[theThinker (OP)]

I assume the single phase substation taps power from one of the 3-phase transmission line. One terminal of the primary connects to the transmission line. Does the other terminal need to be earthed ?

[Bored chemist]

No, but it's safer that way, and easier in practice.
The single phase 230 volt power delivered to my house is actually one line from a 3 phase system. My neighbours are on the other two phases.
Light industry is supplied with all 3 phases via the same system.

[theThinker (OP)]

Thanks.

I have just read a little about Wye Delta connection and have a slightly better understanding. 

[theThinker (OP)]

Some related questions:
1) Assume the power station generator is Y connected. Can the neutral point be earthed?
( I think it would be a "fault" condition).
2) Assume we take out the iron core in the step-up transformer - 30kV to 300kV. If we try to run the system with this air core, what will happen?

[vhfpmr]

Some related questions:
1) Assume the power station generator is Y connected. Can the neutral point be earthed?
( I think it would be a "fault" condition).

It's not a fault, an earthed system needs to be star connected so that the star point provides a common earth for all phases. In a delta system you'd have to earth one of the lines, in which case you'd end up with one phase live at both ends.

2) Assume we take out the iron core in the step-up transformer - 30kV to 300kV. If we try to run the system with this air core, what will happen?

The magnetising current will go though the roof because you've lost most of your inductance, and the leakage inductance will go up because you're losing a lot of your coupling.

[Bored chemist]

Can the neutral point be earthed?

Yes
https://en.wikipedia.org/wiki/Three-phase_electric_power#/media/File:3_Phase_Power_Connected_to_Wye_Load.svg

If we try to run the system with this air core, what will happen?

Something will fail. If you are lucky, it will be a fuse.

[theThinker (OP)]

Some related questions:
1) Assume the power station generator is Y connected. Can the neutral point be earthed?
( I think it would be a "fault" condition).

It's not a fault, an earthed system needs to be star connected so that the star point provides a common earth for all phases. In a delta system you'd have to earth one of the lines, in which case you'd end up with one phase live at both ends.

2) Assume we take out the iron core in the step-up transformer - 30kV to 300kV. If we try to run the system with this air core, what will happen?

The magnetising current will go though the roof because you've lost most of your inductance, and the leakage inductance will go up because you're losing a lot of your coupling.

1) I don't understand. It seems connecting the neutral to earth is like a short circuit. When the generator runs, current will leak, supplying to earth.  Even without the earth, the generator will still work. The neutral is at 0 potential, but a "zero" is only relative to all the line voltages and should be OK.
2) I think correct. With nearly no inductance, the stator coil may even explode. 

[Bored chemist]

When the generator runs, current will leak, supplying to earth.

That is like saying the current will leak along the wires.
It isn't a leak if you want it to happen.
If you have unbalanced loads, you need a neutral wire for the out-of-balance current to flow through

You can use the Earth instead.

[theThinker (OP)]

When the generator runs, current will leak, supplying to earth.

That is like saying the current will leak along the wires.
It isn't a leak if you want it to happen.
If you have unbalanced loads, you need a neutral wire for the out-of-balance current to flow through

You can use the Earth instead.

Thanks. I think I got it now. It is AC,not DC circuit.

I think the neutral of Y for transformer/generator could be earthed - or should be earthed.

[evan_au]

3-phase AC electricity was invented by Nikola Tesla, and it is still the dominant form of delivery around the world.

There are two main ways of organising 3-phase electricity delivery:
- "Y" or "Star": This has a central point, which is usually earthed. Star connection is usually used for domestic distribution, as the earth gives additional safety in case of accidental human contact.
- "Δ" or "Delta": This has no central point. It is more often used at higher voltages, and in long distance transmission. In these cases, Delta is not earthed, as it would require bigger insulators on the other two phases, and would cause unbalanced currents in the 3 phases*. It normally connects to a Star at the other end of the high-voltage line, which is earthed

*If you watch the arrangement of the 3 wires on long distance transmission lines, you will see that over several kilometers, they rotate the 3 wires, so that each phase has a turn being closest to the physical ground, and hence an equal capacitance to ground. [Do not try this if you are the driver!]

Most countries with 220-240V AC distribution put a distribution transformer in the street, serving many houses.
The USA and Canada, because of their lower distribution voltage (110V AC) suffer from higher resistance losses, so they  usually put a step-down transformer outside each couple of houses, producing 2 phases of 110V (earthed) + 220V (not earthed).

See: https://en.wikipedia.org/wiki/Delta-wye_transformer