The main focus of this PhD thesis is fundamental investigations into control techniques of inverter-based microgrids. It aims to develop new and improved control techniques to enhance performance and reliability. It focuses on the modelling, stability analysis and control design of parallel inverters in a microgrid. In inverter-based microgrids, the paralleled inverters need to work in both grid-connected mode and stand-alone mode and should be able to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using droop control. Inverters can have different power set-points during grid-connected mode but in stand-alone mode they all need their power set-points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set-points to inverters with low power set-points, which can raise the DC link voltage of the inverters causing them to shut down. This thesis investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the DC link voltage and adjusts the power set-point in proportion to the rise in the voltage. A small signal model of an island microgrid has been developed and used to design the controller. The model and the controller design have been validated by simulation and practical experimentation. The results confirmed the performance of the proposed controller for limiting the DC link voltage and supporting a seamless mode transfer. The limitation of the droop controller, that is utilized to achieve load sharing between parallel-operated inverters in island mode, has also been addressed. Unequal output impedances among the distribution generation (DG) units lead to the droop control being inaccurate, particularly in terms of reactive power sharing. Many methods reported in the literature adopt low speed communications to achieve efficient sharing. However, the loss of this communication could lead to inaccuracy or even instability. An improved reactive power-sharing controller is proposed in this thesis. It uses the voltage at the point of common coupling (PCC) to estimate the inductance value of the output impedance including the impedance of the interconnecting power cables and to readjust the voltage droop controller gain accordingly. In an island microgrid consisting of parallel-connected inverters, the interaction between an inverter’s output impedance (dominated by the inverter’s filter and voltage controller) and the impedance of the distribution network (dominated by the other paralleled inverters’ output impedances and the interconnecting power cables) might lead to instability. This thesis studies this phenomenon using root locus analysis. A controller based on the second derivative of the output capacitor voltage is proposed to enhance the stability of the system. Matlab simulation results are presented to confirm the validity of the theoretical analysis and the robustness of the proposed controller. A laboratory-scale microgrid consisting of two inverters and local load has been built for the experimental phase of the research work. A controller for a voltage source inverter is designed and implemented. A dSPACE unit has been used to realize the controller and monitor the system in real time with the aid of a host computer. Experimental results of the two voltage source inverters outputs are presented.

Ingeniero Civil Eléctrico / En el presente trabajo se propone un modelo de matemático de optimización maestro-esclavo entre un despacho económico (maestro), retroalimentado con un análisis de estabilidad de pequeña señal (esclavo). El modelo propuesto determina si una solución, determinada mediante una optimización algebraica (maestro), es estable (o no) mediante el cálculo de sus valores propios (esclavo) e itera hasta encontrar una solución que es tanto económica como estable. El método consiste en definir un problema maestro como etapa de optimización económica, caracterizada por un modelo de flujo optimo convencional (OPF, por su nombre en inglés: Optimal Power Flow), mientras que el problema esclavo o etapa de estabilidad es el encargado de determinar las ganancias de control droop (de cada unidad de generación conectada mediante un conversor) para estabilizar el despacho obtenido por el problema maestro. Si dicho despacho no puede ser estabilizado, entonces se itera insertando una restricción (o corte ) de factibilidad en el maestro hasta obtener una solución de mínimo costo (de forma local) que sea estable en pequeña señal. Se proponen dos versiones del modelo dependiendo si el maestro corresponde a un DC-OPF o uno AC-OPF. Dichas formulaciones son testeadas con micro-redes de dos y tres micro-fuentes. Esta tesis demuestra que estas formulaciones resuelven exitosamente el despacho coordinado (definiendo tanto la inyección de las unidades en potencia activa y reactiva, como las ganancias de los controladores) para sistemas de dos y tres micro-fuentes. Más aún, en el problema de dos micro-fuentes, la aproximación DC-OPF resulta completamente convexa por lo que la solución encontrada corresponde a un óptimo global.

Η παρούσα διπλωματική εργασία έχει ως αντικείμενο την μελέτη και την παρουσίαση των στρατηγικών για τον έλεγχο της ροής ισχύος στα μικροδίκτυα. Επιπροσθέτως γίνεται μια εκτενής παρουσίαση της δομής και των αρχών λειτουργίας των μικροδικτύων καθώς και των δυνατοτήτων που έχουμε για κατανεμημένη παραγωγή και αποθήκευση και των ηλεκτρονικών μετατροπέων ισχύος που χρησιμοποιούμε για τον έλεγχο αυτών / This thesis regards to the subject of studying and presentation of power control strategies regarding power flow in Microgrid systems. Additionally is a detailed presentation of the structure and operation principles of microgrids and the possibilities we have for distributed generation and storage and power electronic converters used to control them.

