Hi,

As I said, as per design in all three phases with respect to ground voltage should be 700v. But in R phase it is 450 and Y phase 750 and B phase 950v.

 

Paru, following is information that supports the conclusion that the insulators are the source of the severe unbalanced voltage-to-ground problem:

Using an unbalanced, Y-connected, capacitive-load, to represent leakage-current through the insulator, I was unable to reach the phase-to-ground voltages you cited. However, when a lossy-capacitor, that is, a resistance in series (not in parallel) with a capacitor, was modeled, then the values you measured were reached!

This suggests that conductive pollutant from your process has unevenly contaminated the insulators!

Note, that because the resistive component is in series with the capacitor, the Meggar doesnt work!

Reards,
Phil Corso ([email protected])

 

Hi Phil,

Finally I have decided to just watch as the system with me is very bulky. The insulators are plate type mica between live part and earth. There is no earthing to the insulator's other side.

So it is true that this unbalanced voltage is due to uneven capacitance between the phases and ground.

I am worried only that some day some of my insulators will get damaged due to this problem. I got a lot of answers from other people saying that this is a normal phenomenon in furnace transformers with delta secondary.

Still I will try to find out if there is any possibility why this capacitance is uneven.

One more question I would like you to clarify...
As you said, the the opposition for AC current to flow through insulator is the impedence. That is the resistance + capacitive reactance. Ok.
Now say if some how the capacitive reactance has become less, still resistance is in circuit. Which will save the insulator.

Am I right?

 

Responding to Paru’s 10-Jul-08 (12:40) comments:

1) I strongly disagree with those convinced your EAF unbalance is a "normal phenomenon!" While some unbalance is present in ungrounded delta-systems, the voltage excursion you’re experiencing is far from "normal!"

2) The displaced-neutral (earth in this case) resulting from the observed 450, 750, 950V voltage-to-ground measurements is well outside of theoretical neutral (~400V) of the delta system! In any ungrounded, delta connected power system, a displacement of much more than 5% is considered worrisome!

3) What would be learned from an exercise to manipulate resistance and capacitance representing the insulator pollution? The problem is still the high degree of unbalance! And No! You are not right!

4) What metal(s) are processed in the EAF?

5) Insulator pollution is a result of the dust emitted by the EAF process!

6) BTW, pollution is exacerbated when water is present!

7) When were the insulators last cleaned!

8) Have you compared the two EAF circuits for physical anomolies or differences in their installations? Perhaps, busbar or cable lengths? Separation of phases? Etc?

Regards, Phil

 

hi phil
just few days back i had also same thinking like you and i was worried a lot.

the furnace is running perfectly without any problem. the arc voltages are also balanced when the furnace is running. the main insulators were recently changed which are between conducting arm and supporting structure i.e ground. some more insulators are there to support the delta closure.

It is very difficult to check the insulator other than visually checking as the system is very bulky. all insulators are plate shaped. there are big water cooled cables.

i dont know how to get rid of this problem.