2011-39011 10 May 2014
ENG 10 C-5R Draft No. 2
Introduction
Today, electricity has been a major necessity for people for it gives energy to almost all devices that make their works easier. But before electricity could be consumed to power a load or a certain device, it needs to travel long distances through the utilization of electrical power systems. Electrical Power Systems consists of mainly of generating stations, transmission lines and distribution systems. From the generating stations, different types of energy are converted to electrical energy (high voltage) and then transported to the distribution systems through the transmission lines, where the voltages to be delivered to the consumers by the distribution companies are lowered. There are two designs for distribution systems, one is the Overhead Distribution System (ODS) and the other is the Underground Distribution System (UDS). Each of these has its own advantages and disadvantages. In the earlier century, ODS has been commonly used by distribution companies. It is reliable, low-cost, easily maintained and is an established method to transport bulk electricity across long distances. ODS are the lowest-cost method transmitting electrical power because most of the insulation is provided by air. The disadvantage is that ODS is more prone and subjected to interruptions due to natural causes such as lightning and storms and other failure due to short circuits and line breakage. It is also unpleasant to the eye especially to urban areas. Moreover, the voltage drop in ODS is considerably high. This puts the use of UDS into consideration. In UDS, power lines are buried directly in the ground. Because it is underground, it promotes safety in subjected area; also, it is less subjected to damage from weather conditions and other physical causes. The downside to this is that UDSystems are far more expensive than ODSystems. Its maintenance is also hard since direct access to the underground transmission lines is not available disabling hands-on inspection. Therefore, it is necessary to come up with design for the transmission lines that will facilitate more efficient energy transmission, and at the same time, will not deteriorate easily. | Discusses historical developments regarding the topic. | Description and Rationale of the ProblemDue to the need to transform overhead transmission lines to underground transmission lines, for the efficiency of electrical transmission and convenience, considerations were made on how underground distribution lines will be more efficient in terms of electrical transmission and how it can be designed to be more economical.The efficiency of electrical transmission depends on the physical and electrical properties of the transmission lines used. These physical and electrical properties depend on the insulating and conducting materials composing the transmission cable. PILC consist of a stranded copper conductor, oil-impregnated kraft-paper tapes helically wound over the conductor, copper shielding tapes, and an extruded layer of lead. This material is the widely used type of transmission cables in most underground distribution systems. But paper is highly susceptible to deterioration from moisture for it hygroscopic and it tends to absorb moisture from air. Due to this, there aroused a need to provide a better material that can resist such deterioration. And among the numerous types of transmission lines that can be used, Cross Linked Polyethylene (XLPE) insulated cable is the most probable candidate.To determine if XLPE insulated cables are truly an effective alternative for PILC cables, the physical and electrical properties of the materials used for the cables would be compared. This will show the advantages of using XLPE cables as transmission lines in underground electrical distribution systems. | Contains background information and assumptions of the field of study.Provides reason/ justification for the study as well as the structure to be followed in conducting the study. | The Research Problem The research problem for this study will be: Is Cross Linked Polyethylene (XLPE) insulated cable an appropriate replacement for Paper-Insulated Lead Covered cable as a material for the transmission lines of underground distribution systems? To verify this, the following questions will be asked: 1. What are the characteristics of an ideal underground transmission cable? 2. What are the physical and electrical properties of the insulation material of XLPE and PILC cables? 3. How do PILC cables differ from XLPE cables in terms of its physical and electrical properties? 4. What are the advantages of using XLPE cables instead of PILC cables? | The research problem is clearly stated.Research problem is in question form. | Objectives of the Study This study mainly aims to evaluate Cross Linked Polyethylene (XLPE) Insulated cables as substitute for Paper-Insulated Lead Covered cables for the efficiency of electrical transmission of underground distribution systems. The specific objectives of this study are: 1. To define the characteristics of an ideal underground transmission cables. 2. To identify the physical and electrical properties of the insulation materials of XLPE and PILC cables such as polymers, chemical compositions, dielectric losses, molecular structures, fibre structures, crystallinities, thermal expansion, cross linkings and degradations. 3. To compare the said electrical and physical properties of XLPE and PILC cables. 4. To analyse the comparison between XLPE and PILC cables. 