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Sizing a neutral earthing breaker

Sizing a neutral earthing breaker

Sizing a neutral earthing breaker

(OP)
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.


 

RE: Sizing a neutral earthing breaker

If a line-ground fault occurs how does the fault current get back to the generator? The contactor or breaker must withstand the fault current long enough to allow the gen breaker to clear the fault. If the generator is part of a group in which one machine (the first one up & running) provides the neutral earthing for the group, consider that fault contribution from all machines will pass through the earthing contactor / breaker of one machine.

The neutral breaker must not have settings which would cause it to open ahead of the main output breaker. Really you want a fault-rated switch (i.e. a breaker with no tripping unit). Watch for instantaneous trip levels - that last thing you need is a large fault exceeding the instantaneous trip point of a small breaker and falling just short of the same setting for the larger main breaker.
 

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

RE: Sizing a neutral earthing breaker

(OP)
Scotty,

Thank you for your post. I appreciate the withstand capability of the breaker should be as you describe but my question related to the service rating of the breaker under normal operating conditions.
For example for a 1MVA generator would the rating be 1600A?

RE: Sizing a neutral earthing breaker

In my opinion it is not necessary to size it for full thermal rating because protection on the genny should detect an earth fault and open the main breaker and trip the set long before the thermal rating became an issue. I can not find any reference in any UK code listing requirements, but I don't see why it would be necessary for it to have greater thermal rating than the main earth conductor provided it can withstand a worst case fault long enough for the protection to clear. In the case of a contactor as opposed to a breaker the fault withstand seems to dominate the selection process.
 
With a breaker another possible problem if it uses a motorised actuator could be that the actuators are usually slow to open the breaker in response to a demand which may allow current to flow in the parallel paths of earth and neutral for long enough for the E/F protection to operate.

Anecdotal: we have a couple of 250kVA generators on hire at the moment which use a conventional contactor with the poles paralleled as the neutral earthing device. The earthing contactor looks to be a 37kW AC-3 type. I'm not commenting one way or the other on this design, other than to say it presumably meets the code requirements in Italy where it was made. Draw you own conclusions whether it is a sound design or whether Italian electrical equipment has the reputation it deserves!
 

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

RE: Sizing a neutral earthing breaker

Why on earth would you even consider putting a breaker in a generator neutral?  That is just asking for trouble.  The generator should be grounded and should remain grounded at all times.  Why would you want to switch the neutral, and what happens when the neutral switch is in the wrong position?

RE: Sizing a neutral earthing breaker

davidbeach, I agree.

But on the same note, I would like to hear why and under conditions people want to switch the neutral?

I might actually learn something from this.

RE: Sizing a neutral earthing breaker

I've seen neutral switching done on multiple generator installations where each generator has a grounding resistor, but only one is used at time to limit circulating currents through the grounding resistor and to limit ground fault current.  

Never liked the approach myself.  

The new IEEE hybrid grounding method for generators uses a contactor to switch between high-resistance and low-resistance grounding, IIRC

RE: Sizing a neutral earthing breaker

At least with the hybrid grounding system the generator is always grounded.  The hybrid system is used on systems that need to be low impedance grounded for protection out on the system to work properly; a high impedance ground and a low impedance ground are in parallel with each other.  When the generator breaker trips, a switch in the low impedance ground path opens leaving the generator high impedance grounded to limit ground current into a winding fault.

Any system of switched neutrals where the neutral can be open risks an ungrounded generator (actually capacitance grounded) and transient overvoltages for certain fault conditions.

RE: Sizing a neutral earthing breaker

(OP)
DPC,

To comply with G59/1 it is necessary to open the generator neutral earthing breaker when operating in parallel with the grid so as to prevent circulating currents. The breaker becomes closed when the generator is islanded.
Now, could we stick to the point here. What factors (apart from earth fault scenarios) would dictate the rating of a NEB ?

RE: Sizing a neutral earthing breaker

When the advice is free, you have to take what you get.   

Just fyi, there is no such requirement in the US.  



