Due to high mechanical inertia and rapid variations in wind speed over time, at variable wind speeds, the problem of operation in the optimal energetic area becomes complex and in due time it is not always solvable. No work has been found that analyzes the energy-optimal operation of a wind system operating at variable wind speeds over time and that considers the variation of the wind speed over time. In this paper, we take into account the evolution of wind speed over time and its measurement with a low-power turbine, which operates with no load at the mechanical angular velocity ω_MAX. The optimal velocity is calculated. The energy that is captured by the wind turbine significantly depends on the mechanical angular velocity. In order to perform a function in the maximum power point (MPP) power point area, the load on the electric generator is changed, and the optimum mechanical velocity is estimated, ω_OPTIM, knowing that the ratio ω_OPTIM/ω_MAX does not depend on the time variation of the wind speed.

At the beginning, this article details the manufacturing procedures for varistor materials. Starting from the initial composition of two large series of varistors (those with two additive oxides and those with five additive oxides), there is a major overview of the main stages of the technological process and the equipment used for the production, emphasizing the technological changes that were made. The article continues with the study of the influence of the sintering pressure and the sintering temperature on the electrical properties of the varistor materials made before. There were two experimental series of 7 varistors, one series based on 2 additive oxides and one based on 5 additive oxides. Each varistor of these series was sintered at another temperature, the fundamental purpose being to determine an optimal sintering temperature for each chemical composition. A second activity consisted of manufacturing two more series of varistors with the same chemical composition (2 oxides and 5 additive oxides), which were sintered at two different pressures, for having a set of conclusions on the influence of sintering pressure on the electric performances. All conclusions are underlying a new process for manufacturing metal oxide based varistors.

This paper proposes two low-cost ferrite V permanent magnet assisted synchronous reluctance rotors (VPMSynRM) as an alternative solution to the rare earth inserted tangential permanent magnet synchronous motor (PMSM). Finite element (FEM) approach has been utilized to show the performance of the proposed rotors. The purpose of the proposed solution is to replace only the rotor of such a PMSM that is already in production, so the size of the stator and the number of windings will remain unchanged. The motivation of this study is to reduce the cost of the rotor by changing the type of PM and by the choice of suitable rotor geometry.

The wind turbine’s operation is affected by the wind speed variations, which cannot be followed by the wind turbine due to the large moment of the power plant’s inertia. The method proposed in this paper belongs to the wind turbine power curves (WTPC) approach, which expresses the power curve of the permanent magnet synchronous generator (PMSG) by a set of mathematical equations. The WTPC research papers published before now have not taken into consideration the total power plant inertia at time-variable wind speeds, when the wind turbine’s optimal operation is very difficult to be reached, and its efficiency is thus threatened. The study is based on a wind turbine having a large moment of total inertia, and demonstrates, through extensive simulation results, that the optimal values of the PMSG’s power can be determined based on the kinetic motion equation. This PMSG’s optimal power represents an ideal time-varying curve, and the wind turbine should be controlled so as to closely follow it. For this purpose, proportional integral (PI) and proportional integral derivative (PID) type-based control methods were implemented and analyzed, so that the PMSG’s power oscillations could be reduced, and the PMSG’s angular speed value made comparable to the optimal one, meaning that the wind turbine operates within the optimal operation area, and is efficient. The simulations are actually the numerical solutions obtained by using the Scientific Workplace simulation environment, and they are based on the wind speed measurements collected from a wind farm located in Dobrogea, Romania.

In this paper, the operation of the wind power plants at small values of the wind speed is analyzed. The values of the wind turbine's delivered power are determined based on the experimental data from a wind farm located in Galbiori - Dobrogea, in order to observe, to what extent, its operation is in the optimum area from the point of view of the captured energy. The possibility of capturing the maximum wind energy existent at a certain moment is investigated. The time-evolution of the wind turbine's delivered power, at small wind speed values is also analyzed.

Efficiency improving methods, while maintaining the unity power factor, represent a major research topic in the PFC rectifiers' area. This paper introduces three novel highly efficient three-phase interleaved buck-type unity power factor rectifiers, namely the interleaved six-switch, the interleaved Swiss and the interleaved eight-switch topologies. Compared to the switching strategy of the single-stage topologies, the interleaved switching strategy enables cancellation of the AC-side input current harmonics and reduces the DC-side voltage ripple. Performances of the proposed three interleaved PFC rectifiers are evaluated for various power ranges, based on simulation studies performed in GECKO Circuits environment. Efficiency evaluation for all three PFC rectifiers is also provided. According to the results obtained, for the novel claimed topologies, higher power efficiency than for the single-stage PFC rectifier solutions they are based on is assured. The IEC 6000-3-2 standard requirements are met by all of them.