High Voltage Insulation Diagnostics


Glossary - HV Insulation Diagnostics PI - Polarization Index


Step voltage


As a voltage is applied to an insulat- ing material, the particles inside the materials line up and become polar- ized. The more atoms that become polarizes, the resistance of the mate- rial increases. In a insulating materi- al, the quicker the particles polarize, the better the insulating material is. PI is defi ned in the IEEE Std 43-2000 as the ratio between two insulation resistances readings, during the test, the fi rst is taken after 1 minute and the second is taken after 10min- utes of continuous measurement.


DAR - Dielectric absorption ratio The principle behind an insulating material is to keep two conductive materials separated. If an insulat- ing material becomes contaminated (e.g. through dirt, grease etc) leak- age current can be absorbed by the insulating material. This leakage cur- rent reduces the resistance of the material which results in less power can be delivered to the load. The Dielectric Absorption Ratio (DAR) is defi ned as the between two insula- tion resistances measured after 30 seconds and after 60 seconds (1min) of continuous measurement


IR - Insulation resistance of insula- tor, IEV 151-15-43: resistance under specifi ed conditions between two conductive elements, separated by insulating materials.


DD - Dielectric discharge test After the power is turned off to a high voltage appliance, the built up charge needs time to dissipate away before the appliance is safe. Typi- cally the insulation material is left connected to the test voltage for 10 - 30 min and then discharged before DD test is carried out. After 1 minute, the capacitance, voltage and discharge current is measured and the DD can be calculated.


Step voltage test involves insulation re-sistance measurement at differ- ent test voltages. Good insulation will show very little deviation be- tween particular read-ings. In case of microcracks, dirt or hu-midity in insulation system the insulation re- sistance decreases with rising test voltage. When performing a step volt-age test, it is important to start with the lowest test voltage and then move to a higher voltage level. Typical test duration is 60 seconds.


Withstanding voltage test This function offers Withstanding Voltage test of insulation material. It covers two types of tests: a) Breakdown voltage testing of high voltage device, e.g. transient sup- pressors and


b) DC withstanding voltage test for insulation coordination purposes. Both functions require breakdown current detection. The test voltage increases step by step from the Start up to the Stop value over a prede- fi ned time and it is kept at the Stop value for a predefi ned test time


Guard terminal


The purpose of the GUARD terminal is to lead away potential leakage cur- rents (e.g. surface currents), which are not a result of the measured insulation mate-rial itself but of the surface contamina-tion and mois- ture. Guard terminal is to be used in conjunction with shielded test leads when measuring insulation re-sist- ances in GΩ and TΩ range..


R(t) graphs


Sometimes a simple value does not show the full picture about the quality of an insulating material, R(t) graphs illustrates how resistance (on the Y or vertical axis) changes in rela- tion to time (on the X or horizontal axis).


CAT IV 600 V


Safety category


Good insulation


Bad insulation Time Leakage


Leakage current is an AC or DC cur- rent that fl ows trough or on the insu- lation’s surface.


R 3 3 years A


manufactures waranty


10 kV Test voltage 10 kV


5 kV Test voltage 5 kV


R(t)


R(t) graph insulation resistance


PI / DD/ DAR


Diagnostic test


Downloadable


3. 2


Accessories: page 3.12


W


A


N


R


R


A


E


T


Y


S


Y


Resistance


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