Insulation Resistance Test and Polarization Index Test

 

Insulation Resistance Test and Polarization Index Test:

Both Insulation Resistance Test (IR Value Test) and Polarization Index Test (PI Value Test) are conducted on HV machine to determine service condition of the insulation. In HV machines and winding are likely likely to e a!ected y moisture and contamination. IP test is conducted s"ecially to determine the dryness and cleanliness of winding insulation.

In insulation resistance resistance test#  test# a high $% voltage voltage is  is a""lied across# conductor and ground more s"eci&cally. The voltage voltage   is a""lied across the insulator. insulator. $ue to this a""lied high $% voltage there voltage  there will e a current current   through the electrical insulator.. By dividing the a""lied voltage insulator voltage y  y this current current   we get the actual resistive value of the insulator. insulator.

Say th Say thee app appli lied ed hi high gh voltage voltage   is V and corresponding current current   through the insulator is I. Hence as per Ohm’s law  law  the value of insulation resistance resistance is  is [math] R!"!frac#V$#I$[%math]   &his test is generally done 'y means of megger . (egge (e ggerr gi give vess re re)u )uir ired ed di dire rect ct *d *dc+ c+ voltage voltage   across the insulator an it also shows the resistive value of insulator directly in ( ohm range .&he megger  are generally of ,-- V /., 0V an , 0V. ,-- V megger  are ,-  are used for insulation test upto 1.1 0V rated insulation. 2or  high voltage voltage   transformer other HV e)uipment and machines /., or , 0V megger  are  are used. 3s al alll in insu sula lato tors rs that aree di ar diel elec ectr tric icapplication in na natu ture re of they the y have ha ve  ac alwa al ways ys the a ca capa paci citi tive ve  property. 4ue to during voltage  voltage acro ross ss the electrical

 

insulator  initially there will 'e a charging current. 5ut after some time when the insulator is totally charged the capacitive charging current current   'ecomes 6ero and then only resistive conductive current current   presen presents ts in the the insulator insulator.. &hat is is why it is always recommended to do insulation resistance resistance test  test at least for 1 minut mi nutee as it is proved proved th that at ch char argi ging ng current current   tot totall ally y 'ecome 'ecomess 6ero 6ero afte afterr 1 minute. Only measuring insulation resistance resistance 'y  'y megger  for  for 1 minute does not always give relia'le result. 3s the resistive value of an electrical insulator  also  also varies with temperature. &his difficulty is partially solved 'y introducing polarity inde7 test or in short 8I value test. &he philosophy 'ehind 8I test is discussed 'elow. 3ctually when a voltage voltage is  is applied across an insulator there will 'e a lea9age current   from line to ground. current 3lthough this lea9ag 3lthough lea9agee current current   is very small small is in milli milli ampere ampere or sometimes sometimes in micro ampere range 'ut it has mainly four components. 1+ :apacitive component. /+ Resistive or conductive component. ;+ Surface lea9age component.