Need a little help with open-delta connections

[mxstar211]

I am not very familiar with open delta connections, but I have recently ran into quite a few of these systems. I am a 21P in the Army, and am currently on a training mission for doing power assements to provide back-up power generation during natural diseasters.

Most of the open-delta configurations I have come across have been at lift stations, (sewer pumps) which are usually located out in the country away from three-phase lines.

I understand why they use the open-delta as it is much cheaper to run single phase and get three phase power using an open-delta connection, but I am a little confused as the stuff I have researched has used two different numbers that reduce the output of the transformers when wired in open-delta.

I mostly see that the output when using two transformers is reduced by 57.7%, but I have also seen a few times where when the open-delta is by design, and a winding not failing the output is reduced by 87%.

Sorry for the long post, but if someone could explain this, or provide a link it would be much appreciated.

Thanks a lot!

 

[bob]

The open delta transformer connection can be made with only two transformers instead of three. This connection is often used when the amount of three phase power needed is not excessive, such as a small business. The output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).

Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).

 

[SG-1]

I understand why they use the open-delta as it is much cheaper to run single phase and get three phase power using an open-delta connection...

Open-Delta is three phase in & three phase out.

Bob did a good job explaining the two percentages you are seeing. Here is a link with more of the same. Open-Delta is near the bottom.

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

 

[mxstar211]

The open delta transformer connection can be made with only two transformers instead of three. This connection is often used when the amount of three phase power needed is not excessive, such as a small business. The output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).

Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).

That is exactely what I found when I did a google search. I just didn't read it close enough. I feel like an idiot now. lol. I guess that's what lack of sleep will do to you. Thank you very much for the fast response.

 

[mxstar211]

Open-Delta is three phase in & three phase out.

Bob did a good job explaining the two percentages you are seeing. Here is a link with more of the same. Open-Delta is near the bottom.

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

Yes, that is the same link I was able to find, I just did a bad job of reading it. I guess I am using the wrong terminology. The POCO runs two lines and uses to transformers to get 3 phase power. I thought that was an open-delta. Thanks for the help, I will try do some more searches tomorrow.

 

[SG-1]

Here is another site for 3 Phase information.

http://www.3phasepower.org/

 

[LarryFine]

The POCO runs two lines and uses to transformers to get 3 phase power.

The primary of an open Delta (most of them, anyway) is powered from two lines and the system neutral, just like when we take two lines and the neutral from a 208Y/120v 3ph panel.

I thought that was an open-delta.

Actually, the secondary is the open Delta. If it's a high-leg service, the 'main' transformer is exactly the same as a typical 120/240v 1ph service.

In fact, the high-leg Delta originated as a 3ph modification to existing 1ph services.

 

[mivey]

Yes, that is the same link I was able to find, I just did a bad job of reading it. I guess I am using the wrong terminology. The POCO runs two lines and uses to transformers to get 3 phase power. I thought that was an open-delta. Thanks for the help, I will try do some more searches tomorrow.

You are using two-phase to get three-phase. This two phase system where only two of the three evenly-displaced phases are brought out on the distribution system is sometimes referred to in the utility industry as a "V-phase" system. FWIW, this two-phase system has a 120 degree displacement and is not the same as the old traditionally labeled two-phase system which had a 90 degree displacement.