Hércules Araújo Oliveira

This paper presents and discusses challenges in microgrids (uGrid) that arise when they operate isolated from the main grid. Specifically, these challenges occur because the system becomes an ungrounded delta configuration, and the microgrid power sources exhibit low short-circuit capacity. The important issues addressed include the failure to detect ground overcurrent during an earth fault event, voltage imbalances recorded by voltage transformers (VTs) connected between phases and earth, and the phenomenon of ferroresonance. These issues directly impact the coordination of electrical protection, component integrity, and synchronization checks between different power sources within the uGrid. Therefore, the Energy Management System (EMS) is tasked with managing protection devices and systematically responding to minimize disruptions, thereby ensuring operational security. This paper examines the protection functions within devices for both operational modes (on-grid and off-grid) and the corresponding decisions implemented as rules within the EMS. Effective coordination between protection devices and management systems ensures a rapid and selective response to faults, thereby enhancing microgrid security and facilitating efficient problem detection and resolution.

Blade Element Momentum Theory (BEMT) is the most used method to design horizontal-axis wind turbines worldwide. This is because BEMT has a low computational cost and easy numerical implementation. Additionally, it is demonstrated in the literature that the prediction of output power using BEMT agrees well with experimental data. Another important feature of the BEMT is its applicability to small, medium, and large-sized turbines. However, BEMT models are usually implemented and adjusted for a specific power range turbine, and they are not applied in a more general form. Thus, this article presents an analysis of additional correction methods for tip and root losses, high values of the axial induction factor, and high angle of attack to better represent horizontal-axis turbines in terms of numerical stability. The approach has the intention of combining several complementary correction methods strategically inserted in the BEMT in order to compile an algorithm that is more general, stable, and workable for any turbine size. The main contribution of this work is to propose a stable BEMT numerical algorithm through the assessment of the combination of the correction methods available in the literature, i.e., classical and modern ones. The algorithm ensures applicability for small, medium, and large-sized wind turbines, as well as being fast and easy to implement in any computer and extendable even to turbines with a diffuser. This approach is validated by comparing the results with experimental data from four turbines of different power ranges (1.9 kW to 7.3 MW). The results show the best approximations for performance power curves against the measured values of all turbines. Moreover, it is effective, less complex, and quick in analyzing the performance of those turbines. Furthermore, the need for high-performance computers to analyze the performance of horizontal-axis turbines is avoided.

Isolated microgrids need dispatchable energy sources to ensure continuity of service during lack of the renewable energy sources. One common source is the diesel generator which is used as a backup and for battery charges. Practical experiences have shown that microgrids have operation problems when diesel generators and rectifiers work together to charge battery banks. This is because of resulting diesel generator voltage distortions caused by the interaction of line currents harmonics with equivalent series reactance of the AC source and the notches introduced during the current commutation between 2 SCRs of the bridge rectifier. These distortions eventually inhibit the rectifier operation, causing malfunctioning of the system. One solution is to use generators with rated power greater than the rectifiers, which results in oversized installation and high operating costs. This work proposes a solution to avoid generator oversizing by using resonant filters, notch filter and a capacitive reactive compensation circuit to avoid self-excitation of the generator. Soft-start operation of the rectifier, and disturbance decoupling in the generator are also proposed. The design allows the use of generators and rectifiers of similar power, reducing the installation and operation costs of microgrids. It has been used successfully in two real-life microgrids.