ISEL - Eng. Electrotécn. - Comunicações
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- Model predictive control of dual three-phase four-leg multilevel inverter supplying photovoltaic energy to low-voltage unbalanced gridsPublication . Monteiro, Joaquim; Pires, V. Fernão; Silva, J.; Fernando et al.; Monteiro, JoaquimIn this paper, a new control system is proposed for a grid-connected photovoltaic (PV) generation system based on a dual three-phase four leg multilevel inverter. This power converter topology, built using two four-leg two-level three-phase inverters, connects two PV arrays in the DC side to three open-end-windings of a three-phase transformer on the AC side. This arrangement is controlled to inject the PV generated power into the low voltage (LV) grid allowing support for unbalanced loads. The proposed control system is based in the model predictive control (MPC). It allows the control of the injected grid currents in balanced/unbalanced conditions while ensuring the balance of the capacitor DC voltages. Several simulation tests are made to validate the effectiveness of the control strategy applied to the proposed PV generation system.
- Improving the FMEA method to failure risk assessment of transformers: a case study in Tehran electric power distribution transformersPublication . Shafei, Atefeh Pour; Rezaei, Alireza; Silva, J. Fernando A.; Monteiro, J.; Monteiro, JoaquimDistribution transformers play a pivotal role in power networks, serving as critical assets whose failure can lead to substantial expenses for replacement or repair. This study employs an improved Failure Modes and Effects Analysis (FMEA) method, utilizing transformer failures within the Tehran Electric Power Distribution Company over a decade. The Risk Priority Number (RPN) criterion assesses the failure probability associated with various failure modes for quantization within the FMEA method. Risk prioritization is determined through the RPN formulation. This paper proposes new improvements in FMEA, including an expert-based weighting method applied to the weighted RPN's sub-parameters to enhance precision and calculate RPN values based on the age group of each transformer. Subsequently, strategic corrective actions are proposed and implemented to mitigate the risk of transformer failures in the upcoming year. The culmination of this case study results in improved prediction capabilities and more effective failure prevention strategies for the company's transformers, facilitating purposeful asset management.
- Dual three-phase four-leg multilevel inverter with backstepping prediction for unbalanced low-voltage gridsPublication . Monteiro, Joaquim; Pires, V. Fernão; Cordeiro, Armando; Pinto, Sónia; Silva, José Fernando; Monteiro, Joaquim; Cordeiro, ArmandoWind and solar renewable energy sources (RES) have been fundamental in reducing carbon emissions in electrical energy systems. However, when integrated into low voltage grids, some problems can arise due to the high number of single-phase loads. This situation often leads to unbalanced loads, which can result in increased losses or even under- or over-voltages in one of the phases. This issue is even more critical in the context of isolated microgrids. Therefore, this paper introduces a novel control system for a grid-connected photovoltaic (PV) generation with storage setup based on a dual three-phase four-leg multilevel inverter. This power converter topology, comprising two four-leg two-level three-phase inverters, connects two PV arrays to a three-phase transformer with four wires which in turn connects to a grid. This arrangement is designed to inject PV-generated power into the low voltage (LV) grid, providing support for unbalanced loads. A new backstepping method including the delay simplified dynamics is derived to estimate the optimum converter voltage and near optimum vector to control the load currents, while balancing the capacitor DC voltages. As only one voltage estimate is needed, a fast controller is obtained even if the converter has a high number of voltage vectors. The backstepping method increases the speed of choosing the converter vector, allowing the control of the grid currents injected under both balanced and unbalanced conditions, while also maintaining balance of the capacitor DC voltages in RES. Experimental tests were conducted using a laboratory prototype to evaluate the backstepping performance applied to the proposed PV generation system.
- Particle swarm optimization-based algorithm for optimal reactive power dispatchPublication . Menezes, R.; Fonte, Pedro M; Pestana, RuiOptimal Reactive Power Dispatch (ORPD) is very important for the security and economy of power systems. It is a mixed integer nonlinear optimization problem for which metaheuristic methods have proven to be effective in its solution. This paper presents an implementation of the particle swarm optimization algorithm (PSO) for the solution of the ORPD. Another approach, called Fitness-Distance Ratio PSO (FDR-PSO), is implemented to improve the results of the basic PSO. One other version called Second Order PSO (SO-PSO) is implemented for the same purpose. This second-order principle was combined with the FDR-PSO, making it the SO-FDR-PSO. These versions were tested on the IEEE 14 bus and IEEE 39 bus systems. The results show that the tested PSO algorithm and its improved versions can converge to good global optimal solutions (the best optimal solution the PSO could find), effectively solving the ORPD, with increasing degrees of success.
- ESS design and management considering solar PV to fed off-grid EV chargerPublication . Santos, Diogo; Fonte, Pedro M; Pereira, Rita Marcos Fontes Murta; Barata, Filipe; Almeida, Paulo; Cordeiro, Armando; Luís, Ricardo Jorge Ferreira; Fernao Pires, VitorThe increase of electric vehicles creates several challenges to the electric grid, mainly in those with weak power or off-grid. DC microgrids are becoming more and more important in the context of renewable energy sources, where solar PV systems are dominant. In this paper is proposed the design of a DC system to charge electric vehicles using PV generation and a battery storage system. A single DC-DC converter is used to operate the solar PV array with maximum power point tracking method and controls power flow from PV to storage battery and to the EV, operating as DC EV charger. The operation under various loading conditions is discussed. The performance of the proposed solution was simulated using MATLAB/Simulink software.
