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Residential DC Microgrids: Latest’s techniques and future perspectives
Abstract
In the last decade DC microgrids have seen a significant evolvement on both the research and industrial sectors. In this presentation, residential microgrids are elaborated on from the topologies, energy management, control, and protection perspectives. Moreover, an overview of the microgrid and IoT Labs is given with highlights on the recent attained results and faced challenges. Finally, future perspectives are provided based on the current reached research and real application updates.
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This paper presents a review of coordinated control strategies, stability analysis, and fault management for dc shipboard microgrids (dc-SMGs). As an emerging application, the dc-SMG lacks of a comprehensive summary on the control schemes. Considering the specific load profile in ships, this paper discusses the coordinated control strategies of diesel generators and energy storage system (ESS). Depending on the maritime conditions, different operation modes are switched by the energy management system (EMS). Due to the presence of high-bandwidth controlled constant power load (CPL) converters, negative impedance instability is induced. Meanwhile, the pulsed loads in dc-SMG may affect the voltage stability significantly. Small-signal and large-signal stability analysis can be used to evaluate the effect of these two types of loads. In the aspect of shipboard protection system, this paper presents the specific requirements of dc-SMG based on the standards. Lacking of ground in ships, solutions in designing the grounding system for dc-SMG is described. Besides, the short-circuit fault management consisting of fault detection, fault isolation, and reconfiguration in dc-SMGs are summarized. Finally, existing commercial dc ships are presented and compared to demonstrate the developing trends.
Integrated power systems (IPSs) are popular in the shipbuilding industry. DC shipboard microgrids (dc-SMGs) have many advantages compared with ac ones in terms of system efficiency, operation flexibility, component size, and fault protection performance. Being in the exploring stage, dc-SMGs have several potential configurations with different system architectures and voltage levels. In a dc-SMG, functional blocks integrated include power generation modules (PGMs), propulsion system, high power loads, and pulse loads specifically in naval ships. In modern ships, the PGMs include not only generators and fuel cells (FCs) but also energy storage systems (ESSs), which cooperate with generators to improve the overall efficiency and reliability. High power electric converters are vital interfaces between the functional blocks and the dc distribution system. Rectifiers for generators take the tasks of dc bus voltage regulation and power sharing. Inverters for propulsion motors are responsible for the motor drive in different operating conditions. Bidirectional dc/dc converters for ESSs are used to provide supply-demand balance and voltage fluctuation mitigation. This paper makes a comprehensive review of power architecture, functional blocks including electrical machines and energy storage, as well as power converters in dc shipboard power systems.
Modern maritime microgrid systems are witnessing a revolutionary advancement by integrating more renewable energy sources and energy storage systems. The integration of these sophisticated systems is achieved, however, through the power electronics converters that cause severe harmonic contamination. This problem becomes more serious when some of these harmonics that are non-integer multiples of the fundamental (inter-harmonics) exist concurrently with both system frequency drifts and large-power transients, which is a commune issue in maritime microgrid systems such as shipboard microgrids. Hence, the performance of the widely signal processing algorithms applied in the measurement and communication systems such as the smart meters and power quality analyzers tends to worsen. To address this problem this paper proposes an effective method based on the eigenvalue solution to estimate the harmonics and inter-harmonics of modern maritime microgrid systems effectively. This method, which is a system frequency independent technique can work effectively even under large frequency drifts with short window width. The proposed method is evaluated under MATLAB software, and then the experimental validation is carried out via analyzing the electrical power system current of a bulk carrier ship.
Retrofitting of shipboard microgrids is receiving much attention nowadays due to the flexibility it offers to adapt existing ships with rapid market variations to move towards all-electric ships (A-ESs). This modernity mainly relies on incorporating the power electronics converters that unfortunately draw a large number of harmonics, which affect the energy quality and threaten the system stability and crew/passengers safety. The contribution of this paper lies in proposing two developed open-loop algorithms to assess the harmonics distortion of A-ESs without relying on signal periodicity. The first algorithm is an offline-based analyzing technique developed for the short-term protective action stage, which incorporates the eigenvalue solution inside the short-time fast Fourier transform (ST-FFT) to adapt its window size. Consequently, this algorithm can provide a comprehensive harmonics analysis with a fast transient response even under large system frequency drifts. The second algorithm is an online-based assessing technique, which is very suitable for the long-term preventive action stage to provide accurate harmonics distortion assessment with the fast transient response and efficient computation burden. This approach relies on cascading three moving average filters (MAFs) with particular window sizes to filter and estimate the mean value inside the discrete Fourier transform and the true RMS blocks without the need of frequency information. Hence, it results in enhancing the harmonics rejection capability of the algorithm even under the existence of non-characteristic harmonics and frequency drifts. Simulation and experimental results are provided to validate the efficacy of the proposed algorithms and the results are compared with traditional methods.
