Article

Concept of a distributed photovoltaic multilevel inverter with cascaded double H-bridge topology

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Abstract

This paper presents proof-of-concept of a novel photovoltaic (PV) inverter with integrated short-term storage, based on the modular cascaded double H-bridge (CHB²) topology, and a new look-up table control approach. This topology combines and extends the advantages of various distributed converter concepts, such as string inverters, microinverters, and cascaded H-bridge (CHB) multilevel inverters. The proposed CHB² inverter incorporates individual PV elements into modules that can dynamically connect to their neighbors not only in series but also in parallel, which reduces conduction losses and enables simple module balancing. Maximum-power-point tracking of the PV element is performed in each module. As a demonstration of the flexibility of the approach and to filter large power fluctuations before they enter the grid, each module further incorporates batteries for energy storage, which enables continuous power output by compensating solar power fluctuations, while the CHB² can balance the batteries’ load and state of charge. The inverter provides exceptionally high output quality without large filters, since the multilevel ac output is finely quantized. The elimination of extensive output filtering provides for an extremely rapid dynamic response as well. We furthermore introduce an optimized, computationally efficient, look-up-table-based controller and module scheduler which leverages the large number of degrees of freedom provided by the flexible series-parallel CHB² configurations to optimize conduction loss, switching loss, and load balance. The method can be extended to CHB² circuits in general to improve their performance while simplifying control. Finally, we demonstrate that the inverter is robust to open-circuit failure of power switches or PVs.

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... Multi-level inverter is an excellent source for power generation, such as photovoltaic (Solar) power inverter [18], as shown in Fig 1. We can also utilize it for power, generation, and control, ...
... A multi-level Inverter is an electronic gadget that integrates a coveted AC voltage from a few DC voltage sources [4,18]. Multi-level inverters are a vital topic for research over the most recent quite a while, where the DC levels were viewed as almost identical because these are either batteries, solar panels, or others. ...
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... The practical implementation can find a suitable solution in multilevel inverters (MLI), in which the energy storage elements can be integrated in a distributed way, with evident advantages in terms of power control and management [15], [16]. When using the MLI topology, the power fluctuation problem can be addressed in two different ways: in some solutions there is full integration of the batteries and the PV panel, i.e., one group for each submodule (i.e., inverter stage) of the MLI [17], [18], [19]; other solutions provide for partial integration, with one or more batteries connected to the dc-link instead of to the PV panel [20], [21], [22] (so-called hybrid system). However, in all cases mentioned, the integration of each battery always requires an interface dc/dc converter capable of managing the charging/discharging phases during operation. ...
... A unidirectional dc/dc converter is used only for the connection of the photovoltaic generator (PVG) to the dc-link. The proposed circuit configuration in PV-BESS CHB inverter represents a novelty with respect to the solutions of [17], [18], [19], [20], [21], [22]. ...
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... In contrast to hard-wired batteries, the distributed power electronics can control the power of individual modules [6]. Thus, reconfigurable battery systems offer excellent balancing of the state of charge [7,8] and state of health [9][10][11][12], introduce fault tolerance by bypassing defective modules or even semiconductors [13][14][15][16], and increase the effectively available capacitance of battery systems [7,17]. In addition, reconfigurable systems exhibit high efficiency, low cost, and modularity [4,18,19]. ...
... Diagram of the system topology: (a) battery module with HB 2 switch topology allowing paralleling of modules, (b) phase string topology of the MMSPC converter with integrated batteries. connection modes between modules, also often denoted as MMSPC [6,14,[29][30][31]. CHB 2 promise better load distribution among battery modules, higher failure safety, lower effective source impedance, and lower ripple load are major advantages particularly for battery applications [21,22,32,33]. ...
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... bidirectional operation of the modules [18]; sensorless operation with minimum communication requirements [19]; better fault tolerance [20][21][22]. ...
... This effect will reduce as more modules are added to the arm [28]. The PSC has the lowest voltage quality as was expected from (22). Figure 8 presents simulation results for module voltages and high-frequency current components for a system that begins with a 0.8 pu voltage imbalance (ranging from 0.6 to 1.4 pu), but as the time passes, the voltages converge to a constant value. ...
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Since the introduction of modular multilevel converters (MMC), various additional features have been intro-duced for MMCs, among them parallel inter-module connectivity. However, such a parallel mode requires appropriate modulation and switching strategies to ex-ploit its full potential. Low-frequency switching modula-tion is widely used with half-bridge modules and a large body of research focusses on the optimal control and scheduling of these methods. However, so far, a suitable adaptation for MMCs with parallel mode is missing, and existing methods perform suboptimum. This paper pro-poses a generic scheduling algorithm for simple and low-cost integration with low-frequency switching modula-tion. The proposed method reduces control complexity, minimizes power loss, and can be combined with most modulation techniques, including nearest-level modula-tion (NLM) and selective harmonic elimination (SHE). The text provides a general algorithm for integrating this method with other control levels and analyzes its effect on MMCs with parallel functionality. Moreover, it examines the performance of the system with NLM com-bined with the proposed scheduler and compares it to other alternatives. Results show up to 50% reduction in power loss with a similar modulation technique as well as significantly lower THD and power loss compared to optimized phase-shifted carrier modulation as another sensorless alternative method.
... MLIs are mainly classed into three main categories; cascaded Hbridge (CHB-MLI) with separated DC sources [6][7][8][9][10][11][12][13], neutral point clamped (NPC-MLI) [5,[14][15][16], and flying capacitors (FC-MLI) [17][18][19][20]. Modified CHB-MLIs have been recently applied for hybrid systems [21][22][23][24]. The main concept of these MLIs is to provide an increased output voltage levels without increasing components power/voltage ratings [25,26]. ...
... Recently, invaluable investigation attempts are concentrated on the reduction of power switching devices count of MLIs. Research outcomes were observed in widespread MLI topologies and control techniques as reported in [19][20][21][22][23][24]. In [1], a single-phase transformer-less inverter was proposed considering a boosting factor of 1:10, which is vital in low DC sources or PV applications. ...
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... Therefore, development of solar inverter is becoming more and more valuable to provide clean energy (Clavadetscher & Nordmann, 2007). In grid tied PV systems, the inverter should meet the perquisites of grid parameters such as voltage, current, frequency, harmonics and leakage current (Goetz et al., 2019). ...
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Transformer-less inverter topologies are preferred to use in grid connected photovoltaic (PV) systems because of their compact footprint, greater output, and affordability. To meet the requirements of leakage current in VDE-4105 standard various transformer-less inverter solutions have appeared in the research done so far. This paper presents a detailed performance analysis of a single-phase transformer-less highly efficient and reliable inverter concept (HERIC) topology together with a proportional resonant (PR) current controller. Moreover, a comparison with the traditional PI current controller is also carried out based on the output current harmonics and total harmonic distortion. For the evaluation of the results, simulations are performed in PSIM.
