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.

 

[ScottyUK]

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!

 

[Colinjohnson]

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?

 

[ScottyUK]

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!

 

[davidbeach]

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?

 

[cranky108]

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.

 

[dpc]

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

 

[davidbeach]

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.

 

[Colinjohnson]

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 ?

 

[dpc]

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

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

 

[davidbeach]

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?

 

[09041982]

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.

 

[cranky108]

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?

 

[slavag]

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

 

[Colinjohnson]

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

 

[slavag]

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

 

[slavag]

Hello.
It's UK requrements. I found excellent document ( BTW, I looking for it long time).
Please see attached.
Regards.
Slava

 

[waross]

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
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

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!

 

[slavag]

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