Resistance or capacitance

[panthripu]

Hi
I would like the experts to help me understand the phenomenon of resistance and capacitance in an insulator. I asked the question in another forum but no body replied to my satisfaction.I found some other threads asking same question but no reply.
There are some insulators which are connected to a bus bar to ground.This bus bar is fed through a transformer with delta winding on the secodnary side. When the IR value of the bus bar is check with megger , it shows very high resistance( showing that the insulators are healthy).

When we check the open circuit voltage of the transformer each phase with respect to ground , the voltage shows unbalance ( as floating). As people say it depends on the ground capacitance.
what is the relation between resistance and ground capacitace of the insulators?

 

[kwired]

Is any point on this transformer secondary grounded? If so your voltage readings will be more predictable. If not then your voltage readings will depend on the capacitance values that exist.

A made capacitor is two conductors separated by an insulator. An insulated conductor laying on another conductive surface also creates a capacitor - one conductor is inside the insulation one is outside.The amount of capacitance will depend on conditions at the location.

With a DC source the capacitor will charge to applied voltage and then no more current flows if no other changes are introduced.

With an AC source the capacitor charges as voltage increases in one direction, and then discharges and charges in the opposite polarity when the AC cycle is in the other direction. Reactive current is always there as long as the AC voltage is applied.

This capacitive coupling exists almost everywhere but is usually at low levels. A load of pretty high resistance is typically enough to drop the voltage to virtually nothing.

 

[G._S._Ohm]

what is the relation between resistance and ground capacitace of the insulators?

The insulator can be modeled as an R in parallel with a C and so the impedance Z is 1/[(1/R + 1/(-jXc)] and Xc = 1/wC and w = 2 x PI x f which is 377 @ 60 Hz.

Do you have values for R and C? I'd say 10^9 ohms and 5 pF but I'm just guessing.

 

[mivey]

You guys did great. Panthripu, what forum did you go to that could not provide similar answers? Did the previous two posts get you what you needed?

If not, don't give up. Stay here and you will be sure to get more info than you ever wanted as we have a large number of members who are quite knowledgeable. The more you ask, the more they will tell you.

 

[panthripu]

resistance and capacitance

resistance and capacitance

Thanks guys...
As i told earlier , our transformer with delta secondary gives us open circuit voltage with respect to ground , ph-1==220v , ph-2==230v and ph-3==100v only.Secondary winding is nowhere grounded.However phase to phase voltages are balanced.The bus bar of all three phases is supported with insulators.
Some people says it is because of some insulator in phase 3 is week causing this problem. The IR value when check shows insulation values in gega ohms.Are they talking about capacitance? If yes, then over a period of time if i find the voltage in phase-3 gets lower , let us say 80 v , does it show weekening of the insulators?If answer is again yes, what is happening to capacitance ? Considering that IR values still are very high.

 

[panthripu]

resistance and capacitance

resistance and capacitance

Some people say when we talk about a transformer with delta connected winding on secondary side and no ground point on secondary winding. It shows floating voltage only and can show any value depending on system ground capacitance(topic of confusion).So we should never measure it. As long as phase to phase voltage is good , everything is ok
Other people says that if the difference is very big as i mentioned then there is some problem with transformer.

One group says some insulator is week in system although IR values are very high.

The manufacturer says nothing to worry about it depends on the internal configuration/desing of the transformer.

 

[mivey]

Thanks guys...
As i told earlier , our transformer with delta secondary gives us open circuit voltage with respect to ground , ph-1==220v , ph-2==230v and ph-3==100v only.Secondary winding is nowhere grounded.However phase to phase voltages are balanced.The bus bar of all three phases is supported with insulators.
Some people says it is because of some insulator in phase 3 is week causing this problem. The IR value when check shows insulation values in gega ohms.Are they talking about capacitance?

The ohms is resistance, not capacitance.

If yes, then over a period of time if i find the voltage in phase-3 gets lower , let us say 80 v , does it show weekening of the insulators?If answer is again yes, what is happening to capacitance ? Considering that IR values still are very high.

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. Their are other factors involved with the voltage and it is probably not a dependable reading to be used for most things.

 

[mivey]

Some people say when we talk about a transformer with delta connected winding on secondary side and no ground point on secondary winding. It shows floating voltage only and can show any value depending on system ground capacitance(topic of confusion).So we should never measure it. As long as phase to phase voltage is good , everything is ok
Other people says that if the difference is very big as i mentioned then there is some problem with transformer.

One group says some insulator is week in system although IR values are very high.

The manufacturer says nothing to worry about it depends on the internal configuration/desing of the transformer.

Measuring the voltage to ground through a high impedance is not very useful. You could also have a high impedance fault to ground in some piece of equipment. You could have a variance in capacitance due to cable routing. It could be due to some other loads. Who knows?

Something that is useful is to put in a ground fault detection system. That way you can tell if a phase has faulted to ground.

 

[mivey]

Some people say when we talk about a transformer with delta connected winding on secondary side and no ground point on secondary winding...

A transformer is not perfectly uniform inside the tank and neither are the electromagnetic fields, temperatures, etc and these may even shift around during operation.

 

[G._S._Ohm]

Check how much current is being drawn from the low phase.

 

[Besoeker]

Some people say when we talk about a transformer with delta connected winding on secondary side and no ground point on secondary winding. It shows floating voltage only and can show any value depending on system ground capacitance(topic of confusion).So we should never measure it. As long as phase to phase voltage is good , everything is ok
Other people says that if the difference is very big as i mentioned then there is some problem with transformer.

