Connecting 3 phases triangle-star transformer NEMA

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

| 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.

Hello Mr. Quebec :)

I think the connection whereby the primary AB winding corresponds to the secondary an winding, and the primary BC winding corresponds to the secondary bn winding, and the primary CA winding corresponds to the secondary cn winding automatically gives the 30 degree shift you are talking about, due to the vector math.

The primary windings are driven by line-to-line voltage, the secondary windings are connected line-to-neutral. The primary line-to-line voltages are therefore in phase with the secondary line-to-neutral voltages. The secondary line-to-line voltages lag the secondary line-to-neutral voltages by 30 degrees (sketch a vector diagram). Therefore the secondary line-to-line voltages lag the primary line-to-line voltages by thirty degrees. All secondary quantities lag corresponding primary quantities by thirty degrees.

Delta-wye connections giving the 30 degree phase shift are seemingly the most straightforward, that may be why it is standardized on for standard delta-wye transformers.

Other transformers with other phase shifts are available. Delta-delta or wye-wye give no such phase shift. I think transformers called zig-zag or phase-shifting transformers are used to get other phase shifts, though these may be more autotransformers than isolation transformers.

-------------- 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.

|> 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.