Francisco Kleber A. Lima

A family of turn-on and turn-off nondissipative passive snubbers for soft-switching single-phase rectifier with reduced conduction losses

This paper presents a family of nondissipative passive snubbers applied to the single-phase rectifier with reduced conduction losses, for applications in power supply for telecommunication and UPS. The family of lossless passive snubbers presented in this paper guarantees soft-commutation during the turn-on and the turn-off intervals of the switch. The snubber not only should slow the di/dt and dv/dt rates of the active switch, but also losslessly recover the energy stored in the zero-current inductor and in the zero-voltage capacitor, leading to reduced voltage stress across the switch and diodes. A methodology to generate the family of passive snubbers for the single-phase rectifier with reduced conduction losses is also presented. Simulation and experimental results for a single-phase rectifier 3000 W laboratory prototype using one snubber of the family are also presented.

Study of Passive Snubbers Applied to a Single-phase High Power Factor Rectifier

This paper presents a family of non-dissipative passive snubbers applied to a single-phase rectifier with unity input power factor. This single-phase rectifier presents reduced conduction losses because the current flows through two power semiconductors only, for any stage of operation. The family of lossless passive snubbers presented in this paper guarantees soft-commutation during the turn-on and the turn-off intervals of the switch within a large range of variation of the input current. The snubber circuit not only should slow the di/dt and dv/dt rates of the active switch, but also losslessly recover the energy stored in the zero-current inductor and in the zero-voltage capacitor, leading to reduced voltage stress across the switches and diodes. The association of the family of high performance snubber circuits here presented with the single-phase rectifier with reduced conduction losses results in a high efficiency rectifier, free from overvoltages across the power switches. To access the performance of the proposed family, simulation and experimental results for a single-phase rectifier 3000W laboratory prototype using one snubber of the family are also presented.

Unit power factor single-phase rectifier with reduced conduction loss using a non-dissipative passive snubber

This paper presents a single-phase rectifier with low conduction losses. To have a further reduction in the rectifier losses, a passive non- dissipative snubber is included in the rectifier. The snubber allows non-dissipative commutation at the switches within a large range of the input current. The snubber reduces the rate of the current grow during the switch turn on and also the rate of voltage grow during the switch turn off. The performance of the snubber circuit associated with the low conduction losses results in a rectifier with high efficiency. The design of a three KW rectifier is also presented and simulation results are shown to access the performance of the proposed rectifier.

Efficiency of a yeast-based formulation for the biocontrol of postharvest anthracnose of papayas

To identify formulations of biological agents that enable survival, stability and a good surface distribution of the antagonistic agent, studies that test different application vehicles are necessary. The efficiency of two killer yeasts, Wickerhamomyces anomalus (strain 422) and Meyerozyma guilliermondii (strain 443), associated with five different application vehicles, was assessed for the protection of postharvest papayas. In this study, after 90 days of incubation at 4oC, W. anomalus (strain 422) and M. guilliermondii (strain 443) were viable with all application vehicles tested. Fruits treated with different formulations (yeasts + application vehicles) had a decreased severity of disease (by at least 30%) compared with untreated fruits. The treatment with W. anomalus (strain 422) + 2% starch lowered disease occurrence by 48.3%. The most efficient treatments using M. guilliermondii (strain 443) were those with 2% gelatin or 2% liquid carnauba wax, both of which reduced anthracnose by 50% in postharvest papayas. Electron micrographs of the surface tissues of the treated fruits showed that all application vehicles provided excellent adhesion of the yeast to the surface. Formulations based on starch (2%), gelatin (2%) and carnauba wax (2%) were the most efficient at controlling fungal diseases in postharvest papayas.

Analysis, modelling, and simulation of droop control with virtual impedance loop applied to parallel UPS systems

This paper explores a control strategy for parallel uninterruptible power systems (UPS). The control technique used in that work was based on the droop control method. This method is usually applied to achieve good active and reactive power sharing when communication between the inverters is difficult due to its physical location. This paper has considered that the UPS systems there were no communication between their controls. A detailed mathematical model about the explored system is shown in that work and simulation results are presented in order to prove the theory presented.

Reactive power control of DFIG-based wind turbine during voltage sag

Improving the Fault Ride Through capability (FRT) of Doubly Fed Induction Generators (DFIG) in wind power applications is a very important challenge for wind power industry. The mathematical models of such generators allow to analyze their response under generic conditions. However, their mathematical complexity does not contribute to simplify the analysis of the system under transient conditions, and hence do not help to find straight-forward solutions to enhance their FRT. This paper discusses an important point that is still absent in the Brazilian grid code. This point is the reactive power injection by the wind turbine during a voltage sag, in order to realize the voltage regulation at the point of common coupling (PCC). The advantages of such requirement to the system will be widely discussed in this work. Furthermore, the simulations and experimental results will also be shown in order to demonstrate the proposed ideas.

A single-phase PLL algorithm with disturbances immunity

This paper proposes a new closed-loop synchronization algorithm, PLL (Phase-Locked Loop), for applications in power conditioner systems for single-phase networks. The structure presented is based on the correlation of the input signal with a complex signal generated from the use of an adaptive filter in a PLL algorithm in order to minimize the computational effort. Moreover, the adapted PLL presents a higher level of rejection for two particular disturbances: interharmonic and subharmonic, when compared to the original algorithm. Simulation and experimental results will be presented in order to prove the efficacy of the proposed adaptive algorithm.

FACTS applied to fixed speed wind turbines to meet grid code requirements

This paper presents a new proposal of combining a FACTS device with the fixed speed wind turbine (FSWT) to meet the restrictions found in upcoming grid codes. The applied FACTS device is based on the control strategy of the Dual Unified Power Quality Conditioner - the iUPQC. The power stage of the iUPQC consists of two converters connected back-to-back. One of them is connected in series and the other one is connected in parallel to the grid. In this work the proposed system will be tested under voltage sag conditions. Simulation results carried out in PSCAD/EMTDC are presented to show that this approach can fulfill the grid code requirements. The obtained advantages are the employing of lower power ratings converters while compared to the full scale power converter topology.

Contributions to the study of energy efficiency in dry-type transformer under nonlinear load

Additional losses in transformers due to harmonic currents and voltages cause further heating in their structural parts, and damage its suitable operation, thereby reducing their lifetime. This paper proposes an energy efficiency study in a three-phase dry-type transformer, which compares temperature measurements obtained from a 10kVA three-phase dry transformer on both sinusoidal and non-sinusoidal currents. It is also performed a comparison between experimental and simulation results obtained from a model 3D transformer developed in COMSOL Multiphysics software.

Simplified energy-efficient current control of the PMSG based WECS machine side converter

This work aims to model the machine side of a permanent magnet synchronous generator (PMSG) based wind energy conversion system (WECS), which makes use of full scale power electronic converter (PEC). In order to verify the behavior of the wind turbine control system, the current control of the machine side converter (MSC) is implemented in two different circuits: one with a classical three-phase three-level bidirectional inverter and the other arising from the substitution of each arm of the inverter by a controlled voltage source. Thus, the circuitry is simplified and the simulation time is reduced, without affecting the controllers, and is therefore a simplification option to be implemented in larger models, such as wind farms connected to the electric power system, besides being useful for the design of other current controllers and current signals filters.