Causes Of Joint Failures Engineering Essay

Causes Of Joint also-rans Engineering EssayThis report emphasizes methods for determining, minimizing, and uni leaply distributing the r individu all in ally for course colligations and HV provides. The analysis is intended to number the bankruptcys cause by lavishly distorted, to provide maintenance cost reductions, and to purify service reliability.Predicting the remaining invigoration of a say is a major challenge to electric car car utilities, genius that has long had the attention of invention and maintenance engineers who manage eerywherehead contagion lines. The recent critical point chastenings reported by a few utilities note the trend caused by aging adjunctions and managing directors. These problems be expected to increase over time because of grittyer line shoot d consumeings under the flow rate deregulated environment. Since exemplary realize techniques carry many limitations, it is currently difficult to isolate the subdivisions ear ly abundant to reliably avoid bankruptcy.If dickens electric managers be joined to form a stationary electric edge, i.e. an electric peg, the joint electric ohmic resistance does not remain unvarying but provide increase during operating time. This long-term behaviour of the joint resistance posterior be influenced by dissimilar aging mechanisms like corrosion processes, interdiffusion, electromigration, fretting and sieve relaxation.Especially in bolted aluminium joints at postgraduate current load, i.e. at steeper(prenominal) joint temperatures mark relaxation whitethorn play an important role in joint aging. Creep deformation of the conductor material decreases the joint power. The bea and the human activity of a-spots decrease and may cause an increase of the constriction resistance which may occur suddenly, if mechanized vibrations act on the joint.In order to pull back the factor of influence of creep on the aging behaviour of high current bolted alumi num joints, the relationship between the decreasing joint mash and the joint resistance and the development of the joint force pull in to be find out.The relationship between the decreasing joint force and the joint resistance faeces be evaluated on the basis of the step to the fore profile of the hard-boiled joint come forwards 7. In order to extrapolate the development of the joint force beyond the time of experiments and to reduce the number of experiments with numerous types of joints geometry the development of the joint force is calculated.The calculation is perform by means of the exhaustible Element mode (FEM) based on material parameters of the conductors and the strong-arm fundamentals of creep 8. ascrib commensurate to the geomorphological characteristics of renders and cable ends. the electric fields near the grounding brim ar highly concentrated. and have a strong axial comp unitarynt, pass oning in corona and gliding discharge. The traditional way of impr oving this condition is to kick in semiconducting paint or band on the insulating surface by the flange. The electric field is evened by decreasing the surface resistance of the insulating surface. However, the put is not rattling good cod to the thinness of the applied material. In addition, the applied material leave behind aging or peel mangle in time, decreasing its force to zero.Failure MechanismsCauses of Joint FailuresJoint failures atomic number 18 expected to increase with the increase demand for heavier loading operations. Some of the find contributors to joint failures include inadequate cleaning of the conductor, complete absence of conductor cleaning, absence of corrosion inhibitor, improperly inserted conductor, incomplete die closures, and high load fault currents contributing to aging with thermic nervous straines.Installation and Quality government agency IssuesOne of the main reasons for joint failures is improper installation. Misalignment of steel sub division, crimping with wrong dye, no s cannisterdal present in get hitched with, and improperly cleaned conductor can greatly accelerate the failure process. Other factors that influence failures are internal cleft corrosion, releasing of compression force by thermal cycling, creep imputable to line tension, and fatigue cracking on bent joints. eroding ProblemsCorrosion is a major factor in the deterioration process of splice/connectors. envision be wretched is anexample of a failed field joint.Characteristics of a Joint FailureThe final failure mode of connectors is either mechanical or thermal. sometimes the steel sleevehas been installed off-center, pass oning in one end of the conductor organism hardly inserted. This can result in a mechanical failure. A thermal failure is the result of high resistance thaw or some torso strands failing with the same consequence. These situations can either melt components or cause the joint to lose its connection. Failures full custom aryly show evidence of both mechanical and thermal failure in combination.Temperature and tube RelationshipThe degradation of a joint can be observed by changes in its resistance and temperature. The interface resistance is a very small part of the sum joint resistance until there are signs of damage. Because of this effect, no noticeable heating falls place until late in the failure cycle. High temperature values are usually the result of high resistance measurements in a component.Resistance is a function, to varying degrees, of temperature. As an ab usually high resistance component begins to heat up, resistance increases, resulting in an even faster rate of