Why is delta-wye popular and not Y-Y?

[AggiesNM]

I feel a delta-wye transformer just about complicates everything when it comes to high voltage substation design. Why aren't Y-grnd - Y-grnd the norm in the power system? Only at bulk power EHV do I see Y-Y. Why not in distribution, 138kV and down?

 

[Mbrooke]

If distribution lines are multi grounded neutral with line to neutral connected loads this would cause zero sequence current to pass through to the transmission system which is typically considered a 3 wire system despite the static shield or OPGW.

 

[AggiesNM]

So all this fuss to limit zero sequence currents from appearing on the high side? I feel we can live with this trade-off.(implement high-speed tripping, relocate comms to microwave or fiber to prevent interference et. al.)

 

[stevenal]

Take another look at Mbrooke's answer. It's not about tripping, unless you wish to trip on load. Start stringing load sized neutrals prior to installing the Wye-wyes. I suggest this is the more complicated path.

 

[Mbrooke]

1) You could end up having neutral shift both during T line faults perhaps even during normal operation due to the lack of a low impedance neutral return path (hence why the NESC does not allow the earth alone to be used as a normal current carrying conductor)

2) Its bad enough you have stray current and ground potential voltage differences on the distribution system, do you really want to have the transmission system contributing to this problem?


FWIW, in some parts of the world where distribution is 3 wire (delta-wye 11kv to 230/400Y volts) a Y grounded Y grounded power transformer is actually typical. Similarly in the US there are some utilities which employ Y-Y when stepping 115kv down to 34.5kv 3 wire only sub-transmission.

There are members here who have discussed Y-Y being used in Europe/India in depth. Hopefully they will chime in.

 

[cuky2000]

Here are a few links that may be worth to see:

1) Transformer Connections less Prone to Ferroresonance:

2) Why utilities use Wye-Wye Connected transformer?

3) The Whys of the Wyes