Insulation Resistance Test and Polarization Index

Megger is an electrical device which is used to measure the Insulation Resistance by performing Insulation Resistance Test (IR Test). Basically Megger injects High DC Voltage across the Insulator and ground due to which leakage current flows through the Insulator to the ground. By measuring this Leakage Current, Megger calculates the Insulation Resistance.

Suppose the DC Voltage applied by the Megger = V

And Leakage Current through the Insulator = I

Therefore from Ohm’s Law,

Insulation Resistance = V/I ohm

A typical Megger is shown in figure below.

Meggers are generally of Rating 500 V, 2.5 kV and 5 kV. In modern Megger as shown in figure above these range can be selected. 500 V is used to measure the Insulation resistance of control cable for Insulator which can withstand up to 1.1 kV voltage. For High Voltage Transformer / High Voltage Machine or Equipment, 5 kV range is selected to perform Insulation Resistance Test. Megger has the provision to directly read the Insulation Resistance during Insulation Resistance Test.

All Insulators are supposed to be pure capacitor having very few capacitance so as to take minimum charging current. When an Insulator is connected across a DC voltage it takes leakage current. Leakage current can be divided into following types:

·         Capacitive Charging Current

·         Resistive or Conductive Current

·         Surface Leakage Current

·         Polarization Current.

Capacitive Charging Current:

When a DC voltage is applied across in insulator, because of its dielectric nature there will be an initial high charging current through the insulator from line to ground. Although this current decays exponentially and becomes zero. Generally this current exists for initial 10 seconds of the test. But it takes nearly 60 seconds to decay totally. That is why it is always recommended to do Megger or Insulation Resistance Test at least for 1 minute as it is proved that charging current totally becomes zero after 1 minute. Thus after 1 minute leakage current measured by Megger won’t include the charging current due to Capacitance of Insulator.

Resistive or Conductive Current:

This current is purely conductive in nature, flows through the insulator as if the insulator is purely resistive. This is direct flow of electrons. Every insulator should have this component of electric current. Resistive or Conductive leakage current through the Insulator will be more if moisture and contamination in Insulator is high.

The resistive or conductive component of insulator leakage current remains constant throughout the test.

Surface Leakage Current:

Due to dust, moisture and other contaminants on the surface of the insulator, there is one small component of leakage current through the outer surface of the insulator. Therefore before conducting Megger or Insulation Resistance Test, Insulator should be cleaned properly so as to eliminate the surface leakage component of Leakage Current.

Polarization Current:

Because of presence of impurities and moisture in Insulator, insulator becomes polar in nature. Therefore when we apply high DC Voltage across the Insulator, the polar molecules i.e. dipole try to align themselves in the direction of Electric Field. During the period the molecules try to align along the Electric Field, a current will flow through the Insulator because of movement of dipole along their axis. This current is called Polarization Current and it lasts for short time and as soon as the polar molecules align themselves along the Electric Field, there will be no further movement of dipole and hence polarization current will stop. It normally takes 10 minutes for Polarization Current to become Zero.

Thus if we take the Megger reading after 10 minute then Megger will not consider the Polarization Current for the calculation of Insulation Resistance.

So when we take Megger value of an insulator for 1 minute, the Insulation Resistance value will be free from the effect of capacitive component of leakage current. Again if we take Megger value of an insulator for 10 minutes, the Megger result shows the value, free from effects of both capacitive component and polarization component of leakage current.

Polarization Index (PI):

Polarization index is the ratio of Megger value taken for 10 minutes to the Megger value taken for 1 minute.

Therefore, PI = Megger value after 10 minutes / Megger value after 1 minute

Significance of PI Test:

Let I = Total initial current during Polarization Index Test or PI test.

I_C = Charging current due to Capacitance of Insulator.

I_R = Resistive or Conductive Current.

I_S = Surface leakage current.

I_P = Polarization Current of the Insulator.

Therefore,

I = IC + IR + IS + IP

The value of Megger or Insulation resistance Test / IR Test after 1 minute,

R_1minute = V/( I_R+ I_S + I_P)  …….I_C= 0 after 1 minute

Similarly, Megger Value after 10 minutes,

R_10minute = V/( I_R+ I_S) ………….I_P = 0 after 10 minutes

So, from Polarization Index Test or PI Test,

PI Value = R_10minute / R_1minute

               = (I_R+ I_S + I_P)/( I_R+ I_S)

               = [1+ I_P/( I_R+ I_S)]

So, PI Value = [1+ IP/( IR+ IS)]

From the above it is clear that, if the value of (I_R + I_S) >> I_P, the PI of insulator approaches to 1. Large value of I_R or I_S or both indicate unhealthiness of the insulation.

The value of PI becomes high if (I_R + IS) is very small compared to I_P. This equation indicates that high Polarization Index of an insulator implies healthiness of insulator. For good insulator resistive leakage current is very very small.

The value of Polarization Index of an insulator should be more than 2. If the value of Polarization Index is less than1.5 then it means Insulator is unhealthy and shall not be used.