One group says some insulator is week in system although IR values are very high.

The manufacturer says nothing to worry about it depends on the internal configuration/desing of the transformer.

If no part of the delta secondary has a ground referenced connection then measurement to ground, particularly with a high impedance meter, won't provide any meaningful information.

 

[panthripu]

resistance or capacitance

resistance or capacitance

Right. But the concern is not about the ground faults as with ground fault , the voltage of the grounded phase will go down to zero and other two phase increase by 1.732 times and can be seen. But is it right to check healthy ness of the insulators based on the phase to ground voltages ?
Today if i measure the voltage to be 100 volts and after a month it is measured 80v.Should all insulators be checked or changed even when the megger value is very high considering that this drop is voltage is due to change in capacitance ?
Has anybody heard of failure of insulator because of capacitance ?

I dont know whether i am asking the question in correct way or not ?
Now if capacitance is about the distance between the condutors then the insulator which is fixed between a phase and ground has capacitance. If this insulator is moisted or have dust on it , what does change ? resistance or capacitance?

 

[ptonsparky]

The capacitance will change because you have changed the insulation thickness, or spacing, between the conductors of the capacitor. The reistance will change by the type of dust. Coal? Iron? Wood? Where is the moisture comming from? Is the contamination equal at all locations? How are the meter leads connected? Do they have exactly the same spacing and arrangemnt? Good batteries? Has it been calibrated? After a while technical accuraccy is a waste of time.

 

[Besoeker]

Right. But the concern is not about the ground faults as with ground fault , the voltage of the grounded phase will go down to zero and other two phase increase by 1.732 times and can be seen. But is it right to check healthy ness of the insulators based on the phase to ground voltages ?
Today if i measure the voltage to be 100 volts and after a month it is measured 80v.Should all insulators be checked or changed even when the megger value is very high considering that this drop is voltage is due to change in capacitance ?

If the system has no reference to ground, then measurements of voltage to ground are measurements of an indeterminate nature. A change in voltage measured at one point in time to from another measurement at a different point in time could be almost anything connected to the system being in a different state at each of the two points in time. Or the measuring technique. Or even someone else making a conductor to ground measurement with a low impedance meter elsewhere on the system.

 

[panthripu]

resistance or capacitance

resistance or capacitance

I agree that the insulation property of an insulator decreases over a period of time , it may be a long period. When i say insulation property , it means the IR value with a megger will be lower than previous measurement.
Is it possible that IR value of the insulator is very good and it failed. And later on someone comes and say that it failed because of leakege current as the capacitance of the insulator changed.

 

[mivey]

I agree that the insulation property of an insulator decreases over a period of time , it may be a long period. When i say insulation property , it means the IR value with a megger will be lower than previous measurement.
Is it possible that IR value of the insulator is very good and it failed. And later on someone comes and say that it failed because of leakege current as the capacitance of the insulator changed.

No. Well, I guess anybody can say anything, but they shouldn't.

You are not going to be able to get any useful insulation data out of these high impedance voltage measurements.

 

[panthripu]

resistance or capacitance

resistance or capacitance

Now things are getting clear.....
What will be the effect on the insulator if the capacitance will become very very high or very very less.(or in other words if capacitive reactance becomes very very less or very very high ?
and
What can change the capacitance or capacitive reactance of a insulator ?

 

[ptonsparky]

Now things are getting clear.....
What will be the effect on the insulator if the capacitance will become very very high or very very less.(or in other words if capacitive reactance becomes very very less or very very high ?
and
What can change the capacitance or capacitive reactance of a insulator ?

How are you getting capacitance of an insulator?

Go back to post 2 for the construction of a capacitor. A capacitor requires three items. One of which is an insulator. The insulator could be air. Varnish. Plastic. Glass. etc.. The insulator by itself has no capacitance value.

 

[mivey]

Now things are getting clear.....
What will be the effect on the insulator if the capacitance will become very very high or very very less?

If you can isolate the component you are interested in, apply a known excitation, and make some measurements to a known reference, you can find out something useful.

Your problem is that you have a circuit with any number of parallel & series resistances and reactances. You now want to look at the circuit's reactance change to determine what is going on with a particular component in the circuit. You are trying to do this with an unknown source impedance, and unknown reference point, and unknown parallel and series impedances that all may be changing as you progress.

Any capacitive reactance you measure has to be measured with known inputs and reference points and you do not have this. You have some amorphous floating value that you can't use to draw any conclusions.

As ptonsparky said, the insulator is not a capacitor by design. It may exhibit some small capacitive qualities but I suspect the capacitance you are seeing is from the bus bars themselves, not the insulators that separate them.

But suppose one of the insulators started to break down and began to conduct current. It would start acting like a measurable resistor in parallel with the capacitance. Your problem is that you do not have a controlled setting in which to measure the capacitive reactance delta because there are too many other parasitic elements involved.

The easiest way to make a measurement and reduce the effect of the parasitic elements is to not measure the capacitance but to measure the resistor in parallel with it. That is what the meggar test will do. To identify which part of the circuit has actually had a decrease in resistance will take a little more work.

 

[panthripu]

resistance or capacitance

resistance or capacitance

Thanks guys
So what i can say now , its really not a good idea to monitor the open circuit phase voltage (voltage of each phase with respect to ground) in a transformer with delta winding to see the healthiness of the insulation value of the system.
2)Capacitance value of an insulator does not change untill either thickness of the insulator changes .By dust or moisture the resistance value changes.
3)With aging of the insulator also the resistance of the insulator changes ,however there will be negligible effect on its insulator.