This report emphasizes methods for determining, minimizing, and uniformly distributing the stress for cable joints and HV bushings. The analysis is intended to reduce the failures caused by high stressed, to provide maintenance cost reductions, and to improve service reliability.
Predicting the remaining life of a joint is a major challenge to electric utilities, one that has long had the attention of design and maintenance engineers who manage overhead transmission lines. The recent joint failures reported by a few utilities follow the trend caused by aging joints and conductors. These problems are expected to increase over time because of higher line loadings under the current deregulated environment. Since typical inspection techniques have many limitations, it is currently difficult to isolate the components early enough to reliably avoid failure.
If two electrical conductors are joined to form a stationary electric contact, i.e. an electric joint, the joint resistance does not remain constant but will increase during operating time. This long-term behaviour of the joint resistance can be influenced by different aging mechanisms like corrosion processes, interdiffusion, electromigration, fretting and stress relaxation.
Especially in bolted aluminum joints at high current load, i.e. at high joint temperatures stress relaxation may play an important role in joint aging. Creep deformation of the conductor material decreases the joint force. The area and the number of a-spots decrease and may cause an increase of the constriction resistance which may occur suddenly, if mechanical vibrations act on the joint.
In order to describe the factor of influence of creep on the aging behaviour of high current bolted aluminum joints, the relationship between the decreasing joint force and the joint resistance and the development of the joint force have to be determined.
The relationship between the decreasing joint force and the joint resistance can be evaluated on the basis of the surface profile of the rough joint surfaces [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 performed by means of the Finite Element Method (FEM) based on material parameters of the conductors and the physical fundamentals of creep [8].
Due to the structural characteristics of bushings and cable ends. the electric fields near the grounding flange are highly concentrated. and have a strong axial component, resulting in corona and gliding discharge. The traditional way of improving this condition is to apply 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 effect is not very good due to the thinness of the applied material. In addition, the applied material will aging or peel off in time, decreasing its effect to zero.
Failure Mechanisms
Causes of Joint Failures
Joint failures are expected to increase with the increase demand for heavier loading operations. Some of the key 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 thermal stresses.
Installation and Quality Assurance Issues
One of the main reasons for joint failures is improper installation. Misalignment of steel sleeve, crimping with wrong dye, no grease present in splice, and improperly cleaned conductor can greatly accelerate the failure process. Other factors that influence failures are internal crevice corrosion, releasing of compression force by thermal cycling, creep due to line tension, and fatigue cracking on bent joints.
Corrosion Problems
Corrosion is a major factor in the deterioration process of splice/connectors. Figure below is anexample of a failed field joint.
Characteristics of a Joint Failure
The final failure mode of connectors is either mechanical or thermal. Sometimes the steel sleevehas been installed off-center, resulting in one end of the conductor being barely inserted. This can result in a mechanical failure. A thermal failure is the result of high resistance heating or individual strands failing with the same consequence. These situations can either melt components or cause the joint to lose its connection. Failures generally show evidence of both mechanical and thermal failure in combination.
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Temperature and Resistance Relationship
The degradation of a joint can be observed by changes in its resistance and temperature. The interface resistance is a very small part of the total joint resistance until there are signs of damage. Because of this effect, no noticeable heating takes 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 abnormally high resistance component begins to heat up, resistance increases, resulting in an even faster rate of temperature increase. This physical property of electrical conductors can quickly make a bad situation worse!
In addition, as the component heats it may reach the material melting point. Often a complete structural failure results and the line drops. More often, however, the melted metal re-solidifies; as this happens resistance decreases, and thus temperature, may be reduced. This is only a 2-3 temporary phenomenon, but if the inspection is conducted at this point in the failure cycle, the data will certainly be misleading! Eventually both resistance and heating will again increase, and the material will again melt. This melting and re-welding can take place many times before total failure occurs, especially when copper alloy components are involved.
Design of High Voltage Cable Joints
General
For the stress control, basically the following methods are known:
Geometrical, where the contour of conducting elements is controlling the electrical field at the end of a high voltage 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 first method which refers to the geometry of the joint, is more of a mechanical way of reducing or controlling stress. If we have to cables and there is a joint connecting them, what ever the stress distribution 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 decreased 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 refractive 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 design 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. Another 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 support tube, which can be removed while being placed around the cable on site. It has the advantage that no push-on tools must be used.
Electrical Design
One basic function of every termination or joint is to control the electrical field at the endof a cable or between two cables. This means that the electrical field is controlled by the contour of conducting elements integrated into the joint body. During design stage FEM (Finite Element Method) calculation programs are an important tool as the latest versions of these programs offer a vast range of possibilities such as:
Calculation of the electrical field in any direction of the joint body
Optimization tools for calculating the optimum shape of stress control elements
Solving of coupled fields, like thermo mechanical stresses
Models for non-linear behavior of materials, like stresses in polymeric materials
Simulation of slip-on procedures
Choice of Material
Nowadays the materials used for high voltage joints are silicone rubber and EPDM. The basic requirements for an elastomeric material are as follows:
Sufficient mechanical properties in order being expandable in the required range.
Capability to withstand the required temperature range.
Availabilty of material with constant quality and constant purity.
Low ageing with respect to electrical and mechanical properties.
According the requirements given above silicone rubber is an ideal and preferred 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.
Ageing
An important aspect, which we still have to consider is the ageing factor of insulating material and interface. The ageing can be described by the life time law as follows:
EN * t = const.
Where,
E = Electrical field in the insulation
t = Time, where the electric field is applied
N = Lifetime coefficient
Stress Analysis
For the Stress 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 about the software first and then we proceed towards our analysis.
