The weight and brittle nature of porcelain insulators make them especially susceptible to destructive harmonic frequencies. Indeed, material characteristics play a major role in equipment design under dynamic forces. While steel and aluminum are ductile and have predictable strength, porcelain is non-ductile and strength can vary greatly. Nevertheless, with good design, advanced materials and modern manufacturing methods, porcelain can still prove a dependable form of insulation.
This edited INMR article, based on a contribution by Patrick Maloney of PPC Insulators, discusses the forces from seismic activity with a view to improved application of porcelain insulators in such regions.
Porcelain insulators have been a crucial part of energy systems for over a century. Porcelain’s rigid nature maintains minimal flex and assures alignment of components in substation equipment. However, porcelain can also be seriously impacted by the forces unleashed during seismic events.
Over the past 25 years, there have been important advancements in better understanding seismic events. Moreover, insulators are but one component of a complex array that makes up substation equipment and therefore entire devices need evaluation. For example, in many cases, insulators are mounted onto concrete or steel structures and support equipment. Bushings, by contrast, are located at the top of equipment. The response of the equipment and components to input frequencies will depend on several factors.
When the natural frequency of a piece of equipment closely matches the input frequency, resonance occurs, amplifying the resulting dynamic motion and acceleration response. The required response spectrum (RRS) simulates amplitudes, frequencies and energy during typical seismic events. Equipment having 1.1 to 8 Hz natural frequencies is covered most closely in the RRS.
Generally, high voltage equipment has several characteristics that make it more responsive to seismic inputs. Being high and heavy, such equipment exhibits lower levels of natural frequency normally found during seismic events. When two items vibrate at the same natural frequency, increased motion results and this induces high cantilever loads.
Understanding the forces a porcelain insulator can be subjected to in relation to the strengths and weakness of ceramic materials is a first step. An insulator’s mechanical ratings are a) cantilever/bending moments, b) torsion, c) tension and d) compression. Cantilever loads determine core diameter and therefore weight.
D – core diameter; F – required strength (min. breakage load);
l – length; specific strength of porcelain
Ceramic materials have very high compression ratings, but low tension ratings. Bending moments induce compression and tension stress. Tensile stress is amplified by the lever action of the height of the insulator (as in Fig. 1).
Bending moments increase with greater force and/or taller insulators (as in Fig. 2). In the case of dynamic motion, the force is based on 1) mass of the insulator and mass mounted above the insulator and 2) acceleration due to the seismic event.
Trying to make design changes to ensure survival of equipment is often impossible since natural frequency is outside seismic event frequency. When calculating the force/energy that goes into equipment during a seismic event, weight is a key factor. The challenge therefore is to optimize design to maximize strength to weight ratio.
There are several methods to reduce weight of a porcelain insulator of given strength. For example, the insulator should be specifically designed for the application need and maximum section lengths help reduce weight in the case of multi-stack insulators. Moreover, manufacturers have material choices that offer higher strength and maintaining tight quality assurance standards further enhances overall strength of ceramic material.
The design of an insulator needs to account for the specific application under seismic conditions. Often, porcelain insulators used at substations are based on standard designs for general purpose and intended to perform in different applications. One example is an insulator with uniform cylindrical core and which can be applied upright or under hung, but is considerably heavy. Tapered insulators are therefore often preferred in HV applications due to their size but in this case selecting the optimal taper is important.
When a piece of equipment is first being considered for application under seismic conditions, the entire assembled and mounted structure needs to be evaluated using special software. Finite element analysis (FEA) will identify high stress areas in any given configuration. At the same time, lower stress zones will also be identified. The equipment designer or consultant should work closely with the insulator manufacturer to ensure all zones have equal safety margin. Several iterations may be necessary to fully identify all the optimal increases and decreases in strength at given locations across the insulator.
As lower stress areas are identified and remedied, weight is being reduced in that area. Top section weight reductions can then reduce strength needs in lower sections. This process leads to less mass, less motion caused by the mass and less overall stress. The cost of shaker table testing can exceed US$150,000 for large substation equipment. A thorough evaluation by a competent seismic specialist can yield significant savings by avoiding any need to re-test.
The location of an insulator in the equipment is also fundamentally important. In many cases an insulator is used to support a heavy piece of equipment. If that equipment is compact, with the mass near the top, there will be very little bending stress on the top fitting ad analysis shows that an extreme tapered insulator will be appropriate (Fig. 3).
If the equipment has a high center of gravity, with the mass well above the insulator, the top fitting will be subjected to a much greater bending stress. This calls for a more robust design for the top portion. In Fig. 4, the top of the insulator is subjected to 50% of the maximum bending loads.
Mass at the top of an insulator has the greatest bending effect. For example, in the case of an air break switch in the open position with the mast fully extended, high bending moments are found at the top of the insulator.