- Protection against direct atmospheric discharges and overvoltages in photovoltaic installationsPublication . Coelho, Gonçalo; Barata, Filipe; Elvas, Luís; Viveiros, CarlaThe increasing global energy demand has led to the expansion of photovoltaic (PV) power plants, particularly in response to climate goals like the European Union's "Fit for 55" initiative to reduce CO2 emissions by 55% by 2030. However, as PV installations grow, their vulnerability to atmospheric discharges, such as lightning, also increases. This study investigates the design and dimensioning of lightning protection systems for PV plants, focusing on two methods: traditional protection per IEC 62305 standards and ionizing devices based on NP 4426 standards. Both methods were analyzed for technical feasibility and cost-effectiveness. A case study of the PV installations at the Lisbon Engineering School (ISEL) was conducted, and a risk analysis determined that Level III protection was required. The study demonstrated that both methods provide effective protection, but ionizing devices offer a more cost-efficient solution, with lower material and labor costs. Consequently, the study showed that ionizing method is around 35% affordable than traditional rods for protecting PV plants in urban environment against atmospheric discharges.
- A single-switch DC-DC converter with a high voltage gain capability and reduced voltage stress of the switch for renewable energy applicationsPublication . Fernao Pires, Vitor; Cordeiro, Armando; Foito, Daniel; Monteiro, Joaquim; Silva, J. F.The growing trend toward high-efficiency, high gain converters that minimize component count, has spurred the development of many new step-up topologies. In this context introduces a new single-switch, non-isolated DC-DC step-up converter topology specifically designed for renewable energy applications. Key advantages of this topology include a high boost factor, continuous input current, reduced voltage stress on the switch, and a suitable number of elements relative to the achieved boost factor. These features address many of the critical design objectives. The steady-state analysis of the proposed converter is presented and discussed. Simulation and experimental results are presented to verify the theory and performance of the converter.
- Isolated quasi-switched boost integrated T-type DC-DC converter for DC microgridPublication . Ferreira, Daniel; Cordeiro, Armando; Rocha, Luís; Romero-Cadaval, Enrique; Roncero Clemente, Carlos; Silva, J. Fernando; Foito, Daniel; Martins, João F.; Fernao Pires, VitorOver the last few years, there has been a high demand for new or improved isolated DC-DC to fulfill requirements of applications related with RES. In this context, this paper introduces a new isolated DC-DC converter, for use in renewable energy applications. The topology proposed contains an Active quasi-switched Boost Three-Level T-type DC-DC converter with continuous input current at the converter input side, connected to a high-frequency step-up isolation transformer, and a voltage doubler rectifier (VDR) on the output side. The steady-state analysis of the proposed converter is presented and discussed. Simulation results are presented to verify the theory and performance of the converter.
- Transformerless ultra-high gain buck-boost DC-DC converter with single-switch of reduced voltage stressPublication . Cordeiro, Armando; Gamboa, Paulo; Luís, Ricardo; Fonte, Pedro M; Monteiro, Joaquim; Martins, João F.; Silva, J. Fernando; Foito, Daniel; Fernao Pires, VitorThis paper introduces a new DC-DC power converter topology capable of both step-up and step-down voltage conversion, with an exceptionally high voltage gain ratio.. Besides the high extension of the voltage gain range, the converter is also characterized by the use of a single switch. Moreover, the stress imposed on the switch's voltage is minimized, enabling the utilization of low-voltage, low RDS-ON MOSFETs. Consequently, this modification leads to reduced costs and losses associated with switch conduction and turn ON. Another aspect concerning the proposed converter is that the input current exhibits a continuous behavior, which can be significant for various applications. The paper provides insights into the operational performance, steady state behavior, and mathematical underpinnings of the proposed dc-dc converter. Comparative evaluation of the static voltage gain of the proposed converter and other topologies with comparable characteristics will also be shown. Verification of the presented converter's key features are conducted through both simulation and experimental assessments using a 440-W laboratory prototype. Through these analyses, the efficacy and viability of the modified coupled-inductor SEPIC converter with enhanced voltage gain capability are confirmed.
- ESS design and management considering solar PV to fed off-grid EV chargerPublication . Santos, Diogo; Fonte, Pedro M; Pereira, Rita; Barata, Filipe; Almeida, Paulo; Cordeiro, Armando; Luís, Ricardo; Fernao Pires, VitorThe increase of electric vehicles creates several challenges to the electric grid, mainly in those with weak power or off-grid. DC microgrids are becoming more and more important in the context of renewable energy sources, where solar PV systems are dominant. In this paper is proposed the design of a DC system to charge electric vehicles using PV generation and a battery storage system. A single DC-DC converter is used to operate the solar PV array with maximum power point tracking method and controls power flow from PV to storage battery and to the EV, operating as DC EV charger. The operation under various loading conditions is discussed. The performance of the proposed solution was simulated using MATLAB/Simulink software.