Several space organizations have been planning to establish a permanent, manned base on the Moon in recent years. Such an installation demands a highly reliable electrical power system (EPS) to supply life support systems and scientific equipment and operate autonomously in a fully self-sufficient manner. This paper explores various technologies available for power generation, storage, and distribution for space microgrids on the Moon. Several factors affecting the cost and mass of the space missions are introduced and analysed to provide a comprehensive comparison among the available solutions. Besides, given the effect of base location on the design of a lunar electrical power system and the mission cost, various lunar sites are introduced and discussed. Finally, the control system requirements for the reliable and autonomous operation of space microgrids on the Moon are presented. The study is complemented by discussing promising future technological solutions that could be applied upon a lunar microgrid.
In recent years, due to the large spread of power electronics converters (PECs) onboard shipboard power systems (SPSs), harmonic contamination became an inherent sproblem. Moreover, as the SPSs are mostly operating in an islanded mode with a limited rating of the generators that feed heavy variable loads, voltage stability issues became a challenge as well. To address these issues, implementing PWM-based FACTS devices using IGBTs such as the active power filters may not be very practical for high-power SPSs due to their limited rated powers, high implementation cost, and high losses. The passive power filters (PPFs), and fixed capacitor-thyristor controlled reactors (FC-TCR) can be effective alternatives due to their capability to operate in high power applications, smaller losses and, lower cost. The aim of this paper is to provide more in-depth analytical investigations to evaluate and compare the behaviour of the PPFs and the FC-TCR compensator in reducing the harmonics and enhancing the voltage stability concurrently of high-power SPSs. In addition, details of the voltage stability concept are addressed to visualize the potential risk of the voltage collapse. Besides, the modeling of the FC-TCR and design of the closed-loop control algorithm to ensure the voltage stability is conducted. Via intensive simulation studies of a practical ship, which are carried out in MATLAB/Simulink environment, it has been demonstrated that the FC-TCR-based on the proposed design can enhance the power quality and voltage stability of SPSs better than traditional PPFs filters.
Addressing power quality issues in shipboard micro-grids (SMs), which are mainly attributable to the increased installation of power converters, has received much attention recently. To this end, static var compensators (SVCs), such as thyristor switched capacitors (TSCs) and fixed capacitors-thyristor controlled reactors (FCs-TCRs), can be effective solutions. Controlling these compensators, however, is not a trivial task as it involves sophisticated operations, especially estimating the firing angle, which should be carried out based on some nonlinear equations. This paper aims to propose the application of some simple yet numerically efficient algorithms based on Bisection, Newton-Raphson, False Position, and Scant methods for estimating the firing angle of the FC-TCR. The effectiveness and robustness of these algorithms are demonstrated via modeling of the FC-TCR with the electrical power system of a practical hybrid ferry under MATLAB/Simulink environment, where the results proved that the enhanced power quality issues respect the IEC standards 61000-4-7/30. Furthermore, an experimental setup consists of a digital signal processor and a programmable source is used to demonstrate that these techniques can be effectively applied in real-time applications.
In recent years, the significant revolution of shipboard micro-grids (SMs) towards all-electric ships offered several advantages such as high efficiency, better maneuverability, and controllability. However, the application of power electronics onboard SMs, particularly for DC and hybrid ships leads to serious power quality issues. The application of passive power filters (PPFs) has been largely used to solve some of the power quality issues, such as harmonics contamination, power factor compensation, and voltage drop enhancement. However, analyzing some crucial factors such as the harmonic attenuation factor (HAF) and filters sensitivity factor (FSF) of these filters have not been attracted much attention. In this paper, a more in-depth mathematical analysis of the HAF and FSF of these filters is conducted with details. Moreover, some developed solutions to improve these factors are suggested. Based on intensive simulation studies of a practical hybrid ferry, which are carried out under MATLAB/Simulink environment, it has been demonstrated that the conducted analysis and proposed solutions can enhance the filtering performance of the PPFs.
