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Polymeric Housed Bushings: Utility Viewpoint in a Regulated Market

Polymeric Housed Bushings: Utility Viewpoint in a Regulated Market

June 18, 2016 • ARTICLE ARCHIVE, Bushings
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The former IEC 1109, considered a set of tests including design test and type test. Now, an updated document published in 2008 established the 1000-h a.c. tracking and erosion test of IEC 62217 as a minimum requirement for the tracking resistance of a housing material. The new version of IEC 61109 changes the concept of tracking and erosion test and includes the tracking wheel test to ‘screen’ designs. The standard would be acceptable for overhead lines insulators. The recent IEC 62217-2012, Polymeric HV insulators for indoor and outdoor use, considers the 1000 hour salt-fog tracking and erosion test as a screening test intended to reject materials or inadequate designs. Of course, this is a minimum requirement for polymeric insulators used outdoors or indoors, and includes solid core as well as hollow core insulators. From this standard two alternative tracking and erosion tests were removed – the 5000-h multi-stress test and the tracking wheel test now both included in IEC/TR 62730-2012 with the same screening criteria.

IEC 60815-3, published in 2008, considers a Unified Specific Creepage Distance (USCD) varying from 22 mm/kV to 53.7 mm/kV depending on the site pollution severity (SPS) class. The USCD is the same for ceramic and glass insulators for a.c. systems according to IEC 60815-2. Moreover, IEC 60815-3 includes recommendations for polymeric profiles, correction factors for altitude and insulator diameter as well. The most important point is that the standard considers three operating areas as a function of pollution severity and USCD (for a fixed insulating length). In Area 1, creepage distance is too low, which will result in increased flashover probability. Within Area 2, design and creepage distance are correct, which will result in minimal probability of flashover and damage. Finally, in Area 3, the creepage distance appears correct but is obtained by incorrect design, which will result in increased risk of damage. However, the relationship between size and position of the areas and safe operating regions (SOR) and the values of the diagram axes depends on pollution, climate and the properties of the housing material and design; hence, a general recommendation is not given in IEC 60815-3. For the last point, the standard recommends outdoor testing for at least one year, monitoring leakage current, flashovers and degradation to housing and interfaces. Tracking and erosion tests are considered in IEC 60587 for insulating materials and, based on experience with high voltage insulators, the recommended minimum requirement is 1A3.5 according to IEC/TR 62039-2007, which means 3.5 kV. However, in very polluted areas, as found in Mexico, this value is now questionable since equipment with insulating materials that presumably passed the 4.5 kV level failed due to pollution.

Before 2007, CIGRE WG D1.27 dealt with properties and methods to evaluate polymeric materials, specifically the ‘fingerprint’, and dealt with loss of hydrophobicity and ageing. The main methods identified included:

• Differential scanning calorimetry (DSC) to measure the endothermic or endothermic properties of the material;
• Thermal gravimetric analysis (TGA) to measure the thermal stability of the silicone rubber. As temperature increases, weight loss occurs due to release of moisture or gases from the decomposition of the silicone rubber. In the end, only a non-volatile residue remains;
• FTIR to analyze the absorption of infrared radiation due to vibrational motions in the molecules. Characteristic vibrational frequencies in spectra provide distinct identification of the silicone rubber molecules;
• Gravimetric analysis for determination of filler content.

Actual bushing standards do not consider additional tests, as considered in IEC 62217 for hollow core insulators and IEC 61109 for overhead transmission lines. Also, standards for HV equipment consider a correct procedure to compute the contamination behavior for a new polymeric design, but without specific experimental support to confirm it.

Fig. 14: Failure of 230 kV bushing in KM 20 substation. housed bushings Polymeric Housed Bushings: Utility Viewpoint in a Regulated Market Screen Shot 2016 06 17 at 10

Fig. 14: Failure of 230 kV bushing in KM 20 substation.
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Conclusions

Cost has become a main driver and it would be a good decision to agree to standardization in both materials and manufacturing. Also, additional tests, e.g. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), IR thermography, computer tomography and shearography should all be used during acceptance tests to identify the presence of any voids, cracks, de-laminations or poorly bonded areas and should also be considered in the standards. Customers typically do not have information about possible changes in materials and manufacturing or the tests performed to validate these. Customers and manufacturers need to focus experience and new technical developments to write new standards that better discriminate good from bad formulations and designs.

Polymeric-housed bushings offer the promise of much better flashover performance but their failure can lead to heavy damage to substation equipment and great cost to replace it (see Fig. 14). Due to this specific failure, as well as those highlighted in Figs. 5 and 6, the CFE decided to develop its own national standard for HV equipment with silicone rubber housings. The main point in this standard has been to require several tests not included in the international standards such as: inclined plane tests with a test voltage of at least 6 kV; adhesion tests for the housing; FTIR; TGA; and a tracking wheel test as an alternative to the 1000-h salt fog test.

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