Utility specs large transformers for motors

[kwired]

I don't see why. I am usually able to obtain the available primary fault current (3ph & SLG), associated X/R, fuse data, transformer data, riser cable (if padmount txf) from the utility without a challenge. Larger utilities may take a month to turn the request around but only once did I have an issue. I ended up filing a complaint with the state PUC and the utility eventually relented.




I think you mean secondary impedance. Assuming a 2% impedance on the dist pots, this is a fault current of about 30kA at the txf sec terminals. I don't think you will get a significant reduction in fault current with the secondary quadraplex. If you are using the infinite bus, I think your equipment should be rated for greater than 30kA

If available fault current is 30kA at transformer terminals, and that overhead quad is only 1/0 AWG (seems to be common size and it sort of looks to be about that from pic) I put this information into the fault current calculator that is on Mike Holt's free stuff page and with only 50 feet of quad that 30kA is nearly cut in half a the load end.

IMO that quad is where the most drop in fault current will likely be and how much all depends on it's length.

 

[electrofelon]

I don't see why. I am usually able to obtain the available primary fault current (3ph & SLG), associated X/R, fuse data, transformer data, riser cable (if padmount txf) from the utility without a challenge. Larger utilities may take a month to turn the request around but only once did I have an issue. I ended up filing a complaint with the state PUC and the utility eventually relented.




I think you mean secondary impedance. Assuming a 2% impedance on the dist pots, this is a fault current of about 30kA at the txf sec terminals. I don't think you will get a significant reduction in fault current with the secondary quadraplex. If you are using the infinite bus, I think your equipment should be rated for greater than 30kA

Well both, but I know what the transformer impedance is - or at least from their spec book they give a value which may be "worse case". Hopefully the primary gets me down a bit closer to the AIC. of the equpiment.

 

[electrofelon]

Around her you don't get a new bank that large on a pole, anything over 150 KVA will be pad mount transformer.

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One of the utilities here will only go max 600 amp overhead, but that is silly because they will still not necessarily give you full transformer capacity for your service. Guys, why dont you just say the max bank you want to put up on a pole and let me know if that will serve my stuff :rant:

I see 3x500 banks fairly often, usually distribution though where they are feeding an older 4800 line off a newer 12.47 or 13.2 line. Thats a lot of iron up there!

If available fault current is 30kA at transformer terminals, and that overhead quad is only 1/0 AWG (seems to be common size and it sort of looks to be about that from pic) I put this information into the fault current calculator that is on Mike Holt's free stuff page and with only 50 feet of quad that 30kA is nearly cut in half a the load end.

IMO that quad is where the most drop in fault current will likely be and how much all depends on it's length.

Yeah thats good to know. I remember now running those numbers when I got the utility plans and they stated they were using 3 100's - Figured I'd be all right. You are close, the service drop is about 60 feet.

 

[kwired]

One of the utilities here will only go max 600 amp overhead, but that is silly because they will still not necessarily give you full transformer capacity for your service. Guys, why dont you just say the max bank you want to put up on a pole and let me know if that will serve my stuff :rant:

I see 3x500 banks fairly often, usually distribution though where they are feeding an older 4800 line off a newer 12.47 or 13.2 line. Thats a lot of iron up there!



Yeah thats good to know. I remember now running those numbers when I got the utility plans and they stated they were using 3 100's - Figured I'd be all right. You are close, the service drop is about 60 feet.

So a 22 or 25kA main breaker and series rated 10kA branch breakers (which is typical) should be fine. Most 10 kA branch breakers will be series rated with RK5 fuses also.

But this is 480 volt right? Seem to recall NF branch beakers are usually 14kA if that is what you are using. If you have fuses you shouldn't even be that concerned with AIC,

 

[electrofelon]

So a 22 or 25kA main breaker and series rated 10kA branch breakers (which is typical) should be fine. Most 10 kA branch breakers will be series rated with RK5 fuses also.

But this is 480 volt right? Seem to recall NF branch beakers are usually 14kA if that is what you are using. If you have fuses you shouldn't even be that concerned with AIC,

Its a mlo siemens w bqd, which are 14k. Also there is a nonfused disco (meter disconnect) so that is 10k.

 

[kwired]

Its a mlo siemens w bqd, which are 14k. Also there is a nonfused disco (meter disconnect) so that is 10k.

Looking like your concerns are valid, you are going to end up right about borderline for too much AIC with what you have.

If they would have just sized the transformer(s) like most POCO would have for your load, there wouldn't be a problem.

Presuming there is no other customer fed from this transformer bank?

 

[electrofelon]

Looking like your concerns are valid, you are going to end up right about borderline for too much AIC with what you have.

If they would have just sized the transformer(s) like most POCO would have for your load, there wouldn't be a problem.

Presuming there is no other customer fed from this transformer bank?