... Step-up converters are broadly designed and developed for dc microgrid [1,2], electric vehicles [3,4], distributed generation [5], and many other applications. There are several methods to design step-up converters [6], which are grouped into six categories, as illustrated in Figure 1. ...
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Step‐up converters are increasingly developed for renewable energy and storage systems in order to raise the output voltage of those resources to higher voltage levels. Conventional step‐up structures, such as P‐type boost converter, provide low voltage gain, and they must be cascaded with the same modules to produce higher boost factors. This conversion method reduces overall efficiency. Affording higher gains with lower duty cycles and single‐stage conversion increase efficiency. This paper proposes a new step‐up P‐type dc/dc converter with high voltage gain even with lower duty ratios. Additionally, it has continuous input current, high efficiency, and common ground between the input and the output. Detailed analysis is presented, and the performance of this new topology is compared with other high‐gain step‐up converters. Simulations and experiments evaluate the achievements of this topology. Results prove the proposed structure.
... The first ones usually adopt step-up converters to match the low-voltage provided by the PV modules with the high-voltage required by commercial grid-connected inverters [5][6][7][8][9][10]. Instead, the second systems have more diverse requirements; for example, PV systems interacting with stand-alone AC loads usually adopt step-up converters [5,[11][12][13], but PV systems designed for battery charging or to interacting with DC loads often use step-down converters [14,15]. Figure 1 depicts some of those types of PV systems: Type I uses a step-up dc/dc converter to perform the maximum power point tracking (MPPT) control on the PV source, which ensures the extraction of the maximum power from the PV string. ...
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The wide range of step-up and step-down input-output voltage characteristic of the Cuk converter makes it a good candidate to interface photovoltaic arrays in both classical and distributed maximum power point tracking systems. Because its two inductor structure, Cuk converters have continuous input and output currents, which reduce the additional filtering elements usually required for interfacing dc/dc converter topologies. However, PV systems based on Cuk converters usually do not provide formal proofs of global stability under realistic conditions, which makes impossible to ensure a safe operation of the PV installation. Therefore, this paper proposes a high performance sliding-mode controller for PV systems based on Cuk converters, which regulates the PV voltage in agreement with the commands imposed by a MPPT algorithm, rejecting both load and environmental perturbations, and ensuring global stability for real operation conditions. Finally, the performance of the regulated PV system is tested using both simulations and experiments.
... Every different stage multilevel inverter has contained different value of efficiency. Cascaded H-bridge is simpler than other multilevel inverters because of the number of components [9][10][11][12], voltage sharing and switching redundancy. Thus, these papers focus to analyse the performance of single phase Cascaded H-Bridge multilevel inverter five level by using 12 V input and each level can generate three voltage outputs +Vdc, -Vdc and zero. ...
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Multilevel inverter becomes an alternative for high power and medium voltage applications recently and is more popular in renewable energy. The cascaded H-bridge multilevel inverter is more suitable for the Photovoltaic application because it acts as a separate DC source for each bridge. This paper presents a selective harmonic elimination (SHE) modulation for single-phase cascaded H-bridge (CHB) five level inverter. The optimum angle of the transcendental equations is solved the fundamental and harmonic elements using the Newton-Raphson (NR) process. The proposed selective harmonic elimination (SHE) scheme is tested using MATLAB/Simulink simulation. These simulation results are then verified through experiment using microcontroller. The proposed SHE is efficient in eliminating the 5 th order harmonics and producing a higher quality output waveform with a better harmonic profile.
... Conversely, the topologies referred to as unipolar MLIs produce negative voltage levels through an Hbridge. Due to the use of an H-bridge, unipolar MLIs require fewer components and impose lower voltage stress across the switches [41][42][43]. The NPC and FC topologies, which are among the most studied topologies, are categorized as bipolar topologies. ...
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Total harmonic distortion (THD) and voltage stress across the switches are critical issues in power electronic systems. Although multilevel inverters (MLIs) were initially used to minimize these issues, doing so is challenging when simultaneously attempting to minimize the number of components such as switches, DC sources, and gate drivers. To address this problem, a new pd-type MLI is presented with two back-to-back connected d-type modules with an H-bridge, that generates the negative voltage levels. The proposed topology with 10-unidirectional switches, and 4 DC sources operates in symmetric and asymmetric configuration to generate 9, 13 and 17-voltage levels. The presented inverter is extended using cascaded connections to attain more output voltage levels, making it usable for the applications with diverse number of DC links for medium and high voltage applications. The proposed topology also exhibits small THD, low number of power electronic components, and low total voltage stress across the switches in each cycle. Furthermore, a widely used nearest level control modulation technique is used to generate output voltage levels with minimum amount of THD at the output. Finally, simulations were performed using MATLAB/Simulink and experiments were conducted to validate the performance of the proposed topology.
... Therefore, development of solar inverter is becoming more and more valuable to provide clean energy (Clavadetscher & Nordmann, 2007). In grid tied PV systems, the inverter should meet the perquisites of grid parameters such as voltage, current, frequency, harmonics and leakage current (Goetz et al., 2019). ...
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Waste of energy due to the usage of high-density material in Automobile is a cause of energy wastage, by increasing the strength to weight ratio energy efficiency can be increased. Material strengthen can be increased by number of processes and each process produce specific material features. The ECAP is one of the most effective and dominant technique with the help of which strengthen of engineering material is done. In this research aluminum alloy-2024 is investigated. The samples are prepared and passed through ECAP die channels, intersecting at angle of 1000 and preheated along with die up to 4000C using electric furnace. Hardness and tensile test were carried out and were compared with As-received material. It was observed that hardness is increased to 10.3%, yield strength and ultimate tensile strength (UTS) increased to 11% due to grain refinement. Microstructure analysis were carried out and it was observed that grain size is reduced to 1μm ≤ 0.75μm with some lager particles.
... In contrast to hard-wired batteries, the distributed power electronics can dynamically reconfigure the module interconnection and control the power of individual modules. Thus, reconfigurable MMC-battery systems offer excellent balancing of the state of charge [5,6] and state of health [7][8][9][10], introduce fault tolerance by bypassing defective modules or even semiconductors [11][12][13][14], and increase the effectively available capacitance of battery systems [6,15,16]. Several companies are developing or already market commercial systems based on battery-integrated CHB/MMC [17][18][19]. ...
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... In both simulation and experiments, the proposed scheduler combined with PSANLM is evaluated and compared to phase-shifted carrier (PSC) modulation as an alternative that includes scheduling. Furthermore, we compare the performance of the proposed scheduler to a conventional cell-sorting scheduler (which highly relies on measurement of module voltages) in both MMSPC and MMC topologies [57] as well as a fixed-switching-pattern scheduler in MMSPC [58], [59]. The implemented cell-sorting algorithm is based on the literature [60]. ...
