The phase to ground fault is the most probable fault in power system. Generally, the grounding resistor in neutral point of transformers is utilized to limit the current for this fault. However, this resistor decreases the sensitivity of the differential protection. Fault current limiter reduces the high fault currents, however, the signals used as inputs of the differential protection may be adversely affected by the Fault current limiter and this may lead to wrong operation of this protection. Thus, the performance of the transformer differential protection performance evaluation is required in the presence of a fault current limiter in the neutral point.
In this thesis, besides studying the phenomenon of inrush current in the transformer, two methods of differential current gradient and median absolute deviation of discrete wavelet transform are introduced to discrimented it from internal fault. The results, are compared by modeling in a software environment. The algorithms are implemented on a typical power system including a three-winding power transformer (220/132/50 kV) which is supplied with a 220 kV transmission line.The sample power system is simulated using the PSCAD/EMTDC software and the proposed algorithm is implemented in MATLAB software. In both algorithms, median absolute deviation of data is used to discrimination the inrush current and internal fault. The effect of different parameters, like fault type, fault resistance, fault inception angle, transmission line length and presence of the fault current limiter has been simulated.
The simulation results show that the error in performance of the current gradient method is increased in the presence of a fault current limiter in the neutral point of the transformer. In addition, in contrast to the gradient method, the algorithm based on discrete wavelet transform, for all cases, is able to correctly identify the internal fault from the inrush current, in the presence of fault current limiter.

 

Due to the many advantages of series compensators in transmission lines, the use of these elements in the transmission lines is increasing rapidly. Due to the effect of the compensator on the positive sequence impedance of the line as well as existence of a coupling between two circuits in the double circuits line, it is difficult to accurately locate the faults in series compensated double-circuit transmission lines by conventional relays. In these conditions the distance relays are under-reached or over-reached that results in their inaccurate performance. Three Algorithms are proposed in this thesis for the fault location of the series compensated double-circuit transmission lines. The proposed algorithms are traveling wave–based and the wavelet transform is used to extract the exact time of the arrival of waves at relay location.Initially, the effect of the series compensator on traveling waves and the fault location algorithms in the series compensated single transmission line is discussed. In these algorithms, Clark's transform has been used for phase-to-modal conversion. Then the proposed algorithms for fault location of the series compensated double-circuit transmission lines. At first, two algorithms have been proposed by using the traveling waves at one end of the line, one for faults on one circuit and the other one for faults between the two circuits. Then, an algorithm has been proposed for all faults in these lines using the traveling waves receiving at two ends of the line. The Karen-Bauer transform has been used for phase-to-modal conversion in all the three proposed algorithms. Also, polarities of the first two receiving traveling waves have been used for determining the half-side location of the faults. Finally, the effect of the presence of TCSC on the proposed algorithms has been studied. The test system has been simulated in PSCAD/EMTDC software and the algorithms's performance has been evaluated in MATLAB software. The effect of different parameters such as fault type, location, inception angle and resistance, compensation degree, the compensator location, impact of the MOV, sources impedance and presence of the TCSC has been investigated. The simulation results indicate the high accuracy of the proposed algorithms at various conditions

 Most part of protection task of power system is covered by distance protection and hereon is covered by distance relays. This relay has numerous weaknesses that leads to its maloperation and causes disturbance in power system protection process especially in stress condition. These weaknesses eclipse power system security and if not removed, may cause black outs in vast geographical scales. Therefore presenting solutions for these cases had always been preferences of power system and many solutions have been presented over the years. Newest devices which are used by researchers to solve distance relays problems are phasor measurement units. These units with real time measuring and other capabilities like connecting to global positiong system, almost are capable to solve distance relay problems. Purpose of performing this project is to study and observe feasibility of phasor measurement units to decrease protection problems. In this project, a new protection scheme based on phasor measurement units would be introduced which has not got the problems of distance relays and would cover system protection with high security factor. With respect to high price of phasor measurement units and impossibility installing this device on whole system busses, minimal number of this device must be installed on network in such a way to perform our purposes in covering complete protection of transmission network. Covering complete protection of transmission network is accessible with fault location. So optimized fault locating of phasor measurement units should be performed with purpose of fault observability. In section optimized placement of phasor measurement units, first fundamental concepts and different algorithms of optimal phasor measurement units placement would be explained. Then a comparison would be performed between different algorithms and then final algorithm would be selected. After that, conventional concept of fault observability and new impression of it would be discussed. Then, conventional and proposed protection scheme would be presented. To validate optimal placement which has done and also proposed protection scheme, some sample networks are selected and simulations of each section are stablished on these networks. IEEE 9 bus system is our main network, so optimal phasor measurement unit placement plus all parts of protection scheme would be simulated on it. It should be noted that present work includes most parts of other big project called "Feasibility study of using capabilities of phasor measurement units (PMUs) to improve the operation of distance relays in transmission lines of the Khouzestan power network". In section of phasor measurement unit placement, more over results of optimal placement on sample networks, resuls of optimal placement on 230 and 400 kilovolts transmission system of province Khouzestan would be presented.

