Purpose of Disconnecting The Neutral 2

[Energyfit24381]

Good afternoon,

Im working on a distributed generation application where i will need to disconnect the neutral when switching from utility power to generator power. We have a bonded neutral to ground source (utilty transformer NTRL will be bonded to ground inside of the switchgear it feeds) and the back ATS controlling the gens will be grounded there as well while the gens are equipped with NGR’s that tie into that same common geound. Just looking for help understanding the purpose of having to disconnect the neutral in the event of a utility outage and switching to generator backup.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[waross]

Many standby generator installations have a common grounding electrode to which both the generator and grid neutrals are connected.
This is the simplest connection.
It is a dependable connection and eliminates some failure modes that are possible with other schemes.
BUT
There are exceptions.
If special conditions prohibit a common neutral/ground, then the most used option is to use a four pole ATS and switch the neutral.
That is, when transferring to generator power, the utility neutral to ground connection is opened and the generator neutral to ground connection is closed.
Why?
To comply with the rule (Both codes and good engineering practice) that there may be only one connection between the neutral and ground on a separately derived system.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

It’s not clear to me - is the transformer solidly grounded or is there a resistor? It sounded like you have a solidly grounded transformer and generators with neutral resistors. Do you have line to neutral loads?

It seems odd to me to have resistively grounded generators supplying a solidly grounded system with a neutral nor do I typically see a neutral where resistive grounding is used.

If it’s a resistive ground, no neutral
If it’s solidly grounded, I don’t see neutral grounding resistors on generators.

I don’t understand this system.

 

[Energyfit24381]

@waross & @wcasey thanks so much! The utility transformer is Delta-Y, Grounded. We will be pulling the XFMR neutral to the main swgr where everything will be commonly bonded. (The remote ATS and Gen grounds will tie their grounds to main swgr as well). We do have 120/277 L-N loads for controls. As you stated, we are not running any neutrals from the generator but we do have a neutral bus in the ATS for our 120/277 CPT. The NGR will be tied to the ATS ground bus which is bonded in the main switchgear. Please let me know if that clears it up a little bit. I agree its a lot going on.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[waross]

If you ave line to neutral loads, you must run a neutral conductor and it must be grounded.
Resistance grounding is not acceptable when feeding line to neutral loads.
As I pointed out, the neutrals may have a common ground point or the neutrals may be switched by a 4 pole ATS.
My preference is a common neutral bus and one ground connection from the bus to ground.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

That is an odd setup, and your L-N loads are not going to work very well or at all when you are on the generator, and if there’s any protection using the NGR voltage (59G/64G) the gen will trip (depending on the NGR resistance of course). As Waross mentioned, Having L-N loads without a continuous solidly grounded neutral I’m sure is a major code violation too.

That being said, I am not sure why you’d need to switch the neutral in your case, especially since there’s no neutral from the gen.
The only possible reason I can think of is this: the utility transformer neutral is tied to the utility neutral at the transformer (as they typically are in the US) and the facility wants to disconnect from the utility ground to eliminate stray voltage (elevated electric ground to earth voltage) when on the gen.

 

[Sn00ze]

The neutrals from each source should always be switched to avoid transfer of potential.

 

[waross]

The neutrals from each source should always be switched to avoid transfer of potential.

Have you ever seen the carnage that results when a switched neutral neutral pole is slow closing?
It's called an open neutral.
It is not kind to line to neutral loads.
I have never done that but I have more than once had to make repairs when someone else has done so.
If the neutral's are common and grounded, there is no potential to transfer.
Is this an AI answer?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

There are configurations of how things are grounded and how ground fault protection is being provided that do result in the need to switch the neutral. But that should only ever be done using a four pole transfer switch in which the neutral position on the switch is early make-late break.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Energyfit24381]

Thanks so much guys for the feedback. For clarity i put together a rough sketch of the system so everyone sees the application. Let me know if this clears things up.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[wcaseyharman]

That is a very strange design.

