Reconstruction of the Free Energy in the Metastable Region using the Path Ensemble

By quenching into the metastable region of the three-dimensional Ising model, we investigate the paths that the magnetization (energy) takes as a function of time. We accumulate the magnetization (energy) paths into time-dependent distributions from which we reconstruct the free energy as a function of the magnetic field, temperature and system size. From the reconstructed free energy, we obtain the free energy barrier that is associated with the transition from a metastable state to the stable equilibrium state. Although mean-field theory predicts a sharp transition between the metastable and the unstable region where the free energy barrier is zero, the results for the nearest-neighbour Ising model show that the free energy barrier does not go zero

A Skewness Based Method for Diagnosis in Quasi-Z T-Type Grid-Connected Converters

Trabalho apresentado em 8th International Conference on Renewable Energy Research and Applications, 3-6 novembro 2019, Brasov, RomeniaOne of the very interesting converters for transformerless grid-connected PV systems is the multilevel quasi-Z T-Type inverter. Besides their multilevel operation, it presents voltage Boost capability and robustness to instantaneous switching between power semiconductors of the same leg and capability to work in fault tolerant condition. However, to ensure this capability, it is essential to ensure a fast and correct diagnosis of the component under fault. Thus, this paper proposes a new method for the detection of an open switch fault. The proposed approach uses the inverter AC line currents that will be analyzed by a Skewness based method. The method has the capability to distinguish the switch under fault. Besides that, it presents robustness to interferences such as noise, as well is insensible to variations of the injected power into the grid. Several simulation and experimental results will be presented in order to verify the capability of the proposed method.N/

Power Electronics Didactic Modules for Direct Current Machine Control

Several didactic modules for an electric machinery laboratory are presented. The modules are dedicated for DC machinery control and get their characteristic curves. The didactic modules have a front panel with power and signal connectors and can be configurable for any DC motor type. The three-phase bridge inverter proposed is one of the most popular topologies and is commercially available in power package modules. The control techniques and power drives were designed to satisfy static and dynamic performance of DC machines. Each power section is internally self-protected against misconnections and short-circuits. Isolated output signals of current and voltage measurements are also provided, adding versatility for use either in didactic or research applications. The implementation of such modules allowed experimental confirmation of the expected performance

A Water Pumping Photovoltaic Powered System Based on a Merged DC-DC Sepic-Cuk Converter

Trabalho apresentado em 2019 8th International Conference on Renewable Energy Research and Applications (ICRERA), 3-6 novembro 2019, Brasov, RomeniaThis paper has the purpose to present a non-isolated dual output DC-DC converter for a water pumping photovoltaic powered system. The proposed topology is based in the integration of a traditional Sepic and Cuk converters but requires only a single power semiconductor switch. It is characterized by an extension of the voltage static gain when compared with the classical boost topology and by a reduced voltage stress across the power switch and diodes. The system will be implemented through a classical MPPT algorithm that will control the proposed DC-DC converter. Several simulation and experimental results are also presented in order to confirm the characteristics of the proposed power converter and global water pumping systems.N/

Modbus Protocol as Gateway between different Fieldbus devices: a Didactic Approach

Trabalho apresentado na 20th International Conference on Interactive Collaborative Learning, 27-29 setembro de 2017, Budapeste, HungriaThis paper is dedicated to improve the skills of Electric Engineering students about Industrial Automation. This is done creating a set of practical exercises in the laboratory that allow students to interact with didactic and industrial equipment understanding the compatibility problems and give students the tools, the knowledge and even some creative freedom to solve problems that happens in real situations. In this case, it was focuses in the interconnection of equipment from different manufacturers using the Modbus protocol. To allow to the students this practical study, it was developed a solution with a Modbus RTU protocol directly implemented in program memory of both S7-200 (using the Freeport configuration mode) and S7- 300 PLCs. The solution allows a gateway between Modbus-TCP (over a Master TSX PLC from Schneider-Electric) and Profibus-DP (over a Master S7-300 PLC from Siemens).N/

Fault-tolerant multilevel converter to feed a switched reluctance machine

The switched reluctance machine (SRM) is one of the most interesting machines, being adopted for many applications. However, this machine requires a power electronic converter that usually is the most fragile element of the system. Thus, in order to ensure high reliability for this system, it is fundamental to design a power electronic converter with fault-tolerant capability. In this context, a new solution is proposed to give this capability to the system. This converter was designed with the purpose to ensure fault-tolerant capability to two types of switch faults, namely open- and short-circuit. Moreover, apart from this feature, the proposed topology is characterized by a multilevel operation that allows improvement of the performance of the SRM, taking into consideration a wide speed range. Although the proposed solution is presented for an 8/6 SRM, it can be used for other configurations. The operation of the proposed topology will be described for the two modes, fault-tolerant and normal operation. Another aspect that is addressed in this paper is the proposal of fault detection and diagnosis method for this fault-tolerant inverter. It was specifically developed for a multilevel SRM drive. The theoretical assumptions will be verified through two different types of tests, firstly by simulation and secondly by experiments with a laboratory prototype.info:eu-repo/semantics/publishedVersio

