost.
9.1.2. Legal requirements Galvanic Isolation Galvanic isolation is one of the significant requirements for the safety reasons. Galvanic isolation is achieved using transformers (low or high frequency) or using switch (in case of transformerless inverters). In Spain under grid code RD 1699/2011, this feature is required for the connection of PV to low-voltage
distribution system and is also adopted in the other countries of the world. Thus, depending on application, the selection of galvanic isolation can be made.
Anti-Islanding Detection
Islanding is the process in which the PV system continues to supply power to the local load even though the power grid is cutoff [231]. A safety feature is to detect islanding condition and disable PV inverters to get rid of the hazardous conditions. The function of inverter is commonly referred to as the anti-islanding. Some of the hazardous conditions are (a) damaging the equipment, re-tripping the line with an out of phase closure and (b) a safety hazard for utility line workers who assume that the lines are de-energized. The feature of anti-islanding protection is required under the standard IEEE/UL1741 1547 [232]. 9.2. Ideal Features for standalone inverters Ideally, the standalone inverters should have the following features [233], (a) sinusoidal output voltage, (b) low radio frequency and audio noise, (c) disconnection under low DC-link voltage, (d) output voltage and frequency within permissible limits, (e) low idling and no-load losses, (f) cable to withstand large fluctuation in the input voltage, (g) output voltage regulation, (h) high efficiency at light loads, (i) surge handling capacity, (j) low THD generation by the inverter, (k) protection against under/overvoltages and frequency variations, short circuit, etc., and (l) handling of overloading for a short period of time due to higher starting currents from refrigerators, pumps, etc. 9.3. Ideal features for grid-connected inverters The characteristics of the grid-tied inverters are as follows [233]: (a) faster dynamic response, (b) power factor should be close to unity, (c) adequate frequency control, (d) low harmonic output, (e) efficient synchronization with the grid, (f) tolerance to fault currents, (g) DC current injection, and (h) protection to under/over frequency and under/over voltage. 9.4. Comparative assessment of industrial inverters The evolution in the power electronic converter technology for PV applications, the growth in the PV installed capacity and the search for the ultimate PV inverter have led to the existence of a wide variety of power converter topologies used in practice. Fig. 16 shows several industrial PV inverter topologies for central, string, multistring, and ac-module configurations [234]. Several features of these inverters topologies are presented in Section 6. The basic control structures for both single- and three-phase systems are detailed in Section 7. According to HIS report 2015, an SMA German company has the highest share of 14% on the basis of revenue earning from the PV inverter, followed by Huawei (9%) and small percentages for Sungrow, ABB, and SolarEdge inverter manufactures. For different countries the inverter specifications are different as each country has their own standards and grid codes. A comparative assessment for grid-connected PV inverters is carried out in Table 11 for various inverter supplier companies [235]-[244].
10. Future scope of the research To meet the future energy demand, the major focus nowadays is to further increase the penetration level of renewable energy sources. However, a major disadvantage is the uncertain nature of these source in terms of reliability, system security and system stability. Thus, for the robust and accurate integration of solar energy to the utility grid, there is need to examine the modern power electronics converters to meet the
