Electrical performance of every insulator depends greatly on choice of design given the intended service environment. However, the outcome of improper specification can prove much different between ceramic and composite type insulators.
As explained in this edited contribution to INMR by Alberto Pigini, inappropriate or too superficial specification of ceramic insulators can lead to flashover. But the result in the case of composite type insulators can be permanent damage.
Composite insulators offer a number of well-known benefits due to their high strength to weight ratio. But experience has demonstrated that they are far from ‘indestructible’, meaning that they must always be handled, stored and installed properly. Similarly, to assure service performance comparable to or better than that of ceramic insulators, care must also be taken in their specification. Indeed, analysis of reported failures with these insulators over the years has shown that many are due to deficiencies in specification and selection from the electrical point of view.
Electrical design of composite insulators should not be made looking solely at flashover performance during short-term tests. Rather, it must be based on risk of surface degradation from partial discharges that, over the long-term, can cause tracking, erosion and eventual failure. This is critical 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 documented failures 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 also been the result of inaccurate estimation of the actual pollution environment where they were put into service.
Experience from laboratory ageing tests as well as from field trials has demonstrated that there are three distinct 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 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, erosion and possibly permanent failure.
As a result, there should always be sufficient design margin between withstand severity and actual environmental pollution when 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 (both AC and DC), the conventional approach based on pollution classes, as for example defined in IEC 60815, is not ideal. Rather, to assure satisfactory service performance, a statistical approach must be made that accounts for environmental parameters as well as specific insulator characteristics.
In particular, specification in terms of required creepage distance alone is not sufficient. For example, the efficiency of a profile may become very 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 any insufficiency in design and trigger washing should average leakage current values reach the destructive ‘b’ type.
While only electrical design is considered in the above discussion, 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 with recent generation composite insulators, have been due to inaccurate mechanical specification or by mishandling and improper installation practices that did not take into account that these can induce permanent damage.
In principle, the maturity and intrinsic reliability of composite insulators can be considered satisfactory and of the same high level as ceramic insulators. However, reliability of these insulators in practice will depend on whether electrical and mechanical specifications are accurate and also account for the special characteristics, response to specific service stresses and methods of installation.