UK embedded generator neutral earthing

[Colinjohnson]

Hello,

I am designing a power system which comprises a 11/0.4kV 1000kVA grid connected transformer feeding an LV switchboard. In addition a 1000kVA generator will be used in parallel with the transformer for occaisional peak lopping (TRIAD operation). To meet the requirements of G59 and in order to avoid circulating currents I am proposing to operate the generator neutral earthing breaker as follows :

When the generator is started the NEB will close and the generator will run up to speed and synchronise but at the moment of synchronism the NEB will open leaving the grid transformer to carry the system neutral. Because the supply agreement if for occaisional paralleling only when the generator is carrying the entire load the grid transformer ACB will open and the generator NEB will close (Island operation).

The problem is in my office the opinion of my colleugues is that the generator neutral should be solidly bolted to earth at all times which would mean that two neutrals would be in circuit when the generator is in parallel.

Am I right and what technical arguments could I advance to win my case ?

 

[motorspert]

In my opinion, you are right. If you earth the two neutrals then you will have circulating third harmonic currents which will overheat the NERs and the generator windings.

If your colleagues ar unconvinced, then there are a numer of power books that may be of use. There may be some papers on the ENA website

 

[Colinjohnson]

Hi,

Thanks for your post. What is the ENA website ?

 

[motorspert]

http://www.energynetworks.org/spring/indexpages/enaindex_ndate.asp

Electrical netwoks association, they used to be part of the CEGB once upon a time, now they issue the G59 G5/4 pubilcations etc

 

[ScottyUK]

One technical argument which they can not argue against under UK regulations is that a protective earthing conductor shall not be used to carry current under normal operation. If the neutrals are earthed at two discrete location then the neutral current will split according to Ohms Law between the parallel path formed by the neutral conductor and the combined impedance of the earthing conductors and the mass of earth between them.

The generator frame must be bonded solidly to earth for safety.

Another technical problem is the behaviour of any earth fault detection which uses a residually connected CT group: any current which passes through the earthing conductor does not contribute to the vector sum of the CTs, which should be zero under normal conditions, causing an apparent earth fault to be detected and tripping the breaker. If this is a critical load that could be inconvenient to say the least.


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

 

[Hoxton]

As always: What is the object?

1) Not to connect two neutrals.
2) To detect a generator earth fault before it is synchronised to the mains.

Just pulled up a single line of a proposed similar scheme, at a UK hospital.
Generator to be mains parallel. Generator neutral not earthed. Protected by a neutral voltage displacement relay whilst starting (I assume it is disabled when the generator is synchronised).

So no generator neutral breaker

System earth is the star point of the incoming transformer at 400V.

The scheme I am looking at has a proposed standby genset as well, which is earthed at its neutral, but interlinked so this can only be connected if the incoming transformer is switched out.

Should be carefully reviewed against the actual site layout.

 

[ScottyUK]

Short term paralleling of a solidly earthed generator primarily designed for standby duty to a solidly earthed network is a violation of the regulations because of neutral current flowing in the earth conductor. Long term paralleling is a violation of the regulations which takes place over a longer period of time.


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

 

[Colinjohnson]

Dear Hoxton,

Thanks for your post. If the generator does not have a neutral earthing breaker how does it run islanded ?

CJ

 

[Hoxton]

Ah!

Your post did not refer to islanding, only "peak lopping" (against the mains?)

If you want to "island" (run in mains failure to support your site load), then you would need a system earth connection, and your generator neutral would probably be best placed to provide this via a neutral contactor.

Note: You do not need a "breaker" since there is no fault current to interrupt in normal circumstances.

Please check the interlocks and sequence of operation. There may be a short period at changeover where the system is earthed at two points. Check with an expert who is familar with your exact system and regulations! It may be better to shut the system down and then start the genset (earthed) rather than have a hot changeover in either direction.

I have seen 3 pole breakers used with all three poles connected up in parallel as neutral breakers. Check the rating of the breaker under these circumstances with the manufacturer.

 

[rbulsara]

Collinjohnson:

Not familiar with UK codes but if this were in the USA, one of the follwings would be a normal set up for this type of application: I can't imagine UK codes differing too much as they both are based on sound engineering principles and meet NEC.

A. The generator neutral is not grounded at the generator, instead the generator neutral would be permanently connceted to the swithboard (tranformer) neutral which is would be grounded. A 3 pole- breaker would be used to parallel with the transformer secondary at the switcboard. This is what is called "non-separately derived source", the generator that is. The netural is grounded at one point only all the time. This is most common and cost effective.

B. If the breaker (or the automatic transfer switch) were to be a 4-pole device which also swithes the neutral, the generator neutral would the be separately grounded at the generator. OR in other words if the generator is kept separately grounded, you need to use a 4-pole breaker.

The scheme as you describe is very unusual, to say the least. I would not do it that way.