For the protection power transformers in return of short-circuit internal, is typically used a differential relay. Differential relay should act quickly when internal errors occur, while in non-error conditions, such as the inflow of an offset current, it must remain stable. In this thesis, we present a multi-criteria stabilization algorithm based on fuzzy logic that is used to improve the performance of differential relay in the protection of power transformers in the event of internal faults and also the transformer energization. The aim of this thesis is provide an algorithm based on fuzzy logic so that it can use Fuzzy combination of suitable criterion signals that are used to analyze the current signals measured by the current transformers, Different energization of the transformer, regardless of the amount of the second harmonic component, whose value changes due to the saturation of current transformers and the use of amorphous materials in the core of the transformers, until correctly detects the inrush current. These factors cause operation of the differential relay to be inaccurate with the second harmonic restraint method. Also, by employing the fuzzy combination of criterion signals, the speed of the algorithm's performance for internal fault and its increases the sensitivity rate to low current internal fault (turn to turn internal fault). To evaluate presented algorithm in this study, the different conditions of the transformer operation are simulated by MATLAB software. Finally, the presented simulation results will be compared with the traditional method.

State estimation is a basic operation for monitoring, protection, and control of power systems. Implementation of state estimation algorithms in low voltage power systems like micro-grids is a sophisticated process due to the complicated nonlinear model of renewable power sources and the effect of load and power line on the their dynamics. Thus the dynamic state estimation of common synchronous and variable speed generators, such as doubly fed induction generators (DFIGs) and permanent magnet synchronous generators, (PMSGs) connected to the infinite bus, is studied in this thesis.In this condition, the state variables are not affected by the grid parameters. Then, using the load and line model these energy resources have been generalized to a sample microgrid connected to a grid. In order to get closer to the real situation of dynamic state estimation in the practical conditions, a multi-agent and a centralized scheme has been proposed for this microgrid. In the centralized scheme, all the measurement variables and constant parameters are sent to the control center, and are estimated there. In the multi-agent scheme, a local controller is considered for each generation unit which estimates the load, line and connected generator state variables. The dynamic states are estimated by estimator algorithms explained below for both of the aforementioned schemes.
Selecting a proper method for estimation of power system dynamic variables is complicated because of the large number of state variables and strongly nonlinear dynamic models of the asynchronous and synchronous generators. Therefore, seven different methods, based on Extended Kalman Filter (EKF), Unscented Kalman filter (UKF), and Particle Filter (PF), are used in this study to estimate these variables. A structure formerly used for UKF has been added to the EKF to reduce the measurement errors. A structure formerly used for UKF is added to the EKF to reduce measurement errors. In addition to UKF standard structure, corrected UKF is used to improve estimation accuracy. The PF method is well known as a strong method in dynamic state estimation. PF implements a resampling method which may cause problems in estimation of a high-order state matrix. In addition to standard structure, three different resampling methods, including Deterministic Particle Filter, Stratified Particle Filter and Residual Particle Filter, are used to overcome this problem. This methods are compared with each other based on estimation time and accuracy factors.
 

Under voltage and frequency load shedding schemes implemented to overcome events separately. Hence, in presented schemes considered only voltage or frequency conditions that these schemes couldn’t operate properly in load shedding and support the networks in severe and combinational events. In recent years, hybrid decentralized and centralized load shedding schemes presented to prevent from instability network. These methods could apply load shedding with considering voltage and frequency conditions at the same time. Relays setting was determined by try and test in previous combinational load shedding methods and these settings wasn’t optimized. In this thesis, proposed a combinational decentralized scheme based on intelligence optimization algorithm that minimizes load shedding value and maximizes voltage and frequency of power systems to employ an optimal and sufficient load shedding scheme. Although, relay structures differ from previous investigations in a proposed scheme. Voltage stability evaluated by using L-index and relays using from local data such as current, voltage and frequency. This load shedding scheme is using from Particle Swarm optimization (PSO). MATLAB software interlink with DIgSILENT software to set relays with using optimization algorithm. First, relay setting information received from MATLAB, then the proposed relay set in plotted network in DIgSILENT software. Power system stability evaluate with simulating different events automatically and eventually obtain relay settings. The proposed load shedding method was tested on dynamic and widespread Khorasan network that considered as large and real network. Comparison simulation results between the proposed scheme and traditional load shedding scheme and two previous load shedding schemes demonstrated the best performance of the proposed scheme. Despite significant reduction in load shedding value, this scheme has more proper performance in marginal stability and steady state frequency.

