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Chapter One Introduction CHAPTER ONE INTRODUCTION 1.1 General Concepts The practice of grounding of electrical systems is almost as old as the development and widespread use of electric power itself, Poor grounding not only contributes to unnecessary downtime, but a lack of good grounding is also dangerous and increases the risk of equipment failure. Without an effective grounding system, we could be exposed to the risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas. If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people. However, good grounding isn‟t only for safety; it is also used to prevent damage to industrial plants and equipment. A good grounding system will improve the reliability of equipment and reduce the likelihood of damage due to lightning or fault currents. Billions are lost each year in the workplace due to electrical fires. This does not account for related litigation costs and loss of personal and corporate productivity.(" Practical Grounding, Bonding, Shielding and Surge Protection") 1.2 Problem Statement This research aims to study the effect of earthing resistance on the service voltage of distribution transformers. 1.3 Objectives This research aims to study the effect of earthing resistance on the service voltage of distribution transformers, decrease of the earthing resistance of distribution transformers, and also balance of distribution of loads to face transformers faults. 1 1.4 Methodology Earth resistance readings obtained by using a device named (Earth/Ground tester 1625). Voltage readings of earth to neutral, line to neutral, line to earth, and loads obtained by using a device named Clampmeter 1.5 Project layout This Project consists of five chapters, Chapter one deals with an introduction that consists of, general concepts, problem statement, objective and project organization. .Chapter two consist of an introduction of distribution transformers, its classification, its connection, its component, grounding, earthing, terms used in electrical earthing, its function, ungrounded systems, its types, grounding resistance, and point of grounding. Chapter three consist of an introduction, grounding electrode, soil resistance, measurement of soil resistivity, resistance of a single rod electrode, Current-Carrying Capacity of an Electrode, Use of Multiple Ground Rods in Parallel, Measurement of Ground Resistance of an Electrode, Concrete-Encased Electrodes, Corrosion Problems in Electrical Grounding Systems, Maintenance of Grounding System and Chemical Electrodes. Chapter Four includes, Introduction, Results and Analysis.Chapter five include conclusions and recommendation. 2 CHAPTER TWO DISTRIBUTION TRANSFORMERS AND GROUNDING 2.1 Introduction Distribution transformer is a static device consisting of a winding, or two or more coupled windings, with or without a magnetic core, for inducing mutual coupling between circuits. They are exclusively used in electric power systems to distribute power by electromagnetic induction between circuits at the same frequency, usually with changed values of voltage and current. Distribution transformers is one of the important electrical isolation transformers which provide the final output voltage to the end users. as the name specifies it distributes the power to costumers depends on their need. The basic function of distribution transformers is to step down the line voltage (33KV–11KV–6.6KV) to the consumer low voltage (415V) for supplying it to them. Because normally the transmission line voltages will be high to increase the power transfer without losses but the electrical devices which the consumer uses will operate for low voltage. So the line voltage will be stepped down to consumer level and distribute it to them efficiently. Distribution transformers are used extensively by traditional electric utility companies, power plants, and industrial plants. As mentioned above, they perform a very simple function and they can have many applications. Transformers are used in every power plant, all grid substations, buildings, in the industry, the underground installations, wind turbines, on platforms, marine vessels, under the sea, etc. Due to peculiarities of all these applications, many different types of distribution transformers have been developed in the course of history.(" Practical Troubleshooting Electrical Equipment") 3 2.2 Classification To simplify the overview of many distribution transformer types, it is useful to have some kind of systematic classification. However, this is not easy to do because there are many ways of doing it. The distribution transformer types could be classified according to their power rating, voltage, current, weight, type of cooling etc., but such approach would have a limited applicability. 2.2.1 classification due to number of phases in A- Single a phase transformers. B- Three-phase transformers. In a three-phase system, the single-phase units are used in a bank of three transformers linked together. A single three-phase transformer costs approximately 15% less and occupies less space than one unit of three single-phase transformers within the same tank. However, due to limitations during the manufacturing and mainly transportation, particularly of large units, the transformers sometimes must be produced as single-phase transformers. Another reason for using a single-phase unit rather than a three-phase unit is the possibility of having a fourth identical unit as a spare. Despite its simplicity and clarity, this type of classification does not overly help in classification of the whole distribution transformers family.("Practical Troubleshooting Electrical Equipment") 2.2.2 According to basic technology, design and manufacturing There are two main technologies for designing and manufacturing the distribution transformers: A- Core type B- Shell type In a shell-formed transformer, the primary and secondary windings are quite “flat” 4 and are positioned on one leg surrounded by the core. In a core-formed transformer, cylindrical windings are like “coils” and cover the core legs. However, this classification is also limited in the large portfolio of either of those two distribution transformer types. 2.2.3 According to the insulating/cooling fluid in A- Liquid-filled transformers B- Gas-filled transformers (mainly with SF6) C- Dry-type transformers 2.2.4 According to location A. INDOOR TRANSFORMERS – Is one which because of construction much be protected from weather. Usually dry type or non-flammable oil immersed type. Shown in appendix(a). B. OUTDOOR TRANSFORMERS – Is of weather resistant construction suitable for service without the additional protection from weather. Usually of the mineral oil immersed type. Shown in appendix(b). 2.3 Connection of Distribution Transformers Modern electrical systems are almost exclusively three-phase systems, notwithstanding the many miles of distribution circuits that are configured a single- phase taps off of these systems. In addition, there still exist remnants of two-phase systems (typically in mining operations) that were fairly common years ago. When two polyphase systems have different voltages and/or phase angles, these systems can be interconnected using transformers having various possible types of connection.AS any one of these connections can be accomplished either with a bank of single phase transformers or by a single polyphase transformer. As we shall see in this chapter, it is in fact possible to interconnect two poly-phase systems having a different number of phases using special transformer connections. Single-phase two-winding transformer is nothing more than a primary and a 5 secondary winding wound around the same magnetic core. Single-phase two- winding transformers can be used in either single-phase circuits or poly-Phase circuits. A polyphase two-winding transformer contains a number of sets of primary and secondary windings. Each set wound around a separate magnetic core leg. A three-phase two-winding transformer has three sets of primary and secondary windings, and a two-phase two-winding transformer has two sets of primary and secondary windings. 2.3.1The Y-Y connection in three-phase systems The most obvious way of transforming voltages and currents in a three-phase electrical system is to operate each phase as a separate single-phase system. This requires a four-wire system comprised of three phase wires plus a common neutral wire that is shared among the three phases. This is commonly referred to as the Y- Y connection, as illustrated in Figure 2.1.The left-hand part of Figure2.1 shows the physical winding connections as three separate two-winding transformers. Both the primary and secondary windings of each of these transformers are connected between one phase, labeled A, B, and C, and the neutral, labeled N. The right-hand part of Figure2.1 shows the winding connections as a vector diagram. The direction of the phase rotation is assumed to be A-B-C expressed in a counterclockwise direction. 6 Figure 2.1 : Y-Y transformer connection and vector diagram. Figure2.2 depicts the situation where the primary neutral is returned to the voltage source in a four-wire three-phase circuit. Each of the magnetizing currents labeled iA, iB, and iC contain the 60Hz fundamental current and all of the odd harmonic currents necessary to support sinusoidal induced voltages. Figure 2.2:Y-Y Connection
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