Q. What is the need of doing this test?
resistance ∝ 1 / temperature
A. We all know the winding in transformer (either primary or secondary) should be isolated from the ground and primary and secondary winding should be connected magnetically but not electrically (not in case of auto transformer). so transformer should maintain some insulation from ground and other winding.
- while doing the test, specify the temperature
resistance ∝ 1 / temperature
procedure
primary winding to earth,apply the megger voltage(connect the +ve lead to primary and -ve lead to the ground) and the note down the insulation resistance value which was appeared in the megger (primary winding should be isolated electrically from the earth and bus bar connected or bus duct connected and any other).
secondary winding to earth,apply the megger voltage(connect the +ve lead to primary and -ve lead to the ground) and the note down the insulation resistance value which was appeared in the megger (secondary winding should be isolated electrically from the earth and bus bar connected or bus duct connected and any other).
primary to
secondary winding ,apply the megger voltage(connect the +ve lead to primary and -ve lead to secondary) and the note down the insulation resistance value which was appeared in the megger (primary and secondary winding should be isolated electrically from the earth and bus bar connected or bus duct connected and any other).
dielectric absorption coefficient : It is the ratio of megger value for 1 minute to 15 seconds. The value should be greater than 1.25
polarization index value:It is the ratio of megger value for 10 minute to 1 minute.The value should be greater than 1.3
Q. What is dielectric absorption coefficient?
A. When we apply megger voltage first one minute is taken for settling capacitance in machine,so we are finding rate of settling of capacitance.this is due to pole arise in dielectric medium.
Q. What is polarization index value?
A. The polarization index is a ratio of the Insulation resistance at the end of a 10 min test to that at the end of a 1 min test at a constant voltage.
The total current that is developed when applying a steady state dc voltage is composed of three components:
Since leakage current increases at a faster rate with moisture present than does absorption current, the megohm
readings will not increase with time as fast with insulation in poor condition as with insulation in good condition.
This results in a lower polarization index.
An advantage of the index ratio is that all of the variables that can affect a single mega ohm reading, such as temperature and humidity, are essentially the same for both the 1 min and 10min readings.
The following are guidelines for evaluating transformer insulation using polarization index values:
Less than 1 Dangerous
1.0 - 1.1 Poor
1.1 - 1.25 Questionable
1.25 - 2.0 Fair
Above 2.0 Good
Q. What is dielectric absorption coefficient?
A. When we apply megger voltage first one minute is taken for settling capacitance in machine,so we are finding rate of settling of capacitance.this is due to pole arise in dielectric medium.
Q. What is polarization index value?
A. The polarization index is a ratio of the Insulation resistance at the end of a 10 min test to that at the end of a 1 min test at a constant voltage.
The total current that is developed when applying a steady state dc voltage is composed of three components:
- Charging current, due to the capacitance of the insulation being measured. This current falls off from maximum to zero very rapidly.
- Absorption current, due to molecular charge shifting in the insulation. This transient current decays to zero more slowly.
- Leakage current, which is the true conduction current of the insulation. The leakage current varies with the test voltage. It may also have a component due to the surface leakage that is due especially to surface contamination.
Since leakage current increases at a faster rate with moisture present than does absorption current, the megohm
readings will not increase with time as fast with insulation in poor condition as with insulation in good condition.
This results in a lower polarization index.
An advantage of the index ratio is that all of the variables that can affect a single mega ohm reading, such as temperature and humidity, are essentially the same for both the 1 min and 10min readings.
The following are guidelines for evaluating transformer insulation using polarization index values:
Less than 1 Dangerous
1.0 - 1.1 Poor
1.1 - 1.25 Questionable
1.25 - 2.0 Fair
Above 2.0 Good
how can we justify the result? i mean u have not mentioned how can we say that there is a fault or it is normal, based on the test.
ReplyDeletewe cannot fix the certain value for all transformers, depending upon transformer ratings it will changes,manufacture will do the all tests at factory lab,and he know the how much value is reasonable,WE JUST COMPARE OUR VALUES AND FACTORY VALUES AND DETERMINE THE CONDITION OF MACHINE.
ReplyDeletehow we get the polarization index value?
ReplyDeleteapply megger for 10 min and take the value in megger at 10th minute,similarly apply megger for one minute and take the reading at end of minute,now divide first value by second value.
ReplyDelete