Power Utility Perspective on Insulator Selection for Polluted Environments

Insulators

Selecting porcelain and glass insulators for three-phase a.c. systems up to 525 kV phase-to-phase using IEC/TR 60815 “Guide for the selection of insulators in respect of polluted conditions” has been based on service experience and laboratory tests under both naturally and artificially polluted conditions. As stated in IEC/TR 60815: “Simple general rules should assist in choosing the insulator which should give satisfactory performance under polluted conditions”.

Although it is advised to determine type and severity of pollution and then use laboratory tests to help select suitable insulators, many utilities have simply standardized on one or more values of recommended minimum Specific Creepage Distance (SCD), i.e. ”Ratio of the leakage distance measured between phase and earth over the r.m.s. phase to phase value of the highest voltage for the equipment”. As given in IEC/TR 60815, recommended SCD values are 16, 20, 25 and 31 mm/kV(Um) for Light, Medium, Heavy and Very Heavy polluted environments respectively.

Pollution levels (I to IV) have mostly been determined from IEC/TR 60815 Tables, from service experience or when possible, by conducting site pollution severity (SPS) measurements using ESDD or DDDG methods. Table 1, for example, shows typical ESDD and DDDG values used to classify site pollution level and required minimum SCD.

Table 1: Typical ESDD and DDDG Values Used to Classify Site Pollution Level & Minimum SCD Required

Basic profiling rules recommended in IEC/TR 60815 are normally followed (summarized in “The Practical Guide to Outdoor High Voltage Insulators) are shown in Fig. 1 and Table 2.

Fig. 1: Insulator profile parameters, plain shed (a), alternating shed (b) and ribbed shed (c) [3].
Table 2: Recommended IEC/TR 60815 Profile Parameter Limits

Influence of diameter on pollution performance has also been considered for insulators with average diameters between 300 and 500 mm. In this regard, it is recommended that specific creepage distance be increased by 10% and 20% for diameters above 500 mm.
Although laboratory tests in accordance with IEC 507 have been recommended to evaluate an insulator’s pollution performance, these are rarely conducted. Rather, greasing or washing have been utilized as remedial measures for areas where insulators are unable to cope with severe pollution or low natural washing.

Typically, for simplicity and ease of use, many power utilities specify maximum connection length for a.c. porcelain and glass insulators (taking into account live line work), minimum dry arcing distance and minimum creepage distance, They then state that insulator profile must comply with IEC/TR 60815.

Attend the 2022 INMR WORLD CONGRESS in Berlin to listen to a presentation illustrating how the old technical recommendation, i.e., IEC/TR 60815 and the new technical specification i.e., IEC/TS 60815 are being used by a large power utility when specifying outdoor insulators for application in polluted environments. Emphasis is placed on three different approaches.