This report emphasizes methods for determining, reducing, and uniformly distributing the stress for cable bones and HV bushings. The examination is intended to reduce the failures brought on by high stressed, to provide maintenance cost reductions, and also to improve service consistency.
Predicting the rest of the life of any joint is a significant concern to electric resources, the one that has long experienced the attention of design and maintenance technicians who manage over head transmitting lines. The recent joint failures reported by a few utilities follow the craze caused by increasing age bones and conductors. These problems are expected to increase over time because of higher series loadings under the existing deregulated environment. Since typical inspection techniques have many limits, it happens to be difficult to isolate the components early on enough to reliably avoid inability.
If two electronic conductors are signed up with to form a fixed electric contact, i. e. a power joint, the joint amount of resistance does not continue to be constant but will increase during functioning time. This long-term behaviour of the joint resistance can be affected by different maturing mechanisms like corrosion operations, interdiffusion, electromigration, fretting and stress relaxation.
Especially in bolted aluminum bones at high current insert, i. e. at high joint temps stress relaxation may play an important role in joint maturity. Creep deformation of the conductor materials lessens the joint drive. The area and the number of a-spots decrease and may cause an increase of the constriction amount of resistance which may arise suddenly, if mechanised vibrations act on the joint.
In order to spell it out the factor of influence of creep on the maturing behavior of high current bolted aluminum joints, the relationship between the reducing joint force and the joint amount of resistance and the introduction of the joint force have to be determined.
The relationship between the decreasing joint drive and the joint resistance can be examined based on the surface profile of the difficult joint floors . To be able to extrapolate the introduction of the joint push beyond enough time of experiments and also to reduce the range of tests with numerous types of joint parts geometry the introduction of the joint make is computed.
The calculation is performed through the Finite Aspect Method (FEM) based on material parameters of the conductors and the physical basics of creep .
Due to the structural characteristics of bushings and cable ends. the electric fields near the grounding flange are highly focused. and have a solid axial component, leading to corona and gliding release. The traditional way of bettering this problem is to use semiconducting color or music group on the insulating surface by the flange. The electric field is evened by reducing the surface amount of resistance of the insulating surface. However, the effect is of low quality because of the thinness of the applied materials. In addition, the applied material will maturing or peel from the lime in time, lessening its result to zero.
Causes of Joint Failures
Joint failures are anticipated to increase with the increase demand for bulkier loading operations. Some of the key contributors to joint failures include: limited cleaning of the conductor, complete absence of conductor cleaning, lack of corrosion inhibitor, incorrectly inserted conductor, incomplete pass away closures, and high weight fault currents contributing to aging with thermal stresses.
Installation and Quality Assurance Issues
One of the main known reasons for joint failures is poor installation. Misalignment of material sleeve, crimping with incorrect dye, no grease within splice, and improperly cleaned out conductor can greatly accelerate the failure process. Other factors that affect failures are inside crevice corrosion, launching of compression force by thermal cycling, creep scheduled to line anxiety, and fatigue cracking on bent joints.
Corrosion is a significant factor in the deterioration process of splice/connectors. Number below is anexample of the failed field joint.
Characteristics of a Joint Failure
The final inability method of connectors is either mechanical or thermal. Sometimes the material sleevehas been installed off-center, resulting in one end of the conductor being barely inserted. This can cause a mechanical inability. A thermal failing is the consequence of high resistance warming or individual strands declining with the same effect. These circumstances can either melt components or cause the joint to reduce its interconnection. Failures generally show evidence of both mechanised and thermal failure in blend.
Temperature and Amount of resistance Relationship
The degradation of a joint can be viewed by changes in its level of resistance and temp. The interface level of resistance is an extremely small area of the total joint amount of resistance until there are indicators of damage. As a result of this effect, no obvious heating takes place until later in the failing cycle. Temperature worth are usually the consequence of high resistance measurements in a component.
Resistance is a function, to varying degrees, of temp. As an abnormally high resistance component commences to warm up, resistance increases, leading to an even faster rate of temperature increase. This physical property of electric conductors can quickly make a negative situation worse!