 

[davidbeach]

I agree wholeheartedly with rbulsara (except for "B.", more in a moment). Talk of switching the generator neutral seems to be asking for trouble; will you trip the generator if there is ever a discordance between neutral breaker and system configuration? Why not just build the system in such a manner as to not need a neutral breaker in the first place. Bring phase and ungrounded neutral from the service and the generator to the service switchgear and ground the neutral at that point. Use 3-pole breakers and the generator neutral is always grounded but never creates circulating ground currents.

rbulsara's "B." option would work almost as well as option "A." IF there were no possibility of parallel operation with the utility. But since the discussion involves parallel operation it can't be used because with both breakers closed there would be two ground points. The "B." option is worth considering when using breakers to replace multiple transfer switches and it is desired to keep multiple neutrals separated when fed from different sources but does not work with parallel sources.

 

[Colinjohnson]

Dear Hoxton and Rbulsara,

Perhaps I should have explained. The generator has 2 functions.

Firstly TRIAD or peak lopping. At the beginning of a TRIAD period the generator will start and run up to synchronism against the mains with the NEB closed. At the moment of synchronism the NEB will open leaving the mains transformer to provide the system neutral. The generator will then drive the transformer load down to zero at that point the NEB will reclose and the mains breaker will open leaving the generator islanded carrying the whole site load. (The DNO agreement is for occasional paralleling only)
At the end of the TRIAD period the mains breaker will back synchronise to the generator and the NEB will open when the mains breaker closes. The generator will then shed load and at float will open its main breaker and shut down.
The site is an important water treatment plant and bumpless transfer in both directions is essential.

The other mode of operation will be on a complete loss of mains condition when the generator will start and take the site load automatically. During this period the NEB will be permanently closed. When the mains returns the back synchronising described above will take place.

I hope this clarifies.

My original question regarding two earthed neutrals being in service simultaneously appears to have been answered.

1. Circulating currents are prevented.
2. In normal operation the circuit earth is not a current carrying

 

[davidbeach]

Wow. I would certainly not want to go down that type of route; there are way too many things to go wrong. Sounds to me like the kind of installation Murphy's law was written about. How can you get exactly simultaneous operation of the phase and neutral breakers? What happens on an incomplete sequence? Why do the complex with the simple will work better?

 

[rbulsara]

collinjohnson:

I would still stick with my opinion and davidbeach's point is valid as to my point B. Nonetheless those are two configurations permitted by NEC. In your case A applies, I would think.

You will be well served to consult more experienced local electrical engineer for the project.

 

[burnt2x]

collinjohson,

Might be off the OP but I see something that could be trouble here! With different power sources, I see different levels of short circuit available. Are you sure the SC available from the distribution network equal that from your 1MVA generator? If not(sure about this), use multi-function relay with 2 protection groups, one with the logic for supply grid settings and the other group set for your backup genset. I have used a single relay like this with the said option.
Another thing, that "bumpless" transition you've mentioned holds true only when you "manually" initiate transfer from grid supply to genset and back, with the normal supply still there. Complete loss of mains really means a few seconds before genset revs up and comes on line. It is really theoritical to think about transitions that are fully bumpless.
Back to the OP, you are right to propose that scheme but it seems too complicated! Suggest to consider options made by others in the loop. Murphy's law says negative on systems with too many source of errors! KISS it!

 

[Colinjohnson]

Gentlemen,

Thanks for your posts. We seemed to have moved away from the 2 neutrals question to the power system configuration.

Firstly on neutral changeovers; When the generator is being used in the TRIAD mode and becomes sychnronised and the generator breaker closes the NEB will not OPEN until the generator breaker has signalled it is closed. So yes, there will be 2 neutrals in circuit for a few milliseconds. When the mains breaker is about to open and leave the generator islanded the generator NEB will close first then the mains breaker will open. The overlap will prevent voltage fluctuations. (I know I've tried it ).
Again when the mains breaker is about to back-synchronise the NEB will be shed immediately after the mains breaker closes.

On the 'too complicated' question. I have designed and build a similar system for the same client with 6 generators with individual NEBs spread either side of a bus section each side being fed by a separate mains transformer both with on load tapchangers. The station could run Mains - Island, double island, island-island, double mains or mains-mains. It worked very well and apart from the computer battery going down had no operational problems. The NEB system was software controlled with a relay logic back up.

On the question of the fault levels and disconnection times. Yes this is always a problem especially on very remote feeder circuits and special measures may be required. However all this will come out in the protection study I'm sure.

 

[Marmite]

Colinjohnson,
I agree with Rbulsara and Davidbeach that the system sounds unnecessarily complicated. Engineering Technical Report 113, which is the practical guide to the application of G59 paragraph 4.2.1 allows for the generator neutral of a generator used for standby or occasional paralleling to be connected solidly to the Utility neutral earth terminal where there is a HV/LV substation on site (which there is as per your initial description)and waives the requirement for the generator(Customer) to provide an independent earth electrode.A three pole changeover contactor is sufficient in these cases, which is the system "A" described in Rbulsara's post.
Regards
Marmite

 

[slavag]

Only my 0.01$.
I agree with Rbulsara, David and Marmite.
regards.
Slava.