Quick three phase question

[La Poste]

Greetings.

I apologise if this is a bit of a daft question but I don't know much about three phase.

In a three phase system there is only current in the neutral if there is an imbalance of the phases right?

So looking at the transformer that supplies our street it is a three phase transformer that supplies various houses on different phases.

Each house has a line and neutral so does the above principle mean that the neutral cable in my own house will be carrying less than the phase?

If the current in the neutral cable is a result of an imbalance between the phases then does this principle apply to houses that are supplied from the same three phase transformer?

Any opinions appreciated.

Thanks.

 

[ferg]

you are only using 1 of the phases in your house so there will be no other phases to cancel each other out hence the neutral current will not be lower. probably a poor way of explaining it but hopr it helps

 

[Silly Sausage]

Greetings.

I apologise if this is a bit of a daft question but I don't know much about three phase.

In a three phase system there is only current in the neutral if there is an imbalance of the phases right?

A perfectly balanced load, eg a motor, no Neutral Current, hence no N conductor.

So looking at the transformer that supplies our street it is a three phase transformer that supplies various houses on different phases.

Each house has a line and neutral so does the above principle mean that the neutral cable in my own house will be carrying less than the phase?

If the current in the neutral cable is a result of an imbalance between the phases then does this principle apply to houses that are supplied from the same three phase transformer?

Any opinions appreciated.

Thanks.

As for the rest, I assume, house will just be spurred off the the street cable, so your house L & N currents will be equal.
Overall I should imagine there will be an imbalance across the 3Ps, so there will be a N current.

Then you've got Harmonics!!!

At this point I'll gracefully bow out and leave it to the Forum superiors!

 

[RogertheBodger]

Not sure what you are asking here La Poste, but if you have three houses in your street, each supplied from a different phase, the load at each house would have to be identical to have no neutral current at the 3 phase transformer. the loads would be different of course, how you would calculate the neutral current at the transformer, especially if you had a long row of houses is a whole another question.

 

[Knobhead]

Your house will be 1 phase and neutral

View attachment 13161

 

[Knobhead]

Not sure what you are asking here La Poste, but if you have three houses in your street, each supplied from a different phase, the load at each house would have to be identical to have no neutral current at the 3 phase transformer. the loads would be different of course, how you would calculate the neutral current at the transformer, especially if you had a long row of houses is a whole another question.

Neutral curent is given by:

In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

 

[Moses]

Your house will be 1 phase and neutral

View attachment 13161

Tony is that the pyramid of the threefaces ?

 

[Silly Sausage]

Neutral curent is given by:

In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

Tony,

email me with the proof of that, please.
I just can't work it it out!

Complex notation for me every time

 

[Knobhead]

Tony is that the pyramid of the threefaces ?

The lines in red are the phase to neutral voltage, black are phase to phase.

 

[RogertheBodger]

Neutral curent is given by:

In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

Ah yes, Thanks Tony, I think I spilled the inkwell over that page in my schoolbook, have to write it down again. lol

 

[Knobhead]

Tony,

email me with the proof of that, please.
I just can't work it it out!

Complex notation for me every time

It's in the XLS sheet I sent you. At least I think I sent it. If not PM your e-mail address.

 

[Moses]

The lines in red are the phase to neutral voltage, black are phase to phase.

as opposed to pyramid of Tutankhamen!!! LOL

 

[Silly Sausage]

It's in the XLS sheet I sent you. At least I think I sent it. If not PM your e-mail address.

I never did. PM en route...

 

[Knobhead]

Sent..

 

[La Poste]

Thanks people that's food for thought.

As for the rest, I assume, house will just be spurred off the the street cable, so your house L & N currents will be equal.
Overall I should imagine there will be an imbalance across the 3Ps, so there will be a N current.
Then you've got Harmonics!!!

So there are two elements to this neutral current, one is the neutral current from each house which will be equal to the phase current as they are single phase and the second element is a neutral current due to imbalances of the three phases that you call Harmonics.

Would it be fair to say that sometimes the current in my neutral will be larger than my phase current?

I'm in dark territory here but it bugs me that I don't understand.