Intelligent transition control between grid-connected and standalone modes of three-phase grid-integrated distributed generation systems

This paper proposes an intelligent seamless transition controller for smooth transition between grid-connected (GC) and standalone modes of distributed generation (DG) units in the grid. The development of this seamless controller contributes to two main processes in the transition modes: the synchronization process and an islanding process. For the synchronization process, the stationary reference frame phase-locked loop (SRF-PLL) associated with the voltage source inverter (VSI) is modified using the frequency, voltage deviation, and phase angle information. Furthermore, the islanding process is classified as intentional and unintentional islanding scenarios for achieving efficient transition control. Here, the intentional islanding process is achieved with the information that is available in the system due to the planned disconnection. For the unintentional islanding process, a fuzzy inference system (FIS) is used to modify the conventional droop control using the information of change in active power, voltage, and frequency. To identify the action of the proposed approach during the transition process, numerical simulations are conducted with the hardware-in-loop (HIL) simulator by developing a 10kWp three-phase grid-connected DG system. The results identified the efficient control of the VSI for both islanding and grid connection processes. In the islanding conditions, the proposed controller provides advantage with less detection and disconnection time, and during synchronization, it instantly minimizes the phase-angle deviation to achieve efficient control

Inter-Microgrid Operation: Power Sharing, Frequency Restoration, Seamless Reconnection and Stability Analysis

Electrification in the rural areas sometimes become very challenging due to area accessibility and economic concern. Standalone Microgrids (MGs) play a very crucial role in these kinds of a rural area where a large power grid is not available. The intermittent nature of distributed energy sources and the load uncertainties can create a power mismatch and can lead to frequency and voltage drop in rural isolated community MG. In order to avoid this, various intelligent load shedding techniques, installation of micro storage systems and coupling of neighbouring MGs can be adopted. Among these, the coupling of neighbouring MGs is the most feasible in the rural area where large grid power is not available. The interconnection of neighbouring MGs has raised concerns about the safety of operation, protection of critical infrastructure, the efficiency of power-sharing and most importantly, stable mode of operation. Many advanced control techniques have been proposed to enhance the load sharing and stability of the microgrid. Droop control is the most commonly used control technique for parallel operation of converters in order to share the load among the MGs. But most of them are in the presence of large grid power, where system voltage and frequency are controlled by the stiff grid. In a rural area, where grid power is not available, the frequency and voltage control become a fundamental issue to be addressed. Moreover, for accurate load sharing a high value of droop gain should be chosen as the R/X ratio of the rural network is very high, which makes the system unstable. Therefore, the choice of droop gains is often a trade-off between power-sharing and stability. In the context, the main focus of this PhD thesis is the fundamental investigations into control techniques of inverter-based standalone neighbouring microgrids for available power sharing. It aims to develop new and improved control techniques to enhance performance and power-sharing reliability of remote standalone Microgrids. In this thesis, a power management-based droop control is proposed for accurate power sharing according to the power availability in a particular MG. Inverters can have different power setpoints during the grid-connected mode, but in the standalone mode, they all need their power setpoints to be adjusted according to their power ratings. On the basis of this, a power management-based droop control strategy is developed to achieve the power-sharing among the neighbouring microgrids. The proposed method helps the MG inverters to share the power according to its ratings and availability, which does not restrict the inverters for equal power-sharing. The paralleled inverters in coupled MGs need to work in both interconnected mode and standalone mode and should be able to transfer between modes seamlessly. An enhanced droop control is proposed to maintain the frequency and voltage of the MGs to their nominal value, which also helps the neighbouring MGs for seamless (de)coupling. This thesis also presents a mathematical model of the interconnected neighbouring microgrid for stability and robustness analysis. Finally, a laboratory prototype model of two MGs is developed to test the effectiveness of the proposed control strategies

Effect of State Feedback Coupling on the Design of Voltage Source Inverters for Standalone Applications

