Pole mounted utility transformer

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Re: Pole mounted utility transformer

sparks1:

Something is wrong here. My thinking is that when the primary of a single phase transformer like you are talking about is connected to a three phase system, there is only one voltage applied to the primary and there is only one current going through the primary, it is single phase, and you cannot talk about the two phases being 120 degrees out. And the secondary has no 120 degrees out of phase relationship either. It is a center-tapped winding. The phases are 180 degrees out of phase to each other in respect to the neutral.

BTW there is a such thing as two phase but not in this case.

edit: jim beat me to it...

[ January 26, 2005, 04:13 PM: Message edited by: crossman ]
 
Re: Pole mounted utility transformer

I'm a three phase rookie, so help me out here. I was thinking the same thing, 120? out of phase. But I'm better now. Thy're only 120? out in relation to their neutral.

So now I'm wondering, how can they be 120? from each other in delta?

And if they can be, now again, why can't they be 120? from each other if you ignore one phase?
 
Re: Pole mounted utility transformer

Or should I be looking at it with one phase as the rail?
 
Re: Pole mounted utility transformer

Thank you Jim,

I agree with everything you have stated regarding the three phase Line to line voltage system and relationship.
I was refering to phase voltages or coil voltages with respect to the neutral, not the line voltage.
Two phases don't you get it!

BWT
If the Primary is Delta connected there are two current involved, coil current and line current.

The phase coils are 180 degees out of phase with respect to neutral for single phase.

Time for cold one!
 
Re: Pole mounted utility transformer

If the rated secondary current is 104.166 Amps and the transformer has, lets say 3% impedance, not figuring in the line impedance. This will deliver 3.4722 KA not enough to blow Charlie's fuse of 10KA. But maybe something is missing here?

Charlie, the assumption infinite buss, does this mean a short circuit that has welded itself to the equipment?

If so, would this drive the short circuit current even higher then 3.4722 KA, am I on the right rail here? What is it going to take to blow your 10KA fuse?

The assumptions bolted fault! Is this a ground fault that has welded itself to the equipment and introduces additional impedance?

I don't think its a lighting bolt! Right?
Finally, can I use the quick formula (I Rated /%Z) on a large job to calculate the available short circuit current?

Did I miss anything Phil?
 
Re: Pole mounted utility transformer

The 1? transformers of that size will generally have an impedance of 2% so you would divide 104.166 by .02 to get the projected fault current. That will give you about 5 kA on the secondary side of the transformer or about 165 amperes on the primary side of a 7.62 kV primary. A 15 ampere fuse will let go pretty fast under than load.

The fuse and cutout assemble is rated to handle 10 kA of fault current without blowing apart. This is the same as the amount of fault current your circuit breakers and fuses are able to handle.

When the term infinite bus is used, it means that the primary voltage will not sag under fault conditions and deliver the required amperage at the rated voltage.

A bolted fault is one in which the fault itself has no arcing and no impedance to be introduced into the circuit.

Your formula is correct but the outcome is not. Consideration must be given to the other parameters. We will give you the worst case condition, it is up to you how that information is used. This calculation is great for sizing the withstand rating of equipment but you cant begin to use it for calculating incident energy. :p
 
Re: Pole mounted utility transformer

So now I'm wondering, how can they be 120? from each other in delta?
Click to expand...
Physis, your wondering is well founded.

The phase voltages are 60 degrees out-of-phase with each other, in a delta connection.

If they were not, the phase voltages would not add to 0, and the delta could not be "closed" without creating a short circuit.

Ed
Delta2.gif
 
Re: Pole mounted utility transformer

Thanks Ed,

I was having a hard time visualizing the waves in different configurations. I took out some graph paper and I'm all better now. After drawing it I'm not even sure what I was confused about. :confused:
 
Re: Pole mounted utility transformer

Referring back to the Delta secondary.
Lets say we take phase A for single phase with the center tapped neutral to provide 120/240 volts.
What is the degree of phase angle with respect to this neutral?
Is it 60 degrees?

Charlie,
Your fuses are working fine now! Thank for the feedback.
What is incident energy all about?
Phil
 
Re: Pole mounted utility transformer

Lets say we take phase A for single phase with the center tapped neutral to provide 120/240 volts.
What is the degree of phase angle with respect to this neutral?
Is it 60 degrees?
Click to expand...
No. The term "out-of-phase" should not be used in comparing any of the terminals of a single winding of a three phase transformer. It means displaced in time, or occurring at different times.

