نشریه الکترومغناطیس کاربردی
سال هشتم شماره 2 (پیاپی 21، پاییز و زمستان 1399)

Doubly Fed Induction Generator (DFIG) is one of the important generators that is used in the wind turbine structures. In the past few years, DFIG-based wind turbine systems that have been installed in the onshore and offshore wind farms are greater in number, than any other generator. Since the number of references that present the DFIG designing field are limited and equations and the design process are incompletely reviewed, in this paper the complete design process of a 250kW DFIG is presented. First, the optimal design equations of DFIGs are reviewed and then a model for the study of generator operation is presented. The detailed design of the DFIG is done next, and finally the Finite Element Analysis (FEA) is used to verify the design. The results show the effectiveness of the presented design process for the DFIG.

In this article a miniaturized ultra-wideband (UWB) bandpass filter (BPF) using the microstrip rectangular ring loaded by two open stubs is presented. The proposed structure consists of two doublet parallel coupling gaps at each side of a microstrip ring. In comparison with some other filters, this structure shows a significantly wider passband due to the introduction of a cross-coupling between the feed lines (input and output) which generates four pairs of transmission poles in the passband. In order to control the transmission poles in the pass band and adjust the transmission poles near the lower and upper edges of the filter, two open stubs have been loaded on the microstrip rectangular ring. Consequently, a compact four poles ultra-wide bandpass filter is designed which exhibits extremely sharp rejection skirts around the target passband. The proposed filter has a passband that covers 3.2 to 10.6 GHz and its measured 3 dB fractional bandwidth is about 107.5%. In order to validate the performance of the proposed technique, the designed UWB band-pass filter is fabricated and experimental verification is provided. A good agreement has been found between the results obtained by simulation and the measurements. Furthermore, rejection level better than 20 dB in upper stopband is extended to around 15 GHz. To our knowledge, the size of the proposed ultra-wideband filter is more compact in comparison with known similar filters.

Non-radiative wireless power transfer systems based on magnetic resonant coupling have received the attention of many researchers, due to their high efficiency over distances greater than the diameter of their coils and their mid- range of operations. In this article, the design and implementation of a 4-wireless power transmission system consisting of a power coil (drive loop), a transmitting coil (Tx coil), a receiving coil (Rx coil) and a load coil (load loop) is presented. Power is transmitted when the frequency of the E class amplifier is equal to the self-resonant frequency of the transceiver coils. The purpose of this paper is to achieve the highest efficiency without designing an additional impedance matching system. This condition is achieved by varying the coupling factor between the power coil (load) and the two resonators. The maximum efficiency is 93% at 15 cm and using this method results in efficiency improvements of 56.3% and 35.6% at the distances of 60 cm and 100 cm respectively, compared to the case of the coils being fixed.

In this paper, a novel high pass waveguide filter of the ferrite blade with a tunable rejection band is designed and analyzed. The first band-pass of this filter is obtained in the frequency bands where the proposed structure is in the left-hand region. The reason for the left-hand property is the negative effective permeability of the ferrite, and the effective permittivity of the structure is due to the plasmonic property of the waveguide below the cutoff frequency. Stopband occurs in the frequency bands that have positive permeability while the effective permittivity is still negative. As the effective permittivity of the structure is positive, the proposed structure acts as a regular waveguide with a right-hand nature and acts as a high pass filter. The rejection band of the proposed filter shifts due to the ferrite-adjustable property of the magnetic bias. The use of this method dramatically reduces the complexity of the external control system. To analyze the proposed waveguide filter, we proceed by solving the Maxwell equations and obtaining the distribution of the electric and magnetic fields inside the regions of the structure and applying the boundary conditions, to obtain the characteristic equation and then the dispersion diagram is achieved by the numerical solution. To confirm the analytical solution, the proposed structure is simulated using Ansoft HFSS software and finally, its scattering parameters are investigated.

Different factors such as nonlinear phenomena and mode instability affect the output of the high-power fiber lasers and amplifiers. Since these devices have many applications in the industry, medicine, and military facilities, the study of the various factors’ effects on their output is directly reflected on the design of high-power lasers and amplifiers. In the present paper, the mode instability which is the major limiting factor on the output of high power lasers and amplifiers has been studied, simulated, and investigated. In these high-power devices, the temperature increases due to quantum defects, background loss, and light scattering, which change the refractive index of the fiber material. Variation of the refractive index is the main reason of mode instability in high power fiber lasers and amplifiers. Mode instability causes the coupling of the fundamental mode  to the upper-mode and thus decreases the fundamental mode’s energy. In this paper, different factors that affect the threshold of the mode instability and power transfer from the fundamental mode to the upper mode have been investigated.

