Реферат: Development of Composite Insulators for Overhead Lines
(3) To have a switching impulse withstand voltage of 625 kV or greater.
(4) To have a withstand voltage of 161 kV or greater when polluted with an equivalent salt deposition density of 0.03 mg/cm2.
(5) To have satisfactory arc withstand characteristics when exposed to a 25kA short-circuit current arc for 0.34 sec.
(6) Not to produce a corona discharge when dry and under service voltage, and not to generate harmful noise (insulator string).
• Mechanical performance (insulator alone)
(1) To have a tensile breakdown load of 120 kN or greater.
(2) To have a bending breakdown stress of 294 MPa or greater.
(3) To show no abnormality at any point after being subjected to a compressive load equivalent to a bending moment of 117 N. m for 1 min.
(4) To show no insulator abnormality with respect to torsional force producing a twist in the cable of 180°.
(5) To be for practical purposes free of harmful defects with respect to repetitive strain caused by oscillation of the cable.
Table 1 shows the characteristics of an insulator
designed to satisfy these specifications.
3. PREDICTING SERVICE LIFE
The service life of a composite insulator involves both electrical and mechanical aspects. Electrical aging involves damage from erosion or tracking due to the thermal or chemical effects of discharge occurring when the insulation material is polluted or wet, and may even result in flashover.
Mechanical aging includes long-term drop in the strength of the core material or in the holding force of the end-fittings, as well as brittle fractures of the core material, and can on occasion result in breakage of the insulator string. A drop in core strength or holding force of end-fitting can be countered by adopting an appropriate safety factor and using a reliable method of compression.
Brittle fractures, on the other hand, occur mostly near the interface between the insulation material and the end-fitting, and provided this area has been properly manufactured, the probability of their occurrence will be lower than that of electrical aging. To estimate service life from the electrical aspect, actual-scale composite insulators were exposed to electrical stress, and were subjected to an exposure test under a natural environment. A test chamber simulating environmental stress was also constructed, and accelerated tests were carried out according to international standards (IEC 61109 Annex C). Further, by comparing leakage current waveform and cumulative charge, which may be characterized as electrical aging, evaluation of composite insulator service life was carried out. Furthermore, since in Japan, a drop in insulation performance due to rapid pollution during typhoons is a familiar henomenon, an investigation was made based on the characteristics of leakage current obtained during a typhoon into the effect of rapid pollution on electrical aging in composite insulators.
4. CONCLUSION
Composite insulators are light in weight and have demonstrated outstanding levels of pollution withstand voltage characteristics and impact resistance, and have been widely used as inter-phase spacers to prevent galloping.
They have as yet, however, been infrequently used as suspension insulators. The composite insulators for suspension use that were developed in this work have been proven, in a series of performance tests, to be free of problems with regard to commercial service, and in 1997 were adopted for the first time in Japan for use as V-suspension and insulators for a 154-kV transmission line. To investigate long-term degradation due to the use of organic insulation material, outdoor loading exposure tests and indoor accelerated aging tests are continuing, and based on the additional results that will become available, work will continue to improve characteristics and rationalize production processes in an effort to reduce costs and improve reliability.
5. WORLD LIST
Conventionally
Outstanding
The Disadvantages
Fractured
To Degradation
Withstand
Polluted
a desire