This Ph.D. thesis aims at investigating the effect of state feedback cross‐coupling decoupling of the capacitor voltage on the dynamics performance of Voltage Source Inverters for standalone microgrids/Uninterruptible Power Supply systems. Computation and PWM delays are the main factors which limit the achievable bandwidth of current regulators in digital implementations. In particular, the performance of state feedback decoupling is degraded because of these delays. Two decoupling techniques aimed at improving the transient response of voltage and current regulators are investigated, named nonideal and ideal capacitor voltage decoupling respectively. In particular, the latter solution consists in leading the capacitor voltage on the state feedback decoupling path in order to compensate for system delays. Practical implementation issues are discussed with reference to both the decoupling techniques. Moreover, different resonant regulators structures for the inner current loop are analysed and compared to investigate which is the most suitable for standalone microgrid applications. A design methodology for the voltage loop, which considers the closed loop transfer functions developed for the inner current loop, is also provided. Proportional resonant voltage controllers tuned at specific harmonic frequencies are designed according to the Nyquist criterion taking into account application requirements. For this purpose, a mathematical expression based on root locus analysis is proposed to find the minimum value of the resonant gain at the fundamental frequency. The exact model of the output LC filter of a three‐phase inverter is derived in the z‐domain. The devised formulation allows the comparison of two techniques based on a lead compensator and Smith predictor structure. These solutions permit the bandwidth of the current regulator to be widened while still achieving good dynamic performance. As a consequence, the voltage regulator can be designed for a wide bandwidth and even mitigates odd harmonics arising with unbalance loads supply. Discrete‐time domain implementation issues of an anti‐wind up scheme are discussed as well, highlighting the limitations of some discretization methods. Experimental tests performed in accordance to Uninterruptible Power Supply standards verify the theoretical analysis

Power management and control stategies of renewable energy resources for micro-grid application

Microgrids (MGs) have become an increasingly familiar power sector feature in recent years and goes through the increase of renewable energies penetration. MG is defined as a group of interconnected loads and multiple distributed generators that is able to operate in grid-connected or islanding mode. Recent reports claim dramatic growth in projects planned for hundreds of GWs worldwide. Notably, following to many natural disasters, the concept of MG and its perceived benefits shifted beyond economic and environmental goals towards resilience. Consequently, MGs have begun to find a natural place in the regulatory and policy arena. Remote areas, facilities with low-quality local energy resources and critical infrastructure are all potential need the MGs solution. However, MGs have some disadvantages as the complexity of control and integration to keep the power quality to acceptable standards. The energy storage system requires more space and maintenance. Finally, protection is one of the important challenges facing the implementation of MGs. The present doctoral research is based on the philosophy of MGs for optimal integration and power management in an effective and efficient way to provide a sustainable and reliable power supply to consumers while reducing the overall cost. This work proposes a novel control strategies and design approaches of micro-grids for remote areas and grid connected system in which both the reliability of continuous power supply and power quality issues are treated. Moreover, this thesis also introduces the concept of Net Zero Energy House in which the system is designed in such a way that the house produces as much energy as it consumes over the year. Many controls algorithms have been investigated in order to find the best way to reduce the sensors’ number and the degree of control complexity while keeping better power quality as well as the system reliability. The developed concept is successfully validated through simulation as well as extensive experimental investigations. Particular attention is paid to the optimal integration of MGs based on the climate data of Central African States

Development of controllers using FPGA for fuel cells in standalone and utility applications

In the recent years, increase in consumption of energy, instability of crude oil price and global climate change has forced researchers to focus more on renewable energy sources.Though there are different renewable energy sources available (such as photovoltaic and wind energy), they have some major limitations. The potential techniques which can provide renewable energy are fuel cell technology which is better than other renewable sources of energy. Solid oxide fuel cell (SOFC) is more efficient, environmental friendly renewable energy source. This dissertation focuses on load/grid connected fuel cell power system (FCPS) which can be used as a backup power source for household and commercial units. This backup power source will be efficient and will provide energy at an affordable per unit cost. Load/grid connected fuel cell power system mainly comprises of a fuel cell module, DCDC converter and DC-AC inverter. This thesis primarily focuses on solid oxide fuel cell (SOFC) modelling, digital control of DC-DC converter and DC-AC inverter. Extensive simulation results are validated by experimental results. Dynamic mathematical model of SOFC is developed to find out output voltage, efficiency, over potential loss and power density of fuel cell stack. The output voltage of fuel cell is fed to a DC-DC converter to step up the output voltage. Conventional Proportional-Integral (PI) controller and FPGA based PI controller is implemented and experimentally validated. The output voltage of DC-DC converter is fed to DC-AC inverter. Different pulse width modulation-voltage source inverter (PWM-VSI) control strategy (such as Hysteresis Current Controller (HCC), Adaptive-HCC, Fuzzy-HCC, Adaptive Fuzzy-HCC, Triangular Carrier Current Controller (TCCC) and Triangular Periodical Current Controller (TPCC)) for DC-AC inverter are investigated and validated through extensive simulations using MATLAB/SIMULINK. This work also focuses on number of fuel cells required for application in real time and remedy strategies when one or few fuel cells are malfunctioning. When the required numbers of fuel cells are not available, DC-DC converter is used to step up the output voltage of fuel cell. When there is a malfunction in fuel cell or shortage of hydrogen then a battery is used to provide backup power