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... Despite indisputable advantages, MMCs are criticized for their complexity in terms of both circuit hardware and control software due to the large quantities of active switches and passive components involved. Since MMCs, particularly in highvoltage applications, consist of numerous submodules, the simplification of submodules or proper use of redundancy will greatly reduce system complexity and failure rate [13], [44]. Fig. 1(a) shows the schematics of two commonly used MMC submodules-H-bridge and asymmetrical half-bridge submodules. ...
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... The authors use the method as a "... superior solution for PV system grid integration due to its simple implementation, signal stage power conversion, no added complexity with increasing the number of connected modules, and it eliminates the need for individual control loop for each module..." It is important to mention that most conventional techniques do not achieve a distributed MPPT, which decreases the efficiency of the system. Goetz et al. in [104] propose a modular Double Cascading H-bridge (CHB 2 ) topology. ...
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... In contrast to hard-wired batteries, the distributed power electronics can dynamically reconfigure the module interconnection and control the power of individual modules. Thus, reconfigurable MMC-battery systems offer excellent balancing of the state of charge [9,10] and state of health [11][12][13][14], introduce fault tolerance by bypassing defective modules or even semiconductors [15][16][17][18], and increase the effectively available capacitance of battery systems [9,19]. Several companies are developing or already market commercial systems based on batteryintegrated CHB/MMC [20][21][22][23][24][25][26][27]. ...
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... Different solutions have been proposed in the literature in order to integrate a storage system in PV CHB converters. In [26], a modular cascaded double H-bridge (CHB 2 ) topology with battery directly connected to dc-link is used, thus enabling continuous power output by compensating solar power fluctuations, while the CHB 2 can balance the batteries' load and state of charge. In [21,22], the battery takes place of a PV module in one or more power cells in a single-stage CHB configuration designed to coordinate power allocation among PV, BESS, and the utility grid. ...
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... To maximise the efficiency, it is required to track the maximum output power out of given solar irradiance. This technique is known as Maximum Power Point Tracking (MPPT) and is being under extensive research by researchers all over the world [7] [8]. Some of the conventional MPPT techniques are [9][10]: ...
Conference Paper
This paper focusses on three phase Cascaded H bridge Multi-level Inverter (MLI) topology fed by solar Photovoltaic (PV) module. For maintaining maximum power as well as constant voltage for MLI, Perturb and Observe (P&O) technique is applied. Sine Pulse Width Modulation (SPWM) is implemented along with unipolar and bipolar switching scheme. Performance parameters like Total Harmonic Distortion (THD), switching loss, switching stress, Root Mean Square (RMS) fundamental voltage and efficiency of MLI output voltage is calculated and graphically compared for both unipolar as well as bipolar schemes. It is found that unipolar scheme outperforms bipolar scheme in all parameters and hence is better than the later. The control algorithm is also validated in Hardware-in-the-Loop using Typhoon HIL 402 emulator and results are presented and discussed.
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In this manuscript, an extended topology based switched coupled inductor quasi-z-source with nine-level inverter is proposed for three-phase grid-tie photovoltaic power system using hybrid technique. The proposed method is the combined execution of Gradient Boosting Decision Tree (GBDT) and Rat Swarm Optimizer (RSO), hence it is called GBDTRSO method. The major objective of the GBDTRSO method is “compute the performance of photovoltaic system by the maximal power extraction”. Here, the designing of the proposed inverter is optimized to supply the maximal power from photovoltaic power generating system. Besides, the higher voltage gain diminished current ripple switched coupled inductor quasi-Z-source inverter i.e extended boost switched coupled inductor quasi-Z-source inverter topology is proposed. The vital aspects of proposed topology is higher voltage gain, ripple free continual source current, typical ground amid the direct current source and inverter circuit, therefore it is optimized for photovoltaic utilizations. The objective function is deemed depending upon its controller parameters with limits like, power, voltages, current, modulation index. These parameters have been employed to the inputs of proposed method. Ensure the maximal power supply to the load using Gradient Boosting Decision Tree technique in terms of maximum power point tracking. Furthermore, the Rat Swarm Optimizer diminished the feed power and controls the direct current link current, voltage and frequency levels. The proposed technique is activated in Matrix Laboratory (MATLAB)/Simulink platform, also the efficiency is likened with different existing methods. The statistical analysis of proposed and existing methods using energy sources is also analyzed. Finally, the experimental outcomes demonstrate that the proposed method is more efficient than the other existing methods.
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Energy savings, clean energy, savings in utility and energy governance tools are buzzwords in the healthcare industry. Healthcare sectors become largest consumer of energy in the modern world. Based on data of American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), a moderate hospital consume 2.5 times higher energy than commercial buildings. As a result, increased worries about energy costs and environmental issues, as well as the anticipation of rising energy prices in the future and the need to enhance the dependability of healthcare facilities, have led to a focus on in-house power generation systems and the importance of energy management in hospitals and their health care facilities. Solar power systems that are clean and ecologically friendly have grown in popularity as distributed power generation (DPG) systems in recent years. In this work, a Grid-tied Solar PV system incorporated with Battery energy storage technology is considered in conjunction with health informatics and the hospital Energy Management System reduces energy consumption cost and improves the reliability of the power supply to run all clinical equipment available in the hospital’s Intensive Care Unit (ICU) and other premises. In this context, the Energy management controller utilised in the hospital Energy Management System will effectively use the electricity supplied by the Solar PV system while minimising grid demand and stabilising the voltage in the DC bus, which must be inverted into AC using an inverter to feed clinical loads. Furthermore the maximum power point tracking method is adopted, which enhances the quality of DC voltage generated by solar PV panels and feed to the DC bus. Sliding mode controller (SMC) is adopted in the inverter side and the quality of the inverted voltage is optimized using artificial bee colony (ABC) method. The proposed solar PV system in conjunction with health informatics and the hospital Energy Management System is developed and simulated in the MATLAB Simulink. The response of the suggested SMC and ABC techniques are compared and their outcomes are shown to confirm the performance of hospital energy management system.
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Maximum Power Point Tracking is an extensive key component of a Solar Photovoltaic system, which can be controlled with different techniques. Applying soft computing techniques for efficient grid integration and cleaner energy production is the aim of this work. This work focused on an MPPT-driven Fuzzy Logic Controller integrated with a 15-level Multilevel Inverter for a single-phase SPV system. MPPT will extract the maximum power from the solar power plant during different irradiation levels of the sun in the daytime. The FLC analyses the ideal duty cycle to validate the highest output of the DC-DC boost converter using output power from the SPV cell with input factor of temperature and sun irradiation. FLC can work with vague input, doesn't require precise mathematical formulations, and can effectively handle nonlinear functions. Furthermore, when compared to other control methods, FLC is more effective. As per IEEE standard 519, Total Harmonic Distortion is limited and maintained in the given range. The proposed model is economically viable and simple for implementation. The simulation studies of the projected MLI are performed and evaluated using Matlab/Simulink. The performance of voltage THD in FLC-MPPT is 0.91% which is lower than P&O- MPPT of 3.51% harmonics in the system.
