Cause of Equipment Damage?

[tigerbait]

Situation:
A business is fed from a grounded utility source through a grounded wye-wye 13.2kV/480V padmount transformer. A dry-type, single phase, 480V/120-240V transformer is connected to the secondary system. Two remote tap fuses open on the utility line feeding the padmount transformer.

Question:
Could this somehow result in a sustained overvoltage beyond the dry-type transformer that would be bad enough to damage equipment, or would damage have to have been caused by transients associated with the fault itself?

 

[jraef]

If 2 fuses opened simultaneously on the primary, there would be NO voltage on the secondary at all. Any damage was likely the result of the original fault that was the cause of the primary fuses blowing.

"Venditori de oleum-vipera non vigere excordis populi"

 

[davidbeach]

jraef-
When fed through a grounded wye-wye transformer, the loss of only two primary fuses does not cause the loss of all voltage on the secondary so long as there is a current path from the primary neutral point back to the source. In that case, the 480V winding of the second transformer has a 2 in 3 chance of being connected to the one live phase and a dead phase, the 480V transformer winding will be series with a winding of the first transformer on a 277V circuit. Should result in a sever undervoltage condition.

 

[jraef]

You're right, I missed the Y-Y detail. Need new glasses I guess.

"Venditori de oleum-vipera non vigere excordis populi"

 

[alehman]

Perhaps one fuse opened first, leading a ferroresonance condition, followed by the second fuse opening. Ferroresonance could account for overvoltage condition. Is the 13.2kV source connected via shielded cable?

 

[PWR]

Perhaps it was the sustained undervoltage condition that damaged equipment. What was the nature of the damage and to what type of equipment did it occur?

 

[tinfoil]

jreaf:

I am a bit confused about your first post. In a grounded Y-Y transformer (assuming both Xo and Ho are solidly grounded), wouldn't one of the phases still be delivering nearly-nominal voltage even if the other two incoming phases were isolated?

For the sake of discussion, lets assume that the H2 ands H3 connections are open. H1 is still connected to a live 7.6kV L-N source (13.2 is presumably the L-L voltage).

Normally, in a Y-Y, the primary coil attached to the H1 and Ho bushing is wound on the same leg as the secondary coil connected to the X1 and Xo bushings.

So, from the point of view of phase 1, nothing has changed on the source side OR the load side of the xmfr. So shouldn't near-nominal voltage be delivered to the X1 bushing?

I would also think that there would be a small amount of voltage induced on the X2, X3, H2 and H3 bushings due to flux linkage within the multilegged core, but the impedance of the connected load on those phases would keep this voltage low.

 

[tinfoil]

Soory, that last post was intended for davidbeach

 

[davidbeach]

tinfoil-
Yes, I agree with your development as far as it goes. Continuing on then, to the single phase transformer, say that it is connected X1 to X2. The X2-X0 winding, with H2-H0 open circuited, is basically an inductor. So, you now have a circuit that (among other things) starts at X1, goes through the 480V transformer winding, to X2, and through the X2-X0 winding. The 480V winding is normally connected with 480V (X1-X2) and is now connected 277V (X1-X0) and in series with the X2-X0 winding.

 

[tinfoil]

davidbeach:
I assumed (apparently wrongly) that most single-phase loads would be connected line-to-ground on a 4-wire service (which is the normal practice here in Canada), and missed the original post reference to a 480V120-240 dry-type, which would obviously be connected L-L.

We do not usually even use 277/480V, as 347/600V is used virtually everywhere for industrial loads.

Sorry for my confusion.