Browsing by Author "Silva, J. Fernando"
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- Backstepping predictive control of hybrid microgrids interconnected by neutral point clamped convertersPublication . Barros, J. Dionísio; Rocha, Luis Lamy; Silva, J. FernandoIn this work, DC and AC parts of hybrid microgrids are interconnected by a neutral point clamped-NPC converter controlled using a new backstepping predictive (BP) method. The NPC converter is controlled to operate in the DC microgrid voltage control mode or in the AC microgrid power control mode. The novel backstepping predictive controller is designed using the dq state space dynamic model of the NPC converter connected to the hybrid microgrid. The designed BP controller regulates the DC voltage or AC injected power, balances the capacitor voltages, controls the AC currents, and enforces the near unity power factor. Simulation (MATLAB/Simulink) and experimental (laboratory prototype) results show that the converter can regulate the DC voltage in the DC microgrid interconnection point, by adjusting the AC power conversion to compensate variations on the loads or on the distributed renewable energy sources in the DC microgrid. AC currents are sinusoidal with low harmonic distortion. The obtained BP controller is faster at balancing capacitor voltages than PWM (pulse width modulation) control with carrier offset. The fast AC power response allows the converter to be used as a primary frequency regulator of the AC microgrid. This research is appropriate for power and voltage control in hybrid microgrids with renewable energy.
- Control of bidirectional quadratic DC-DC converters for storage support of DC power gridsPublication . Fernao Pires, Vitor; Cordeiro, Armando; Foito, Daniel; Silva, J. FernandoMany applications require a DC bus supporting connections to several renewable energy sources, storage systems and loads. However, due to the intermittent nature of renewable energy sources and load variations it is essential to stabilize the voltage of the DC bus. Usually a battery is used to support the DC bus voltage, but their continuous charge and discharge cycling will affect his lifetime. Thus, the use of a storage system based in supercapacitors provides an interesting alternative to stabilize the DC power grid. In this context, this work presents a study of a storage system based in supercapacitors combined with a bidirectional quadratic DC-DC converter to support the DC bus. A full design of the controllers for the quadratic converter is also presented. The behaviour of the system will be tested through several simulation results.
- Control of PV distributed systems based on three-phase triple inverters to support grids with unbalanced loadsPublication . Fernão Pires, V.; Foito, Daniel; Cordeiro, Armando; Silva, J. FernandoThis paper is focused on the control system for photovoltaic (PV) generators using three-phase triple inverters. These inverters allow for a distributed PV system, and at the same time, they provide AC multilevel operation. The proposed controller is designed to allow the operation of the inverter in a way that it can extend support to the grid with ancillary services. One of the ancillary services that the three-phase triple inverter can support is the injection of unbalanced currents in order to eliminate or attenuate grid load unbalances. Therefore, the proposed topology will be extended to accommodate four wires. The capability of the system to support the grid with ancillary services, especially the compensation of unbalanced loads, the four-wire multilevel topology operation and the proposed controller will be verified through several simulation tests.
- Control transition mode from voltage control to MPPT for PV generators in isolated DC microgridsPublication . Fernao Pires, Vitor; Cordeiro, Armando; Foito, Daniel; Silva, J. FernandoThe integration of photovoltaic (PV) power sources on a large scale into grid-tied and islanded solutions is now a reality. Despite the fluctuating characteristics of solar energy, the direct current (DC) nature of PV sources is highly compatible with DC microgrids compared with AC solutions. However, as more PV sources are being installed, challenges related to the control of DC microgrids are also increasing, especially for islanded applications. To ensure an appropriate transition from the maximum power point to DC bus voltage regulation of DC microgrids, novel control strategies must be designed, and this aspect is particularly critical when these grids operate without energy storage systems. We therefore propose a new control strategy for a PV generator for islanded DC microgrids without storage systems. Our approach allows for smooth switching from the maximum power point mode to the voltage control regulation mode (V control) when the available PV power is excessive, thus limiting the generated PV power and returning to maximum power point operation when all the available power is required. This allows us to control a DC microgrid without a storage system and provides a new alternative that offers a cost-effective solution for rural and deprived regions. Our solution combines the maximum power point tracking algorithm with a proportional integral compensator for V control and minimises undervoltage/overvoltage problems in the DC bus. The behaviour of the isolated DC microgrid under different conditions, for example in the case of transitions, perturbations, and different types of loads, is also studied and verified. The performance of the proposed control strategy is confirmed by several simulation and experimental tests.
