مجله مهندسی برق و الکترونیک ایران
سال شانزدهم شماره 1 (بهار 1398)

In this paper, a novel real time pricing design is presented for Demand Response (DR) programs. A Load Serving Entity (LSE) is responsible to provide energy for flexible loads, inflexible loads and thermal loads. The LSE consider operation conditions of system and uncertainty of renewable energy resources and it designs a Real Time Price (RTP) demand response. The inflexible and thermal loads cannot participate in this model and the costumers who using these loads, pay their costs according to the retail market price. On other hand, the flexible loads can be participated in this proposed model. It’s assumed that flexible loads of customers are aggregated by several aggregators which took part in this model. The advantage of proposed model is that it ensures cost saving for customers. Numerical results show that proposed model increases LSE’s profit, decreases the costumer’s payment and improves characteristics load curve such as load factor. The presented model is formulated as a bi level program which it maximize the profit of the LSE and utility of the customers. This problem is solved by using GAMS software.

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.

Fast charging stations are one of the most important section in smart grids with high penetration of electric vehicles. One of the important issues in fast chargers is choosing the proper method for charging. In this paper, by defining an optimization problem with the objective of reducing the charging time, the optimal charging levels are obtained using a multi-stage current method using a genetic algorithm. Another important parameter in station design is determining the number of charging units to optimize the input power capacity. In this paper, for the first time, a new coordination policy has been proposed in order to increase the number of charging units for a given power capacity and thus reduce the charging time of vehicles. Regarding the change of current during the charging time, it is possible to reduce the chargers for work with less power capacity or at the same capacity, the number of chargers can be increased to reduce the charging time. In this condition, the chargers cannot start to work simultaneously and the proposed coordination between the chargers determine the starting time of chargers. The simulation results in MATLAB environment show that by defining the problem of coordination between the chargers, the number of charging units increases and the waiting time of electric vehicles at station will decreased.

Power capacity of EV charging stations could be increased by installing PV arrays on their rooftops. In these charging stations, power transmission can be two-sided when needed. In this paper a new method based on queuing theory and random forest algorithm proposed to calculate net power of charging station considering random SOC of EV’s. Due to estimation time constraints, a queuing model with limited input and capacity used to model arrival and departure of EV’ and a relationship proportional to SOC of EV’s in which they arrive to the charging station has been suggested. To demonstrate impact of photovoltaic rooftop, a model based on Random Forest suggested for estimation of maximum output power of PV array. In simulation process, charge and discharge operations for Nissan and Tesla EV’s is done by writing a priority list and real efficiency of charger/inverter in AC level II operation conditions. Simulation results show that proposed method is a precise and efficient model for scheduling and capacity estimation of V2G parking lots with PV rooftop.

In this paper, using the state-dependent Riccati equation (SDRE) technique, a suboptimal fault-tolerant control scheme is designed for a DC microgrid in the islanded mode. The objectives are the voltages control of the photo-voltaic cell, the battery, the capacitor bank, and the DC bus as well as on time fault detection. In the design procedure of the SDRE observer-controller, a nonlinear mathematical model is utilized to model the behavior of the microgrid in different working conditions. The performance of the microgrid is evaluated in the presence of uncertainties in its parameters and measurement noises. The obtained simulation results show that the proposed method is so effective in the fault detection, not detecting the disturbance instead of fault, and having a robust performance even in the presence of external disturbances.

Recently, increasing of non-linear loads in the power distribution network has been increased harmonics in these networks. The harmonics problems get worse and complicated by installation of power factor correction capacitors and filters. But the Distributed Generations (DGs) interface inverters with properly control can help to improve power quality, harmonic compensation and voltage unbalance. Until now, much research has been performed in identifying harmonics and compensation by DGs. This paper presents a hierarchical control method by DGs to compensate voltage disturbances on the Sensitive Load Bus (SLB) in a Micro Grid (MG), which includes two levels of control: primary and secondary. In secondary control has been proposed an appropriate non-uniform compensation method of harmonics and voltage unbalance, based on impact of DGs in the resonance frequencies. With doing analysis of the network modal impedance and its results, the DGs with higher participation factors will have greater duty in compensation of the stimulus harmonics, and the DGs that being less engaged with stimulating of harmonics will have greater portion in compensation of the network voltage unbalance. The primary control also includes current controller, virtual resistance and load compensation controller. The simulation results show the effectiveness of the proposed control system.

In this paper, an optimal integrated inner controller is designed for the microgrid primary control level. The main task of the primary control level is to maintain stability and proper power sharing in microgrids. Non-optimal controllers have been generally used to design the inner controller in this level in the majority of researches. On the other hand, accurate and complete models of microgrids can be obtained. Therefore, it is an advantage to use the model based optimal control approaches. In this paper, an optimal inner controller is designed to simultaneously control of the microgrids voltage and current using a technique called linear quadratic tracking controller (LQT). In order to evaluate the performance of the proposed controller, it is applied to a microgrid including two distributed generators (DGs) in MATLAB/Simulink. Results of the simulations illustrate the proper performance of the controller in comparison with conventional methods.