requirement for new grid codes specified by the utility operators and to result a high-quality output with minimum harmonic content. Furthermore, there is a need to advance the design procedure of the PV arrays in order to obtain higher efficiencies. Thus, a continuous research is need towards improving the PV efficiency by introducing new and advanced materials that can be used for the fabrication of PV panels. In the last decade, a progressive research is carried out on the development of new topologies for grid connected power converters. The reliability, power density, highest possible efficiency, and overall performance of the power converters are the areas where research is headed. Few of the booming research topics in transformerless converters are: (a) utilizing transformerless multilevel converter to enable medium voltage for grid connection, (b) qausi-Z-source-network for future power conditioners, and (c) developing power converters with additional power storage and low voltage ride through capabilities. In near future, it is anticipated that the PV market will be captured by newly developed power converter based on SiC semiconductor devices. The next-generation GaN PV converters along with these new SiC power converters will enable new era of grid-connected PV system by enhancing efficiency and performance of the power converters. 11. Conclusion and Future Work Solar PV has gained exceptional importance as one of the emerging technologies to overcome the increasing demand for the electricity and the need for the reduction of carbon dioxide emissions and depleting fossil fuels. In this paper global energy status of the PV market, classification of the PV system i.e. standalone and grid-connected topologies, configurations of grid-connected PV inverters, classification of inverter types, various inverter topologies, control procedures for single phase and three phase inverters, and various controllers are investigated, reviewed, and described in a schematic manner. Considering the configurations of grid-connected PV inverters, centralized inverters, string inverters, multiple string inverters, and AC module integrated inverters are discussed and described. According to Table 2, the power rating of the centralized inverter is 1-50 MW suitable for commercial applications. The power rating for string inverter is 1-50 kW and is utilized for commercial and residential applications. Similarly, the power rating for module integrated inverters is 500 W and are suitable in grid-connected, street-lightning, and residential applications. Furthermore, in this review, the classifications of inverter categories consisting of line commutated and self-commutated inverters, current source and voltage source inverters, the commonly used switching devices, and the current and voltage control modes for VSI converter are comprehensively reviewed. Nowadays, inverters are mostly using either power IGBTs or MOSFETs. Power MOSFETS are used for high frequency and low power switching operations, whereas IGBTs are employed when high power and low-frequency operations is required. Between the CCM and VCM mode of VSI, the CCM is preferred selection for the grid-connected PV systems. In addition, various inverter topologies i.e. power de-coupling, single stage inverter, multiple stage inverter, transformer and transformerless inverters, multilevel inverters, and soft switching inverters are investigated. It is also discussed that the DC-link capacitor of the inverter is a limiting factor. For PV inverter applications, the electrolytic capacitors available in the market are not considered as a suitable option due to their high dependency on the operating temperatures. It has been recommended that inverters should be designed with improved capacitors capable of handling the temperature variations. A few novel topologies for grid-connected inverter are also described in this survey, which uses small film and lower capacitance of the capacitor. Furthermore, the selection of transformer plays an important role in the design of inverters. It is a fact that line-frequency transformers-based inverters have more weight and volume than transformerless inverters
