Electrical reliability assurance is a very important aspect of electrical power systems; significant consideration should be given to reliability at both the planning and operation stage of power systems. A decrease in reliability levels can lead to enormous economic losses, especially for certain industrial facilities, and utilities could be penalized for violation of the mandatory reliability standards. Besides the traditional methods for electrical reliability enhancement, it is highly recommended to consider the adoption of innovative technologies, such as the integration of Distributed Generation (DG) units into the electrical network, especially those which are based on renewable energy source (wind and photovoltaic). Distributed Generation technologies can be beneficial to the electrical distribution system performance. However, these pose certain technical challenges to the reliable operation of the system. In this work, we also focus on the micro-grid operation security during islanding mode of operation in the presence of DG units. In this thesis, the unique aspects of reliability evaluation for an electrical distribution system has been performed using system-independent analytical expressions, considering probabilistic load and DG unit modeling, under different scenarios including dispatchable and renewable DG units with reasonable penetration levels. Further, a modified adequacy formulation has been adopted during the islanding mode of operation in order to consider micro-grid load correlation and an additional load curtailment level introduced in this work. The extra curtailment is needed to ensure adequate technical constraints and allow successful micro-grid operation, when the dispatchable DG units rating in a micro-grid is less than a defined percentage of the micro-grid peak load at time of islanding. Afterwards, during islanding, a second load curtailment level is adopted as needed to ensure service continuity under different operational conditions. A distribution test system is considered, and accordingly reliability indices are evaluated for both the worst case load scenario (islanding occurs at peak load), and for a realistic case (islanding might occur at any load level). Further, Expected Energy Not Served is evaluated. In conclusion, the impacts of DG units and islanded operation of micro-grids have been analyzed for the enhancement of the overall reliability of the distribution system and the successful islanding mode of operational conditions.

El presente trabajo ha sido desarrollado con la finalidad de la obtención del título de Doctor en Electrónica por la Universidad Politécnica de Valencia. Este documento reporta el diseño y la construcción de un convertidor electrónico destinado a operar en el entorno de las microrredes con energía fotovoltaica. Las microrredes cada vez están tomando más importancia y su implementación a gran escala está más cercana. Por este motivo resulta interesante trabajar en la obtención de equipos destinados a mejorar las capacidades de las microrredes. Las Microrredes son agrupaciones de recursos de generación, almacenamiento, distribución eléctrica y cargas gestionadas desde la propia instalación, que pueden funcionar tanto conectados a la red eléctrica como aislados de la misma. El quipo desarrollado es capaz de operar tanto con la microrred conectada a la red principal como con la microrred aislada. El cambio entre ambos estados se realiza mediante transiciones suaves que no afectan a las cargas. El equipo emplea diferentes controles para los diferentes estados. El equipo presenta igualmente la posibilidad de trabajar en paralelo con otros equipos sin necesidad de comunicaciones entre ellos. El sistema dispone de un sistema de almacenamiento de energía en baterías que permite mejorar el servicio cuando la microrred se encuentra aislada. El almacenamiento de energía se realiza en corriente continua mediante un convertidor DC/DC conectado al bus de continua en paralelo con el inversor. El sistema de almacenamiento de energía permite energizar las cargas a pesar de que la energía requerida por estas sea mayor que la entregada por los paneles fotovoltaicos. En el caso en el que la generación es mayor que la carga, el exceso de energía puede ser almacenado en las baterías. El documento presenta tanto el diseño como los resultados obtenidos en simulación y experimentales del equipo trabajando en diferentes situaciones. Los resultados obtenidos confirman el buen funcionamie / Velasco De La Fuente, D. (2012). Reconfiguración de controladores para inversores fotovoltaicos con almacenamiento de energía en DC operando como generadores autónomos en una microrred [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16183 / Palancia