Future electric grid will evolve from the current centralized and radial model toward a more distributed one. In recent years, distributed generation (DG) units have been playing an important role in electric generation due to their promising advantages in reducing air pollution, improving power system efficiency, and relieving stress on power transmission and delivery systems. Despite the increased penetration of DG systems, the application of individual DG system always has its limitation such as high cost/W, limited capacity and reliability, and safety concerns. A better way to utilize the emerging potential of DG is to take a system approach viewing generation and associated loads as a subsystem called a “microgrid”. Forming an electric island, the microgrid can work autonomously following a disturbance. In the islanded microgrid, micro sources are responsible for maintaining the voltage and the frequency of the microgrid system within their specified limits and sharing the load between the generators in a stable manner. However, a robust and stable operation of a microgrid depends on a robust control scheme of the microgrid sources. The most common technique to control microgrid sources is based on conventional droop characteristics. Although the conventional frequency/voltage droop technique properly shares a common active load, the reactive power sharing accuracy can be strongly affected by system parameter and active power control. In addition, frequency variations of different sources in transient mode can cause poor active power sharing. To override the above-mentioned problems, a novel frequency/voltage droop scheme is proposed in this thesis. The proposed scheme improves the performance of the microgrid in terms of power sharing and voltage regulation and smooths the system’s dynamic and transient responses. This work has developed the modeling, control parameters design, and power-sharing control starting from a single voltage source inverter to a number of interconnected DG units forming a flexible microgrid. Specifically, this thesis presents: • A control-oriented modeling based on active and reactive power analysis. • A control synthesis based on enhanced droop control technique. • A small signal stability study to give guidelines for properly adjusting the control system parameters according to the desired dynamic response. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2012-04-25 12:08:48.634

Presented in this thesis are extensive theoretical investigations into the causes and effects of carrier localization in InGaN/GaN quantum wells. The results of the calculations agree well with experimental data, where it is available, and provide additional insights into the mechanisms that lead to some of the experimentally observed effects of localization. Firstly, the wave functions of the electrons and holes in InGaN/GaN quantum wells have been calculated by numerical solution of the effective-mass Schrödinger equation. In our calculations we have assumed a random distribution of indium atoms, as suggested by the results of atom probe tomography: this allows us to find the contributions to the carriers' potential energy that arise from band gap fluctuations, the deformation potential and the spontaneous and piezoelectric fields. We show that the fluctuations in alloy composition can be sufficient to localize the carriers; our results are in good agreement with the results of experiment and more detailed ab-initio calculations, but we also obtain information about the distribution of localized states which those methods cannot yet provide. We find that the holes are localized on a short scale in randomly-occurring regions of high indium content, whereas the electrons are localized on a longer length scale. We consider the effect of well width fluctuations and find that these contribute to electron localization, but not to hole localization. We also simulate the low-temperature photoluminescence spectrum and find good agreement with experiment for the energy, width and shape of the photoluminescence peak. Secondly, we have used first-order time-dependent perturbation theory to study the diffusion of the carriers between their localized states at non-zero temperatures. The rates for scattering via the interaction with acoustic phonons are calculated using the carrier wave functions, and the resulting master equation for the distribution of the carriers is solved by a Monte Carlo method. We find that, even towards room temperature, the carriers are localized to a small number of states, and that their diffusion lengths are proportional to a combination of the density of localized states and the localization length. The experimentally-observed `S-shape' of the photoluminescence peak energy as a function of temperature is reproduced in our results and is explained by the thermal redistribution of holes among the localized states. A reduction of the depth of this S-shape is found as the excitation power is increased, as has been observed experimentally, and which we attribute to the saturation of the localized states.