One of the main capabilities of the perovskite solar cell is their high absorbability in the visible range. However, they have low absorption coefficient in the infrared region, which is a large part of the solar spectrum in this area, as well as the toxicity of perovskite compounds. Metal nanoparticles have been introduced to overcome this problem. In this research, we studied the effect of size, distance, position, and composition of plasmonic nanostructures on light absorption, optical current, short-circuit current density, and maximum generation rate in perovskite solar cells. The results revealed that the optical properties mentioned above will be enhanced up to 1.33 times, compared to the reference state in which no nanoparticles are present, by using silver nanoparticles with 50 nm radius and 20 nm distance. In the following, different thicknesses of the absorbent layer were examined, as thickness has a direct relationship with the absorbent layer toxicity

Blumlein bipolar pulse former is a widely used bipolar pulse generator with good performance. This structure produces a bipolar pulse from a unipolar pulse using the reflection theorem of the waves in transmission lines. Practical considerations in the design process of Blumlein bipolar pulse former is an important issue from the view point of output voltage waveform quality and construction cost. In this paper, a Blumlein bipolar pulse former has been simulated and analyzed to investigate the effect of different physical parameters on the output voltage waveform. The results of simulations can be used to design a bipolar pulse generator with acceptable output voltage waveform.

Electric arc furnaces are the largest concentrated, nonlinear and chaotic loads in power distribution networks. Magnetic modeling and simulation of such loads have the least error with respect to the physical nature of the arc and play a significant role in the accuracy of the studies. In this paper, the electromagnetic simulation of a low-frequency AC arc furnace is performed based on finite element method, utilizing the powerful COMSOL Multiphysics software. The finite element method for magnetic, thermal and dynamic analysis is a commonly used method and its capability in various physical fields has been repeatedly proven. In this finite element simulation, the corresponding electrical circuit, the electrode physical dimensions, the initial thermal condition and the furnace power supply system are also considered. By simultaneous solving of different physical equations in COMSOL Multiphysics software and applying the magnetohydrodynamic principles, Ampere's law, Ohm's law, and Maxwell’s equations, the arc model and plasma generation are investigated. Then, by extracting the results and comparing them with previous studies, the stability of the arc is evaluated. A stable arc decreases the harmonic components and also diminishes the voltage flicker severity imposed on the grid and thus, reduces the need for compensator equipment.

In this paper, a Ka band low-noise amplifier realized in 0.15μm InGaAs pHEMT technology for satellite applications is presented.  The proposed three stages amplifier is designed and simulated using the equivalent circuit model and its layout is studied by full-wave electromagnetic simulation. Full-wave simulation results in the frequency range of 32GHz to 37GHz show a maximum noise figure of 1.8 dB and gain of 20.7 dB with 0.4 dB ripple. Also, the input and output return loss is better than 16 dB and output 1dB gain compression point is equal to 13 dBm. The total area occupied by the final design is 1.6 × 1.3 mm 2. All three amplifier stages have source-degenerated configuration and to realize the impedance matching network whilst reducing the size of LNA, transmission line is used instead of inductors. A parallel LC tank circuit in series with a resistor in biasing network is used to improve the stability in a wide frequency range up to 45GHz.

Since a high percentage of faults in underground power cables occur at the joint location of these cables, identifying the cable joints location is one of the most important challenges for power distribution companies. In this paper, a passive method to detect the underground electrical cable joints based on the variations of magnetic fields at the ground surface is proposed. For this purpose, the magnetic fields above a long cable containing a joint at the middle were first simulated using CST software. Simulation results showed that when scanning along the cable, the magnetic field changes at the joint position which can be used for finding its location. Simulations were repeated for different joints located at different depths and in different soils. The advantage of the proposed method is that it does not need any external source and causes no interruption of electric power in underground power cables.

In this paper, the influence of the rotor circuit on the characteristics of the brushless doubly-fed induction machine (BDFIM) is investigated. For this purpose, a dynamic model is provided for the nested-loop BDFIM. The model is based on the coupled circuits of the stator windings and the rotor loops. By using the winding function method, the self- and mutual-inductances of the coupled circuits are obtained as functions of the rotor position and design parameters. This model is used as a fast tool to predict the dynamic performance of the BDFIM and is verified by means of time-stepping Finite Element Analysis (FEA). Using the provided dynamic model, some characteristics of the BDFIM such as the Joule loss and the magnetic coupling between the power and control windings at no-load conditions are studied. Using the developed model, the effect of the number and the arc span of the rotor loops on the no-load characteristic of the BDFIMs is studied.

The purpose of passive defense plans is to reduce the vulnerability of human resources and critical equipment in the country. Therefore, compliance with non-operating defense principles is essential to prevent and reduce the risks of natural disasters such as lightning. Direct and indirect lightning strikes on buildings and transmitting current through incoming service lines can cause step-and-contact voltages that are harmful and dangerous to the humans and, the buildings and their valuable contents. In general, the step voltage is scattered to a radius of several meters around the location of the lightning stroke, causing injury to the people nearby. Therefore, from the passive defense perspective, the present study aims to minimize the damage of lightning strikes on sensitive centers placed at high altitudes and located in rocky areas. Since telecommunications and data stations are generally positioned in highlands or open areas, lightning protection is very important. Observing the compliance with international standards and recommendations in such environments, this paper uses the CDEGS Software to present the best and most optimal way to implement the ground system in order to minimize the step-and-contact voltages of telecommunication sites.