To increase the allowable capacity of wind generation and system reliability of distribution system for islanding operation, an actual Taipower feeder has been selected for computer simulation. The voltage enhancement of distribution feeders with fixed speed wind generator has been obtained by using the static synchronous compensator(STATCOM). The model of Doubly-Fed induction generator(DFIG) in Matlab/Simulink has also been applied in the feasibility study of islanding operation for Taipower feeder. The critical clearing time of fault contingency is solved by considering the low voltage ride through (LVRT) capability of wind generators (WG) and the CBEMA curve of sensitive loads. In this way, feeder circuit breaker(CB) can be tripped in time to achieve islanding operation of distribution feeders without causing the tripping of WG due to voltage disturbance for the system fault contingency. To restore the stable operation of distribution feeders after being isolated from the rest of the power system, an adaptive load shedding scheme has been presented to disconnect the proper amount of system loading to achieve the balance of wind power generation and load demand according to the variations of feeder load profile and wind speed. It is found that system voltage sag problem due to severe fault contingency can be mitigated effectively by using the STATCOM for the support of low voltage ride through capability of WG. With the proper design of protection relay settings for feeder CB tripping and load shedding scheme, the WG and critical loads can be prevented from tripping during transient disturbance to ensure the successful islanding operation of distribution system.

This thesis proposes and evaluates the application of a low-frequency disturbance injection, as an active islanding detection method, in a microgrid with multiple electronically-interfaced Distributed Generation (DG) units. Each DG unit is interfaced to the microgrid through a two-level Voltage-Sourced Converter (VSC). The low-frequency disturbance signal for islanding detection is injected through the q-axis control of each VSC unit. The low-frequency signal is at 1 Hz with an amplitude of up to 2.5 % of the rated VA of the VSC unit and augments the reference signal of the q-axis control. The effectiveness of the low-frequency injection for islanding detection is examined under two distinct VSC control scenarios. In the first scenario, each VSC only injects pre-determined real- and reactive-power components in the system and does not participate in frequency/voltage control. In the second scenario, the VSC controls are also equipped with frequency/real-power and voltage/reactive-power droop characteristics and thus share power and participate in frequency and voltage control of the microgrid, specifically in the islanded mode. The investigations reported in this thesis show that the proposed islanding detection method can effectively detect an islanding event under both VSC control strategies, subject to the conditions that UL and/or IEEE anti-islanding standards impose. The studies show that an islanding event can be detected within 536 ms subsequent to the instant of islanding. As part of this thesis, an eigen analysis software tool has been developed that can systematically investigate the impact of low-frequency disturbance injection on the small-signal stability and dynamic performance of the microgrid, prior and subsequent to an islanding event. This thesis concludes that the low-frequency disturbance injection-based method can be successfully applied to a multi-DG system, since (i) islanding detection is achieved within applicable standards requirements by all DG units in the system, and (ii) the low-frequency disturbance injection signal has no noticeable impact on the dynamics nor the small-signal stability of the system if its magnitude is kept below a pre specified limit.

This thesis proposes and evaluates the application of a low-frequency disturbance injection, as an active islanding detection method, in a microgrid with multiple electronically-interfaced Distributed Generation (DG) units. Each DG unit is interfaced to the microgrid through a two-level Voltage-Sourced Converter (VSC). The low-frequency disturbance signal for islanding detection is injected through the q-axis control of each VSC unit. The low-frequency signal is at 1 Hz with an amplitude of up to 2.5 % of the rated VA of the VSC unit and augments the reference signal of the q-axis control. The effectiveness of the low-frequency injection for islanding detection is examined under two distinct VSC control scenarios. In the first scenario, each VSC only injects pre-determined real- and reactive-power components in the system and does not participate in frequency/voltage control. In the second scenario, the VSC controls are also equipped with frequency/real-power and voltage/reactive-power droop characteristics and thus share power and participate in frequency and voltage control of the microgrid, specifically in the islanded mode. The investigations reported in this thesis show that the proposed islanding detection method can effectively detect an islanding event under both VSC control strategies, subject to the conditions that UL and/or IEEE anti-islanding standards impose. The studies show that an islanding event can be detected within 536 ms subsequent to the instant of islanding. As part of this thesis, an eigen analysis software tool has been developed that can systematically investigate the impact of low-frequency disturbance injection on the small-signal stability and dynamic performance of the microgrid, prior and subsequent to an islanding event. This thesis concludes that the low-frequency disturbance injection-based method can be successfully applied to a multi-DG system, since (i) islanding detection is achieved within applicable standards requirements by all DG units in the system, and (ii) the low-frequency disturbance injection signal has no noticeable impact on the dynamics nor the small-signal stability of the system if its magnitude is kept below a pre specified limit.