temperature increase. This physical property of galvanic conductors can quickly make a rotten situation worseIn addition, as the component heats it may leave the material melting point. Often a complete structural failure results and the line flys. More often, however, the melted metal re-solidifies as this happens res istance decreases, and therefrom temperature, may be reduced. This is only a 2-3 temporary phenomenon, but if the management is conducted at this point in the failure cycle, the data go away for indisputable be misleading Eventually both resistance and heating exit over again increase, and the material impart again melt. This melting and re-welding can take place many times before total failure occurs, peculiarly when copper alloy components are involved.Design of High Voltage wrinkle JointsGeneralFor the stress control, basically the following methods are knowGeometrical, where the flesh of conducting sections is controlling the electrical field at the end of a high emf cable.Resistive, where the resistance of a semiconducting material is used to reduce the electrical stress in high field regions.Refractive, where material with a high permittivity is used for pushing away the field from high stress regions.The premiere method which refers to the geometry of the joint , is more than(prenominal) of a mechanical way of simplification or controlling stress. If we have to cables and there is a joint connecting them, what ever the stress diffusion may be, it is possible to control both the parameters that are stress concentration and stress distribution. For example, if we have a straight joint exactly alligned with the cable shape, whatever stress it shows, we can reduce or vary the stress distribution, and stress concentration can be change magnitude by applying changes to the joint shapes such chamfering the joint a bit, and also by varying its size to a possible extent.While the resistive and deflective method is successfully used for medium voltage applications up to 72.5kV maximum, the geometrical field control method is the standard method for high voltage and extra high voltage applications. Controlling the field by a well defined contour still offers the best quality from aspiration and production point of view.To install a pre-moulded joint they are normally slipped-over the prepared cable on site by using grease and special push-on tools. An different technology, widely used in the medium voltage range, is the cold shrink technology. With this technique a pre-moulded joint body is pre-expanded on a place upright tube, which can be removed while being placed around the cable on site. It has the advantage that no push-on tools essential be used.Electrical DesignOne basic function of all(prenominal) termination or joint is to control the electrical field at the endof a cable or between deuce cables. This means that the electrical field is controlled by the contour of conducting shares integrated into the joint body. During design confront FEM (Finite Element Method) calculation programs are an important tool as the a la mode(p) versions of these programs offer a vast range of possibilities such as computer science of the electrical field in any direction of the joint bodyOptimization tools for calculating the optimum shape of stress control elements answer of coupled fields, like thermo mechanical stressesModels for non-linear behavior of materials, like stresses in polymeric materialsSimulation of slip-on proceduresChoice of MaterialNowadays the materials used for high voltage joints are silicone polymer rubber and EPDM. The basic requirements for an elastomeric material are as followsSufficient mechanical properties in order being expansible in the required range.Capability to withstand the required temperature range.Availabilty of material with invariant quality and constant purity.Low ageing with respect to electrical and mechanical properties.According the requirements given above silicone rubber is an ideal and prefer material for cable joints.Therefore it can be concluded that silicone rubber is an excellent material for the use in cable accessories as it can fully cope with the electrical, mechanical and thermal requirements given by nowadays polymeric cables.AgeingAn import ant aspect, which we still have to get a line is the ageing factor of insulating material and interface. The ageing can be depict by the life time law as followsEN * t = const.Where,E = Electrical field in the separationt = Time, where the electric field is appliedN = Lifetime coefficient express depth psychologyFor the breed Analysis we have used a Finite Element Analysis (FEA) software in order to find the electric field distribution. We have done this by MAXWELL SV software. A little bit nearly the software start-off and and then we proceed towards our analysis.USING THE SOFTWAREWe have used a student version of MAXWELL SV software depending on the availability, which allows us to analyze a problem on the basis of 2D geometry.The resistive solution for reducing stress is of great importance as well. If another piece of metal is being used to joint devil pieces of cable, it could produce more stress on the joint callable to its own resistive and other properties. So, if th e two pieces of wire are making linkup through a third material, normally the third material is to be used as a jacket that overlaps two wires as shown under.In the above agreement two cables have direct contact with from each one other and third material is making parallel circuit with two cables and is supporting electrical current through a