In addition, as the element heats it may reach the material melting point. Ordinarily a complete structural inability results and the lines drops. More regularly, however, the melted material re-solidifies; as this happens resistance decreases, and so heat, may be reduced. This is only a 2-3 temporary phenomenon, if the inspection is conducted at this time in the failure cycle, the data will certainly be misleading! Eventually both amount of resistance and heat will again increase, and the materials will again melt. This melting and re-welding may take place many times before total inability occurs, particularly when copper alloy components are participating.
Design of High Voltage Cable television Joints
Geometrical, where the contour of conducting elements is managing the electric powered field at the end of a higher voltage cable television.
Resistive, where in fact the resistance of a semiconducting material is used to lessen the electro-mechanical stress in high field regions.
Refractive, where material with a higher permittivity is used for "pushing" away the field from high stress regions.
The first method which refers to the geometry of the joint, is more of any mechanical way of reducing or handling stress. If we have to cables and there is a joint connecting them, what ever the stress circulation may be, you'll be able to control both the guidelines that are stress amount and stress circulation. For instance, if we have a upright joint exactly alligned with the cable form, whatever stress it shows, we can reduce or change the stress circulation, and stress concentration can be reduced by applying changes to the joint shapes such chamfering the joint a little, and also by differing its size to a possible level.
While the resistive and refractive method is efficiently used for medium voltage applications up to 72. 5kV maximum, the geometrical field control method is the typical way for high voltage and further high voltage applications. Managing the field by way of a well identified contour still offers the best quality from design and development point of view.
To install a pre-moulded joint they are normally slipped-over the well prepared cable tv on site by using grease and special push-on tools. Another technology, widely used in the medium voltage range, is the cold shrink technology. With this technique a pre-moulded joint person is pre-expanded on the support tube, which is often removed while being positioned around the cable on site. It gets the gain that no push-on tools can be used.
Calculation of the electric powered field in virtually any course of the joint body
Optimization tools for determining the optimum condition of stress control elements
Solving of coupled areas, like thermo mechanical stresses
Models for non-linear tendencies of materials, like tensions in polymeric materials
Simulation of slip-on procedures
Choice of Material
Sufficient mechanised properties in order being expandable in the mandatory range.
Capability to hold up against the required temp range.
Availabilty of material with continuous quality and constant purity.
Low ageing regarding electrical and mechanical properties.
According the requirements given above silicone rubber is a perfect and preferred material for cable joints.
Therefore it can be concluded that silicone rubber is an outstanding material for the use in cable connection accessories as it could fully handle the electrical, mechanical and thermal requirements given by nowadays polymeric cables.
An essential requirement, which we still have to consider is the ageing factor of insulating material and interface. The ageing can be detailed by the life time law as follows:
EN * t = const.
E = Electro-mechanical field in the insulation
t = Time, where in fact the electric field is applied
N = Life time coefficient
For the strain Analysis we have used a Finite Component Research (FEA) software in order to find the electric field circulation. We have done this by MAXWELL SV software. A bit about the software first and then we proceed towards our analysis.
USING THE SOFTWARE
We have used a student version of MAXWELL SV software with regards to the availability, that allows us to analyze a problem based on 2D geometry.
The resistive solution for lowering stress is of great importance as well. If another piece of metal has been used to joint two pieces of cable, it might produce more stress on the joint because of its own resistive and other properties. So, if the two pieces of wire are making contact through a third material, normally the 3rd material is usually to be used as a jacket that overlaps two wire connections as shown below.
In these arrangement two wires have direct connection with the other person and third material is making parallel circuit with two cables and is assisting electrical current through the joint. Within the arrangement which involves a joint in series, the third material is making a string circuit with two cables and adding extra amount of resistance to the joint with will improve the stress on the joint.
On the other palm if we speak about the refractive part the situation could be described as, if an individual little bit of sleeve is considered and we plot the rise in temperature due to sleeve. Open part of the wire will have low temperature as compared with the portion under sleeve and it will create difference of resistance at both ends of cable under sleeve, because of this difference thermal stresses on the ends of the two wires being joint. To reduce the stress it is strongly recommended that sleeve should be good conductor to heat up and the temps of exposed part will be as same as the part under the shielded part.
