Providing a neutral grounding point for 11kV generators

[digitrex]

Hello,

I have an offshore installation with an 11kV bus, like this:
- Five 11kV generators connected to the 11kV switchboard, 4nos 2.7MW and 1no 4.2MW, installed at different time.
- All generator neutrals are brought to a switching (contactor) panel so that only one neutral is grounded.
- The NGR at the neutral switching panel is 31.75ohm, limiting ground fault currents to 200A.

Now we have a plan to add 3 more generators. Unfortunately I am coming to a problem with the neutral switching panel because:
- there is no isolation switch to isolate half of the switching panel for extension to add more generator neutrals.
- I need to shutdown all the generators in order to make the neutral switching panel safe for extension, which I need to avoid.

I am considering 2 options to overcome this:

(1) Install a new neutral switching panel so that the existing neutrals can be transferred over to the new panel in stages.

(2) Eliminate all the generator neutral groundings and replace by two nos of earthing transformers that can be used to supply power generator auxiliaries. This means that the 2nos existing delta-star transformers to aux switchboard are to be replace by 2nos zig-zag transformers. The neutral grounding for the 11kV system will be provided by the 2nos zig-zag transformers.

Option (1) is possible.
Option (2) is my idea of avoiding Option (1) in order to avoid searching for new spare spaces for the new switching panel. Moreover, I won't have to worry about how many new generators to be added in the future.

I am not sure whether Option (2) is a good idea and I would like to have your views.

 

[davidbeach]

Why not just put a grounding resistor on each generator? I've never understood why go through the complexity of switching neutrals into a grounding resistor when you can apply a resistor for each generator and not worry about switching.

 

[trosepe]

I agree with David; can't think of any good reason to switch the generator neutrals to a common resistor. The individual resistor for each generator is simple, effective and from my experience is more common. The changing system zero sequence impeadance, which has been used to defend the single resistor system, was managed with electro-mechanical protective relays 40 years ago and can certainly be managed with todays protective wonders.

 

[slavag]

It's depend.
Actually it ( used one resistor)reduce GF level.
In lot of applications ( specialy offshore generators) is used only one resistor/grounding-neutral point per system.

digitrex , is very difficult say something.
Zig-zag is good idea too.
You can isnstall new generators with switch (grounding switch)and resistor, for every one and add some logic for rgulation of number of connected resistors in the system.
For my pinion, it's more simple, then change now exsisting installation.
Good Luck.
Slava

 

[dpc]

I prefer a dedicated resistor for each generator, but there are pros and cons. The single resistor approach is apparently much more common outside the US.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[nawao]

A common NGR allows to monitor GF level at only one sole and lonely earthing general system point in order to activate selective GF protection. Because this common resistor is the 'zero sequence generator', its tripping will be delayed. (Probably 4th time step)

 

[trosepe]

The single resistor makes for a simpler protection scheme but creates a single point of failure. One of the primary goals of a power distribution design is to eliminate all single points of failure possible. In my opinion the neutral contactor also represents an unneccesary failure point. Circulating neutral current should not be an issue assuming all generators have same winding pitch.

 

[digitrex]

Thank you very much for your views.
I was away for a few days and only able to respond today.

I am weighing the different options to see which is the better for the existing site, bearing in mind that spaces at offshore location is scarce and precious.

I was hoping for more comments from you folks on my Option 2.

Now I have 3 options to be considered:

(1) Installing a new neutral switching panel and transfer existing generators’ neutral to the new panel. This will need to be done at stages and each generator will be shutdown for the transfer work.

(2) Install zig-zag transformers to provide fixed neutral groundings. Existing generators’ ground fault protection relay settings will be adjusted/reduced to the lowest possible. Each of the 2 existing distribution transformers will be shutdown for changeout to zig-zag transformers.

(3) Install individual NGR for each generator, including the existing units. Each of the existing generators will be shutdown for the NGR installation. Ground fault protection relays of the existing generators and feeders will be reviewed to accommodate the variable ground fault levels depending on no. of online generators.

Of the 3 options above, I think Option 2 has less interfacing works with the existing running plant, eventhough it would involve lifting/installation of heavier equipment.

These are my views, I will be glad if you can give me advises/comments on Option 2, something which I need to consider.

 

[slavag]

Hi digitrex.
You can go to option2 w/o any problems.
Standard configuration. Don't worry.
BTW, option3, I meant other, only NEW additional generators with individual NGR for each generator, exsisting installation will no change.

Good Luck.
Slava

 

[trosepe]

Space, budget and operational considerations are all important. All three options are going to be workable and all have pro's and con's. Choose the one that fits best with the conditions you are faced with. Option #2 seems as if it would accomplish the most for the least ammount of space and is probably the best retrofit option. Develop a good plan up front and work out all the issues you can in the planning stage. It will be time well spent.

 

[EddyWirbelstrom]

Where generators are directly connected to a common bus without a unit transformer, I have seen systems where each generator star point is earthed via a switched neutral earth resistor. This neutral switch is closed on only one generator to avoid circulating 3rd harmonic currents between generators due to different generator pole pitch.