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In this paper, an effective control strategy based non-isolated Quasi-Z-Source (QZS) novel multilevel inverter topology (NIQZS-NMLI) is proposed for interfacing photovoltaic (PV) system. The proposed approach is the combination of Archimedes optimization algorithm (AOA) and Recalling-Enhanced Recurrent Neural Network (RERNN) called AOA-RERNN approach. Here, the modelling design of non-isolated QZS-NMLI topology is developed with new storage devices to distribute the maximum power from the photovoltaic power generating system. This novel multilevel inverter topology reduced the number of switches and the total harmonic distortion of the system, and also it is used to achieve the higher boost capability, lesser voltage stress across the active switching devices, and greater modulation index for the inverter. The objective function is determined depending on its controller parameters with constraints, like voltages, current, power and modulation. These parameters have been employed as the inputs of AOA-RERNN approach. This AOA-RERNN approach increases the voltage profile, power supply and decreases the power oscillations when sharing the power to the load. The maximum power distribution is guaranteed to the load by RERNN depending on the extraction of maximum power from the PV source. The proposed approach is executed in MATLAB/Simulink site; its performance is analyzed with existing approaches.
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The paper focuses on the energy management of a single-phase PV-BESS hybrid distributed system sized for residential applications, using a CHB converter topology as grid interface. The CHB is driven with a hierarchical energy management architecture, with a single centralized controller for the upper layer, and with multiple decentralized controllers for the lower layer. The upper layer generates the reference signals to be tracked. In ideal conditions, the CHB should work with all PV modules at their MPP and with unitary power factor. This is not always possible because of some functional constraints (e.g., voltage/current constraints, partial shadowing, SoC mismatches). Therefore, a weighted sum optimization method is proposed to explicitly considers these effects and to compute a set of reference variables to be tracked, in a way to optimize the steady-state system performances while, at the same time, guaranteeing the respect of the aforementioned constraints. In this framework, different functional requirements have been included in the optimization algorithm with higher or lower priority according to different operating conditions. The optimal references are then tracked by a decentralized control layer, using standard feedback control techniques. Numerical analysis and experimental results are carried out in order to validate the optimal algorithm in typical operating conditions and to assess the effectiveness of proposed CHB configuration with the whole control strategy.
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Photovoltaic energy has grown at an average annual rate of 60% in the last 5 years and has surpassed 1/3 of the cumulative wind energy installed capacity, and is quickly becoming an important part of the energy mix in some regions and power systems. This has been driven by a reduction of cost of PV modules. This growth has also triggered the evolution of classic PV power converters from conventional single-phase grid-tied inverters to more complex topologies in order to increase efficiency, power extraction from the sun, reliability, while not impacting the cost. This paper presents an overview of the existing PV energy conversion systems, addressing the system configuration of different PV plants, and the PV converter topologies that have found practical applications for grid-connected systems. In addition, recent research and emerging PV converter technology are discussed, highlighting their possible advantages compared with existing technology.
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In the electricity market, customers have many choices to reduce electricity cost if they can economically schedule their power consumption. Renewable hybrid system, which can explore solar or wind sources at low cost, is a popular choice for this purpose nowadays. In this paper optimal energy management for a grid-connected photovoltaic-battery hybrid system is proposed to sufficiently explore solar energy and to benefit customers at demand side. The management of power flow aims to minimize electricity cost subject to a number of constraints, such as power balance, solar output and battery capacity. With respect to demand side management, an optimal control method (open loop) is developed to schedule the power flow of hybrid system over 24 h, and model predictive control is used as a closed-loop method to dispatch the power flow in real-time when uncertain disturbances occur. In these two kinds of applications, optimal energy management solutions can be obtained with great cost savings and robust control performance.
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In this paper a single-phase Cascaded H-Bridge (CHB) inverter for photovoltaic (PV) applications is presented. Based on the presented mathematical analysis, a novel controller is introduced which adjusts the inverter power factor (PF) and manipulates the distribution of the reactive power between the cells to enhance the operating range of the CHB inverter. The adopted control strategy enables tracking of the maximum power point (MPP) of distinct PV strings and allows independent control of the dc-link voltages. The proposed controller also enables the inverter to operate under heavily unbalanced PV conditions. The performance of the CHB inverter and the proposed controllers are evaluated in the PSCAD/EMTDC environment. A seven-level CHB-based grid connected laboratory prototype is also utilized to verify the system performance.
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The modular multilevel converter is a suitable converter topology for high-voltage high-power applications and consists of series-connected submodules. Typically, these submodules are half-bridges with dc capacitors. A voltage ripple in the submodule capacitors is inevitable due to the current flowing in the arms. The converter should therefore be controlled in such a way that the capacitor voltages are kept balanced and close to their nominal values over time. This paper presents a new submodule circuit which alleviates the balancing of the capacitor voltages. The proposed submodule circuit consists of two capacitors and eight switches, forming a three-level submodule. Ideally, the voltage and current rating of the switches can be chosen such that the combined power rating of the semiconductors is the same as for equivalent half-bridge submodules. The proposed submodule circuit provides the possibility of connecting the two capacitors in parallel when the intermediate voltage level is used. This will reduce the capacitor voltage ripple, especially at low switching frequencies and thus allow for a reduction of the size, weight, and cost of the submodule capacitors. The proposed submodule circuit is validated by both simulation results and experiments on a laboratory prototype. It is found that the parallel connection of the submodule capacitors will, in fact, significantly improve the balancing of the capacitor voltages.
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Multi-level Cascaded H-bridge (CHB) converters are ideal for implementing large scale photovoltaic systems. The improved quality of the voltage waveforms, high efficiency and ability to employ multiple Maximum Power Point Tracking (MPPT) algorithms are just some of the advantages. In this paper a three-phase CHB converter supplied with photovoltaic arrays is considered. A control and modulation structure based on Model Predictive Control (MPC) is described. The scheme inherently controls the DC link voltages while also providing the ability to modify any of those voltages to meet MPPT requirements. This avoids the cost and added complexity of extra DC/DC converters that are typically required to keep the DC link voltages uniform. Simulation and experimental results are presented that confirm the correct operation of the proposed approach.