This paper deals with a shunt active power filter (SAPF) integrated in a photovoltaic (PV) system, which is interfaced to the grid via a double-stage configuration, for simultaneously improving the power quality in the existence of non-linear loads and injecting the PV harvested power to the power grid. The direct power control (DPC) based on the conventional Proportional-integral (PI) suffers from some shortcomings in the transient state, such as large overshoots and undershoots in the voltage. Long response time is another disadvantage when using such a controller. To overcome this situation, the proposed control method is equipped by an anti windup fractional order proportional-integral differentiator (AW-FOPID) regulator, replacing the standard PI or PID regulators to maintain the DC link voltage at its desired value with small overshoots and undershoots in the voltage, while maintaining a short response time. The AW-FOPID controller, however, has five parameters, which makes it troublesome to tune. Accordingly, to adjust this AW-FOPID parameters, the Particle Swarm Optimization (PSO) algorithm is employed by minimizing the Integral Time Absolute Error (ITAE). Furthermore, an intelligent algorithm for tracking the maximum power point (MPPT) based on fuzzy logic has been applied to eventually resolve the drawback of the rapidly changing weather conditions. The overall control scheme is examined by simulation using MATLAB/Simulink software. The obtained simulation results and comparative study demonstrate the feasibility and performance of this control strategy.
The electric power systems (EPSs) are of a paramount importance in satellites. These systems are responsible for power generation, storage, delivery, and conditioning. A failure in an EPS may hinder all the aforementioned functionalities, which consequently, leads to a failure of the whole space mission. There are many technologies available in both industry and literature of EPSs, where each has its own fault tolerance capabilities. On the other hand, the efficiency of the EPS is an important factor, which promotes having higher power reserve; thus, feeding the payloads at all needed times. In this paper, the EPS for Satellites-based microgrids are reviewed with more focus on their energy generation and storage, including their protection schemes. Moreover, sizing guidelines for the energy generation and storage systems are also provided.
Recently, the evolutionary development in power electronics (PE) technologies onboard shipboard micro-grids (SMs) has been receiving significant attention. Indeed, this evolution added enormous advantages such as enhanced maneuverability and controllability. However, the installation of PE converters onboard SMs results in serious power quality issues. The application of the fixed capacitors-thyristor controlled reactors (FCs-TCRs) is largely used to improve some of the power quality issues. However, the non-linearity of the firing angle equation increases the complexity of controlling this sophisticated technology. In this paper, three simple and yet effective numerical algorithms are proposed to estimate the firing angle of the FC-TCR compensator. These algorithms that are based on Newton-Raphson, Bisection, and false position methods have proved their reliability and efficacy in estimating the firing angle under load and frequency variation. The effectiveness and robustness of the proposed methods are demonstrated via modeling an electrical power system of a practical hybrid ferry under MATLAB/Simulink environment, then the results are compared with the traditional technique
This article proposes an improved maximum power point tracking (MPPT) method that features a simple design, and improved efficiency in fast-changing irradiance conditions. The method uses three consecutive measurements and compares the power difference between each two consecutive samples, furthermore the voltage variation between the last two successive samples is observed. According to the obtained result of these comparisons, the algorithm applies the suitable action either increasing or decreasing the voltage. This simple concept allows easy implementation and reduces the implementation cost and calculation burden. Second, the method has a prompt tracking response during fast changes in solar irradiance (e.g., due to passing clouds). The proposed method is validated through experimental tests using solar irradiance profiles according to the EN50530 standard and is compared to the classical Perturb and Observe method. The experimental results show that the proposed MPPTeffectively identifies the change in solar irradiance, and maintains high tracking efficiency even in fast-changing conditions.
In recent years, the impedance source network has been studied by many researchers focusing on high conversion gain power electronics. Several number of topologies have been developed. The combination of the Switched inductor cells and Z-source networks has shown its capability of achieving high voltage gains and good power conversion at low shoot through duty cycle values as compared to traditional Z-source network. This paper contributes as an overview of different Switched inductors cells combined with Z-source or quasi Z-source net-work. General analytical classification, main charachterestics, and comparison of different topologies and their modifications, are provided in this paper. This study should be of interest for engineers and researchers when selecting their high voltage gain impedance source converter.
