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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ANSI/UL2737 (Withdrawn)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MIL-L-24410 (Withdrawn)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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389 Flashover voltage measured with the "up and down" method is marked by a dot on Figure 6 for comparison. It can be seen that the flashover voltage obtained with constant energization corresponds to the 7545% flashover probability with one second energization. Figure 6 compares the 1 second and 10 second energization. The figure shows that the energization time effects only high probability, above 42% flashover values. Even in this region, the effect is not significant. ge Current Figure 7 shows the variation of leakage current, when lightly polluted Link 1 was energized for 1 second with 30 kV for 25 times. It can be seen that each energization generates different leakage current. The maximum and minimum values were between 32 mA and 58 mA. The test of other links produced similar values. Leakage current is the measurement of protection provided by the insulator link if the link does not flashover. The less than 100 mA short duration (1 sec) current pulse is not considered dangerous. Figure 7 indicates that the link protects the operator, if the link withstands the voltage, APPLICATION OF RESULTS Practically speaking, the most important result is the estimation of link performance. The presented curve gives probability of link flashover if the link is wetted thoroughly by steam fog. However, wetting does not occur every day. 60 5 50 40 5 30 z c L Q) Y 0 9 20 f 10 0 0 5 10 15 20 25 Number of Energization Figure 7. Leakage current variation during test of Link 1 (light pollution and 1 sec energization). The statistical variation of the environmental conditions and flashover voltage suggests the estimation of link performance using probabilistic techniques. The flashover of an insulator link occurs if: a) the link is contaminated. b) the link is wetted by fog, drizzle or light rain. c) the operating voltage is within the flashover limits shown in Figure 4 or 5. These three independent events occur in nature with certain probability. The probability of link pollution cannot be estimated easily. It depends on the environment, maintenance frequency, operator skill, etc. It is safe to assume that the probability of light or medium contamination is 100%. If heavy pollution is visible, the operator will clean the link. Wetting of insulators occurs in foggy, wet and drizzling weather. Heavy rain cleans the insulators and eliminates the danger of flashover. The number of wet and foggy days per year can be obtained from meteorological records. As an example, in North Dakota the average number of foggy days is 6-8 per year. Accordingly, the probability of wetting of the link is 2.2- 1.65%. The probability of flashover can be determined from Figure 4-5. As an example, we assume that the crane touches a line with line to ground voltage of 46 kV and it is protected by Link 3.. Figure 5 gives a flashover probability of 72% at 46 kV if the link is heavily polluted (0.3 mg/cm2 ). Risk of failure is the probability that the link fails to protect the operator, which is the product of wetting and flashover probability. Our example of the probability of risk of failure is 0.72 * (0.022 or 0.016) = 0.11 - 0.15. This means that once or twice out of every 10 cases, the link fails to protect the worker. This example shows that even an imperfect link provides some level of protection and reduces the number of accidents significantly. However, the link should provide 100% protection. This can be determined using flashover probability curves. As an example, Figure 5 indicates that the link provides 100% protection if the line to ground voltage is less than 40 kV. In practical terms, it means that a heavily polluted new link protects the operator if the line voltage is less than 69 kV. This example shows the importance of the flashover probability measurement. The risk or the 100% protection level cannot be estimated using the data obtained by standard contamination testing.
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