Bank of Secondary 120/240V, 1PH Xfmr into 208Y/120V

[naminorite]

I'm stumped. A EE specified a bank of three, single phase transformers factory wired 120/240V on the secondary side, but required the contractor to wire it into a 208Y/120V. When I approached the EE, he said that the 208Y/120V can be achieved by tying the neutral terminals and leg terminals together. I presumed he meant one of the legs of each single-phase transformer. As I sat and sketched it out, I find that I'm shorting half of my secondary windings and that when I looked at the turns ratio, ended up with a 480V(L-L). I don't believe my L-N voltage would be the 277V, but that is besides the point. Am I missing something here? Can the specified bank of single phase transformers be wired into a 208Y/120V system?

 

[davidbeach]

If you have a center tap on a 240V winding you can't get there from here. If you have two 120V windings that are connected in series to get 240V, then it would be easy enough to connect those windings in parallel to have 120V.

 

[naminorite]

David,

Thanks for the quick response, the nameplate info on the 120/240V transformers show LV/2, so it is center tapped. So you're saying that to your knowledge, what the EE is expecting is not possible?

 

[davidbeach]

If it is a single winding with a center tap, about the only way to get a 208Y/120V secondary is to apply half the rated voltage to the primary (you haven't said what that is) but you wouldn't use the center tap for anything.

 

[waross]

First get permission from the supplier to change connections.
Then in a clean dry place, open the transformer and change the internal connections. On the back of the center low voltage bushing you will find two cables or straps. One must be moved to one line bushing and the other must be moved to the other line bushing. Check your X₁, X₂, X₃, X₄ connections in your text book for the proper connections. Use an Ohm-meter to check continuity before making the new connections. (I saw a crew mess that up on a bank of 100 KVA pigs and stall the generator in the small diesel generating plant that supplied the area.)
If this is oil filled, all tools must be clean and dry. The workers should wear oil proof gloves, both for their own protection and to avoid contaminating the transformer oil. A tiny amount of dirt may lead to early failure of the transformer, possibly in hours or weeks.
If the transformer is oil filled with the HV bushings in the top, be careful not to damage the high voltage leads when you remove the top. There should be enough slack to move the top enough to change the secondary connections but not much more.
After the change you will have 120 V transformers with the center secondary bushing not used.
If this is your first time, try to find someone to assist you.
The supplier may prefer to supply transformers that are already configured for 120V rather than allowing you to make the changes in the field. There may be warranty issues if you raise the top of the transformer.
Utility workers routinely do this reconnection in the field but utilities have much better margins than contractors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jmmee97]

As waross explains, you are parallelling the two low voltage windings of the transformer. If you can find an old Westinghouse or GE distribution transformer manual, the complete connections are shown. The engineer that retired when I came on board gave me his.

 

[jghrist]

See Drawing G3.3G (p. 227 of 345) in

http://www.usda.gov/rus/electric/pubs/1728f-804.pdf

for connection details of grd Y - grd Y connection of single phase transformers for 120/208 V service.

 

[naminorite]

David: Let me provide you the nameplate information on the transformer:
Manufacturer: ABB; Oil-filled; Pole-mounted
HV 7200/12470Y; LV 120/240; 37.5kVA
Taps 1/A 7200; 2/B 7020; 3/C 6840; 4/D 6660; 5/E 6480

I've also attached the photo of the nameplate.

waross: Thank you for the insight. I unfortunately don't have the manpower/tools/skills, etc. to perform the task that you have laid out. Based upon your experience, looking at the exterior connections, can I connect the 120/240 into the 208Y/120 that the EE states is possible?

jmmee97: Would it be possible to pdf the transformer manual that you have and possible post it on

http://www.scribd.com?

It's a free posting site for published material.

jghrist: Thank you. I was pleasantly surprised to find EE stuff on the USDA website and have that pdf file that you mentioned in my library. Based upon your experience though, is it possible to externally connect the 120/240V transformers into the 208Y/120V that the EE says can be done?