L’augmentation de la proportion d’énergie renouvelable sur le réseau électrique amène de nouveaux défis dans le domaine des réseaux électriques dont notamment la détection de l’îlotage non-intentionnel. Il s’agit de la situation pendant laquelle une partie du réseau de distribution, bien que déconnectée du réseau global après l’ouverture d’une protection, reste alimentée par la production locale. Ce phénomène, autrefois improbable, engendre de nombreux problèmes car la fréquence et la tension dans l’îlot ne sont plus imposées par le réseau global et deviennent donc incontrôlées. La probabilité d’apparition de ce phénomène semble augmenter du fait de l’insertion grandissante de la proportion de production renouvelable sur le réseau. De plus, de nouvelles réglementations tendent à élever le seuil de fréquence des protections de découplage, augmentant ainsi la probabilité que les producteurs ne détectent pas de situation anormale. Les travaux dans le cadre de cette thèse sont une contribution à une meilleure compréhension globale de ce phénomène et à sa détection. Des expériences en laboratoire ont été menées et ont notamment permis de prouver l’existence du phénomène. Ensuite, une étude de données réelles a permis de définir la durée typique de présence tension après l’ouverture d’une protection en tête de départ HTA sur lequel se trouvent des installations de production. Deux cas d’étude sont ensuite présentés. Le premier consiste à analyser la probabilité qu’un défaut monophasé sur la HTA ne soit pas détecté par la production en BT. Le second concerne des simulations numériques et montre l’impact du modèle de charge sur le comportement de l’îlotage non-intentionnel. La troisième étape de ces travaux concerne l’analyse de l’impact de nouvelles régulations dans les onduleurs photovoltaïques sur la taille de la zone de non-détection des protections de découplage. Enfin, la dernière partie présente une étude de l’impact de la dispersion naturelle des réglages des protections de découplage sur le comportement du système électrique, lors d’un événement de grande ampleur. / A high penetration of distributed generators (DGs) on the electrical networks brings new challenges for distribution system operators (DSO). Unintentional islanding is one of them; it is a situation when a part of the network is disconnected from the main grid but remains supplied by local generation. Though this was unlikely to occur before, the likelihood of this situation seems to increase with a higher penetration of DGs, and it leads to several issues since the frequency and the voltage in the island are not governed by the main grid anymore. Moreover, new requirements tend to increase the upper frequency threshold of the interface protection to prevent a system wide event. This could lead to an increase of the likelihood of undetected islanding. This thesis is a contribution to a better understanding of this phenomenon and its detection. First, laboratory experiences were conducted to prove its existence. Then, real data have been analyzed in order to define the typical duration of voltage presence after the trip of a protection feeder. Two case studies are introduced in the next part. The first one consists in assessing the likelihood of a single-phase fault on medium voltage networks that would generally not be detected by low voltage (LV) generation. The second one concerns numerical simulations and shows the impact of the load model on the frequency behavior of the island. The third part of this work assesses the impact of new regulations in the photovoltaic inverters on the non-detection zones of interface protections. Finally, the last part introduces a study regarding the impact of the natural dispersion of interface protection settings on the stability of an electrical system when a system wide event occurs.