Strictly speaking, the term "phase" should not be used to describe terminals at all.
Different voltages can be in-phase or out-of-phase with each other, as can different currents, or a voltage and a current.

The various terminals of a winding, such as the center-tapped winding of a 4-wire delta transformer that you referred to, can have polarity in relation to each other, but not phase.
They are just the terminals of one voltage.

Ed

[ January 27, 2005, 01:10 PM: Message edited by: Ed MacLaren ]
 
Re: Pole mounted utility transformer

The "phase" word.

In a Wye system.
When voltage is measured across a single winding it will read 120v or phase voltage. Line voltage is measured across the lines for 208 volts. In a wye system the line voltage is higher then the coil voltage by the square root of 3. However, phase current and line current are the same for wye. Three Phase voltages are 120 degrees out of phase with each other in a wye or a delta system. Adding voltage vectors of the two 120v two phase voltage values will equal the sum of 208 volts.
If a single phase connection derived from the center tap connection in a delta secondary were measured across the two lines it will read 240 volts. One line to neutral will read 120 volts. The reason for this is the mid point of the two in phase voltages and their vector sum is 240 volts. In a three phase delta system the phase voltage and the line voltage are the same, However to find the line current the coil current is * 1.732 .The term "phase" is confusing. Maybe we should call it three Line voltage instead of three phase.
Phil
 
Re: Pole mounted utility transformer

The term "phase" is confusing. Maybe we should call it three Line voltage instead of three phase.
Click to expand...
Phil, that is your opinion but I suspect that most of us have no problem with the term. I have always thought of the ends of a 120/240 volt coil as phases but they do not make a two phase system. Add another transformer and you can get another phase for a 3? system. They are A? and C? on a 3? system where the higher phase to ground is B?.

This discussion reminds me of a straight angle in geometry. :D
 
Re: Pole mounted utility transformer

Charlie,
The term "Phase" may have derived from the fact that in a Delta their is only one voltage.
Coil voltage or phase voltage and line voltage are the same thing.
But, in a wye the term "Phase" could be misunderstood, line voltage in a wye is equal 120v*1.732 .
The term "phase refers to coil voltage not line voltage, as I understand it, it is the coil to coil relationship. Power is transferred from Primary to the secondary coils irregardless of how the coils are connected.

Should we stick with the term "phase" for line voltage after all its been around for a long time?
Phil
 
Re: Pole mounted utility transformer

OK, the way I use I use the term phase is to talk about phase to phase voltage or phase to neutral voltage. When dealing with a vector diagram, I think of the ends of the vectors as points and call them A?, B?, or C?. I think of conductors as phase conductors or grounded conductors.

I don't know what this does for you but it seems fairly simple to me when I look at it that way. :D
 
Re: Pole mounted utility transformer

Now I'm picturing points on a graph being stationary and the paper spinning. :D :D
 
Re: Pole mounted utility transformer

OK, the way I use I use the term phase is to talk about phase to phase voltage or phase to neutral voltage.
Click to expand...
Me too. I think most of us do. And I have no problem with the term "phase" being used to identify a terminal or conductor, as in Phase A, Phase B, etc.

What gets me going is the incorrect use of the term "out-of-phase", for example, when someone says that the two ends of a single phase voltage source are out-of-phase by 180 degrees. :(

Ed
 
Re: Pole mounted utility transformer

Ed,
A single AC sine wave has 360 eletrical time degrees. Three phase is three sine waves that are each displaced 120 electrical time degrees apart.They are out of phase for this reason. Think of it this way,(3)120 electrical time degrees equals 360 electrical time degrees or one complete sine wave.
In your diagram,you are saying that they are out 60 degree, I think you need to walk a little further. In a straight three phase detla for example, there is no neutral, therefore there is no "with respect to neutral" In three phase it doesn't matter whether it's a Delta connected three phase four wire; straight delta or a wye system they are all 120 electrical time degrees degrees??out of phase. For example,what would happen if you tied A phase and B phase to together, dead short right, this is because they are 120 electrical time degrees out of phase with each other. Now if they were all"In Phase" with each other it wouldn't be three phase, and if you tied A phase and B phase together again not much will happen because they would be "In phase" with each other. In your diagram it shows a simple triangle that has three 60 degree angles that totals 180 degees. That is not a sine wave. Its a triangle!

The 180 degrees means the applied primary current is 180 degrees difference to the secondary induced current.
 
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