Avoiding Insulator Failures Due to Improper Selection

Composite insulators offer a number of well-documented advantages. However, contrary to what was promoted during their early years, they are certainly not ‘indestructible’. Therefore, to ensure performance comparable to or better than that expected of ceramic insulators, great care must be taken in their:

• specification,

• handling, and

• installation.

With respect to specification, a large proportion of the problems reported with these insulators over the years can be traced back to deficiencies in selection – especially from the electrical point of view. This is because electrical design of composite insulators should not be made looking solely at their flashover performance during short-term tests. Rather, it must ideally be based on the risk of surface degradation from partial discharges that, over the long term, can cause tracking, erosion and eventual failure.

This is a critical shortcoming since composite insulators are highly vulnerable to damage should there be continuous partial discharges and arcing activity on or near their surfaces. For example, many reported cases of failure have been due to insulators being installed without suitable shielding electrodes to limit electric field gradients near their high voltage end and even at their earth end in the case of very high system voltages.

Similarly, failures have sometimes been the result of inaccurate estimation of the real pollution service environment. CIGRE Brochure 142-1999 explained that experience from laboratory ageing tests as well as field trials has confirmed that there are three classes of leakage current on composite insulators under normal wetting conditions:

1. a low-value, highly intermittent class;

2. a relatively high average current of a few mA, but far from values typical of pre-flashover conditions;

3. a high current value class (i.e. some hundreds of mA) indicating that the insulator is nearing flashover.

While ceramic insulators are designed looking mainly at the 'c type' class of leakage current, composite insulator units should be designed instead taking 'b type' currents into account. In fact, research has indicated that while class 'a' currents have little influence on long-term performance, class 'b' currents can lead to tracking and erosion and possibly permanent failure.

As a result, there should always be sufficient design margin between withstand severity and actual environmental pollution whenever selecting composite insulators. The critical need is to limit leakage current over the full service life taking into account the possible influence of service stresses on surface hydrophobicity and wettability. Therefore, in the case of composite insulators, whether AC or DC, the conventional approach based on pollution classes as defined in IEC 60815 can be considered questionable.

Rather, to assure satisfactory service performance, a statistical approach must be made that accounts for both environmental parameters and specific insulator characteristics. In particular, specification in terms of required creepage distance alone is itself not sufficient. For example, the efficiency of a profile may become low if too much creepage is forced on a given arcing distance. Indications as per IEC 60815 should ideally be regarded more of an 'orientation tool' than a substitute for the information that comes from testing.

For those composite insulators already installed on lines and where it is too late to change specifications, diagnostics based on measuring leakage current along selected units can help identify possible insufficiency in design and trigger washing should average leakage current values reach the destructive class ‘b’ type.

While only the aspect of electrical design is considered here, suitable specification from the mechanical point of view is of course also important and possibly even more so than for ceramic insulators. Again, many reported failures, especially affecting recent generations of composite insulators, have been due to inaccurate mechanical specification or by mishandling and improper installation practices that do not take into account the permanent damage that can occur.

In principle, the maturity and intrinsic reliability of composite insulators can now be considered satisfactory and of the same high level as ceramic insulators. However, reliability in practice will depend on whether the electrical and mechanical specifications are accurate and also account for their special characteristics, response to specific service stresses and methods of installation.

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