SIMULATION Structured ANALYSIS
For our simulation we've considered two cords joined together, where the joint between them is assumed to be always a perfect one, and therefore we can address it as you perfect conductor. Therefore, the conductor can be considered a single copper cable connection as confirmed in the results below. In reality when the cords should be became a member of, the ends of both separate wires must be stripped of its insulation. For modeling purposes it has been represented with a distance in the insulation between your conductor and exterior sheath. This space will in the beginning be left clear to see the distribution without insulation. In try to distribute the strain uniformly the space will be filled with a number of material. The coating of the insulation at the joint is generally a whole lot thicker than the rest of the wire, and the simulations below make uses of the. The conductor has been designated as a source of 500KV, and the outer sheath at ground probable. The insulating materials for the two cords is XLPE.
The results presented below show stress distribution in various cases where the geometry has been unchanged, and different insulation materials have been examined. Because of different properties of different insulating materials the stress distributions also vary.
The first final result reveals the field distribution, where the difference has been still left empty, to examine the original stress without insulation in any way.
Figure 1: Cable joint with no insulation at the joint
XLPE Cable connection Insulation
Outer Sheath (0V)
In this situation the stress in the gap at the joint and encompassing insulation in high. This might eventually bring about failure of the joint because of the extreme strains.
Now that the original stress has been motivated, it is necessary to find how to reduce and uniformly spread the stress. As mentioned previously the difference is now filled up with a covering of a number of materials. The covering at the joint is thicker than the insulation of a standard cable concerning attempt to lessen and create a more uniform circulation of stress. Below the syndication for a number of materials used to insulate the joint is shown.
Figure 2: Field circulation for Silicon Insulated cable television joint
Figure 3: Field syndication for FR4-Epoxy insulated wire joint
Figure 4: Field distribution for Polyimide-Quartz insulated cable joint
Figure 5: Field circulation for Polyethylene insulated cable connection joint
Figure 6: Field distribution for Teflon insulated cable joint
Figure 7: Field syndication for Polystyrene covered cable
Figure 8: Field circulation for Porcelain covered cable
Notice that we now have two effects of using the solid insulation at the cable joint, first the concentration of the stress is brought down, and second the stress is allocated more uniformly. These are both important in guaranteeing the insulation is effectively used, and maximizes the life span of the insulation. Notice that in the case of all but Polyimide-Quartz insulation the concentration is minimized relatively, though the main result is the even distribution of stress. Alternatively Polyimide-Quartz insulation greatly decrease the stress concentration, though the distribution is unwanted as the majority of the stress is concentrated at the conductor surface which will result in unequal wear of the insulation. Thus the optimal insulation will have even syndication of stress, and relatively minimize the strain concentration. It had been motivated that the silicon insulation is best suited for this purpose. The distribution of the strain over the joint and encompassing insulation is very consistent, and the focus is somewhat reduced as well. This effect is constant with industry routines as generally a thick covering of silicon is used at the joint for insulation .
Conclusion - Wire Joints
The Electric Stress has been motivated for the general structure of a higher voltage cable television using finite component analysis, and a way suggested to create a more uniform, and less focused syndication of stress. The results obtained agree with what is currently standard practice in the high voltage industry.
FINITE Aspect ANALYSIS
Finite Element Evaluation (FEA) was initially developed in 1943 by R. Courant, who utilized the Ritz approach to numerical analysis and minimization of variational calculus to obtain approximate solutions to vibration systems. Quickly thereafter, a paper posted in 1956 by M. J. Turner, R. W. Clough, H. C. Martin, and L. J. Topp established a broader meaning of numerical research. The paper devoted to the "stiffness and deflection of sophisticated structures".
By the early 70's, FEA was limited to expensive mainframe pcs generally had by the aeronautics, motor vehicle, security, and nuclear establishments. Since the swift decline in the price of computers and the phenomenal increase in processing ability, FEA has been developed to an incredible precision. Present day super computers are now able to produce accurate results for a myriad of parameters.
