There is much debate on the state-of-the-art of composite insulator technology and a spectrum of contradictory case histories with different types of insulators. The only consensus is that there has been a lot of improvement compared to previous generations, which often account for most of the unsatisfactory experience. The underlying issue behind contradictory views on reliability is the simple fact that insulator performance depends on choice of design given the intended service environment. In the case of composite insulators, in particular, this has not always been done properly. For example, inappropriate or too superficial a specification from the electrical standpoint could lead to flashover of ceramic insulators but can result in irreversible damage to composite insulators.
Composite insulators offer a number of well-recognized advantages. However – contrary to what was promoted originally – they are not indestructible. Therefore, to ensure performance, care must be taken in their specification, handling and installation. In fact, failure analysis of most reported problems with these insulators has shown that these were due to deficiencies in specification 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 which, over the long term can lead to tracking, erosion and eventual failure. Indeed, this is critical since composite insulators are 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 high system voltages). Similarly, failures have sometimes been the result of wrong estimation of the pollution service environment.
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:
a. a low-value, highly intermittent class;
b. a relatively high average current of a few mA, but far from values typical of pre-flashover conditions;
c. a high current value class (i.e. some hundreds of mA) indicating that the insulator is approaching 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 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 adverse influence of service stresses on hydrophobicity and wettability.
Therefore, in the case of composite insulators for AC and DC applications, the conventional approach based on pollution classes as defined in IEC 60815 is not ideal. 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 not sufficient. For example, 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 as an ‘orientation tool’ but not as 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 class ‘b’ type.
While only the aspect of electrical design is considered here, proper specification from the mechanical point of view is also important and possibly even more so than for ceramic insulators. Again, many reported failures, especially on recent generations of composite insulators, have been due to inaccurate mechanical specification or by mishandling and poor installation practices that do not take into account the risk of permanent damage.
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 electrical and mechanical specifications are accurate and also how these insulators respond to specific service stresses and different methods of installation.