- Design of single phase SiC bidirectional DC-AC converter with low-cost PLL for power factor correctionPublication . Bento, Alexandre; André, Sérgio; Luís, Ricardo; Silva, J. FernandoThe paper presents the design stages of a single-phase Silicon Carbide bidirectional DC-AC converter. This includes the LCL filter design responsible to meet grid connection requirements. A 3kW laboratory prototype of the power converter is built employing a low-cost phase locked loop and its results are presented. The design of the low-cost phase locked loop and its implementation are depicted in some detail.
- Diagnosing Power Transistor Faults in Multilevel T-Type Based Nine Switch Inverter Using Center of Mass IndexesPublication . Monteiro, Joaquim; Amaral, Tito; Silva, J. Fernando; Pinto, Sonia; Fernao Pires, VitorNine-switch voltage source inverters (NSVSI) are DC-AC converters that utilize a reduced number of switches, making them advantageous for dual or six-phase motors. To enhance the quality of the output voltage and provide fault tolerant capability, NSVSI topologies, like the T-Type-based NSVSI, have been modified to operate as multilevel converters. However, to ensure fault-tolerant capability, a fault diagnosis algorithm for power transistors must be developed. Therefore, this paper proposes a novel fault detection and diagnosis algorithm to identify faulty transistors in a multilevel T-Type-based NSVSI. This method is based on the development of specific indices derived from the center of mass of the output currents. The proposed technique offers a fast and reliable solution, demonstrating robustness under various load conditions. The effectiveness of this method will be validated through a series of simulation tests.
- Direct Power Control of Matrix Converter Based Unified Power Flow ControllersPublication . Monteiro, Joaquim; Silva, J. Fernando; Pinto, S. F.; Palma, João Carlos Pires daThis paper presents the Direct Power Control of Three-Phase Matrix Converters (DPC-MC) operating as Unified Power Flow Controllers (UPFC). Since matrix converters allow direct AC/AC power conversion without intermediate energy storage link, the resulting UPFC has reduced volume and cost, together with higher reliability. Theoretical principles of DPC-MC method are established based on an UPFC model, together with a new direct power control approach based on sliding mode control techniques. As a result, active and reactive power can be directly controlled by selection of an appropriate switching state of matrix converter. This new direct power control approach associated to matrix converters technology guarantees decoupled active and reactive power control, zero error tracking, fast response times and timely control actions. Simulation results show good performance of the proposed system.
- Dual inverter with a floating bridge controlled by a vectorial voltage modulatorPublication . Fernao Pires, Vitor; Cordeiro, Armando; Silva, J. FernandoThis paper presents a new control strategy based on a voltage modulator for a dual inverter with a floating bridge. This converter is characterized by a multilevel operation with two conventional two-level voltage source inverters and a single DC voltage source. The proposed control and modulation scheme allows a fast and robust operation of the inverter. The voltage modulator will allow the regulation and stability of the voltage capacitor of the floating bridge. It was also developed to provide an asymmetrical operation of the DC voltages (single DC source and capacitor). Due to this, high output voltage levels will be ensured. Several simulation results will be provided to show the characteristics of the proposed system.
- Fault-tolerant design of three-phase dual-buck VSI topologiesPublication . Cordeiro, Armando; Fernao Pires, Vitor; Silva, J. FernandoThis paper presents fault tolerant topologies for the three-phase dual-buck voltage-source inverter (VSI). Using the proposed topologies, increased reliability is obtained, which is fundamental in many safety-critical applications. The solutions intend to achieve fast changing between main and redundant branches through the combination of several power devices. The present study focuses on solutions that can be used to replace the classical two-level three-phase voltage-source inverters to extend the fault-tolerant capacity. Several aspects of failure modes, detection and isolation processes within power electronic converters are discussed regarding the requirements of safety related applications. The theoretical validity of the proposed solution is confirmed by simulation results.
- High-frequency transformer isolated AC-DC converter for resilient low voltage DC residential gridsPublication . Santos, Nelson; Silva, J. Fernando; Soares, VascoIn a global and growing society, it is necessary to rethink strategies in order to minimize the environmental impact resulting from the progressive increase of energy consumption and the misuse of energy resources. Energy power system converters must be considered as a global way to spare most of the wasted energy. Today, in the power distribution infrastructure, including modern residential buildings, most equipment have power supplies with imbedded AC-DC power converters which may have overall losses as high as 25% regarding the equipment output. Therefore, common DC buses for residential applications are being studied to increase equipment efficiency. This paper presents and designs an AC-DC isolated converter that uses a full-bridge matrix topology with high-frequency isolation transformer and non-electrolytic capacitors to integrate into the future residential buildings DC bus, presenting a reliable alternative to AC power. Non-linear control techniques (sliding mode control and backstepping control) are employed to guarantee stability and disturbance robustness to the output DC low voltage, while enforcing sinusoidal input AC current and power factor correction. Control strategies are described and simulation results are presented and discussed.
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