Although many power engineers still tend to regard composite insulators as a relatively new technology, these products have already been on the market for five decades, starting in the 1970s. In fact, they have undergone a steady succession of design improvements based on accumulating field experience that now make this technology seem to be approaching maturity.
One can also say that, today, depending on quality of manufacturer, composite insulators offer performance that is basically equivalent to and sometimes even exceeding that of high quality porcelain and glass alternatives. This means that operators of power networks can confidently select which insulator technology (ceramic versus non-ceramic) is most suitable to meet their needs based on such as considerations as line design, service environment and acquisition as well as total life cycle costs.
This article, prepared by INMR columnist and industry specialist, Alberto Pigini, examines the global insulator marketplace and reviews the growing impact composite insulators have had on this business, especially since 1990.
According to studies published by researchers in the United Kingdom, the annual worldwide demand for electrical insulators is estimated at approximately US$ 4.5 billion. Up to 2008, there was relatively strong growth in this market as shown in Fig. 1, anticipating the market for power generation development (Fig. 2), while more recent years have seen a tendency toward market saturation.
Fig. 3 shows the increase in total power generation capacity expected in coming years – regarded as one of the key drivers of future growth in this business. According to these projections, in spite of the recent slowdown due to economic problems in Europe and elsewhere, a more or less continuous increase in annual sales volume can be expected over the next 15 to 20 years, with projected installation of new generation almost doubling.
At the same time, while sustained growth is expected for the overall insulator market, the relative dynamics of each alternative technology are likely to be different. For example, based on reports from several sources, as shown in Fig. 4, composite insulators have seen a steady increase in market share of since 1995, in spite of competitive measures taken by suppliers of porcelain and glass insulators (both of which have seen a corresponding drop in their respective shares).
This trend, as shown in Fig. 5, has especially been evident in China, the world’s most rapidly developing power market over that period. Indeed, on Chinese electrical projects commissioned over the last 5 years, about 44% of the insulators installed for applications above 66 kV were apparently composite type, while the shares accounted for by porcelain and glass were about 27% and 29% respectively.
The market position of composite insulator technology in China is especially strong for UHV AC and DC lines, where their share is as much as 67% at ±800 kV DC due to expected superior performance under pollution. This reduces required string length, effectively decreasing tower profiles/sizes and resulting in lower overall project costs.
In regard to actual numbers of units, Fig. 6 shows the estimated population of composite insulators installed worldwide on overhead lines and at substations with voltages higher than 66 kV.
Application of composite insulator technology is now also increasing in the fast expanding DC world where the rapid increase in the number of these systems will drive growth of the entire market (see Fig.7). It seems reasonable to assume that a large share of this market will be captured by composite insulators, as indicated by the Chinese experience.
Insulator Acquisition Cost
It is difficult to make a generalized comparison of the relative acquisition costs of alternative insulator technologies since these vary by manufacturer, by country and also due to continuous changes in the competitive environment. As such, what is presented in Fig. 8 is intended more as a qualitative indication representing only a ‘snapshot’ of a typical situation. This evaluation was made considering the normal insulator rating requirements for each system, the usual mechanical and electrical stresses and a service environment characterized by medium contamination.
The corresponding lower values shown for DC insulators than for AC can be explained by the fact that specialized ceramic insulators are necessary for DC applications, with a lower number of competitors presently able to supply this market.
In these charts, only insulator acquisition cost is considered. However, the impact of insulator technology on total project costs must of course also be considered when deciding on the optimal choice. For example, in the case of composite insulators, reduced string lengths could be specified versus ceramic types, with correspondingly lower weight, thereby decreasing tower profiles/sizes and with consequent cost savings. Also, transportation and installation costs can be reduced in each case.
A number of surveys have been undertaken over the years regarding comparative service experience with different types of insulators for AC line applications. Unfortunately, these have not been regularly updated although such a task is presently under consideration within CIGRE. It is important to note, however, that findings from such surveys have varied, often because the definition of ‘failure’ may have been different in each case.
A line insulator has two basic functions: mechanically, it holds the conductor at a prescribed clearance from the tower while, electrically, it also provides the necessary insulation to ground. Therefore, one way to define insulator failure is when either one or both functions are no longer being fulfilled. Indeed, this has now become the latest philosophy.
A less technical and perhaps misleading way to define failure, however, is to simply state that an insulator is deemed to have failed whenever it is removed from service due to unsatisfactory service experience with certain units from the batch. For example, that criterion was used in the CIGRE survey of service experience with composite insulators carried out by WG 22-03 in 1986 and led to an unjustifiably high figure for number of reported failures. In some cases, reporting utilities removed all insulators of a particular design (batch) simply because a few had already failed.
The recent EPRI philosophy sees definition of failure as follows:
• the insulator was unable to insulate the line from ground;
• the insulator has lost mechanical strength.
EPRI’s database in this context has been active since 1997 and recording of insulator failures has continued. In one of the most recent such reports, for example, it was estimated that the total number of composite insulator failures in the world, excluding China, was around 400, of which 89 involved a ‘vintage’ insulator from one manufacturer that is no longer in production. Considering the projected population of insulators in service, this suggests that corresponding failure rate is in the range of only 10-4 to 10-5. Similar values have been reported in China, where millions of composite insulators are already installed. This proportion takes into consideration failures of first and second generation composite insulators and one might therefore expect that modern designs probably have an even lower failure rate, perhaps closer to 10-6.
