نشریه مهندسی برق و مهندسی کامپیوتر ایران
سال هجدهم شماره 4 (پیاپی 59، زمستان 1399)

In this paper, a control scheme is presented for an isolated DC microgrid including wind turbine connected to permanent magnet synchronous generator, electrical energy storage unit, and variable electrical loads. Energy sources are connected to a common bus through DC buck and buck-boost converters. The local distributed controllers are located in the first control layer. These controllers are designed based on a Lyapunov stability analysis and thereby its stability is guaranteed. Moreover, the current and voltage, injected to the network, are adjusted by controlling the switching functions of the converters. The decentralized secondary controllers determine the contribution of the local units for supplying the local loads. In this control layer, a model predictive controller for the wind generation unit as well as a proportional-integral controller for preserving the bus voltage are proposed to determine the reference currents for the local controllers. In addition to the practical simplicity, complete isolation of the secondary controllers, minimum requirements to data transfer, and providing a control structure without any need to change in development plan are the important advantages of the proposed control scheme. The performance of the controllers is investigated and verified using the simulations in MATLAB software performed for different cases.

In this paper, we propose a cognitive radio network which consists of a number of secondary users and one primary user. The primary user utilized a successive relay performance. The successive relaying technique creates a full duplex relay performance by two half duplex relays, which improves spectral efficiency. In the presence of secondary users, we use successive relay technique in the primary user to ensure its acceptable performance. Also, the sum rate of secondary users is increased. The challenges of this network are inter-relay interference and inter user interference. The interference alignment method is utilized to eliminate the interferences in the successive relay technique and in the cognitive radio network. Besides, the minimum transmitted power of the primary user is derived to guarantee its quality of service requirement. Two power allocations algorithms are proposed to maximize the sum rate of secondary users and the energy efficiency of the network. In both power allocations algorithms, satisfying the quality of service of the primary user is considered. The closed-form solutions of these algorithms are studied. The fractional programing approach was employed to solve the energy efficiency optimization in two steps.

In this study, a forward converter with phased shifted active clamp is presented for the driving of magnetron lamp (4 kV, 300 mA, 1000 ±40 W).The presented converter is of a boost and high-gain type. To reduce the voltage stress, the active clamp structure is used. In addition, using the clamp switch phase shifted method, while the maximum flux density of the transformer core retains power, it is possible to increase the time the main key is turned on. Thus, with the same nucleus, the possibility of increasing power transmission is created. Therefore, in the same power, the volume, weight and price of the core used should be reduced.Also, a series resonance circuit provides soft switching conditions. Maximum and average power supply power is controlled for minimum losses. The power supply delivers about 1 kW with an average power of about 250 W by adjusting the time the converter is enabled. Other advantages of the proposed circuit include simply power circuit, reduction of the number of switching elements and reduction of switching losses. The design results have been simulated and verified by PSCAD software.

The presence of uncertain parameters in the power system has created many challenges for designers andoperators of the system including the problem of capacitors in the presence of wind power plants. The answer depends on the amount of load and output power of the wind power plant that has uncertain values. In this paper, the information gap based decision theory method is used model the uncertainty in load and output power of the wind power plant. The objective function includes the cost of capacitive banks and energy losses, used to of load Flow based on unscented transformation for calculate energy losses. A genetic algorithm is used to optimize the above problem. Finally, the efficiency of the proposed method has been investigated by carrying out numerical studies on the IEEE 33-bus network.

MIMO systems, and particularly massive MIMO systems, achieve high spectral efficiency by using a large number of antennas. An important issue in these systems is beamforming. In fully digital baseband beamforming, an RF chain is required for each antenna, which leads to high cost and power consumption. In analog beamforming, only one RF chain is used and beamforming is performed by using phase shifters. This method does not provide optimal spectral efficiency and thus, analog-digital hybrid methods for beamforming are considered. In this paper, a hybrid beamforming method is proposed in which the required number of RF chains is much less than fully digital method. The precoder and receiver filter are designed by maximizing the spectral efficiency. To this end, the optimal beamforming matrix (which contains the right singular vectors of the channel matrix) is approximated by the product of two analog and digital beamforming matrices. This approximation is improved by an iterative method. The criterion for the proximity of the two matrices is considered to be the Frobenius norm of their differences. In the receiver, the design of the hybrid beamforming is performed in a similar way, using the mean squares error criterion. Also, to improve the method, a gradient-based algorithm is proposed to further reduce the error. The simulation results show the performance superiority of the proposed method over similar methods as well as its less complexity.

In this paper, a procedure for feed placement in a collocated Multiple Input Multiple Output (MIMO) antenna is presented based on characteristic mode theory. The orthogonality of characteristic currents is used to determine optimal feed locations in the presented antenna. First, characteristic mode analysis is done to determine the resonance modes of the antenna; Then the optimal feed locations are determined to excite these orthogonal modes. Moreover, we modify the shape of the radiation pattern of the antenna by changing amplitude excitation of these modes. It is observed that only by changing amplitudes of the antenna modes and their composition at the desired frequency, the shape and the beam width of the antenna pattern is changed.

A single electron transistor is a nanoscale device comprised of three metallic electrodes and one island or quantum dot. The island can made of carbon nano materials like a graphene nanoribbon. The number of carbon atoms along the width of the graphene nanoribbon affect on the speed of transistor operation and coulomb blockade region. In this research, the current for a single electron transistor utilizing a graphene nanoribbon island is modeled. The impact of several parameters on the transistor current is investigated including the number of carbon atoms along the width, length of nanoribbon, and the applied gate voltage. The modeling results show that increasing the number of carbon atoms along the width of the nanoribbon results in reduced coulomb blockade region. Moreover, reducing the length of nanoribbon and increasing the applied gate voltage cause a decrease in the zero current range of the transistor. Increasing the number of atoms along the width of three islands also gives a boost in the electron tunneling region and thus, the transistor performance will be improved.

Recently, color filters have been used for high quality resolution color imaging and printing in subwavelength scale. In this paper, a reflective subtractive color filter with excellent color contrast has been demonstrated. In the proposed filter, titanium dioxide nanocuboids have been integrated with aluminum mirrors at the top and bottom of them. Due to the creation of magnetic dipole in titanium dioxide nanocuboids, a resonance occurs in the visible spectrum, which the resonance wavelength could be tuned over the whole visible range through the adjustment of the nanocuboid side size. By utilizing aluminum patches on both sides of the nanocuboids, the efficiency of more than 70% and the bandwidth of less than 35 nm have been attained. The proposed filter exhibits high incident angular tolerance such that by increasing the incident light angle from 0 to 60 degree, the resonance wavelength has very poor variation and maintains the same bandwidth and efficiency. Also, the proposed filter is polarization-independent due to its symmetric geometry. These advantages facilitate the function of the suggested filter in imaging and displays with high brightness.