or high-frequency transformers-based inverters. Nowadays, transformerless inverters developed and adopted in many applications since they are the most efficient, compact and cost-effective systems. However, leakage current problem exists in the transformer-less inverters. In some of the transformer-less topologies discussed in this review, the problem of leakage current has been resolved to a greater extend. These topologies include the half-bridge multilevel topologies with neutral point connected to the midpoint of the input voltage, the H5 inverter and HERIC inverter. However, due to the cost of additional essential transistors and power diodes, transformer-less topology has not been considered as a standard and is only used in low-power applications. The effect of soft-switching on the overall operation of inverter has also been discussed in this review work. High switching losses occur in case of hard-switching PWM converters. On the other hand, at the expenses of high current and voltage on power switching devices, the resonant converters utilizes ZCS and ZVS softswitching technique that can greatly minimize the switching losses and also increases efficiency. To conclude, some soft-switching inverter topologies i.e. modified time-sharing dual mode controlled soft switching inverter, series-resonant dc-dc converter with bang-bang dc-ac inverter, some multilevel concepts i.e. cascaded inverter, full-bridge single-leg switched clamped inverter, and half-bridge diode clamped inverter, and some transformer-less topologies i.e. HERIC, H6, H5 are considered as attractive regarding high efficiency, compact structure, higher lifetime, and low cost. It is also discussed in this paper that the proper operation of grid connected PV system is ensured by the fast and accurate design of its control system. The control structures for single-phase grid-connected inverters are mostly classified into three categories: (1) control structure for single-phase inverter with DCDC converter, (2) control structure for single-phase inverter without DC-DC converter, and (3) control structure based on Power Control Shifting Phase (PCSP). The methods used to control the three-phase inverters are the synchronous referbce frame control, the stationary reference frame control, and the natural abc-control. Consequently, six categories of several controllers’, that are the linear controllers, the nonlinear controllers, the robust controllers, the adaptive controllers, the predictive controllers, and the intelligent controllers are critically and analytical investigated. In the near future, it is expected that overall performance of the grid-connected solar PV system will be improved and the cost will be minimized. According to the specific power requirements, location, and capacity for grid connection, this review study will help the engineers in selecting the most suitable and appropriate control technique and inverter topology. It is also anticipated that this survey will be advantageous to the engineers, researchers, manufacturers, and users working in the field of solar energy for enhancing the harnessing of solar energy and its grid integration. In addition, it will also help them in selecting appropriate topology for their particular application. 12. References [1] Tsengenes G, Adamidis G. Investigation of the behavior of a three phase grid-connected photovoltaic system to control active and reactive power. Electr Power Syst Res 2011;81(1):177–84. [2] Annual report, photovoltaic industry association, EPIA; 2012. [3] El Nozahya MS, Salama MMA.Technical impacts of grid-connected photo- voltaic systems on electrical networks—a review. J. Renewable Sustainable Energy 2013;5:032702 [4] Tsao-Tsung Ma. Power quality enhancement in micro-grids using multifunctional DG, inverters. In: Proceedings of the international multi conference of engineers Comp. sent., Vol.II.HongKong,March14–16,2012.
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Fig. 1. A typical structure of off-grid system
Fig. 2. Solar PV global capacity and annual additions, 2006-2016
Inverter
Line Commutated Inverter
Self Commutated Inverter
Voltage Source Inverter
Voltage Control Mode
Current Source Inverter
Current Control Mode
Fig. 3. Classifications of power electronic based converters [41]
Igrid
|Igrid|
Fig. 4. Grid-connected Line-commutated CSI [47]
Grid
+
Vdc
Lgrid
Igrid
Fig. 5. Grid-connected Self-commutated VSI [48]
Grid
PV Module
Grid
PV Module
(a)
(b)
Fig. 6. PV inverter types (a) Single stage inverter, (b) Two stage inverter [67]
Grid
PV Module
Grid
PV Module CDC
CPV
