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Cooling pipes for electrical equipment
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Our material is the leader among non-conducting materials. The periodical is called the Power Engineering Journal. The issue dates back to November 1991. The ten-page article is written by D.M. Hodgson, and deals with the evaluation of PEX for use as cooling tubes in HVDC thyristors and in power factor correction equipment.
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GEC Ahlstom Engineering Research Centre in Stafford, Great Britain undertook the test assignment when the need arose for a new method of cooling high voltage equipment. Liquid was to be used for cooling instead of air.
The reason was that liquid cooling would enable the equipment to be made smaller and more efficient.
The most serious problem was that metal tubes conduct electricity. The tube needed would also have to have the same useful life as the remainder of the installation. PTFE (also known under its Teflon trade name) tube would have been an expensive and cumbersome alternative
ABS plastic had insufficient strength at temperatures above 60ºC. This factor also dismissed a number of other plastic materials. In thyristor applications, more than 100 tubes connected in series are needed to circulate deionized water in an electrically stressed environment
In a cooling system employing deionized water, the temperaturevaries between +5ºC and +80ºC, while in an environment cooled by a deionized water/glycol solution, it varies between -50ºC and +80ºC.
According to the article, Wirsbo-PEX tube was the only alternative worth testing. The long-term strength of the tube at temperatures of up to 95ºC was documented, and the tube is approved for domestic water distribution.
Moreover, the connections themselves had to be specially manu-factured and had to be electrically non-conductive. Wirsbo-inPEX was to tailor the connections to the thyristor valves.
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All of this looked good, but no one had earlier subjected the material to high-voltage tests and simultaneous pressure and temperature cycles in an electrically stressed environment. The tube was subjected to
laboratory tests at various temperatures and electrical stresses to determine its oxygen diffusivity and the way it would be affected by ozone. The test was to simulate more than 30 years of maximum loading. The tube was subjected to between 10 000 and 30 000 full-scale temperature cycles at working pressure.
A few small adjustment were made to the design of the couplings, but no other changes were necessary. When the test was
comple-ted, all test pieces were subjected to a gauge pressure of 30 bar, but no leakage or other problems were found. Bursting tests at 85 - 90 bar showed that the aged tubes on test had the same strength as new tubes.
Material specimens from the tubes that had been tested displayed no ageing symptoms as compared to new tubes. The test results were positive in all respects |
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Finally, we cannot refrain from quoting the following from the
article...
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XLPE tube present and future
Quality
The features of XLPE tubing which contribute to its suitability for meeting the special requirements of cooling systems for high-power electrical equipment at acceptable costs are:
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I high reliability as a dielectric material
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II combination of mechanical properties, i.e. selfsupporting but semiflexible
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III pre-formable to complex shapes and
tolerant of minor abrasion and accidental damage
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IV exceptionally smooth internal and external surfaces for cleanliness and low hydraulic resistance
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V resistance to attack by ozone
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VI adequately stablised against ultraviolet radiation
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VII low permeability to atmospheric gases, in particular oxygen and carbon dioxide
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VIII proven potential for extremely long and reliable service life for the specific duties of water or glycol/water inside, air outside for the required ranges of temperature and hydraulic pressure.
We wish to extend our thanks to GEC Ahlstom and Power Engineering Journal for their permission to use extracts from the article.
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