What is Finite Factor Analysis?
FEA consists of a computer style of a materials or design that is stressed and examined for specific results. It is employed in new product design, and existing product refinement. A company can verify a proposed design will be able to perform to the client's features prior to creation or construction. Changing a preexisting product or framework is useful to qualify the product or structure for a fresh service condition. In case of structural inability, FEA may be used to help determine the look modifications to meet up with the new condition.
There are generally two types of evaluation that are being used in industry: 2-D modeling, and 3-D modeling. While 2-D modeling conserves straightforwardness and allows the research to be run on a comparatively normal computer, it tends to yield less exact results. 3-D modeling, however, produces more exact results while sacrificing the capability to run on all however the fastest personal computers effectively.
How Does indeed Finite Element Analysis Work?
FEA runs on the sophisticated system of factors called nodes which make a grid called a mesh. This mesh is designed to support the material and structural properties which define how the structure will react to certain loading conditions. Nodes are assigned at a certain density throughout the materials with regards to the anticipated stress degrees of a particular area. Regions which will receive large amounts of stress will often have a higher node density than those that experience little or no stress. Sights may contain: fracture point of previously tested materials, fillets, corners, complicated depth, and high stress areas. The mesh acts like a spider web in that from each node, there stretches a mesh element to each one of the adjacent nodes. This web of vectors is what carries the material properties to the thing, creating many elements.
Introduction - Bushings
In today's competitive market, there is a dependence on the bushing creation industry to improve bushing efficiency and also to keep your charges down; because high-quality low-cost products and techniques have become the main element to success in the global current economic climate. The consistency of equipment and facilities found in a ability system is an essential precondition of the energy transmitting security.
High voltage bushing malfunction is one of the major contributors to the transformer failures. Since the electronic design of the HV bushings is the most crucial part of the processing process, finding an algorithm for the electrical design of bushings in an optimum way is very important. Bushing failing is one of the leading causes of transformer failures. The electrical power design of capacitive grading bushings is one of the top parts of making of these kinds of bushings.
Capacitive grading bushings contain inserted in their insulation primary concentric conductive foils, that happen to be isolated from each other. By altering the diameter and amount of these cylinders, the electrical stress and voltage drop in the core and along its surface can be influenced by deviation of the proportion of the incomplete capacitances between the doing cylinders, .
The grading of ac-bushing is achieved from the capacitances that are shaped between the grading foils and so dependant on the permittivity of the insulating materials.
HIGH VOLTAGE BUSHINGS
Bushings provide a point of user interface in a way that the electric energy can go away to and from the equipment. The current reaches some potential above surface and must be electrically protected from the fish tank walls which are at ground potential. It could be thought of like a bridge where in fact the potential is the length of the bridge and the longer the bridge a lot more support it will need to have such that it will not come into contact with the ground. The existing path is the number of lanes. If the amount of lanes are reduced on part of the bridge under heavy traffic stream, a multi-car pile up will occur. The two key factors are: 1) Insulating System - to prevent a failure function of over voltage. 2) Conductor Way - to prevent a failure setting of over current. Over voltage will cause a flash over in the insulation and over current may cause overheating in the conductor due to I^2 * R deficits.
Figure 9: Diagram of typical high voltage bushings
Composite Bushing. - A bushing where insulation includes two or more coaxial layers of different insulating materials.
Compound-Filled Bushing. -A bushing in which the space between your major insulation (or conductor where no major insulation can be used) and the within surface of the protecting weather casing (usually porcelain) is filled with a mixture having insulating properties.
Resin-bonded newspaper insulation;
Oil-impregnated newspaper insulation; or
Dry or Unfilled Type Bushing. - Consists of porcelain tube without filler in the space between your shell and conductor. These are usually rated 25 kV and below.
Oil-Filled Bushing. - A bushing where the space between the major insulation (or the conductor where no major insulation can be used) and the inside surface of any protecting weather casing (usually porcelain) is filled up with insulating petrol.
Oil Immersed Bushing. - A bushing made up of a system of major insulations totally immersed in a bath tub of insulating petrol.
