Resistance or capacitance

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resistance or capacitance

resistance or capacitance

I really disagree that there is no capacitance. If i take example of a transformer where the secondary terminals are not connected to any cable or load. There exists capacitance.....i.e capacitance between phases, capacitance between phase and tank of ground and the insulation media here is filled oil.Correct?
When the two bus bar are connected to ground through insulators, in this case we can assume two insulators in series connected between two phases.Am i right or no?
 
There may be capacitance (and I'm sure there is in the system, lots of them), but what ptonsparky was saying is the insulator is not a capacitor. It might be part of one.

Saying the insulator has a capacitance value is like looking at a plate capacitor and talking about the capacitance of one of the pieces of glass in it.

Resistance is not the same as permittivity. One resists current flow, the other resists the forming of electric fields. Decreasing resistance is not the same as moving the plates of the capacitors closer together.

Look here:
http://www.allaboutcircuits.com/vol_1/chpt_13/3.html
 
110306-1601 EST

panthripu:

You have indicated you are an engineer. Somewhere in the classes you have taken there should have been presented some basic discussion of the basic electrical elements --- resistance, capacitance, and inductance.

A basic capacitor is composed of two separated conductive elements (often times called plates). The media between these conductive elements relative to capacitance has a characteristic called dielectric constant. This is a number that relates to the media compared to air as a reference. So if you had a capacitor with air as the dielectric and in place of the air substituted Mylar, and made no other changes to the capacitor, then the capacitance of the capacitor would increase by an amount determined by the dielectric constant of Mylar.

A real world capacitor may and does have other characteristics that are represented by inductance and resistance in an equivalent circuit. There are no real world capacitors that only have the theoretical characteristics of a capacitor.

Some capacitors are more stable than others, and more closely approximate an ideal capacitor than others. Polystyrene and mica capacitors are quite close to ideal in certain frequency ranges. Electrolytic capacitors are mostly not very ideal, but still are very useful.

The measurements you are making are probably not very useful unless you note some very large disparity from one time to another.

Suppose you have a ceramic insulator. At 60 Hz and room temperature this will have some value of capacitance and shunt resistance. The capacitance won't be large, and the shunt resistance (leakage resistance) should be very large. Surface contamination should have negligible effect on capacitance, but possibly a very large effect on shunt resistance.

I believe your real concern is how can you do PM checks with the system in operation. How to predict potential failure? I do not think that in general in your field of interest you will have a failure relating to the capacitance of a component. Rather somehow there is a failure of the dielectric media or some sort of surface contamination. Dielectrics fail as a result of high electric field intensity. In an air capacitor when you reach a certain voltage the air breaks down and there is an arc. Remove the current and the arc is extinguished. Reapply a lower voltage and the capacitor dielectric has repaired itself (became deionized), and it is as good as before the breakdown if the arc did not damage the plates. Most other insulators are not self healing. High voltage over the surface or sharp points may cause radio noise that would be a useful detector.

I think your real question needs to be what are the best techniques to use to predict potential failure.

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resistance or capacitance

resistance or capacitance

Well , I really appriciate you .
My concern as i mentioned earlier , I have a system with a transformer having secondary winding connected in delta configuration.There is no doubt that the voltage between phase to ground if check will measure any value as it is floating.And ground capacitance , either due to design or transformer or routing of cables or busbars affects the ground capacitance.
My transformer has busbar on seconday side which is supported on the wall(ground) with mica insulators.
The voltage between phase to ground is checked after everymonth although it is meaning less as i believe. And results shows that one of the phase voltage is decreasing slowly.
Should i believe that some insulator is deterioting or i should confirm by megger (IR values).
 
panthripu said:
Well , I really appriciate you .
My concern as i mentioned earlier , I have a system with a transformer having secondary winding connected in delta configuration.There is no doubt that the voltage between phase to ground if check will measure any value as it is floating.And ground capacitance , either due to design or transformer or routing of cables or busbars affects the ground capacitance.
My transformer has busbar on seconday side which is supported on the wall(ground) with mica insulators.
The voltage between phase to ground is checked after everymonth although it is meaning less as i believe. And results shows that one of the phase voltage is decreasing slowly.
Should i believe that some insulator is deterioting or i should confirm by megger (IR values).
Click to expand...
If you believe what we have been telling you:
mivey said:
You can't make that conclusion. The megger test compared with previous readings over time will tell you what is going on with the bus insulation.
Click to expand...
 
resistance or capacitance

resistance or capacitance

Thanks guys
It was really informative discussion for me.Doubts are nearly clear.See you again.
 
ptonsparky said:
Forget the capacitance value of the insulator. Erase it from your mind. There is none.

Add conductive material to both sides, then you have a capacitor.
Click to expand...
Another way to look at it is that every capacitance has a geometric component. If you don't know the area and thickness of the insulator in question, then you don't know the capacitance. Like my dad always used to say in response to questions like this, "how long is a piece of string?"
 
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