No other customers on that bank, and unlikely there ever will be. In hindsight I should have installed a fused disco for the meter disconnect and then the breakers in the MLO would (presumably ) series rate to it. Figured I'd be fine with 10k even if the POCO gave me full 200A. This was a really weird job and due to some people at the utility not knowing what they were doing, told me to build it and get it inspected without an design first. I think I'll email the PM and see if he knows why they threw in 167's.

 

[electrofelon]

Here is the response I got this morning:

Ethan,



It is my bad I did not notify you about this change prior the job going to construction.



About a week before construction date I was made aware of PSE being out of 100 kva transformers and the soonest we can get it would be 6 - 8 weeks. So I asked them if we have next size up available and was told yes. That's why we installed those so we don’t delay you any more.



The fault current of 500 kva bank is 31,700 ams. See below a quick image shot of the table.



Let me know how it effects electrically your equipment.



Thank you!

 

[kwired]

Here is the response I got this morning:

Couple things here.

1) how far in advance did they get notification of the project? We are now around three months since you started this thread.

2) 300 kVA is still overkill for 200 amps. If only load is a 20 and 30 HP motor many POCO's would likley only supply it with 75kVA bank

 

[electrofelon]

Couple things here.

1) how far in advance did they get notification of the project? We are now around three months since you started this thread.

2) 300 kVA is still overkill for 200 amps. If only load is a 20 and 30 HP motor many POCO's would likley only supply it with 75kVA bank

Application was put in around December I think. They asked a few questions and told me to go ahead and call them when it was inspected. I thought it was a bit lax for a commercial service, bit figured since it was a small service for two machines maybe it was just easy. Also I hadn't worked with this POCO before. As I stated earlier in the thread, they seemed to size the transformers to the LRA, which I guess kinda makes sense from a "power quality" standpoint, but dosent seem necessary.

 

[kwired]

Application was put in around December I think. They asked a few questions and told me to go ahead and call them when it was inspected. I thought it was a bit lax for a commercial service, bit figured since it was a small service for two machines maybe it was just easy. Also I hadn't worked with this POCO before. As I stated earlier in the thread, they seemed to size the transformers to the LRA, which I guess kinda makes sense from a "power quality" standpoint, but dosent seem necessary.

Not necessary at all IMO if two motors is all you have for loads. LRA should be limited in duration, or the motor is burning up.

Voltage sag during motor starting? That could be a problem for other customers if there were any on same transformer. If it is a problem for your premises, then you should use reduced voltage starting methods, VFD's, etc.

If across the line starting of your motor(s) causes a voltage sag on the primary distribution- bigger transformers isn't helping that one bit, in fact may even make it a little worse. A smaller transformer and it's impedance will have some reduced voltage starting effects as seen by the primary circuit.

 

[electrofelon]

Not necessary at all IMO if two motors is all you have for loads. LRA should be limited in duration, or the motor is burning up.

Voltage sag during motor starting? That could be a problem for other customers if there were any on same transformer. If it is a problem for your premises, then you should use reduced voltage starting methods, VFD's, etc.

If across the line starting of your motor(s) causes a voltage sag on the primary distribution- bigger transformers isn't helping that one bit, in fact may even make it a little worse. A smaller transformer and it's impedance will have some reduced voltage starting effects as seen by the primary circuit.

On the utility design plans, there was a field for "flicker" so it seems they take that into account. Just seems a little ridiculous.

 

[kwired]

On the utility design plans, there was a field for "flicker" so it seems they take that into account. Just seems a little ridiculous.

Is that a field for your/owners design decision? If only load is two motors, I wouldn't care about this value.

If I had large office building and was concerned about flicker when the air conditioning starts up - then it gets more attention.

 

[electrofelon]

Is that a field for your/owners design decision? If only load is two motors, I wouldn't care about this value.

If I had large office building and was concerned about flicker when the air conditioning starts up - then it gets more attention.

No it was something the utility computed and I was never asked for any input.

 

[Tony S]

Not necessary at all IMO if two motors is all you have for loads. LRA should be limited in duration, or the motor is burning up.

Voltage sag during motor starting? That could be a problem for other customers if there were any on same transformer. If it is a problem for your premises, then you should use reduced voltage starting methods, VFD's, etc.

If across the line starting of your motor(s) causes a voltage sag on the primary distribution- bigger transformers isn't helping that one bit, in fact may even make it a little worse. A smaller transformer and it's impedance will have some reduced voltage starting effects as seen by the primary circuit.

I’ve had experience of that with upgrading the 550V supply to a plant.

We changed two 500kVA transformers for two 1000kVA transformers. The 500kVA transformers had a choking effect on the DOL (Direct On Line) high inertia loads. The lights did dim during a plant start up. The larger transformers didn’t give that buffer. It caused problems elsewhere in the works 11kV system due to phase distortion to the point that MBFU lamps were exploding on a remote plant.