Magíster en Ciencias de la Ingeniería, Mención Eléctrica / Ingeniero Civil Electricista / Las microrredes se presentan como una solución frente a los problemas de integración de energías renovables a los sistemas, alimentación de zonas aisladas y una alimentación eléctrica sin interrupciones. Una solución para ello es la implementación de sistemas de control jerárquicos compuestos por un control primario, un control secundario y un control terciario. En general se trabaja con un control primario mediante curvas de estatismo que permite evitar el uso de líneas de comunicación al ser un control distribuido; y un control secundario y terciario centralizados cuyas funciones son restaurar la frecuencia y voltaje a su valor nominal y optimizar la operación de la microrred en cuanto a características económicas. En esta tesis se presenta un sistema de control de tres niveles, de los que se diseña e implementa en Simulink el control primario y secundario, dejando planteada la conexión con el terciario. El control primario utilizado es distribuido, opera mediante curvas de estatismo y tiene la función de permitir compartir potencia entre los inversores; en cambio, el control secundario es centralizado y restablece el voltaje y frecuencia de un sistema a sus valores nominales. El control terciario consiste en un EMS que minimiza los costos de operación de la microrred de acuerdo a predicciones de los recursos disponibles y sus costos. El problema en estudio se compone de dos partes. En primer lugar, se debe analizar la estabilidad de la microrred, para lo cual ésta se analiza de forma independiente en el control primario y secundario. Para ello, el primario utiliza un análisis de estabilidad en pequeña señal, mientras que el secundario un análisis de polos a través de la ecuación característica del sistema. En segundo lugar se estudia el retardo en la comunicación entre el control secundario de frecuencia y los inversores, para lo que se estudian estrategias de control que utilizan predictor de Smith y variaciones éste, así como también control predictivo. Los resultados obtenidos permiten observar que el sistema comparte carga de acuerdo a lo indicado por la teoría, independiente de la carga conectada a cada inversor y los valores de potencia máxima de cada uno, manteniéndose la estabilidad del sistema. Además, se presenta el controlador PI con predictor de Smith con filtro pasa-bajo como la solución al problema de retardo en la comunicación debido a que al ser diseñado para un retardo dado es más robusto que los otros controladores, ya que a medida que aumenta el retardo de la planta y se mantiene el de diseño mantiene la estabilidad para retardos mayores que los otros controladores, sin tener un tiempo de estabilización mayor como ocurre con el controlador predictivo. Finalmente, la simulación permite comprobar que se cumplen los valores máximos determinados mediante un estudio de sensibilidad del sistema, así como también los valores de diseño de los controladores.

Microgrid (MG) concept has been emerged to enable integration of renewable energy sources and storage devices using power electronic converters. An MG can be grid connected to exchange power with the main grid, isolated that is completely separated from the grid, or islanded that is temporarily separated from the grid. The P and Q-V drooping approach is commonly used to control and achieve power sharing among the generators.\\ This study presents an approach for systematically modeling a class of microgrid (MG) systems. The derived model 1) accommodates grid-connected and islanded operation of the MG simultaneously, and 2) allows modeling of converter-based as well as directly-interfaced resources. The originally nonlinear model is then converted to a linear model whose eigenvalues determine local stability of the MG. \\The model is used to analyze the impacts of adding cross-coupling droop terms (P-V and Q) on an MG's performance. Various performance aspects such as stability, stability robustness, transmission power loss, voltage profile, and power sharing are considered. The conclusions are as follows. (1) Addition of a small portion of cross-coupling will reduce the losses without compromising other aspects in a grid-connected MG. Larger cross-coupling terms will compromise the system stability. (2) Large cross-coupling terms can be added to reduce the power loss and to improve the system stability in an isolated MG. Simulation and experimental results are presented to verify the derivations.

The objective of this thesis is to discuss the solutions investigated by AMDG (Advanced Materials and Devices Group) to reduce the "efficiency droop" effect that occurs in III-Nitrides Light Emitting Diodes (LEDs) when driven at high injection current densities. The efficiency droop refers to a decrease of the LED light emission efficiency when increases the current density from low values ~10 A/cm2 to higher values >100A/cm2. Many scientific papers have been written about the possible reasons for this phenomenon. Therefore, this thesis will discuss the different effects suspected to contribute to the droop, and discuss LED structure modifications studied by Dr. Dupuis' research group to reduce their impact. In addition to a description of a conventional LED structure, a discussion of the device fabrication process will be provided including the solutions investigated in our group to improve LED performance. Because measurement is critical to our studies, a description of the equipment used by the AMDG will be provided, e.g., the Electroluminescence (EL) and Photoluminescence (PL) test stations, Atomic Force Microscopy (AFM) for surface topology, TLM for metallic contact resistivity, X-Ray diffraction for crystal quality and epitaxial layer structure, and Hall-Effect measurement for doping concentration characterization and material resistivity. Because the IQE gives us a direct assessment of the active region's crystal quality, the setup and operation of a new Temperature-Dependent PL (TD-PL) system to measure the Internal Quantum Efficiency (IQE) was the main focus of this research. The External Quantum Efficiency (EQE) is measured using electroluminescence measurements. The EL measurements involve the acquisition of the emitted light spectrum along with different processed data such as the Full-Width at Half Maximum (FWHM) of the spectral intensity, the peak wavelength, output power, etc., which allows a comparison of the different LED structure performances. Within this work, a new LabVIEW© program (called QuickTest 2.0) has been developed in order to automate the instrumentation setup and improve both the speed and accuracy of EL acquisition. A brief description of the G language used by the LabVIEW© software will be provided along with the objective and motivation for upgrading the program, the general features of the program, and a comparison of spectrum acquisition and processed data results. The benefit for the research in the AMDG was to reduce measurement time, improve efficiency, supply a more user-friendly front-panel, and to enable transfer to other computers.

