Self-starting Condition Research Articles

3D numerical simulation of the Darrieus vertical axis wind turbine with J-type and straight blades under various operating conditions including self-starting mode

A comprehensive three-dimensional numerical analysis is applied in this study to evaluate the competence of the J-type blades in the vertical axis wind turbines under wide range of operating conditions including various wind speeds, tip speed ratios and wind turbulence intensities. It is found that the vortices which are encapsulated in the concave part of the J-type blades escape from the tips of the blades and become stronger than those originated from the straight blades. Such strong wakes behind the J-type blades are responsible for larger negative effects on generating the torque at high tip speed ratios. Neither of the wind speed, nor the wind turbulence intensity have significant effects on performance of the J-type blade turbines. Of particular interest is to discover the behavior of the J-type blade under self-starting conditions which is a major shortcoming with Darrieus turbines. The results show that replacing the straight blades with the J-type blades causes the generated torque to increase by 26.9% and 37.6% at the wind speeds of 10 m/s and 5 m/s, respectively. Better performance of the J-type blades under self-starting conditions, makes them a promising candidate, especially at low wind speed regions such as urban areas.

On ensuring dynamic voltage stability of critical loads in the electric power systems of industrial facilities

Voltage sags and interruptions in the electric power systems (internal grids) of industrial facilities are caused by various short circuits, including those that occur in external grids. If the depth and duration of such sags and interruptions exceed the rated limits for critical loads, they cause continuous processes to shut down, which produces defective products and entails substantial losses. Russian standards and regulations do not set forth requirements with respect to voltage sags and interruptions, which means that industrial companies cannot sue grid operators for damages. This is why dynamic voltage stability of critical loads involved in continuous processes has to be addressed within the internal grid. They analyze the effectiveness of solutions designed for dynamic voltage stability of critical loads; these can be shared, used on a single site or designed to power secondary circuits. It shows that use of dynamic voltage restorers (DVR) and dynamic voltage sag correctors (DVSC) can be a cost- effective solution if the topology and specifications are configured appropriately. The paper presents general technical requirements for sag and interruption compensators, which are adjusted for cost-effectiveness. Case-specific feasibility testing should be based on daily readings of electrical parameters and electric power quality readings, calculated electromechanical transients, and a feasibility study. Use of DVRs at an industrial facility helped stabilize the voltage curve and reduce the span of irregular voltage fluctuations by a factor of 3, reduce the average voltage at the 6 kV busbars without compromising equipment performance, and improve the starting conditions for AM clusters and self-starting conditions for large AMs. Thus, lowering the elevated voltage at the 6 kV busbars helped cut electricity purchase costs. In most cases where accidents cause disturbances in the external 110 kV grid, DVRs coupled with the automatic voltage controls of power transformers were able to prevent process shutdowns, which constituted the key cost effect.

Experimental Study Wind Turbine Performance of Straight-Savonius and Ice-Wind Type on the Similar proportion Aspect Ratio

The Performance of wind turbines at low speed can be improved by Ice-Wind model, particularly in self-starting conditions. Compared to a traditional wind turbine with two blades of the similar area and material, Ice-Wind can increase efficiency by 19%. Research on the Savonius turbine, particularly the Ice-Wind turbine, is challenging. It is because it has many restrictive parameters, such as the height, diameter, and area of the turbine blades. The Ice-Wind turbine shape is obtained by cutting a Savonius turbine. This process led to research on Ice-Wind turbines only under the similar parameters. The aspect ratio of a Savonius turbine has a significant effect on the speed, mechanical power and static-torque produced by the wind turbine. The research was done on Savonius and Ice-Wind turbines with the similar aspect ratio. The results show that the speed, power factor and efficiency of the Savonius turbine are higher than those of Ice-Wind. However, Savonius produces a smaller static-torque coefficient value than Ice-Wind. The results of this research contrast with other studies comparing Savonius and Ice-Wind turbines. In other researches, Savonius and Ice-Wind turbines have the similar area but different aspect ratios.