5. To present the advantage of using XPLE insulated cables as an alternative for PILC cables. | Objectives are specific and attainable, realistic and time-bound.Can provide answers to the research problems. | Importance of the Study The lack of direct access to the transmission lines of underground distribution systems makes the maintenance of the system a challenge. Visual examinations cannot be made in case a failure or a problem in the system occurs. Also, fixing or replacing these damaged transmission lines would take a considerable amount of time because it needs to be excavated first from the ground. Therefore, to avoid these problems, the distribution system should be designed to last for a long period of time for which maintenance will not be required to be done regularly. The distribution system’s transmission line should have an insulating layer that can withstand aging and immediate deterioration caused by the earthly substances surrounding it. To be able to know whether a certain material can withstand the said troubles, it is important to know the properties of the insulating and conducting materials used for the cables in the transmission lines. The comparison between XLPE and the commonly used PILC cables will present the advantages, as well as the disadvantages of using each of these cable types as underground transmission lines. Through this, it can be evaluated whether the replacement of PILC cables with XLPE insulated cables would be more efficient in the transmission of electrical power and more economical in a way that regular maintenance will be required. | Basis of comparison is stated.Importance of study is stated. | Limits of the StudyThere are many types of cables that can be used as transmission lines for underground distribution systems but this study will focus on the comparison of Cross Linked Polyethylene (XLPE) insulated cable and Paper Insulated Lead Covered (PILC) cables. The electrical and physical propertiesof the materials used will be discussed such as its polymers, chemical compositions, dielectric losses, molecular structures, fibre structures, crystallinities, thermal expansion, cross linkings and degradations. The process of manufacturing the cables will not be considered since it is only the said properties that will be compared. The analysis will be limited to the data acquired from the medium-voltage cable segments of a particular company. | Limits are clarifiedThere is a justification on why some factors are not considered. | Definition of TermsPaper Insulated Lead Covered (PILC) CablesPaper insulated cables are one of the oldest types of insulation readily available from natural resources used for power cables. Even, throughout the twentieth century, majority of the primary cables of large distribution systems of the world were PILC cables (Bernstein and Thue, 1999). This type of insulation uses paper impregnated with a dielectric fluid.The advantages of PILC cables are: lack of sensitivity to DC Testing and has better known history of reliability (Bernstein, 1999). Moreover, PILC cables are used for carrying large power (Ray, 2007). On the other hand, disadvantages are that paper ishighly susceptible to deterioration from moisture for it hygroscopic and it tends to absorb moisture from air. Due to this, insulation must be sealed using special jointing methods to avoid contact with air as well as sheathed with a material that is water resistant, such as lead. Because of this difficulty as well as weight of the cable, the use of Polyvinyl Chloride (PVC) and XLPE insulated cables became widespread when they were developed (Ray, 2007).Cross-Linked Polyethylene (XLPE) CablesPolyethylene is a hydrocarbon polymer also known as polyolefins. Its different polyethylene chains can be linked together and this process is called crosslinking. Crosslinking allows stability at elevated temperatures be maintained (Bernstein, 1999). With the introduction of Polyethylene, in 1941, extruded dielectric cables were developed such as Cross-Linked Polyethylene (XLPE) Cables (Zuidema et al., 2011).Advantages of XLPE include low dielectric constant, low dielectric loss, low power factor, high dielectric stress, excellent moisture resistance and extremely low moisture vapor transmission. The dielectric loss factor is about one decimal power lower than that of paper insulated cables. The mutual capacitance of XLPE cables is also lower due to its dielectric constant, thus reducing charging currents and earth-leakage currents in the networks. It also has reduced susceptibility to water treeing (Bernstein, 1999).Cable AmpacityAccording to Kelly and Landinger (1999), the term “ampacity” was used by William Del Mar in the early 1960’s to pertain to “Current Carrying capacity.” Eventually the term “Ampacity” was published by AIEE/IPCEA and used in general. It is stated in PEC (Article 1.1) that “Ampacity is the current, in amperes, that a conductor can carry continuously under conditions of use without exceeding the temperature rating.” This means in a cable, there is a certain maximum amount of current the conductor can carry, given the environment and time, without experiencing immediate or progressive deterioration. | Clear and precise definitions.Sources are cited. |

References:

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Electrotechnik: A Magazine for Electrical Engineering. (n.d.). A comparison of Overhead and Underground Transmission. Retrieved May 2, 2014 from http://www.electrotechnik.net/2011/11/comparison-of-overhead-and-underground.html
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