A Sensorless PMDC Motor Speed Controller with a Logical Overcurrent Protection

A method to control the speed or the torque of a permanent-magnet direct current motor is presented. The rotor speed and the external torque estimation are simultaneously provided by appropriate observers. The sensorless control scheme is based on current measurement and switching states of power devices. The observer’s performances are dependent on the accurate machine parameters knowledge. Sliding mode control approach was adopted for drive control, providing the suitable switching states to the chopper power devices. Despite the predictable chattering, a convenient first order switching function was considered enough to define the sliding surface and to correspond with the desired control specifications and drive performance. The experimental implementation was supported on a single dsPIC and the controller includes a logic overcurrent protection

PV generator-fed water pumping system based on a SRM with a multilevel fault-tolerant converter

This paper presents a pumping system supplied by a PV generator that is based on a switched reluctance machine (SRM). Water pumping systems are fundamental in many applications. Most of them can be used only during the day; therefore, they are highly recommended for use with PV generators. For the interface between the PV panels and the motor, a new multilevel converter is proposed. This converter is designed in order to ensure fault-tolerant capability for open switch faults. The converter is based on two three-level inverters, with some extra switches. Moreover, to reduce the number of switches, the converter is designed to provide inverse currents in the motor windings. Due to the characteristics of this motor, the inverse currents do not change the torque direction. In this way, it was possible to obtain an SRM drive with fault-tolerant capability for transistor faults; it is also a low-cost solution, due to the reduced number of switches and drives. These characteristics of fault-tolerant capability and low cost are important in applications such as water pumping systems supplied by PV generators. The proposed system was verified by several tests that were carried out by a simulation program. The experimental results, obtained from a laboratory prototype, are also presented, with the purpose of validating the simulation tests.info:eu-repo/semantics/publishedVersio

DC Microgrids: Benefits, Architectures, Perspectives and Challenges

One of the major paradigm shifts that will be predictably observed in the energy mix is related to distribution networks. Until now, this type of electrical grid was characterized by an AC transmission. However, a new concept is emerging, as the electrical distribution networks characterized by DC transmission are beginning to be considered as a promising solution due to technological advances. In fact, we are now witnessing a proliferation of DC equipment associated with renewable energy sources, storage systems and loads. Thus, such equipment is beginning to be considered in different contexts. In this way, taking into consideration the requirement for the fast integration of this equipment into the existing electrical network, DC networks have started to become important. On the other hand, the importance of the development of these DC networks is not only due to the fact that the amount of DC equipment is becoming huge. When compared with the classical AC transmission systems, the DC networks are considered more efficient and reliable, not having any issues regarding the reactive power and frequency control and synchronization. Although much research work has been conducted, several technical aspects have not yet been defined as standard. This uncertainty is still an obstacle to a faster transition to this type of network. There are also other aspects that still need to be a focus of study and research in order to allow this technology to become a day-to-day solution. Finally, there are also many applications in which this kind of DC microgrid can be used, but they have still not been addressed. Thus, all these aspects are considered important challenges that need to be tackled. In this context, this paper presents an overview of the existing and possible solutions for this type of microgrid, as well as the challenges that need to be faced now.info:eu-repo/semantics/publishedVersio

Testing System for PV Grid-Connected Power Condition Systems with Support for Ancillary Services

Funding Information: This work was supported by national funds through FCT—Fundação para a Ciência e a Tecnologia with reference UID/CEC/50021/2020 and UID/EEA/00066/2020. Publisher Copyright: © 2023 by the authors.Due to the high spread of photovoltaic (PV) systems in the low voltage distribution grids there is a substantial number of requirements for the connection of these systems. Therefore, several tests should be performed before the integration of the PV systems into the grid. Moreover, new requirements have been established that are most likely to be implemented in the near future. To provide these tests and verify if PV systems are in accordance with requirements and recommendations, a testing setup for the PV systems was developed. This testing system consists of a controllable power source prepared to receive energy and to inject it into the grid. In fact, that system imposes a controllable voltage, in amplitude and frequency, to the PV system to simulate the perturbations of the grid. Since the system under test must inject energy in accordance with the standards’ specifications, then the controllable power source that emulates the grid must receive that energy. Moreover, it should also be prepared for PV systems that are able to provide ancillary services, including new ones that support imbalanced networks. A fast and robust control system will be used for this controllable power source. Several experimental tests from the developed prototype are presented.publishersversionpublishe