Power Management Strategies for a Wind Energy Source in an Isolated Microgrid and Grid Connected System

This thesis focuses on the development of power management control strategies for a direct drive permanent magnet synchronous generator (PMSG) based variable speed wind turbine (VSWT). Two modes of operation have been considered: (1) isolated/islanded mode, and (2) grid-connected mode. In the isolated/islanded mode, the system requires additional energy sources and sinks to counterbalance the intermittent nature of the wind. Thus, battery energy storage and photovoltaic (PV) systems have been integrated with the wind turbine to form a microgrid with hybrid energy sources. For the wind/battery hybrid system, several energy management and control issues have been addressed, such as DC link voltage stability, imbalanced power flow, and constraints of the battery state of charge (SOC). To ensure the integrity of the microgrid, and to increase its flexibility, dump loads and an emergency back-up AC source (can be a diesel generator set) have been used to protect the system against the excessive power production from the wind and PV systems, as well as the intermittent nature of wind source. A coordinated control strategy is proposed for the dump loads and back up AC source. An alternative control strategy is also proposed for a hybrid wind/battery system by eliminating the dedicated battery converter and the dump loads. To protect the battery against overcharging, an integrated control strategy is proposed. In addition, the dual vector voltage control (DVVC) is also developed to tackle the issues associated with unbalanced AC loads. To improve the performance of a DC microgrid consisting wind, battery, and PV, a distributed control strategy using DC link voltage (DLV) based control law is developed. This strategy provides simpler structure, less frequent mode transitions, and effective coordination among different sources without relying on real-time communication. In a grid-connected mode, this DC microgrid is connected to the grid through a single inverter at the point of common coupling (PCC). The generated wind power is only treated as a source at the DC side for the study of both unbalanced and balanced voltage sag issues at a distribution grid network. The proposed strategy consists of: (i) a vector current control with a feed-forward of the negative-sequence voltage (VCCF) to compensate for the negative sequence currents; and (ii) a power compensation factor (PCF) control for the VCCF to maintain the balanced power flow between the system and the grid. A sliding mode control strategy has also been developed to enhance the overall system performance. Appropriate grid code has been considered in this case. All the developed control strategies have been validated via extensive computer simulation with realistic system parameters. Furthermore, to valid developed control strategies in a realistic environment in real-time, a microgrid has been constructed using physical components: a wind turbine simulator (WTS), power electronic converters, simulated grid, sensors, real-time controllers and protection devices. All the control strategies developed in this system have been validated experimentally on this facility. In conclusion, several power management strategies and real-time control issues have been investigated for direct drive permanent magnet synchronous generator (PMSG) based variable speed wind turbine system in an islanded and grid-connected mode. For the islanded mode, the focuses have been on microgrid control. While for the grid-connected mode, main consideration has been on the mitigation of voltage sags at the point of common coupling (PCC)

Experimental validation of solar photovoltaic and battery storage-based microgrid energy management system

viii, 71 leaves : colour illustrations ; 29 cmIncludes abstract and appendices.Includes bibliographical references (leaves 61-71).The purpose of this work is to design and implementation of an energy management system for a solar Photovoltaic (PV) and battery storage-based microgrid system. The control architecture of PV, battery, and inverter units are presented here. A rule-based energy management system is proposed by considering some crucial factors such as, battery capacity, economical operation, profit-making, utility billing, efficiency etc. The seamless operation of the microgrid has been achieved for grid-connected and islanded condition. A set of power electronics interfaces, based on various converters and inverters, are used to integrate solar PV and battery storage to the DCbus. Optimal power exchange between the main grid and microgrid unit is ensured, while power balance is achieved in every cases. In addition, Water Cycle algorithm (WCA) is employed for PI parameters optimization rather using conventional trail-error tuning method. The proposed EMS and control scheme are implemented using Festo-LabVolt and real-time Opal-RT interfaces. Experimental results demonstrate the accuracy and robustness of the proposed microgrid energy management system under different operating conditions

Integration of distributed generation along with energy storage system to reduce the high penetration impacts of renewable energy sources into the power grid.

