This thesis derives the proper load shedding scheme for a distribution system with wind power generating units to maintain the power supply reliability for the islanding operation of micro grid systems after fault disturbance. The comparison of operation performance and control scheme between the fixed speed and the variable speed wind power generators are made. The seasonal wind power energy by the wind turbine is calculated by applying the exponential rate (Power Law) and Weibull possibility distribution model with the actual minutely wind speed data in Hengchun and Penghu area in 2005. The mean values of seasonal wind power output and standard deviation are determined for the design of load shedding scheme for the islanding operation of the distribution feeder. Moreover, a practical distribution feeder BX31 in Fengshan District in Taiwan Power Company (TPC) is selected for the computer simulation of micro grid systems. The systems voltage and power variation of the distribution feeder are investigated when the wind turbine is connected to the feeder by considering the weekday and weekend load models. The voltage sag on the distribution feeder and transient stability of wind turbine are simulated for the fault contingency with three-phase short-circuit fault occurred on the feeder outlet. Consequently, synchronous condensers are connected and the blade angle of wind turbines is then adjusted to improve the transient response of output characteristic of wind turbine. It is found that the power supply reliability of micro grid with wind power generations will be deteriorated following the tripping of wind turbines due to the oscillation of terminal voltage introduced by long fault clearing time. To solve the problem, the proper design of the tripping for the micro grid systems with wind turbines is derived according to the transient stability analysis. To restore the systems stability of the islanding systems, the proper amount of load shedding is determined and the switching of control modes of the wind turbine is executed, according to the power mismatch between wind turbine generation and load demand of the distribution feeder.

Opportunities for enhancing the security and reliability of power supply as well as the utilization of renewable and efficient energy sources have generated major interest in Microgrids. A microgrid typically consists of interconnected loads, distributed generators (DG) and energy storages, and should be able to operate in parallel with the utility grid or as a power-island. The main focus of this thesis is on the transition between parallel and islanded operation of a microgrid. A literature review on existing microgrids was carried out. Based on the survey, a microgrid test system was implemented on PSCAD/EMTDC simulation program. The microgrid controls essential for the study and a load shedding scheme were designed and implemented. When the microgrid changes from parallel to islanded operation, its controls need to be changed. It was found that delays in microgrid control mode transition can impact the amount of load need to be shed to preserve the frequency stability and the power quality of the islanded microgrid. The importance of fast detection of islanding was therefore highlighted. The IEEE standard 1547.4-2011 recommends application of the existing DG synchronization criteria for microgrid synchronization. The adequacy of these criteria for synchronization of a microgrid with highly unbalanced loading was investigated. It was found that the required criteria can be met with the support of switched capacitors for voltage balancing, and a circuit breaker supervised by a synchro-check relay is sufficient to successfully reconnect an islanded microgrid back to the utility. In order to meet the requirement for fast detection of islanding of microgrids, new islanding detection technique was proposed. In the proposed scheme, Discrete Wavelet Transform was used to extract features from transient current and voltage signals, and then a Decision Tree classifier was employed to distinguish islanding events from other transients. Simulation based tests asserted that the proposed technique has a high reliability and fast response compared to most existing islanding detection methods. Also, the detection time of the proposed method was invariant with the power imbalance in the microgrid, and gave a zero non-detection-zone with any type of generator.

Opportunities for enhancing the security and reliability of power supply as well as the utilization of renewable and efficient energy sources have generated major interest in Microgrids. A microgrid typically consists of interconnected loads, distributed generators (DG) and energy storages, and should be able to operate in parallel with the utility grid or as a power-island. The main focus of this thesis is on the transition between parallel and islanded operation of a microgrid. A literature review on existing microgrids was carried out. Based on the survey, a microgrid test system was implemented on PSCAD/EMTDC simulation program. The microgrid controls essential for the study and a load shedding scheme were designed and implemented. When the microgrid changes from parallel to islanded operation, its controls need to be changed. It was found that delays in microgrid control mode transition can impact the amount of load need to be shed to preserve the frequency stability and the power quality of the islanded microgrid. The importance of fast detection of islanding was therefore highlighted. The IEEE standard 1547.4-2011 recommends application of the existing DG synchronization criteria for microgrid synchronization. The adequacy of these criteria for synchronization of a microgrid with highly unbalanced loading was investigated. It was found that the required criteria can be met with the support of switched capacitors for voltage balancing, and a circuit breaker supervised by a synchro-check relay is sufficient to successfully reconnect an islanded microgrid back to the utility. In order to meet the requirement for fast detection of islanding of microgrids, new islanding detection technique was proposed. In the proposed scheme, Discrete Wavelet Transform was used to extract features from transient current and voltage signals, and then a Decision Tree classifier was employed to distinguish islanding events from other transients. Simulation based tests asserted that the proposed technique has a high reliability and fast response compared to most existing islanding detection methods. Also, the detection time of the proposed method was invariant with the power imbalance in the microgrid, and gave a zero non-detection-zone with any type of generator.

