Potential Transformer Overexcitation ? 2

[dpc]

A question for those of you with expertise in instrument transformers:

For a 25kV PT (solid dielectric), would a primary voltage of 115% at a frequency of 150% cause sufficient core saturation to blow a 1E primary fuse?

(This is strictly a theoretical question, of course! )

Thanks.

 

[RajT]

Interesting question. I would suspect that the fuse would not blow immediately as it would need to see 150% increase in current for some 4hrs before it would go but you may smell the insulation getting hot after a while and subsequent failure of the transformer over a period.
It would be worth putting the question to the Tx manufacturer as they would have data on such effect as part of their destructive testing tests on the unit.

 

[busbar]

Aside from C57.13 which seems mum on the subject, another classical reference is GET-97 [

http://www.geindustrial.com/products/applications/GET-97D.pdf

] that discusses frequency and voltage, but doesn’t mention the combination. V/Hz ratio is lower, and 115% overvoltage is within ANSI tolerance, so insulation degradation is not likely an issue here.

If they are open-delta connected, some caution about using just one fuse serving two cores {usually on ‘B-phase’} as a situation to avoid. My best guess is that 15-35kV PT-primary fuses can blow just looking at them crosseyed. Maybe just chalk it up to “fatigue.”

 

[scottf]

dpc-

I do not believe that the fuse would blow. Under normal conditions, the current flow through a VT primary winding is a function of the applied secondary burden and core excitation, in which the applied burden is the dominating factor.

In normal cases, this would be in the mA range.

Depending on the specification and rated over-voltage factor, most VTs would not be in saturation at 115% of rated voltage.

Now, you also mentioned 150% of rated frequency. Since core saruration is also a function of frequency (the higher the frequency, the lower the flux density), operating at a higher frequency would not cause more current to flow in the primary.

Primary fusing of VTs is really only to protected the system when the VT has an insulation failure, i.e. an internal line-to-ground failure, in which case much larger currents flow in the primary winding...or from the primary winding to ground.

 

[dpc]

Thanks guys. I was thinking the higher frequency would contribute to saturation, but I had that backwards.

I agree that those 1E PT fuses are very sensitive. I've had them operate for other phantom faults.

 

[busbar]

Anecdotally, lightning seems to take out “E” PT fuses even when gear is not connected to overhead lines. [E-rated fuses cannot be considered "economy priced" either.]

 

[jbartos]

Suggestion (for a theoretical question, a theoretical answer): It depends (Assuming that very small E rated fuse is available. They start from .5A. Visit

http://www.bussmann.com/library/bifs/6002.pdf

):
1. Theoretically, how much PT is loaded on the secondary (some PTs load heaters).
2. Theoretically, how sensitively the PT is protected by E fuse with respect to its VA rating, e.g. 600VA PT may be protected for a fraction of its VA.