Sustainable operation analysis of the mining industry power supply system

The paper presents an analysis of operating currents and torques during self-starting of motors with a voltage of 6 kV for cases of short circuit in the head feeders from the main step-down substation and in the feeders to the most distant substations of mining enterprises. For the network under consideration, the characteristic modes are: short circuit on the buses of shop substations. For the mining industry when selfstarting asynchronous motors with a squirrel-cage rotor and synchronous motors with asynchronous startup, the current multiplicity will be determined by the duration of the power failure (run-down duration). It has been established that the acceleration of motors during self-start will be somewhat slower than the usual start, since self-start begins at a speed of 0.89–0.94 of the synchronous speed and a resistance torque is equal to 0.65–0.75 of the maximum motor torque. The self-starting time of the motors is from 9 s to 42 s. To ensure self-starting conditions in the most severe modes in mountain conditions, it is necessary to make changes to the protection settings presented in the work.

Dynamics of passively mode-locked lasers with saturable absorber and saturable nonlinearity

The dynamics of a model of passively mode-locked laser with saturable absorber, the optical amplifier of which possesses a saturable nonlinearity, is considered with interest in both continuous wave and pulse regimes of operation. The study rests on the laser self-starting picture which assumes that the laser should operate instantly in the pulse regime, when continuous waves become unstable in the system. Within the framework of the modulational-instability analysis, a global map for the laser self-starting conditions is constructed in terms of a two-dimensional complex parameter space, mapped by the real and imaginary parts of the modulation gain over a broad range of values of the modulation frequency. The map suggests that the saturable nonlinearity lowers the threshold value of the input intensity required for laser self-starting, analytical expression for this threshold input field is derived in the particular case of a zero modulation frequency. Treating the system dynamics in the full nonlinear regime using numerical simulations, time series of the laser amplitude and instantaneous phase, as well as of the gain, are obtained and their changes with the variation of the homogeneous gain are examined. It is found that a relatively small value of the equilibrium gain will favor gain decrease, and pulse generation and buildup into either simple-periodic or multi-periodic pulse trains, depending on the magnitude of the saturable nonlinearity coefficient.

Study of Self-starting High-voltage Variable Frequency Electric Drives

The paper compares transient processes of self-starting non-controlled and variable frequency electric drives (VFED) at short-term power supply interruptions. It offers simplified dependencies for calculating duration of transient processes of the individual self-starting asynchronous motor. The study defines reasons for increasing speed recovery time of this assembly. It distinguishes main differences of self-starting the variable frequency and non-controlled electric drives with or without the Flying Start function in the algorithms of the frequency converter. Results of the experimental studies of self-starting high-voltage VFED with the Power Flex 7000 frequency converter are presented. The paper proposes guidelines to provide continuous operation of the essential units at the power supply interruptions. It also emphasizes developments aimed to improve VFED self-starting conditions.

Optimal blade shape of a modified Savonius turbine using an obstacle shielding the returning blade

Due to the worldwide energy crisis, research and development activities in the field of renewable energy have been considerably increased in many countries. Wind energy is becoming particularly important. Although considerable progress have already been achieved, the available technical design is not yet adequate to develop reliable wind energy converters for conditions corresponding to low wind speeds and urban areas. The Savonius turbine appears to be particularly promising for such conditions, but suffers from a poor efficiency. The present study considers a considerably improved design in order to increase the output power of a classical Savonius turbine. In previous works, the efficiency of the classical Savonius turbine has been increased by placing in an optimal manner an obstacle plate shielding the returning blade. The present study now aims at improving further the output power of the Savonius turbine as well as the static torque, which measures the self-starting capability of the turbine. In order to achieve both objectives, the geometry of the blade shape (skeleton line) is now optimized in presence of the obstacle plate. Six free parameters are considered in this optimization process, realized by coupling an in-house optimization library (OPAL, relying in the present case on Evolutionary Algorithms) with an industrial flow simulation code (ANSYS-Fluent). The target function is the output power coefficient. Compared to a standard Savonius turbine, a relative increase of the power output coefficient by almost 40% is finally obtained at λ = 0.7. The performance increase exceeds 30% throughout the useful operating range. Finally, the static torque is investigated and found to be positive at any angle, high enough to obtain self-starting conditions.

Inhibition of modulation instability in lasers by noise.

It is shown that additive noise can inhibit modulation instability in laser equations of motion. A related self-starting condition for pulsation is obtained by employing a fluctuation--dissipation relation between noise and losses and a statistical mechanics approach. Entropy considerations are shown to play a crucial role. The quantum limit for self-starting is estimated.

Self-starting laser oscillator with a nonlinear nematic liquid-crystal mirror

A novel laser oscillator with a cavity completed by the refractive-index grating induced by generating beams in a nematic liquid-crystal cell is studied experimentally and theoretically. It is shown that both the thermal and orientational nonlinearity of the liquid crystal can provide the holographic mirror formation and the self-starting condition in the laser oscillator. The self-starting laser with a nonlinear mirror demonstrates a single-longitudinal-mode generation with good beam quality.