CPV
(b)
(a)
Fig. 7. Power de-coupling capacitor different positions for single stage and multiple stage Inverter [14]
PV Module
PV Module
Grid
Grid
Low frequency trans former
High frequency trans former
(a)
(b)
PV Module High frequency trans former (c)
Fig. 8. (a) Placement of the Line-frequency transformer between the inverter and the grid. (b) HF-link gridconnected ac/ac inverter. (c) High-frequency transformer is embedded in a dc-link PV-module-connected dc-dc converter [77]
S1
PV Module
S2
D1 Grid
S3 D2
PV Module
S4 Fig. 9. Three-level half-bridge diode clamped inverter [95]
S3
PV Module
S5
S1
D1
Grid PV Module
D2 S2
S6
S4
Fig. 10. Full-bridge single leg switch clamped inverter [93]
S1
S3
S4
S2
PV Modul e
Grid
S5
S7
S8
S6
PV Modul e
Fig. 11. Cascaded inverter [97]
Fig. 12. Ratio of off-grid versus grid-connected solar PV distribution between 1993-2012
PV strings
PV modules
PV strings
PV strings
Module inverter
String inverter
Multi-string inverter AC bus
AC bus AC bus
(a)
(b)
AC bus
(c)
(d)
Fig. 13. Configurations of grid-connected PV inverter [125, 152] Vgrid PLL
Vdc.ref
Sin(wt)
IPV
+
MPPT DC-DC
Voltage Control Vdc
Ir
Iref
Igrid.ref
+
Vdc
PV Module
VPV
DC
Current Controller
PWM AC
Iout
Vgrid.RMS PPV
Grid
PDC . 2 Vgrid . RMS
Fig. 14 (a) Vgrid PLL
Vdc.ref
Sin(wt)
IPV
+ MPPT
Voltage Control Vdc
Ir
Iref
+
Vdc
PV Module
VPV
Igrid.ref
Iout
Vgrid.RMS PPV
PDC . 2 Vgrid . RMS
Fig. 14 (b)
DC
Current Controller
PWM AC
Grid
IMPPT
+
MPPT DC/DC
PV Module
L
Iout
C
Vinv
Vgrid
Grid
Iout
Vdc Loop_up Table (δ )
DSPWM
PI
Vdc.ref
PI
+
+
Iref
ZCD
Digital control
Vgrid.ref
Fig. 14 (c)
vd Pref +
Idref
+
PI
Id
P
+
PI ωL
PWM
ωL
Qref +
Iqref
+
PI
Q
+
PI
Vabc Filter Grid
abc
vq
Iq
Iabc
DC/ AC
Id
θ PLL
dq
Iq
Fig. 15 (a)
Pref +
+
PI
Idref
P Qref +
Iqref
Iα
Iαref
dq αβ
Iβref
+
HC
PWM
Iabc
DC/ AC
Vabc Filter Grid
HC
+
PI
Q
PR
PR
+
abc
Iα
Iβ
Iβ
Fig. 15 (b)
θ αβ
PLL
Pref + PI
Idref
P Qref
+
Q
Iaref
dq
Iqref
+ Ia
Ibref abc
Icref
PI
Current Controller
+
Current Controller
PWM
DC/ AC
Iabc
Vabc Filter Grid
Ib
+
Current Controller
Iα Ib Ic
Ic
Fig. 15 (c)
PLL
θ
Fig. 16. Industrial inverter topologies for String, Multistring, Central, and ac Module configuration. (2LVSI: two-level voltage–source inverter; MV: medium voltage) [234].
Table 1: Comparative analysis of various surveys on inverter and control schemes Ref
[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [OS]
GS PV
AG PVI
C PV
CI
VIT
RE
CG PV
CO PV
IGCPVIC
FT
Focused Area
Inverter and control Inverter topologies Grid-connected PV Grid-connected PV Solar energy Solar energy Inverter and control Grid-connected PV Solar PV system Grid-connected PVI Grid-connected PVI Grid-connected PVI Grid-connected PVI Grid-connected PVI Solar PV system Solar PV system Hybrid energy systems
MPPT Grid-connected PVI Multilevel inverter Grid-connected IC 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).
Table 2: Difference between the VCM and CCM of VSI [41]. Parameter Inverter type Fault short circuit current Control parameter
VCM Self-commutated VSI High AC voltage
CCM Self-commutated VSI Low (Limited to rated current) AC current
Table 3: Dissimilarity between the VSI and CSI [49]. Parameters Dependency on load Power Source Related loss Input parameter
VSI
CSI
The output voltage amplitude is independent of load. On the other hand, the output current magnitude and waveform are dependent on the nature of load
The output current amplitude is independent of load. On the other hand, the output voltage magnitude and waveform are dependent on the nature of load
A DC voltage source with lesser or insignificant impedance is the input of VSI. The total power loss is low because of low conduction loss and high switching loss. A constant input voltage is maintained. In parallel to the input DC side of a VSI, a capacitor is connected. Whereas DC capacitor is efficient, cheap, and small energy storage.
Changeable current from a DC voltage source having high impedance is the input of a CSI The total power loss is high because of high conduction loss and Low switching loss. The input current is continuous however changeable. In series to the input DC side of a CSI, an inductor is connected. Whereas, DC inductor contributes more losses, expensive, and bulky.