Due to increasing demand for energy, power systems are operated near about their stability. That is why maintaining system stability under fault conditions, is a very important matter to avoid the blackout in power systems. After all compensatory measures and controls to maintain voltage stability, lose their effectiveness may only be shed load. In this thesis, to find a suitable location of load shedding, a method based on sensitivity coefficients voltage path (TSF) is used. This method determines the amount and locations of load shedding so that the greatest improvement in voltage profile results. The method presented in this thesis, apply offline and aspects of the study - research. Due to the increasing presence of wind turbines in power systems, in this thesis, the performance of load shedding voltage, in the presence of wind turbines connected to the DFIG (DFIG), is examined. To evaluate the improvement of network voltage stability by load shedding, fast voltage stability index (FVSI) is used. In addition, by combining two indicators TSF and FVSI, a new index called the Fast Voltage Stability Index Terajectory Sensitivity (FVSITS) has been introduced. Performance of two ways is evaluated in 14 bus network standard ieee, The simulation results show that the new indicator disclosed better results than the index TSF.

Convenient economical consideration is the main benefit for using over current relays in the distribution networks. Directional over current relay is carried out to get proper setting that the nearest relay could operate in fault location and minimize load clipping to increase reliability of the distribution networks. Over current relay studies generally evaluate based on a fixed network topology. Distribution network utilization could cause conversion in the network topology, like input and output of loads or a generator that could disturb relay coordination and increase time operation of the relays. In the other hand, high penetration of DG’s might increase relatability and supply backup power during outage of units and, etc. Although, it could make directional over current relay coordination to a challengeable problem. DG’s high penetration could increase fault level current rating in the distribution networks, also it might convert these networks from one way to two ways power transmission. It is real challenge for system, and directional over current relays is using to overcome this problem. The principle objective of this thesis, is to propose an adaptive scheme for direction over current relay coordination in distribution networks with considering distributed generations. This scheme could be updated directional over current relays setting in dynamic changes of network topology. Also, it was used from a new time-current-voltage characteristic for directional overcurrent relays, in addition to a bus current, this new characteristic depends on voltage and current of a bus. Directional over current relay coordination is formulated as an optimization problem and was used from a genetic algorithm to solve it. Genetic algorithm has a good accurate and high speed performance. The distribution network analysis like load currents and short-circuit currents obtained from DIGSILENT software and genetic algorithm applied in MATLAB software. Also, the operation time of directional over current relay optimized with Multi-Objective genetic algorithm and compare it with single-object genetic algorithm. The proposed characteristic is tested on the power distribution system of the IEEE 14 bus and IEEE 30 bus.

 

In recent years, occurance of blackouts due to severe and combined faults has become one of the most important problems in power systems. Traditional methods of under frequency and under voltage load shedding are used as the last line of defense for power system security and stability. These methods are act independently and based on fixed setting. As an result, these methods are ineffective against sever combinational events. To tackle these problem, some combined local and centralized load shedding methods have been recently suggested. The aim of this thesis is to propse some adaptive centralized combinational load shedding schemes for self-healing of power system that maintaine the power system voltage and frequency stability, minimize the load shed amount and the economic cost of load shedding. For this purpose, in this thesis three centralized combinational load shedding schemes are suggested. In the first scheme, selection of suitable buses for load shedding is based on the voltage drop and the voltage stability risk index. In the second scheme, in addition to the above indices, the load shedding costs are considered in selection of suitable buses for load shedding and the total load shedding power is divided between all the loads with a certain ratio. In the third scheme, the power system protection against cascading faults is also planned and in addition, to increase the social welfare, selection of the load shedding buses is done in such a way that the number of customers who are cut off is reduced compared to the second scheme. In this scheme, in order to compensate some of the problems related to the VSRI index, a voltage stability index based on the maximum power limit through the transmission lines index is used. Based on the results of simulations obtained for the standard IEEE 39-bus network, the propsed schemes has a better performance compared to the previously proposed methods. The third schem has a better performance, in comparison to the second scheme, since its results in a better power stability margin, steady-state frequency and social welfare, but its economic benefits are slightly lower.