Differential gain and buildup dynamics of self-starting Kerr lens mode-locked Ti:sapphire laser without an internal aperture

The small-signal differential gain was calculated from the measured average cw and Kerr lens mode-locked (KLM) output powers over the least-misalignment-sensitive region of a completely soft-aperturing, self-starting KLM Ti:sapphire laser. The results indicate that the self-starting condition is reached with this laser operating near the center of the stable range. The simulated location of maximum differential gain and pulse buildup process show good agreement with the experimental results.

Influence of diffraction on hard-aperture Kerr-lens mode locking

It is demonstrated that with rigorous inclusion of diffraction on an intracavity hard aperture the values of the nonlinear differential loss coefficient Gamma (which characterizes the efficiency of Kerr-lens mode locking) are several times higher than in the commonly used simplified approach in which diffraction effects are neglected. Diffraction changes modal properties and causes a faster decrease of mode losses with increasing power than predicted by the traditional approach. With this correction, the discrepancy between the Kerr-lens mode-locking theory and the theoretical estimates of the self-starting condition is substantially decreased.

Effects of dynamic gain saturation on the self-starting condition for additive-pulse mode locking

We investigated the self-starting condition for additive-pulse mode locking in an arc-lamp-pumped cw Nd:YLF laser. We measured the first beat note of the free-running laser to estimate the cavity correlation time. We supplement the theoretical model for the self-starting condition, which was previously given in terms of the cavity correlation time and the critical buildup time, by including the effect of dynamic gain saturation. This modified model is compared with the experimental results.

Starting dynamic, self-starting condition and mode-locking threshold in passive, coupled-cavity or Kerr-lens mode-locked solid-state lasers

The starting dynamic of mode-locked solid-state lasers is analyzed taking into account the gain reduction by the total random radiation. A simple self-starting condition is derived, depending on the pump-rate, the gain cross section, the nonlinear coefficient, the gain bandwidth and the resonator length. It is shown that an additional slow saturable absorber can greatly improve the self-starting conditions. Experimental results are in good agreement with the predictions of the theory.

Analysis of a passively mode-locked self-starting all-fiber soliton laser

Modeling results are presented for a passively mode-locked figure-eight laser (F8L) supporting a linear polarization state. Starting from amplified spontaneous emission, the model produces soliton, chirped, red-shifted, ultra-short pulses. The self-starting conditions of the laser are investigated, and the erbium-doped fiber amplifier gain threshold to passively mode-lock the laser is determined as a function of the center coupler splitting ratio (C/sub r/). The influence of C/sub r/, as well as the effects of the cavity length (L/sub c/), first- and second-order group velocity dispersion to the F8L operating, parameters, and output pulse characteristics such as width, peak power, chirp, self-frequency shift and envelope distortion, are studied. >

Analysis of passive additive-pulse mode locking with eigenmode theory

The equation of motion for the coupled cavities of additive-pulse mode locking is solved by the eigenvalue method. The pulse evolution and self-starting condition for passive operation is analyzed. It is shown that the laser configuration is equivalent to an intracavity interferometer, and a phase modulation across the pulse is the essential element for mode locking. With the method developed, transient pulse evolution of an initial seed pulse can be calculated and thereby optimized. It is found that the pulse shortening rate is linear with respect to the number of round trips. The duration for the initial pulse to shorten to a steady state is proportional to its initial width and inversely proportional to the nonlinear coefficient. >

Self-starting of passively mode-locked lasers

The self-starting condition of passively mode-locked lasers, including those locked by additive-pulse mode locking, is reexamined. A quantitative model is developed for a threshold intensity for self-starting based on the hypothesis that spurious reflections have to be overcome by coherent injection signals of sufficient amplitude.

Self-starting additive-pulse mode locking of a Nd:YAG laser

We report additive-pulse mode locking of lamp-pumped Nd:YAG lasers at both 1.06 and 1.32 microm. Under completely self-starting conditions, stable cw mode-locked trains of 6-psec pulses have been achieved at an average output power of 2.4 W.

Self-starting condition for additive-pulse mode-locked lasers

We analyze self-starting in additive-pulse mode-locked lasers and find that dynamic gain saturation can play a determining role. A simple condition for self-starting is presented for comparison with experiments on different laser systems.