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The increase in the power levels of photovoltaic (PV) energy conversion systems has resulted in new large-scale grid connected configurations that have reached the megawatt level. This substantial increment in the power levels imposes new challenges to the grid interfacing converter, and therefore results in new opportunities to be explored. This work introduces a new medium voltage multilevel scheme based on a three-phase cascaded H-bridge (CHB) converter and multiple PV strings. The proposed configuration enables a large increase of the total power capacity of the PV system, while the introduction of a multilevel converter helps to improve both power quality and efficiency and medium voltage operation at the grid side. The main challenge of the proposed configuration is to handle the inherent power imbalances that occur not only between the different cells of one phase of the converter but also between the three phases. Simulation results of a 7-level CHB PV system are presented to validate the proposed topology and control method.
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This paper presents a new micro-inverter topology that is intended for single-phase grid-connected PV systems. The features of the proposed topology are: (1) eliminating the double-frequency power ripple using small film capacitor; (2) improving the maximum- power-point tracking (MPPT) performance; (3) using long life-time film capacitors, which will improve the reliability of the inverter; and (4) requiring no additional circuitry to manage the transformer leakage energy.
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The integration of large-scale PV systems to the grid is a growing trend in modern power systems. The cascaded H-bridge (CHB) converter is a suitable candidate for the grid interconnection due to its modular characteristics, high-quality output waveforms and capability of connecting to medium-voltage grids. However, the CHB converter requires isolated DC sources. In order to avoid the leakage currents caused by the high potential differences across the parasitic capacitance of the PV panels to ground, an isolated DC-DC conversion stage is required when the CHB topology is used. The objective of this paper is to compare two PV system configurations based on the CHB multilevel converter using two isolated DC-DC converter topologies, namely the boost-half-bridge (BHB) and the flyback, for their performance on providing isolation and achieving individual MPPT at the DC-DC power conversion stage of large-scale PV power systems. Simulation results from a 263 kW PV system based on a seven-level CHB converter with the two aforementioned isolated DC-DC converters are provided for comparison and evaluation with different input PV voltages.
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Partial shading of PV arrays is one of the main causes for reduced energy yield of many PV systems. However, up to now very little attention has been drawn on assessing the performance of MPP trackers due to the complexity and extensive measurement equipment required for this purpose. Against this background, the presented work fills this gap by determining the actual impact of non-ideal, irregular conditions on MPPTs and develops solutions for improved performance. In total 13 MPPTs integrated in state-of-the-art PV inverters were tested with I/V curves measured at a real, shaded PV array. While all inverters have a very high MPPT accuracy under steady state, ideal conditions, shaded conditions led to difficulties as the MPPTs tend to keep a local maximum as long as it exists and are not able to recognise the evolution of another maximum on the I/V curve. This local maximum might not be the overall MPP. In total, this resulted in a reduction of energy yield during a whole simulated day of 1% to 2%. In addition, start-up tests with single partially shaded I/V curves showed very low MPP match which led to a power loss of up to 70%.
Conference Paper
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Large scale grid connected photovoltaic (PV) energy conversion systems have reached the megawatt level. This imposes new challenges on existing grid interface converter topologies and opens new opportunities to be explored. In this paper a new medium voltage multilevel-multistring configuration is introduced based on a three-phase cascaded H-bridge (CHB) converter and multiple string dc-dc converters. The proposed configuration enables a large increase of the total capacity of the PV system, while improving power quality and efficiency. The converter structure is very flexible and modular since it decouples the grid converter from the PV string converter, which allows to accomplish independent control goals. The main challenge of the proposed configuration is to handle the inherent power imbalances that occur not only between the different cells of one phase of the converter but also between the three phases. The control strategy to deal with these imbalances is also introduced in this paper. Simulation results of a 7-level CHB for a multistring PV system are presented to validate the proposed topology and control method.
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There is a high development of grid-connected photovoltaic systems that utilize static converter which influence the efficiency of the system. The energy conversion depends on the architecture of the converter, different solutions are available and H-bridge multilevel converters seem to be an optimal solution also for the power quality. It is important to utilize a proper architecture of converter but also to set up a control optimized to have the energy conversion at maximum efficiency. The persistence of the maximum efficiency condition is related, depending on the atmospheric conditions, to the tracking of the Maximum Power Point (MPP) by modifying the operating conditions of the system (Maximum Power Point Tracking -MPPT). In the paper is proposed a sensorless control set up to deliver the maximum power to the grid in presence of variations of incident irradiation on the photovoltaic arrays. The control technique is presented and validated by simulation implemented on a photovoltaic system with H-bridge 5-levels converter. The simulation results confirm that the control is able to effectively track the MPP and to stabilize immediately in the new steady-state condition.
Conference Paper
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This paper introduces a new control method and proportional PWM modulation of the cascaded H-bridge multilevel converter for grid-connected photovoltaic systems. This control makes each H-bridge module supply different power levels, allowing therefore for each module an independent maximum power point tracking of the corresponding photovoltaic array.
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Maximum power point tracking (MPPT) techniques are used in photovoltaic (PV) systems to maximize the PV array output power by tracking continuously the maximum power point (MPP) which depends on panels temperature and on irradiance conditions. The issue of MPPT has been addressed in different ways in the literature but, especially for low-cost implementations, the perturb and observe (P&O) maximum power point tracking algorithm is the most commonly used method due to its ease of implementation. A drawback of P&O is that, at steady state, the operating point oscillates around the MPP giving rise to the waste of some amount of available energy; moreover, it is well known that the P&O algorithm can be confused during those time intervals characterized by rapidly changing atmospheric conditions. In this paper it is shown that, in order to limit the negative effects associated to the above drawbacks, the P&O MPPT parameters must be customized to the dynamic behavior of the specific converter adopted. A theoretical analysis allowing the optimal choice of such parameters is also carried out. Results of experimental measurements are in agreement with the predictions of theoretical analysis.
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When providing ac output, modular multilevel converters (MMCs) experience power fluctuation in the phase arms and the individual modules comprising them. This power fluctuation causes voltage oscillations on the module capacitors, which grow with the output power and inversely to the output frequency. Thus, low-frequency operations of MMCs, e.g., for motor drives, require injecting circulating currents and common-mode voltages, and strict dc voltage output relative to ground is impossible. To address this problem, this paper introduces a novel module topology that allows a parallel module connectivity state in addition to positive series and bypass interconnections. The parallel state enables direct power transfer across modules and converter arms to directly cancel the arm power fluctuation and hence suppress the capacitor voltage ripple. The proposed series/parallel converter can operate at a wide frequency range down to dc without common-mode voltages or circulating currents; it also allows fully sensorless operation as well as full utilization of the additional components at higher output frequencies. We present detailed simulation and experiment results to characterize the advantages and limitations of the proposed solution.
