جستجوی مقالات مرتبط با کلیدواژه "تشدید" در نشریات گروه "برق"

Resonance in transmission lines can cause unusual over-voltages and put the equipment at risk. One of conditions that can lead to resonance in transmission lines is the asymmetrical connection (single-phase and two-phase) of transmission lines compensated by shunt reactor. The asymmetrical connection of the line can be caused by the failure of the circuit breaker, activation of the single-phase recloser, or the presence of switchsync relay. In this paper, the feasibility of resonance in the 400 kV Abu Talib-Torbat Jam transmission line is investigated using EMTP-RV software. According to the results, in the case of asymmetrical connection (single-phase and two-phase) of the Abu Talib-Torbat Jam transmission line, resonance does not occur in the power frequency. But this line has a resonant frequency in the range of harmonics. In order to investigate the effect of harmonics, harmonic studies have been conducted and it is shown that harmonics cannot cause unusual over-voltages in this line.

In this paper, design, simulation, and implementation of an induction heater with the ability of heating metals up to 300°C is carried out. The respective heater is made utilizing the relevant circuit, proper capacitor selection, and inductor design, and the experimental results are in good agreement with simulation results. In this paper, the induction heater simulation is carried out using the finite element method with COMSOL Multiphysics software. The finite element method for magnetic and thermal analysis is a conventional method and its capability has been proven in various physical fields. Therefore, finite element software has been applied to calculate and analyze the magnetic field and heat induced by induction heating. In this simulation, the relevant electric circuit is considered and all the elements are taken into account. According to the proposed electrical circuit and the related control method, a laboratory prototype proportional to the simulated dimensions is implemented in finite element software and the waveforms of the circuit parameters are analyzed. These induction heaters can be utilized in a variety of small and labware industries. In addition, they can be used as a preheater or heater for various applications (e.g., metallurgy industry, materials and metals processing, and testing in small dimensions). It should be noted that the circuit presented in this article is very cost effective and comparison of a laboratory-scale sample with simulation results demonstrates the validation and accuracy of the analysis.

Assessment of complex harmonic current contribution of each wind turbines at connection point of a wind farm to the grid (primary emission) is presented in this paper. Moreover, contribution of grid background harmonic voltage distortion on harmonic current distortion (secondary emission) of each turbine is also evaluated. Both assessments are represented based on primary and secondary transfer functions and modeling of wind farm components in frequency domain. In order to increase the accuracy of these transfer functions, wind turbine impedance must also be included in transfer functions while studying aggregation effect of complex harmonic current emissions. Accordingly, a new model of primary and secondary emission transfer functions for a type-III wind turbine is proposed to study aggregation effect of harmonic currents in a wind farm. This model is based on average modeling of back-to-back converters of a doubly-fed induction generator, in order to extract total impedance of a wind turbine in its linear working range. By applying this impedance into transfer function models, if turbines emit current with identical magnitude and phase angle, then the total primary emission aggregate less than the number of turbines, and if turbines emit current with identical magnitude but uniform phase angle distribution, then the total primary emission aggregate less than square root of the number of turbines and doesn't follow square root rule.

Application of power electronic converters in wind turbines results in generation of harmonic and interharmonic. Also, the switching of converters results in turbines active output power distortions. Harmonic interaction of wind farms with external grid, harmonic propagations in it and amplification of these propagations due to resonance of underground transmission cables, can leads to increase in power quality problems. This paper presents a method for study and classification of conventional current harmonic emissions in a wind farm. This method uses the concept of transfer function in order to evaluate the harmonic contributions of each wind turbines at point of connection as well as the contribution of external grid background harmonic voltage on each turbine current harmonic distortion. In this regard, extraction of voltage and current transfer functions, transfer admittance and impedance is done based on circuit theory and harmonic modeling of wind farm components from medium voltage (MV) side of turbine transformer in frequency domain. Two classification of harmonic propagation called as “primary emission” and “secondary emission” is evaluated based on these functions. With use of this method, the contribution of each turbine and also the contribution of external grid at overall wind farm emission can be decomposed. Also, the impact of transfer function of conventional LCL filter used in wind turbines, on each those transfer functions, is studied. The important conclusion is that the presence of this filter causes that the amplitude of primary harmonic transfer function from one turbine to another turbine, in special frequency range, be more than the amplitude of primary harmonic transfer function from one turbine to the grid.

In this research, the nonlinear rolling motion of ships is studied. After obtaining the governing equation of roll motion, the method of multiple scales perturbation technique is applied to solve the nonlinear differential equation. The ship response is studied with and without harmonic excitation. In order to validate the responses obtained by the method of multiple scales, the response was compared with the numerical solution. Finally, the effects of damping coefficient and restoring arm on the frequency response function and resonance frequency have been studied.

Regarding the necessity of low noise and low vibration operation of the BLDC motors in some especial applications, a novel method is proposed in this paper in order to shift the natural frequencies of a BLDC motor’s stator to higher values. Therefore the noise and vibration of the structural resonances can be reduced since the electromagnetic excitation forces in the BLDC motor have lower magnitudes at higher frequency components. The proposed method is based on placing a single hole, with definite diameter and location, on definite regions of the stator cross sectional area (each region contains one stator tooth and its upper parts in the stator’s yoke). The frequency analyses have been done in the SolidWorks software environment and the best diameter and location of each hole is extracted by the Response Surface Methodology (RSM).Preventing the magnetic saturation of the stator structure and the motor cogging torque enhancement prevention are considered as constraints for the problem. The new method also can be used for other types of electrical machines’ stator.