 

[jmmee97]

naminorite,
Sorry, I would probably break the binding trying to scan the pages. Also, several sections are outdated such as the oil-filled cutouts and gap arresters. Come to think of it, we still have some gap arresters on the system.

 

[naminorite]

jmmee97: Thank you for trying. Could you provide the title of the manual, some sort of book number associated with it and a publishing date? Maybe I can find it somewhere on the web?

 

[davidbeach]

For 240/120, the transformer is connected (internally) per the left diagram. For the 208Y/120V bank you would use the right internal connection. There is nothing that can be done external to the tank to allow the change.

With the transformer internally connected per the left diagram, any external connection between X1, X2, and/or X3 will create a fault.

 

[naminorite]

David: So, it seems that I would need to follow waross's instructions (paralleling the secondary windings internally) to be able to connect the 3 xmfrs into a 208Y/120V system. Thank you for your insight.

On a slightly happier note: Merry Christmas to all and a Happy New Year!

 

[naminorite]

waross/jmmee97: How long would it take to do the work that you are suggesting?

 

[jghrist]

I agree with David and Bill. If you don't want to make the internal reconnections yourself, you may find a transformer repair shop in your area that will do it. See

http://www.thomasnet.com/products/transformer-repair-services-67520841-1.html

 

[waross]

I would do the change to three transformers in less than 1/2 a day on a good day. Some crews may take longer. Come to think of it, I take longer than I used to.
If you do it yourself and the crew gets any dirt in the transformer you may find that you have voided the warranty.
The shop idea is very good.
If you do the work yourself, I would connect "A" and "C" on X3, "B" and "D" on X3. The leads may be marked. Others may connect it differently.
Moisture and/or dirt contamination is deadly. I saw the connections changed with a dirty wrench. The adjustable wrench looked clean but there was some hidden dirt in the adjusting thumb screw and the transformer oil washed a small amount of dirt out of the wrench. And the transformer failed within a few hours. That was an expensive bit of education which is yours free if you choose to be guided by it.
PS: The transformer may be under a slight vacuum. You may have to pull the ring on the two way pressure relief valve before you are able to remove the top. ONLY DO THIS IN A DRY AREA. Checking and recording the relative humidity before opening the transformer is a good "Cover Your Ass-ets" procedure.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racobb]

I think Bill meant X1 and X3. It is also pretty common practice to either remove or insert the middle bushing lug backwards to indicate that there is no longer an internal connection to the external terminal that was the neutral/common point of the 2 windings. Done this conversion many times without issue.

The idea is that if you have a single phase 25 kVA transformer with 2-120 volt secondary windings, they will each be rated 12.5 kVA at 120 volts. When you reconnect them AC-BD for 120 volt operation you have the 2-12.5 kVA 120 volt windings in parallel for the same total kVA rating of 25. Hang 3 and hook them up for a 75 kVA bank of transformers 208/120 volts wye.

The AC-BD thing is because the ends of one winding are A and B, the ends of the other winding are C and D.....at least it used to be that way!

Alan

 

[stevenal]

All on X3 Bill? Perhaps you mistyped?

 

[itsmoked]

Or a shill for a transformer company..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[naminorite]

One last question to close-out my concerns on this topic. What is the revised impedance with this reconnection? The nameplate impedance says 2.4%. Is it half of that since I'm reconnecting the windings in parallel?

 

[waross]

Sorry for the typo, folks. One of the X3's should be an X1.
The last time the boys did that, (actually crossed "B" and "C", each winding was shorted) we didn't burn the transformer, but we did:
1> Find out that he transformer was about 800% over fused.
2> Stall the diesel back in the power plant.
3> Inconvenienced 7000 or 8000 utility clients before we got the gen-set up and running again.
Ah, the joys and surprises of working in the turd world.
Too bad you couldn't have been there with me, 'smoked.

Calculate your available short circuit current at 240 Volts.
Then double your current. It will take the same primary voltage as before to force rated current through a shorted secondary, so the %impedance should remain the same. Your available short circuit current will double so check your switchgear ratings.

Bill
--------------------
"Why not the best?"
Jimmy Carter