2 phases of 3 phase 12kv to 120/240 single phase

[ankledeep]

I should know this, but..... my home is on it's own transformer it's fed with two phases of a three phase 12 KV system. Since the high side of the transformer is fed with two phases 120 degrees out of phase, how does it make 120/240 single phase? the primary (high side) of the xformer never gets "0" volts across it??? the more I think about it the more confused I get!!!

 

[itsmoked]

The 12kV is run thru a transformer.
Just the transformer's primary.
One winding - two terminals.
The two phases fed to the transformer equal one phase across its primary. It has no 'electrical clue' about the phases. They add vectorially and the result is all the transformer sees.

The transformer's secondary is another winding. The step down ratio results in 240V across these terminals. However. This winding is center-tapped which results in 120V between the center tap and either end.

The transformer isolates the house side from the 12kV side. This means you could ground any one of these three terminals and the others would be their appropriate values above ground. By convention the center tap is grounded. This limits the potential between the two end terminals to no more than 120V above ground.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Compositepro]

To clarify further, your house transformer is not being fed two phases of power. You have two wires which have single phase power between them. Three phase power is sent over three wires and there is single phase power between any pair of the three wires.

Your house transformer is a simple single phase transformer dropping a high voltage to 240 volts with a center tap on the 240 volt winding. You get 120 volts between the center tap and either end of the winding. The center tap is is grounded at your service panel and becomes your "neutral".

 

[rbulsara]

What may help is the definition of a single phase circuit.

A single phase circuit has only two wires, one supply and other return, the load is in between the two. The return wire does not have to be a neutral or grounded. So two lines wires taken out of a 3 phase system still constitutes a single phase system, for that part of the circuit.

Three phase system involves 3 hot wires plus a neutral if it is 3ph, 4 wire. or just 3 hot wires and no neutral.

A two wire circuit derived using a phase and neutral of a 3ph, 4-wire system will also be a single phase circuit and so will a two wire circuit using two hots or "phases" or lines.

 

[rbulsara]

So in other words there is no "2 phase" system for all practical purposes.

 

[ccjersey]

The primary (and the secondary) do cross 0 volts 2 times each cycle, This is normal AC electricity, no problem.

If the power co wants to upgrade the capacity of the transmission lines, they may increase the voltage on the same size "phases" and use transformers connected between a phase and neutral. Still single phase, but the transformer is exposed to about 57% of the primary voltage that it would be if connected across 2 "phases". This would allow the use of a cheaper transformer.

 

[BJC]

Your transformer is not fed by two phases of a 12 KV system.
You have a 7.2 KV single phase transfromer. The transformer has "feed thrugh" bushing that leaves your transfromer and goes on to your neighbor. The two bushings are connected directly to each other inside the transfer case. The other side of the transfromer is connected to the bare neutral ( or ground conductor ).

 

[rbulsara]

ankledeep:

On second part of your question:

There is "0" crossing, even between two lines (12 kV). The key is not to confuse the "0" of a sinewave with the ground or neutral potential.

If you put a oscilloscope (of course via proper PT's) between two lines (or "phases" as you say) of 12kV system, you will see a sine wave, just as you will see a sinewave for any other single phase AC source, such as between a phase and neutral. Differences will be just in mangnitudes and phase angle. If you see both waveforms simultaneously on the two channels of an oscilloscope, you will see that the two sine waves are off by 30 degrees, the phase angle difference between line to line voltage and the line to neutral voltage of a 3phase, 4 wire system. But the sine wave will be symmetrical around "0" axis in both cases.

 

[ankledeep]

rbulsara, thank you ..... that is what I was confused about!!! I was thinking of setting up a test with some 120/208 three phase, a couple of PTs and a scope and playing with it to get a clear picture in my mind of what is really going on in this kind of configuration...

Thank you one and all for your answers. It helps stir the mind!!!(and hopefully clear the dust out!!!)

 

[davidbeach]

BJC, There are utilities that run 12kV delta systems and connect distribution transformers to two phase conductors. Then, years later they upgrade to 21kV wye and modify the insulators and connect the transformer line to neutral.

 

[cuky2000]

Check if the enclose reference could help

http://cuky2000.250free.com/Wye_Open_Delta_Open_Rev_b.jpg

 

[itsmoked]

davidbeach; that's exactly what they did in my neighborhood.

Now down the semantical highway:
Hence the transformers were hooked "a" phase to "a" phase initially. Which is two, (2), count'em, II, phases. As I originally stated though, as far as the transformer is concerned, it is still a single phase.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[oldfieldguy]

Somebody's missing the point. In a delta connection,you have only three real potentials, a-b, b-c, and c-a. I fyou connect a single-phase transformer to any two conductors, you've just connected to a single (one only) phase.

With a wye or star system, you have six potentials, a-b, b-c, and c-a, AND a-n, b-n, c-n. Assuming you know your voltage, a single phase transformer can ONLY be connected to one of those SIX pairs, each giving a (Yep! again!) single phase.

It doesn't much matter if you've got a three-phase, two-phase or twelve-phase system: if you only connect to two conductors, you're only going to get ONE (single) phase.

old field guy

 

[itsmoked]

Both statments are correct.

Those three individual phase lines are each a phase if you hook to two of them you are hooking to two of the phase conductors. Your load is between two of the phases. Yes it is a "single phase" load.

Yes an a-b connection between two phases a and b does result in a single phase at the load. I agree.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ankledeep]

OK, NOW we're seeing the big picture!!!!! Thanks Wirechief for the great screen shots of what I was trying to picture in my mind!!!! THIS IS PERFECT!!!!! I hope you don't mind, I will share this screenshot for anyone who wants to see it....
download and open the word document to see the "traces"

http://www.rlfarms.com/2of3phases/

Thanks again