USING THE SOFTWARE
We 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 two pieces of cable, it could produce more stress on the joint due to its own resistive and other properties. So, if the two pieces of wire are making contact through a third material, normally the third material is to be used as a jacket that overlaps two wires as shown below.
In the above arrangement two cables have direct contact with each other and third material is making parallel circuit with two cables and is supporting electrical current through a joint. In the arrangement 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 will increase the stress on the joint.
On the other hand if we talk 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 create difference of resistance at both ends of wire under sleeve, due to this difference thermal stresses on the ends of the two cables being 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 shielded portion.
SIMULATION BASED ANALYSIS
For our simulation we have considered two cables joined together, where the joint between them is assumed to be a perfect one, and hence we can treat it as one perfect conductor. Therefore, the conductor can be a single copper cable as demonstrated in the results below. In reality when the cables are to be joined, the ends of the two separate cables must be stripped of its insulation. For modeling purposes this has been represented by a gap in the insulation between the conductor and outer sheath. This gap will initially be left empty to see the distribution with no insulation. In attempt to distribute the stress uniformly the gap will then be filled with a variety of material. The layer of the insulation at the joint is generally a lot thicker than the rest of the cable, and the simulations below make uses of this. The conductor has been assigned as a source 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 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 joint
XLPE Cable Insulation
Outer Sheath (0V)
Gap
HV 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 extreme stresses.
Now that the initial stress has been determined, it is necessary to find 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: Field distribution for Silicon Insulated cable joint
Joint Insulation
Figure 3: Field distribution for FR4-Epoxy insulated cable joint
Figure 4: Field distribution for Polyimide-Quartz insulated cable joint
Figure 5: Field distribution for Polyethylene insulated cable joint
Figure 6: Field distribution for Teflon insulated cable joint
Figure 7: Field distribution for Polystyrene insulated cable
Figure 8: Field distribution for Porcelain insulated cable
Notice that there are two effects of using the thick insulation at the cable joint, first the concentration of the stress is brought down, 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 minimized 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 majority 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 across 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 Joints
The Electric Stress has been determined for the general structure of a high voltage cable using finite element analysis, and a method suggested to create a more uniform, and less concentrated distribution of stress. The results obtained agree with what is currently standard practice in the high voltage industry.
FINITE ELEMENT ANALYSIS
Background
Finite Element Analysis (FEA) was first developed in 1943 by R. Courant, who utilized the Ritz method of numerical analysis and minimization of variational calculus to obtain approximate solutions to vibration systems. 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”.
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By the early 70’s, FEA was limited to expensive mainframe computers generally owned by the aeronautics, automotive, defense, and nuclear industries. Since the rapid decline 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 accurate results for all kinds of parameters.
What is Finite Element Analysis?
FEA consists of a computer model of a material or design that is stressed and analyzed for specific results. It is used in new product design, and existing product refinement. A company is able to verify a proposed design will be able to perform to the client’s specifications prior to manufacturing or construction. Modifying an existing 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 meet 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 conserves simplicity and allows the analysis to be run on a relatively 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 fastest computers effectively.
How Does Finite Element Analysis Work?
FEA uses a complex system of points called nodes which make a grid called a mesh . This mesh is programmed to contain 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 material depending on the anticipated stress levels of a particular 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 spider 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 – Bushings
In today’s competitive market, there is a need for the bushing 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 transmission 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 grading bushings is one of the important parts of manufacturing of these kinds of bushings.
Capacitive grading bushings contain embedded in their insulation core concentric conductive foils, which are isolated from each other. By adjusting the diameter and length 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 capacitances between the conducting cylinders, [1].
The grading of ac-bushing is achieved from the capacitances that are formed between the grading foils and thus determined by the permittivity of the insulating material.
HIGH VOLTAGE BUSHINGS
Bushings provide a point of interface such that the electric current can pass to and from the apparatus. 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 where the potential is the length of the bridge and the longer the bridge 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 pile up will occur. The two key factors are: 1) Insulating System – to prevent a failure mode of over voltage. 2) Conductor Path – to prevent 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 I^2 * R losses.
Figure 9: Diagram of typical high voltage bushings
General Types:
High-voltage bushings for use on transformers and breakers 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 of a protective weather casing (usually porcelain) is filled with a compound having insulating properties.
Condenser Bushing.- A bushing in which cylindrical conducting layers are arranged 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 types:
Resin-bonded paper insulation;
Oil-impregnated paper insulation; or
Other.
Dry or Unfilled Type Bushing.- Consists of porcelain tube with no filler in the space between the shell 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 immersed 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 Failures
Operating records show that about 90 percent of all preventable bushing failures are caused by moisture entering the bushing through leaky gaskets or other openings. Close periodic inspection 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 worth the effort. High-voltage bushings, if allowed to deteriorate, may explode with considerable violence and cause extensive 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 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 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 grade 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 fluid such as oil. These layers of insulation can be checked by measuring the power factor of the bushing when the parent apparatus is out of service [2]. While the parent apparatus is in service, an infrared camera 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 drill through the tank wall of transformer. The floating potentials of capacitor core can be calculated with several methods[6], that are, the minimized energy algorithm, the partial capacitance algorithm and the electric charge conservation algorithm[6].
Some times dummy dielectric constant method are also used to solve the failure problems. When the electric flux line leaves higher dielectric constant region to lower dielectric 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 Analysis
For 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.
Figure 11: Distribution of stress for first arrangement on bushing
Figure 12: Distribution of stress for capacitively graded bushing arrangement
In 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 Bushings
The 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.
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