فهرست مطالب عارف درودی

The inertia constant, a key parameter that makes power systems robust to disturbances and disruptions, has today been changed by factors such as the influence of renewable generation units and the reduction of traditional synchronous generators. Changes in the inertia of a power network play an important role in its performance, and a lack of knowledge of its status will have severe consequences for the network's online protection and control systems. Hence, knowledge of its online changes is essential for transmission system operators. This paper presents an online inertia estimation method that is based on inter-area electromechanical oscillations and simultaneously estimates the network inertia constant and the system damping coefficient. Unlike the previous methods that have used the flow of active power between two areas, the oscillation parameters are extracted from the voltage angle of the buses, improving the accuracy of the estimate. For this purpose and based on the classical swing equation, the relationship between the inertia constant of the system and the electromechanical oscillation parameters is first determined by the Prony technique. Then, the network equivalent inertia and damping coefficient are estimated. The efficiency and accuracy of the proposed method are verified by DigiSilent and MATLAB software and in several scenarios implemented on the IEEE 39-bus standard network. The simulation results show the appropriate accuracy of the proposed method. It is shown that the results of estimating the inertia and damping coefficient based on the active power signal have an average error percentage of 10 and 17.32, respectively. After using the voltage angle signal, the results are improved to 9.03 and 11.94, respectively. Due to the simplicity and small burden of calculations, the presented method can be easily used by network operators.

Induction motors have numerous applications in various industries. In order to avoid industrial processes failures, fast and timely fault detection of motors is essential. In this paper, a novel method is presented for on-line detection of broken rotor bars which is a common rotor faults in induction motors. The method is based on the stator current analysis and signal processing using continuous wavelet transform (CWT). The most advantage of the method is that it detects the broken rotor bars in normal operations and at different loading conditions. In this method, the stator current signal should first be analyzed in the healthy and faulty conditions and then the broken bar can be diagnosed using a proposed index. Finite element method (FEM) is applied to simulate the motor under healthy and faulty conditions.

Power Systems are continuously exposed to large and small disturbances. Disturbances can stimulate the electrical modes of power system and produce transient oscillations in system quantities. The precise modeling of all equipment including loads is very important for power system stability analysis. In this paper, the effect and importance of different load models on the load angle stability of a typical power network are first shown. It will be shown that Dynamic load model is necessary for stability analysis. Then, the parameters of the chosen dynamic model are estimated using unscented Kalman Filter (UKF) and "Event Playback" method. Voltage and frequency of the busbar after occurrence of an event are injected to the bus and the parameters are estimated.

Fault location is an interesting subject in power grids. On the other hand, the application of fault current limiter (FCL) has been increasing in power network and causes some problems in operation of protection systems. This paper studies the application of FCL in power system and its effect on one-ended simple reactance fault location methods. In these methods, only voltages and currents of transmission line sending-end terminal which equipped with measuring equipment are used to estimate the fault location. Presence of FCL in the time of fault occurrence makes the existing reactance methods inaccurate and causes malfunction of fault location devices. In this paper, a new method has been proposed to improve fault location in transmission lines equipped with FCL. To evaluate the effectiveness of the proposed method, various faults on different locations of transmission lines have been applied to the simulated power grid. This method implies an irritation process to calculate the remote terminal impedance and subsequently, fault current and location. The accuracy and effectiveness of the proposed method have been proved through simulations.

One of the important power quality issues is voltage sag. The voltage sag can generate many undesirable effects on electrical equipment. The major effect of voltage sag on electrical machine is torque pulsations after and during voltage sag. In this paper¡ using a simple theoretical analysis called stator flux trajectory method¡ producing manner of the torque pulsations and the impact of sag characteristics on the pulsations severity are investigated. Also¡ the critical characteristics that have the worst effects on machine behavior are identified. sag magnitude and duration and initial point-on-wave are the major characteristics that can influence the motor behavior. To justify the theoretical analysis¡ a typical synchronous motor simulated by applying different sag types and the results compared with the theoretical analysis. The results of the simulation confirm the validity of the proposed analysis.

One of the most important power quality aspects in wind farms is voltage fluctuation or flicker which should be investigated due to the nature of wind speed variations. These variations result in power and voltage fluctuations at the load bus. Moreover، the wind generation systems may be assumed as a harmonics source because of their power electronic converters. There are numerous factors that affect flicker and harmonic emission of grid-connected wind turbines during continuous operation، such as wind characteristics (e. g. mean wind speed، turbulence intensity)، type of generator and grid conditions (e. g. short circuit capacity، grid impedance angle). In this paper، an IEC based flickermeter is first modeled and then a variable speed wind turbine has been simulated by Matlab/Simulink software. The flicker and harmonics emissions of wind turbines equipped with DFIG during continuous operation and using output reactive control are investigated. The simulation results show that control of wind turbine output reactive power is an effective means for flicker mitigation during continuous operation. However، there should be a compromise between flicker reduction and harmonics level increase to enhance the whole power quality of wind turbine.

In this paper, derating factor of induction motors that are subjected to regular fluctuations (flicker) has been calculated. Voltage flicker is one of the most important phenomena in power systems study. The phenomenon may cause additional losses and decrease the efficiency of the induction motors. Due to huge amount of induction motors in the different industries, it is important to study the effect of voltage fluctuation on the induction motors behavior. The paper applies electrical, mechanical and thermal models of induction motors to estimate additional losses, temperature rise and derating factor.