Failure of Line Insulators

What is the cause of the failure of line insulators?

Basically there are two functions about HV line insulators: mechanically speaking, it keeps the conductor from the tower and the ground at a certain distance. Electrically, it provides the necessary insulation for the ground. Therefore one way to define failure is when either or both of these functions are no longer being fulfilled. A less technical way of defining a fault is when an insulator is removed from service due to an unsatisfactory use experience. But this definition of failure can be misleading. For example, this standard was used many years ago in the experience survey conducted by CIGRE on the use of composite insulators. 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. The reason is simply that, because of a few failures, some of the utilities that supplied the data decided to remove all insulators of a particular design.

Different types of insulators have different failure modes. The failure mode of tempered glass or porcelain insulators is very different from that of composite insulators, which may affect the continuity of use. Therefore, it is difficult to truly compare the failure rates of different types of insulators.

Insulator Failure Rate

The failure rate of any particular type of insulator can only be assessed by the utilities that use these insulators. They are the only ones who know how many such insulators have been installed and how many have failed. Understandably, insulator manufacturers are also interested in knowing these figures because they have the ability to take the necessary measures to increase this ratio.

The failure rate of a specific insulator type can be used as a basis for evaluating its quality. In this case, all failures, including mechanical failures and electrical failures, should be considered together. But from the utility point of view, it is also important to understand the failure rate evaluated based on its impact on the reliability of the electrical system in which the insulator is installed. In this case, It is more logical to determine the failure rate of an insulator type based on the number of faulty insulators that actually caused a power outage. When the selection of a particular insulator type is mainly based on the failure rate, it is particularly important to distinguish between the two types of assessments that actually constitute a failure.

Conclusions

Knowing the failure rate of a particular type of insulator is a very useful tool, which can be used as the basis for long-term performance expectations. However, the failure rate value itself may not be sufficient to fully assess the impact on the performance of the electrical system. It is also important, especially for utilities, to know whether the failure of a given insulator will result in line drop and/or service interruption. In this regard, utility engineers should realize that line design is also important.

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