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Modules with series and parallel connectivity add new features and operation modes to modular multilevel converters (MMCs). Compared to full- and half-bridges, the series/parallel modules allow sensorless module balancing and reduce conduction loss with the same semiconductor area. However, in high-voltage applications with limited switching rates, the sensorless operation of the series/parallel modules suffers from large charge-balancing currents. This paper introduces a series/parallel module variant with a small port inductor. The port inductor suppresses the charge-balancing current despite low switching rates. We also propose a carrier-based modulation framework and show the importance of the carrier assignment in terms of efficiency and balancing. The proposed module and the modulation method are verified on a lab setup with module switching rates down to 200 Hz. The module voltages are kept within a narrow band with the charge-balancing currents below 5% of the arm current. The experimental results show practicality and advantages of the new series/parallel modules in high-voltage MMC applications.
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This paper presents a modular multilevel series/parallel converter (MMSPC) with intrinsic inter-module switched-inductor power transfer. The switched-inductor voltage conversion feature allows controllable and efficient transfer of energy between modules with nonnegligible voltage difference, providing both step-down and step-up functionalities. Thus, this converter can accurately control and rapidly adjust the voltage of each module to generate an ac output voltage waveform with a controllable number of levels, increasing the quality of the output. Moreover, the intrinsic dc-dc conversion feature can generate a dc controllable output voltage and enable new applications. In this text, we specifically demonstrate how the fiexibility of obtaining both ac and dc output with the same setup renders the topology promising for battery energy storage systems (BESS) and dc microgrid applications. Experimental results validates the topology and concept of an MMSPC with intrinsic switched-inductor conversion IEEE
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Due to the unequal solar radiations or dust accumulation of photovoltaic module in single-phase cascaded H-bridge photovoltaic inverter, the unbalanced output power among photovoltaic modules will make the H-bridges with higher power over-modulation, resulting in deteriorated grid current. Concerning this issue, this paper proposes an optimized third harmonic compensation strategy that injects moderate amount of third harmonic into the over-modulation H-bridges to keep the amplitudes of their modulation waveforms just being unity, then compensates same amount of opposite harmonic sequence and properly distributes it to the rest H-bridges. The proposed method can ensure that all the power units will be free from over-modulation under some heavy power imbalance conditions and not increase the third harmonic component of grid current. The validity and effectiveness of the proposed method is verified by simulation and experimental results.
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The Hybrid modulation strategy using both low frequency square wave modulation and high frequency pulse-width modulation (PWM) can extend stable operating region of cascaded H-bridge converter. However, it does not control accurately dc voltages of converter but balances them by charging or discharging dc capacitors based on the state of system, which could aggravate fluctuation of dc voltages. If cascaded H-bridge converter is used in photovoltaic field, the aggravated fluctuation on dc capacitor voltages will result in further losses in energy harvesting of solar cells. To address this issue, a modified hybrid modulation strategy is presented in this paper. When dc-bus measured voltages are close to their reference voltages, these H-bridges operate in zero mode. Adding appropriate zero state can prevent grid current charging or discharging the dc capacitors whose voltages have already reached their control goals. Therefore, the proposed technique is capable of suppressing dc voltage fluctuation and thus improving the output power of solar cells. Simulation and experimental results validate the effectiveness and feasibility of the proposed method. IEEE
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This paper presents a multi-objective real-time controller for a modular multilevel converter capable of parallel module connectivity, the so called modular multilevel series parallel converter (MMSPC). The MMSPC topology allows the batteries to be dynamically rewired in various series-parallel configurations, generating a wide range of output voltage levels. The novel control method parallelizes the modules to balance their voltages without the need for individual module voltage monitoring. Additionally, the controller optimizes across the large number of feasible system configurations to minimize switching and conduction losses. Finally, the controller efficiently encodes the system configuration with module interconnection states rather than the module switch states, which substantially simplifies control. Furthermore, this work experimentally validates the MMSPC topology and concept. In the prototype, the parallel mode reduced the system losses at 5 kW output power by 18% and 24% for load power factors of 1.0 and 0.8, respectively. Sensorless balancing via parallelization maintained well-matched module voltages (standard deviation = 0.045 V) over a 5-hour battery discharge with highly variable load current. The reduced conduction losses and simple balancing capability of the MMSPC can enable new applications at medium and low voltages that benefit from its high-quality output, elimination of filtering magnetics, fast response, and modularity.
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Featuring high efficiency, low harmonic distortion, high modularity and scalability, the modular multilevel converter (MMC) is particularly suitable for high voltage direct current transmission applications. As an advanced control strategy, model predictive control (MPC) has the advantage of direct modeling and fast dynamic response. It can simultaneously control multiple variables through an appropriate cost function. The conventional MPC can achieve an optimal control objective by evaluating all the candidate switching states for the MMC; however, with increasing number of submodules, there is an increasing number of candidate switching states that place an enormous burden on the control. In this paper, a grouping-sorting-optimized MPC (GSOMPC) strategy is proposed for the MMC with the number of submodules for each arm increases to hundreds. It divides all submodules of each arm into M groups, with each containing X submodules. By the implementation of the first level and second level optimized MPC between groups and submodules, respectively, the computational load of each phase decreases from C-2N(N) to 2X + M + 3(N = M x X). In addition, to reduce the strict requirements of control hardware for sorting and calculation, the proposed strategy is able to simultaneously control the submodule voltage, ac current, circulating current, and switching frequency. Applied to a 2.7-kV/60-kW MMC back-to-back dynamic test system, experimental results verify the feasibility and effectiveness of the proposed GSOMPC strategy.
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As the modular multi-level converter is now developing rapidly, especially in high power applications, and the large amount of sub-modules in the modular multi-level converter will increase the probability of failures, fault detection and diagnosis of the sub-modules are of importance for continuous operation and post-fault maintenance. In this article, a fault diagnosis technique is proposed for the short circuit fault in a modular multi-level converter sub-module using the wavelet transform and adaptive neuro fuzzy inference system. The fault features are extracted from output phase voltage by employing wavelet transform under different fault conditions. Then the fuzzy logic rules are automatically trained based on the fuzzified fault features to diagnose the different faults. Neither additional sensor nor the capacitor voltages are needed in the proposed method. The high accuracy, good generalization, as well as the time-saving characteristic is verified by conducting a comparison with the neural network method.
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Preface. 1. Introduction. I: Converters in Equilibrium. 2. Principles of Steady State Converter Analysis. 3. Steady-State Equivalent Circuit Modeling, Losses, and Efficiency. 4. Switch Realization. 5. The Discontinuous Conduction Mode. 6. Converter Circuits. II: Converter Dynamics and Control. 7. AC Equivalent Circuit Modeling. 8. Converter Transfer Functions. 9. Controller Design. 10. Input Filter Design. 11. AC and DC Equivalent Circuit Modeling of the Discontinuous Conduction Mode. 12. Current Programmed Control. III: Magnetics. 13. Basic Magnetics Theory. 14. Inductor Design. 15. Transformer Design. IV: Modern Rectifiers and Power System Harmonics. 16. Power and Harmonics in Nonsinusoidal Systems. 17. Line-Commutated Rectifiers. 18. Pulse-Width Modulated Rectifiers. V: Resonant Converters. 19. Resonant Conversion. 20. Soft Switching. Appendices: A. RMS Values of Commonly-Observed Converter Waveforms. B. Simulation of Converters. C. Middlebrook's Extra Element Theorem. D. Magnetics Design Tables. Index.
