Good afternoon all.

I have just got off the phone with an engineer who is telling me that the neutral of a generator is being switched to open when in parallel with the mains.

We have mains incomer @ LV 4 pole ACB
Generator incomer @ LV 4 pole ACB
Load connected in between.
The set can operate in island and in parallel.

In island the neutral (not neutral earth connection) contactor remains closed. However when run in parallel the contactor opens.

It is not a neutral earth connection that is being switched but the neutral itself.

Therefore when in parallel with the mains this generator is connected to it with its neutral disconnected.

Also the generator neutral is bonded to earth directly in the alternator.

Does this seem normal to you guys?

Normally in my mind we have only one star point connected to ground when in parallel. Usually the mains neutral earth connection remains and we open the generator neutral earth connection.

Any questions or ideas?

Thanks for your time.

 

during parallel operation, any one of the generator should be neutral grounded. This is to maintain the earth fault current limited to its designed value.

If you ground the neutral of two generator or more, in case of any earth fault, the earth fault current may be double or more which may be detrimental to the system and cater a capital equipment loss.

Another reason is to avoid circulating current between generators.

 

Thanks for your time in replying.

Maybe I didn't explain it to you correctly. It is not the connection to ground that is being switched! It is the neutral. The connection to ground is solid.

The neutral terminal exits the alternator. It is then switched by a contractor. It then continues to the acb. Where it then joins the mains acb. We are talking about losing the neutral connection from the alternator onwards.

I'm aware that we only maintain 1 star point connection when running multiple generators together. But we only have one in this scenario anyway.

My question again is if parallel with the mains and the NEUTRAL is lost. (Because it's being opened) star point of alternator still grounded. Where will the imbalanced load current go?

It just does not seem right to me to open the NEUTRAL. (Not neutral earth connection)

Cheers.

 

Dobbs...

Search Control.com Archive for "Neutral Interruptus".

Regards,
Phil Corso

<b>Moderator's Note:</b> Enter the following in the search box:
+"Neutral Interruptus"

The quotes and + are part of the search term.

 

Thanks for that Phil.

Had a quick look found a few posts from yourself. So you agree. Open neutral unbalanced load not good.

Regards

 

Doobs...

Can you provide a hand-drawn sketch or a Single-Line-Diagram detailing your system layout?

Phil ([email protected])

 

Please elaborate your question clearly.
Neutral is coming out of alternator and opened at contactor.
Are you grounding it separately?

If not then how can you tell that it is connected to ground?

 

Doobs...

are you referring to a situation in which an engine-driven- generator is paralleled with the utility source, via an Automatic-Transfer-Switch (ATS)?

Phil Corso

 

> Please elaborate your question clearly.
> Neutral is coming out of alternator and opened at contactor.
> Are you grounding it separately?
> If not then how can you tell that it is connected to ground?

Right let me try to explain it again. And thanks for your time.

One switchboard with two acbs fitted.
One is the mains incomer from a local transformer. It is a 4 pole acb. The transformer star point has a solid star point connection to ground. This is all ok.
The other acb is the generator incomer. This is also a 4 pole acb.

The system can operate in island mode in the event of a loss of mains supply. If a mains failure occurs the mains acb opens the generator starts and its acb will close. It will remain like this for the duration of the mains failure. (i guess you know all this).
The system is also setup to export generator power back to the grid.
The generator is 3p+N. The star point of the alternator is connected to ground within the alternator. A cable link connects the neutral (star point) to an earthing point. This earthing point is connected to an earth rod. It also connects to all other external metal work. Including the star point of the mains transformer. So it is grounded at the star point.

Now then..

The neutral cable that leaves the alternator does not go straight to the generator acb in the switchroom. it goes to a contactor first and then onto the switchboard. At the switchboard it connects to the generator incoming 4 pole acb (neutral terminal). This contactor opens when the generator is running in parallel with the mains supply. This cannot be right. Whoever installed this is opening the neutral. Not the neutral earth connection. When the generator runs in parallel we lose the neutral. An earth fault current can then be seen to flow as the unbalanced load flows on the earthing conductors to get back to the star point of the alternator.

Does this make more sense?

Thanks again.

 

Hi Phil.

>Can you provide a hand-drawn sketch or a Single-Line-Diagram
>detailing your system layout?

I have forwarded a very poor sketch to your email. Also to answer below yes it is an ats but not interlocked to prevent both acbs from closing at the same time.

