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Better Understanding Erosion Testing

September 9, 2017 • ARTICLE ARCHIVE, Silicone Technology Review, Testing
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International standardization is definitely helpful. Standards for electrical insulating materials and insulating systems allow power engineers and other technical experts across the globe to better understand each other and develop international exchange of goods and services. Imagine the chaos if we still had to apply old local measures such as ‘cubits’.

Standardization activities, however, are driven mainly by new products, new applications and new technologies. For example, DC transmission at EHV and UHV is still a relatively new technology. Already in practical use for several years, it now faces increasing demands for new standards, e.g. evaluating the properties of different insulating materials used in HVDC and UHV DC applications.

In this regard, erosion resistance of polymeric outdoor insulation materials has become a ‘hot topic’ of late. A status report was published by CIGRE in March 2015 (CIGRE Brochure 611: Feasibility Study for a DC Tracking & Erosion Test) and international round-robin-tests are ongoing. Taking the findings of the Working Group into consideration, it seems that a wet DC test (such as the inclined plane test) will be complicated to work with. But is there any alternative? Will an arc test on the basis of IEC 61621 provide the applicable desired results?

Findings up to now look promising and therefore I’d like to make readers more familiar with this interesting tool to evaluate and compare the stress that test specimens are exposed to.

The methodology allows comparative evaluation of cumulative thermal stress during an arc test and is based on infrared measurement of the hot-spot temperature of the specimen.

This temperature depends on the value and clocking frequency of imposed current as well as on the material being tested. During the test, temperatures of the silicone elastomer specimens rise with increasing stress to reach up to 1600°C. The computed integral of the maximum temperature less the minimum deterioration temperature multiplied by a given time unit (temperature-time-area) is then understood to be a measure of the deterioration thermal stress during the test.

Better Understanding Erosion Testing Screen Shot 2017 09 08 at 15

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Better Understanding Erosion Testing Screen Shot 2017 09 08 at 15

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The graph below charts the TTA (mean values of 5 specimens each) versus test time of a silicone elastomer (VMQ 1) in an arc test for both an AC test according to standard IEC 61621 and also the same test carried out under DC stress. It shows that test specimens are exposed to the same cumulative thermal stress under AC and DC. Consequentially, these specimens exhibited the same failure times. The procedure allows an improved understanding, evaluation and comparison of test results and will help decide whether or not DC erosion testing is necessary and, if so, which test conditions need to be applied.

Better Understanding Erosion Testing Screen Shot 2017 09 08 at 15

Temperature-time-areas (TTA) as measure of thermal stress in arc test carried out at DC and AC.
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Resources and time will be necessary to finalize this work. Nevertheless, if we compare this to the time it took to arrive at a common standard for measuring length then we seem to be on track. The so-called meter convention was founded in 1875 by 12 charter members. New Zealand became a member in 1991 and even now not all countries in the world are associated. Taking this into account, we can perhaps conclude that suppliers in the T&D industry are doing a rather creditable job.

Jens Lambrecht