Open Delta Fusing 1

[buzzp]

We have an open delta transformer configuration used for metering purposes. This is an old installation so only two legs of the delta are fused with the middle leg tied to ground and no fuse.
Is it still standard practice to fuse only the two outter legs and not the third (grounded) leg? I can't picture a scenario where the two fuses will not see the current (and therefore open the fuses). With motors, this set-up is not allowed any more (overloads). I was wondering if the same can be said for an open delta transformer configuration?

 

[dpc]

The open delta connection is still standard for metal-clad switchgear voltage transformers. Only two fuses are used.

 

[rbulsara]

NEC does not require fuses in "grounded" conductor, even for motors. Fuses/ OL protection is required only in ungrounded condcutors.

See NEC 430.36 and 37. (2002 Edition).

 

[buzzp]

I understand about motor overloads (not short circuit protection). If you recall, the overloads for motors used be found in only 2 of the 3 phases. This has since been disallowed due to the potential of the unfused (no overload) possibility of an increase in current in the one leg with no corresponding increase in the other two. I believe this is due too single phase conditions along with regenerated voltages. I merely wanted to know if there were any similarities in the reasoning too require three overloads and three fuses for an open delta. I believe I answered this part of the question myself unless I am wrong about why they required the third motor leg to be protected from an overload.
Even if I am wrong about that then I can not see why the grounded leg would see an OC under any conditions (fault line-line will be picked up by other two fuses and fault to ground can not happen since the leg is already grounded).
Thanks for the posts and helping me to answer the question with reasoning.

 

[busbar]

 
buzzp, are you asking about metering-VT primaries or secondaries? For 2-element 3ø 3-wire metering with two VTs, 3 fuses are typically used with an ungrounded open-delta primary, but the corresponding open-delta secondary often has the jumpered connection {bø} grounded, with a pair of fuses in the other two secondary leads—consistent with IEEE C57.13.3.

Another case with 3ø 4-wire 2½-element metering is an open-wye primary using two VTs/two primary fuses with their common connection grounded, with the same for the VT-pair open-wye secondary.

 

[buzzp]

Yes I was referring to the secondaries. All the primaries are fused (ends up being four fuses because our PTs have fuse blocks on top and they use both blocks on both PTs).

You described our installation very well in your first paragraph. If I would have thought about it some more I would have understood why this third fuse is not needed. Thanks for the IEEE reference. This power stuff is new to me so you have to be patient Thanks again to all.

 

[busbar]

The ‘four fuse’ practice is mentioned in GEP-9186-6 for current-limiting primary fuses in open-delta applications. General Electric’s rationale is that a current-limiting fuse should not simultaneously energize two VT primaries. [I have discovered too many MV fuses that seem to have opened from merely looking at them crosseyed, and no other apparent reason.]

I misstated the IEEE reference and should have said, “…[fusing of ungrounded-secondary leads] consistent with the secondary-grounding recommendations of IEEE C57.13.3.”

 

[buzzp]

Thanks for that tidbit of info. I was wondering why 4 fuses would be used. I thought it may have been just a matter of convenience since each PT had two fuse blocks. Kind of makes sense with GEs logic. Thanks again.

 

[scottf]

Busbar is correct on the 4 primary fuses. The biggest problem with primary fused VTs is sizing the fuse low enough to operate before the VT fails given a primary/secondary short, but having it large enough to withstand energizing currents. Typically a 0.5A fuse is used, since the primary winding impedance is so high.

 

[Bung]

Leaving the grounded leg unfused does have the advantage of hoding the transformer secondary winding down to earth as long as possible - the metering guys tend to be less than keen to have the meters floating at some unknown potential in space...

Bung
Life is non-linear...

 

[buzzp]

Okay then another delta question: Is it common practice to ground a leg on an open delta CT connection for metering?

I would think it would be but have found that our plants do not have a ground on the open leg.

 

[busbar]

 
It is assumed that the subject CTs are for an application like 2-element metering of a 3-wire delta (or 3-wire wye) circuit.

ANSI/IEEE C57.13.3-1983 calls out secondary-side instrument-transformer grounding, but may be indirect where used in applications like transformer-differential relaying where, for delta-wye power transformers, delta-connected CTs are grounded via the interconnected-wye CTs at the oppsite boundary of the differrential zone. See C57.13.3 §2.

Electrostatically-induced potential may exceed the secondary-component insulation-withstand rating if left ungrounded.

 

[buzzp]

Yes this is 3 wire Y connection for metering. I missed the mention of open delta connections in this IEEE standard the first time but I see it is metioned in section 2.3. Thanks again busbar.
I will be requesting a ground be added to these CT circuits.

 

[buzzp]

Ok, one more question. I have thought about this you will be happy to know.
Okay, we have 2 CT's metering a 3 phase Y circuit. Each phase has its own ammeter (no power meters are hooked to these). The B-phase is grounded. My question: is there a preference as to whether this grounding would be after the b-phase meter or before the b-phase meter? The IEEE likes to see them grounded at the first point of application. This would mean it would be grounded before the meter. However, all of our open delta CT metering circuits are set-up with the ground after the meter input. I can see a difference if we were measuring power flow but the meter does not care which side of its input is grounded. Anyone have any reasons why I should keep the ground intact after the meter? I appreciate the help.

 

[busbar]

 
buzz, are you referring to the case where two CTs are configured to indicate all three phase currents? See page 37+ of GET-97D, at

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

[~6MB] Is the grounding you describe on the CT prmaries or secondaries?

 

[buzzp]

Yes, the two CTs are wired in an open delta (2 CTs) and is used to indicated current on all three phases using seperate meters. The grounding is on the secondaries.
I found the drawing in the GE document on page 37 figure 39. This figure shows a ground at the CT locations (presumably since the R's in this figure are the wire lead resistances).
If you imagined, now, the R's are our current meters with the Z's being replaced with wire. Would you put the ground where it is on this figure or move the ground to the other side of "R2" ('B' phase meter)? Thank you.

 

[busbar]

 
Either way will work to preserve secondary insulation, in indstrial switchgear it is most common to ground at the CT-common jumper via a “shorting block” similar to a Marathon 150xSC-series device.

 

[buzzp]

Thats what I was thinking too. Thanks again Busbar. Going to give you another star for your efforts in helping me. Thanks.

 

[stevenal]

It occurs to me that the GE method does not depend on the integrity of the ammeter to stay grounded. Might be the least of your concerns at that point, but a no cost option doesn't need much advantage.

 

[buzzp]

Stevenal, I am going to move the grounds too be before the meters instead of after. I am not a metering type but what we have here at this particular plant is very goofy in my opinion. We have two seperate transformers for station service (6.9kV to 480V) fed from two different sources (generators). They use one WH meter for metering both station service busses. They simply assume the voltage is the same on the output of both transformers (have seen differences on the two 480V busses of 10%). The currents from both busses goes to the WH meter. I have requested this be changed but it has to wait. Probably more than you wanted to know. Thanks for the post Stevenal.
Busbar, I can apparently only give one star per thread per person so sorry I can not give you another. Thanks though.