The increase in solar energy installation capacity and the versatility of modern power inverters have enabled widespread penetration of distributed generation in modern power systems. Islanding detection techniques allow for fast identification and corrective action in the face of abnormal events. Current standards specify the operational limits for voltage, frequency, and detection time. Grid codes specify the procedures for disconnection to establish safe network maintenance conditions. Passive, active, and remote techniques require voltage, current, and frequency measurements and the definition of thresholds for detection. Operational parameters such as load mismatch and quality factors influence the detection capabilities. False-positive triggering due to grid transients can lead to unnecessary disconnection of distributed generation resources. Cybersecurity threats pose a critical challenge for power systems and can result in significant operational disruptions and security risks. In particular, when a power system initiates communication links between different nodes or ends, it becomes more vulnerable to various forms of cyber-attacks. As such, it is imperative to address the potential cybersecurity risks associated with communication links. Through a literature review, this work analyzes the performance of several islanding detection techniques and proposes a modified 9-bus benchmark system to verify the robustness of passive and active methods against false-positive detections upon severe grid-side transients. Furthermore, this thesis conducts a detailed analysis of cyber-attacks on the remote islanding detection technique, using a real-time simulator to assess the potential impact of such attacks on the technique's effectiveness by simulating various attack scenarios. The findings of this analysis can help power system operators to better protect their systems from cyber-attacks and ensure the reliable operation of their distributed generation resources. Moreover, it discusses a conceptual implementation of hardware-in-the-loop testing. The modeling of the systems is discussed. Guidelines and international standards are presented. Various setups for experimental work are suggested and implemented.

Significant penetration of distributed generation (DG) and the increasing automation level available for distribution networks have opened an option of splitting a network into subsystems and operating each as an "autonomous island". This is particularly important when a major contingency occurs. However, there are issues and challenges that must be addressed before islanded operation becomes viable, among which, ensuring seamless switching of a distribution subsystem from grid-connected to islanded mode is critically important. Unless the subsystem is a predesigned microgrid, it is highly possible that the subsystem load demand will exceed the generation capacity of island DGs. Therefore, an appropriate load shedding scheme must be implemented to ensure the islanded subsystem is power balanced. In this thesis, a switching control strategy is designed to deliver seamless islanding switching. This strategy comprises a multiple-DG coordination method and a single-step load shedding scheme. Mathematical studies and time-domain simulations that investigate the transients observed during the islanding switching process are both conducted, and together, they are used to address the transient stability issues of an islanded subsystem. This thesis focuses on a distribution subsystem consisting of a mix of synchronous and inverter-based DGs and a combination of static and dynamic loads. DG modelling and control is first introduced, and based on that, various types of method to achieve multiple-DG coordination, including an innovative multiple-master strategy, are investigated. The widely accepted master-slave strategy is used to coordinate DGs when the subsystem is islanded. The strategy demands a single dispatchable and controllable DG, such as a synchronous generator, to be the master, whilst requires the others, such as intermittent renewable-based DGs, to be the slaves. Dynamic load modelling is another critical part of this thesis. The transient stability of dynamic loads after major disturbances is investigated and then used to design the stability-oriented load shedding priority. The single-step load shedding scheme calculates the load shedding amount based on the power flow at the point of common coupling (PCC) and the spinning reserve available in the island. This scheme is activated by the tripping event of the PCC circuit breaker between the grid and the island, and then priorities the load to be shed according to the priority predetermined from the stability perspective. Mathematical analysis is first conducted on a simple subsystem to investigate the impact of DG settings on the islanding transients. A full-scale subsystem is also simulated in PSCAD/EMTDC and used to verify the effectiveness of the switching control strategy. In time-domain simulations, the subsystem is islanded following either a routine switching event or a permanent grid fault. Various factors that may affect the transient performance are analysed, such as the severity of the fault, the DG penetration level, the fault clearance time and the switching control delay. This thesis concludes that based on the proposed switching control strategy, the concept of seamless switching from grid-connected to islanded operation is technically viable.

Distributed generation is becoming increasingly prevalent on power grids around the world. Conventional designs and grid operations are not always sufficient for handling the implementation of distributed generation units; the new generation may result in undesirable operating conditions, or system failure. This paper investigates the primary issues involved with the implementation of distributed generation and maintaining the integrity of the power grid. The issues addressed include power flow, system protections, voltage regulation, intermittency, harmonics, and islanding. A case study is also presented to illustrate how these issues can be addressed when designing distributed generation installation on an existent distribution system. The case study design is performed on the campus distribution system of California Polytechnic State University, San Luis Obispo, with the design goal of implementing renewable energy sources to make the campus a net zero energy consumer.

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.