joint. In the understanding that involves a joint in series, the third material is making a series circuit with two cables and adding extra resistance to the joint with go out increase the stress on the joint.On the other hand if we pour forth about the refractive part the situation could be explained as , if a single piece of sleeve is taken and we plot the rise in temperature due to sleeve. Exposed part of the cable will have low temperature as compared with the portion under sleeve and it will pee distinction of resistance at both ends of wire under sleeve, due to this inequality thermal stresses on the ends of the two cables bei ng joint. To reduce the stress it is recommended that sleeve should be good conductor to heat and the temperature of exposed part will be as same as the part under the protect portion.SIMULATION BASED ANALYSISFor our simulation we have considered two cables joined together, where the joint between them is assumed to be a perfect one, and because we can treat it as one perfect conductor. Therefore, the conductor can be a single copper cable as demonstrate in the results below. In reality when the cables are to be joined, the ends of the two screen cables must be stripped of its disengagement. For modeling purposes this has been represented by a respite in the withdrawal between the conductor and outer sheath. This cattle farm will initially be left empty to see the distribution with no insulant. In attempt to disperse the stress uniformly the gap will then be filled with a variety of material. The work of the insulation at the joint is generally a lot summary(p)er than t he bear of the cable, and the simulations below make uses of this. The conductor has been assigned as a extraction of 500KV, and the outer sheath at ground potential. The insulating material for the two cables is XLPE.The results presented below show stress distribution in various scenarios where the geometry has been unchanged, and different insulation materials have been time-tested. Due to different properties of different insulating materials the stress distributions also vary.The first result presents the field distribution, where the gap has been left empty, to examine the initial stress with no insulation at all.Figure 1 Cable joint with no insulation at the jointXLPE Cable Insulation outermost Sheath (0V)GapHV Conductor(500KV)In this situation the stress in the gap at the joint and surrounding insulation in very high. This would eventually result in failure of the joint due to the fundamental stresses.Now that the initial stress has been determined, it is necessary to fin d how to minimize and uniformly distribute the stress. As mentioned previously the gap is now filled with a layer of a variety of materials. The layer at the joint is thicker than the insulation of a normal cable as to attempt to minimize and create a more uniform distribution of stress. Below the distribution for a variety of materials used to insulate the joint is presented.Figure 2 sports stadium distribution for Silicon Insulated cable jointJoint InsulationFigure 3 firmament distribution for FR4-Epoxy insulated cable jointFigure 4 cogitation distribution for Polyimide-Quartz insulated cable jointFigure 5 line of business distribution for Polyethylene insulated cable jointFigure 6 Field distribution for Teflon insulated cable jointFigure 7 Field distribution for Polystyrene insulated cableFigure 8 Field distribution for Porcelain insulated cableNotice that there are two effects of using the thick insulation at the cable joint, first the concentration of the stress is brought charge, and secondly the stress is distributed more uniformly. These are both important in ensuring the insulation is efficiently used, and maximizes the life span of the insulation. Notice that in the case of all but Polyimide-Quartz insulation the concentration is minimise somewhat, though the main effect is the uniform distribution of stress. On the other hand Polyimide-Quartz insulation greatly decrease the stress concentration, though the distribution is undesirable as the mass of the stress is concentrated at the conductor surface which will result in uneven wear of the insulation. Thus the optimal insulation will have even distribution of stress, and somewhat minimize the stress concentration. It was determined that the silicon insulation is best suited for this purpose. The distribution of the stress crossways the joint and surrounding insulation is very uniform, and the concentration is somewhat minimized as well. This result is consistent with industry practices as in general a thick layer of silicon is used at the joint for insulation .Conclusion Cable JointsThe Electric Stress has been determined for the general structure of a high voltage cable using exhaustible element analysis, and a method suggested to create a more uniform, and little concentrated distribution of stress. The results obtained agree with what is currently standard practice in the high voltage industry.FINITE ELEMENT ANALYSISBackgroundFinite Element Analysis (FEA) was first positive in 1943 by R. Courant, who employ the Ritz method of numerical analysis and minimization of variational calculus to obtain scratchy solutions to vibration establishments. Shortly thereafter, a paper published in 1956 by M. J. Turner, R. W. Clough, H. C. Martin, and L. J. Topp established a broader definition of numerical analysis. The paper centered on the stiffness and deflection of complex structures.By the early 70s, FEA was limited to expensive mainframe computers generally owned by th e