1-phase 230 V 30 kVA load fed by 500/230 3F 30 kVA Transformer

[radug]

Hello,

Could you tell me how to calculate the current that will be drawn in the primary side of a 30 kVA 500/230 3F transformer when the load is 30 kVA 1-phase plus neutral? I suppose there will be an unbalance in the currents. If so, how to protect that from overcurrents?

And also, are there 3F to F+N transformers?


Thanks.

 

[Skogsgurra]

This is a very tough thing to do. 30 kVA is considered three-phase land.

You could, perhaps, use a T transformer. See

http://www.rtri.or.jp/infoce/wcrr97/C135/C135.html

It makes two phases at 90 degrees out of a three phase grid. I imagine that you could connect between the two diagonally situated ednpoints of the secondary windings (connected together at the "right angle") and get a balanced load on the three-phase grid. That is what the cited site says, anyhow.

Good luck!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jghrist]

How is the transformer connected? Dy or Yy?

 

[waross]

I would use the same connections as are used to derive single phase from a three phase generator.

If your transformer has a wye secondary you can use a zigzag connection. Equal current will flow in all phases but will be out of phase in two phases. If the secondary connections are X1, X2, X3 and X0, remove the X0 end of the X3 winding and connect it to X2. Insulate and ignore it.
Now, you will have V/2 from X1 to X0, and V/2 from X0 to X3.
The voltage will be about 15% higher than the original line to line voltage.

You may also use a delta or double delta connection. If a single phase load is connected to a delta transformer bank, approximately equal currents flow in all windings, but, again, the current will be out of phase in two windings.
This gives you the same voltage as the original line to line voltage.
In both connections, all currents will be in phase with each other. This means for a unity power factor load, One phase current will be in phase with its phase voltage, one phase current will be lagging 60 deg. and one phase current will be leading 60%. This applies to delta, to double delta (where a single phase neutral or center tap is needed) and to zig-zag connections.
Because of the current phase shifts in two phases, the transformer bank must be de-rated by 1/3. Your 30 kVA single phase load will require a 45 kVA transformer bank.

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

 

[ijl]

It should not be too difficult just to calculate the currents (or have I missed something?). If you have a Ynyn transformer, then two phases are idle, and the current in the third phase can be calculated in the usual way, from the transformation ratio. But if you have a Dyn transformer, then current flows in two primary phases (not windings), and an additional factor of sqrt(3) must be included.

 

[Skogsgurra]

Yes, I think you are missing the idea with a Scott T transformer.

It converts a three-phase system to a two-phase system with equal loading on all three phases. What I think is possible is to connect the two phases at one point and use the resulting "slanting" voltage (as they call it in the reference) to supply the single phase load.

What I am uncertain about is if such an arrangement will still load the three phase system symetrically. I think it will.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[CJCPE]

It seems to me that if you used a 500:200 transformer with a wye secondary and disconnected the X0 end of X3 and connected it to X2 you would have 230 V from X1 to X3. I think that you would have 115 V from X1 to X0 and X3 to X0.

 

[waross]

Gunnar. I think that you may be able to achieve equal current loading with a Scot connection or some derivation of it. However, I suspect that equal the kW loading may not be equal.
He folks, this is a common connection to derive single phase power from a three phase generator. Either the zig-zag connection or the delta connection results in equal current loading of all three phases but not power loading because of the phase shifts.
The Zig-Zag connection is not related to the connection of the same name used to derive an artificial neutral.
Consider an open delta bank consisting of two 50 kVA transformers on "B" and "C: phases. When you draw vector diagrams of the voltages and voltage drops across the open ends you will find that the two transformers form a "virtual" transformer on "A"phase. This virtual transformer may be replaced with a 50 kVA transformer. When loading a delta bank with a single phase load, consider the transformer bank as a virtual transformer on "A" phase in parallel with a virtual transformer on "A" phase formed by the open delta connected on "B" phase and "C" phase.
It takes two 50 kVA transformers in open delta to develop one 50 kVA virtual transformer. As a result, the overall rating of the transformer bank is 100 kVA rather than 150 kVA.
If you think that the numbers don't add up, you may not have considered the current phase shift relative to the voltages on "B" phase and "C" phase.
Many generators in the range that I work with have nameplates showing a number of ratings and connections, both single phase and three phase.

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

 

[waross]

Hi CJCPE,
You are correct. This is the generator Zig-Zag connection that I have been trying to describe.
It is commonly used to derive single phase,120:240 volts from a three phase 120:208 volt generator. It gives equal kVA lading and equal currents on all three phases (for 240 volt loads) BUT NOT equal kW loading.
The other connection that is used on newer generators is the double delta connection. Many NEMA generators have two 120 volt windings per phase. The windings are series connected for 240 volt delta output. For single phase the windings are connected in two separate 120 volt deltas. If the deltas are A1,B1,C1, and A2, B2, and C2, for single phase C1 is connected to A2 and forms the neutral. Single phase power with a center tap or neutral is developed from A1 through C1-A2 and C2.
Same comments apply, equal currents, equal kVA, UNEQUAL kW.
2/3 times original kVA rating.
If a 100 kW single phase load is applied to a 150 kVA three phase generator or transformer bank, the heating will be based on a 150 kVA load, but the power delivered will be only 100 kW.
There is no need for special transformers (Scott or different voltages). The existing transformers may be used if de-rated by 1/3 or larger standard transformers may be used.

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

 

[Skogsgurra]

I think I was too trigger-happy here.

Bill has the right solution.

Forget Scott and friends.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...