Hybrid AC/DC Microgrid Mode-Adaptive Controls

The lack of inertial response at microgrids is usually compensated by configuring primary controllers of converter‐interfaced devices to contribute in the transient response under power disturbances. The main purpose of this chapter is to study the modes of operation of primary level techniques of generation, storage, loads, and other devices attached to hybrid ac/dc microgrids. Although the chapter includes an analysis of the modes of operation of lower‐level regulators, the focus is on upper‐level or primary controllers. In this context, we analyze mode‐adaptive controls based on voltage and frequency levels and we evaluate their behavior by simulation in the Matlab/Simulink® environment. The results demonstrate that mode‐adaptive techniques are adequate for maximizing the energy extracted by distributed generation (DG) systems and limit demand side management actuations while ensuring an adequate regulation of the microgrid

Reliability Studies of Distribution Systems Integrated with Energy Storage

The integration of distributed generations (DGs) - renewable DGs, in particular- into distribution networks is gradually increasing, driven by environmental concerns and technological advancements. However, the intermittency and the variability of these resources adversely affect the optimal operation and reliability of the power distribution system. Energy storage systems (ESSs) are perceived as potential solutions to address system reliability issues and to enhance renewable energy utilization. The reliability contribution of the ESS depends on the ownership of these resources, market structure, and the regulatory framework. This along with the technical characteristics and the component unavailability of ESS significantly affect the reliability value of ESS to an active distribution system. It is, therefore, necessary to develop methodologies to conduct the reliability assessment of ESS integrated modern distribution systems incorporating above-mentioned factors. This thesis presents a novel reliability model of ESS that incorporates different scenarios of ownership, market/regulatory structures, and the ESS technical and failure characteristics. A new methodology to integrate the developed ESS reliability model with the intermittent DGs and the time-dependent loads is also presented. The reliability value of ESS in distribution grid capacity enhancement, effective utilization of renewable energy, mitigations of outages, and managing the financial risk of utilities under quality regulations are quantified. The methodologies introduced in this thesis will be useful to assess the market mechanism, policy and regulatory implications regarding ESS in future distribution system planning and operation. Another important aspect of a modern distribution system is the increased reliability needs of customers, especially with the growing use of sensitive process/equipment. The financial losses of customers due to industrial process disruption or malfunction of these equipment because of short duration (voltage sag and momentary interruption) and long duration (sustained interruption) reliability events could be substantial. It is, therefore, necessary to consider these short duration reliability events in the reliability studies. This thesis introduces a novel approach for the integrated modeling of the short and long duration reliability events caused by the random failures. Furthermore, the active management of distribution systems with ESS, DG, and microgrid has the potential to mitigate different reliability events. Appropriate models are needed to explore their contribution and to assist the utilities and system planners in reliability based system upgrades. New probabilistic models are developed in this thesis to assess the role of ESS together with DG and microgrid in mitigating the adverse impact of different reliability events. The developed methodologies can easily incorporate the complex protection settings, alternate supplies configurations, and the presence of distributed energy resources/microgrids in the context of modern distribution systems. The ongoing changes in modern distribution systems are creating an enormous paradigm shift in infrastructure planning, grid operations, utility business models, and regulatory policies. In this context, the proposed methodologies and the research findings presented in this thesis should be useful to devise the appropriate market mechanisms and regulatory policies and to carry out the system upgrades considering the reliability needs of customers in modern distribution systems

Voltage and Reactive Power Control in Islanded Microgrids

Previous studies put on view lots of advantages and concerns for islanded microgrids (IMGs), whether it is initiated for emergency, intentionally planned or permanent island system purposes. From the concerns that have not been addressed yet, such as: 1) The ability of the distributed generation (DG) units to maintain equal reactive power sharing in a distribution system; 2) The ability of the DG units to maintain acceptable voltage boundary in the entire IMG; 3) The functionality of the existing voltage and reactive power (Volt/Var) DG, this thesis analyzes the complexity of voltage regulations in droop-controlled IMGs. A new multi-agent algorithm is proposed to satisfy the reactive power sharing and the voltage regulation requirements of IMGs. Also, the operation conflicts between DG units and Volt/Var controllers, such as shunt capacitors (SCs) and load-ratio control transformer (LRT) during the IMG mode of operation, are investigated in this thesis. Further, a new local control scheme for SCs and LRTs has been proposed to mitigate their operational challenges in IMGs

Analysis of an On-Line Stability Monitoring Approach for DC Microgrid Power Converters

An online approach to evaluate and monitor the stability margins of dc microgrid power converters is presented in this paper. The discussed online stability monitoring technique is based on the Middlebrook's loop-gain measurement technique, adapted to the digitally controlled power converters. In this approach, a perturbation is injected into a specific digital control loop of the converter and after measuring the loop gain, its crossover frequency and phase margin are continuously evaluated and monitored. The complete analytical derivation of the model, as well as detailed design aspects, are reported. In addition, the presence of multiple power converters connected to the same dc bus, all having the stability monitoring unit, is also investigated. An experimental microgrid prototype is implemented and considered to validate the theoretical analysis and simulation results, and to evaluate the effectiveness of the digital implementation of the technique for different control loops. The obtained results confirm the expected performance of the stability monitoring tool in steady-state and transient operating conditions. The proposed method can be extended to generic control loops in power converters operating in dc microgrids

Modeling and Simulation of Protective Relay for Short Circuits in AC Micro-grids using Fuzzy Logic

The duo of high human appetite for electricity in the 21st century and high human population growth rate entail inadequacy of contemporary electric power protective systems for the emerging micro-grid. This thesis presents results of a research which seeks to propose a new model of protective device for short circuits in ac micro-grids. Response of the proposed relay is consistent with a reliable device. Consequently, a protective relay for short circuits in micro-grids is proposed