Oil-Impregnated Paper- Insulated Bushing. - A bushing in which the internal structure is constructed of cellulose materials impregnated with petrol.
Resin-Bonded, Paper- Insulated Bushing. - A bushing where the major insulation is provided by cellulose materials bonded with resin.
Solid (Ceramic) Bushing. - A bushing in which the major insulation Is provided with a ceramic or analogous material.
Operating records show that about 90 percent of most preventable bushing failures are brought on by moisture going into the bushing through leaky gaskets or other opportunities. Close regular inspection to find leaks and make vehicle repairs as needed will prevent most outages due to bushing failures. This exterior inspection requires short amount of time and expense and will be well worth your time and effort. High-voltage bushings, if allowed to deteriorate, may explode with appreciable assault and cause comprehensive damage to adjacent equipment.
Flashovers may be brought on by debris of mud on the bushings, especially in areas where there are contaminants such as salts or conducting dusts in the air. These debris should be removed by regular cleaning.
Figure 10: Picture of High Voltage Bushing that has failed due to penetration of moisture
One of the failures can also be a dielectric failure occurring with the paper insulation punctured through from the center draw rod, at a location about 1 / 3 of the way down from the top terminal, to the grounded capacitance faucet.
HOW May THE BUSHINGS WITHSTAND THE Strains?
The bushings must contain many layers of capacitors to level the voltage down consistently from the potential at the centre conductor to earth potential. These capacitors are made of many layers of paper and foil and usually filled up with an insulating smooth such as olive oil. These layers of insulation can be examined by measuring the energy factor of the bushing when the father or mother apparatus is out of service . As the parent equipment is operating, an infrared camera may be used to look for low petrol levels. The engine oil level relates to the insulation quality of the grading capacitors. Infrared method is only going to work when the parent apparatus produces high temperature because it relies on the thermal mass difference between your fluid and the void near the top of the bushing. Bushings in transformers are ideal examples because of the heat produced by deficits in the windings and central.
The capacitor center of high voltage bushing is widely used to diminish the electric stress and also to avoid field centralization where in fact the high voltage lead drill through the container wall membrane of transformer. The floating potentials of capacitor center can be computed with several methods, that are, the reduced energy algorithm, the partial capacitance algorithm and the electric fee conservation algorithm.
Some times dummy dielectric continuous method are also used to resolve the failing problems. Once the electric flux range leaves higher dielectric frequent region to lower dielectric frequent region, if the ratio of higher dielectric continuous to lessen dielectric constant is a lot larger than 1, then it is practically vertical on the software in low dielectric regular material.
For the simulation we've tried two arrangements. First, the high voltage conductor is covered by one large thick layer of silicon from the porcelain outside layer, which is performed set up by two steel flanges at surface potential. Second, a capacitive graded design where the conductor is covered by several layers of silicone of varying axial length segregated by slender layers of foil (form a large capacitor), again with two flanges at ground potential retaining the structure in place. A voltage of 132KV is supplied to the conductor.
The results shown below show the stress distribution where to distribute the stress uniformly the geometry of the composition has been improved. It really is expected that with both arrangements the circulation of the strain will change greatly.
Figure 11: Syndication of stress for first agreement on bushing
Figure 12: Distribution of stress for capacitively graded bushing arrangement
In the first layout the electric stress is targeted around the top of conductor, and the metal flange. As the other regions are under substantially less stress. The result is steady with known theory. That is inefficient use of the insulation, as the wear of the insulation is not even. It is thus necessary to find a far more desirable arrangement. Number 12 shows the consequence of a capactively graded layout. Not only is the strain allocated more uniformly throughout the insulation, making sure maximum efficiency and extended life span, though the concentration is also reduced. This is the ideal design for high voltage bushings and is currently found in many high voltage applications. Again the effect found is steady with theory, as capacitive grading is significantly used to send out stress uniformly.
Conclusion - High Voltage Bushings
The Electric Stress has been identified for two different bushing plans using finite component analysis. The capacitive grading arrangement was found to be the best at distributing, and reducing the focus of stress. The results obtained trust theory, and are applied throughout the industry.
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