نشریه مهندسی برق و مهندسی کامپیوتر ایران
سال پانزدهم شماره 3 (پیاپی 44، پاییز 1396)

Stability challenges in Microgrids (MGs) usually arise from low inertia of Distributed Generation (DG). In this paper, a voltage stability improvement method is proposed in order to improve MG operation. Voltage Stability Index (VSI) is applied to evaluate and improve voltage stability of MGs including different types of DGs. A new hybrid optimization is introduced to find the optimal operation of autonomous MG and to improve VSI. Operational optimization is performed by finding optimal droop parameters of DGs and sitting wind DGs to reduce energy generation cost. Optimization is defined as a multi-objective function and a hybrid HS-GA algorithm is applied to solve the optimization problem. A new power flow formulation is also proposed in which the steady state frequency, reference frequency, droop coefficients and, reference voltage of droop based DGs are considered as optimization variables. Results of proposed approach are compared with other methods for 33 and 69-bus IEEE systems using MATLAB software. Results prove the efficiency of proposed approach for operational improvement of MGs.

In recent years, there has been an increasing attention in energy exchange among the various countries or areas. One of the methods for energy exchange, with coordinated by the area operators, is participation of producers and consumers in other area markets directly. In this paper, for multi-area power systems that there is a single market in each area (multiple markets structure), a mechanism for power exchange management is proposed. In the mentioned structure, the participation of external bus players in each market is possible. In the designed mechanism, the power exchange management is done by a proposed central coordinator entity. The coordinator performs the management of power exchange in an iterative decision-making process to maintain system security by using a technical approach. The simulation studies for a triple-market case in the standard three areas system (IEEERTS-96) are presented to validate the effectiveness of the proposed mechanism.

In some applications that we need a high voltage gain such as the photovoltaic cell and fuel cell, high step up dc-dc converters must be used, but conventional boost converter cannot provide the high voltage gain. For this reason, in this paper, a single switch transformerless high step-up buck boost dc-dc converter with reduced voltage stress on the semiconductors is proposed. The proposed converter has higher voltage gain in step-up mode in comparison with conventional boost and buck-boost converters. Reduced voltage stress on the active switch allows to choose lower voltage rating MOSFETs to reduce both switching and conduction losses. Low voltage stress on the diodes allows the use of Schottky rectifiers for alleviating the reverse-recovery current. The proposed converter can be operated in the continuous conduction mode (CCM) and the discontinuous conduction mode (DCM). In this paper, different operation modes of the proposed converter, calculation of the voltage gain, the currents that flow through the components, efficiency and capacitors voltage ripple are presented. To verify the operation of the proposed converter, simulation results via PSCAD software and experimental results are provided.

Teleoperation systems have attracted more attention in processes that human operator’s availability is difficult. In this paper, using retarded functions, teleoperation systems have been modeled as a special case of Network Control Systems (NCS) with nonuniform sampling and network delays. It is assumed that slave and master robots are linear and continues-time systems and input-delay approach is used for the stability analysis. Using the proposed Lyapunov function, the sufficient conditions for the stability of discrete network-based teleoperation system is proposed. It will be represented that the proposed conditions are less conservative than previous recent researches. Also an upper bound of sampling time for discrete control signals is computed in a manner that does not disturb the stability conditions. To meet this condition the problem is defined as the convex optimization program and is represented by the LMI terms. In the simulation part, the behavior of the teleoperation system under the nonuniform sampling is represented and the effect of sampling time on the trade-off between the stability and transparency has been studied.

A novel model for impact ionization metal-semiconductor (IMOS) device with an engineered bandstructure (heterostructure) has been proposed and simulated. The IMOS intrinsic, wherein the carrier generation is mainly due to impact ionization, controls the band to band tunneling. In the proposed model, it is assumed the intrinsic region to be SixGe1−x (0.5≤x≤1) whose bandgap varies linearly from that of Si at the source edge to that of Si0.5Ge0.5 at the gate edge. Maximum gap difference of ΔEG=ΔEC=0.32 eV appears at the source /intrinsic region interface. As a consequence, the probability of band to band tunneling and hence the device dark current is reduced. The numerical result shows that the breakdown voltage and the dark current for the proposed heterostructure IMOS are respectively ~0.3 V and four times smaller than those of the homojunction Si0.5Ge0.5-IMOS, with the same dimensions.

In this paper a new topology of three-level inverters with ten-switch is proposed to control low voltage three phase induction motor. Ten-switch inverter is a reduced-switch-count three level inverter which compared to conventional topologies such as NPC, CHB and flying capacitor has lower count of semiconductor switches. Thus, it enjoys from lower cost, lower volume and lower weight. In this paper, a direct torque control (DTC) scheme based on switching table is developed to control a ten switch inverter. Simulation results of controlling induction motor fed by a conventional two-level inverter, NPC and proposed 10 switch inverter are presented and they are compared together in different aspects. Results confirm the effectiveness of the proposed topology and its control method.

This paper proposes a new application of sensorless control method for doubly fed induction generator (DFIG) using rotor-current-based MRAS observer (RCMO) under grid voltage dip. MRAS means model reference adaptive system. In this paper the method of control of DFIG is vector control VC (or field oriented control FOC). The position and speed of rotor are estimated by RCMO instead of measuring. DFIG is connected to a grid with a balanced voltage dip on PCC and is tested by large variation in rotor speed. Simulation results using MATLAB/Simulink are presented for a 2-MW DFIG. The simulation results show the decoupled control of active and reactive power of DFIG in three conditions: a) normal voltage of grid b) grid voltage dip c) huge variation at wind speed. The results show that the estimated speed of rotor and power produced in DFIG carefully follows the references. The conclusion of simulation results is that for decoupled control of active and reactive power of DFIG, application of RCMO method is favorable and acceptable under balanced voltage dips and variable speed wind as well as conventional VC.

In this work, novel gallium-nitride (GaN) high electron mobility transistor (HEMT) with a p-layer in the barrier at source and drain sides (SD-PL) is reported. Important parameters such as gate-source and gate-drain capacitances, maximum DC trans-conductance (gm), cut off frequency (fT), maximum lateral electric field, breakdown voltage, DC output conductance (go) and saturated drain current of the proposed structure are studied in details using two-dimensional and two-carrier device simulations. The simulation results of the proposed structure are compared with those of the source side p-layer in the barrier (S-PL), drain side p-layer in the barrier (D-PL) and conventional structures. According to the extracted results, the proposed structure improves the gate-source capacitance, maximum gm, cut off frequency and go compared to the D-PL structure. Also this new structure reduces the peak electric field at the gate corner near the drain and consequently increases the breakdown voltage significantly in comparison with the conventional structure. Increasing p-layer length (LP) and thickness (TP) in the SD-PL and S-PL structures, improves the breakdown voltage, gate-source capacitance, gate-drain capacitance and go.

In this paper, we propose a coordinated fair scheduling (CFS) scheme for LTE-Advanced networks where the cells are equipped with multiple sector antennas. To enhance the network spectral efficiency and throughput, the sectors use the same frequency bands. However, to reduce the co-channel interference, the transmissions from the sectors to the users are coordinated. In other words, multiple sectors are allowed to transmit simultaneously, if the occurred co-channel interference is less than a threshold value. The scheduling scheme takes advantage of the user's diversity in space to transmit to the users with good channel conditions while maintaining fairness among the users using the alpha-fair criterion. Furthermore, a heuristic approach is proposed to reduce the computational complexity of the scheduling scheme. The performance of the proposed CFS scheme and the heuristic approach are evaluated using simulation results. The simulation results show that using coordinated fair scheduling improves system performance and increases cell throughput.