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Standards for Insulating links
ANSI/CPLSO-14
ANSI/UL2737 (Withdrawn)
ASTM F2973
MIL-L-24410 (Withdrawn)
Tests by Independent Organizations
Load Insulator
Miller & Hirtzer
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387 Figure 3. New insulator Link 50 kV, 30 ton (300 kN) NEW INSULATOR LINK The presented analysis clearly indicates that use of a reliable insulating link would eliminate most of the dangers caused by transmission line and crane contact. However, the reliability of a contaminated insulator link is in question. This problem initiated a search for a better insulator link. Electricite De France uses 400 kV insulator rated to 600 kN or 60 ton. The one minute failing load of these insulators is about 800 kN and their maximum working load is 450 kN or 45 ton. Reliable operation of these insulators suggests the development of an insulator link using similar technology. Figure 3 shows a photograph of the prototype insulator link. This link is rated to 50 kV and 30 ton (300 kN). Two heavy hot dip galvanized forged steel end-fittings are compressed, crimped on a 38 mm (around 1.5 inches) impregnated fiberglass rod. The rod is covered by an injected EPDM housing with seven skirts. The leakage distance to strike distance ratio is 2.22. The EPDM housing is elastic and impact resistant. The available insulator links are rated between 3 to 50 ton and tested 4 times the rated load. Two links can be connected in parallel using a simple load equalizing mechanism to increase capacity. Figure 3 indicates that this link is similar to utility insulators. The skirts provide protective areas and improve contamination performance. INSULATOR LINK TESTS Insulator links were tested to evaluate contamination caused flashover voltage reduction. The contamination flashover test method used for insulators cannot be applied directly to insulator links because of the different operating conditions. A modified test method was developed to simulate the links in actual operating conditions. Testmethod The proposed contamination flashover test method is similar to the standard clean fog method [12]. The main difference is in the application and duration of the test voltage. Contamination flashover is a slow process where the leakage current caused heating produces dry bands. Repeated flashover of the dry bands ultimately leads to flashover of the insulator. Evidently the duration of energization affects the contaminated link flashover voltage. Taking this into account, the insulator links were tested with repeated, short duration applications of ac voltage. The voltage application times were 1 sec or 10 sec and in each test, the insulator links were energized 20-25 times to obtain statistical distribution of flashover voltage. The detailed test schedule is: 1. 2. 3. 4. 5. 6. 7. 8. Prior to application of contaminants, the insulator link was washed and cleaned with detergent, using a soft scrubber. Furthermore, the new link was contaminated 4- 6 times and the contamination was washed down by gently rubbing the surface. This conditioned the insulator surface and assured uniform contamination distribution. The clean, dry link was sprayed with standard contamination slurry. The slurry contained 40 fliter kaolin and variable (15-150 fliter) salt. The insulators were dried overnight in a vertical position. The well dispersed distribution of the contamination was observed. The ESDD was measured by washing off the contamination. The link was contaminated by spraying again, using the same slurry and dried overnight. This permits recovery of the surface. The link was wetted in a fog chamber by steam fog. The rate of steam generation was: 0.06 kg/m3/hr. During the wetting, the surface resistance was measured using 1500V. When resistance was at a minimum, the 60 Hz test voltage was switched to the insulator for 1 to 10 seconds. The voltage was applied 25 times using a 20-40 second waiting period between each energization. The test was repeated with different voltages and different contamination levels. The test voltage and leakage current were measured by a digital oscilloscope. This allowed an accurate recording of voltage duration, rms and peak voltages. The number of flashovers in each series is recorded and the flashover probability is calculated.
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