Once you see the drawing you will see the placement of the contactor on the right. IMO it should be installed between the neutral and the earthing connection. Not in the neutral conductor itself.

The imbalance in the load is causing current flow on the earthing conductors.

Am i right?

Thanks

 

Doobs...

thank you for your prompt reply. And, Yes, the sketch shows that with both sources closed, the neutral (to the load) is isolated.

This is the classic case of "Neutral Displacement". That is, Phase-to-Centroid (point at which Y-loads are connected) will vary for each phase. The variation, depending on the degree of unbalance, can result in abnormal voltages across the ph-neutral loads!.

I sent you a sketch illustrating this phenomenon. If others want the sketch, contact me.

Phil Corso

 

Doobs...

further to my earlier post, details can be found in the Control.Com Archives, specifically:

o http://www.control.com/thread/1026242921

Phil

 

Thanks Phil.

Without the earthing conductor on the drawing that you can see toward the bottom of the drawing the neutral or should i say path back to the star point would be lost totally. No doubt resulting in the situation you described above.

Thanks for the email back its been a while since i used a vector diagram to understand an issue.

Thanks to all!!

 

Doobs...

after further review I find the existing arrangement will not be exposed to the Neutral Displacement phenomenon because it insures that one of the two sources, i.e., the Gen'r or Utility source is "always" connected to the load's NEUTRAL-BUS.

However, there is a problem related to Ground-Fault Duty, as well as Ground-Fault Protection Coordination. It arises because even though the gen's Neutral is isolated from the NEUTRAL BUS, it's still connected to the GROUND-BUS, and so is the the Utility source.

Phil

 

Haha.

Exactly what i wanted to hear.

The original problem is (i left it out on purpose) is that the ground protection on the generator circuit breaker is operating. Its residual type and balances current of L1,L2,L3 and N.

Because the unbalanced current has to find its way back to the star point of the alternator it has to return via this earth connection also via the mains n/e connection. If that makes sense.

Ive just been told the currents will balance themselves out because we are in parallel with the mains? How so?

I thought current (diesel generator generated) had to return to its source? Regardless of being in parallel. Unless there has been some new changes in physics that i am not aware of?

Cheers.

 

Doobs

you have an interesting sense of humor! I wish you had mentioned the relaying problem first. Replying to your comments:

a) When a ground-fault occurs, and the neutral of one source is grounded, then the fault-current's return path will be to that source, regardless of how circuitous the path.

b) If the neutrals of both sources are grounded then there is a division of current-flow inversely proportional to the impedance from each source to the fault location.

c) For case b), the action of the GFP (Ground-Fault-Protection) sensor(s) will be erratic, causing false (unwarranted) trips.

d) False-trips can also occur, even if there is no evidence of a fault, because of system's capacitance-to ground magnitude.


e) The 4-pole ACB arrangement is supposed to ameliorate the problem, but in your case it's incorrectly applied.

f) A second reason for your problem is (regardless of the 4-pole ACB app) the neutrals are paralleled because both are connected together via the GROUND-BUS (or block) which also leads to a circulating-current, and harmonics.

Question to you: was the addition of the 5th-pole contactor to the gen's neutral, an afterthought? If so it's misapplied.

Lastly, FMI, can you provide the Capacity and % Imp of both the Utility Xfmr, and the Gen'r?

Phil

 

Doobs... correction to my earlier post:

b) If the neutrals of both sources are grounded then there is a division of current-flow inversely proportional to the impedance between the each source and fault location.

 

Thanks Phil.

Its not so much the earth fault current that bothers me (although it is an important function).

Its the fact that when we run in parallel with the scenario we have the owners of the installation say that its okay to have an open circuit neutral. they think that with it in parallel and the star point of the TX earthed (forget that the alternator is earthed for a moment) that because the alternator is in effect connected to a star winding (TX) that the imbalanced load will right itself and not have to return to the star point on the neutral conductor.

In other words they are telling me the imbalance won't have to return to its source as the mains tx will "sort it out". The other explanation i have had is that the mains will feed the imbalance. (even tho we are exporting and in effect site is supplied from the gen.)

All very confusing :-/

Yes i've been told before my sense of humor is odd! Haha.

Cheers

 

Doobs...

hopefully you are immune from future litigation should there be a mishap.

Phil

 

Dear Doobs,

if you don't mind can you forward me the SLD at "[email protected]".
I am a little bit confused still.