Table 4: Differences between transformer based and transformer-less inverters [72] Inverter Advantages
Disadvantages
Line-frequency transformer based inverter Safer due to galvanic isolation, high reliability, simpler design
High-frequency transformer based Inverter safer due to galvanic isolation, high efficiency, lightweight, compact and simpler design
High volume and weight, low efficiency
Costly technology, and complex
Transformer-less inverter high efficiency, light weight, compact, design Additional safety essential
Complex measures
Table 5: Evaluation of different inverter topologies Category of inverter Single-stage inverter
Multiple-stage inverter
Power rating 500-3 kW
Switch
Diode
PD
ToTI
ELT
EC
Topology type
Four
Two
LIEC
T-L
M
M
Four switching devices based Single stage buck-boost inverter topology [106]
> 3 kW
Four
LIEC
T-L
M
M
Single stage boost inverter [107]
500-3 kW
Four
LIEC
T-L
M
M
Zeta-Cuk configuration derived Single-stage inverter [108]
500 W
Three
Two
LIEC
H-FT
M
M
Single-stage isolated flyback inverter [109]
> 3 kW
Four
Two
LIEC
T-L
M
M
Single-stage inverter based on buck-boost configuration [110]
2 kW
Six
Two
LIEC & IDI
H-FT
M
H
Two-stage non-isolated buck-boost inverter [111]
2 kW
Five
One
LIEC & IDI
H-FT
M
H
Two-stage isolated buck-boost inverter [111]
4 kW
Eight
Four
LIEC
H-FT
M
H
Multiple stage boost inverter by GEC [112]
5 kW
Five
Two
SFC
T-L
L
H
Novel selective dual-mode timesharing sine wave controlled soft-switching inverter [113]
160 W
Six
Four
LIEC
H-FT
M
H
Two-switch flyback inverter [114]
> 3 kW
Four
Two
LIEC
T-L
M
M
Half-bridge diode inverter [115]
> 1.5 kW
Six
Two
LIEC
T-L
M
H
Full bridge single leg switch clamped inverter [116]
High power application
Eight
LIEC
T-L
M
H
Cascaded inverter [117]
250 W
Six
LIEC
H-FT
M
H
Series resonant dc-dc converter with bangbang dc-ac converter [118]
Inverter using electrolytic capacitor of low capacitance or using film capacitor in place of a large electrolytic capacitor High-frequency transformer inverter Multilevel inverter
Soft-switching inverter
Six
clamped
three-level
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 Frequency Transformer, ELT: Expected Life Time, L: Low, M: Moderate, H: High, EC: Expected Cost,
Table: 6. Historical overview of grid-connected PV inverter [14, 234] Topology
Central
String
Multi-String
AC Module 1. No mismatch losses between modules 2. Deals with the problem of partial shading and that is why it is more appropriate 3. Flexible and expandable in design 4 Elimination of bulky electrolytic capacitor 5. Longer average life (around 25 years) 5. Individual modules failure detection and debugging is easy 6. Modularized nature can produce mass production 1. In case of faults the replacement of inverter is not simple
Advantages
1. Central inverter presents lower cost
1. String diode losses are eliminated 2. Partial shading energy loss is reduced 3. Flexible in structure and design 4. Decent consistency 5. Since each PV module contribute AC signal to a common AC bus that is why it provides improved safety and stability
1. String diode losses are eliminated 2. Partial shading energy loss is reduced 3. The DC-DC converter can be used for voltage amplification 4. Separate current control and MPPT is utilized
Disadvantages
1. Mismatch in PV modules, string diodes, and centralized MPPT power losses 2. DC losses due to high voltage DC cables 3. Power feeding to the utility grid is cut off in case of inverter failure 4. The working of solar module is interrupted under partial shading 5. Non-flexible in design 6. Low reliability Lower cost in comparison to string inverter
1. Suitable for low power ratings
1. Surplus losses inside the DCDC converter 2. The reliability of the system decreased as all the strings are coupled to a single inverter
Its cost is higher in comparison to centralized inverter
In comparison to centralized Topology its cost is higher
Cost is low when compared to string inverter but at high levels it may presents high cost
1-50 MW
1–5 kW /string
50 kW
up to500– 600 W
Costing
Power Rating
Table: 7. PV technology characteristics [140, 141–143]. PV technology significant features Fuel used
Parameter’s details Solar power
Operating range Efficiency of PV cells Application types Benefits
1 kW up to 300 MW 6–7% organic cells, 11–14% for thin film, and 12–16% for crystalline silicon Utility-scale, commercial, and residential Low maintenance and operating costs, sustainable and emission-less technology, and modular type. No direct CO2, CO, NOx emissions 0.004 USD/kWh for utility scale generation and 0.07 USD/kWh (AC) for grid-connected residential. 600 − 1300 (USD/kW) Fluctuating output power due to the deviation in weather patterns, higher installation costs, require electronic & mechanical tracking devices and back up storage for maximum efficiency.
