Sizing a neutral earthing breaker 1

[Colinjohnson]

Gents,

Best practice would suggest that a DG Neutral Earthing Breaker would be sized as the Phase circuit breaker but is this really necessary given the low neutral current which will flow in service ?
Are there any factors which would justify derating the device and what might be a sensible rating ?

Your comments would be appreciated.

 

[Colinjohnson]

Scotty, Slavag,

I have to come clean here. I have specified a 1600A Neutral Earthing Breaker (no resistor in neutral circuit) for this occaisionally parallelled 1MVA DG set. The panel builder has not allowed sufficient space for the NEB and is looking for technically sound reasons for reducing the current rating and hence the spacial requirement in his switchboard. I can deal with any earth fault issues but not too sure whether to reduce the current rating given all the harmonics returning via neutral circuits these days. The load has a large VF drive content.

Regards

PS thanks for the file - Extremely interesting reading.

 

[slavag]

Hello Colin.
It's standbay generator? Are you paralleld to grid for the short time? If yes, for my pinion you don't need any neutral disconnection. For short time you can work with two
earthing points.
Regards.
Slava

 

[Colinjohnson]

Hi Slavag,

Yes the parallelling is for a short time at the moment but we hope to secure a permanent grid connection some time in the future so we need to disconnect the generator star point to meet the requirements of G59/1.

Regards

 

[davidbeach]

Unless the neutral breaker is an absolute requirement of some code or standard, i.e. that is the only means of compliance, there are always going to be better solutions for any given situation that don't require the use of a neutral breaker. It sounds like something from a grid operator's perspective with no consideration for the health of the generator. If you have any other options and you are working for the benefit of the generator owner, I would call it negligence to have used a neutral breaker.

 

[waross]

I think that there may be some ambiguity in terms here.
A breaker in the neutral must be sized to carry and possibly interupt the currents that may flow in the neutral during fault conditions. These currents will be some multiple of full load current. Large breaker.
A breaker that connects the neutral point to the ground system is NOT a neutral breaker. You may call it a grounding breaker BUT NOT a neutral breaker.
In a solidly grounded system, the current expected at a grounding breaker or switch may be of a similar magnitude to the neutral current.
In an impedance grounded system, the impedance limits the maximum current and in a high impedance ground system, a ground breaker may need to carry or interrupt only a relatively small current.
If you have a solidly grounded system, the neutral grounding breaker must be expected to at some time carry full line-to-ground fault current. If it is an automatic breaker it must be capable of interrupting full line-to-ground fault current.
But keep trying and I am sure that you will find someone who will tell you that ground faults will only occur less than 1% so you are safe using a breaker that is less than 1% of 1600 amps.
Do get the panel builder to supply a stamped engineering report that the reduced size breaker will be safe.
LOL

Is this the standard that you must comply with?
The main documents for the connection of embedded generation are G75 (for connection of generation above 5MW or at voltage levels above 20kV), G59/1 (for connection of generation that does not fall into these categories).

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

Dear Folks.
Maybe we try think on another way.
I asked several designer, several opinios.
UK standard it's UK standard, but I'm sure possible found additionals. Now, we are EE, we always try found other solution.
1. It's LV 400V
2. According to UK code must have only one (one) earthing/grounding point.
I think it's no problem. Possible connect neutral of generator to LV SWG and neutral of SWG is earthed only in one point.
Regards.
Slava

 

[ScottyUK]

Bill,


I'm sure you didn't mean this:

If it is an automatic breaker it must be capable of interrupting full line-to-ground fault current.

Interrupting??

The neutral earthing switch absolutely must not open under fault conditions otherwise the neutral will not be connected to earth, it must remain closed until the main breaker on the generator clears the fault. That is why the fault rating is so important, and why I suggested that this is an application for a fault-rated switch rather than a breaker.

Why would you need a thermal rating of 1600A on something that does not carry continuous current and only ever sees fault current? Do US codes allow an earthing conductor smaller than the phase conductors, provided loop impedance and voltage rise criteria are met? By the logic in your previous post then all earthing conductors should be sized to the full thermal rating of the main breaker. Maybe they are in the US - I don't know the NEC too well - but my impression was that this is not the case.

Thanks for trying to get some sane terminology into this - I agree it is definitely not a 'neutral breaker'. In my world that would be a breaker in the neutral conductor, and therefore one pole of a four pole breaker, which is entirely different from the function of a neutral earthing switch.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

I agree with you that the earthing breaker should not open under fault conditions.
But, if the breaker may open automatically, possibly from an unintended operation of automatic controls brought on by fault transients and or voltage dips, then it may open on fault current.
I think that we both agree that the device must be fault rated.

Why would you need a thermal rating of 1600A on something that does not carry continuous current and only ever sees fault current?

You probably don't. However a breaker with an appropriate fault and withstand rating may be almost as large. Maybe not.

I can find references to G59/1 but I am unable to locate the actual standard.
If G59/1 allows high impedance grounding I would be considering that solution.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

Hello.
Colin, Scotty, possible see the G59/1 standard/code.
Regards.
Slava

 

[alehman]

Two conditions must be considered:
1. Fault outside of generator. For this case, the output breaker must be tripped and the neutral should remain closed.
2. Fault within generator. For this case, the output breaker and field should be tripped immediately. Once that is done, if there is a grounding breaker it could be tripped to minimize internal damage from ground fault. The grounding breaker must be rated to break available L-G fault current? I don't think the NEC addresses grounding breakers. I think the continuous rating should handle the unbalanced load.

Am I over-simplifying?

 

[davidbeach]

alehman, yep you are over-simplifying. Don't feel bad about it though, I asked a similar question here years ago.

If that grounding breaker opens during a fault condition, and the fault in the generator is arcing rather than solid, as will generally be the case, you will develop very large voltages between the ungrounded stator windings and the grounded stator iron. During a fault it is absolutely important to maintain the ground reference. The ground reference has to allow enough current flow to exceed the current through the capacitance between the winding and the core plus any surge or grading capacitors connected to the generator windings.

Because the generator stator windings remain capacitively grounded even if the grounding breaker opens, you will still have some ground current to the fault, but the overvoltages will cause additional damage elsewhere in the stator.

 

[slavag]

David. Great!!!!
Now we have answers on all Q's. At Alehman's thread, I ask why hybrid system, why not only disconnect resisstor.
Here is answer and in additional I start think about generator energy discharge, that it's possible only via grounding and prefer via high-resistance
Regards.
Slava
Four great threads!!! about neutral grounding