I know although its a stupid question still i need some clarification.
An insulator between line and ground, When i check the line with megger, it gives us resistance (gega ohm). Should i say it is resistance of the insulator? Then what about the capacitance.

This question came to my mind after reading some threads here related to transformers.

When i use this insulator in a DC line, only resistance comes in to picture. Where as when i use it in AC line both resistance & capacitance comes in to picture (as impedance). Right ?

 

Parish... You are right! An insulator has both parameters.

Additionally, capacitance is more difficult to calculate because of the insulator's configuration!

Rregards, Phil Corso

 

In the transmission line case the capacitance to ground is only important when its ac impedance (at 60 Hz) is drastically smaller than the resistance to ground.

It is a little different in the case of transformers, but it still has to do with relative impedance magnitudes

 

Right, often in very complicated ways. You can have "leakage" in AC circuits from both. But the stray capacitance is generally fixed and ignored at power line frequencies. The resistance is important as an indicator of the insulation integrity.

Regards
cww

 

OK , till now it is clear to me.
With aging or dust or moisture the resistance of the insulator tends to decrease , what about the capacitance ?

If the insulation resistance is very high measured with megger , then should one care about the capacitance.

 

Capacitance is all about spacing and dielectric constant.
It shouldn't change much without mechanical movement.

Regards
cww

 

I agree , but as i mentioned earlier this question came to my mind when i was going through some thread here about transformer with delta connected winding on secondary side.

The no load voltage on secondary side with ground can show any value and purely depends on the ground capacitance.

Now when the secondary side of the transformer gives very high Insulations Resistance, it shows all the insulators connected to the bus on secondary side are in good condition. Right?

Then how the capacitance comes in to picture.
If the no load voltage changes from previously measured/noted values, does it show the weakening of some insulator?

 

No reply?

 

Parish... Your question is a good one. The simple answer is, yes! A decrease of IR is indicative of a potential problem. Furthermore, unbalanced phase-to-ground voltages are also problematic, especially if the variation differs greatly from test to test!

Do you have specific set of IR or voltage measurements you would like evaluated?

Regards, Phil Corso