 

[Energyfit24381]

No argument there, i agree. Its not ideal for sure as this is primarily a retrofit im tearing into that was previously designed.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[davidbeach]

Uh, no.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Energyfit24381]

More details please. Im not asking for anyone to redesign this for me but am looking for pointers in the right direction if you can. I’ve torn into this a few times, just looking for extra eyes.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[waross]

I think that we need some additional information.
Is this a standby generator or or you exporting power to the grid?

If you are feeding the grid, there is a problem with the utility transformer.
When you back feed, the delta-wye transformer becomes a grounded wye-delta.
That is a problematic connection.
Utilization people make a clear distinction between the grounded circuit conductor and the neutral conductor.
Transmission and distribution people tend to use the terms interchangeably.
Admittedly most of the issues are on the wye side, but the utility may have a general ban on grounded wye-delta connections.
If that is the reason then they may accept the NGRs or they may demand that there be no connection from the generator neutrals and ground.
If you are feeding the grid you won't have an ATS.
If you are feeding the grid and supplying local loads then both the Main contacts and the Generator would need to be closed simultaneously. Automatic Transfer Switches are designed and constructed so that it is not possible for both Main and Generator contacts to be closed simultaneously.

If that is a standby generator then it is not going to work or be acceptable to the utility as shown.
Your ATS must have the Main or Grid contacts wired between the utility transformer and the main switch.
Your ATS must have the Generator or Standby contacts wired between the generator and the main switch.
Your ATS must have the connection to ground removed. That is the connection near what you have labelled ATS NTRL
There will not be a need to interrupt any neutral connections.
XFRM NTRL
GND.....Should be the only connection to ground.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

And we have not yet discussed islanding provisions if and when the grid fails.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jghrist]

On the sketch, what is the ATS transferring? Normally, an ATS would transfer the load between the utility feed and the generators in the event of loss of utility. I don't see that in the sketch. What is G and M in the ATS?

 

[waross]

What is G and M in the ATS?

WAG....
Generator and Mains.
Which can never be closed simultaneously in a proper transfer switch.
David summed it up well...

Uh, no.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

Well, for starters it doesn’t seem your transfer switch is doing anything and won’t work anyway (mains and gen can’t close at the same time, assuming as waross guessed m is for mains and g is for gen).
You have a solidly grounded source and a resistive grounded source to serve the same load that is supposed to have a neutral with line to neutral loads, but the gens have no neutral.
It appears you can backfeed the grid with a delta bank (no ground reference, can energize with a groundfault) unless there’s interlocks in the switchgear not shown.
With no UV tripping the gens will try to pickup the grid when the power goes out and will hopefully trip on under frequency before they burn up or kill a lineman.

So yes, there a bit of improvement necessary. Switching the neutral is the least of your concerns.

As a start, the transfer switch should go between the transformer and the main (M should to the transformer, g to the gens, and the feed to the switchgear from between M and G in the ATS). Since you have L-N loads the gens should be solidly grounded and include a neutral. Ideally per waross the bonding should be in the switchgear for both sources and the neutral not switched.

That’s my $0.03 (adjusted for inflation).

 

[Energyfit24381]

Thanks so much guys, the feedback is awesome
and much appreciated as usual. I am leaving quite a bit out since i was just concentrating on what condition would trigger the need to disconnect/switch ntrl sources. From the feedback im seeing where everyone is getting hung up. The ATS will work, that part has been vetted. We are not doing a traditional tranformer/Main intercept but we do have CT/PT’s on the Mains (being fed by transformer) to sense loss of utility. That information will travel to the gen controllers to start the gen startup process. Upon that condition the mains will open, gens will start and synch (the “G” breaker will be open during that sequence) and when controller sees all gens on line the “G” breaker will close feeding the normally closed “M” breaker which will then back feed into the main swgr breaker as shown to feed the loads. Again the Main Swgr breaker will remain open during this condition which @waross is island mode. We will be exporting as well though.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-