Environmental impact Maintenance & operation annual costs Installation costs Drawbacks
Table 8: Control configurations for single-phase inverters [153] Topologies Single phase inverter with DC/DC converter Single phase inverter without DC/DC converter Single phase inverter with PCSP
o o o o o o o o o
Advantage Fast Dynamic Instantaneous current control Simplicity of the conversion system Instantaneous current control Fast Dynamic Simplicity Less circuitry Few resources Reactive power controlled
o o o o o o o
Inconvenient Complex hardware circuit No full control of power factor Complex hardware circuit No full control of power factor No full control of current No fast dynamics
Figures 14 (a) 14 (b)
14 (c)
Table 9: Control structures for three-phase inverters [154, 155] Topologies dq control
𝜶𝜷-control
o o o o o
abc control
Advantage Simplicity Controlling and filtering can be easier accomplished The steady-state error is removed Around the resonance frequency, a very high gain is acquired High dynamic
o o
Inconvenient The steady-state error is not removed Compensation capability of the low-order harmonics is very poor
o o
Complex Hardware circuit No complete control of power factor
o
The transfer function is complex
PI PR
o
Simple transfer function
o
More complex than hysteresis and Deadbeat
o o
High dynamic Rapid development
o
High complexity of the control for current regulation.
o o o
High dynamic. Simple control for current regulation. Rapid development
o
Implementation in high frequency microcontroller
Figures 15 (a)
Table 10: Main features of the proposed controllers in literature
15 (b)
15 (c)
Controller Type PI
PR
Hysteresis Dead-Beat
I
M
F
Ref.
Reference Frame
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
Table 11: Comparative assessment of industrial grid-connected PV inverters [235]-[244] Model
ULX Indoor
SunnyTripower 25000TL
HPC-004SL
Frontius International GmbH
Zigar Solar Outs
Vista Save SS1000TL
RM-1000
POM 500
Sunny Boy 8000-US
SG100K3
Country
Denmark
Germany
Korea
Austria
Spain
Australia
Italy
India
Germany
India
Company name
Danfoss A/S
SMA Solar Technology AG
Hyundai Heavy Industries CO., Ltd
Fronius Agilo 75.0-3
Zigor Cooperation
Gista Sunnyenrgy Pty Ltd
Tilsstems S. r. l
SMA Solar Technology AG
Max. DC power (kW) Max. DC Voltage Nominal DC Voltage Min. DC Voltage to start feed in Max. DC Current MPP Voltage Range No. of MPP Trackers DC Inputs
5.85
25.22
390
78.1
5.8
1.1
-
Power Micro system Pvt. Ltd 550
8.6
Neowatt Power Solutions Pvt Ltd 110
450
1000
250
950
-
550
400
900
600
880
310
600
-
460
5.5
-
150
-
345
-
125
188
-
475
-
100
70
-
365
-
30
33
25
170
20
8.5
11
1200
30
250
180-350
390-800
100-380
460-820
235-750
135-500
80-180
450-850
300-480
450-820
3
2
-
1
-
1
-
1
1
-
-
6
-
4
-
1
-
-
4
-
Max. AC Power Output AC Voltage Range Nominal AC Voltage Max. AC Current Frequency
4.5
25
4
75
5
1
1.2
550
8
100
208.5-251.5
160-280
177.76-222.2
170-270
-
180-270
210-275
229.5310.5
211-305
310-450
230
220,230,240
202
230,400
230
230
270, 315
240-277
400
23
36.2
-
29
5
1
1176
32
50
50,60
50,60
50,60
50
50
50
50,60
60
50, 60
Power Factor No. of feed-in Phase Max. Efficiency Euro Efficiency Power Consumption at Night THD
0.97
1
0.95
-
0.99
0.99
1
1
1
0.99
1
3
1
3
1
1
3
1
3
94.3
98.3
95
97.3
-
97
98.7
96.8
94
93.4
98.1
94.5
96.7
94
96.5
98.2
-
-