Although wind power as a renewable energy is assumed to be an all-round advantageous source of energy, its intermittent nature can cause difficulties, especially in the islanding mode of operation. Conventional synchronous generators can help to compensate for wind fluctuations, but the slow behavior of such systems may result in stability concerns. In this study, the virtual inertia method, which imitates the kinetic inertia of a synchronous generator, is used to improve the system’s dynamic behavior. Since the proposed method incorporates no long-term power regulation, it requires no mass storage device and is thus economical. To preclude additional costs, a rotating mass connected to the Doubly Fed Induction Generator (DFIG) shaft or a super-capacitor connected to the DC-link on a back-to-back converter of a wind power generator could be used. The concept and the proposed control methods are discussed in detail, and eigen-value analysis is used to study how the proposed method improves system stability. As well, the advantages and disadvantages of using DFIG rotating mass or a super-capacitor as the virtual inertia source are compared. The proposed approach also shows that while virtual inertia is not incorporated directly in long-term frequency and power regulation, it may indirectly enhance the system’s steady-state behavior. A time domain simulation is used to verify the results of the analytical studies.

Η παρούσα διπλωματική εργασία ασχολείται με το σχεδιασμό και την εγκατάσταση φωτοβολταϊκού συστήματος. Έλαβε χώρα στο Εργαστήριο Παραγωγής, Μεταφοράς, Διανομής και Χρησιμοποίησης Ηλεκτρικής Ενέργειας του Τμήματος Ηλεκτρολόγων Μηχανικών και Τεχνολογίας Υπολογιστών της Πολυτεχνικής Σχολής του Πανεπιστημίου Πατρών. Στόχος της εργασίας αυτής , είναι η μελέτη του συστήματος που δημιουργήθηκε για να λειτουργεί με 3 διαφορετικούς τρόπους: α) Μονοφασικό Διασυνδεδεμένο στο δίκτυο διανομής, β) Τριφασικό Διασυνδεδεμένο στο δίκτυο διανομής και γ) Μονοφασικό αυτόνομο. Στην εισαγωγή γίνεται μια γενική αναφορά στις Ανανεώσιμες Πηγές Ενέργειας και περιγράφεται η κατάσταση στην Ελλάδα και την Ευρώπη σχετικά με αυτές. Εν συνεχεία παρουσιάζονται τα χαρακτηριστικά μεγέθη της ηλιακής ακτινοβολίας , εξηγείται ο τρόπος λειτουργίας των φωτοβολταϊκών και γίνεται αναφορά στις διάφορες τεχνολογίες των φωτοβολταϊκών που έχουν αναπτυχθεί μέχρι σήμερα. Συνεχίζοντας, παρατίθενται τα στοιχεία και οι γενικές αρχές δημιουργίας ενός φωτοβολταϊκού συστήματος. Έπειτα ανάλύεται εκτενώς το πρόβλημα της Νησιδοποίησης και μέθοδοι αντιμετώπισής του. Τέλος, παρουσιάζεται η μελέτη, ο τρόπος λειτουργίας και ο εξοπλισμός του συστήματος που εγκαταστάθηκε στο Εργαστήριο. / This thesis deals with the design and installation of a photovoltaic system, and connection with the distribution network. It was held in Generation, Transmission, Distribution and use of electricity Laboratory of the Department of Electrical Engineering and Computer Engineering of the University of Patras . The aim of this work is to study the system set up to work in three different ways: a) Single-phase grid connected distribution, b) Three-phase grid connected distribution and c) Single phase alone. The introduction contains a general reference to the Renewable Energy and describes the situation in Greece and Europe about them. It then presents the features sizes of solar radiation, gives an explanation about the mode of photovoltaic and mentioned in various photovoltaic technologies have been developed to date. Then are presented the data and principles to create a PV system. Afterwards, the problem of islandig and dealing methods are analyzed. Finally, we present the study mode and System equipment installed at the Laboratory.