This letter proposes a multilevel inverter (MLI), which can be considered to be belonging to the cascaded H-bridge (CHB) family. In the proposed MLI, some of the H-bridges are connected to dc-sources, while most of the H-bridges are connected to only floating capacitors. A boosting circuit is integrated in the CHB, which permits charging the floating capacitors, enabling a higher voltage at the inverter's output with respect to the dc-sources total one. This boosting feature is essential in some applications like photovoltaic and fuel cell systems. Compared to the popular solutions in the literature, the proposal still offers a high boosting ratio with fewer employed components. A prototype of the proposed MLI has been built, and the results confirming its validity are shown in this letter. Efficiency analysis and evaluation with respect to one of the conventional solutions is also presented, where it is demonstrated that the proposal shows an improvement of 3.94% according to the European efficiency.
Photovoltaic (PV) technology is rapidly developing for grid-tied applications around the globe. However, the high level PV integration in the distribution networks is tailed with technical challenges. Some technical challenges concerns the stability issues associated with intensive PV penetration into the power system are reviewed in this paper. To mitigate the voltage disturbances in a system with massive PVs integration, some techniques are devoted such as frequency regulation techniques, active power (AP) curtailment, reactive power (RP) injection, and storage energy. Also, with a high penetration level of distributed generators, the potential of dynamic grid support is discussed. Islanding operation and microgrid (MG), operating using different control techniques, which ensure a smooth transition (ST) between grid-connected and islanded operation modes as well as synchronization between the two modes, are discussed.
The difference in power cell irradiances in cascaded multilevel converters results in different duty cycles among those cells when maintaining the maximum power point tracking (MPPT). However, the difference in cell duty cycles is undesired since it is proportional to the output voltage and current distortions. To this regard, a multilevel topology for photovoltaic (PV) applications is proposed, where an H6 bridge power cell is used instead of an H-bridge one. In case of solar irradiance mismatch among the power cells, the proposed converter injects power with lower voltage from the shaded cells without altering the PV voltage; hence maintaining MPPT operation. This modification allows retaining an equal duty cycle in all the power cells whatever the meteorological conditions are present; consequently, maintaining good output voltage and current waveform qualities. To test the effectiveness of the proposed solution, a detailed simulation model, as well as experimental prototype were built. The obtained results show that the proposed topology provides a significantly improved output voltage and current qualities compared to the cascaded H-bridge one. The performance of the proposed topology compared to one offering improved harmonics performance, according to the European efficiency, has been also compared, where an enhancement of 2.64% has been registered.
This paper proposes an efficient external energy maximization strategy (EEMS) for a residential micro grid system (MGS). The system sources decomposed into three parts, a photovoltaic (PV) array being a green energy source, an AC grid and a hybrid energy storage system (HESS) (a battery (BT) and super-capacitor (SC)). The proposed strategy aimed at minimizing the grid energy consumption and improve the overall system efficiency by maximizing the BT and SC energy demands within their effective operating limits during the no PV electricity production. This enhance the efficiency of the energy management system during load profile variations. The performance of the proposed EEMS is compared with the PI controller and the equivalent consumption minimization strategy (ECMS) through simulations. A simulation model are developed using Matlab/Simulink to validate all performances.
Renewable energy systems (RESs), such as photovoltaic (PV) systems, are providing increasingly larger shares of power generation. PV systems are the fastest growing generation technology today with almost ~30% increase since 2015 reaching 509.3 GWp worldwide capacity by the end of 2018 and predicted to reach 1000 GWp by 2022. Due to the fluctuating and intermittent nature of PV systems, their large-scale integration into the grid poses momentous challenges. This paper provides a review of the technical challenges, such as frequency disturbances and voltage limit violation, related to the stability issues due to the large-scale and intensive PV system penetration into the power network. Possible solutions that mitigate the effect of large-scale PV system integration on the grid are also reviewed. Finally, power system stability when faults occur are outlined as well as their respective achievable solutions.
In the conventional photovoltaic (PV) fed quasi-Z (qZ) network-based impedance-source-converters (ISCs), the PV array is connected to their input, whereas in the proposed topology in this paper, an additional array is paralleled to the second qZ-network's capacitor (C 2). This modification allows harvesting more PV power through full utilization of the employed qZ-network in the classical ISCs. Moreover, the proposal offers higher conversion efficiency since the current in the second qZ-network's inductor (i L2) is smaller. The voltage of the added PV array (PV 2) is independent of the voltage of the primary array (PV 1) in a wide range, which promotes tracking their maximum power points (MPPs) separatly, achieving a higher efficiency even under partial shading. The use of two separate MPPTs in one converter is a challenge since the perturbation done by the first MPPT alters the power in PV 2 as well, and vice-versa. Therefore, a two MPPs tracker (MPPsT) algorithm, which perturbs two parameters and observes four variables, is proposed for the presented converter. The theoretical analysis is validated through real-time hardware-in-the-loop tests, and it demonstrates that at least 11% more power can be harvested compared to the conventional qZ-network-based ISC.
