Puzzled by Medium Voltage

[HoosierSparky]

I am starting to get a migrane looking at a one line that has me scratching my head.

I have a system that begins at an SES (208/120) with a 200a 3phase disconnect feeding a 75kva single phase 240/120v to 2400v transformer. From there, two separate feeders of #2 15KVA Okoguard "URO-J" Al jacketed cable with full neutral in PVC conduit go to 2 37.5KVA 2400v/240v single phase transformers. These each feed a 100a MCB panels.

All transformers have an internal "Loadbreak switch".

Looking at this, I start thinking 450.3 and Table 450.3(A). None of these are supervised locations.

The BIG question is, should there be a disconnecting means at the 75KVA transformer for each feeder to the 37.5 transformers or at each in addition to the "coordinated thermal overload protection that is integral to the transformers? :?

 

[HoosierSparky]

Diagram

Diagram

View attachment Medium Voltage.pdf Thought I would add a one line to show what I am seeing.

 

[templdl]

Strange to say the least. Who drew this?

 

[HoosierSparky]

I drew it copying it from the submitted plan. You can see why I am puzzled.

 

[Steve Merrick]

Ummm...

Ummm...

View attachment 13032 Thought I would add a one line to show what I am seeing.

You're feeding 208/120V 3-phase into a 120/240V single phase transformer? I will bet it's running hot.

 

[HoosierSparky]

You're feeding 208/120V 3-phase into a 120/240V single phase transformer? I will bet it's running hot.

In Arizona, EVERYTHING runs hot! In this case they are only using 2 legs.

 

[Smart $]

In Arizona, EVERYTHING runs hot! In this case they are only using 2 legs.

That would still be 208V supply to a 240V primary... :?

At the other end, you will get 208V secondary output (barring compensation with voltage taps, probably both ends).

 

[Steve Merrick]

That would still be 208V supply to a 240V primary... :?

At the other end, you will get 208V secondary output (barring compensation with voltage taps, probably both ends).

Yes, and your two 120 volt legs will be 30 degrees out of phase, leading to high circulating currents in the transformer core. At the very least, it violates the "intended use" clause under 110.3(B).

 

[templdl]

You're feeding 208/120V 3-phase into a 120/240V single phase transformer? I will bet it's running hot.

Precisely, that was my thought. That's why ,I asked who drew the schematic.

 

[HoosierSparky]

I contacted the engineer and suggested using 3-phase throughout. His response:

"We did consider it but had norequirementsFor three phase and loads wereso low that we
decided to design with singlephase and transformer
should balance load at secondaryside"

Transformers spec'd are EATON MaxiShrub pad mounted, oil filled.

I may be overthinking this. All that is being powered are 2 "Comfort Stations" for a golf course. Due to the distance from any power source, they are stepping the voltage up and then down to power some lighting and soda machines.

My concern has mostly been about the 2400v portions, the system not being under observation. I am requiring Special Inspection. I am not familiar with the Loadbreak switches or the Bayonet type fuses that are part of the transformers. Will these suffice as required disconnects per 403.3? Would this shut down the power to the loadcenters without shutting down both 37.5KVA's?

 

[jim dungar]

Is the 75kVA a 1-phase transformer? If so, you should be running only 2 conductors +g, a connection to the X0 terminal will result in a transformer failure.

As was pointed out, the 37.5kVA output will be 208V not 240V, unless the transformers have 'BN taps'. As currently drawn the output will be 104/208V.

Overall, this is not an unusual installation, it just appears someone is overlooking some details.

 

[HoosierSparky]

Is the 75kVA a 1-phase transformer? If so, you should be running only 2 conductors +g, a connection to the X0 terminal will result in a transformer failure.

As was pointed out, the 37.5kVA output will be 208V not 240V, unless the transformers have 'BN taps'. As currently drawn the output will be 104/208V.

Overall, this is not an unusual installation, it just appears someone is overlooking some details.

Good catch! Thanks.

 

[templdl]

?

?

I've supplied a number of step up, step down transformers for airports to supply power for there avionics on the long runways.
It was an extremely simple set up. These are 1ph transformer the first transformer had a simple 240v 2w primary w/taps and a 2400v 2w secondary. The second transformer had a 2400v 2w pri w/taps and a 120/240v secondary.
I did that all of the time.
Remember that a transformer's taps are located on the primary windings which are wound normally on top of the LV windings. Normally the primary windings are HV. Since you have a step up transformer the pri winding is LV which is wound on the outside w/taps, top on the HV winding. It is also important to note that you may not get the customary (2)FCANand-(4)FCBN taps because of the turns ration on the LV winding.
The step down transformer would have the customer
Taps are important to compensate for voltage drops.

