Failure Rates of High Voltage Line Insulators

Dr. Claude de Tourreil

1. Introduction

Failure of a HV suspension or tension insulator is clearly something that any electric utility or insulator manufacturer wishes would never occur. However, it is unrealistic to expect that any piece of equipment can exist forever without failing. 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 kept as small as possible, meaning that the failure rate of HV line insulators should be extremely low. Fortunately, most modern insulators have now reached such a level of reliability. But how low is this rate, what does it really mean and is any particular failure rate really meaningful if taken only by itself?

To answer these questions, it is first of all necessary to define what a failure is and, for the term failure rate to become truly meaningful, to examine the consequences of an insulator failure.

2. What is Failure?

A HV line insulator basically has 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 an insulator failure is when either or both of these functions are no longer being fulfilled.

The failure modes of toughened glass or porcelain insulators are different from the failure modes of composite insulators and this may have a consequence for the continuity of service.

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 second failure criterion can actually be quite misleading. For example, that criterion had been used in the survey of service experience of composite insulators done by CIGRE WG 22-03 in 1986 and this ultimately led to an unjustifiably high figure for the number of reported failures. This occurred because, in some cases, reporting utilities decided to remove all insulators of a particular design simply because a very small number of them had already failed.

Different insulator types fail in different ways. The failure modes of toughened glass or porcelain insulators are different from the failure modes of composite insulators and this may have a consequence for the continuity of service. Because of this, it is perhaps quite difficult to compare failure rates of different types of insulators.

3. Causes and Modes of Insulator Failures

Many factors can bring about the failure of an insulator. The insulator can contain a tiny defect which, with time, will grow and lead to failure. That defect may 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.