Compte tenu du comportement aléatoire et fluctuant des sources d'énergie renouvelable (SER), l'équilibre entre la génération et la demande ne sont pas faciles à contrôler. Par conséquent, la stabilité dynamique du flux d'énergie et le contrôle de la fréquence deviennent de plus en plus difficiles en raison des impacts de la pénétration élevée des SER dans les micro-réseau électrique. Des stratégies de contrôle des convertisseurs/onduleurs avec filtre sont nécessaires pour maintenir l'alimentation électrique appropriée dans l'ensemble du micro-réseau. L'objectif de notre travail est d'explorer les aspects critiques de la génération distribuée (GD), de l'intégration des énergies renouvelables et des systèmes de stockage de l'énergie, en mettant l'accent sur l'amélioration de l'efficacité du réseau électrique tout en minimisant la pollution atmosphérique. Cette thèse reconnaît les avantages environnementaux et économiques de la GD tout en soulignant les défis inhérents à la gestion des sources d'énergie renouvelable fluctuantes. Un algorithme de contrôle pour un système de stockage d'énergie hybride diesel-éolien à forte pénétration est conçu pour maintenir la stabilité dynamique du flux d'énergie et le contrôle de la fréquence du réseau. Les principaux résultats comprennent la réduction efficace du temps de transition dans le flux d'énergie éolienne et des fluctuations de fréquence. D'autre part, cette étude répond aux défis posés par la nature intermittente des SER et leur impact sur la stabilité dynamique et le contrôle de la fréquence. Nous avons introduit un algorithme de contrôle utilisant la logique floue pour un système de stockage d'énergie éolienne en utilisant la méthode de partage de puissance. En comparant cette approche au contrôleur conventionnel, l'algorithme proposé a démontré des améliorations substantielles dans la réduction du temps de transition dans le flux d'énergie éolienne et des fluctuations de fréquence. Dans le cadre de cette thèse, une étude complète de divers convertisseurs statiques est réalisée afin de déterminer le dispositif de stockage d'énergie le plus approprié pour les applications de réseaux intelligents. Ce système de stockage joue un rôle essentiel dans le maintien de la stabilité du réseau tout en minimisant les pertes d'énergie. L'objectif est d'identifier le dispositif de stockage d'énergie le plus adapté à cette application. Les avantages de cette technologie sont d'une grande efficacité et fiabilité, qui peuvent connecter diverses sources d'énergie et réduire les pertes de conduction dans les convertisseurs de puissance. On a analysé l'efficacité et la fiabilité de différents convertisseurs et évalué leur performance dans des conditions de charge et de décharge du système de stockage. Les plages de fonctionnement des convertisseurs élévateur-abaisseur, abaisseur-élévateur et abaisseur-élévateur (-Vout) ont été analysées pour optimiser le système de stockage d'énergie. Cette thèse présente également une analyse complète d'un schéma de simulation qui exploite un système solaire composé de panneaux photovoltaïques intégrés au réseau électrique, à diverses charges, et à un dispositif de stockage d'énergie. Après la modélisation des panneaux photovoltaïques et de leurs caractéristiques opérationnelles, un filtre adaptatif est développé pour atténuer les fluctuations du courant d'entrée. On a exploré en outre l'efficacité et les mécanismes de contrôle des convertisseurs de puissance et des onduleurs, facilitant ainsi l'intégration du système de stockage d'énergie avec le réseau électrique. Plusieurs techniques de contrôle non linéaires sont utilisées pour évaluer les performances du système avec différentes configurations, y compris un onduleur simple, un filtre multi-variable, un filtre passe bande et une configuration sans filtre. Cette recherche nous a permis de proposer une régulation efficace du bus DC au sein du réseau électrique. L'avantage clé de ces régulateurs non linéaires est leur capacité à compenser la puissance réactive et les courants harmoniques, ce qui se traduit par un réseau électrique sans perturbations et une réduction du taux de distorsion harmonique totale (DHT) des onduleurs, améliorant finalement l'efficacité globale du réseau électrique. Cette thèse apporte des connaissances précieuses pour optimiser les performances des systèmes éoliens et solaires ainsi que du dispositif de stockage d'énergie, et leur intégration au réseau grâce à des techniques de contrôle et de filtrage avancées, avec des implications significatives pour l'amélioration de la stabilité et de la fiabilité des sources d'énergie renouvelable dans le réseau électrique. Abstract Being the fluctuation behavior of Renewable Energy Sources (RESs), generation, balance, and demand are not easy tasks to