Failure of a suspension or tension insulator is clearly something that every utility or insulator manufacturer wishes would never occur. However, it is unrealistic to expect that any network component can exist forever without failure. Therefore, one can reasonably anticipate that a few insulators will fail in service after a certain period of time. What manufacturers and users alike would hope is that this number be as small as possible, i.e. failure rate of HV line insulators should be extremely low. Fortunately, most insulators available today have reached such a level of reliability. But is any particular failure rate really meaningful if taken only by itself? During the late 1990s, renowned insulator expert, the late Claude de Tourreil, discussed these questions by defining what failure really is and examining its consequences.
What Constitutes Failure?
A HV line insulator basically performs two functions: mechanically, it holds the conductor at a certain distance from the tower and the ground; and electrically it provides the necessary insulation to ground. Therefore, one way to define failure is when either or both of these functions are no longer being fulfilled. A less technical way to define failure is to say that an insulator has failed whenever it is removed from service because of unsatisfactory service experience. But this definition of failure can be misleading. For example, that criterion was used in a survey of service experience with composite insulators conducted by CIGRE many years ago. This led to an unjustifiably high figure for reported failures simply because some utilities supplying data decided to remove all insulators of a particular design because a small number had failed.
Different insulator types fail in different ways. The failure modes of toughened glass or porcelain insulators are quite different from the failure modes of composite insulators and this could have consequences for continuity of service. Because of this, it is difficult to really compare failure rates of different types of insulators.
Causes & Modes of Insulator Failure
Many factors can bring about the failure of an insulator. For example, the insulator can contain a tiny manufacturing defect that will grow over time and lead to failure. That defect can be located within the dielectric part of the insulator, the metal fittings or in the materials used to keep the dielectric part and the metal fitting together. The failure of an insulator could also be caused by service conditions, such as insulator separating when abnormally high ice accretion or heavy winds cause mechanical load to exceed the unit’s rated value. Similarly, extreme pollution levels can cause degradation of the dielectric or corrode metal fittings, thereby preventing the insulator from fulfilling its electrical or mechanical functions. A flashover leading to a power arc of extended duration can also bring about the failure of the dielectric or the failure of the metal fittings. Finally, various acts of vandalism can also cause insulator failure. Close examination of a failed insulator can best determine the actual cause.
Effect of Insulator Failure on Electrical System
When an insulator fails mechanically, it is no longer relevant to consider any electrical function. By contrast, should an insulator fail electrically it might lose its mechanical function or more likely still offer the strength required to allow normal operation of the line for an extended period of time. These different scenarios are usually a consequence of the inherent characteristics of various types of insulators.
The effect of insulator failure on the operation of an electrical system also depends on tower design. Mechanical failure of a composite insulator, a glass or porcelain cap & pin or a porcelain long rod insulator will in all cases cause a drop of the conductor if no other insulator is utilized in parallel. The situation becomes more complicated when an insulator fails electrically but not mechanically. It is assumed that a long rod porcelain insulator cannot fail electrically without also failing mechanically. By contrast, a composite insulator that has failed electrically could still have sufficient mechanical strength to hold the conductor. But since there is only one insulator between conductor and tower it can no longer hold the service voltage.
The most favorable case is for cap & pin glass or porcelain insulators. Since strings are made up from more than two insulators, the failure of one unit usually allows the string to still hold the nominal voltage of the line. A failed glass disc, for example, has practically no detrimental impact on performance of the string. The same would be true if a string consists of porcelain cap & pin insulators.
Detection of Failed Insulators
Determination of the failure rate of insulators requires knowing both the total number of insulators which have been installed and the number of insulators which have failed. It is usually fairly simple to determine the number of insulators of any type that have failed mechanically. As indicated above, it is most unlikely or even impossible for a porcelain long rod insulator to only fail electrically.
Detection of glass cap & pin insulators that have failed electrically is simple and based only on visual examination. Should the cap & pin string be of a porcelain, however, it would become more difficult to locate the defective unit even though methods for such detection are available. At the same time, electrical failure of any single cap & pin insulator is generally not critical since it usually does not lead to service interruption. Therefore, it is sufficient to detect failed cap & pin insulators only once failure of the entire string has occurred.
Since electrical failure of a composite insulator will affect continuity of service, it is highly desirable to be able to foresee such failure early enough to avoid a service interruption. The electrical degradation of composite insulators takes place over a period of time which, in most cases, is long enough to permit inspection to locate the defect before there is a service interruption.
Insulator Failure Rates
The failure rates of any given type of insulator is really something that can only be assessed by the utilities which use these insulators. They are the only ones to know how many such insulators have been installed and how many have failed. Understandably, insulator manufacturers are also interested to know these figures because they are in a position to take the measures necessary to improve that rate.
The failure rate of a specific insulator type may be used as a basis for assessing its quality. In this case, all failures, both mechanical and electrical, should be taken together. But from the utility point of view, it is also important to know the failure rate evaluated in terms of its impact on reliability of the electrical system where these insulators are installed. In this case, it is more logical to determine the failure rate of an insulator type based on number of failed insulators that actually caused an outage. When selection of a particular insulator type is based in large part on failure rate, it is particularly important to distinguish between these two different assessments of what really constitutes failure.
Knowing the failure rate of a given type of insulator can be a very useful tool on which to base expectations for long-term performance. Failure rate value by itself, however, may not be sufficient to fully assess the impact on the performance of an electrical system. It is also important, especially for the utility, to know if the failure of a given insulator will cause a line drop and/or an interruption of service. In this respect the utility engineer should realize that line design is also important.