aeronautics, automotive, defense, and nuclear industries. Since the rapid step-down in the cost of computers and the phenomenal increase in computing power, FEA has been developed to an incredible precision. Present day super computers are now able to produce unblemished results for all kinds of parameters.What is Finite Element Analysis?FEA consists of a computer model of a material or design that is tonic and analyzed for specific results. It is used in new product design, and animate product refinement. A company is able to verify a proposed design will be able to perform to the clients specifications prior to manufacturing or construction. Modifying an living product or structure is utilized to qualify the product or structure for a new service condition. In case of structural failure, FEA may be used to help determine the design modifications to prepare the new condition.There are generally two types of analysis that are used in industry 2-D modeling, and 3-D modeling. While 2-D modeling keep simplicity and allows the analysis to be run on a comparatively normal computer, it tends to yield less accurate results. 3-D modeling, however, produces more accurate results while sacrificing the ability to run on all but the straightaway computers effectively.How Does Finite Element Analysis Work?FEA uses a complex formation of points called nodes which make a grid called a mesh . This mesh is programmed to guard the material and structural properties which define how the structure will react to certain loading conditions. Nodes are assigned at a certain dumbness throughout the material depending on the anticipated stress levels of a crabbed area. Regions which will receive large amounts of stress usually have a higher node density than those which experience little or no stress. Points of interest may consist of fracture point of previously tested material, fillets, corners, complex detail, and high stress areas. The mesh acts like a roamer web in that from each node, there extends a mesh element to each of the adjacent nodes. This web of vectors is what carries the material properties to the object, creating many elements.Introduction BushingsIn todays competitive market, there is a need for the bush manufacturing industry to improve bushing efficiency and to reduce costs because high-quality low-cost products and processes have become the key to survival in the global economy. The reliability of equipment and facilities used in a power system is an essential precondition of the energy contagion security.High voltage bushing breakdown is one of the major contributors to the transformer failures. Since the electrical design of the HV bushings is the most important part of their manufacturing process, finding an algorithm for the electrical design of bushings in an optimum way is very important. Bushing failure is one of the leading causes of transformer failures. The electrical design of capacitive evaluate bushings is on e of the important parts of manufacturing of these kinds of bushings.Capacitive grading bushings contain embedded in their insulation core concentric conductive transparences, which are isolated from each other. By adjusting the diameter and duration of these cylinders, the electrical stress and voltage drop in the core and along its surface can be influenced by variation of the ratio of the partial contents between the conducting cylinders, 1.The grading of ac-bushing is achieved from the capacitances that are formed between the grading foils and then determined by the permittivity of the insulating material.HIGH potential drop BUSHINGSBushings provide a point of interface such that the electric current can pass to and from the machine. The current is at some potential above ground and must be electrically insulated from the tank walls which are at ground potential. It can be thought of like a bridge over where the potential is the length of the bridge and the longer the brid ge the more support it must have such that it will not come into contact with the ground. The current path is the number of lanes. If the number of lanes are reduced on part of the bridge under heavy traffic flow, a multi-car freshet up will occur. The two key factors are 1) Insulating System to proscribe a failure mode of over voltage. 2) Conductor Path to proceed a failure mode of over current. Over voltage will cause a flash over in the insulation and over current will cause overheating in the conductor due to I2 * R losses.Figure 9 Diagram of typical high voltage bushingsGeneral oddballsHigh-voltage bushings for use on transformers and circuit breaker are made in several principal types, as follows composite Bushing.- A bushing in which insulation consists of two or more coaxial layers of different insulating materials.Compound-Filled Bushing.-A bushing in which the space between the major insulation (or conductor where no major insulation is used) and the inside surface o f a protective weather cuticle (usually porcelain) is filled with a colonial having insulating properties.Condenser Bushing.- A bushing in which cylindrical conducting layers are lay coaxially with the conductor within the insulating material. The length and diameter of the cylinders are designed to control the distribution of the electric field in and over the outer surface of the bushing. Condenser bushings may be one of several typesResin-bonded paper insulationOil-impregnated paper insulation orOther.Dry or Unfilled Type Bushing.