IEC TS 61956:1999 pdf download – Methods of test for the evaluation of water treeing in insulating materials

IEC TS 61956:1999 pdf download – Methods of test for the evaluation of water treeing in insulating materials.
This technical specification describes methods of test for the evaluation of water treeing in polyethylene (PE) and crosslinked polyethylene (XLPE) compounds, as a means to assess their relative performance under alternating (ac.) electric stress, in the presence of water. Two methods are described, method I covering the evaluation of insulating materials alone, and method II covering the evaluation of insulating sandwiches consisting of an insulating material in intimate contact with semiconducting screens.
1.2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this technical specification. For dated references, subsequent amendments to. or revisions of, any of these publications do not apply. However, parties to agreements based on this technical specification are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below, For undated references, the latest edition of the normative document referred to applies. Members of IEC and ISO maintain registers of currently valid International Standards.
IEC 60243-1 :1998. Electrical strength of insulating materials — Test methods — Part 1: Tests at power frequencies
IEC 61072:1991 Methods of test for evaJuating the resistance of insulating materials against the initiation of electrical trees
1.3 DefinitIons
Water treeing is a degradation process (11) as observed in low density polyethylene (LDPE) and XLPE under a.c. stress and moist conditions, which results In the formation of dielectrically weakened areas of so-called water trees.
Water trees are hydrophilic dendritic, tree-like features (specifically, they appear initially to be chains of water.fllled cavities which later become bushes of microscopic channels with hydrophilic surfaces), which grow typically under wet and electrical operating conditions and may reach lengths of the order of 1 mm within several years. Two different types of water trees can be distinguished:
a) Bow tie tree resembles a bow tie and consists of divergent straight branches radiating in opposite directions from a central point. The bow tie trees, contained within the bulk Of the insulation, are generally lined up in the direction of the electric field.
b) Vented tree resembles a tree whose trunk is vented to the surface of the insulation or the insulation/screen Interlace. The branches are generally oriented in the direction of the electric field, away from the insulation surface or the interface.
2Test method l (plaque test)
2.1 Principle
The plaque tests (variant A and variant B) are intended to assess the development of watertrees in low density polyethylene (LDPE) and crosslinked polyethylene (XLPE) based insulatingmaterials.
Both test variants make use of plaque-shaped test specimens and of the same test cell. Bothtests are screening tests to differentiate and to preselect insulating compounds with regard towater treeing.
The test with stress concentrations (variant A) is primarily intended to assess the developmentof vented trees from the tips of protrusions of the screen or intrusions in the insulation at theinsulation/screen interface, simulated by needle-shaped and water-filled indentations. A plaquewith a number of identical indentations is exposed simultaneously to water and to an electricfield,producing electric field stress concentration at those points， within a test cell accordingto 2.3.1.
By means of this test, bow tie treeing can be additionally assessed within the bulk of the testspecimen, away from the stress concentration areas.
The test with uniform field stress (variant B) is intended to assess the development of bow tietrees, when a plaque is exposed simultaneously to water and to a uniform electric field within atest cell described in 2.3.1.
2.2Test specimen
Test specimens in the form of disc-shaped slabs with a diameter of (35 ± 1) mmcan be achieved by punching them out from plaques of one of the following thicknesses;(4,0±0,1) mm; (3,0 ± 0,1) mm; (2,0±0,1) mm.Comparison between materials should bemade at equal thicknesses only.
For vented tree testing, plaques may be manufactured from pellets by pressmolding. For bowtie tree testing, it is recommended to homogenize the compound by extrusion in order to avoidconcentration of additives and impurities on the surface of the pellets. In every case extremecare should be taken to prevent contamination of the materials as well as of the resultingplaques and slabs.