Impedance network inverters are a good alternative for voltage-source and current-source inverters. The shoot-through solution and the boosting capability of such converters make them an excellent solution for photovoltaic (PV) application. Furthermore, energy storage integration in these inverters does not require any additional components in the converter; indeed, a battery can be directly connected in parallel with one of the capacitors of the Z-or quasi Z-network. However, for an optimal control of these converters, complex control and modulation strategies are required. Model Predictive Control (MPC) provides high control performance at the expense of the computational effort. In this paper, a low computational control method where both MPC and proportional resonant (PR) controller are combined, is proposed. This makes the proposed controller perform two iterations only instead of iterating for all the available switching states. As shown in the obtained results, the proposed controller conserves the high performance of the conventional MPC with 50% less computational burden.
The high variability of solar irradiance, originated by moving clouds, causes fluctuations in Photovoltaic (PV) power generation, and can negatively impact the grid stability. For this reason, grid codes have incorporated ramp-rate limitations for the injected PV power. Energy Storage Systems (ESS) coordinated by ramp-rate (RR) control algorithms are often applied for mitigating these power fluctuations to the grid. These algorithms generate a power reference to the ESS that opposes the PV fluctuations, reducing them to an acceptable value. Despite their common use, few performance comparisons between the different methods have been presented, especially from a battery status perspective. This is highly important, as different smoothing methods may require the battery to operate at different regimes (i.e., number of cycles and cycles deepness), which directly relates to the battery lifetime performance. This paper intends to fill this gap by analyzing the different methods under the same irradiance profile, and evaluating their capability to limit the RR and maintain the battery State of Charge (SOC) at the end of the day. Moreover, an analysis into the ESS capacity requirements for each of the methods is quantified. Finally, an analysis of the battery cycles and its deepness is performed based on the well-established rainflow cycle counting method.
This paper presents a multilevel topology of photovoltaic (PV) system with integrated energy storage system (ESS). In this proposed topology, half of the cells are connected to PV panels, the rest are connected to batteries. The sum of the cells are connected in series to form a dc-link, which is then connected to an H-bridge to convert the dc voltage to an ac one. The advantage of the proposed topology compared to the modular multilevel converter (MMC) as well as compared to the cascaded H-bridge converter is that less active switches are needed here. More switches are saved as the level of the output voltage increases, which results to a cheaper and less sizable converter. A local maximum power point tracking (MPPT) algorithm is proposed for this converter, which requires the voltage and current of each sub-module. Hence, there are no losses related to partial shading effect here. The excess of power in case of high irradiation levels is stored in the batteries, and is recovered when needed. A detailed simulation program has been built to test the effectiveness of the proposed converter; the obtained results are shown in the paper.
This paper presents a method that overcomes the problem of the confusion during fast irradiance change in the classical MPPTs as well as in model predictive control-MPPTs available in the literature. The previously introduced MPC-MPPTs take into account the model of the converter only, which make them drift during fast environmental conditions. Therefore, the model of the PV array is also considered in the proposed algorithm, which allows it prompt during rapid environmental condition changes. It takes into account multiple previous samples of power, and based on that is able to take the correct tracking decision when the predicted and measured power differ. After the decision is taken, it will be sent to a second part of the algorithm as a reference. The second part is used for following the reference provided by the first part, where the pulses are sent directly to the converter, without a modulator or a linear controller. The proposed technique is validated experimentally by using a buck converter, fed by a PV simulator. The tracking efficiency is evaluated according to EN50530 standard in static and dynamic conditions. The experimental results show that the proposed MPC-MPPT is a quick and accurate tracker even under very fast changing irradiance.
The main objective of this work is to provide an overview and evaluation of discrete model predictive controlbased maximum power point tracking (MPPT) for PV systems. A large number of MPC based MPPT methods have been recently introduced in the literature with very promising performance, however, an in-depth investigation and comparison of these methods have not been carried out yet. Therefore, this paper has set out to provide an in-depth analysis and evaluation of MPC based MPPT methods applied to various common power converter topologies. The performance of MPC based MPPT is directly linked with the converter topology, and it is also affected by the accurate determination of the converter parameters, sensitivity to converter parameter variations is also investigated. The static and dynamic performance of the trackers are assessed according to the EN 50530 standard, using detailed simulation models and validated by experimental tests.
