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Working Group Reports

Retention of hydrophobicity under DC voltage stress Evaluation with the dynamic drop test



Polymeric high voltage insulators are applied in all voltage levels under AC as well as under DC voltage stress.  They are considered reliable and in some aspects even superior to ceramic insulators and correspondingly their share in the insulator market is considerable. Housing materials for outdoor insulation are predominantly silicone rubbers (SiR), ethylene propylene diene monomer (EPDM) and ethylene-vinyl acetate rubbers (EVA). For indoor insulation cycloaliphatic epoxies are commonly used [1]. The applied materials have to provide minimum requirements in respect to all relevant properties, which should be verified by suitable test methods. Based on the work of CIGRE WG D1.14 the technical report IEC/TR 62039 was developed, which defines twelve important physical properties for polymeric materials in outdoor insulation [2]. Up to now minimum requirements and test methods for ten of these properties exist. Still missing are test methods and evaluation criteria for the resistance to corona and ozone and for the dynamic properties of hydrophobicity [2], [3]. The distinct hydrophobic properties of some polymeric materials are essential for their ability to prevent the onset of pollution initiated leakage currents and surface discharges. In this aspect the three dynamic properties of hydrophobicity (retention, transfer and recovery) are key factors for the insulations long term performance and especially their performance under heavily polluted environmental conditions [4]. Distinct hydrophobic properties improve equipment reliability and long term durability. In case of a reliable and long lasting hydrophobicity economic benefits can be acquired by a possible reduction of creepage distance [5]. Possible test methods for evaluating the retention of hydrophobicity are the salt fog test [6] or the Modified Rotating Wheel Dip Test [7], [8]. Obtaining valid results requires in both cases cylindrical test specimens, which are difficult to manufacture, especially with defined surface structure. But even with adequate specimens at hand the randomness of the occurring stress will result in insufficient reproducibility and high scatter. This motivated the development of the Dynamic Drop Test (DDT) after [9], which aims to control precisely the location and magnitude of the pollution initiated surface discharges. One aim of CIGRE WG D1.27 was to contribute to the advancement of this promising but not yet matured test method. Numerous relevant publications have their share in identifying relevant
influences like the composition of the base polymer [10], the filler content [7] or the reorientation and diffusion of low molar mass siloxanes [11]. This article summarizes the contribution of CIGRE WG D1.27 concerning the retention of hydrophobicity. The main aspects were the evaluation of a DC type DDT and the influence of the insulating materials surface structure.

File Size: 194,1 KB

Pages NB: 4

Study Committee : D1 ?

WG (TF): WG D1.27

Year: 2014

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