REALISATION OF INSULATION REISTANCE AND POLARISATION INDEX
( A TOOL FOR PREDICTIVE MAINTENACE & ANALYSES )
Insulation resistance measurement has been a common test and procedure adapted by service personnel for determination of condition of electric machineries, cables, breakers, switch gears etc and has been recommended and used for more than half a century to evaluate the condition of electrical insulation. The process being so common that its effectiveness is quiet often blurred by its simplicity? But being properly utilized it can be effectively utilized to improvement equipment insulation condition and thus overall life of the equipment. Simple individual insulation resistance measurements may be of questionable value, but the carefully maintained record of periodic measurements, accumulated over time, is of unquestioned value as a measure of critical aspects of the condition of the electrical insulation. The insulation measurement is a standardized procedure and should be evaluated on basis of those laid out procedures for effective analyses, here we would review the basic standards utilized in Insulation measurement, rectify many mistakes that are made during insulation measurement and understands it results.
Now the basic question is what insulation is. In common terms anything that resists the flow of current through and keeps the current to its path can be classified as a insulation. Insulation resistance is the capability of the electrical insulation of a winding to resist direct current. The quotient of applied direct voltage of negative polarity divided by current across machine insulation, corrected to 40 °C, and taken at a specified time (t) from start of voltage application. The voltage application time is usually 1 min (IR1) or 10 min (IR10).
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Its purpose is to evaluate the condition of the insulation between conductors and ground. By applying a direct voltage between the conductors (windings) and the casing of the electric motor (machine) and measuring current leakage across the insulation system the IR can be easily determined. The readings are applied to Ohm s law (R = V/I) and an resistance is provided.
Electrically Insulation can be modeled as a capacitor in parallel with a resistor as shown in Figure 1. The current thus flowing will comprise of two components: the capacitive current (Ic) and the resistive current (Ir).Whenever Dc current flows another component comes into effect that is the dielectric absorption current Ida which can be modeled by another parallel capacitor.
It
Ic Ia Ig
E
Capacitive Current
The capacitive current is the current due to the sudden charging of the capacitance
in the system. It is a reversible current of comparatively high magnitude and short duration, which decays exponentially with time of voltage application, and which depends on the internal resistance of the measuring instrument and the geometric capacitance of the winding.
It normally stops flowing after some time when it completely decays to a minimum value.
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Dielectric absorption (polarization) current (Ia): A current resulting from molecular polarizing and electron drift, which decays with time of voltage application at a decreasing rate from a comparatively high initial value to nearly zero, and depends on the type and condition of the bonding material used in the insulation system. The applied insulation voltage puts a
stress on the molecules of the insulation.The positive side of the molecules are attracted to the negative conductor and the negative side of the molecules are attracted to the positive conductor.
.Like Ic, Ia usually dies off fairly quickly as the molecules realign to their maximum extent.
Conduction current (Ig): A current that is constant in time, that passes through the bulk insulation from the grounded surface to the high-voltage conductor, and that depends on the type of bonding material used in the insulation system.