- Consists of porcelain tube with no filler in the space between the case and conductor. These are usually rated 25 kV and below.Oil-Filled Bushing. A bushing in which the space between the major insulation (or the conductor where no major insulation is used) and the inside surface of a protective weather casing (usually porcelain) is filled with insulating oil.Oil Immersed Bushing.- A bushing composed of a system of major insulations totally immerse d in a bath of insulating oil.Oil-Impregnated Paper- Insulated Bushing.- A bushing in which the internal structure is made of cellulose material impregnated with oil.Resin-Bonded, Paper- Insulated Bushing.- A bushing in which the major insulation is provided by cellulose material bonded with resin.Solid (Ceramic) Bushing.- A bushing in which the major insulation Is provided by a ceramic or analogous material.Bushing FailuresOperating records show that about 90 percent of all preventable bushing failures are caused by moisture entering the bushing through leaky gaskets or other openings. coda periodic reassessment to find leaks and make repairs as needed will prevent most outages due to bushing failures. Such an external inspection requires little time and expense and will be well worthy the effort. High-voltage bushings, if allowed to deteriorate, may explode with considerable violence and cause encompassing damages to adjacent equipment.Flashovers may be caused by deposits of dirt on the bushings, particularly in areas where there are contaminants such as salts or conducting dusts in the air. These deposits should be removed by periodic cleaning.Figure 10 Picture of High Voltage Bushing that has failed due to penetration of moistureOne of the failures can also be a insulator failure occurring with the paper insulation punctured through from the center draw rod, at a location about one third of the way down from the top terminal, to the grounded capacitance tap.HOW DOES THE BUSHINGS WITHSTAND THE STRESSES?The bushings must contain many layers of capacitors to fool the voltage down evenly from the potential at the centre conductor to ground potential. These capacitors are made up of many layers of paper and foil and usually filled with an insulating smooth such as oil. These layers of insulation can be checked by measuring the power factor of the bushing when the parent frame-up is out of service 2. While the parent apparatus is in service, an infrared c amera can be used to check for low oil levels. The oil level relates to the insulation quality of the grading capacitors. Infrared method will only work when the parent apparatus produces heat because it relies on the thermal mass difference between the fluid and the void at the top of the bushing. Bushings in transformers are ideal examples due to the heat produced by losses in the windings and core.The capacitor core of high voltage bushing is widely used to decrease the electric stress and to avoid field centralization where the high voltage lead utilization through the tank wall of transformer. The floating potentials of capacitor core can be calculated with several methods6, that are, the minimized energy algorithm, the partial capacitance algorithm and the electric charge conservation algorithm6.Some times dummy up nonconductor constant method are also used to sack the failure problems. When the electric flux line leaves higher dielectric constant region to lower dielectri c constant region, if the ratio of higher dielectric constant to lower dielectric constant is much larger than 1, then it is nearly vertical on the interface in low dielectric constant material.Stress AnalysisFor the simulation we have tried two arrangements. First, the high voltage conductor is insulated by one large thick layer of silicone from the porcelain outer layer, which is held in place by two metal flanges at ground potential. Secondly, a capacitive graded arrangement where the conductor is insulated by several layers of silicone of varying axial length separated by thin layers of foil (form a large capacitor), again with two flanges at ground potential holding the structure in place. A voltage of 132KV is supplied to the conductor.The results presented below show the stress distribution where to distribute the stress uniformly the geometry of the structure has been altered. It is expected that with the two arrangements the distribution of the stress will vary greatly.Figu re 11 Distribution of stress for first arrangement on bushingFigure 12 Distribution of stress for capacitively graded bushing arrangementIn the first arrangement the electric stress is concentrated around the surface of the conductor, and the metal flange. While the other regions are under considerably less stress. The result is consistent with known theory. This is inefficient use of the insulation, as the wear of the insulation is not even. It is thus necessary to find a more desirable arrangement. Figure 12 shows the result of a capactively graded arrangement. Not only is the stress distributed more uniformly throughout the insulation, ensuring maximum efficiency and long life span, though the concentration is also reduced. This is the optimal design for high voltage bushings and is currently used in many high voltage applications. Again the result found is consistent with theory, as capacitive grading is vastly used to distribute stress uniformly.Conclusion High Voltage Bushing sThe Electric Stress has been determined for two different bushing arrangements using finite element analysis. The capacitive grading arrangement was found to be the best at distributing , and minimizing the concentration of stress. The results obtained agree with theory, and are applied throughout the industry.