Defects that generate a distortion of the E-field measurements (EF) can be detected by comparing the EF pattern obtained on the defective insulator with a reference fingerprint obtained on the sound insulator.

E-field measurements (EF):
When a composite insulator is electrically defective, the electric field changes in the vicinity of the defective area. A portable, in-service and manually-operated diagnostic probe was developed from the version used for cap-and-pin insulators and has been tested in the laboratory and in the field to check its suitability. The principle of operation is based on the measurement and recording of the AC axial electric field along the insulator. The EF probe can easily be used by skilled LLW personnel. Defects that generate a distortion of the EF (i.e. conductive or semi-conductive defects) can be detected by comparing the EF pattern obtained on the defective insulator with a reference fingerprint obtained on the sound insulator (see example in Fig. 8). To enhance the detection of defects, the data can be normalized by dividing the electric field value measured for each shed by the corresponding value of the plot taken as reference.

Fig. 8: Examples of E-field probe application on a 420 kV insulator): left: defect at the fitting; right: defect in the middle

The maximum EF variation found between the corresponding values on defective and sound insulators (in p.u. of EF of the sound insulator) are shown in Fig. 9 as a function of the defect length and position. It can be shown that even relatively small conductive and semi-conductive defects can be identified by this method. In the attempt to derive general rules, measurements were performed on insulators having different defects, generated during the manufacturing process (lack of primer, carbonization on the rod, break of the rod) or resulting from a long duration ageing test in different environmental conditions. Electrical field deviations higher than the intrinsic sensitivity of the methodology were confirmed for all types of defects investigated.

Fig. 9: Maximum electric field deviation (p.u) as a function of the defect length and position for conductive type defects

Apart from being very sensitive, the advantage of this method relies on its capability of indicating defect size and location, as shown in Fig. 8. However, the method is quite demanding in terms of time/cost and expertise required and thus it is not economical to scan an entire overhead line. On the contrary, it may be very effective to ensure the safety of LLW on a specific insulator or insulator vintage. It was found empirically, that the EF-probe may not detect low severity level defects near the end fitting, due to electric field shielding by the corona ring. This by the way does not allow measurements closer than approximately 15 cm from the end fitting with corona rings. The atmospheric humidity during EF-probe measurements must be recorded. If an insulator is covered by a hygroscopic pollution layer, this situation must be considered when taking the reference measurement and analyzing the results.

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