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240v 3-phase with Open Delta Transformer

240v 3-phase with Open Delta Transformer

240v 3-phase with Open Delta Transformer

I have an old building that is getting its power from an open delta utility company transformer.  This is giving us 240v 3-phase power.  I have been doing some research on this subject and have not come up with a lot of information, what I am wondering is the following.  Do I need to avoid putting single phase power on the 'C' phase due to it being a "stinger" leg that has a different voltage?  if so what is this voltage?  how do I do my load calculations on this panelboard?  there isn't a relationship with the square root of 3 for me to work with so I am not sure how to input the 3-phase power.....any advice or points in the right direction would be extremely helpful.


RE: 240v 3-phase with Open Delta Transformer

I had meant to say 'B' phase not 'C' phase....sorry

RE: 240v 3-phase with Open Delta Transformer

B Phase is 208 V to ground = sqrt(240^2 - 120^2).

Do not connect any loads to B Phase.  Per NEC it must be identified at every point where it is visible with orange ape.

For your panel schedule, put one third of each three phase load on each phase.  Treat A & B Phase as a normal 120/240 V panel and add the VA to the 3-phase load.  The resulting current will be a litte off due to phase angles, but it will be close.

RE: 240v 3-phase with Open Delta Transformer

Thanks a million.  That is the first thing I have heard on this subject that makes sense.  I definately appreciate the help....By the time I was done I had called several other engineers I knew at different companies and nobody knew how to do it....go figure.

RE: 240v 3-phase with Open Delta Transformer

  Our system has a lot of 240V open delta transformer banks. I am curious how you would connect single phase load to any phase A, B or C.  Often times we have little dry type's to step the 240V to 120 for lighting, but without a ground point, you would never know what your phase to ground voltage would be on a 240V open delta.  At least this is what I thought, but let me know if this is not correct.

RE: 240v 3-phase with Open Delta Transformer

I think the original post may be refering to an open delta connection with a grounded center tap on one transformer. This gives 120V from, say A and C to ground and 208V from B (the 'stinger') to ground.

For a standard ungrounded open delta, the voltage from any leg to ground is indefinate. If one corner of the open delta is grounded, there will be 240V L-G from each of the other two legs. Unless you have a center tapped winding, there's no way to get 120V.

RE: 240v 3-phase with Open Delta Transformer

That is exactly the case, I have to center tap one of the transformers.  

RE: 240v 3-phase with Open Delta Transformer

Open delta banks do not regulate voltage as well as closed delta banks. Check your 3 phase motor loads once you're up and running for voltage balance.

RE: 240v 3-phase with Open Delta Transformer

Why do open delta banks not regulate as well?  How do you calculate the limits on loading on an open delta bank?  Anyone recommend a good reference book that discusses open delta transformers?


RE: 240v 3-phase with Open Delta Transformer

Because your transformer is missing a winding.  As load is increased, the voltage imbalance increases.  

GE used to have a little pamphlet on transformer connections.  I'm sure a Google search for open delta transformer connections will turn up something.  

RE: 240v 3-phase with Open Delta Transformer

When looking at the ability to calculate the fault currents and capacity of the bank you need to keep the following in mind:

1) You are typically dealing with transformers of vastly different sizes. Something like 50 kVA and 15 kVA or any other combination.  As a result when doing per unit calculations you have two totally different PU bases.

2) The lighting transformer (Larger one) is connected with its secondary windings in series with the center tap grounded to provide single phase 120/240 V service.  

3) The smaller transformer will typically have its two secondary windings connected in series to provide a 240 Volt side to a delta type service.

The calculations can get a little messy however the key is establishing a common PU base, keeping track of your winding connections, and knowing how the loads will be distributed.

RE: 240v 3-phase with Open Delta Transformer

One of the most popular uses of open delta is to supply a small three phase load and a large single phase load. Typically, a small transformer is installed alongside an existing 120/240 volt lighting transformer.
Short circuit calculations are for the purpose of selecting properly rated switches, breakers and other equipment.
Calculate the available fault current for the large transformer. This will be the available fault current on "A" phase.
Calculate the available fault current on the small transformer. This will be the available fault current on "C" phase.
Add the currents vectorially. This will be the available fault current on "B" phase.

RE: 240v 3-phase with Open Delta Transformer

Old thread, timely response. I spent some time today figuring the vector diagram for the 230-3 phase 115/230 4-wire service after being somewhat embarrased talking to the utility planner. Never thought that only 2-transformers and 4-wires could give you so much functionality (Does not make the same impact when in a class room). What I figure is that referencing the 115/230 neutral the (abc) is van=115<0, vbn=115<-180, and Vcn-208<90. However, from the above post, the b phase should be considered the 208V line. Does this mean that the rotation should actually be considered acb?

RE: 240v 3-phase with Open Delta Transformer

Just out of curiousity, if the customer closes the open
leg with a transformer of appropriate size, does the
balance even out?  In other words, can one feed the primary
of a 25 kVA transformer from a 50 kVA open-delta service,
and expect decent 3-phase on the secondary?