As the continuously fluctuation of the operating point due to the permanent research of maximum power point which the photovoltaic source can product, the dynamic behaviour of the used DC/DC converter -adopted to realize the maximum power point tracking- will be disturbed at all time. For the aim to design an efficient photovoltaic system, the dynamic behaviour of the converter is studied based on the damping factor and the natural frequency of the system. As an example, the system under study is composed with a photovoltaic module connected to an equivalent resistive load through a DC/DC boost converter, assuming that this last one has been controlled with a direct method of maximum power point tracking. The results of analytical studies allow having an overview about the dynamic behaviour performance of the used DC/DC converter for the aim to implement a prompt maximum power point tracking algorithm.
Today, a predictive controller becomes one of the state of the art in power electronics control techniques. The performance of this powerful control approach will be pushed forward by simplifying the main control criterion and objective function, and decreasing the number of calculations per sampling time. Recently, predictive control has been incorporated in the Z-source inverter (ZSI) family. For example, in quasi ZSI, the inverter capacitor voltage, inductor current, and output load currents are controlled to their setting points through deciding the required state; active or shoot through. The proposed algorithm reduces the number of calculations, where it decides the shoot-through (ST) case without checking the other possible states. The ST case is roughly optimized every two sampling periods. Through the proposed strategy, about 50% improvement in the computational power has been achieved as compared with the previous algorithm. Also, the objective function for the proposed algorithm consists of one weighting factor for the capacitor voltage without involving the inductor current term in the main objective function. The proposed algorithm is investigated with the simulation results based on MATLAB/SIMULINK software. A prototype of qZSI is constructed in the laboratory to obtain the experimental results using the Digital Signal Processor F28335.
This paper proposes a new fault-tolerant strategy for the purpose of revamping the performance of three-phase multilevel (ML) converters with faulty switches. The proposed fault-tolerant strategy is based on an adaptation of the modified Space Vector Modulation (SVM) technique to the Fundamental Phase-Shift Compensation (FPSC) method. The proposed fault-tolerant strategy generates balanced line-to-line voltages with increased voltage levels in the faulty condition. In this paper, first a brief background about Cascaded H-bridge (CHB) multilevel converters and SVM technique is provided. Then the proposed fault-tolerant strategy and the modified SVM technique are presented. Finally, several experimental results are provided to validate operation of the proposed strategy.
In this work, a cost function design, based on Lyapunov stability concepts, for Finite Control Set Model Predictive Control is proposed. This predictive controller design allows one to characterize the performance of the controlled converter while providing sufficient conditions for local stability for a class of power converters. Simulation and experimental results on a buck dc-dc converter and a 2-level dc-ac inverter are conducted to validate the effectiveness of our proposal.
The perturb and observe (P&O) algorithm is one of the most commonly utilized maximum power point tracking (MPPT) control schemes for photovoltaic (PV) generators. However, the operation of this algorithm at high perturbation frequencies, when the system response to MPPT perturbations is never allowed to settle, has not been given adequate attention in the literature. This paper characterizes system behavior in this mode of operation for standalone PV systems feeding resistive loads and motor-pump loads. Simulation and experimental results show that the P&O algorithm operating at a high perturbation frequency may offer higher energy utilization efficiency and better system performance, despite the resulting nonperiodic waveforms of the system.
In a DC distribution system, where multiple sources supply a common bus, current sharing is an important issue. When renewable energy sources are used, such as photovoltaic (PV), DC/DC converters are needed to decouple the input voltage, which can be due to operating conditions such as Maximum Power Point Tracking (MPPT), from the DC bus voltage. Since different sources may have different power delivery capacities that may vary with time, coordination of the interface to the bus is of paramount importance to ensure reliable system operation. Further, since these sources are most likely distributed throughout the system, distributed controls are needed to ensure a robust and fault tolerant control system. This paper presents a Model Predictive Control-Maximum Power Point Tracking (MPC-MPPT) and a MPC-DROOP current regulator to interface PV in smart DC distribution systems. Back-to-back DC/DC converters control both the input current from the PV module and the droop characteristic of the output current injected into the distribution. The predictive controller speeds up both of the control loops since it predicts and corrects for error before the switching signal is applied to the respective converter.
