Help with testing partial? ground fault on floating secondary

[Ingenieur]

Math error
looks balanced

0.058
0.055
0.057

The 'good' transformer
0.060
0.056
0.057

The ph-ph tolerance is amazing <0.5% from avg and 0.6% low-hi
even more so considering a 4.16 kV primary and a tap changing xfmr with a high ratio
25:1 at 170 V
and 2 rods have the exact same current!

so not the wire
transfomer
rods
glass
can't be ground current since delta and windings >1G Ohm
what's left lol

 

[Ingenieur]

Can you calculate where between the two lower voltage electrodes the hypothetical fault in the vessel insulation might be from those vectors?

it can't be ground current
it's a delta-delta xfmr-load
and windings are >1M ohm which is uA's

that calculation would be next to impossible
tank geometry
rod geometry
the current flow in the glass is a gradiant field
modeling the glass a fluid conductor would be extremely difficult
not solidly bonded but almost like an earth reflection on an OH conductor

The vectors are phasors, basically simplified math for a rotating sinusoidal signal of fixed freq relationship
not a current field vector

waaaaay above my pay grade lol

 

[GoldDigger]

it can't be ground current
it's a delta-delta xfmr-load
and windings are >1M ohm which is uA's

OK. Then what could be shifting the center point? The difference in current between the electrodes coupled with a higher than normal impedance in one or more transformer windings?
See my last paragraph for why current could flow with no persistent ground fault measured.

One additional thought that occurred to me is that a turn to turn short in one of the delta windings could reduce the output voltage of that winding without necessarily causing enough primary overcurrent to trip.
If the OP measures the current on each secondary phase line and checks to see if he sees the same ratios on the primary phase lines it could rule out that kind of transformer problem. ]

A turn to turn short deep inside a winding would not necessarily show up on the megger test as a low impedance to ground. And the DC resistance of the deenergized winding would not necessarily be different enough to be an indication either. Only an AC impedance test of the deenergized transformer windings would be a conclusive indication.

But in the end it still comes down to the fact that the OP can pull a significant current from somewhere on the secondary side to ground so there has to be a much lower than capacitive impedance in there somewhere.
A single fault between electrodes and ground through the molten glass will not show up on a megger when cold nor will it cause any fault current during operation. It will just offset the center of the delta from ground. And then any second fault (deliberately adding a load to ground) will cause current to flow. Another comparative test would be to add a load (lamp, whatever) from anywhere on the secondary to earth ground. We would expect that no current will flow, even during smelting while the electrodes are hot.

 

[Ingenieur]

At this voltage and with these run capacitive coupling would be nil
from conductor or glass via earth or bonding to windings?

i think he is getting a high impendance coupling to ground making a 'lightly' corner grounded delta with no current flow

rod/phase via glass to vessel to earth via bonding netwrork

think delta and corner ground thru a resistor/impedance
only drug down to 76V not 0

I can't resolve the ph-N voltage unbalance and the balanced ph V
the only way is move a corner towards ground

 

[milldrone]

A single fault between electrodes and ground through the molten glass will not show up on a megger when cold nor will it cause any fault current during operation. It will just offset the center of the delta from ground. And then any second fault (deliberately adding a load to ground) will cause current to flow. Another comparative test would be to add a load (lamp, whatever) from anywhere on the secondary to earth ground. We would expect that no current will flow, even during smelting while the electrodes are hot.

GoldDigger,
We did try to add a 500 watt quartz light load from electrode conductor to ground. This produced a "nominal 4 amp current". It also lowered the voltage to ground a nominal 4 volts on the the higher voltage legs and 2 volts on the lower voltage leg. If I remember correctly Gar has concluded that we have an approximate 180 amps of ground current. This leads me to believe that we must have multiple ground "leaks" in the melter and the pool is acting like a voltage divider.

On a side note, I did get a chance to talk with a corporate engineer about "the good child" melter. We had a racoon event at the substation yesterday. This caused a 70 minute power interruption. The emergency generators did function correctly as well as the backup cooling. Somehow during the event there were some compromising conditions that harmed the orifice the glass flows through.

This event gave me some access to the engineer that I would normally not have ( he is not an electrical engineer). He explained that the "off center vector measurement" is not normal but does happen time to time. His recommendation was to not worry about it!

I was able to get the site engineer and the corporate engineer face to face and let them "discuss the situation"

 

[gar]

160203-0814 EST

To clarify my 180 A or whatever it was statement. I estimated an internal impedance for the B path to ground. From this Z (probaby mostly R) and the open circuit voltage from B to an idealized neutral point I calculated a possible short circuit current of 180 A or whatever it was. This is the current that would flow if B was externally shorted to ground. But as the system is normally operated there is virtually no current flow from the kettle to ground.

The field distribution within the kettle in relationship to where the leakage to the outside metalic part of the kettle determines to where the displaced neutral point moves.

.