Sharif Digital Repository

Since the power systems usually are being operated close to their nominal capacity due to economical reasons, the probability of wide blackouts in power grids have been increased. However by employing suitable strategies the probability of huge blackouts is still very small but if they occur may put the power system operators in trouble while restoring the system. Thus many studies have been conducted in recent years dealing with power system restoration planning to achieve fast and reliable restoration plans. The scope of this project is overvoltage assessment during system restoration. In this manner we can find the optimal strategy with the least overvoltage problems to prevent the... 

Nowadays, due to the indensible role of electrical energy in life of societies, the reliability of service should be provided in an economic form. Researches has shown that, about 80% of faults occur in the distribution network. In a distribution network that is equipped with automation systems, through the acceleration in the determination of the fault location, and also possibility of reacting against transient and steady faults, the electricity interruption time duration is minimized. This thesis aims to determine the number and to locate the auto reclosers and fault indicators in a distribution network optimally. To make it economically feasible, a limited number of these devices and... 

Partial discharges that occur in a transformer insulation, generate current pulses. If these pulses be recorded, they can be used for transformer insulation condition assessment. Through processing of these recorded partial discharge signals, the PRPD patterns are generated and used to identify the source type of partial discharge defect. If multiple partial discharge defects exist in a transformer insulation, the related PRPD pattern, doesn’t look like any PRPD patterns of single defects. In this case, we need in the first step to discriminate the partial discharge signals stemmed from all the existing multiple partial discharge sources. To simulate the occurrence of multiple partial... 

In power system operation, the term "operation planning" carries a wide range of meaning. For some utilities, operation planning denotes primarily short-term planning tasks (up to several days), such as load forecasting, unit commitment, hydro-thermal coordination, transaction pricing, fuel allocation, and security analysis while for other companies operational planning is interpreted in a wider context, and include mid-term planning activities (up to several months), such as maintenance planning, fuel budgeting, rate forecasting, network planning, relay coordination, etc. The mid-term planning provides the link between the long-term and short-term planning programs. The duration of this... 

In two past decades, a revolution has occurred by emerging of substation automation system in field of substation control and protection system. The traditional control system has changed to the modern automation systems which are based on computer networks. The development of this project become possible by having experience in and being familiar with automation systems, studying equipment specifications, tasks, and criteria and analyzing operation of various systems. The main target of this thesis is evaluating the automation system precisely and exhaustively and developing a model based on mathematical relations. This model is founded based on the application and analysis of the networks.... 

Inrush Current is a form of over-current that occurs during energization of a transformer. For power transformers, the magnitude of the first peak of inrush current is initially several times the rated load current but slowly decreases until it finally reaches the normal exciting current value. This process typically takes several minutes. Time-variant axial and radial electromagnetic forces under inrush current and short-circuit current are calculated (by finite element method; FEM), and compared. An analytic method is presented for computation of inrush current in transformer. The simulation results obtained by this proposed method are compared with the measured values. A compact... 

Capacitor setting and reconfiguration are of main means of loss reduction and voltage profile improvement in distribution system operation. Cooperation between these two strategies can effectively minimize power losses in distribution system automation. Also, from the medium and long term viewpoint, capacitor placement and reconfiguration can be used for loss reduction and reliability enhancement. In this section, the sum of losses cost, capacitor investment cost and reliability cost should be minimized. Regarding the need to solve two non-linear combinatorial optimization problems simultaneously, metaheuristic algorithms are used in this thesis. Specifically, Elitist Genetic Algorithm,... 

The price that customers pay for electric energy is determined by cost of generation, transmission and distribution parts. In Practice all customer have main concern about reliability of electric energy, because of it’s major effect on different aspect of life. Although customers are interested to have fully reliable service, but power companies are aware that it is impossible to achieve this goal from technical and economical aspect. The utilities and customer benefits can be achieved through minimization of the total investment cost of network, while improving the reliability and quality of electric service. This also requires to minimize the customers service interruption time. To meet... 

With new progresses in technology, a growing need to power energy has emerged. In Iran most of power plants are far from cities and therefore expansion of power network will cause long transmission lines with high capacities which connect power plants to cities. There are three solutions for these kinds of lines: 1) two-circuit transmission lines 2) higher voltages 3) new forms of transmission lines (HVDC and compact). In this thesis some of these suggestions have been analyzed to find the best solution for a 560 Km transmission line in Iran.
Designing transmission lines has a lot of steps which all are related to each other. In this project a comprehensive algorithm has been utilized to... 