Model predictive control (MPC) for power converters and drives is a control technique that has gained attention into the research community. The main reason is that although MPC presents high computational burden it can handle multivariable case and system constrains and nonlinearities easily in a very intuitive way. Taking advantage of that, MPC has been successfully used for different applications such as active front end, power converters connected to RL loads, uninterruptible power supplies and high performance drive for induction machines among others. This paper provides a review of the application of MPC in the power electronics area.
This paper provides a comprehensive review of the maximum power point tracking (MPPT) techniques applied to photovoltaic (PV) power system available until January, 2012. A good number of publications report on different MPPT techniques for a PV system together with implementation. But, confusion lies while selecting a MPPT as every technique has its own merits and demerits. Hence, a proper review of these techniques is essential. Unfortunately, very few attempts have been made in this regard, excepting two latest reviews on MPPT [Salas, 2006], [Esram and Chapman, 2007]. Since, MPPT is an essential part of a PV system, extensive research has been revealed in recent years in this field and many new techniques have been reported to the list since then. In this paper, a detailed description and then classification of the MPPT techniques have made based on features, such as number of control variables involved, types of control strategies employed, types of circuitry used suitably for PV system and practical/commercial applications. This paper is intended to serve as a convenient reference for future MPPT users in PV systems.
The increased use of DC microgrid for critical application lead to the necessity of advanced control design for a stable operation of the system. The loads connected to DC microgrid are controlled with power electronic devices and shows the behaviour of constant power load (CPL), which poses a serious challenge to stability as it adds nonlinearity and minimises the effective damping. This paper presents a robust controller design approach based on Hardy Space (H)-infinity control norms to tackle the nonlinearity added by CPL by expanding the region for stability. The design criteria are based on Lyapunov theory of nonlinear systems within the framework of Linear Matrix Inequality (LMI). The necessary and sufficient conditions are obtained in terms of linear matrix inequalities to ensure the transient performance and stability of the system. The LMI equation is solved to maximize the size of the estimated domain of stability. The performance of the proposed controller is verified using simulation in MATLAB/Simulink. The DC microgrid in this paper consists of a solar photovoltaic (PV) unit and a battery as an energy storage system together with loads. The Proposed controller not only ensures the stability of the system but also guarantees the improved transient performance of the closed loop system by expanding the size of the stability region.
cascaded multilevel inverter (CMI) is one of the most popular multilevel inverter topologies. This topology is synthesized with some series connected identical H-bridge cells. CMI requires several isolated dc sources which brings about some difficulties when dealing with this type of inverter. This paper addresses the problem by proposing a switched capacitor (SC) based CMI. The proposed topology, which is referred to as switched capacitor single source CMI (SCSS-CMI), makes use of some capacitors instead of the dc sources. Hence it requires only one dc source to charge the employed capacitors. Usually, the capacitor charging process in a SC cell is companied by some current spikes which extremely harm the charging switch and the capacitor. The capacitors in SCSS-CMI are charged through a simple auxiliary circuit which eradicates the mentioned current spikes and provides zero current switching condition for the charging switch. A computer-aid simulated model along with a laboratory-built prototype are adopted to assess the performances of SCSS-CMI, under different conditions.
Mismatching and partial shading in photovoltaic (PV) energy harvesting systems are the main causes for performance degradation and efficiency drop. A power electronic energy harvesting topology based on cascaded power optimizers that use distributed maximum power point tracking (MPPT) is believed to be one of the promising solutions to address these issues. In this scheme, each PV module is interfaced to the energy system through a separate dc/dc converter with maximum power point tracking capability. This paper presents application of the model predictive control technique to a distributed maximum power point tracking algorithm for maximizing the energy harvest performance of a cascaded power optimizer based system under dynamic weather conditions. The developed technique employs two control loops: a submodule maximum power point tracking model predictive control loop for each converter and a supervisory maximum power point tracking loop for power optimization of all cascaded PV modules. The provided experimental results confirm high energy capture, fast dynamic response, and negligible oscillations around MPP using the proposed method.