control because it is not desirable to have constant power generation from RESs due to natural prospects. As a result, the dynamic stability of power flow and control of frequency is becoming more challenging due to the high penetration impacts of RESs. Control strategies of converter/inverter with filter are also required to maintain the proper power supply in the entire microgrid where energy storage device plays crucial roles. The objective of this study is to explore critical aspects of distributed generation (DG), renewable energy integration, and energy storage systems, focusing on enhancing power network efficiency while minimizing power losses and environmental air pollution. This doctoral thesis acknowledges the environmental and economic benefits of distributed generation (DG) while highlighting the inherent challenges in managing fluctuating renewable energy sources (RESs). A control algorithm for a high-penetration hybrid diesel-wind-based energy storage system is designed to maintain dynamic stability in power flow and control network frequency. The key findings include the effective reduction of transient time in wind power flow and frequency fluctuations through the use of an integral-derivative (I-D) controller. On the other hand, it recognizes the challenges posed by the intermittent nature of renewable energy sources (RESs) and their impact on dynamic stability and frequency control. This thesis introduced a control algorithm employed with a Fuzzy Logic (FL) controller for a wind-based energy storage system using the power-sharing method. By comparing this approach to the traditional Proportional Integral Derivative (PID) controller, the study demonstrated substantial improvements in reducing transient time in wind power flow and frequency fluctuations. A storage system (battery) plays a crucial role in maintaining network stability while minimizing energy losses. As a part of this thesis, a comprehensive survey of various DC-DC converters is done to determine the most suitable energy storage device for smart grid applications. The main objective is to identify this application's most appropriate energy storage device. The advantages of this technology are high efficiency and reliability, which can connect various energy sources and reduce conduction losses in the power converters. The study analyzed the efficiency and reliability of different converters and evaluated their performance in charging and discharging conditions of a battery. The operating ranges of boost-buck, buck-boost, and buck-boost (-Vout) converters are analyzed to optimize the energy storage system. This doctoral thesis also presents a comprehensive analysis of a simulation scheme that leverages a solar system composed of photovoltaic (PV) panels integrated with the electrical grid, various loads, and an energy storage device. The research begins by investigating the modeling of PV panel cells and their operational characteristics. Subsequently, an adaptive notch filter synthesis is developed to mitigate input current fluctuations. The research further explores the efficiency and control mechanisms of power converters and inverters, facilitating the seamless integration of the energy storage system with the electrical grid. Multiple simulations are conducted, employing nonlinear control techniques to evaluate the performance of the system with different configurations, including a simple inverter, a multi-variable filter, a notch filter, and a filter-less setup. The research aims to achieve effective regulation of the DC bus within the proposed grid. The key advantage of these nonlinear controllers is their ability to compensate for reactive power and harmonic currents, resulting in a disturbance-free power network and a reduction in the Total Harmonic Distortion (THD) rate of the inverters, ultimately enhancing the overall efficiency of the power grid. This thesis contributes valuable insights into optimizing the performance of wind and solar systems along with energy storage device and their integration with the grid through advanced control and filtering techniques, with significant implications for improving the stability and reliability of renewable energy sources in the power grid

Application of Power Electronics Converters in Smart Grids and Renewable Energy Systems

This book focuses on the applications of Power Electronics Converters in smart grids and renewable energy systems. The topics covered include methods to CO2 emission control, schemes for electric vehicle charging, reliable renewable energy forecasting methods, and various power electronics converters. The converters include the quasi neutral point clamped inverter, MPPT algorithms, the bidirectional DC-DC converter, and the push–pull converter with a fuzzy logic controller