نشریه مهندسی برق
سال چهل و چهارم شماره 3 (پیاپی 69، پاییز 1393)

In this paper, a 6-switche converter is suggested which consists of a capacitor divider in one of its legs with a Voltage source inverter in order to integrate the 2 renewable three-phase AC sources to utility. Given that in the suggested structure the number of switches is reduced so, it causes a considerable reduction in price of the system and switching losses. Also the presented control scheme can be used in the 9-switches converter when one of the legs is failed. This subject increases the reliability of system. The control scheme and modulation method guarantee the quality of input and output current waveforms and unity power factor of each renewable sources. The presented simulation results validate the efficiency of the proposed control system and modulation method for suggested converter. Also the presented simulation results show the dependent power flow control for each one of distributed sources. Due to the nonlinearity of the system, in this paper the particle swarm optimization (PSO) is applied to find the optimum Values of parameters of power flow controller.

After fault occurrence, service restoration in Power Distribution Networks (PDNs) is an emergency condition. Therefore, immediate restoration of out-of-service areas requires fast evaluation and effectiveness solution for keeping customer’s satisfaction and increasing network’s reliability. This paper proposes a fast and effective method based on a new heuristic algorithm for service restoration implementation in PDNs considering Load Shedding. The proposed problem formulation consists of four different objective functions: First, maximizing the amount of total load restored, second, minimizing the number of the switching operations, third, considering customer’s priority, and finally, minimizing total Load Shedding. For showing the validity of results in different condition, three levels for load demand have been considered. The fastness and effectiveness of this approach has been tested on the IEEE 119-bus PDN.

In this research, placement of UPFC is presented on dynamic stability by using the residue factor and critical modes of power system. The placement by using critical modes determines the requirement number of UPFC to enhance damping of power system oscillations. The supplementary controller is used to control UPFC The input signal for this controller is selected based on the participation of the generators speed state variables in the critical modes of power system. The effectiveness of UPFC in power system stability enhancement are demonstrated on IEEE 9-BUS system by drawing the speed and angle deviations curve of generators, the system poles locations and Bode diagram.

Voltage stability margin can be calculated by tracing PV curves using the Continuation Power Flow (CPF) method. This method needs many power flow solutions and consequently a large number of computation times. It is well known that the behavior of PV curves, especially near the critical point, is similar to a quadratic function. This is used in some papers to approximate PV curves and calculate maximum loading point by minimum number of power flow runs. This paper aims to complete previous works so that the computational efforts are reduced and the accuracy is maintained. To do this, a quadratic function with two constant coefficients, instead of three ones, is used. This makes the calculation of the quadratic function simpler because the determination of the second-order sensitivities is no longer needed. Also a method is proposed to select the initial operating point near the critical point. For this purpose, in the power flow equations, the voltage magnitude at one of the weak buses is assumed to be known and the loading Factor is unknown instead. This is also used to determine the next operating point, after every approximation of PV curves. The approximations continue until the difference between the calculated voltage stability margins in successive iterations becomes less than 0.0005. The simulations are performed on three IEEE test systems show the effectiveness of the proposed method.

One of the three aspects of the stability in power systems is voltage stability. Since voltage collapse can be a reason of the global blackout in power networks, the importance of this study is revealed. In this regard, this paper focuses on transient voltage stability problem and to achieve this goal we use wide area measurement system. To transient voltage stability assessment, a novel fast index is used. The index in one of its three parts monitors the produced reactive power in the network. Important events like generator outage, line outage and load increasing are studied in the paper. Simulation results in the DIgSILENT Power Factory software on IEEE 39 buses dynamic system demonstrate the efficiency of proposed approach with respect to other methods.

In this paper combination of battery, ultra capacitor and fuel cell are used to feed induction motor which used in hybrid vehicle. Fast dynamic response is one of the critical criteria in hybrid vehicles. So in this paper Direct Torque Control (DTC) method has been used. Fuel cell can solve some of electrical vehicle problems but large variation of fuel cell output voltage can derange DTC. In this paper an accurate fuel cell dynamic model has been extract and effect of important parameters such as temperature and pressure have been analyzed. The proposed structure for hybrid vehicle has cold start capability. Also in order to increase efficiency and controllability, two induction motors are used in vehicle which a common inverter is used to drive them.Finally hybrid vehicle has been tested in standard driving cycle which results show ability of proposed structure and control method.