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This paper presents a modular cascaded H-bridge multilevel photovoltaic (PV) inverter for single- or three-phase grid-connected applications. The modular cascaded multilevel topology helps to improve the efficiency and flexibility of PV systems. To realize better utilization of PV modules and maximize the solar energy extraction, a distributed maximum power point tracking control scheme is applied to both single- and three-phase multilevel inverters, which allows independent control of each dc-link voltage. For three-phase grid-connected applications, PV mismatches may introduce unbalanced supplied power, leading to unbalanced grid current. To solve this issue, a control scheme with modulation compensation is also proposed. An experimental three-phase seven-level cascaded H-bridge inverter has been built utilizing nine H-bridge modules (three modules per phase). Each H-bridge module is connected to a 185-W solar panel. Simulation and experimental results are presented to verify the feasibility of the proposed approach.
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A novel auto-tuning digital DC–DC converter is presented. In order to reduce the recovery time and undershoot, the auto-tuning control combines LnL, conventional PID and a predictive PID with a configurable predictive coefficient. A switch module is used to select an algorithm from the three control algorithms, according to the difference between the error signal and the two initially predefined thresholds. The detection and control logic is designed for both window delay line ADC and Σ Δ DPWM to correct the delay deviation. When the output of the converter exceeds the quantization range, the digital output of ADC is set at 0 or 1, and the delay line stops working to reduce power consumption. Theoretical analysis and simulations in the CSMC CMOS 0.5 μm process are carried out to verify the proposed DC–DC converter. It is found that the converter achieves a power efficiency of more than 90% at heavy load, and reduces the recovery time and undershoot.
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This paper presents a new strategy, diffusion charge redistribution (DCR), for balancing power among photovoltaic cells to increase energy extraction and to improve maximum power point tracking (MPPT) efficiency under partial shading conditions. With DCR, testing and binning during cell manufacturing can be eliminated, and significant cost savings can be achieved during production. The proposed technique performs power balancing by taking advantage the intrinsic diffusion capacitance of the solar cells and requires no external passive components for energy storage, thereby minimizing power electronics cost and complexity. Strings balanced by this technique exhibit power versus current curves that are convex, which also greatly reduces the cost and complexity of the required MPPT algorithm.
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The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.
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This paper introduces a novel modular multilevel series/parallel converter that allows switching modules dynamically not only in series, as in the traditional modular multilevel converter (M2C), but also in parallel. As in M2C, the semiconductor voltages do not exceed the module capacitor voltage for any module state. While the new topology is a generalization of M2C and could, therefore, be operated identically to it, the additional states provide degrees of freedom that the controller can dynamically employ to achieve several advantages. Whereas in M2C many modules are bypassed if the instantaneous converter voltage is lower than the system's peak voltage, the parallel connectivity enables these modules to contribute to the current load, thus reducing conduction losses. In addition, the parallel configuration of modules can be used for balancing the modules’ state of charge (SOC). The parallelization losses are moderate or negligible, dependent on the switching rate. Since the parallel connection of capacitors can ensure balancing, it enables stable operation of a multilevel converter without the need for monitoring the module SOCs. While such economical control hardware may be appropriate for low-power systems, we also present more sophisticated control that uses the additional degrees of freedom to minimize losses. Finally, we point to further extensions of the circuit topology to multipole module connectivity that could enable additional functionality and applications.
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Modular multilevel converters (M2LCs) are typically controlled by a hierarchical control scheme, which essentially requires at least two control loops: one to control the load current and another to control circulating currents. This paper presents an M2LC with a single controller, which is based on model predictive direct current control (MPDCC) with long prediction horizons. The proposed MPDCC scheme maintains the load current within tight bounds around sinusoidal references and minimizes capacitor voltage variations and circulating currents. An internal prediction model of the M2LC is used to minimize the number of switching transitions for a given current ripple at steady state while providing a fast current response during transient conditions. A state-space model, which is generalized for an $N$ number of modules per each arm of the M2LC, is also presented to investigate the dynamic behavior of arm currents and capacitor voltages. Simulated performance of the converter, under various operating conditions, is presented in comparison to measured performance of a single-phase, three-level 860-VA M2LC prototype to demonstrate the proposed MPDCC philosophy.
Conference Paper
A curve fitting method that approximates the P-V characteristic curve of a given solar array with a fourth order polynomial is presented. The coefficients of the approximated P-V characteristic curve are strongly dependent on cell temperature. The strong dependence of the P-V curve on cell temperature allows the P-V characteristic curve of a given system to be written as a function of both panel voltage and cell temperature. The curve fitting method presented can be used in an indirect maximum power point tracking algorithm for use with solar powered systems. Experimental results are presented to confirm the validity of the method.
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In this paper, the cascaded H-bridge (CHB) inverter is evaluated as a PV power conversion solution for large-scale PV power plants. The advantages and disadvantages of the CHB inverter for large-scale PV power plants are discussed. Comparison between the CHB and conventional central inverters is also presented in terms of efficiency and cost. For comprehensive assessment of the CHB inverter on the system level, a system-level evaluation is also performed in this paper. The levelized cost of electricity (LCOE) is used as the evaluation metric. LCOE of PV system with either the conventional central or CHB inverters is calculated for different geographic locations and PV module technologies. The comparative evaluation demonstrates the promising benefits in using the CHB inverter for large-scale PV power plants.
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This paper presents a single-phase grid-connected photovoltaic (PV) module-integrated converter (MIC) based on cascaded quasi-Z-source inverters (qZSI). In this system, each qZSI module serves as an MIC and is connected to one PV panel. Due to the cascaded structure and qZSI topology, the proposed MIC features low-voltage gain requirement, single-stage energy conversion, enhanced reliability, and good output power quality. Furthermore, the enhancement mode gallium nitride field-effect transistors (eGaN FETs) are employed in the qZSI module for efficiency improvement at higher switching frequency. It is found that the qZSI is very suitable for the application of eGaN FETs because of the shoot-through capability. Optimized module design is developed based on the derived qZSI ac equivalent model and power loss analytical model to achieve high efficiency and high power density. A design example of qZSI module is presented for a 250-W PV panel with 25-50-V output voltage. The simulation and experimental results prove the validity of the analytical models. The final module prototype design achieves up to 98.06% efficiency with 100-kHz switching frequency.