En la actualidad, los conceptos de generación distribuida y microrredes (microgrids) están despertando un gran interés en la comunicad científica, al presentar ventajas claras sobre los esquemas centralizados como son: su inherente redundancia, la mejora de la eficiencia en el transporte de energía eléctrica al aproximar los puntos de generación y de consumo, así como la posibilidad de integración de fuentes de energía renovable. En este contexto, la tesis plantea como aspectos novedosos la propuesta de un esquema de control reconfigurable para la operación de un microinversor fotovoltaico (convertidor electrónico concebido para procesar la energía procedente de un panel fotovoltaico individual) tanto en modo aislado como interconectado con una red eléctrica. Normalmente, tales unidades operan en conexión a la red eléctrica, funcionando como una fuente de corriente. El objetivo de la tesis es la concepción de técnicas que permitan dotar a los microinversores con la funcionalidad adicional de poder trabajar como fuente de tensión para operar en modo aislado, sin necesidad de utilizar sistemas adicionales de almacenamiento de energía (en la actualidad, baterías). Por otra parte, en la tesis se ha propuesto un nuevo algoritmo de detección de funcionamiento en isla (o modo aislado), basado en la inyección de un segundo armónico de corriente en la corriente de salida del microinversor y posterior evaluación de cómo éste interactúa con la red eléctrica. La técnica propuesta se basa en el procesado de la señal devuelta por la red mediante el algoritmo Goertzel de identificación de componentes armónicas. En la tesis se demuestra que las propuestas realizadas permiten la reconfiguración de los controladores del modo fuente de corriente a fuente de tensión, y viceversa, sin transitorios peligrosos para el microinversor ni para las cargas locales cercanas al mismo. Los resultados analíticos se han validado mediante simulaciones detalladas y ensayos experimentales llevados a cabo en un microinversor fotovoltaico 180 W. / Trujillo Rodríguez, CL. (2011). Concepción de controladores reconfigurables para microinversores fotovoltaicos operando como unidades autónomas de generación de energía en microrredes [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/14275 / Palancia

Indiana University-Purdue University Indianapolis (IUPUI) / The interconnection of distributed resources requires specific voltage regulation, monitoring, protective relaying, power quality, and islanding detection. For this reason IEEE established standard IEEE 1547 that ensures the compliance with such requirements and it will help formulate technical specifications for grid interconnection with Distributed Generator (DG) resources. In search of meeting the IEEE 1547 standard requirement of detecting unintentional islanded operation, there has been ongoing research to develop anti-islanding methods that can detect the different changes that can occur when the grid is disconnected. A team of Electrical Engineering faculty at Indiana University Purdue University Indianapolis has worked previously on testing a DG unintentional Islanding Detection Scheme. This scheme uses an active anti-islanding method in which a small 1 Hz perturbation signal is added into the DG system and it helps detect when the grid is disconnected. The scheme uses the premise that a frequency deviation caused by perturbation to the system is smaller when the grid is connected than when it is in an island. In an initial dynamic simulation model for the islanding detection scheme, a two-machine microgrid system is used to explore frequency and voltage responses when the grid is disconnected. In this thesis, the two-machine microgrid is expanded to a ten-machine system so it can be shown that the frequency deviation caused by a perturbation signal is much smaller when the grid is connected even for a larger DG network. The 1 Hz component of the DG electrical frequency in a multiple machine microgrid system is also calculated in this thesis. This project was conducted in different stages. First, it was necessary to calculate the steady state power flow and electric power of a three-machine system and update the two-machine MATLAB program with the necessary changes. After making the changes, it was necessary to simulate the system and adjust the inertia of the machine that represents the grid to ensure that the simulation output was close in magnitude to previous testing results. When the three-machine system was successfully generated, a brand new program was created so a multiple machine system could be simulated. Then the multiple machine program was used to simulate and experiment with up to a ten-machine system. Finally a program to calculate the 1 Hz component of the DG electrical frequency was generated and used to show that the magnitude squared of the 1 Hz component is inversely proportional to the number of machines connected to the system. These last findings will later help set the threshold for islanding detection appropriately for different numbers of DG.

With the increasing interest in distribution automation, distribution power flow is important for applications like VAR planning, switching, state estimation and especially optimization. Typically, a distribution system originates at a substation and continues to a lower voltage for delivery to the customers. There are several tools for transmission system analysis. These tools include Newton Raphson, Gauss Seidel and fast decoupled techniques. These techniques however sometimes fail to converge when applied to distribution systems due to their higher resistance/reactance (R/X) ratio of the lines, making them ill conditioned. Distribution systems typically have a radial topological structure where the loads are not always constant power. With the increase in distributed generation (DG) there is a critical need to develop analysis tools to study the effect they will have on the distribution systems. Also, shipboard power systems are different from terrestrial distribution systems, as they are tightly coupled and have multiple generators. This dissertation focuses on developing a software program to perform the power flow analysis of terrestrial as well as shipboard power systems. Components are modeled considering the mutual coupling of cables and the tightly coupled nature of the ship systems. The algorithm is built and tested on I test cases. The distributed generator is modeled as both a PQ (constant power factor) and a PV (constant voltage) node. This dissertation also focuses on reconfiguration for restoration of unbalanced distribution systems. Reconfiguration is changing the status (OFF/ON) of switches and reconfiguration for restoration is changing the switch status to maximize the supply to loads that are left unsupplied after fault removal. Methods exist for restoration of distribution systems and can be categorized into heuristics, knowledge based, meta-heuristics and intelligent techniques. However, the application of these methods have not considered the unbalanced nature of distribution system operation with mutual coupling. The restoration in this dissertation is achieved using optimization with multiple objectives; that of maximizing the load giving priority to vital loads and minimizing the number of switch operations. Also a restoration scheme for shipboard power systems with an IPS and distributed generation has been developed. Restoration with possible islanding is demonstrated.

