Triangle-star 3-phases transformer NEMA

[Steve & Julie]

Hi,

I wonder why NEMA standard tells to connect a triangle-star 3-phases
transformer with the secondary is lagging the primary. I know the phase
difference is 30 deg but why lagging not leading? Could it be connected so
it could be leading anyway?
Anyone has an explanation?

Thanks

Steve Quebec, Canada

 

[[email protected]]

| I wonder why NEMA standard tells to connect a triangle-star 3-phases
| transformer with the secondary is lagging the primary. I know the phase
| difference is 30 deg but why lagging not leading? Could it be connected so
| it could be leading anyway?
| Anyone has an explanation?

A single phase load with 0 phase angle (pf 1) on the secondary will result
in current on 2 lines coming in. If the source transformer upstream is
also a D-Y or T-S, then the current on those 2 lines will be 30 degrees
off of the voltage that is relative to neutral. But one will be lagging
and the other will be leading.

If you have 3 such single phase loads, evenly distributed over each of the
three phases, all those currents will sum up to being 0 degrees phase angle
and the end result will be the same angle as the load. So a balanced three
phase load won't be an issue.

You get as much leading as you get lagging and it cancels out when all the
phase loads are the same.

 

[Don Kelly]

----------------------------

Hi,

I wonder why NEMA standard tells to connect a triangle-star 3-phases
transformer with the secondary is lagging the primary. I know the phase
difference is 30 deg but why lagging not leading? Could it be connected
so it could be leading anyway?
Anyone has an explanation?

Thanks

Steve Quebec, Canada

--------------
It's a convention. A bit like having cars in North America drive on the
right side of the road, rather than it being the individual driver's choice.
If all follow the same convention, life is easier and there are fewer
accidents. Such useful conventions become standards.
It has nothing to do with the phase angle between voltage and current in any
phase on either side of the transformer but is of concern in protective
relaying.

 

[[email protected]]

| ----------------------------
| |> Hi,
|>
|> I wonder why NEMA standard tells to connect a triangle-star 3-phases
|> transformer with the secondary is lagging the primary. I know the phase
|> difference is 30 deg but why lagging not leading? Could it be connected
|> so it could be leading anyway?
|> Anyone has an explanation?
|>
|> Thanks
|>
|> Steve Quebec, Canada
|>
|
| --------------
| It's a convention. A bit like having cars in North America drive on the
| right side of the road, rather than it being the individual driver's choice.
| If all follow the same convention, life is easier and there are fewer
| accidents. Such useful conventions become standards.
| It has nothing to do with the phase angle between voltage and current in any
| phase on either side of the transformer but is of concern in protective
| relaying.

So which is the best to drop MV distribution voltage to LV delivery voltage
and/or utlization voltage? D-Y or Y-Y?

I'm anticipating "it depends" as the answer.