Power electronics are becoming established in ever broadening areas of industry. The transition from previous generation technology is driven by the oportunity for improvements in controllability, efficiency, and longevity. A wide variety of power semiconductors are available, however power handling capacity is still a significant limitation for many applications. An increase in the capacity of a single device is usually accompanied by a drop in switching frequency, and hence achievable system bandwidth. Increased capacity can be attained without this loss in bandwidth by using multiple lower power devices in parallel. Products based on parallel device topologies are already present in the marketplace, however there are many associated complications. The nature of these complications depends on the control method and topology used, but no system combines high performance and high power with high reliability and easy maintainability. This research aims to identify and develop a method that would provide a system of voltage source inverters with a total capacity in excess of 10MVA, with effective control bandwidth comparable to a 100kVA system. Additionally, the method should be equally applicable at still higher power levels in the future with the anticipated development of higher capacity power semiconductors. The primary goal when using paralleled devices is to achieve an even distribution of system load between them, as unbalanced load leads to poor system utilisation. Devices can be paralleled either passively, in which devices are controlled in common and characteristics inherent to the device are relied upon to balance load; or actively, in which devices are individually controlled and monitored to improve load balance. A key component of the thesis is the identification and analysis of the inadequacies inherent to passively paralleled systems. It is the limitations of passive paralleling that provide the motivation to develop an active parallel control mechanism. Following the analysis, an active control algorithm is developed and implemented on a paralleled system. The proposed system topology consists of an array of medium power Voltage Source Inverter (VSI) modules operating in parallel. Each module is controlled semi-independently at a local level, with an inter-module communications network to enable active equalisation of module load, and redundant fault management. An innovative load equalisatiion algorithm is developed and proven, the key feature of which is this inclusion of a synthetic differential resistance between modules within the system. The result is a modular expandable structure offering the potential for very high power capacity combined with quality of response usually only found in low power systems. The system as a whole is extremely reliable as any module can be isolated in the event of a fault without significantly affecting the remainder of the network. Performance results from both simulation and experimentation on a two module small scale prototype are given. Using the developed topology and control method extremely accurate load balancing can be achieved without degradation of the response characteristics. The system is tested up to only 2.4kW in the course of this research, but the correlation with simulation is high and gives confidence that the developed mechanism will allow the 10MV A goal to be achieved. Following the developmental stage of this research the technology has been applied to a commercial system comprising parallel structures of up to 8 modules with a total power handling capacity of 1MVA with no deterioration in performance. 2MVA systems are deliverable with the current technology without any changes, and higher power levels are expected to be easily achieved.

Tesis para optar al grado de Magíster en Ciencias de la Ingeniería, Mención Eléctrica / Memoria para optar al título de Ingeniero Civil Eléctrico / Microgrids are one of the key technologies to facilitate the integration of large amounts of renewable generation technologies to the main grid. The main power supplies inside the microgrid are power electronic devices which are responsible for energy conversion and provide the necessary control. Dynamical interactions between the microgrid and newly connected power electronic-based sources can lead to small-signal instability. Hence, several stability analysis approaches have been developed over the recent years, particularly methods to ensure stability by first dividing the system into source and load subsystems and then applying the Nyquist criterion to the respective source/load impedances ratio. Nevertheless, this aspect has been rarely studied considering droop-controlled inverters, as the active power droop control also impacts the output frequency of micro-sources and has a deep impact in the small-signal impedances of the inverters. The main objective of this thesis is to characterize the small-signal impedance of droop-controlled inverters typically used in microgrids through simplified models, in order to achieve a comprehensive understanding of their behavior. This work postulates as hypothesis that the general behavior of the small-signal impedance of droop-controlled inverters when operation conditions change can be characterized through the analysis of the transfer functions of linearized multi-input multi-output reduced-order inverter models, by contrasting them with the resulting small-signal impedances of more complex models. The obtained results show that the small-signal impedance of these inverters were effectively characterized, specially by one of the proposed models. Two indices were developed in order to quantify the graphically obtained results, which confirmed the performance of the developed models, specially with respect to DD, DQ and QD-Channels.The indices confirmed the identification of the operating variables that impact the small-signal impedance the most when perturbed. The results also indicate that the low-frequency range of the small-signal impedance is the most affected range when changing the operating conditions, as the high-frequency range tends to converge to the large-signal impedance. This work could lead to improved small-signal stability studies, in which one of the biggest problems nowadays is the dependence of the small-signal impedance on the changing operating point. / Conicyt Proyecto PFCHA/MagísterNacional/2017- 22172061