There is an increasing trend nowadays toward Distributed Generation (DGs). DG poses positive and negative side effects for both utility and customers. Reduction of system expansion costs, decreasing power loss and reliability enhancement are some benefits of DGs. Connecting DGs to the distribution system can lead to some troublesome issues. Anti-islanding protection is one of these issues. Islanding is the situation in which a distribution system becomes electrically isolated from the remainder of the power system, yet continues to be energized by DG connected to it. Traditionally, a distribution system doesn’t have any active power generating source in it and it doesn’t get power in case... 

Transformers are one of the most important equipments in transmission and distribution networks. Transformer unplanned outages have severe impacts on the continuity of power system operation. To improve the reliability of transformers and to achieve an optimum operation cost, online condition monitoring of transformers is inevitable. Information about the quality of the transformers insulation system is known as the best parameter to be monitored in a transformer. Since partial discharge signals are initiated long before the beginning of a severe damage, partial discharge monitoring and its evaluation canbe employed to warn the operator.Data mining on the partial discharge signals extracts... 

Feeder reconfiguration is one of the most prominent tasks for loss reduction, reliability improvement and also congestion alleviation in distribution networks. Besides improving abovementioned operational issues, during a long period of time, proper switching in appropriate times can causes a lot of economic savings. Furthermore, it can be intensified when capacitors are implemented and controlled in the network. To reach this purpose, most of the previous literature focuses merely on the power grid reconfiguration. Few of these researches have considered this problem as a dynamic one and studied that in a period of time, e.g. one day, considering fixed level of loads despite the others that... 

Transformer are one of the most important equipment in transmission and distribution network. Transformer unplanned outage have severe impacts on the continuity of power system operation and is also an irreparable economic harm to power network operators. To improve the reliability of transformers and to achieve an optimum operation cost, online condition monitoring is inevitbale. Information about the quality of the transformer insulation system is known as the best parameter to be monitored in transformer. Since partiale discharge (PD) signals are initiated long before the beginning of a severe damage, monitoring and its evaluation can be employed to warn the operator. Data mining on the... 

The goal of this master thesis is to develop a new accurate numerical algorithm for digital differential relay to protect a power transformer. This work employs specially developed software to simplify its input signal processing task which helps to quickly and accurately detect and clear the transformer internal faults. The available commercial transformer digital differential relays lack the required sensitivity to detect turn to turn faults when a few numbers of turns (less than 10% of the winding) are involved. This work presents a new equivalent circuit for transformer when a turn to turn fault occurs on either winding. This equivalent circuit helped to develop an accurate algorithm for... 

Electrical energy customer and load growth on the one hand and the problems of allocation of land for the development of the power networks on the other hand, make the tendency to use less space in the electrical industries. One of the solutions used in power distribution networks, especially in crowded areas is compact substation which requires less space than ordinary substations and also the prefabricated types of this substation can be used in the distribution system. In a compact distribution substation, all components must be designed with a minimum size. Therefore, the insulation system of the compact substation should be designed based on these requirements.
In the distribution... 

The work starts with comparison of 3D-wound core transformers against the conventional transformers using the published research results. The past works on transformer short circuit force calculation are reviewed. An algorithm is developed to design 3D-wound core distribution transformers. To determine the maximum short circuit forces, currents in different type of short circuits have been calculated. The worst case in different core configurations and different type of winding connections is determined. Different analytical methods are investigated for transformer short circuit force calculation. Two-dimensional and three-dimensioal finite element methods (using Comsol software) employed to... 

The purpose of carrying out this Master thesis is to detect occurrence of CT saturation which leads to consecutive failures in operation of protective current transformers, as a major component in protection of power systems. The protective relay installed in the secondary side of the protective CT fails to act properly under occurrence of this phenomenon, and it doesn’t reflect proportionally the high value of fault currents passing through CT’s primary winding. As a consequence, faults become undetectable. Meanwhile, the proposed methods for detection of CT saturation must be reliable under a noisy condition, realizing that power systems have some inherent noises always available in the... 

Transformers are the key component in power system transmission and distribution networks. Condition based maintenance will increase their expected life and online monitoring is essential to ensure operation reliability. In this work a new approach to transformer online monitoring is provided based on partial discharge (PD) measurement.Multi-source PD signal separated using time-frequency S transform (ST) that is applied to the PD signal waveforms. The resultant ST matrix is then converted to gray scale image from which high level features are extracted using Bag of Words (BoW). Gaussian mixture model (GMM) clustering is used to discover clusters in the feature space. For recognition of... 

During natural disasters, especially an earthquake the distribution systems operation is severely affected and interrupted by facing many failures. More than 80% of these outages are due to failures in distribution Systems. Therefore, it is crucial to enhance the resilience of the distribution systems (DSs) against these disasters. However, nowadays, due to limitation of resources for investment in these networks, only the most efficient measures should be implemented. In this work, studying the recent earthquakes in different countries, we have tried to identify the fundamental causes of failure of important network equipment, including distribution substations equipment and network...