Optimal Power Flow in three-phase islanded microgrids with inverter interfaced units

In this paper, the solution of the optimal power flow (OPF) problem for three phase islanded microgrids is studied, the OPF being one of the core functions of the tertiary regulation level for an AC islanded microgrid with a hierarchical control architecture. The study also aims at evaluating the contextual adjustment of the droop parameters used for primary voltage and frequency regulation of inverter interfaced units. The output of the OPF provides an iso-frequential operating point for all the generation units and a set of droop parameters for primary regulation. In this way, secondary regulation can be neglected in the considered hierarchical control structure. The application section provides the solution of the OPF problem over networks of different sizes and a stability analysis of the microgrid system using the optimized droop parameters, thus giving rise to the optimized management of the system with a new hierarchical control architecture

Control of an islanded microgrid

This thesis presents a detailed investigative process into the study of the control of an islanded microgrid. This investigation is done through the research and exploration of multiple existing control techniques for the control of a microgrid and then by analysing them to identify the areas where the existing methods can be altered in order to reduce or mitigate common operational issues. The final goal was to use the gathered information to develop an innovative strategy that may be used to control an islanded microgrid. However, due to various challenges faced over the course of the project – this goal was not achieved. In light of this, the aim of this thesis was for it to became a research focused development of a body of work that may be useful or potentially serve as a point of reference for future studies in the control of an islanded microgrid. 1. P & PI Controller Regulation & Response Times 2. Natural Load Sharing Amongst Distributed Generators 3. Secondary Frequency-Load Control Mechanisms 4. Controllable Storage Systems 5. Automated Load Shedding in Microgrids 6. Stabilizer Control Strategies By developing this list of factors and considerations, this thesis project aims to be a useful resource for future studies performed in the topic of islanded microgrid control. The aspiration is that by collating extensive background, theoretical and technical research in this project, the efficiency of those who may want to continue work in this area of study will be improved

Optimal operation of active distribution networks

This document presents a generic Optimal Power Flow Formulation (OPF) for operating Active Distribution networks during grid-connected and gridislanded modes. The optimization model is intended to be executed in real-time, considering the effect of droop controls. Hence, fast convergence and global optimum are required to guarantee the grid’s optimal operation during these two operation modes. For this reason, the mixed integer nonlinear programming model is relaxed into a convex optimization model. Several approaches are discussed to evaluate the convergence and computation time performance. The results demonstrate that the Wirtinger linearization presents the best performance; furthermore, the optimization model guarantees a proper and safe operation while minimizing operations costs.Este documento presenta una formulación genérica de flujo de potencia óptimo (OPF) para la operación de redes de distribución activas en modo conectado a la red y en modo aislado. El modelo de optimización está pensado para ser ejecutado en tiempo real, teniendo en cuenta el efecto del droop control (control primario). Por lo tanto, se requiere una convergencia rápida, además el óptimo global es necesario para garantizar el funcionamiento óptimo de la red durante estos dos modos de funcionamiento. Por este motivo, el modelo de programación no lineal entera mixta es convertido en un modelo de optimización convexo. Varias aproximaciones son empleadas para evaluar la convergencia y el tiempo de cómputo. Los resultados demuestran que la linealización de Wirtinger presenta el mejor rendimiento; además, el modelo de optimización garantiza un funcionamiento correcto y seguro al tiempo que minimiza los costes de operación.MaestríaMagíster en Ingeniería EléctricaContents 1 Introduction 10 1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.2 State of the art . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3.1 General objective . . . . . . . . . . . . . . . . . . . . . 14 1.4 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.4.1 List of Publications . . . . . . . . . . . . . . . . . . . 15 1.5 Document organization . . . . . . . . . . . . . . . . . . . . . 17 2 Hierarchical Control in Active Distribution Networks 19 2.1 Level-zero and primary control . . . . . . . . . . . . . . . . . 20 2.1.1 Level-zero control . . . . . . . . . . . . . . . . . . . . . 20 2.1.2 Primary control . . . . . . . . . . . . . . . . . . . . . . 23 2.2 Secondary control . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3 Tertiary control . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4 Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4.1 Grid-connected operation . . . . . . . . . . . . . . . . 28 2.4.2 Grid islanded operation . . . . . . . . . . . . . . . . . 28 3 Optimal Operation in Active Distribution Networks 30 3.1 Model of the grid . . . . . . . . . . . . . . . . . . . . . . . . . 30 4 3.2 Model of distributed energy resources . . . . . . . . . . . . . 33 3.2.1 Distributed generators . . . . . . . . . . . . . . . . . . 33 3.2.2 Model of solar panels . . . . . . . . . . . . . . . . . . . 34 3.2.3 Model of wind turbines . . . . . . . . . . . . . . . . . 34 3.2.4 Model of energy storage systems . . . . . . . . . . . . 35 3.3 Technical constraints . . . . . . . . . . . . . . . . . . . . . . . 36 3.4 Balanced case (single-phase equivalent) . . . . . . . . . . . . . 37 3.5 Three-phase case . . . . . . . . . . . . . . . . . . . . . . . . . 39 4 Convex approximations 42 4.1 Second-order cone approximation . . . . . . . . . . . . . . . . 42 4.2 Sequential convex optimization . . . . . . . . . . . . . . . . . 44 4.3 Wirtinger calculus . . . . . . . . . . . . . . . . . . . . . . . . 45 4.4 Effect of the frequency . . . . . . . . . . . . . . . . . . . . . . 48 5 Results 50 5.1 Methodological framework . . . . . . . . . . . . . . . . . . . . 50 5.2 Variation of the Ybus as function of the frequency . . . . . . . 51 5.3 Single phase case . . . . . . . . . . . . . . . . . . . . . . . . . 56 5.3.1 Connected mode . . . . . . . . . . . . . . . . . . . . . 57 5.3.2 Islanded mode . . . . . . . . . . . . . . . . . . . . . . 57 5.3.3 Generic operation . . . . . . . . . . . . . . . . . . . . 59 5.3.4 Extension to meshed grids . . . . . . . . . . . . . . . . 62 5.4 Three-Phase Case . . . . . . . . . . . . . . . . . . . . . . . . . 64 6 Conclusions, discussion and future work 67 6.1 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 6.2 Applicability to the Colombian Case . . . . . . . . . . . . . . 69 Apendices 7

Load flow calculation for droop-controlled islanded microgrids based on direct Newton-Raphson method with step size optimisation

Load flow calculation for droop-controlled islanded microgrids (IMGs) is different from that of transmission or distribution systems due to the absence of slack bus and the variation of frequency. Meanwhile considering the common three-phase imbalance condition in low-voltage systems, a load flow algorithm based on the direct Newton-Raphson (NR) method with step size optimisation for both three-phase balanced and unbalanced droop-controlled IMGs is proposed in this study. First, the steady-state models for balanced and unbalanced droop-controlled IMGs are established based on their operational mechanisms. Then taking frequency as one of the unknowns, the non-linear load flow equations are solved iteratively by the NR method. Generally, iterative load flow algorithms are faced with challenges of convergence performance, especially for unbalanced systems. To tackle this problem, a step-size-optimisation scheme is employed to improve the convergence performance for three-phase unbalanced IMGs. In each iteration, a multiplier is deduced from the sum of higher-order terms of Taylor expansion of the load flow equations. Then the step size is optimised by the multiplier, which can help smooth the iterative process and obtain the solutions. The proposed method is performed on several balanced and unbalanced IMGs. Numerical results demonstrate the correctness and effectiveness of the proposed algorithm

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