 

[Steve Merrick]

Your engineer is wrong. What we have been trying to say is that your proposed transformer will simply not work for the application. Not only will it not give you 2400 volts when you are feeding 208 into a 240/2400V transformer - it will only supply 2080 volts to your load, it will almost certainly burn up and catch on fire without tripping your overloads - primarily because you are showing a center-point grounded neutral feeding 120V at 0 degrees into one leg and 120V at an phase angle of 120 degrees into the other leg. That will destroy the transformer.

You could disconnect your primary neutral on your step-up tranformer, but you would still be feeding only 208V into a 240V winding. What you need to specify is a 208V/2400V single phase transformer. Eaton has them in the model you are considering. Do not center-point ground the primary (LV) windings.

As to your disconnects, I typically provide a heavy-duty fused disconnect at the secondary (2400V) terminals. In your case you would have one disconnect for each circuit. I size the switch for the full kVA rating of the downstream transformer for each circuit. I size the fuse to protect both the feeder cable based on its ampacity and the step-down transformer at no more than 125 percent of its primary FLA per NEC 450.3(B).

 

[HoosierSparky]

Your engineer is wrong. What we have been trying to say is that your proposed transformer will simply not work for the application. Not only will it not give you 2400 volts when you are feeding 208 into a 240/2400V transformer - it will only supply 2080 volts to your load, it will almost certainly burn up and catch on fire without tripping your overloads - primarily because you are showing a center-point grounded neutral feeding 120V at 0 degrees into one leg and 120V at an phase angle of 120 degrees into the other leg. That will destroy the transformer.

You could disconnect your primary neutral on your step-up tranformer, but you would still be feeding only 208V into a 240V winding. What you need to specify is a 208V/2400V single phase transformer. Eaton has them in the model you are considering. Do not center-point ground the primary (LV) windings.

As to your disconnects, I typically provide a heavy-duty fused disconnect at the secondary (2400V) terminals. In your case you would have one disconnect for each circuit. I size the switch for the full kVA rating of the downstream transformer for each circuit. I size the fuse to protect both the feeder cable based on its ampacity and the step-down transformer at no more than 125 percent of its primary FLA per NEC 450.3(B).

OK. I get what you all are saying. Thank you! Here is where I think I am going with this now. 2 #3/0 w/ #6 grnd to 75kva. Keep the other wiring the same. Because of the switch, cartridge fuse and type of transformer, I don't think a disconnect is now needed. Unqualified personnel would not have access to any of the medium voltage. The installation will be under Special Inspection and Observation. I am requiring this due to it being private Medium Voltage and not a POCO install. The panelboards at the end are protected by the 100a MCB. Should anyone need to isolate a transformer, they can use the switch or pull the fuse. This would all have to be done by qualified personnel using the correct PPE.

Am I missing anything?

 

[templdl]

OK. I get what you all are saying. Thank you! Here is where I think I am going with this now. 2 #3/0 w/ #6 grnd to 75kva. Keep the other wiring the same. Because of the switch, cartridge fuse and type of transformer, I don't think a disconnect is now needed. Unqualified personnel would not have access to any of the medium voltage. The installation will be under Special Inspection and Observation. I am requiring this due to it being private Medium Voltage and not a POCO install. The panelboards at the end are protected by the 100a MCB. Should anyone need to isolate a transformer, they can use the switch or pull the fuse. This would all have to be done by qualified personnel using the correct PPE.

Am I missing anything?

I looks as is you have a much better handle on things than the person who provided you with that one line.
The only correcting that I would make on my example is that it used a 240v 1ph 2w supply. Your is supplied by a 208y/120 system. Instead of the 240v as I used in my example your sterp up would have a 208v pri supplying the transformer with a 208v L-L voltage.
I am not aware if you are doing the installation but unless you have do a 2400v installation I would definitely get a hand from somebody who has. In my example my transformers were padmounted transformers with underground feed. I don't know if you have a game plan for that.

 

[HoosierSparky]

I looks as is you have a much better handle on things than the person who provided you with that one line.
The only correcting that I would make on my example is that it used a 240v 1ph 2w supply. Your is supplied by a 208y/120 system. Instead of the 240v as I used in my example your sterp up would have a 208v pri supplying the transformer with a 208v L-L voltage.
I am not aware if you are doing the installation but unless you have do a 2400v installation I would definitely get a hand from somebody who has. In my example my transformers were padmounted transformers with underground feed. I don't know if you have a game plan for that.

On this job I am lucky, I only have to do the plan review. Initially the plans did not even have a one-line! The one-line I did receive, left a lot of questions, as everyone has commented on and caused me to initiate this thread.

I have received the "cut sheets" for the pad mounted, oil filled transformers that are being spec'd for the job. All are Eaton, utility type. I am VERY comfortable with them. They seem to be very robust for the actual loads at the back end.

As I stated above, I think I have it where I can approve it, provided a few changes are made. The special inspector that has been hired is one of the best EE's in the state. I wish he had done the design. We then would not be talking about it.