A typical 500 kV air break switch is mounted 4.6 m up on a structure. The switch in the open position can be 9.75 m such that from ground level to top of mast can be 14.35 m. Optimizing the needed strength for the insulator’s top can be a critical material reduction zone, since the weight reduction is at the top on the insulator where the mass is furthest from the bending moment.
Shed profile is a means to increase creepage distance but sheds also contribute weight to a porcelain insulator. In the past, sheds have typically been up to 19 mm at the core, tapering down to 12 mm at the tip. Based on improved material science, shed sizes can be reduced resulting in 20% reduction in shed weight.
Porcelain insulators are comprised of either single or multiple sections bolted together. Typically, insulators are single piece construction up to 750 kV BIL. High voltage insulators, by contrast, can be made up of many sections, depending on voltage level. Stress concentrations are found at the joints where the cast iron fittings are cemented onto the porcelain. The diameter of the porcelain at the fitting is increased due to the concentrated stress levels. Reducing the number of sections will help reduce high stress locations and also the weight of additional fittings.
Porcelain insulators are technical ceramics containing a mix of kaolin, alumina, feldspar and silica (quartz). IEC 60672-3 refers to three main types: C-110, C-120 and C-130 of which the C-110 is called quartz porcelain while C-120 and C-130 are known as alumina porcelains. C-120 contains 20%-30% alumina whereas C-130 normally has an alumina content >30%, which increases strength and obtains the highest strength to weight ratio.
The strength values shown in Table 1 are in fact minimums and can be far exceeded. Porcelain insulators manufactured from C-130 clay with much higher than these minimum levels can have its weight reduced by up to 40%.
Table 1: IEC 60672-3: 1984
Importance of Production Process
The manufacture of clay materials has an inherently wide range of resulting material strengths, with possible variation occurring within a single batch or between batches. Achieving consistent body strength can be challenging if processes are not tightly controlled. In fact, ceramic material strength can have over 35% standard deviation. The larger this deviation, the heavier the design of the insulator must be to guarantee that the specified mechanical load (SML) is met. Reducing standard deviation directly reduces weight of any given manufacturer’s design parameters, as illustrated by the following cases based on 2 standard deviations:
A porcelain insulator design with an SML of 10 kN has a std. dev. of 3.5 kN. The design must therefore be such that the average is 17 kN. On the other hand, if the standard deviation is only 1 kN, the design can be based on an average of only 12 kN. This results in an approximately 40% weight reduction.
To understand the causes of body strength variation one must understand the manufacturing process, which in the case of porcelain insulators typically involves wet or plastic methods. Clay recipes are measured and mixed with water to create the base material, called slip. A ball mill grinds this slip to ensure proper particle size and contains approx 50% water and this is then filtered to remove natural contaminants found in clays – from organic to iron. The slip is then pressed into filter cakes at approx 22% moisture content. The filter cakes are chopped and extruded into blocks. Finally, cylindrical blanks are extruded. Over a 5-6 week period the blank is turned and dried to less than 1% moisture content.
To have consistent body strength, all steps leading to a finished product must also be consistent. Particle size, chemical composition, water content of filter cakes, hardness of blanks and drying techniques will all determine predictability of body strength. The multiple drying steps of the wet clay – from pressing the filter cakes to use of dryers preparing the turned insulators for firing – are all important. The most critical is the drying step that sees the wet turned shape dried from 18% moisture to less than 1%. This is due to the thin sheds and the thick core needing to dry at the same rate, while thin sheds are far more like to give off water compared to the core. Up to 6 weeks is required to slowly dry an insulator. Many manufacturers have proper controls in place, but the entire process still requires skilled employees with attention to detail.
An alternative porcelain manufacturing method – called isostatic – has been developed that eliminates the many steps in the drying process. An important feature here is that it is now a much more consistent process, reducing the wide variation of measured material strength. The key to isostatic production is drying the slip to a fine powder and pressing it under high pressure into a dry cylinder.
The isostatic process offers many other inherent advantages since dry cylindrical blanks are produced in a relatively short time. Insulators produced with this method have a production time of less than two weeks versus some six or more weeks generally common with plastic production. Moreover, the turning step is performed dry thereby eliminating any shrinkage from wet turned profiles to dried, ready-for-firing state. The result is tighter tolerances.
The dry pressed blanks also have no particular grain orientation, as found in wet extruded blanks. As the wet body is extruded through the extruder throat, the clay flow can be much slower along the walls due to friction between clay and extruder wall. Internal to the blank, shear will occur causing internal stress, which can lead to failures in the kiln or reduction in mechanical strength. Depending on where in the blank the insulator comes from, these shear areas can end up located near to the surface. One notable trait is the camber formed as an insulator is dried.
Improving the performance of porcelain insulators in service under seismic conditions is achievable primarily through weight reduction methods. Optimizing design based on actual needs as well as using high strength materials and consistent manufacturing processes together ensure designs with optimal performance.