RE: Sizing a neutral earthing breaker

Why not just run the neutral from the generator to the point it connects with the rest of the system and use the system ground to ground the generator?  That way you have a grounded generator that is always grounded and no worries about a grounding breaker that will ultimately be in discordance with the utility breaker?

RE: Sizing a neutral earthing breaker

To the basics:

The Breaker is supposed to put there, where there is a chance of short circuit which is flow of current at its maximum. The main purpose of any Breaker is to clear the short circuit. Putting it on neutral is not standard.

Please explain why and what you want to learn from this non-standard practice.

RE: Sizing a neutral earthing breaker

Most of the larger generation that I've seen uses a wye/delta transformer as a step up which prevents system zero sequence cruuents from being passed on to the generation. Thus most of the larger generation that I've seen is high impedance grounded.

Switching the neutral sounds like a good way to leave a generator ungrounded. So with the problems/risks why would you do that? What system configuration would warrent such an installation?
 

RE: Sizing a neutral earthing breaker

Hello.
I assume it's some UK ( maybe others too) standard for the generator's direct connection to the grid. If I understand right Scotty's posts: not needed open this CB by protection trip. It used for islanding-grid paralleling modes only. I see applications with grounding switch in the neutral of transformers, generators instead CB. Grounding switch used for provide logic: only one grounding/earthing point in case of paralleling devices.
Regards.
Slava

RE: Sizing a neutral earthing breaker

(OP)
Mr Slavag,

You got it! Now how would you determine the rating of the switch you describe ?
Take neutral cables for example. It is common practice to install half size neutrals from a transformer or generator star point. Could this logic be extended to the rating of a neutral switch ? What do you think ?
(Lets forget about fault current for now)

Regards

RE: Sizing a neutral earthing breaker

Hi.
I assume, you connect generator star point to grounding/earth via resistor ( low impedance/resistance). For example, resistor for 300A 10sec. For my pinion you can used grounding/earthing switch for the 300A or lower and phase to earth voltages. I'm not sure, but I think your option is also good option, neutral is half size and switch also half size of main breaker.
Regards.
Slava

RE: Sizing a neutral earthing breaker

Half size neutrals would be sized on the basis of withstanding the maximum possible fault currents.
A neutral switch may be sized on the basis of the maximum possible current. If the current is limited by a grounding resistor it may be sized on the basis of withstanding the maximum current through the resistor under all conditions. Remember that with some transformer arrangements, switching transients may develop up to twice normal voltages and currents through the resistor. But with a limiting resistor the total current will still be quite small in comparison to the load current.

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

RE: Sizing a neutral earthing breaker

Colin,

Under normal operating conditions the neutral-earth bond does (should) not carry any current, so I am not sure why there is any need to look at the thermal rating. Thermal ratings normally only apply to equipment which is continuously carrying current, or at least is carrying current for a period much longer than the thermal time constant of the equipment so it reaches thermal equilibrium with the environment it is in. The only time current will be present in the neutral-earth bond is under fault conditions which should be for a predictable period of time given a known fault current. Are you making this more complicated than it needs to be? I'm trying to follow your reasoning for looking at the thermal rating.
 

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

RE: Sizing a neutral earthing breaker

Scotty, you are right, under normal condition grounding/earth current will be about "zero". Thermal rating isn't important. So, what will installed 10A, 50A or 300A. Must be some rule of thumb. Q is where.
Regards.
Slava

RE: Sizing a neutral earthing breaker

(OP)
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.

RE: Sizing a neutral earthing breaker

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

RE: Sizing a neutral earthing breaker

(OP)
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

RE: Sizing a neutral earthing breaker

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.

RE: Sizing a neutral earthing breaker

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

RE: Sizing a neutral earthing breaker

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

RE: Sizing a neutral earthing breaker

Bill,


I'm sure you didn't mean this:

Quote:

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.
 

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

RE: Sizing a neutral earthing breaker

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.

Quote:

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

RE: Sizing a neutral earthing breaker

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

RE: Sizing a neutral earthing breaker

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?

RE: Sizing a neutral earthing breaker

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.

RE: Sizing a neutral earthing breaker

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

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