1. Apr 2, 2013

psparky

Let's talk about 3 phase feeders (NO neutral) on a aluminum ladder in a factory. When grounded properly.....when hot wire touches ladder....we pop the breaker. A live cut high voltage line on the ground is another good example....how does it return to the generator at the electric company?

I'll buy that it travels all the way back thru the earth to the power company......but how exactly does it get back into that generator?? Or how does it get back into the magnetic field or coils or generator.

If the generator is grounded that's not going to help because the chassis of the generator is insulated from the windings. Or perhaps they are using a Y configured generator and they just ground the Y similiar to a center tap transformer? Don't know if that makes sense or not.

Someone said the Earth just absorbs it....but I'm not buying that.

I have asked several intelligent people this question....but still no exact answer.

Anyone?

2. Apr 2, 2013

sophiecentaur

So you have a 3phase generator (delta connection). You could connect one of the phases to earth and nothing would happen - the volts two other phases would just be jacked up wrt ground. The currents flowing around the three loads would be the same. No current would, essentially, flow to ground, except due to stray capacity to earth from the lines. It's the same with a floating secondary winding on a single phase supply - connect either side to Earth and it merely becomes a 'neutral' of sorts, with the other side taking on the full AC volts wrt earth.

If, otoh, you have a star (WYE) arrangement, with a neutral, currents will flow through the neutral to the earth at the sub station transformer and through your fault to ground.
In both these cases, the protection circuits should kick in pdq.

3. Apr 2, 2013

TurtleMeister

Unless you are getting your power through a direct connection from the power company (without transformers in between), this would not happen. The earth current would only have to flow to the nearest transformer.

4. Apr 2, 2013

psparky

So on the Delta you simply ground one of the line phases into Earth and absolutely nothing happens except for trickle back current? I guess that would make sense because that live phase has nothing to return to....it effectively becomes the return line.

The WYE is easy to understand.

Cool, thanks, Shoulda known Sophie would know this.

5. Apr 2, 2013

sophiecentaur

But an earth current will only need to flow if there is a PD. With a floating system, there would be no PD, once the infinitessimal current had caused the whole system to 'pivot' about the one phase that has been earthed.

6. Apr 2, 2013

psparky

If you are referring to WYE connection, this makes perfect sense.

What's your thought on the Delta generator or delta transformer?

7. Apr 2, 2013

psparky

Lots of abbreviations to look up today!! :) Had to look up wrt as well!

8. Apr 2, 2013

jim hardy

Well hiya, sparky , been a while!

And I see several other posts appeared - i'm a slow typer...

Remember what is a "Separately derived system" . Same logic applies.

Kirchoff says 'current must get back to wherever it came from'.
So your cut line will pass current into earth and that current will find its way back to whatever transformer winding it came from, the one that feeds the faulted line. It could be the transformer at the power plant or one at a substation somewhere in between.

Our plant's transformer had a wye secondary feeding the outgoing lines, neutral earthed. So a fault on one of those lines would return current to the transformer secondary. Primary and secondary don't exchange current.

That transformer had a delta primary. It was hardwired to generator terminals, generator was wye connected.
But since the fault current we're discussing stays on secondary side of the transformer the fault shows up as just a brief unbalanced load making a current blip on transformer's primary side .

Generator's neutral is tied to earth through a grounding resistor whose purpose is to limit current that'll flow should a ground fault occur in the generator or transformer primary side..
Ground current there is limited by a huge resistor to about ten amps, so as to not allow an electrical explosion at the fault.
It works - once found a wrench left in generator that fell down onto one of the terminals.
Can you imagine how much welding 22KV at 20kiloamps would have done were the machine solidly grounded?

dlgoff has posted some great pictures of grounding. I hope he chimes in.

Anyhow - that's how it works.

old jim

9. Apr 2, 2013

sophiecentaur

rtfm, as they say!

10. Apr 2, 2013

TurtleMeister

My previous post does not apply to Delta or floating systems.

11. Apr 2, 2013

Aussielec

I'm going to take a guess and say that it would be a corner grounded delta supply. I do believe all your protective earthing conductors are bonded to the grounded phase back at the main switchboard, this provides your low impedance fault clearing path.

12. Apr 3, 2013

sophiecentaur

I have not come across a system like that. Why would it be used?
Grounding one phase would mean that single phase loads would be engineered differently, depending on which phases were connected to it - not very satisfactory. It sounds more like a what-if exercise which might be given to a student to suss him out!
The OP is quite clear, I think and we have, sort of answered it. It's worth pointing out explicitly, perhaps, that to get (single phase) power from a delta system, you need to connect between two of the phases. In practice, this would always (?) involve a transformer, of course.

13. Apr 3, 2013

psparky

Sophie, you stated this above:

So you have a 3phase generator (delta connection). You could connect one of the phases to earth and nothing would happen - the volts two other phases would just be jacked up wrt ground.

Then you just sorta disagreed with grounding the corner of the Delta.

It would appear to me that it's the same thing. What am I missing here....please clarify.

14. Apr 3, 2013

psparky

Interesting stuff. The part in bold catches my eye. What if there is a really big fault? 10 amps isn't enough to trip most breakers. Do the transformers have this big resistor as well? I'm guessing no because they wouldn't be able to clear a fault from the big breakers in a factory.

15. Apr 3, 2013

sophiecentaur

I'm not disagreeing. Just questioning whether such a system is used in practice. The PD situation isn't in question, imo.

16. Apr 3, 2013

jim hardy

Ahhh that's the trick - we sense voltage across that grounding resistor and trip on a fraction of that ten amps. That way there's not a lot of arc damage inside the generator.

The in-house medium voltage power distribution is also high resistance grounded and just alarms on a ground. That's so you stand a chance of switching off a faulted piece of equipment instead of tripping the whole plant. Gives you way better plant reliability

Hopefully ground indicating circuit breakers are making inroads.
Nuke plants REALLY need them for 120 volt instrument power(where it's resistance grounded like my plant), and for DC distribution. They'd work just like a household GFCI but indicate instead of trip on ground current. Opportunity ? Every plant I know fights with DC grounds in every rainstorm.