Comparing these statistics with corresponding data for cap & pin insulators is not quite so straightforward since strings are comprised of many insulator units and failure of only a few usually still allow the string to hold the nominal line voltage. According to the some sources, failures not affecting service (i.e. electrical failures of only one or a few units) are on the order of 10-4, while failures affecting the entire string are in the range from 10-5 to about 10-6, depending on voltage.
It would be useful to update information on failures of porcelain insulators to better track the evolution of this segment in terms of the overall market and the different competing suppliers. Some manufacturers of these insulators that participated in earlier studies have ceased operations while new ones have appeared. While this has increased competition, it will also undoubtedly impact future overall reliability – but in a way that is not yet quantifiable. Recent information from China, for example, suggests that the overall failure rates for composite and porcelain insulators are currently similar, with a slightly lower rate for glass discs.
It is important to emphasize that, based on a growing body of service experience, manufacturers of composite insulators have greatly improved their designs, both electrically and mechanically. Indeed, some of these manufacturers are now supplying a 5th generation design that promises to eliminate all well-known failure modes of this technology. One can therefore expect a more or less similar low failure rate for insulators of all technologies, assuming these are correctly designed and manufactured using good quality control procedures.
In spite of their overall reliability, one aspect that penalizes composite insulators is the relative complexity of the diagnostics required to find in service units that are either close to critical or which have failed but without apparent external signal. Many improvements have been made over the years towards more reliable and effective diagnostics of composite insulators in service. However, while the complexity of these procedures is now close to that required for porcelain insulators, it is certainly more complex than for glass, where a simple visual inspection is usually sufficient.
The investment and other costs of preventive diagnostics vary quite a bit according to the specific experience of different power system operators. For comparison purposes, the following costs were estimated by one source for the inspection of 1 km of overhead line:
Total cost US$ 3710/km based on 20 inspections over a useful service life of 40 years, i.e. about US$ 200/km for each inspection.
Total cost US$ 989 per km based on 20 inspections conducted over 40 years, i.e. about US$ 50/km per inspection.
The above estimates suggest that the cost of diagnostics for composite insulators is about 4 times that of porcelain. However, these relative costs can vary by country, depending on the inspection procedures used as well as the relative cost of maintenance personnel. No information was given in the above estimate for glass insulators but, since visual inspection alone may be sufficient, it can be estimated that the cost of diagnostics here is one order of a magnitude lower than for composite insulators.
It must also be pointed out that the cost of preventive diagnostics will depend on such issues as criticality of the line and overall maintenance policies at the power utility involved, thereby leading to different inspection cycles, e.g. once every 2 years, 5 years, 10 years, etc.
Estimating the relative cost effectiveness of each alternative insulator technology can best be achieved by comparing their respective life cycle costs (LCC). The LCC of an overhead transmission line insulator is defined as the sum of its initial acquisition cost, replacement cost and accumulated maintenance costs over its expected service life.
The main advantages of composite types in terms of LCC are realized when they are applied to substitute for ceramic insulators in an environment of critical pollution levels. A number of possible mitigation procedures are available to line operators should the design of ceramic insulators not prove adequate in severe environments. As shown in Fig. 9, which refers to the experience at a substation in Iran characterized by extremely high contamination (ESDD=1.338 mg/cm2, NSDD=6.8716 mg/cm2), these can prove very costly.
In particular, the high cost of water is but one of the limiting factors in deserts and similar harsh environments (e.g. see cost details in Table 1 which refer to the Iranian substation case). In this regard, preparing distilled water and transporting it to site constitutes a major part of the total washing cost. Moreover, the already high cost for washing a substation pales in comparison with what must be spent to regularly wash long lines in harsh service environments.
A realistic alternative to washing is application of composite insulators, as illustrated by the case of a line in Saudi Arabia where standard fog type disc insulators needed to be washed every two months to avoid pollution flashover. Based on this experience, the LCC cost of the ceramic insulators proved to be approximately 9 times higher than for composite insulators, based entirely on the high cost of washings.
Another example is shown in Fig. 10, which illustrates an evaluation made with reference to the service environment in Egypt. In this evaluation, the same acquisition and installation costs were taken for composite and ceramic types in order to make the comparison as conservative as possible. The LCC of the composite insulator alternative was then also conservatively projected assuming that these could be used for only a limited time due, for example, to premature ageing. These were then compared to the LCC of ceramic insulators, for which a service life of 40 years was assumed, but with the need for periodic washing (c.w.). It quickly becomes evident that, in the case of heavy contamination requiring more than 6 washing per year, even in the very pessimistic scenario that the composite insulators would have to be replaced after 15 years, selecting this solution would still be far more cost effective.
1. Composite insulators now represent a significant and growing part of the overall market.
2. The acquisition cost of composite line insulators is currently very competitive compared to other technologies.
3. The reliability of different insulator technologies, if well designed and manufactured under high quality control, is essentially comparable.
4. Preventive diagnostics are typically more costly for composite types than for other technologies.
5. The total life cost of composite insulators may prove significantly lower than that of ceramic technologies, especially in harsh environments where there is a need for costly maintenance.