A power conditioning system (PCS) using multiple module-integrated converters and a single sourced 27-level asymmetric cascaded H-bridge multilevel inverter (MLI) without regeneration for photovoltaic (PV) applications is proposed. A newly suggested hybrid-switching closed-loop strategy is proposed to enable the use of a 27-level inverter in PV systems. The suggested hybrid-switching strategy implements fundamental line-frequency switching in the main H-bridge to ensure a high efficiency and low leakage current operation. Furthermore, high-frequency switching is used in the auxiliary H-bridge cells to achieve a higher bandwidth control loop. A cost-effective unidirectional single-ended 1-kW DC-DC module based on a boost coupled inductor and charge-pump circuits is proposed to achieve a single PV source per module for a multi-string configuration. To couple the unidirectional converters with the central inverter, a jumping element in the hybrid switching strategy is introduced to enable an optimal non-regenerative operation. The proposed PCS with the control scheme is analyzed and verified using hardware results in a grid connected mode of operation
Variability in the solar irradiance level and ambient temperature of photovoltaic (PV) systems, necessitates the use of maximum power point tracking (MPPT) of PV systems to ensure continuous harvesting of maximum power. This paper presents a sensorless current MPPT algorithm using model predictive control (MPC). The main contribution of this paper is the use of model based predictive control principle to eliminate the current sensor that is usually required for well-known MPPT techniques such as perturb and observe (P&O). By predicting the PV system states in horizon of time, the proposed method becomes an elegant, embedded controller that allows faster response and lower power ripple in steady state than the conventional P&O technique under rapidly changing atmospheric conditions. This becomes possible without requiring expensive sensing and communications equipment and networks for direct measurement of solar irradiation changes. The performance of the proposed sensorless current model predictive control MPPT (SC-MPC-MPPT) with reduced load sensitivity is evaluated on the basis of industrial European Efficiency Test, EN 50530, that assesses the performance of PV systems under dynamic environmental conditions. The proposed control technique is implemented experimentally using dSPACE DS1007 platform to verify the simulation results.
Solar panels have a nonlinear voltage-current characteristic, with a distinct maximum power point (MPP), which depends on the environmental factors, such as solar irradiance and ambient temperature. In order to increase the power extracted from the solar panel, it is necessary to operate the photovoltaic (PV) system at the maximum power point (MPP). In this paper a novel maximum-power-point tracking (MPPT) method based on current perturbation algorithm (CPA) with a variable perturbation step and fractional short circuit current algorithm (FSCC) to determine an optimum operating current. An experimental comparative study of these maximum power point tracking methods using dSPACE is presented in this article. The effectiveness of proposed algorithm in terms of dynamic performance and improved stability is validated by detailed simulation and experimental studies.
This paper presents a model predictive-based maximum power point tracking (MPPT) method for a photovoltaic energy harvesting system based on a single-stage grid-tied Z-source inverter. First, it provides a brief review of Z-source inverters, MPPT methods and model predictive control. Next, it introduces the proposed model predictive-based MPPT method. Finally, it provides experimental results to verify the theoretical outcomes.
Large-scale grid-connected photovoltaic (PV) systems significantly contribute to worldwide renewable energy growth and penetration, which has inspired the application of cascaded modular multilevel converters due to their unique features such as modular structures, enhanced energy harvesting capability, scalability and so on. However, power distribution and control in the cascaded PV system faces tough challenge on output voltage overmodulation when considering the varied and nonuniform solar energy on segmented PV arrays. This paper addresses this issue and proposes a decoupled active and reactive power control strategy to enhance system operation performance. The relationship between output voltage components of each module and power generation is analyzed with the help of a newly derived vector diagram which illustrates the proposed power distribution principle. On top of this, an effective control system including active and reactive components extraction, voltage distribution and synthesization, is developed to achieve independent active and reactive power distribution and mitigate the aforementioned issue. Finally, a 3-MW, 12-kV PV system with the proposed control strategy is modeled and simulated in MATLAB and PSIM cosimulation platform. A downscaled PV system including two cascaded 5-kW converters with proposed control strategy is also implemented in the laboratory. Simulation and experimental results are provided to demonstrate the effectiveness of the proposed control strategy for large-scale grid-connected cascaded PV systems.
A novel maximum power point tracking (MPPT) algorithm is proposed in this paper based on the linear iteration method for photovoltaic (PV) power generation for improving steady-state performance and fast dynamic response simultaneously. In this MPPT algorithm, linear prediction estimates the MPP of PV with a very high accuracy due to the constant-voltage and constant-current characters of PV. Then, further iterations for error correction are executed to get a better approximation to the MPP. Experiments verify theoretical analysis and demonstrate its fast dynamic response and high working efficiency.