RE: 240v 3-phase with Open Delta Transformer

If you remove one transformer from an existing three transformer delta bank, you have a an open delta bank. Rotation stays the same. Capacity drops to about 57.7%. That is, a hypothetical 100 KVA bank,(Three 33 KVA transformers) will have a capacity of about 59 KVA on open Delta. You can connect  single phase 240 volt loads to any two phases, but it is possible to overload the second transformer. Remember it may be a 15 Kva transformer.
Full load current; 15000KVA / 240 Volts = 62.5 Amps. There will be enough available fault current to assure tripping of the breakers on short circuits. However, the only over load protection for the transformer may be the primary fuses on the transformer. If you connect all your 240 Volt single phase loads on  "A" "C" and use "B" for three phase loads only you should not have a problem. Typically, the panel will have every third breaker missing. (Except for three pole breakers.

Standard practice is to use 4 wires and ground the neutral of the 120/240 winding. The corner ground on a three wire sytem is new to me, but I wouldn't remove it. I usually recommend that Delta systems be reconnected Star, and have done so several times. Here though, I understand that only three phase wires are brought in. Once you undertake to change an existing system you are responsible for complying with current codes. Without inspecting the site to see the scope of work required to bring the grounding of the neutral up to current codes, I wouldn't recommend any changes. It's worked for a long time and it's grounded.
If you have unexplained transformer overheating, and or a high transformer burnout rate, then bite the bullet and change the connections to star and bring the grounding up to code. Otherwise, say "That's interesting", and forget it.
Note: In North America, conventional three phase is 120/208 Volts.
In the open Delta, the voltages are 120-240 single phase and 240V three phase.

RE: 240v 3-phase with Open Delta Transformer

AFAIK, most utilities will NOT provide an 'open delta' connection for new services,(and furthermore many will not provide new 'delta' 3-wire services of any kind), due to the problems with the 'stinger' phase as described above.

However, in the utility where I work, we have several 'legacy' open delta services where one of the xmfrs is 'center-tapped' to provide 120/240 TO A SEPARATE SERVICE to the same customer.  This was done to ensure that all of the single phase load came off the appropriate xmfr which was sized larger to handle this extra load.

Our policy is to force all customers to a 4-wire wye service anytime they make any changes to their main service entrance.  Delta-connected distribution loads (except in certain specialized cases) are no fun for distribution companies to work with.

RE: 240v 3-phase with Open Delta Transformer

Hi tinfoil
I'd like to see your standards adopted here.
Here if a load is over 15 KVA you have to buy your own transformer. Past a certain size, the utility requires two transformers. They would be connected Open Delta. I think the load limit for two transformers is 200 KVA. Past that you must use three transformers. Many engineers here still use Star/Delta. The first time a phase goes out on the primary circuit a fuse blows at the transformer bank. The linemen instead of replacing the fuse then take away the fuse holder.
The cut-out is empty  and the bank is running Open/Delta with the unfused transformer floating across the Open/Delta..
Fortunately the engineers here have a habit of grossly oversizing transformers.

We have legacy systems here where the normal residential transformer has a small transformer beside it to supply a small three phase load. The utility is in the process of checking loads and requiring the customer to buy his own transformer bank if the load is over 15KVA.

RE: 240v 3-phase with Open Delta Transformer

There are only two single-phase transformers but 3 phase in the high voltage side and 3 phase in the low voltage side.
Below is a pictured obtained from the net for physical illustration of the open delta.
I hope this help.

Electrical diagram:

RE: 240v 3-phase with Open Delta Transformer

The trick is the the neutral connection. The neutral actually carries the same current and sometimes more current than the other lines.
Have you studied Wye/Wye systems yet. In a Wye secondary, as you know you have three voltages out of phase by 120 deg.
If you measure to the neutral from any phase you get 120 volts. If you measure phase to phase you get 208 volts.
Now if you remove one of the three transformers; you still get 120 volts line to neutral on two phases. You can still read 208 volts across the two lines that are left. if you reverse the secondary connection on one of the transformers, you move the phase angle through an angle of 180 deg. The phase displacement on a wye circuit is 120 deg. if you rotate that line 180 degrees (By reversing the connections) you end up with 300 degrees. The difference between 300 deg. and 60 deg. is whether you measure the inside angle or the outside angle. Now you still measure 120 volts from line to neutral from either line, but from line to line where you previously read 208 volts, you now read 120 volts because the phase angle has been changed. You now read 120 volts line to line as well as line to neutral. You now have a corner grounded 120 Volt Open Delta bank.
There is not much call for 120 Volts Delta three phase so change the internal transformer connections back to 120/240 volts. Now you have a corner grounded 240 Volt Three Wire Open Delta. Much more usable.
Do you have single phase loads?
Move the ground connection from the corner to the center tap of one of the transformers and from there run a neutral to the service head. I know it has to go all the way to the panel, but the electricians need work too.
You now have a four wire Open Delta with 120/240 Single Phase as well as 240 Volt three phase.

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