Electric power transmission and distribution lines have a vital role in the power system. Consequently, proper functioning of protective relays in transmission and distribution lines is of great importance. Moreover, quick elimination of a fault in transmission lines is essential for the improvement of system stability and reliability. In this thesis, a new fault location method for locating faults for Cross-country earthed and nonearthed faults in parallel double-circuit transmission lines is proposed. The proposed method is based on traveling waves and the wavelet transform. A very appropriate method to locate the fault is taking the wavelet transform of the Clarke transformation components in order to obtain the traveling waves caused by the fault. By obtaining the time when traveling waves reach the busbar or busbars, we can calculate the distance of the fault from the busbars. In this thesis, fault location is first done for simple lines using two methods: analyzing the signals of current or voltage from both ends of the line and from one end of the line. Afterwards, fault location is done for simple faults in parallel transmission lines using data from both ends of the line. Finally, a new algorithm using data from both ends of the line as well as data from one end of one of the lines is proposed for faults between two circuits. In the main part of this algorithm, in which the fault is located using data from one end of the line, fault location is done for the faults between two circuits. To assess the proposed method, a grid model (parallel double-circuit line) is implemented in the PSCAD/EMTDC environment, and then the output data is analyzed in the MATLAB software. The simulation results show that the proposed method calculates fault location accurately for faults between a nonearthed and a earthed circuit. Furthermore, fault resistance, charge flow, source impedance, and even fault distance do not influence the accuracy of this algorithm. To compare the proposed algorithm with other fault location methods, the distance protection method, based on symmetrical components (using data from one end of one of the lines) is also simulated for faults between two nonearthed circuits. The simulation results show that the proposed algorithm has a much lower error rate in fault location than the symmetrical components method. In fact, the location of the fault is calculated very accurately.

In this thesis, a two-area power network is studied for two cases of thermal-thermal and thermal-hydro generation. The load distortion in this system leads to deviation of the system frequency and oscillations of the transmission line power flow. The purpose of this research is to reduce the frequency deviation and oscillations of the transmission line power flow during the system faults. When the load disturbances are applied, the governor nonlinear reactions will produce a sustaining oscillation in the area frequency and tie-line power transient responses. To stabilize the system for such load disturbances, two methods has been utilized and the system performance has been analyzed for the two cases. In the first case, a Thyristor controlled phase shifter (TCPS) is installed in series with the tie-line between the two areas and a superconducting magnetic energy storage (SMES) is installed at the terminal of the second area. These devices are coordinately controlled. In the second case, SMES is installed at the terminals of both areas. The simulation results indicate that the system stability is improved in the first method for the both cases of thermal-thermal and thermal-hydro generation. In the second method (the use of SMES in the terminals of both areas),the system becomes more stable than the first method. Next, The particle swarm optimization (PSO) algorithm has been used to improve the the system performance. This algorithm is used to optimize the coefficients of the integral controller and the fuzzy logic PID (FLPID) controller as well as the parameters of SMES and TCPS. In the thermal-thermal system, to improve the dynamic performance of the system in addition to using SMES-SMES, a fuzzy logic controller has been used instead of the integral controller. Simulations results show that using of the fuzzy logic controller with SMES in two areas improve the dynamic performance of thermal-thermal system.

In this study, a new optimized combinational load shedding method is proposed to improve inadequate performance of the conventional under frequency load shedding scheme and the uneconomical performance of previous combinational load shedding schemes. The proposed method uses locally measured frequency and voltage signals to react such events. The main contribution of this thesis is prposing an algorithm for optimal tuning of the setting of the load shedding relays.These settings are derived using a local voltage-stability index called L alongside considering frequency status as an index and also, using a new flexible and simple logicalbasis for relay operation. The new approach makes it possible to determine the optimal speed, location, and amount of load shedding automatically and adaptively depending on the disturbance location, voltage status and the frequency decline in any network. The new index was rigorously tested on different test systems and the proposed load shedding method have been simulated in an actual large network by DIgSILENT software. Obtained simulation results confirm that by using the proposed algorithms, various power system blackouts could be prevented and power system stability could be preserved adequately and economically.