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Multilevel Converters and battery energy storage systems (BESS) are key components in present and future medium voltage networks, where an important integration of renewable energy sources takes place. The Modular Multilevel Converter (MMC) offers the capability of embedding such energy storage elements in a split manner, given the existence of several sub-modules operating at significantly lower voltages. This paper analyzes such a converter structure under different operating modes. In order to eliminate the low frequency components of the sub-module output currents, the latter are interfaced to the batteries by means of non-isolated DC/DC converters. Control algorithms are developed for the balancing of the battery State of Charges and the respective gain limitations are established. Unbalanced grid conditions are also taken into account through the theory of symmetrical components and solutions are proposed. Finally, the development of a down-scaled prototype is described and experimental results are presented.
Article
Multilevel voltage source inverters offer several advantages compared to their conventional counterparts. By synthesising the AC output terminal voltage from several levels of DC voltages, staircase waveforms can be produced, which approach the sinusoidal waveform with low harmonic distortion, thus reducing filter requirements. The need of several sources on the DC side of the converter makes multilevel technology attractive for photovoltaic applications. This paper provides an overview on different multilevel topologies and investigates their suitability for single-phase grid connected photovoltaic systems. Several transformerless photovoltaic systems incorporating multilevel converters are compared regarding issues such as component count and stress, system power rating and the influence of the photovoltaic array earth capacitance.
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This paper presents an energy-balance control strategy for a cascaded single-phase grid-connected H-bridge multilevel inverter linking n independent photovoltaic (PV) arrays to the grid. The control scheme is based on an energy-sampled data model of the PV system and enables the design of a voltage loop linear discrete controller for each array, ensuring the stability of the system for the whole range of PV array operating conditions. The control design is adapted to phase-shifted and level-shifted carrier pulsewidth modulations to share the control action among the cascade-connected bridges in order to concurrently synthesize a multilevel waveform and to keep each of the PV arrays at its maximum power operating point. Experimental results carried out on a seven-level inverter are included to validate the proposed approach.
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A conventional photovoltaic module has been prepared with the purpose of accessing its cells either individually or associated. Measurements of every cell and of the whole module have been performed in direct and reverse bias, with the objective of documenting the scattering in cell parameters, working point of the cells and shading effects. Several shading profiles have been tested, and the influence of the reverse characteristic of the shaded cell in module output is stressed. (c) 2005 Elsevier B.V. All rights reserved.
Conference Paper
The multi-string photovoltaic (PV) inverter is of interest for building grid-connected PV systems because it offers a number of advantages compared to conventional centralized or single-stage inverters. Optimal operation of the system can be achieved since the maximum power point (MPP) of each string is controlled by a local dc-dc converter input stage. The input dc-dc converter also provides voltage decoupling, allowing the storage capacitance to be minimized by permitting increased voltage fluctuations on the intermediate dc bus. The electrolytic bus capacitor can therefore be replaced by a smaller, more-reliable film capacitor in order to increase the lifetime of the converter. This minimization of capacitance is not possible in a single-stage string inverter as a large input capacitor must directly decouple the PV string from the pulsating single phase output power. In this paper, the capacitance requirements of a two-stage string PV inverter are compared to those of a single-stage inverter and evaluated in light their impact on PV utilization efficiency. A complete simulation model, developed in PLECS and Simulink, is used to determine the interaction between the power conversion stages and the PV supply for the two systems and to demonstrate the relationship between capacitance, dc bus voltage fluctuations and overall system efficiency.
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This paper describes the operation of modular multilevel converter, an emerging and highly attractive topology for medium- and high-voltage applications. A new pulsewidth-modulation (PWM) scheme for an arbitrary number of voltage levels is introduced and evaluated. On the basis of this PWM scheme, the semiconductor losses are calculated, and the loss distribution is illustrated.
Article
This paper presents a single-phase cascaded H-bridge converter for a grid-connected photovoltaic (PV) application. The multilevel topology consists of several H-bridge cells connected in series, each one connected to a string of PV modules. The adopted control scheme permits the independent control of each dc-link voltage, enabling, in this way, the tracking of the maximum power point for each string of PV panels. Additionally, low-ripple sinusoidal-current waveforms are generated with almost unity power factor. The topology offers other advantages such as the operation at lower switching frequency or lower current ripple compared to standard two-level topologies. Simulation and experimental results are presented for different operating conditions.
Conference Paper
A photovoltaic panel fitted with a large collection of low-power inverters integrated at the level of individual solar cells is used to design an ac module. To facilitate dc-ac power conversion, the inverter aggregate is controlled using interleaved carrier pulse width modulation. Every solar cell operates at its maximum power point even when the photovoltaic panel is partially shaded. Additionally, a very low switching frequency can be used to minimize switching losses without increasing output distortion. Both system-level and local control strategies are developed to regulate power output, energy storage, and ensure stable operation. Experimental and simulation results are presented to verify and demonstrate the proposed method.
Conference Paper
This paper proposes a MATLAB Simulink simulator for photovoltaic (PV) system. The main contribution of this work is the utilization of the two-diode model to represent the PV cell. This model is known to have better accuracy at low irradiance level which allows for a more accurate prediction of PV system performance. To reduce computational time, the input parameters are reduced to four and the values of Rp and Rs are estimated by an efficient iteration method. Furthermore, all the inputs to the simulator are information available on standard PV module datasheet. The simulator supports large array simulation that can be interfaced with MPPT algorithms and power electronic converters. The accurateness of the simulator is verified by applying the model to two PV modules. It is envisaged that the proposed work can be very useful for PV professionals who require simple, fast and accurate PV simulator to design their systems.
Article
Conventional popular maximum power point tracking (MPPT) methods are effective under uniform solar irradiance. However, under solar irradiance mismatching conditions [partially shaded conditions (PSCs)], these MPPTs can fail for real MPPT (RMPPT), because multiple local maxima can be exhibited on the power-voltage characteristic curve. Although some researchers have worked on RMPPT under partial shading conditions, the methods have some drawbacks in terms of complexity and requirements for additional circuits, etc. In this paper, a novel MPPT method capable of RMPPT under PSCs is proposed. The performance of the proposed MPPT method is analyzed according to the RMPP position and is verified by simulation and experimental results.
Conference Paper
This paper presents an energy-sampled data model of a cascade H-bridge multilevel converter for single-phase multi-string grid-connected photovoltaic systems. Based on this model a control scheme for the power converter is obtained. The achievement of the DC-AC conversion with an efficient PV energy extraction, low harmonic distortion and output unity power factor is validated with simulation results
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The design and control issues associated with the development of a 1.8 kW prototype single-phase grid-connected photovoltaic system incorporating a multilevel cascaded inverter are discussed in this paper. For the current controller a ramptime zero average current error control algorithm combined with an optimised cyclic switching sequence is suggested. Simulation results are presented to demonstrate the suitability of the control method.