Restoration entails the development of a plan consisting of opening or closing of switches, which is called reconfiguration. This dissertation proposes the design of a fast and efficient service restoration with a load shedding method for land-based and ship systems, considering priority of customers and several other system operating constraints. Existing methods, based on centralized restoration schemes that require a powerful central computer, may lead to a single point of failure. This research uses a decentralized scheme based on agents. A group of agents created to realize a specific goal by their interactions is called a Multi-Agent System (MAS). Agents and their behaviors are developed in Java Agent DEvelopment Framework (JADE) and the power system is simulated in the Virtual Test Bed (VTB). The large-scale introduction of Distributed Generators (DGs) in distribution systems has made it increasingly necessary to develop restoration schemes considering DG. The separation of utility causes the system to decompose into electrically isolated islands with generation and load imbalance that can have severe consequences. Automated load shedding schemes are essential for systems with DGs, since the disconnection of the utility can lead to instability much faster than an operator intervention can repair. Load shedding may be the only option to maintain the island when conditions are so severe as to require correction by restoration schemes. Few algorithms have been reported for the problem of maintaining the island, even though load shedding has been reported for power systems using underrequency and under-voltage criteria. This research proposes a new operational strategy for sudden generator-load imbalance due to loss of utility that dynamically calculates the quantity of load to be shed for each island and the quantity of load that can be restored. Results presented in this dissertation are among the first to demonstrate a state-of-the-art MAS for load shedding under islanded conditions and restoration of the shed loads. The load shedding and restoration schemes developed here have behaviors that can incorporate most of the distribution topologies. Achieving service restoration with DG is complicated but new automated switch technologies and communications make MAS a better scheme than existing schemes.

Distributed generation (DG) is getting more and more popular due to the environmentally-friendly feature, the new generation unit developments, and the ability to operate in a remote area. By clustering the paralleled DGs, storage system and loads, a microgrid (MG) can offer a power system with increased reliability, flexibility, cost effectiveness, and energy efficient feature. Popular energy sources like photovoltaic modules (PV), wind turbines, and fuel cells require the power-electronic interface as the bridge to connect to the utility grid for usable transmission. The inverter-based microgrid system, however, suffers more challenges than traditional rotational power system. Those challenges, including much less over current capability, the nature of the intermittent renewable energy sources, a wide-band dynamic of generation units, and a large grid impedance variation, call for more careful system hardware and control designs to ensure a reliable system operation. Major design interests are found in (i) precision power flow control, (ii) proper current sharing, (iii) smooth transition between grid-tie and islanding modes, and (iv) stability analysis. This dissertation will cover a complete design and implementation of an experimental microgrid with paralleled power conditioning systems operating in the gridtie mode, islanding mode, and mode transfers. A universal inverter is proposed with the LCL filter to operate in both grid-tie and standalone mode without any hardware modification. Next, controllers of individual inverters running in basic microgrid modes will be discussed to ensure high quality output characteristics. The admittance compensation will also be proposed to avoid reverse power flow during the grid-tie connection transient. Combining previous designed single inverters, a CAN-bus multiinverter microgrid system will be established. The current sharing with the proposed frequency-decoupled transmission will be implemented to extend the transmission distance. Next, smooth mode transfer procedures between grid-tie mode and islanding mode will be suggested based on the circuit principles to minimize the excessive electrical stresses. Finally, the state-space analysis of the proposed multi-inverter microgrid system will be conducted to investigate the stability under system variations and optimize the system performance. Experimental and simulation results show that the designed universal inverter can provide stable outputs in different basic microgrid operation modes. With the proposed current sharing scheme, the output current is equally shared among paralleled inverters without a noticeable circulating current. Both the simulation and experimental results of mode transfer show that the multi-inverter based microgrid system is able to switch between grid-tie and islanding modes smoothly to guarantee an uninterrupted power supply to the critical loads. Based on eigenvalue analysis, the study of stability analysis also shows the agreement of the design, simulation and test results which further verifies the reliability of the designed multi-inverter microgrid system. / Ph. D.