This paper presents an optimal method of frequency load shedding (UFLS) based on the rate of change of frequency of load (ROCOFL) in islanded distribution networks (DN). The proposed method minimizes the average amount of penalties paid by the independent operator of the network by utilizing the relevant cost function. The function of islanded distribution network occurs after an error in the sub-transmission network and separation from the upstream network, in an emergency condition, is one of the most important facts that has been recently addressed. The use of installed intelligent equipment to measure and send information, related to power, frequency, frequency changes, voltage range, and condition of generator switches makes it possible to provide an appropriate protection for the islanded networks. The under frequency load shedding is used to improve the network frequency due to the imbalance in the network. In this paper, it is tried to use the two proposed control modules to eliminate the optimal load with the consideration of the minimum penalty. The method presented in this paper is evaluated in the sample network of 14-bus Danish distribution network.

In this paper, a self-healing approach for smart distribution network is presented based on Graph theory and cut sets. In the proposed Graph theory based approach, the upstream grid and all the existing microgrids are modeled as a common node after fault occurrence. Thereafter, the maneuvering lines which are in the cut sets are selected as the recovery path for alternatives networks by making cut sets of the faulted line. After forming the candidate networks with the minimum number of switching, an unbalanced three phase power flow is performed to be assured that the new network configuration does not violate technical constraints. In the case of the constraints violation, the other candidate networks with the same number of switching or more are assessed to find a proper scheme with high loadability and low voltage unbalance values. The proposed approach is scrutinized in two different case studies: with and without microgirds connection to the distribution network. The simulation results applied on a typical distribution network in the MATLAB software show the valuable performance of the proposed approach. The using of the Graph theory has reduced the number of switching and excitation time in comparison with evolutionary search approaches.

Allocation of measurement devices is a necessity of distribution system which is an application of state estimation. In this paper, the problem of active and reactive measurement devices is modeling using a multi-objective method. The objectives of the problem are to minimize the use of measurement devices, increase in state estimation output, improve the state estimation quality and reduce costs. The costs are calculated based on line loss, line capacity, lines connected to the specific line and the change in line flow direction. The Multi-Objective Evolutionary Algorithm based on Decomposition (MOEA/D) is used to optimize the measurement devices allocation within the problem of distribution system state estimation. The simulation and optimization is tested on IEEE 33-busand IEEE 123-bus radial networks and state estimation problem is optimized by particle swarm optimization (PSO) algorithm. The Monte Carlo simulation is used to develop some conditions for network to guarantee the robustness of the proposed method.

In this paper, a new structure of HFTI-qZS-CMI has been proposed. The proposed structure is capable of generating high-voltage for medium to high power applications. By adding a switch, a capacitor and a high-frequency transformer and also eliminating an inductor from each qZS-CMI cell, the HFTI-qZS-CMI structure will be created. Using the high-frequency transformer in the proposed structure, leads to electrical isolation between the input and the output, increase or decrease the output voltage, increase the system reliability and reduce the electromagnetic interference. The number of elements in the proposed structure is less than three stage cascade multilevel inverter. Moreover, in comparison to the qZS-CMI with the same input and output voltages, it has lower voltage stress on the diode and the switches as well as higher boost factor and larger modulation index.To control the on and off state of the switches, the Improved phase-shifted sinusoidal pulse width modulation with third harmonic injection is used which increases the modulation index more than one and improve the quality of inverter output power. The proposed structure simulation has been carried out in MATLAB / SIMULINK software. The results of analysis and simulation show better performance for the proposed inverter comparing to other structures.

In some industries applications such as fuel cells, we must use a high voltage gain dc-dc converters for increasing voltage, but, the conventional converters cannot provide the high voltage gain with increasing duty cycle and the converters efficiency is limited by the equivalent series resistances. For this reason, in this paper, a single switch transformerless high step-up dc-dc converter with reduced voltage stress on the semiconductors is proposed. The proposed converter voltage gain is higher than the conventional converters. In this converter, the power switch voltage stress is low which allows to choose lower voltage rating MOSFETs to reduce both switching and conduction losses and 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 and efficiency are presented. To verify the operation of the proposed converter, simulation results via PSCAD software are provided.

This paper deals with the design and simulation of a RHCP Fabry-Perot antenna for 14.5 GHz. Based on transmission line model, partially Reflecting Surface (PRS) was designed so thatgain enhancement and circular polarization simultaneously obtained. PRS consist of a square loop and meander-line at the top and bottom respectively. In order to achieve the resonance condition in orthogonal components of the electric field at the cavity ,rectangular slot used as a unit cell in ground plan. The dimensions of the slots are appropriate so that the slot unit cell is closer to the ideal ground plan and slots radiations are negligible. Coaxial cable feed the rectangular patch as a source radiator while rotation implemented to the patch for producing orthogonal field component. Suggested antenna has a 16dBi broadside gain and return loss and AR bandwidth of 1.7% and 7% respectively. Antenna was fabricated that had a 4λ×4λ size and measured results are presented that shows good design.